Download Calculation, verification and monitoring of patient dose in Diagnostic

Calculation, verification and monitoring of patient dose in
Diagnostic and Interventional Cardiology
By
Sandra Ann Hopkins
A dissertation submitted to the Department of Physics,
University of Surrey, in partial fulfilment of the degree of
Master of Science in Radiation and Environmental Protection
Department of Physics
Faculty of Electronics and Physical Sciences
University of Surrey
August 2010
© Sandra Ann Hopkins 2010
Abstract
Routine Coronary Angiography is a relatively high dose diagnostic procedure. The results from a
diagnostic procedure may indicate the need for an interventional examination, which has the
potential for even higher patient dose. Doses for these procedures are monitored via an integrated
dose area product meter (DAP). Calibrated dose area product meters were used to measure DAP for
a group of standard sized patients and compared against recommended reference levels. This was
carried out on two virtually identical systems that were installed approximately 5 years apart.
Results indicated that patient doses were higher on the new lab despite cardiologist technique
appearing to have improved (25.1Gy.cm2 for the old lab and 32.2Gy.cm2 for the new lab). The main
reason for this increase was due to dose saving filters not being automatically inserted by the system
during acquisition runs in the new lab. Medical Physics are working with applications specialists
and users with the aim of reducing doses in the new cath lab below the 29Gy.cm2 reference level.
The equipment also has a skin dose indicator, which can be used to monitor skin dose to warn users
if dose levels might reach the deterministic 2Gy threshold. This skin dose value assumes all
radiation is directed at the same region of the skin. However, the change in x-ray tube orientation
throughout a procedure means that the dose is distributed around the patient’s back. A Matlab
program was previously developed for a different piece of imaging equipment to replicate the dose
distribution and determine the region of maximum dose on the patient’s back. This was adapted for
the local equipment. Gafchromic film was used to produce a visual indication of how the dose
would be distributed in order to compare with the local Matlab programme. An exact match was not
found but the programme was felt to be sufficient to warn users whether skin effects were a
possibility. Measurements using TLDs highlighted the fact that skin dose values are an under
estimate as they do not include any contribution from scattered radiation from nearby acquisition
runs. The programme was used on a sample of routine diagnostic coronary examinations. This
demonstrated that there should not be any risk of skin effects for this standard procedure. The
programme was also used to demonstrate skin dose distribution in a more complex interventional
case. These complex procedures need to be analysed on a case by case basis as it is difficult to
generalise on the dose distribution and the possibility of skin effects. However, it is possible for the
2Gy limit to be reached during a complex interventional examination and the local programme
would be able to determine which regions of the patient’s back would be of most concern. It is
expected that doses should be lower with digital flat plate detectors but evidence demonstrates that
this is not always the case. Optimisation work must be carried out on new equipment to ensure that
doses are always as low as reasonably practical in line with current legislative requirements.
I
Acknowledgements
I would like to express my gratitude to the following people for their assistance and advice. In the
cardiac catheterisation laboratories at Portsmouth Hospitals NHS Trust: Clare Smith for her support
with the collection of patient dose data. To all my colleagues in the Radiological Sciences Group
for covering for me and encouraging me whilst I worked on this project. A special thank you to
Antonio De Stefano for working through the programming aspects of the project. Finally, many
thanks to my supervisors: Anne Davis from Portsmouth Medical Physics and Professor Patrick
Regan of the University of Surrey for their support and advice.
II
Contents
1. Introduction
2. Theory
2.1 Cardiac Angiography
2.2 Interventional Cardiography
2.3 Imaging
2.3.1
X-ray Generator and Tube
2.3.2
Flat Plate Detector
2.3.3
Equipment Configuration
2.4 Patient Radiation Dose
2.4.1
Stochastic Effects
2.4.2
Deterministic Effects
2.4.3
Dose Reference Levels
2.4.4
Guidance for High Skin Dose Procedures
2.5 Measuring Patient Dose –Dose Area Product Meter and Skin Dose Indicator
2.6 Patient Dose Record
2.7 Gafchromic Film
2.8 Thermoluminescence and Thermo Luminescent Dosimeters
2.9 Skin Dose Distribution Software
3. Method
3.1 Verification of Patient Dose Indicators
3.1.1
Dose Area product
3.1.2
Skin Dose Indicator
3.2 Measuring Patient DAP and Comparison with Previous System
3.3 Comparison of Equipment
3.3.1
Detector Dose Rates
3.3.2
Simulated Patient Skin Absorbed Dose Rates (SADRs)
3.4 Practical Verification and adaptation of Skin Dose Distribution Software
3.5 Analysis of Skin Dose Data using in house Distribution Software
4. Results
4.1.1
Verification of Dose Area Product Calibration
4.1.2
Verification of Skin Dose Indicator Calibration
4.2 Measuring Patient Doses and Comparison with Previous System
4.3 Comparison of Equipment
4.3.1
Detector Dose Rates
III
4.3.2
Simulated Patient Skin Absorbed Dose Rates (SADRs)
4.4 Practical Verification and adaptation of Skin dose Distribution Software
4.5 Analysis of Skin Dose Data using in House Skin Dose Distribution Software
4.5.1
Diagnostic Cardiac Angiography
4.5.2
Therapeutic Examinations
5. Discussion
5.1 Verification of Dose Area Product and Skin Dose Indicator Calibration
5.2 Measuring Patient DAP doses
5.3 Practical Verification and Adaptation of Skin Dose Distribution Software
5.4 Equipment Considerations and Dose Reduction Techniques
6. Conclusion
7. References
8. Appendices
A. Example of a Typical Routine Coronary Angiogram Study Report
B. Mathematical modelling and equations of original Matlab dose
distribution model
C. Dose Report for Thermoluminescent Dosimeter irradiation of Gafchromic
film
D. Patient Dose Area Product data for the Old Cardiac Cath Lab
E. Patient Dose Area Product data for the New Cardiac Cath Lab
F. Excerpt from final Matlab programme using definitions and equations
defined in Appendix B
G. Maximum Skin dose values for a set of Routine Coronary Angiograms
H. Output from Matlab Programme – key elements for Routine Diagnostic
Coronary Angiogram
I. Example Study Report for a therapeutic examination of 39 runs.
J. Output from Matlab Programme – key elements for Therapeutic
Coronary examination
IV
1. Introduction
The use of ionising radiation for clinical diagnosis is a long established method for the examination
of a variety of medical conditions. Different imaging modalities and complexities of examinations
mean that some procedures contribute more to patient dose than others. Figure 1 shows the
contribution of the collective dose from the 15 diagnostic examinations which provided the largest
contribution to the collective dose. It can be seen that, with the exception of Computed Tomography
(CT), routine diagnostic coronary angiography provides the largest contribution to the collective
dose in the UK. Although the frequency of the examination is low (0.4%), the dose for each
examination can be high. UK estimates calculate that this exam produces 5.6% of the total
collective dose1. A more recent study for the European population estimates that this contribution
has increased2 and this is attributed to the fact that this procedure can be performed on an out
patient basis and causes less trauma than surgical procedures. There have also been technical
developments which have led to an increase in frequency such as the introduction of digital
detectors and the evolution of stents. Coronary angiography is the standard technique for imaging
the left ventricle and coronary arteries. It is a routine diagnostic procedure with a clearly established
method for performing it. Although patient doses are considered to be relatively high for a
diagnostic procedure, using current imaging techniques it is unlikely to ever reach a level where
deterministic effects could be considered a possibility. However, the results of a diagnostic
examination may indicate that treatment is required. This treatment could involve a more complex
interventional procedure such as a graft or insertion of a stent.
Figure 1. Contribution to UK collective dose and frequency from the 15 exams making the biggest contribution to the
collective dose1.
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16
Collective Dose
Frequency
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These types of interventional procedures can be very complex and there is the potential to achieve
skin entrance doses that could potentially lead to deterministic effects such as skin damage.
The FDA (Food and Drug Adminstration) publication gives advice on precautions to be taken to
reduce skin dose3. The biological effects of ionising radiation on the skin have been analysed by the
International Commission on Radiological Protection (ICRP) who have published advice and
guidance4,5,6.
With increased user awareness and developments in imaging techniques there have been far less
incidence of skin damage. However, it is still possible that skin damage could occur in complex
cases and some imaging equipment designers have included a skin dose indicator on their
equipment. This indicator provides the total accumulated skin dose for a single examination.
However, this value assumes that the x-ray beam orientation has been continually focussed in one
direction so that the same area of skin has been irradiated throughout the entire procedure. In
reality, this is rarely the case and the x-ray beam will be oriented in a variety of ways to improve
visualisation of particular coronary vessels. This will mean that the accumulated skin dose is an
over estimate of the actual maximum skin entrance dose to the patient and can be misleading and
this has been acknowledged in other studies7. Although, the indicator is a good warning device to
let users know that the total patient skin dose is high it does not give them a true indication of the
distribution of the dose on the patient’s skin. Users would benefit from knowing when it is likely
that the skin damage threshold has been reached so that in this case the patient can be followed up
and appropriate treatment and consultation be provided as required. Some manufacturers have
previously developed computer packages to monitor skin dose distribution but these are no longer
commercially available. Attempts have since been made to develop dose distribution software 8,9.
This current project includes adaptation of existing software to provide skin dose distribution
information for local equipment.
2. Theory
2.1 Cardiac Angiography
This is one of the most commonly performed imaging tests for evaluating the heart and its major
vessels. Coronary angiography can show the exact site and severity of the narrowing of arteries.
After the introduction of a catheter into a peripheral vessel, usually the femoral or axilliary vein or
artery, the cardiologist, under direct fluoroscopic visualisation, guides the catheter intravascularly to
the region of interest, injects contrast media to confirm the location of the catheter then injects
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larger amounts of contrast material for diagnostic purposes. The injection of contrast media can be
recorded for later review. Figure 2 shows how the catheter is located and its final position.
Figure 2 : Diagram to show catheter insertion and the main arteries of the heart that are visualised.
The narrowing of the arteries is known to cause angina and is caused by atheroma which are fatty
patches which develop inside the lining of arteries.
In order to accurately assess the condition of the coronary vessels it is necessary to view the heart
from a number of different angles and directions. This requires an extremely manoeuvreable piece
of fluoroscopic equipment. The most common equipment is the ceiling suspended or floor mounted
C arm fluoroscopy imaging system.
In order to visualise all the coronary vessels and assess the
extent of any damage, there are a set of routine standard
orientations that are typically employed. Figure 3 shows
fluoroscopic equipment positioned in the LAO (Lateral
Anterior Oblique) position at an angle of 40 degrees. This is
one of the standard views for the right coronary artery.
During a coronary angiography procedure fluoroscopy is
used in the first instance to localise the catheter and then to
confirm positioning before acquisition. Acquisition runs are
essentially a high frequency pulsed radiography record of the
heart whilst the contrast media is pumped through the
Figure 3 : LAO 40 orientation for the Philips Allura 9
3
Coronary arteries. The length of time of fluoroscopy and acquisition and the number of
acquisition runs will vary depending on the complexity of the procedure and also the
experience of the cardiologist performing the examination.
Figure 4 : Coronary arteries imaged from LAO 40
Figure 5 : Coronary arteries imaged from RAO40Caud25
Standard views employed at Portsmouth Hospitals NHS Trust for routine diagnostic coronary
angiography are as follows:
For viewing the left coronary artery;
i)
Right Anterior Oblique (RAO) 10o
ii)
RAO 30o
iii)
RAO 30o Caudal 25o
iv)
RAO 30o Cranial 25o
v)
LAO 30o Cranial 25o
vi)
LAO 40o Caudal 25o
For viewing the right coronary artery
i)
LAO 40o
ii)
RAO 30o
iii)
LAO 40o Cranial 25o
Figures 4 and 5 show images taken from an LAO40 and a RAO40 Caud25 orientation respectively.
Typically a radiographer will automatically select these tube orientations unless the cardiologist
asks for different/additional views in order to assist with his investigation. These standard views,
however, may not be employed in a different hospital and can depend on the cardiologist’s
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experience10. There are three field sizes on this equipment; Normal (nominally 25cm diagonal at the
detector), Mag 1 (20cm) and Mag 2 (16cm). Typically, for routine diagnostic coronary angiography
the 25cm field is used for fluoroscopy work whilst positioning the catheter and the Magnification
(20cm) field is used during acquisition runs where the contrast media is injected and the flow
through the vessels recorded. Acquisition has been demonstrated locally to provide more than 80%
of the dose during a routine procedure. However, other centres have recorded different proportions
which will be dependent on operator technique and equipment specification11,12.
2.2 Interventional Cardiography
For interventional examinations the tube orientation will depend on the location of the vessel that
needs to be treated. In this case only a few orientations are selected and the number of acquisition
runs will depend on the nature and complexity of the treatment. It is these types of procedures that
present the greatest risk to the patient of deterministic effects.
Cardiac catheterisation is a procedure used to
determine if there is a problem. Once a problem is
identified then treatment is required which can be in
the form of angioplasty, coronary stents or cardiac
surgery where possibly a graft is required. In
angioplasty a deflated balloon is inserted into
narrowed coronary arteries and then inflated in order
to widen it. The balloon is removed once the
widening has been completed (figure 6). A stent is
permanently placed inside an artery to keep it open
(figure 7). These types of interventional techniques
Figure 6 : Insertion of a balloon catheter
Figure 7 : Insertion of a stent.
5
are more complex and can require more extensive imaging. As only one area of the heart may be
treated the x-ray beam may be orientated toward the same area of the patient’s skin for longer
periods of time and has an increased likelihood of developing skin damage.
2.3 Imaging
2.3.1 X-ray generator and tube
The imaging equipment used in this case is a Siemens Axiom Artis which is powered by a
Polydoros 100 kW high frequency multipulse generator. The X-ray tube is a Megalix Cat 125/35/80
and is dual focus (0.4 and 0.8 mm with 8 degree target angle). The imaging chain contains a flat
panel amorphous silicon (CsI scintillator) digital detector. Nominal image field sizes are 25, 20 and
16 cm. Source to image distance ranges from 90 to 120cm. All fluoroscopy is pulsed with optional
pulse rates varying from 0.5 to 30 pulses per second. The digital radiography mode has an
acquisition rate of 15 or 30 frames per second. A 1024 pixel matrix is used. All modes use
automatic dose control with no manual exposure control available to users. There are copper dose
saving filters available on this system with filter thicknesses which vary from 0.1 to 0.9 mm Cu.
The use of these filters is automatically linked to the programme selected by the user and is based
on the X-ray absorption by the patient. Filtration is available for both fluoroscopy and acquisition.
2.3.2 Flat plate detector
Technical developments in imaging have meant that many systems that previously had an image
intensifier as the detecting medium now have a flat plate detector. In this case the caesium phosphor
converts x-ray photons to light photons. The amorphous silicon is sensitive to light and converts the
light to an electrical signal. The light is captured by a ‘pixel’ in the photodiode/thin film transistor
(TFT) array and converted into an electronic signal. Contrast resolution is improved by reducing the
temperature as this results in less noise. A key difference resulting from using a digital detector
instead of an image intensifier is that digital detectors result in very little image distortion.
Additionally high contrast resolution improves with magnification using an image intensifier
whereas resolution remains constant with a digital detector since the resolution is determined by
pixel size.
6
2.3.3 Equipment configuration
Figure 8 shows a schematic of the Siemens Axiom Artis dFC in the AP direction. The distance of
the x-ray tube from the floor is fixed, however the height of the detector can vary so that the
Figure 8 : Schematic of the Axiom Artis dfC
range of distance for the source to detector (SID) can be from 120 cm to 90cm. In clinical use the
couch top is typically positioned 15cm below the iso-centre (centre point of rotation). This is so that
the patient’s heart is at the iso centre and the c-arm rotates around this central point. In this position
the couch top is at 60cm from the focus. This is defined as the point of reference for calculating
patient entrance skin dose. The detector is then moved as close to the patient as possible, typically
5cm from the patient’s surface. Typically the SID will be 95-105cm depending on patient size and
tube orientation.
2.4 Patient Radiation Dose
2.4.1 Stochastic Effects
If a cell is damaged by radiation the repair process may result in mutations and are evident as cancer
and hereditary effects. There may be numerical alterations such that the cell carries more or less
than the normal number of chromosomes (eg Down’s syndrome) or there may be structural
alterations where a chromosome has been broken and a segment has either been lost from the cell
(deletion) or attached to another chromosome (translocation). These types of effects are known as
Stochastic effects. There is assumed to be no threshold dose below which an effect will not occur
and as the exposure increases so the risk of stochastic disease increases. At low doses, it is assumed
7
that the dose response relationship is linear5. For all coronary angiograms there will be a risk to the
patient of inducing cancer. However, the benefit to the patient for having the examination exceeds
the risk to the patient of contracting cancer. Typical effective dose for a standard coronary
angiogram is ~ 7mSv and this equates approximately to a risk of inducing cancer of 1 in 2600.
(based on current risk factors for fatal and non fatal cancers of 5.5% per sievert 13). For
interventional examinations, where the dose may be higher, the stochastic risk will also be
proportionally higher.
2.4.2 Deterministic Effects
If sufficient numbers of cells are damaged there will be temporary or permanent loss of organ
function. This is a termed a deterministic effect. For this type of damage there is a threshold dose
below which no effect will be seen. The severity of the effect depends on the level of exposure
received.
Skin Effect
Approx threshold Time of
dose (Gy)
Early Transient erythema
Description
onset
2
2 – 24
Temporary skin reddening
hours
Main erythema reaction
6
~1.5 weeks
Reddening and oedema of
skin (burning and itching)
Temporary epilation
3
~ 3 weeks
Temporary hair loss
Permanent epilation
7
~ 3 weeks
Permanent hair loss
Dry desquamation
14
~ 4 weeks
Flaky skin
Moist desquamation
18
~ 4 weeks
Blistering of superficial
skin. Possible exposure to
infection
Secondary ulceration
24
> 6 weeks
Delayed healing.
Late erythema
15
8-10 weeks
Mauve skin discolouration
Ishaemic dermal necrosis
18
> 10 weeks
Vascular damage which can
effect dermis function
Dermal atrophy
10
> 52 weeks
Epiermis reduced to a few
layers of cells.
Telangiectasis
10
> 52 weeks
Dilation of superficial
dermal capillaries
6
Table 1 : potential skin effects from radiation exposure .
8
Skin injuries can occur when skin dose levels exceed the 2Gy threshold for deterministic effects.
With protracted examinations there is a risk to the patient of a deterministic effect occurring. A
summary of the potential effects from cardiac exposures on the reaction of the skin is given in table
1. Skin effects have been discussed in the literature and retrospective assessments of skin dose
made14, 15, 16, 17, 18, 19.
2.4.3 Dose reference Levels
A joint document by the Royal College of Radiologists (RCR) and the National Radiological
Protection Board (NRPB) produced a document in 1990 suggesting the concept of a reference dose
in order to indicate ‘abnormally high doses’20. In 1992 a National Protocol was developed
suggesting methods on how local departments could compare their doses with these National
Reference Doses21. The suggestion was to take a representative sample of close to standard sized
patients and compare the mean with the reference dose. The reference dose was the third quartile
value derived from a national database of dose data from many contributing hospitals. If the mean
dose exceeded this reference dose then an investigation was required in order to take corrective
action or to justify the high dose on clinical grounds. This same concept was adopted by the
International Commission on Radiological Protection (ICRP)5, 22 and the Diagnostic Reference
Level was introduced. In 1997 this concept was taken up as a European Medical Exposure
Directive23 and the requirements of this were implemented in the UK by the Ionising Radiation
(Medical Exposure) Regulations 200024. In IRMER, Diagnostic Reference levels (DRL) were
defined as ‘dose levels for typical examinations for groups of standard-sized patients or standard
phantoms and for broadly defined types of equipment’. It is important to note that this relates to
‘diagnostic examinations’ so will apply to routine diagnostic coronary angiography examinations
but does not apply to therapeutic coronary exams such as angioplasties. DRLs are based on a
national database which is extended and revised every five years. Recommended DRLs for the
current database are determined from this data25, 26. For Coronary Angiograms the current and
previously recommended DRL is given in the table 2.
9
NRPB W-14 25
HPA-RPD-02926
(June 02)
(August 07)
Recommended DRL DAP per exam (Gy.cm2)
36
29
Number of hospitals
7
38
Fluoroscopy time per examination (seconds)
294
243
Mean patient range
75 – 85 kg
75 – 85 kg
Number of rooms
17
110
Number of patients
8000
34236
Fluoroscopy time range (seconds)
185 to 385
93 to 606
Third quartile time (seconds)
337
270
Mean DAP (Gy.cm2)
30.4
25.7
DAP range (Gy.cm2)
11.8 to 60.7
11.7 to 72.5
First quartile DAP (Gy.cm2)
22.3
18.9
Third quartile DAP (Gy.cm2)
36.3
29.0
Patient age (years)
60(16-97)
62 (16-99)
Patient weight (kg)
78(35-172)
79(29-183)
Number of images per exam
Not available
737(6-2200)
Table 2 : Summary of data used to derive Diagnostic Reference Levels
2.4.4 Guidance for high skin dose procedures
Although guidance documents25, 26 do give some data on Coronary angioplasties, strictly speaking
Diagnostic Reference Levels do not apply. However there have been studies of doses from
interventional cardiac examinations where reference levels have been proposed 27, 28. More
emphasis is placed, however, on making interventionists more aware of the potential injury from
these procedures and methods for decreasing their incidence by using dose control strategies6.
2.5 Measuring Patient Dose - Dose Area Product Meter and Skin dose indicator
The dose area product (DAP) meter is a measure of dose in Gray multiplied by the area irradiated.
This gives an indication of patient dose that can be used to compare with the same parameter for the
same exam. DAP is an easily available measurement which is commonly used to monitor patient
doses for many routine diagnostic imaging procedures. Some imaging systems calculate the DAP
from known exposure factors but it is understood that the Siemens Axiom Artis uses an integral
ionisation chamber to determine the DAP. In addition to this value, the system also gives a value of
skin entrance dose in mGy. This is calculated at a reference point of 60cm from the focal spot. This
is equivalent to a distance of 15cm in front of the isocentre which is taken to be the most typical
10
couch height position when the centre of the heart is positioned at the isocentre. Siemens apply a
tolerance of +/-30% to this but it is known that greater accuracy can be achieved.
2.6 Patient Dose Record
For every examination, on the Siemens Axiom Artis, there is a patient dose record. This provides
information on every acquisition run including the field size, skin dose in mGy, Dose Area Product,
amount of copper in the beam, frame rate and tube orientation. Fluoroscopy runs are not recorded
individually. However at the end of the dose record the total dose is recorded and from this the
proportion of dose as a result of fluoroscopy can be calculated by subtracting the total accumulated
dose from the individual acquisition runs. Appendix A shows an example patient dose record.
2.7 Gafchromic film
It is possible to get a visible indication of skin dose distribution by using film. Large films with
slow x-ray response are required (doses are too high to use conventional diagnostic imaging film for
this technique)29, 30, 31 although common resin coated photographic paper has also been considered
as an alternative32. Kodak EDR2 film, which was originally used for portal imaging and
radiotherapy has been used for skin dosimetry33, 34, 35. There are limitations to using this film since it
saturates at 1 to 1.5Gy which means it may not be adequate for high dose interventional cardiology.
In addition, the requirement for wet processing can be time consuming and cumbersome.
Gafchromic film does not saturate at higher radiation doses and does not require processing.
Currently Gafchromic film is too expensive to use routinely for patient skin dose mapping but it is a
useful tool to provide an immediate visual indication of the dose distribution36, . The films develop
by changing colour from their original orange to a grey level which becomes progressively darker
in proportion to absorbed dose. The active component in radiochromic dosimetry film is a long
chain fatty acid, which when exposed to radiation, active diacetylenes are polymerised to produce
polydiacetylene which result in the distinctive colour change. Variations in construction provide for
different sensitivities of Gafchromic film. That used in this case is XR Type R dosimetry film. The
response of the film from 80kVp to 120 kVp x-rays is energy independent and is also dose rate
independent. XR Type R dosimetry film can be handled in normal room light for several days
without noticeable effects. The quoted acceptable dose range for this film is from 0.1 Gy to 1.5 Gy.
2.8 Thermoluminescence and Thermo Luminescent Dosimeters
There are many theoretical models to describe thermoluminescence but none can completely
describe the complex mechanism in specific substances. However, a general theoretical mechanism
has been described with reference to the crystal structure of the alkali halides. All crystals contain
11
lattice defects which are an important part of the thermoluminescence process. When the crystal
absorbs ionising radiation free electrons are produced. These may become trapped in the lattice
defects. Additionally, the holes that are produced in conjunction with the free electron, may also
become trapped. Many hole centres are thermally unstable and may decay quickly at room
temperature. The electrons will remain trapped provided they do not gain sufficient energy with
which to escape. The energy required depends on the depth of the trap and the temperature of the
material. Released electrons may recombine with holes at luminescence centres with the excess
energy being radiated as visible or ultraviolet photons. This electron capture and delayed
recombination with a hole at a luminescence centre is what makes up the process of
thermoluminescence. The complete process is displayed in figure 9
Figure 9 : A simple energy band model for thermoluminescence
A glow curve is a plot of thermoluminescence intensity against temperature and can be derived
from the electron release formula. The equation for the glow curve intensity from electrons at a
single trapping level E is given as;
I = noCexp – [ 1/R .s.exp(-E/kT)dT].s.exp(-E/kT)
37
Where E is the trapping level, R is the heating rate, no is the number of electrons present at time to
and temperature To, C is a constant related to luminescence efficiency, k is the Boltzmann constant,
T is the temperature and s is a frequency factor associated with the particular lattice defect.
12
This formula provides the idealised case for a single trapping level. At low T the curve rises
exponentially and after reaching a maximum ‘the glow peak’ it falls to zero. In reality there will be
more than one trapping level each of which will give rise to a glow peak maximum. The area and
peak height of each glow peak depends on the number of lattice defects and the amount of impurity
atoms present.
The TLDs used are known as TLD-100 (LiF : Mg : Ti). This is Lithium Fluoride doped with
magnesium and titanium. This is the most understood and commonly used phosphor.
Although the exact process is unclear, it is suggested that the magnesium ions form electron traps in
combination with certain defect centre in the lattice. The effect of titanium is thought to be in the
formation of luminescence recombination centres.
The reader used is a Harshaw model 5500 automatic TLD reader. Figure 10 shows an example of
this TLD reader.
The reader uses nitrogen gas during the read process mainly to suppress the chemiluminescence
signals from the dosimeter and the reader. In addition it purges the Photomultiplier assembly,
keeping moisture out of this area. The dosemeters are lifted into the read chamber by a vacuum
pick. The TLD is heated as a function of time according to a time temperature profile (TTP). This
profile is divided into three parts; the preheat, acquisition and anneal. The preheat ensures that all
dosemeters in a group have a common starting point and can also be used to eliminate the fasterfading low temperature peaks. The dosimetric data is collected and the glow curve generated during
the acquisition stage. The light detected by the photomultiplier tube at this stage will generate a
current proportional to the amount of light detected. The anneal stage is used to hold the
temperature at a sufficiently high temperature to ensure that all the signal is removed. A calibration
is required to be carried out to produce a reader calibration factor which converts the raw data in
nano coulombs into the desired dose units, in this case mGy.
13
Figure 10 : The model 5500 Harshaw TLD reader
2.9 Skin dose distribution software
The basis for the skin dose distribution software was a previously reported Matlab programme
designed for imaging equipment designed for cardiac work, but by a different manufacturer8, 9. The
aim of the mathematical model was to calculate the dose distribution in the plane of the couch top,
from exposure and projection data stored in the patient dose record. Appendix B describes the
mathematical processes used to develop this original model and includes the main equations used in
the original programme. It is of note that this skin dose software was developed for a system that
had exposure factors available but did not have skin dose available so the Matlab programme also
had to calculate skin dose based on a measured entrance surface dose using 25cm Perspex and then
normalising for the tube potential, current, pulse time and number of frames given for each
acquisition run. This aspect of the programme was not required. The initial programme developed
did not include the contribution due to fluoroscopy and different suggestions were made as to how
to include this. This aspect of the programme was also not required as data from the Siemens
system made it possible to determine the total fluoroscopy skin dose which could again be utilised
to improve the accuracy of an adapted programme.
14
3. Method
3.1 Verification of Patient Dose Indicators.
3.1.1 Dose area product
Since the Dose Area Product data is used to assess the dose to the patient and the values are used for
audit it is necessary to ensure that the meter is correctly calibrated. The manufacturer’s work to a
tolerance of +/- 30% but ideally better accuracy is preferred.
The C-arm is positioned in a Lateral position
as shown in figure 11. An MDH 9020 meter
and 6cc ionisation chamber is placed in the
centre of the image field at a known distance
from the x-ray tube focus. For different set
kV values the dose area product is recorded
from the system and the total output from the
ionisation chamber. In order to calculate the
dose area product it is necessary to obtain the
field area at the ionisation chamber distance.
Figure 11 : Standard set up for measuring skin dose removing table attenuation and backscatter.
This is done by replacing the chamber with an imaging plate at exactly the same distance. This is
exposed and the field area is measured. Typically this is carried out during annual performance
testing of the equipment with all automatic Copper filters removed from the beam. In order to
increase exposure factors during testing it is necessary to place large area copper filters in front of
the detector. The increased attenuation drives up the exposure factors and allows the calibration to
be checked across the full clinical kilovoltage range. Accuracy of the MDH chamber is ensured by
sending it annually to a national calibration test house and applying any correction factors provided.
3.1.2 Skin Dose Indicator
Method 1 – Ion chamber free in air
Since it is known that the skin dose indicator is either calculated from the Dose Area Product data
or directly from exposure factors the set up above in figure 11 can be used since this eliminated
both table attenuation and back scatter. This is a free in air exposure. Some manufacturers calibrate
their dose meters free in air whilst others include the table within their measurement. This is usually
carried out with automatic filters removed from the beam. However, on this occasion some
measurements were also made with Copper attenuation added to confirm that the calibration took
15
this into account. In order to increase exposure factors during testing large area copper filters are
again placed in front of the image detector.
Method 2 – Ion chamber with scatter
It is also possible to make a skin dose calculation under simulated clinical conditions as shown in
figure 12. This will include attenuation due to the table and back scattered radiation from the
patient. This method is a standard method used for measuring skin entrance dose38.
The two different methods were compared in order
detector
to determine the difference in skin dose due to these
factors. The information could then be used to make
adjustments to any differences between indicated
Perspex
and calculated skin dose.
Both methods consider each exposure separately but
Ionisation
chamber
will not take into consideration any additional dose
due to backscatter from nearby exposures on the
skin.
X-ray
Figure 12 : Measuring skin dose under simulated clinical conditions
Method 3 – Thermo Luminescent Dosimeters
Thermo Luminescent dosimeters were placed individually into light proof sachets and positioned on
the film in a grid across the whole area (figure 13). TLDs were positioned 4cm apart. The top of the
couch top was positioned at 60cm so that the indicated/calculated skin dose should correspond to
the actual dose at the couch top. 25cm Perspex was positioned above the Gafchromic film to
provide typical attenuation for a large patient.
In order to be clear about the x-ray field
distribution, the following orientations were
used for separate acquisition runs.
1) AP
2) RAO 30
3) CAUD 30
4) LAO20CRAN30
Figure 13 : TLDs positioned on Gafchromic film
16
The field size was set to Mag 1 (20cm field) to simulate what is typically used clinically and
acquisition runs were made to be long enough to be above the minimum quoted sensitivity of the
Gafchromic film (100mGy). The dose record for these runs was saved to CD for future reference.
4 TLDs were not dosed to act as a reference for background dose and 8 other TLDs were dosed to a
known dose in order to determine the dose to the TLDs placed on the Gafchromic film. The
measured dose from the TLDs was compared to the dose indicated in the final dose record.
Since the Gafchromic film was being placed on top of the table, the attenuation of the table needed
to be taken into account. The attenuation of the table is given in technical details as less than 1.2mm
Aluminium at 100kV.
3.2 Measuring Patient DAP Doses and Comparison with Previous x-ray system
Screening log books were designed to ensure that all necessary information required for the analysis
was recorded routinely. Data required for each examination was as follows;
a) Screening time
b) Fluoroscopy dose
c) Acquisition dose
d) Number of acquisition runs
e) Number of images
f) Patient weight
Figure 14 shows an example of part of a screening log book for coronary angiograms. It is
important that local data is compared with reference data for a similar mean weight and that it is not
skewed by extremely small or large patients. The mean patient weight for the current reference data
is 79kg. This is higher than the mean weight typically used for standard radiographic dose audits.
This is because cardiac patients are typically heavier on average than patients requiring standard
radiographic exams. For this reason only patients within 60kg and 100kg were considered in the
analysis and the mean weight was required to be within 5kg of 79kg.
Data was collected until there was approximately 80 patient dose records available. The data was
Procedure(s)
(Please tick all that apply)
Weight (kg)
Height (cm)
Sex
M / F
Straightforward
Right Coronary
Difficult
Left Ventricle
Exam Completed
Satisfactorily?
Radiographer (initials)
Angioseal
Operator (initials)
Deleted?
Booked/Tracked
Registrar or Consultant?
Figure 14 : Example of Cardiac Angiography screening log book
17
(Please tick one)
Left Coronary
Please attach patient label here
Archiving checked
Degree of difficulty of procedure
Fluoroscopy Dose
Level?
Comments?
Dose (µGy.m2)
Percentage (of 2Gy)
Y / N
Low / Normal / High
then analysed and compared with data taken from the older cardiac catheterisation laboratory which
was essentially the same equipment. The mean of this data is also compared with the National
Diagnostic Reference Level given in section 2.4.3.
3.3 Comparison of X-ray Equipment
Measurements were taken on both x-ray systems to determine the performance of each so that any
differences between the two systems could be considered when comparing patient doses.
3.3.1 Detector Dose Rates
Fluoroscopy imaging equipment is set up by engineers at commissioning and checked routinely
during annual visits to ensure that the detector dose is within specification. For Siemens systems
this is checked using 2mm of Copper on the x-ray tube, removing the anti scatter grid and
positioning a 60cc flat ionisation chamber at the centre of the detector face with the focus to
detector at maximum distance. Ideally detector dose rates should be within 20% of manufacturer’s
specification. All dose levels are checked and the variation of dose with field size is also checked.
3.3.2 Simulated patient Skin Absorbed Dose Rates (SADRs)
In order to interpret patient dose results in each lab it is necessary to compare the SADRs in each
lab under standard conditions and using fluoroscopy and aquisition curves typically used during
clinical examinations. A standard phantom of 19cm Perspex is used and set up as already described
in section 3.1.2 (method 2) for skin dose indicator accuracy measurements.
3.4 Practical Verification and adaptation of Skin Dose Distribution software
Measurements of the field size at the detector face were established as this was required for input to
the programme. This was done by placing a radio opaque ruler directly onto the detector face and
measuring the length of the visible ruler on the display monitor.
Gafchomic film (dimensions 35cm x 43cm) was used to confirm the position and orientation of the
x-ray field as the x-ray tube is rotated in various positions. Measurements were taken from the
Gafchromic film and correlated with images produced by the Matlab programme.
The original equations were used to produce skin dose distributions using Gafchromic film and the
Matlab programme for the following parameters:
0RAO, 35CRAN
0RAO, 35CAUD
35RAO, 0CRAN
35LAO, 0CRAN
This only looked at separate RAO/LAO and CRAN/CAUD rotations and no combinations of both.
18
Further combined orientations were trialled in order to assess how closely the Model recreated the
actual distribution.
It was also possible to manipulate patient dose records to amend the tube angle in order to simulate
particular orientations in order to carry out further analysis of the image display for the Matlab
programme.
3.5 Analysis of Skin dose data using in house dose distribution software
The adapted programme was used to calculate the peak skin dose for a number of routine diagnostic
exams in order to determine whether deterministic effects are of concern for this type of exam and
what would be a typical maximum skin dose value based on the indicated Dose Area Product value.
Additionally the programme was trialled on a therapeutic exam to ascertain how it might be used in
practise in a clinical environment to determine when a patient requires monitoring for skin effects.
19
4. Results
4.1.1 Verification of Dose Area Product Calibration
Table 3 shows the results of measurements carried out to determine the accuracy of the Dose Area
Product meter in the new lab. The field area was measured to be (0.013 +/- 0.002)m2. This was
measured at the same distance as the ionisation chamber.
Set kV Displayed
mA
Measured dose
Calculated Dose
Displayed Dose
Difference between
mGy
Area Product
Area Product
indicated and
µGy.m2
µGy.m2
actual %
+/- 2%
50
762.1
18.3 +/- 0.7
342 +/- 7
336.2
-1.5%
66
329.9
12.6 +/- 0.5
235 +/- 5
234.8
+0.1%
81
74.5
6.5 +/- 0.3
121 +/- 2
122.6
+1.1%
81
74.5
6.5 +/- 0.3
121 +/- 2
122.4
+1.0%
96
39.4
3.9 +/- 0.2
74 +/- 1
78.2
+6.1%
109
44.3
5.5 +/-0.2
103 +/- 2
107.1
+3.8%
Table 3 : Measurements to determine the accuracy of the Dose Area product meter
Thus, for the entire kilovoltage range the indicated Dose Area Product was within 7% of the actual
value. This is considered to be good accuracy. For the old lab the Dose Area Product was measured
to be within 10%
4.1.2 Verification of Skin Dose Indicator Calibration
Method 1 - Ion chamber free in air
Displayed Displayed Indicated
kV
Measured
Difference
mA
mGy/min
mGy/min
between indicated
+/- 2%
+/-5%
+/- 4%
and actual
70
187
72
63.5
+13.4%
60
138
35
31.4
+11.5%
50
752
165
152.4
+8.3%
81
166
82
73.8
+11.1%
90
312
221
196.1
+12.7%
90
555
450
404
+11.4%
70
181
68
61.7
+10.5%
20
70
183
67
60.7
+10.3%
102
205
175
159.3
+9.9 %
90
566
464
403.5
+15.0 %
90
312
222
198.4
+11.9 %
81
167
85
78.8
+7.9 %
70
182
67
61.7
+8.6 %
Table 4 : Measurements to determine the accuracy of the Skin Dose Indicator with no added Copper filters in the beam
With no additional Copper filtration in the beam the indicated skin dose is between 8% and 15%
higher than the actual skin dose (table 4). Additional measurements carried out at the same kV were
for consistency or were with different amounts of attenuation to change the mA and therefore the
indicated mGy/min.
With various amounts of additional copper automatically added into the beam the accuracy of the
skin dose indicator over reads by 15% to 22% (table 5)
Displayed Displayed
Added
Indicated
Measured
Difference
mA
Cu by
mGy/min
mGy/min
between indicated
+/-2%
system
+/-5%
+/- 4%
and actual
83.6
813
0.1 mm
226
191.7
+17.9 %
87.2
817
0.1 mm
409
341.6
+19.7 %
81.8
809
0.1 mm
338
286.2
+18.1 %
86.7
807
0.1 mm
291
255.3
+14.0 %
75.6
690
0.2 mm
189
167.2
+13.0 %
83.4
681
0.2 mm
180
155.6
+15.7 %
64.5
532
0.3 mm
44
38.2
+15.2 %
73.5
740
0.3 mm
74
60.5
+22.3 %
73.5
471
0.3 mm
60
50.3
+19.3 %
64.5
492
0.3 mm
28
22.9
+22.2 %
64.5
537
0.3 mm
44
38.4
+14.6 %
81
492
0.3 mm
73
60.7
+20.3 %
76.1
452
0.6 mm
30
25.5
+17.6 %
67.4
646
0.9 mm
18
15.2
+18.4 %
73.6
311
0.9 mm
9
7.5
+20 %
kV
Table 5 : Measurements to determine the accuracy of the Skin Dose Indicator with various Copper filters in the beam
21
Method 2 – Ion chamber with scatter
Table 6 shows the difference between indicated and measured skin dose for different exposure
conditions and different scattering conditions. All measurements were carried out with the table set
to –16 (16cm below the iso-centre) which would provide a focus to couch top distance of 60cm.
This would mean that the detector would be positioned at the same place that the indicated skin
dose indicator calculates its dose. All measurements were taken using the 20cm field which is used
for the majority of acquisition runs during a coronary angiography procedure.
Set
Source to
Thickness
Displayed
Displayed
Added
Indicated
Measured
Difference
kV
Imaging
of Perspex
kV
mA
Cu by
mGy/min
mGy/min
between
Plate
(cm) +/0.5
+/- 2%
system
+/- 5%
+/- 4%
indicated and
distance
measured %
(cm) +/- 0.5
70
91
16
70
158
0
57
50.8
+12.2%
81
91
16
81
93
0
42
37.9
+10.8%
60
91
16
60
299
0
87
74.3
+17%
60
94
21
60.8
800
0
302
274.6
+10.0%
70
94
21
70
404
0
188
157
+19.7%
81
94
21
81
227
0
126
117.6
+7.1%
96
94
21
96
128
0
90
79.7
+12.9%
96
99
25
96
277
0
216
190.6
+13.3%
109
99
25
109
184
0
182
153.6
+18.5%
81
99
25
81
497
0
323
295.2
+9.4%
70
99
25
72.7
800
0
462
439.7
+5.1%
70
99
25
75.2
800
0.1
275
249.4
+10.3%
109
99
25
78.3
660
0.1
250
223.4
+11.9%
102
93
21
66
775
0.2
112
112
0%
109
99
25
81
753
0.3
159
144
+10.4%
109
93
21
76.1
631
0.6
61
55.44
+10.0%
109
93
21
80.5
493
0.6
46
44.01
+4.5%
102
93
21
82.5
600
0.9
40
35.46
+12.8%
Table 6 : Difference in indicated and measured dose for different scattering conditions
It is of note that the indicated dose again over reads with respect to actual measured dose.
22
Method 3 – Thermo Luminescent Dosimeters
The diagram below shows the darkened areas of the Gafchromic film for each of the acquisition
runs. The Gafchromic film would have been turned over on the couch top so that the patient lay on
their back on the back of the film. Consequently the left hand side of the image below (figure 15)
equates to the left hand side of the patient.
Patient’s Head
Caudal
Patient’s
back RHS
Patient’s
back LHS
Cranial
RAO
LAO
Figure 15 : Dose distribution for TLD work
Table 7 gives the dose values of the TLDs (in mGy). The grid corresponds directly to the
Gafchromic image above such that A1 corresponds to the top right of the image above. Those doses
in red were within the darkened areas of the Gafchromic film and so correspond directly to patient
skin dose. The TLDs were from a batch that had a calibration of +/- 10% for the entire batch.
23
I
H
G
F
E
D
C
B
A
1
12.8
21.2
32.7
59.5
445.7
54
29.8
16.7
11.5
2
18.9
30.4
47.4
234.3
411.9
73.6
37.8
24.8
16.8
3
31.1
44.8
76.2
102.4
107.6
87
50.9
28.8
19.6
4
55.5
549.6
583
351.8
399
281.2
66.8
36.1
21.1
5
64.7
572.3 640.6
350.0
449.6
268.9
84.5
44.1
25.5
6
46.7
77.7
53.0
113.2
149.8
201.6
110.5
55.7
28.8
7
25.4
36.5
98.4
65.6
120.6
929.5
1067.8
63.5
27.7
8
15.2
21.0
34.2
24.9
68.5
964.3
1154.3
56.2
26.2
9
12.0
14.4
20.4
25.9
43.8
64.3
64.4
39.4
20.2
Table 7 : Thermoluminescent dosimeter doses in mGy and their location on the Gafchromic film
Taking those TLDs that were closest to the centre of each field to determine the skin dose the
following skin doses for each orientation were determined. These doses were compared with the
indicated doses given by the X-ray system (entire dose report available in appendix C).
Orientation
TLD dose
System indicated
Difference between
(mGy)+/10%
dose (mGy) +/- 5%
measured and indicated
AP
450
298
-33%
RAO 30
641
474
-26%
CAUD 40
446
310
-30%
LAO 20 CRAN 40
1154
785
-32%
The skin dose measured by the TLD is 26% to 33% higher than the skin dose indicated by the
system. This is significantly different to comparisons made using free in air and scatter conditions
using an ionisation chamber. In this case the indicated dose under reads whereas for previous
measurements it was shown to over read. These differences need to be considered when deciding on
the accuracy of the skin dose indicator. It is considered that the TLD dose is likely to be a more
accurate indicator of skin dose than the ionisation chamber as it includes contributions from scatter
from other acquisition runs in the neighbouring region.
4.2 Measuring patient doses and comparison between old and new cath labs
Appendix D and E give the detailed dose data for the old and new cath laboratories respectively.
Both systems are from the same manufacturer and have the same options available so that they can
be programmed in the same way.
24
This data is summarised schematically in figures 16 and 17 and a summary of the data is shown in
table 8
Patient Dose Area Product Data in old Lab
Fluoro Dose
90.0
Acquisition Dose
80.0
Mean
Current DRL
Dose Area product (Gy.cm2)
70.0
Old DRL
60.0
50.0
40.0
30.0
20.0
10.0
85
81
77
73
69
65
61
57
53
49
45
41
37
33
29
25
21
17
9
13
5
1
0.0
Figure 16 : Patient dose area product data in the old cath lab (results using DAP accurate to within +/-10%)
Patient Dose Area Product Data in New Lab
Fluoro Dose
100.00
Acquisition Dose
90.00
Mean
Dose Area product (Gy.cm2)
80.00
Current DRL
70.00
60.00
50.00
40.00
30.00
20.00
10.00
73
70
67
64
61
58
55
52
49
46
43
40
37
34
31
28
25
22
19
16
13
10
7
4
1
0.00
Figure 17 : Patient dose area product in the new cath lab (results using DAP accurate to within +/-7%)
25
Old Lab
New Lab
Mean DAP (Gycm2)
25.1
32.2
Standard Error in the mean
0.5
0.7
DAP range (Gycm2)
6.3 to 85.3
7.5 to 91.6
Mean Acquisition DAP (Gycm2)
21.4 +/- 0.5
27.2 +/- 0.6
Mean Fluoroscopy DAP (Gycm2)
3.7 +/- 0.2
5.0 +/- 0.3
86.5 %
85.8%
88
73
Mean screening time (seconds)
236 +/- 2
162 +/- 2
Mean Weight (kg)
81 +/- 1
80 +/- 1
Weight range (kg)
60 kg to 99.8 kg
60.2kg to 99.0 kg
Mean number of runs
10
9
Mean number of images
879
674
0 to 1545
393 -1357
Proportion Acquisition
Number of patients
Range of images
Table 8 : Summary of key data for comparison of Cath Labs
4.3 Comparison of Equipment
4.3.1 Detector Dose Rates
For the old lab the clinically used fluoroscopy curve used at the time of the patient dose survey was
called Fluoro card Smooth with a specification dose of 32 nGy/pulse. For the new lab the
specification dose was 29 nGy/pulse. To determine the dose per pulse for a particular field size it is
necessary to multiply by a field size factor. This is provided by the manufacturer. The field size
factors given for each lab are as follows :
Field size
Field size factor
Field size factor
(Old Lab)
(New Lab)
25cm (normal field)
0.54
0.83
20cm (Mag 1)
0.85
1.1
16cm (Mag 2)
1.66
1.3
10cm (Mag 3)
Not available
2.2
Table 9 : Field size multiplication factors for determination of detector entrance dose rates
26
The most commonly used field size for fluoroscopy during routine coronary angiography is the
normal (25cm) field. From Table 9 it can be seen that a larger field size factor is required for the
new lab. Since the nominal specification doses are similar this will mean that the detector dose for
the new lab will be higher. Table 10 gives the values of the detector dose determined at the default
pulse rate of 7.5 pulses per second.
kV
Added
Measured
Specification
Copper
nGy/pulse
nGy/pulse
61.5 146.9 0.6 mm
18.8 +/- 0.8
17.3
+ 8.7 %
27.6 +/- 1.1
24.1
+ 14.5%
0.6mm
28.2+/- 1.1
27.2
+ 3.7%
0.6mm
34.5 +/- 1.4
31.9
+ 8.2%
25cm field
Old Lab
New Lab
65
mA
107.6 0.6 mm
% difference
20cm field
Old Lab
63.1 155.2
New Lab
65
141.3
Table 10 : Detector dose rates for fluoroscopy
Measurements are also made on acquisition. Typically the Mag 1 (20cm) field size is used when
digital acquisition is being used. Table 11 gives the measured values for both the 25cm and 20cm
field. Typically the pulse rate used for aquiring images during coronary angiography is 15 pulses
per second. The measurements below were made using this pulse rate and a default specification
dose of 0.17 µGy/pulse.
25cm field
kV
mA
Added
Measured
Specification
+/- 2%
Copper
µGy/pulse
µGy/pulse
% difference
Old Lab
64.5
383
0.3 mm
0.09
0.098
- 8.2%
New Lab
81
70
0
0.14
0.16
- 12.5%
Old Lab
64.5
485
0.3 mm
0.145
0.146
- 0.7%
New Lab
81
94
0
0.20
0.19
+5.3%
20cm field
Table 11 : Detector dose rates for aquisition
There are obvious differences which will be discussed in more detail later. For fluoroscopy the
detector dose for the new lab is higher than for the old lab due to the fact that the field size factor is
higher. Apart from this the exposure factors are fairly similar. For the acquisition dose, the new lab
doses are higher than the old lab. Additionally, there are some significant differences in exposure
factors. These differences will have an effect on patient dose and will again be discussed later.
27
4.3.2 Simulated patient Skin Absorbed Dose Rates (SADRs)
Table 12 and 13 gives the measured SADRs for each lab for fluoroscopy and acquisition. The
differences in exposure factors between the two labs are again particularly noticeable on
acquisition.
25cm field
kV
mA
Added
Measured
+/-2%
Copper
mGy/min
Old Lab
64.7
145
0.6 mm
4.1 +/- 0.2
New Lab
68.4
102
0.3mm
4.0 +/- 0.2
Old Lab
66
164
0.6 mm
5.9 +/- 0.2
New Lab
68.4
102
0.3 mm
5.3 +/- 0.2
20cm field
Table 12 : Patient SADRs for 19cm Perspex for fluoroscopy (7.5 pps)
25cm field
kV
mA
+/-2%
Added
Measured
Copper uGy/exposure
Measured
mGy/min
Old Lab
64.5
561
0.3 mm
34 +/- 1
31 +/- 1
New Lab
81
171
0
78 +/- 2
70 +/- 2
Old Lab
66
615
0.2mm
59 +/- 1
52 +/- 1
New Lab
81
225
0
98 +/- 3
88 +/- 2
20cm field
Table 13 : Patient SADRs for 19cm Perspex for aquisition (15 pps)
SADRs for fluoroscopy in each lab are broadly similar with the new lab having slightly lower doses
than the old lab. However, for acquisition doses the SADRs in the new lab are noticeably higher.
The reasons for this will be discussed later.
4.4 Practical verification and adaptation of skin dose distribution software
Measured field sizes are given in table 14:
Field size
X (cm)
Y (cm)
25cm
17 +/- 0.5
17 +/- 0.5
20cm (Mag 1)
13.5 +/- 0.5 13.5 +/- 0.5
16cm (Mag 2)
11 +/- 0.5
11 +/- 0.5
Table 14 : Image field size at detector.
28
For the purpose of this work a field area of 13.5cm was used as this tied in with the acquisition part
of the examination, which represents approximately 85% of the total skin dose.
Figure 18 shows gafchromic film imaged with the following orientations;
0RAO, 35CRAN
0RAO, 35CAUD
35RAO, 0CRAN
35LAO, 0CRAN
This demonstrated good symmetry.
This was recreated in the original
Matlab programme and is shown in
figure 19. The original programme
demonstrated some non symmetry
such that the field areas in the
RAO/LAO directions were narrower
than the CRAN/CAUD directions.
Figure 18 : Gafchromic image of symmetric rotation
The programme was modified to make it symmetric. The image from the symmetric programme is
also shown in figure 20. For this particular set of parameters the new symmetric programme was a
better representation of the true situation.
Figure 19 : Original programme for 35 degree rotations
Figure 20 : Symmetric programme for 35 degree rotations
29
However, a second set of orientations
was checked which was still expected to
be symmetric but included orientations
that combined the RAO/LAO with the
CRAN/CAUD rotation as follows:
RAO35 CAUD35
RA035 CRAN35
LAO35 CAUD35
LAO CRAN35
Figure 21 shows Gafchromic film imaged
with the above parameters. The results
clearly show non symmetry in the
RAO/LAO rotation compared with the
Figure 21 : Gafchromic film imaging 35 degree combined rotations
CRAN/CAUD rotation. Figure 22 shows the image created with the symmetric programme. The
Gafchromic image shows that when rotated in the RAO/LAO direction and combined with
CRAN/CAUD rotation the left hand and right hand edges do not change their angulation very much
compared to the CRAN/CAUD direction. This is recreated in the symmetric version of the MatLab
programme. Figure 23 also shows the image displayed for the above orientations using the original
programme which appeared to provide a better match.
Figure 22 : Symmetric programme for combined angles
Figure 23 : Original programme for combined angles
30
The reason for this non symmetry is due to the rotation of
the C-arm within the gantry in the cranial caudal direction
which subsequently affects the centre of rotation for the
RAO/LAO direction. Figure 24 shows an example of a
combined LAO/Caudal rotation.
Figure 24 : Combined rotation of the C-arm
A final set of orientations were imaged with Gafchromic film as follows;
RAO0 CRAN 40 ; RAO0 CAUD40
RAO40 CRAN0 ; LAO40 CRAN0
RAO40 CRAN40 ; RAO40 CAUD40
LAO40 CRAN40 ; LAO40 CAUD40
Figure 25 : Gafchromic film imaged with a mixture of combined and single rotations
The Gafchromic film (figure 25) was compared with images produced using the revised symmetric
and original programmes (see figures 26 and 27). Neither programme was a perfect match,
however, it was felt that the original programme was a closer representation and was used for all
31
further analysis work. An excerpt of the Matlab programme used to define the equations using the
original definitions from appendix B is given in appendix F.
Figure 26 : Symmetric programme
Figure 27 : Original programme
4.5 Analysis of skin dose data using in house skin dose distribution software
4.5.1 Diagnostic Cardiac Angiography
The maximum skin dose for 194 diagnostic Coronary Angiograms was checked using the skin dose
distribution software. These were checked by a separate aspect of the programme that did not plot
the distribution pattern for each study but instead produced an Excel document listing the maximum
skin dose for each study on a CD together with the total DAP and the total skin dose. The individual
data is given in appendix G.
From this data the maximum skin dose value was 715.1mGy and the average was 140.3mGy. All
values are well below the 2Gy threshold value for deterministic effects.
This data did not include the fluoroscopy element so results will be an underestimate but will be
sufficient to show whether there is likely to be any skin effects seen for routine diagnostic
cardiography exams.
The fluoroscopy element was added by dividing the total fluoroscopy dose, proportionally, between
the individual acquisition runs. This was included in the part of the programme which analysed
individual exams in more detail and included the dose distribution plot.
32
An example of the skin dose distribution for a typical Coronary Angiogram consisting of 10 runs is
given in figure 28. The different colours relate to the different amount of dose at each position with
red being the highest dose. The total skin dose for this exam was 457.3 mGy and the maximum
dose was 122.4 mGy. The study report that relates to this particular exam is given in appendix A
and the skin dose analysis (including the additional fluoroscopy element) in appendix H.
Figure 28 : Routine standard Coronary Angiogram dose distribution map
4.5.2 Therapeutic Examinations
As the nature of therapeutic examinations are extremely varied it would not be appropriate to
determine an average value. Instead it would be expected that the programme would be used on a
case by case basis. Figure 29 shows an example of a therapeutic exam consisting of 39 runs. The
study report relating to this procedure is given in appendix I and the skin dose analysis in appendix
J. It is of note that the position of the imaged areas are very similar to the routine Coronary
angiogram except there is one region where the dose is significantly higher than all the rest and this
explains the different distribution of colour for this exam. The total skin dose for this exam was
2578 mGy which is above the value for skin deterministic effects. However, the maximum skin
dose was 648.3mGy
33
Figure 29 : Intervention coronary exam dose distribution map
5. Discussion
5.1 Verification of Dose Area Product and Skin Dose Indicator Calibration
It is understood that for the Siemens Axiom Artis there is an integral ionisation chamber which can
be calibrated by the service engineer if required. Siemens have a specified criteria of +/- 30%. The
skin dose indicator is derived from the same output data so it is expected that on one system the two
calibrations would be fairly similar. The Dose Area Product calibration was found to be within 10%
so this was much better than that specified by manufacturers and gave assurance as to the accuracy
of the patient dose audit carried out using this equipment. Some systems have been found to be less
accurate. This can be due to differences in the method of calibration carried out by the manufacturer
to that shown here. It is known that Siemens carry out a free in air measurement and that is the
method employed here. However, some manufacturers keep the patient table in the beam path when
carrying out this calibration and this needs to be taken into account.
The skin dose indicator calibration was checked using more than one method.
34
Firstly a free in air measurement with no additional copper filters in the beam. The calibration was
found to be within 8% to 15%. The accuracy of this measurement is affected by the fact that high
exposure factors are needed to get a high enough dose rate to provide sufficient accuracy. It was
also found that the indicated dose rate was not always stable despite the exposure factors remaining
stable. There was a larger difference between measured and indicated skin dose with Copper in the
beam path. However, the calibration was still inside the manufacturers tolerance and was
considered sufficiently accurate for the current work.
Additional measurements of skin dose with Perspex in the beam to simulate the patient
demonstrated that the calibration was still satisfactory. It was expected that the radiation back
scattered from the Perspex would increase the dose to the ion chamber and lead to a closer
correlation. However, this increase appears to be offset by the reduction in dose to the ion chamber
due to the table attenuation. This implies that calibration of the skin dose indicator can be checked
with sufficient confidence with either the free in air method or with scattering media on the table.
The final method for measuring skin dose was with the use of thermoluminescent dosimeters
(TLDs). Whereas the skin dose indicator appeared to over read when compared to the ion chamber,
the indicated skin dose indicator now appeared to under read by 26% to 33%. This is a highly
significant difference. TLD measurements positioned outside the direct radiation path, received
scattered radiation equal to approximately 5% of the direct dose. This value would be very
dependent on position beyond the primary beam but demonstrates that for a coronary angiography
exam with a minimum of 9 runs there will be additional radiation to the skin from scattered
radiation from nearby runs.
The results given here where only four runs were involved demonstrated that the actual dose could
be higher by up to 15%. The skin dose indicator does not take this into account. Since the skin dose
indicator provides a total and does not give any indication of dose distribution it is not essential.
However, it demonstrates that for any skin dose distribution software the additional contribution
from scattered radiation from adjacent runs needs to be considered.
There have been a number of different studies using both film and/or TLDs to measure the skin
entrance dose to the patient. One study used pre-packaged therapy verification film held in a
specially made gown with TLDs placed in specific locations on the film as a cross check between
film and TLD exposure. TLDs were, on average, within 9% of the film estimate39. By wrapping
film completely around a patient this would ensure that doses could be measured for lateral
examinations which was not possible for film and TLDs lying flat on the patient couch top. TLDs
have also been arranged in a grid and a concentration factor determined, which is the ratio of the
35
maximum skin dose to average dose30. In another study, results from a TLD array wrapped around
the patient were used to measure the maximum skin dose and action levels for DAP were proposed.
A first DAP action level of 125Gy.cm2 corresponded to the 2Gy dose threshold for optional
radiopathological follow up and a second DAP action level of 250Gy.cm2 corresponded to the 3Gy
skin dose which would require systematic follow up40. These values have not been compared in this
study but is worthy of further work. Indeed, another study demonstrated the need for both operator
and procedure specific DAP values for action levels41.
5.2 Measuring patient DAP doses
The old and new labs are from the same manufacturer and any differences are mainly mechanical
and software related so it is expected that with the same x-ray tube and flat plate detector and the
same staff group, patient doses should be very similar between the two labs. However, the data
demonstrates that there has been an increase in patient DAP dose by 28%. This has been as a result
of a 35% increase in fluoroscopy dose and a 27% increase in acquisition dose. The mean dose now
exceeds the current Diagnostic Reference level of 29Gy.cm2 and requires investigation and efforts
to reduce this dose or justify the need for this higher dose. The mean weight of the sample group
has reduced from 81.3kg to 80.4kg so it is clear that patient weight is not a contributory factor.
There may have been a change in technique, however, the data demonstrates that the fluoroscopy
screening time has reduced from 236 seconds to 162 seconds and the mean number of acquisition
images has reduced from 879 to 674. If there has been any change in cardiologist technique this has
resulted in improved technique resulting in reduced fluoroscopy and acquisition and should have
resulted in a patient dose reduction. This demonstrates that the increase in patient dose does not
relate to the examination technique and is related to equipment performance.
Both systems have the same detector, however the manufacturer’s specification dose rates have
increased for the newer system. The older system had field size factors based on image intensifier
technology with the increase in dose increasing in accordance with the square of the field size. For
flat plate detectors a linear relationship has now been employed by manufacturers and the field size
factor at 25cm and 20cm fields is higher than what had previously been recommended for image
intensifier systems. Detector dose measurements found the new system to have higher detector dose
for both fluoroscopy and acquisition. Additionally, for acquisition there were differences in the
amount of copper pre-filtration that is automatically added by the system and differences in
exposure factors. The significance of this to patient dose was assessed by the use of a 19cm Perspex
phantom to simulate the patient. For fluoroscopy patient doses for the two systems were found to be
broadly similar. However, the new lab had less copper pre-filtration in the beam. Copper filtration
hardens the beam. The x-rays are therefore more penetrating and will result in reduced patient dose
at the expense of image contrast. The improved latitude and image processing performance of
36
digital detectors means that the systems should cope with this reduced contrast. The disadvantage of
additional copper is a reduction in the tube output and it is necessary to increase the output from the
x-ray unit to compensate for the absorption in the copper filter. This increases tube loading and
reduces x-ray tube life.
Measured fluoroscopy dose using a standard Perspex phantom were broadly similar on the two
systems whereas the patient audit for fluoroscopy dose was higher on the new system. This may be
due to the fact that dose is very dependent on patient thickness when pre-filters are employed.
Figure 30 shows the variation of kV with mA for the Siemens Axiom Artis. At small patient
thicknesses 0.9mm of copper is placed in the beam. As patient thickness increases so the mA
increases until it comes to a maximum allowable value for the x-ray tube. At this point the amount
of copper reduces from 0.9mm to 0.6mm, the kV increases to improve penetration and the mA
values reduce back to minimum values. As the patient thickness
Voltage v's Current (uA)
115000
105000
95000
85000
75000
65000
55000
45000
14000
19000
24000
29000
34000
39000
44000
49000
54000
Figure 30: kV/mA operating curve for the Siemens Axiom Artis
continues to increase the mA again increases to maximum allowable values and at this point the
copper reduces to 0.3mm, 0.2mm and then later to 0.1mm copper. When the level of Copper is
reduced, the kilovoltage increases, mA reduces and the patient dose is increased. This means that
for measuring patient dose for the Siemens system the point on the kV/mA curve critically
determines the patient dose. A previous study using a standard phantom, determined that copper
filtration of 0.35mm would reduce the entrance dose by 58% with a mean increase in tube loading
37
of 29% 42. Image quality assessment by cardiologists considered there to be insignificant detriment
to image quality in the procedure being investigated.
Measured patient doses for Acquisition using 19cm Perspex using standard clinical settings for the
new cath lab were found to be 126% higher for the 25cm field and 67% higher for the 20cm field
(which is the field size typically employed for acquisition) when compared to the older cath lab.
Part of the reason for this increase in dose is due to the higher specification detector doses described
earlier. Additionally it can be seen that there is no copper pre-filtration employed in the new room
whereas 0.2mm of copper has been inserted in the old room. Manufacturers have made the decision
to remove the copper filtration in order to improve image quality and prolong tube life. This would
result in a significant increase in dose. However, it can be seen from the exposure factors that the
kilovoltage has been increased and the mA reduced in an effort to reduce the patient dose. These
adjustments, however, do not completely compensate for the increase in dose due to the removal of
copper pre-filtration.
Since users were happy with their old lab where pre-filtration was employed it would seem
appropriate to re-programme the new room to have pre-filtration and exposure factors more in line
with the old room.
Current Local DAP values are just above the Diagnostic Reference Level. A previous survey
carried out by the National Radiological Protection board25 recommended a reference level for
Cardiac Angiography procedures of 36Gy.cm2. Current local values would have been below this
value. This demonstrates the fact that nationally, doses for Cardiac angiography are generally
reducing. Data collected in a study from 15 years ago43 determined a mean DAP of 67Gy.cm2 and a
3rd quartile value of 69Gy.cm2. Overweight or very thin patients were excluded from the study in a
similar manner to the current study so that they did not skew the data. However, this mean value is
considerably higher than that obtained in the current study. This is partly attributable to the fact that
older fluoroscopy systems would generally operate at a higher dose and also that the data comes
from examinations performed in other countries where the standard technique may be very
different. A larger and more recent study included England in the data analysis and this provided
DAP values more in line with what is currently achieved locally27. A study on two different systems
in the UK in 199744 gave mean DAP values for Cardiac Angiography of 47.7Gy.cm2 and 23.4
Gy.cm2. The difference in DAP can be explained by the fact that one room has a biplane image
intensifier system which will give greater imaging capability but will also increase the dose. This
study did not remove large or small patients. Instead size correction factors were used that had been
obtained in a previous study45.
38
5.3 Practical verification and adaptation of skin dose distribution software
A Matlab programme prepared to assess skin dose distribution on a different system was adapted
for use with the Siemens Axiom Artis. Comparisons of dose distribution using Gafchromic film
demonstrated that rotations in the RAO/LAO and CRAN/CAUD are not symmetric. It was not
possible to make the programme replicate the dose distribution exactly but it was deemed
sufficiently accurate to be used to determine the approximate maximum skin dose region. Running
the programme on a set of routine diagnostic examinations (consisting of no more than 12 runs)
determined that it was highly unlikely that 2Gy would ever be reached. Routine diagnostic coronary
angiograms therefore do not pose any concern with regard to deterministic effects. The main
concern for routine exams is with respect to stochastic effects and this is where optimisation of
equipment configuration and technique to minimise patient dose whilst still providing satisfactory
image quality is required. Since routine exams within one hospital are likely to be very similar it
would be possible to determine a maximum skin dose based on a fraction of the total dose as a
rough guide to the expected maximum skin dose without any further measurement.
The programme was used to determine the maximum skin dose for a therapeutic examination. The
image of the skin dose distribution is broadly similar to that for a routine diagnostic examination,
however in this case one of the orientations displays a significantly higher dose compared to the
other orientations. This is the orientation that was used most for optimum visualisation of the region
of interest. For the case used where the total skin dose was 2.6 Gy this led to a maximum skin dose
of 0.6mGy which would not be of concern with regard to deterministic effects. However, maximum
skin doses for therapeutic examinations can be much greater than this, particularly for complicated
examinations and there will be cases where the maximum skin dose is likely to reach or exceed the
2Gy limit. The programme will adequately indicate for each examination, the value and region of
the maximum skin dose. The calculated maximum skin dose will be an under-estimate, since it will
not include the dose contribution from scattered radiation from nearby projections.
Other errors using this programme would be that there is no record of any translational movement
of the table. Observations of typical Cardiac Angiography examinations demonstrate that there is
some table movement and the table height may not be positioned at the iso-centre. These errors will
affect the absolute skin dose value. However, the skin dose distribution software should be
sufficiently accurate to act as a device to notify staff of the potential for skin damage for a particular
exam.
5.4 Equipment considerations and dose reduction techniques
The transition from image intensifier detectors to digital detectors has always been suggested as a
means of dose reduction . Whilst developments in image processing and general equipment
development will result in patient dose reduction, the introduction of digital detectors has not
39
always resulted in a reduction in dose. Indeed, the field size factors used on Siemens flat plate
detectors for the smaller field sizes are higher than those previously given for image intensifier
systems. However, some studies have also found the doses have reduced as a result of the
introduction of flat plate detector technology. One comparative study found that a 30% dose
reduction was possible compared with a conventional system when used for interventional
cardiology46. Another study had similar findings47 , however a separate study determined higher
doses on a digital system and this was attributed to the use of higher exposure factors48. It is likely
that the introduction of digital technology ‘should’ reduce patient doses and this needs to be carried
out by optimising specific imaging techniques. Medical Physics, Applications specialists and
Radiology staff all have a part to play in reducing patient doses by careful optimisation of technique
and exposure factors. Examples of equipment options that are routinely employed in imaging
systems now are virtual collimation, wedge filtering, auto positioning and last image hold. Staff
should be trained to routinely take advantage of these options where they exist. Systems with
variable filtration are likely to provide the lowest patient doses40. Adjusting operational parameters,
such as using variable pulsed fluoroscopy, has also been shown to reduce radiation exposure49.
Another study adjusted projection, filtration, field size and collimation in order to reduce dose and
monitored image quality using a contrast-detail phantom. Effective dose was reduced from 13 to
4.6mSv for a standardized PCI procedure50.
As further developments take place other techniques are being introduced that may well reduce
patient dose or possibly change the way in which these examinations are carried out. For example,
Rotational Angiography can give lower patient dose and uses less contrast agent than a standard
series of 5 to 6 views to get the same clinical information51. For standard angiography each separate
run takes 5-6 seconds and is static while contrast media is injected. For Rotational Angiography the
x-ray tube moves automatically whilst contrast media is being introduced and consequently the
vessels can be viewed from more than one angle from this single run. The single run will take
longer than a conventional run but since less runs are required this should lead to a reduction in
overall dose. Magnetic Catheter guidance is in the development stage but has potential. Finally,
there appears to be a trend to move from the standard fluoroscopy technique to using a Computed
Tomography technique to obtain the same diagnostic information. Most CT scanners on the market
will include a Cardiac package option if required and as more departments make the transition to
CT Cardiac angiography manufacturers are further developing their scanners to make them more
user friendly and dose efficient.
40
6. Conclusion
Patient dose was measured in two cardiac catheterisation labs using calibrated dose area product
(DAP) meters. These systems were virtually identical from a technical perspective but were
installed approximately five years apart. Measured DAP determined that the newer lab had higher
patient doses compared with the old lab and they were also above the Diagnostic Reference Level.
This was despite a reduction in the screening time and the number of acquisition runs. The reason
for this higher dose was attributed to manufacturer’s setting higher detector doses in the new lab
and also dose saving copper pre-filtration not being automatically utilised for acquisition runs.
Users need to work with Medical Physics and applications specialists to lower doses whilst
maintaining adequate image quality to ensure that doses are as low as reasonable practicable.
A Matlab programme was adapted for the local cardiac systems to record the skin dose distribution
for coronary angiography procedures and to provide a value and position for the region of
maximum skin dose for each procedure. Skin dose measurements using thermoluminescent
dosimeters (TLDs) determined that indicated skin dose did not include any contribution from
scattered radiation from nearby runs so would always be an under estimate. This is worthy of more
detailed study but the current programme was considered sufficient to determine whether a patient
needed to be followed up for any signs of skin damage. Tests using the Matlab programme
determined that skin effects are not likely to be seen for routine coronary angiography procedures
but complex interventional procedures should be analysed on a case by case basis to determine
whether follow up is required. The accuracy of the programme to display the correct region of
irradiation requires further refinement. Additionally, the programme does not take into account any
translational movement of the patient table. This information is not currently available in the
patient’s dose record. This should be investigated further as the information is available in the x-ray
room on the TV monitor.
41
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