Download Understanding Radiation Units

Biophysics
Understanding Radiation Units
Dr. Yousif Mohamed Y. Abdallah
Educational Objectives
At the end of the lecture, the students should become
familiar with the following:
• Why is it important to measure radiation dose ?
• How radiation dose can and should be expressed?
• Understand the radiation quantities and units used
in diagnostic radiology.
Understanding radiation units
2
Answer True or False
1. The same amount of radiation falling on the
person at level of breast, head or gonads will have
the same biological effects.
2. Effective dose can be easily measured.
3. Diagnostic reference levels are not applicable to
paediatric radiology.
Understanding radiation units
3
Contents
• Dose descriptors outside the patient’s body.
• Dose descriptors for effects that have threshold
(deterministic effects)
• Dose descriptors to estimate stochastic risks
• Diagnostic reference levels
• Dose descriptors and units for staff dose
assessment
Understanding radiation units
Introduction
• Several quantities and units are used in the field of
diagnostic radiology to measure and describe radiation
dose
• Some can be measured directly while others can only
be mathematically estimated
Understanding radiation units
5
Two types of radiation effects
Stochastic effects
– Where the severity of the result is the same but the
probability of occurrence increases with radiation dose, e.g.,
development of cancer
– There is no threshold for stochastic effects
– Examples: cancer, hereditary effects
Deterministic effects
– Where the severity depends upon the radiation dose, e.g.,
skin burns
– The higher the dose, the greater the effect
– There is a threshold for deterministic effects
– Examples: skin burns, cataract
Radiation Protection
in Paediatric
Radiology
Understanding
radiation
units
L02. Understanding radiation units
6
Hot Coffee – Energy contained in a sip
Excess Temperature = 60º - 37 = 23º
1 sip = 3ml
3x 23 = 69 calories
Understanding radiation units
Radiation Dose
Lethal Dose= 4Gy
LD 50/60 = 4 Gy
For man of 70 kg
Energy absorbed = 4 x 70 = 280 J
= 280/418= 67 calories
= 1 sip
Energy content of a sip of coffee if derived in the form of Xrays can be lethal
Understanding radiation units
Dose of Radiation
• Radiation energy
absorbed by a body per
unit mass.
Understanding radiation units
9
Dose Quantities and Radiation units
- Dose quantities external to the patient’s body.
- Dose quantities to estimate risks of skin injuries and
effects that have threshold.
- Dose quantities to estimate stochastic risks.
Understanding radiation units
Why so many quantities?
Radiation dose is a complex topic
• 1000 Watt heater giving off heat (IR radiation) - unit
is of power which is related with emission intensity
• Heat perceived by the person will vary with so
many factors: distance, clothing, room temperature
• As can be seen with the example of heat, the
energy transformation is a highly complicated issue
• This is the case with X-rays - radiation can’t be
perceived
Understanding radiation units
Basic Radiation Quantities
• Used to quantify a beam
of X or γ-rays
• There are:
– Quantities to express
total amount of
radiation.
– Quantities to express
radiation at a specific
point
Total radiation
•Total photons
•Integral dose
Radiation at a
specific point
•Photon fluence
•Absorbed dose
•Kerma
•Dose equivalent
Understanding radiation units
12
Exposure: X
• Exposure is a dosimetric quantity for measuring ionizing
electromagnetic radiation (X-rays & Ɣ-rays), based on
the ability of the radiation to produce ionization in air.
Units:
coulomb/kg (C/kg)
or
roentgen (R)
1 R = 0.000258 C/kg
Understanding radiation units
KERMA
KERMA (Kinetic Energy Released in a Material):
– Is the sum of the initial kinetic energies of all charged ionizing
particles liberated by uncharged ionizing particles in a material
of unit mass
– For medical imaging use, KERMA is usually expressed in air
SI unit = joule per kilogram (J/kg)
or gray (Gy)
1 J/kg = 1 Gy
Understanding radiation units
Absorbed dose: D
Absorbed dose, D, is the mean energy
imparted by ionizing radiation to matter per
unit mass.
SI unit = joule per kg (J/kg) or gray (Gy).
Harold Gray
In diagnostic radiology, KERMA and D are
equal.
Understanding radiation units
Mean absorbed dose in a tissue or
organ
The mean absorbed dose in a tissue or organ DT is the
energy deposited in the organ divided by the mass of
that organ.
Understanding radiation units
Now things get a little more complicated !
Understanding radiation units
17
Radiation Dose Quantities
• Primary physical quantities are not used directly for
dose limitation
• The International Council on Radiation Protection
(ICRP) has defined values for dose limits in
occupational exposure
Radiation Protection
in Paediatric
Radiology
Understanding
radiation
units
L02. Understanding radiation units
Radiation Dose Quantities
Equivalent Dose:
• Accounts for the type of radiation
• Different radiation types have different level
biologic damage per unit absorbed dose
Radiation Protection
in Paediatric
Radiology
Understanding
radiation
units
of
L02. Understanding radiation units
Radiation Weighting Factors, wR
Radiation type
Radiation weighting factor, wR
Photons
1
Electrons and muons
1
Protons and charged pions
2
Alpha particle, fission
fragments, heavy ions
20
Neutrons
A continuous curve
as a function of
neutron energy
Radiation Protection
in Paediatric
Radiology
Understanding
radiation
units
L02. Understanding
radiation units
(Source:
ICRP 103)
Equivalent Dose : HT,R
The absorbed dose in an organ or tissue multiplied by
the relevant radiation weighting factor :
H T , R  wR  DT , R
where DT,R is the average absorbed dose in the organ or
tissue T, and wR is the radiation weighting factor for
radiation R.
Understanding radiation units
Radiation Quantities and Units
Equivalent dose (Unit = sievert, Sv )
– Compares the biological effects for
different types of radiation, X-rays, Ɣrays, electrons, neutrons, protons, αparticles etc.
– For X-rays, Ɣ-rays, electrons : absorbed
dose and equivalent dose have the
same value Gy = Sv.
Understanding radiation units
Rolph Sievert
22
Detriment
• Radiation exposure to different organs and tissues in
the body results in different probabilities of harm and
different levels of severity.
• The combination of probability and severity of harm is
called “detriment”.
• Effective dose reflects the combined detriment from
stochastic effects due to the equivalent doses in all the
organs and tissues of the body.
Understanding radiation units
Effective Dose: ET
• Effective dose takes into account the organ specific
radio-sensitivity to develop cancer and hereditary
effects from radiation
• Unit = sievert, Sv
Understanding radiation units
24
Effective Dose: ET
A summation of the tissue equivalent doses, each
multiplied by the appropriate tissue weighting factor:
E   wT H T
T
where HT is the equivalent dose in tissue T and wT is
the tissue weighting factor for tissue T.
Understanding radiation units
Tissue Weighting Factors, wT
• The organs have different weighting factors, wT.
• These factors are published in ICRP 103 (2007) and
have been changed over the years due to increased
knowledge.
Understanding radiation units
26
Tissue Weighting Factors
• The weighting factors sum up to 1.0.
• They are relative and compares one organ with the
other.
• They are the same for children and adults!
Radiation Protection
in Paediatric
Radiology
Understanding
radiation
units
L02. Understanding radiation units
27
Tissue Weighting Factors
• Data is primarily taken from
knowledge derived from
studying the Japanese
population exposed to
atomic bombs in Hiroshima
and Nagasaki
• On going research has
changed the weighting
factors from 1990 (ICRP 60)
to 2007 (ICRP 103).
Understanding radiation units
28
Tissue Weighting Factors
Multipliers of the equivalent dose to an organ or tissue to account for the
different sensitivities to the induction of stochastic effects of radiation.
Tissue
Bone-marrow (red), Colon, Lung, Stomach, Breast,
Remainder Tissues**(nominal weighting factor
applied to the average dose to 14 tissues)
Gonads
Bladder, Esophagus, Liver, Thyroid
Bone surface, Brain, Salivary glands, Skin
weighting
factor
wT*
∑ wT
0.12
0.72
0.08
0.04
0.08
0.16
0.01
0.04
*ICRP 103
**Remainder Tissues (14 in total): Adrenals, Extrathoracic (ET) region, Gall bladder,Heart, Kidneys,
Lymphatic nodes, Muscle, Oral mucosa, Pancreas, Prostate, Small intestine, Spleen, Thymus,
Uterus/cervix..
Understanding radiation units
Effective Dose (E)
Dose to lungs times their
weighting factor; DL x wL
+
Dose (mean absorbed dose) to
gastrointestinal tract times their
weighting factor; DGI x wGI
+
....(summation over organ after
organ)
=
Effective dose
E
w
T
HT
T
where T stands for tissue
Understanding radiation units
30
Effective Dose (E)
We can compare different paediatric imaging procedures
through their different effective doses, E.
Understanding radiation units
31
Radiation Quantities and Units used in Diagnostic Radiology
– Incident air kerma
– Entrance surface air kerma
– Air kerma-area product
– Air kerma-length product
– Dosimetric quantities for CT
– Dosimetric quantities for interventional radiology
Understanding radiation units
32
Incident Air Kerma
Measured Free in Air on the central beam axis at the
focal spot to surface distance.
Only primary beam is considered, that is, no scatter
contribution.
Unit: joule/kg or gray (Gy)
Understanding radiation units
Entrance Surface Air Kerma (ESAK)
• ESAK measured on the surface of the patient or
phantom where X-ray beam enters the patient or
phantom.
• Includes a contribution from photons scattered
back from deeper tissues, which is not included in
free in air measurements.
Understanding radiation units
Entrance Surface Air Kerma (ESAK)
• If measurements are made at other distances than
the true focus - to - skin distance, doses must be
corrected by the inverse square law and
backscatter factor incorporated into the
calculation.
References:
– Dosimetry in Diagnostic Radiology: An International code of
practice, TRS 457, IAEA, 2007
– Phys. Med. Biol. 43 (1998) 2237-2250.
Understanding radiation units
Dose Measurement
Kerma in X-ray field can be
measured using calibrated:
• Ionization chamber
• Semiconductor dosimeter
• Thermoluminescent dosimeter
(TLD)
Understanding radiation units
36
Kerma-Area Product: KAP
• The kerma - area product (KAP) is
defined as the kerma in air in a
plane perpendicular to the incident
beam axis, integrated over the
area of interest.
• This is the dose related quantity
measured and displayed on all
modern X-ray equipment
excluding CT.
KAP meter
Understanding radiation units
Kerma-Area Product: KAP
• The KAP (Gy·cm2) is constant
with distance since the cross
section of the beam is a
quadratic function which
cancels the inverse quadratic
dependence on dose .
• KAP remains constant along
the beam axis as long as it is
not measured close to the
patient/phantom surface
which introduces backscatter.
Understanding radiation units
Kerma-Area Product: KAP
d1=1
KAP = K x Area
Area = 1
Dose = 1
the SI unit of KAP is the
Gy·cm2
Understanding radiation units
Area = 4
Dose = 1/4
d2=2
39
Kerma-Area Product: KAP
KAP is independent of
distance from the X-ray
source, as:


Air Kerma decreases with the
inverse square law.
d1=1
Area = 1
Dose = 1
Area = 4
Dose = 1/4
d2=2
Area increase with the square
distance
KAP is usually measured at the
level of the tube diaphragms
Understanding radiation units
40
KAP (kerma-area product)
This is a picture of a KAP meter which measures the kerma area product
Unit: Gy·cm2
Understanding radiation units
Example of a dose display during
fluoroscopy or cine runs with dose
rate as shown
Understanding radiation units
42
Kerma-Area Product
In paediatric radiology KAP may be used for:
– Diagnostic reference levels (DRLs)
– By use of conversion factors, it can be converted to
skin dose and/or effective dose
Understanding radiation units
43
Dosimetric Quantities for CT
• Computed Tomography Dose
Index (CTDI)
- determined using scan protocol
parameters.
-useful for comparison of
different scanners.
• Dose-Length Product (DLP)
- measure of dose to patient
- used to estimate effective dose
Understanding radiation units
KERMA
(in phantom)
CT and Risk
CTDI
(dose in phantom per slice)
Length of scan and pitch
DLP
Effective dose
Risk
Understanding radiation units
Measurement of Dosimetric Quantities in CT
• Pencil ionisation chamber with active
length of 100 mm.
•
• Measurements free-in-air or in
standard dosimetry phantom.
• Alternatives: TLD, solid state
detectors.
• CTDIVOLshould be displayed on the
console, reflecting the conditions of
operation selected (IEC, 2003)
Understanding radiation units
Dose Indicators in Interventional Radiology
•
•
For quality assurance purposes
To estimate the probability of occurrence of
stochastic effects use:
Kerma-air product rate (KAP, PKA)
Understanding radiation units
Dose Indicators in Interventional Radiology
•
For quantifying the threshold and severity
of deterministic effects use:
•
•
•
Maximum skin dose (MSD)
Cumulative dose (CD) to Interventional
Reference Point (IRP)
In a complex procedure skin dose is highly
variable
Understanding radiation units
Interventional Procedures: Skin
Dose
• In some procedures, patient
skin doses approach those
used in radiotherapy
fractions
• Maximum skin dose
(MSD) or peak skin dose is
the maximum dose received
by a portion of the exposed
skin.
Radiodermatitis in the right arm. 7
year-old patient. Photograph taken
4 months after radiofrequency
ablation. Surce: ICRP 85
Understanding radiation units
49
Cumulative Dose to
Interventional Reference Point*
•
•
•
•
IRP is located 15 cm from the isocentre towards the focal
spot
The air kerma accumulated at a specific point in space
relative to the fluoroscopic gantry (IRP) during a
procedure
Cumulative dose does not include tissue backscatter and
is measured in Gy.
Cumulative dose is sometimes referred to as cumulative
air kerma
*IRP
Understanding radiation units
Cumulative dose to
Interventional Reference Point
Cumulative dose to IRP is measured with a flat ion
chamber or calculated by the system and
displayed in the angiography room
15 cm
15 cm
IRP
IRP
Isocenter
Isocenter
(IEC-60601-2-43)
Understanding radiation units
MSD vs. Cumulative dose
• In some procedures, cumulative dose to IRP is well
correlated with MSD
• Cumulative dose to IRP can be a good indicator of
doses higher than the thresholds for skin injures
• A “trigger value” for cumulative dose can be
adopted to alert interventionalists the threshold for
skin erythema could be reached.
• A follow-up protocol can be adopted.
Understanding radiation units
Other related dose parameters
Fluoroscopy time:
• Has a weak correlation with KAP
• But, in a quality assurance programme it can be
adopted as a starting unit for
– comparison between operators, centres,
procedures
– for the evaluation of protocol
optimization, and
– to evaluate operator skill
Understanding radiation units
53
Other related dose parameters
Number of acquired images and number of series:
– Patient dose is a function of total acquired
images
– But dose/image can have big variations
– There is an evidence of large variation in
protocols adopted in different centres
Understanding radiation units
54
Diagnostic reference levels (DRLs)
• ICRP, IAEA, EC: introduced the concept of diagnostic
reference levels (DRLs) for patients
• DRLs are a form of investigation level, apply to an
easily measured quantity at the surface of a simple
standard phantom or a representative patient.
• An optimisation tool, not dose limits
Understanding radiation units
Diagnostic Reference Levels (DRLs)
• DRLs calls for local investigation (often very simple) if
constantly exceeded
• DRLs: Management of patient doses must be
consistent with the required clinical imaging
information
Understanding radiation units
Quantities for Establishment of
DRLs
• Incident air kerma and entrance-surface air
kerma
• Incident air kerma rate and entrance-surface
air kerma rate
• Air kerma–area product
• CT Dose index, CT Dose–length product
Understanding radiation units
Quantities and Units for Staff Dose
Assessment
• Personal dosimetry services typically provide
monthly estimates of Hp(10) (mSv), the dose
equivalent in soft tissue at 10 mm depth. This is in
most of the cases used to estimate the effective
dose.
• Sometimes, Hp(0.07) (mSv) is also reported: the dose
equivalent in soft tissue at 0.07 mm depth)
• Personal dosememters (film, thermoluminescent...)
Understanding radiation units
Personal Dosimetry Methods
• Single dosimeter worn
Radiation
– above the apron at neck level
(recommended) or under the
apron at waist level
Lens dose, optional
protection
measures
Finger dose, optional
Second dosemeter
Image
intensifier
• Two dosimeters worn
(recommended in
intrevational procedures)
Patient
outside and above the apron
at the neck, optional
Personal dose
dosemeter behind the lead apron
Dose limits
of occupational exposure
– one above the apron at neck
level
– another under the lead apron at
waist level
(ICRP 60)
Effective dose
20 mSv in a year
averaged over a period of 5 years
X-ray
tube
Anual equivalent dose in the
lens of the eye 150 mSv
skin
500 mSv
hands and feet 500 mSv
Understanding radiation units
Dose Measurement
Dose due to scatter
radiation at a point
occupied by the operator
can be measured with a
portable ionization
chamber
Understanding radiation units
Summary
• Dosimetric quantities are useful to know the potential
hazard from radiation and to determine radiation
protection measures to be taken
• Physical quantities - Directly measurable
• Protection quantities - Defined for dose limitation
purposes, but not directly measurable.
• Application specific quantities - Measurable in medical
imaging.
• Diagnostic Refernce Levels
Understanding radiation units
Answer True or False
1. The same amount of radiation falling on the
person at level of breast, head or gonads will have
same biological effects.
2. Effective dose can be easily measured.
3. Diagnostic reference levels are not applicable to
paediatric radiology.
Understanding radiation units
62
Answer True or False
1. False -Different organs have different radiosensitivity and tissue weighting factors as
given by ICRP.
2. False -It can be only calculated using
different methods.
3. False - DRLs apply for paediatric radiology,
but these are age-specific.
Radiation Protection
in Paediatric
Radiology
Understanding
radiation
units
L02. Understanding radiation units
63
References
• INTERNATIONAL COMMISSION ON RADIATION UNITS AND MEASUREMENTS,
Patient Dosimetry for X Rays Used in Medical Imaging, ICRU, Rep. 74, ICRU,
Bethesda, MD (2006).
• INTERNATIONAL COMMISSION ON RADIOLOGICAL PROTECTION, Radiological
Protection in Medicine, Publication 105, Elsevier, Oxford (2008)
• INTERNATIONAL COMMISSION ON RADIOLOGICAL PROTECTION,
Recommendations of the ICRP, Publication 103, Elsevier, Oxford (2008)
• EUROPEAN COMMISSION, Guidance on Diagnostic Reference Levels (DRLs) for
Medical Exposure, Radiation Protection 109, Office for Official Publications of
the European Communities, Luxembourg (1999)
• INTERNATIONAL ATOMIC ENERGY AGENCY, Dosimetry in Diagnostic Radiology:
an International Code of Practice, Technical Report Series No 457, IAEA,
Vienna (2007)
Understanding radiation units
Additional information
Quantities for radiation measurement
• Physical quantities - Directly measurable
• Protection quantities - Defined for dose
limitation purposes, but not directly
measurable
• Application specific quantities - Measurable in
medical imaging
Understanding radiation units
Radiation quantities and units
• Fundamental dosimetric quantities
• Protection quantities
– Equivalent dose
– Effective dose
• Application specific dosimetric quantities used in DR
– Incident air kerma
– Entrance surface air kerma
– Air kerma area product
– Air kerma length product
– Dosimetric quantities in CT and mammography
Understanding radiation units
Physical Quantities
Understanding radiation units
Physical quantities
• Fluence
• Exposure
• Kerma
• Absorbed dose
Understanding radiation units
Fluence : f
The fluence, f , is the quotient of dN by da,
where dN is the number of particles incident
on a sphere of cross section da, thus
f = dN/da
The unit of fluence is m-2
Understanding radiation units
Exposure: X
dQ
X
dm
where dQ is the absolute value of the total charge of ions
produced in air when all the electrons liberated in air of mass dm
are completely stopped in air.
The SI unit of exposure is the coulomb per kilogram (C/kg)
The special unit of exposure is the röntgen (R).
1R = 2.58 x 10-4 C kg-1
Understanding radiation units
KERMA
The KERMA (Kinetic Energy Released in a MAterial)
dEtrans
K
dm
where dEtrans is the sum of the initial kinetic
energies of all charged ionizing particles liberated
by uncharged ionizing particles in a material of
mass dm
The SI unit of kerma is the joule per kilogram (J/kg),
termed gray (Gy).
.
Understanding radiation units
Exposure and KERMA
Exposure, X, in units of C kg-1, is related to air kerma as
follows:
K a 1  g e
X
W
where W is the average energy spent by an electron to
produce an ion pair, g is the fraction of secondary charged
particles that is lost to bremsstrahlung radiation
production and e is the electronic charge
Understanding radiation units
Absorbed Dose: D
The fundamental dosimetric quantity absorbed dose, D, is
defined as:
d
D
dm
whered  is the mean energy imparted by ionizing
radiation to matter in a volume element and dm is the
mass of matter in the volume element.
The SI unit of absorbed dose is the joule
per kilogram (J/kg), termed the gray (Gy)
In diagnostic radiology, KERMA and D are equal
Understanding radiation units
Exposure and Absorbed Dose or
KERMA
• Exposure can be linked to air dose or kerma by
suitable conversion coefficients.
• For example, 100 kV X-rays that produce an
exposure of 1 R at a point will also give an air
kerma of about 8.7 mGy and a tissue kerma of
about 9.5 mGy at that point.
Understanding radiation units
Application Specific Quantities
Understanding radiation units
Imaging
modality
Radiography
Measurement
subject
Measured radiation
quantity
Remark
Phantom
Patient
Incident air kerma
ESAK, KAP
Fluoroscopy/
Interventional
procedures
Phantom
Patient
ESAK
KAP/Peak skin dose
CT
Phantom
Patient
CT air kerma index
Measured in PMMA head
CT air kerma- length and body phantom
product
Mammography
Phantom
Patient
Incident air kerma,
ESAK
Incident air kerma
Dental
radiography
Patient
Incident air kerma
Air kerma-length
product
Understanding radiation units
Calculated from X-ray
tube output
Calculation of mean
glandular dose
Application Specific Quantities
X-ray tube
focal spot
position
Focal-spot to
image
receptor
distance
(FFD)
Incident air kerma (no
backscatter)
Focal-spot to
patient skin
distance (FSD)
Entrance surface air
kerma (including
backscatter)
Image receptor
Patient
thickness
Schematic diagram showing some dosimetric and geometric quantities
Understanding radiation units
Entrance Surface Air Kerma (ESAK)
 FDD
ESAK  Y (kVp, FDD )  mAs  
 FFD  t
p

2

  BSF


where Y(kVp, FFD) is tube output for actual kVp used during
examination, mAs is actual tube current-time product used
during examination and FFD is focus-to-film distance. BSF is the
backscatter factor that depends on kVp and total filtration of Xrays
Understanding radiation units
Backscatter Factors (Water)
HVL
Field size (cm x cm)
mmAl
10 x 10
15 x 15
20 x 20
25 x 25
30 x 30
2.0
1.26
1.28
1.29
1.30
1.30
2.5
1.28
1.31
1.32
1.33
1.34
3.0
1.30
1.33
1.35
1.36
1.37
4.0
1.32
1.37
1.39
1.40
1.41
Understanding radiation units
Kerma-Area Product: KAP
• If the KAP is calculated by the system, you must know if
the user added filtration you use is included or not !
Understanding radiation units
81
Kerma-Area Product: KAP
• It is always necessary to calibrate and to check the
transmission chamber for the X-ray installation in use
• In some European countries, it is compulsory that new
equipment is equipped with an integrated ionization
transmission chamber or with automatic calculation
methods
Understanding radiation units
Dosimetric Quantities for CT
• Computed Tomography Dose
Index (CTDI)
• CT air kerma index
• Dose-Length Product (DLP)
• Air kerma-length product
Understanding radiation units
ICRU 74 / IAEA TRS 457
• CT air kerma index
– Free-in-air (Ck)
– In phantom (Ck,PMMA)
• Air kerma length product
(PKA)
Understanding radiation units
Dosimetric Quantities for CT
Principal dosimetric quantity in CT is CT air kerma index:
Ca ,100
1

NT
50
 K ( z )dz
50
where K(z) is air kerma along a line parallel to the axis of rotation
of the scanner over a length of 100 mm.
N = Number of detectors in multi-slice CT
T = Individual detector dimension along z-dimension
The product NT defines the nominal scan beam width/collimation
for a given protocol.
Understanding radiation units
Dosimetric Quantities for CT
Weighted CT air kerma index, CW, combines values of
CPMMA,100 measured at the centre and periphery of a
standard CT dosimetry phantoms
1
Cw  C PMMA,100,c  2C PMMA,100, p 
3
Understanding radiation units
Dosimetric Quantities for CT
Pitch (IEC, 2003):
I
p
NT
T= Single detector dimension along
z-axis in mm.
N=Number of detectors used in a
given scan protocol (N>1 for
MDCT), N x T is total detector
acquisition width or collimation
I=table travel per rotation
Radiographic, 2002, 22:949-62
Understanding radiation units
Dosimetric Quantities for CT
• Volume CTDI describes the average dose over the total
volume scanned in sequential or helical sequence,
taking into account gaps and overlaps of dose profiles
(IEC, 2003):
CVOL
NT
 CW
l
• Average dose over x, y and z direction
• Protocol-specific information
Understanding radiation units
Dosimetric Quantities for CT
• Kerma-length product (PKL):
PKL  CVOL  L
where L is scan length is limited by outer margins of the
exposed scan range (irrespective to pitch)
• PKL for different sequences are additive if refer to the
same type of phantom (head/body)
Understanding radiation units
Maximum Skin Dose (MSD)
• Measurement/evaluation of MSD
• Point or area detectors
• Cumulative dose at IRP (interventional radiology point)
• Calculation from technical data
• Off line methods
• Area detectors: TLD array, slow films, radiochromic
films
• From KAP and Cumulative dose measurement
Understanding radiation units
Method for MSD Evaluation:
Radiochromic Large Area Detector
Example: Radiochromic films type Gafchromic XR R 14”x17”
• useful dose range: 0.1-15 Gy
• minimal photon energy dependence (60 - 120 keV)
• acquisition with a flatbed scanner:b/w image, 12-16 bit/pixel
or, measure of OD measurement with a reflection densitometer
Understanding radiation units
Benefits of Radiochromic Films
• The radiochromic film:
– displays the maximum dose and its location
– shows how the total dose is distributed
– provides a quantitative record for patient files
– provides physician with guidance to enable safe planning of
future fluoroscopically guided procedures
– improves fluoroscopic technique and patient safety
– possible rapid semi-quantitative evaluation
Example of an exposed
radiochromic film in a cardiac
interventional procedure
Understanding radiation units
Rapid Semi-Quantitative Evaluation: Example
• For each batch number (lot #) of gafchromic film a Comparison Tablet is
provided
• In the reported example we easily can recognise that the darkness area of
the film, corresponding to the skin area that has received the maximum
local dose, has an Optical Density that correspond at about 4 Gy
Understanding radiation units
DRLs for Complex Procedures
Reference levels (indicative of the state of the practice): to help
operators to conduct optimized procedures with reference to patient
exposure
For complex procedures
reference levels should
include:
Level 2 + DAP
+ Peak Skin Dose (MSD)
• more 3rd
parameters
level
“Patient risk”
• and, must take into
account
the complexity
2nd level
of
the protocol”
procedures.
“Clinical
Level 1
+ No. images + fluoroscopy time
Dose rate and dose/image
(BSS, CDRH, AAPM)
(European Dimond
1st level
Consortium
“Equipment
performance”
recommendations)
Understanding radiation units
94