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Medical exposure in radiology:
Optimization of protection
!! CONTINUATION OF FILE VIII.3 Equipment !
Module VIII.3 - Part 1: Design
considerations for the equipment
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Topic 4: Computed
Tomography
The CT equipment
• The CT scanner
• Hounsfield units
• Difference with projection radiology
• Latest generations of scanners
Module VIII.3-Part 1. Design considerations equipment Continued
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CT: “Computed” and “Tomography”
CT is a tomographic imaging technique,
generating cross-sectional image in the axial
plane
CT techniques use high kV with heavy filtration
A fan beam is passed through the patient
Transmitted radiation is measured by array
detectors
CT images or “sections” are maps of µ
They are derived by mathematical analysis of
multiple projections (use of computer): filtered
back projection
Module VIII.3-Part 1. Design considerations equipment Continued
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Image and “Hounsfield” number
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Pixel size depends on FOV and matrix
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Matrix size 512x512 or 1024x1024
12 bits (4096 gray levels)
Voxel is:
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volume element=pixel area x slice thickness
Relative attenuation coefficient µ is expressed in
Hounsfield units or CT numbers
HUt = 1000 x (µt - µwater) / µwater
By definition HUwater = 0, HUair = -1000
Attention: µ will depend on kV…so will HU
Module VIII.3-Part 1. Design considerations equipment Continued
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Image display: windowing
CT is a DIGITAL IMAGE
Window settings determine HOW tissue attenuation values are displayed
Module VIII.3-Part 1. Design considerations equipment Continued
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Window settings: optimized for mediastinum
Module VIII.3-Part 1. Design considerations equipment Continued
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Window settings: optimized for lung
Module VIII.3-Part 1. Design considerations equipment Continued
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CT scanner
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Computed Tomography (CT) was introduced into
clinical practice in 1972 and revolutionized X Ray
imaging by providing high quality images which
reproduced transverse cross sections of the body.
Tissues are therefore not superimposed on the image
as they are in conventional projections
The technique offered in particular improved low
contrast resolution for better visualization of soft
tissue, but with relatively high absorbed radiation dose
Dose distribution different due to rotation
Module VIII.3-Part 1. Design considerations equipment Continued
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Dose distributions in projection RL and CT
Head exam
Conventional: dose decreases
from entrance to exit, ratio
1/100 …1/1000
Rotational scanning gives dose more
concentrated at center of rotation
Also dose distribution in slices, but
contribution outside imaged slice
Module VIII.3-Part 1. Design considerations equipment Continued
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3rd & 4th generation scanners
3rd generation: rotating
fan beam and array of
detectors (rotate-rotate)
Module VIII.3-Part 1. Design considerations equipment Continued
4th generation: rotating
source and fixed ring of
detectors (rotate-stationary)
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A look inside a rotate/rotate CT
Detector
Array
and
Collimator
Module VIII.3-Part 1. Design considerations equipment Continued
X Ray
Tube
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3rd & 4th generation scanners
• Slice (section) acquisition: 1-2 seconds
• Cables limit rotation to 1 revolution
• Modern scanners: slip ring technology
• Beam is highly filtered (-> 10mmAl HVL)
• Heat loading on tube very high
• Collimation
• defines section thickness
• reduces scatter
• @ 1mm slice also additional collimation at detector
Module VIII.3-Part 1. Design considerations equipment Continued
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Helical or spiral CT
• Slip ring technology allows continuous rotation
• The patient can then be moved continuously
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through the beam, making the examination
much faster
Continuous data acquisition and table feed
X Ray beam
Direction of patient
movement
Module VIII.3-Part 1. Design considerations equipment Continued
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New possibilities for procedures
• The new helical scanning CT units allow a
range of new features, such as :
• CT fluoroscopy, where the patient is stationary, but
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the tube continues to rotate
multislice CT, where multiple slices can be collected
simultaneously
3-dimensional CT and CT endoscopy
• All these new technologies and applications:
• Constant increase in number of examinations
• High contribution to collective effective dose
Module VIII.3-Part 1. Design considerations equipment Continued
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CT contribution to number of exams and to
collective effective dose (FRG 1990-92)
% freq X Ray exams FRG 1990-02
Misc
30%
CT
4%
% coll effect dose X-ray FRG 1990-92
Misc
16%
Spine
10%
Chest
18%
CT
35%
Urography
9%
Angio/DSA
10%
Extremities
20%
Angio/DSA
1%
GI
15%
Dental
17%
Chest
5%
Spine
10%
FRG: Federal Republic of Germany
Module VIII.3-Part 1. Design considerations equipment Continued
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Factors influencing dose in CT
Module VIII.3-Part 1. Design considerations equipment Continued
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Equipment related factors
• Wave form of generator: ~nil, low ripple
• Range of tube current settings, steps
• Beam filtration
• Beam shaper
• Focus axis distance:
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shorter geometry (60 cm instead of 80) gives increase of
wCTDI, but allows less mAs
• Slice collimation
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For ~1mm slices: dose increase
• Scan field and scan angle
Module VIII.3-Part 1. Design considerations equipment Continued
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Dose and spiral CT
• With identical protocol settings
(Slice thickness, table feed, mAs product per scan or rotation,
scan length)
dose in spiral CT would be same as in sequential
scanning
• For interpolation purpose: ~1 additional
rotation => slight increase in dose (<10%)
• If pitch is taken >1: less loss of information, but
dose is reduced
• BUT often INCREASE:
• Scanning over longer distances
• Multi-phase studies, same body scanned repeatedly
Module VIII.3-Part 1. Design considerations equipment Continued
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Summary on this topic CT
• We learned about the principle of CT
scanner, the different generations and the
factors influencing this high dose
examination
Module VIII.3-Part 1. Design considerations equipment Continued
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Topic 5: X Ray beam
characteristics
Beam description, factors affecting beam
quality and effect on imaging process.
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Contents of this topic
Factors affecting X Ray beam and image
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Spectra
Beam quality
Tube current
Influence of mAs
Filtration
Wave-form, ripple
Geometric factors
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Inverse square law
Anode angle
Heel effect
Unwanted radiation
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Stray radiation, scatter, leakage
Fighting against scatter
Enhancing contrast
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Lowering kV
Contrast agents
Module VIII.3-Part 1. Design considerations equipment Continued
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Factors affecting the X Ray beam
•Tube current
•kVp value
•Ripple
•Filtration
Module VIII.3-Part 1. Design considerations equipment Continued
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Photon spectra of X Rays
kVp ≤ charact. line en.
Module VIII.3-Part 1. Design considerations equipment Continued
kVp > charact. line en.
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Beam “Quality”
• “Measures” penetrating power
• Subjectively describes the shape of continuous
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spectrum
Function of:
• kVp
• filtration
• high voltage supply characteristics
• Diagnostic radiology beams are POLYCHROMATIC
• ± Quantification: effective energy
• With moderate filtration: Eeff ≤ 2/3 kVp
• With heavy filtration: Eeff ≤ 1/2 kVp
Module VIII.3-Part 1. Design considerations equipment Continued
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Changing the tube current
Change of QUANTITY
NO change of quality
Effective kV not changed
Module VIII.3-Part 1. Design considerations equipment Continued
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Changing current x time product (mAs)
70 kV- 50 mAs
70 kV- 80 mAs
Module VIII.3-Part 1. Design considerations equipment Continued
70 kV- 25 mAs
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Effect of changing kVp
Change in QUANTITY
&
Change in QUALITY
- spectrum shifts to higher
energy
- characteristic lines appear
Module VIII.3-Part 1. Design considerations equipment Continued
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Importance of correct choice of kV
Right image is made with correct kV; left image
with too high kV, leading to loss of contrast.
Module VIII.3-Part 1. Design considerations equipment Continued
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Remember generator waveforms
Single phase single pulse
kV ripple (%)
100%
Single phase 2-pulse
13%
Three phase 6-pulse
4%
Three phase 12-pulse
Line voltage
0.01 s
0.02 s
Module VIII.3-Part 1. Design considerations equipment Continued
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Effect of waveform or ripple
• During 1/2 mains cycle kV fluctuates between
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min and max
At any moment spectrum defined by kV
The more ripple, the more low energy photons
Less ripple, beam is harder
We do not need these low energy photons:
• Absorbed in patient or scattered
• Dose to patient increases
• Do not reach the image receptor
• HF or 12-pulse will reduce dose to patient
• And give less noisy images (less scatter)
Module VIII.3-Part 1. Design considerations equipment Continued
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Ripple increases image noise
The left image is made with an old (high ripple) generator.
The increased noise led to loss of detail compared to modern equipment
Module VIII.3-Part 1. Design considerations equipment Continued
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Filtration
Change in QUANTITY
&
Change in QUALITY
spectrum shifts to higher energy
1- Spectrum out of anode
2- After window tube housing
(INHERENT filtration)
3- After ADDITIONAL filtration
Module VIII.3-Part 1. Design considerations equipment Continued
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Inherent filtration
•Tube housing window 1
•Inherent filtration:
–Glass envelope
–Cooling oil
–Beryllium window (thin)
•“Radioluscent”
•You can see the filament: -->1
•Filtration expressed in
equivalent Al thickness
•Typically: 1mm Al
•Housing: lead shield !
Module VIII.3-Part 1. Design considerations equipment Continued
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Additional filtration
Additional filtration:
•preferably removes
lower energy photons
• increases effective
energy Eeff
• lowers intensity
• reduces patient
dose !
•but increases tube
loading
Module VIII.3-Part 1. Design considerations equipment Continued
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K-edge filtration
Relative
intensity
Metals with K-edges in
diagnostic range
Ex: Erbium (Mo, Rh,Sn)
Max absorption above
K-edge
Allow modification
spectrum: mammo
K-edge Tin (Sn) allows
lightweight shielding in
protective aprons
kV
Module VIII.3-Part 1. Design considerations equipment Continued
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Geometrical properties
Inverse square law
Anode angle
Heel effect
Module VIII.3-Part 1. Design considerations equipment Continued
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Inverse square law: dose reduction
Fluency of beam constant
Dose (or doserate) per cm2
proportional to 1/x2
Dose = 1
Dose = 1/4
Dose = 1/9
Module VIII.3-Part 1. Design considerations equipment Continued
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Inverse square law
Focus
20cm
Front
20cm
Patient
Image
Receptor
Reduces geometric unsharpness
Reduces patient dose
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100cm
Same dose D at image receptor
Increase at entrance point:
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Patient
Image
Receptor
Left: (40/20)2 => 4 D
Right (120/100)2 => 1.44 D
Module VIII.3-Part 1. Design considerations equipment Continued
Front
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20cm
Anode angle and resolution
Geometrically: size of the
effective focal spot directly
related to the sine of the
angle of the anode.
A the angle of the anode is
decreased, the effective focal
spot is also decreased.
Penumbra and geometrical
unsharpness decreases
Module VIII.3-Part 1. Design considerations equipment Continued
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Heel effect
Plot of intensity
shows variation
along anode to
cathode axis
Heel effect lower by large FID:
not the full cone of radiation (C to
B) is used
Module VIII.3-Part 1. Design considerations equipment Continued
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Unwanted radiation
Module VIII.3-Part 1. Design considerations equipment Continued
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Leakage radiation
•Radiation transmitted thtrough
tube housing
• Image taken on film
• Cathode side
• Need for Radiation protection
•At installation
•In use
•After maintainance
Leakage
Module VIII.3-Part 1. Design considerations equipment Continued
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Stray radiation
• Is the sum of the leakage and
scattered
radiation
• Scattered radiation has been deviated after
leaving the tube
• Scatter occurs from all material exposed to
the radiation: patient, table, filters,room walls
Module VIII.3-Part 1. Design considerations equipment Continued
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Scattered Radiation
• Effect on image quality
• increasing of noise
• loss of contrast
• Effect on patient dose
• increasing of superficial and depth dose
• Possible reduction through :
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 use of grid
 limitation of the field to the useful portion
 limitation of the irradiated volume
(e.g.:breast compression in mammography)
 use of air gap
Module VIII.3-Part 1. Design considerations equipment Continued
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Anti-scatter grid
Source of X Rays
Patient
Scattered X Rays
Lead
Film and cassette
Useful X Rays
Module VIII.3-Part 1. Design considerations equipment Continued
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Anti-scatter grid
• Radiation emerging from the patient
• primary beam : contributes to the image
• scattered radiation does reach the detector and
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contributes to background without
information, and contrast is lowered
scattered radiation also contributes to
unnecessary dose
• the grid (between patient and film)
eliminates most of scattered radiation on
image receptor
Module VIII.3-Part 1. Design considerations equipment Continued
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Anti-scatter grid
BUT:
to keep same dose
on image receptor
increase dose
Grid NOT
recommended for
extremities or
pediatric radiology
Module VIII.3-Part 1. Design considerations equipment Continued
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Scatter reduction by reducing field size
Module VIII.3-Part 1. Design considerations equipment Continued
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Scatter removal by air gap
Module VIII.3-Part 1. Design considerations equipment Continued
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Scatter removal by narrow beam geometry
Module VIII.3-Part 1. Design considerations equipment Continued
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Contrast agent
• Tissue characteristics and photon energy
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determine photoelectric absorption
Basic idea to enhance contrast: lower kV
• cf. mammography
• But lower kV, lower penetration
• How to improve contrast?
• Introducing contrast agents: Ba, Iodine
• High atomic number : photoelectric ~ Z3
• K-edge falls within diagnostic imaging kV
Module VIII.3-Part 1. Design considerations equipment Continued
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Summary on
what we learned about the X Ray beam (1)
• Which factors influence the X Ray spectrum?
• tube potential
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kVp value
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W, Mo, Rh etc.
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inherent + additional
• wave shape of tube potential
• anode track material
• X Ray beam filtration
• How to enhance contrast
• Lower kV
• Use of contrast agents: Barium, Iodine
Module VIII.3-Part 1. Design considerations equipment Continued
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Summary on
what we learned about the X Ray beam (2)
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We became familiar with geometrical properties
• Inverse square law
• Anode angle
• Heel effect
• We learned about some unwanted effects
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Leakage radiation
Stray radiation
Scattered radiation
How to “remove” unwanted scatter radiation
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Reduce field size
Grid
Air gap
Narrow beam geometry
Module VIII.3-Part 1. Design considerations equipment Continued
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Topic 6: Special applications
Dental equipment, Pediatric Equipment
Summary of special applications
• Pediatric radiology
• Specificity
• Requirements on equipment, rooms and accessories
• Positioning and immobilization
• Considerations for the use of the equipment
• Protective shielding
• Dental radiology
• Low dose high frequency
• The equipment
• Radioprotection and patient dose
Module VIII.3-Part 1. Design considerations equipment Continued
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Equipment for pediatric room
Module VIII.3-Part 1. Design considerations equipment Continued
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Specificity of pediatric radiology
• Longer life expectance
• Risk of late detrimental radiation effect greater
• Estimated 2-3 x greater than @ 30-40y
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or 5-7 x greater than after 50y
Justification and optimization even more important
• Smaller size require adapted radiographic
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techniques and exposure factors
Positioning and lack of co-operation requires
short exposures
Functional differences (e.g. higher heart rate,
faster respiration, inability to stop breathing on
command, increased intestinal gas etc.)
Module VIII.3-Part 1. Design considerations equipment Continued
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General, equipment and installation
considerations (1)
• Short exposure times can improve image
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quality. So the generator should have enough
power to allow short exposure times (3
milliseconds) and the timer should allow short
exposure times.
The use of mobile X Ray units in pediatrics
could raise special problems (low
power:motion blurring).
The generator should be of high frequency to
improve the accuracy and reproducibility of
exposures.
Module VIII.3-Part 1. Design considerations equipment Continued
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General, equipment and installation
considerations (2)
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Automatic exposure control (AEC) devices
should be used with caution in pediatrics (they
should be adapted specifically to pediatrics)..
Careful manual selection of exposure factors
usually results in lower doses
X Ray rooms for pediatrics should be designed
for improving the child’s cooperation (control
panel with easy patient visibility and audio
communication, etc.).
Fast film-screen combinations have
advantages (reduction of dose)
Module VIII.3-Part 1. Design considerations equipment Continued
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General, equipment and installation
considerations (3)
• Low-absorbing materials in cassettes,
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tabletops, etc. specially important in pediatrics
radiology.
The antiscatter grid in pediatrics gives limited
improvement in image quality and increases
patient dose given the smaller irradiated
volume (and mass) the scattered radiation is
less
Antiscatter grid should be removable in
pediatric equipment, particularly fluoroscopic
systems
Module VIII.3-Part 1. Design considerations equipment Continued
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General, equipment and installation
considerations (4)
• Image intensifiers should have high conversion
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factors for reducing patient dose in
fluoroscopic systems.
Additional tube filtration may allows dose
reductions.
Pulsed fluoroscopy is recommended since it
allows patient dose reduction as in adults
For CT examinations, the use of specific
technical radiographic parameters for CT
examinations should be promoted (lower mAs
than for adults, and lower kV in some cases)
Module VIII.3-Part 1. Design considerations equipment Continued
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Dental radiology
Module VIII.3-Part 1. Design considerations equipment Continued
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Dental radiology
• High frequency (~1/4 of all examinations) but
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low dose technique
Wide range of entrance doses:0.5 - 150mGy
Dental radiology contributed for 1% of
collective dose from medical diagnostics
Image quality often very low
Many of them in private practice with no
medical physicist or RP officer and have no
medical physics support available
Organs at risk: parathyroid, thyroid, larynx,
parotid glands
Module VIII.3-Part 1. Design considerations equipment Continued
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Intra-oral X-ray dental equipment
Some technical parameters:
-Tube 65kV 7.5 mA
-Filtration: 2mm Al
-Focus skin distance: 20cm
-Field size: 6cm diameter
Module VIII.3-Part 1. Design considerations equipment Continued
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Panoramic dental X-ray equipment
Some technical parameters:
-Tube 60-80kVkV
-Current 4-10mA
Module VIII.3-Part 1. Design considerations equipment Continued
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Image receptors in dental radiology
Intraoral Radiology
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Small films (2 x 3 or 3 x 4 cm) in light-tight
envelopes (no screen)
Digital intraoral sensors.
Compared with category E film, the radiation dose
is reduced by 60%.
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Panoramic Radiology
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Film-screen combination
• Digital sensors.
• Compared with film-screen sensitivity class 200,
the radiation dose is reduced by 50-70%.
Module VIII.3-Part 1. Design considerations equipment Continued
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What we learned about X Ray equipment
• We studied X Ray production (the tube, the
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generator,…) and the variety of radiological
equipment (for plain radiography, fluoroscopy,
CT, …)
We learned about the basic interactions leading
to the image formation
Image intensifiers may drastically improve
image quality and reduce staff dose
and the beam characteristics, including the
« undesirable » apects (scatter, leakage)
Module VIII.3-Part 1. Design considerations equipment Continued
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What we learned about X Ray equipment (ctnd)
• Specific examinations need dedicated
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•
equipment (dental, pediatric, mammographic)
Digital images can enhance contrast and
extract regions of interest
Equipment used by non-radiologists
(surgeons, cardiologists,…) require special
technical requirements
Module VIII.3-Part 1. Design considerations equipment Continued
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Where to get more information (1)
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The Physics of Diagnostic Imaging, David J. Dowsett,
Patrick A. Kenny and R. Eugene Johnston, Chapman &
Hall Medical, ISBN 0-412-40170-1
Equipment for diagnostic radiology, E. Forster, MTP
Press, 1993
The Essential Physics of Medical Imaging, Williams and
Wilkins. Baltimore:1994
Imaging systems in medical diagnostics, Krestel ed.,
Siemens, 1990
The AAPM/RSNA Physics Tutorial for Residents
General Overview of Fluoroscopic Imaging, B. A.
Schueler, Radiographics Vol 20, 1115-1126,(2000)
Module VIII.3-Part 1. Design considerations equipment Continued
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Where to get more information (2)
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European Guidelines on quality criteria for CT. Report
EUR 167262, Office for Official Publications of the
European Communities, 1999, Luxemburg
Radiation exposure in Computed Tomography; 4th
revised Edition, December 2002, H.D.Nagel, CTB
Publications, D-21073 Hamburg
Rational use of diagnostic imaging in pediatrics. WHO,
1987
European guidelines on quality criteria for diagnostic
radiographic images. Report EUR16261 (1996)
Radiation protection and quality assurance in dental
radiology. Radiation protection 81. European
Commission.(1995) CG-89-95-971-EN-C
Module VIII.3-Part 1. Design considerations equipment Continued
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