Transcript IAEA Training Material on Radiation Protection in Cardiology

International Atomic Energy Agency
Patient Dose Management Equipment & Physical Factors
L5
Educational Objectives
1. Physical factors & challenge
to dose management
2. Understanding the role of
operator in patient dose
management
3. How to manage patient dose
using equipment factors
Radiation Protection in Cardiology
Lecture 5: Patient dose management
2
Physical factors and challenges to
radiation management
To create image some x rays must interact with tissues while
others completely penetrate through the patient.
Non-uniform beam exits patient,
pattern of non-uniformity is the
image
X rays interact in patient,
rendering beam non-uniform
Spatially uniform beam enters patient
Reproduced with permission from Wagner LK and Archer BR. Minimizing Risks from Fluoroscopic Radiation, R. M. Partnership, Houston, TX 2004.
Radiation Protection in Cardiology
Lecture 5: Patient dose management
3
Physical factors and challenges to
radiation management
Because image production requires that beam interact
differentially in tissues, beam entering patient must be of
much greater intensity than that exiting the patient.
Only a small percentage (typically ~1%)
penetrate through to create the image.
As beam penetrates patient, x rays interact
in tissue causing biological changes
Beam entering patient typically
~100x more intense than exit beam
Reproduced with permission from Wagner LK and Archer BR. Minimizing Risks from Fluoroscopic Radiation, R. M. Partnership, Houston, TX 2004.
Radiation Protection in Cardiology
Lecture 5: Patient dose management
4
Physical factors and challenges to
radiation management
Lesson:
Entrance skin tissue receives highest dose of x rays and
is at greatest risk for injury.
Only a small percentage (typically ~1%)
penetrate through to create the image.
As beam penetrates patient x rays interact
in tissue causing biological changes
Beam entering patient typically ~100x more
intense than exit beam in average size patient
Reproduced with permission from Wagner LK and Archer BR. Minimizing Risks from Fluoroscopic Radiation, R. M. Partnership, Houston, TX 2004.
Radiation Protection in Cardiology
Lecture 5: Patient dose management
5
Physical factors and challenges to
radiation management
X-ray intensity decreases rapidly with distance from source;
conversely, intensity increases rapidly with closer distances to
source.
64 units of
intensity
4 units of
intensity
16 units of
intensity
1 unit of
intensity
8.8 cm
17.5 cm
35 cm
70 cm
Reproduced with permission from Wagner LK, Houston, TX 2004.
Radiation Protection in Cardiology
Lecture 5: Patient dose management
6
Physical factors and challenges to
radiation management
Lesson: Understanding how to take advantage of the
rapid changes in dose rate with distance from source is
essential to good radiation management. Practical
applications are demonstrated in following slides.
Radiation Protection in Cardiology
Lecture 5: Patient dose management
7
Physical factors and challenges to
radiation management
All other conditions unchanged, moving patient toward or away
from the x-ray tube can significantly affect dose rate to the skin
64 units of
intensity
16 units of
intensity
4 units of
intensity
2 units of
intensity
Lesson: Keep the x-ray tube at the practicable
maximum distance from the patient.
Reproduced with permission from Wagner LK, Houston, TX 2004.
Radiation Protection in Cardiology
Lecture 5: Patient dose management
8
Physical
factors
and
challenges
to
Physical
factors and
challenges
to radiation
management
radiation management
All other conditions unchanged, moving image receptor toward
patient lowers radiation output rate and lowers skin dose rate.
4 units of
intensity
Image
Receptor
2 units of
intensity
Image
Receptor
Image
Receptor
Reproduced with permission from Wagner LK, Houston, TX 2004.
Radiation Protection in Cardiology
Lecture 5: Patient dose management
9
Physical
factors
and
challenges
to
Physical
factors and
challenges
to radiation
management
radiation management
4 units of
intensity
Image
Receptor
2 units of
intensity
Image
Receptor
Lesson: Keep the image intensifier as close to the
patient as is practicable for the procedure.
Radiation Protection in Cardiology
Lecture 5: Patient dose management
Image
Receptor
Reproduced with permission from
Wagner LK, Houston, TX 2004.
10
Physical factors and challenges to
radiation management
Positioning anatomy of
concern at the isocenter
permits easy reorientation
of the C-arm but usually
fixes distance of the skin
from the source, negating
any ability to change
source-to-skin distance.
Radiation Protection in Cardiology
Lecture 5: Patient dose management
11
Physical factors and challenges to
radiation management
When isocenter
technique is employed,
move the image
intensifier as close to
the patient as
practicable to limit
dose rate to the
entrance skin surface.
Radiation Protection in Cardiology
Lecture 5: Patient dose management
12
Physical factors and challenges to
radiation management
Small percentages of dose reduction can result in large
savings in skin dose for prolonged procedures. The
advantages of a 20% dose savings are shown in this Table.
Normal dose
(Gy)
Dose saved (Gy) New dose (Gy)
1
0.2
0.8
2
0.4
1.6
4
0.8
3.2
8
1.6
6.4
16
3.2
12.8
Radiation Protection in Cardiology
Lecture 5: Patient dose management
13
Physical factors and challenges to
radiation management
Lesson: Actions that produce small
changes in skin dose accumulation
result in important and considerable
dose savings, sometimes resulting in the
difference between severe and mild skin
dose effects or no effect.
Radiation Protection in Cardiology
Lecture 5: Patient dose management
14
Physical factors and challenges to
radiation management
Large percentages of dose reduction result in enormous savings
in skin dose when procedures are prolonged. The advantages of
a factor of 2 dose savings are shown in this Table.
Normal dose
(Gy)
Dose saved (Gy) New dose (Gy)
1
0.5
0.5
2
1
1
4
2
2
8
4
4
16
8
8
Radiation Protection in Cardiology
Lecture 5: Patient dose management
15
Physical factors and challenges to
radiation management
Thicker tissue masses absorb more radiation, thus much
more radiation must be used to penetrate the large patient.
Risk to skin is greater in larger patients!
[ESD = Entrance Skin Dose]
15 cm
ESD = 1 unit
Example: 2 Gy
Radiation Protection in Cardiology
20 cm
25 cm
ESD = 2-3 units
ESD = 4-6 units
Example: 4-6 Gy
Example: 8-12 Gy
Reproduced with permission from Wagner LK, Houston, TX 2004.
Lecture 5: Patient dose management
30 cm
ESD = 8-12 units
Example: 16-24 Gy
16
Physical factors and challenges to
radiation management
Thicker tissue masses absorb more radiation, thus
much more radiation must be used when steep beam
angles are employed. Risk to skin is greater with steeper
beam angles!
Quiz: what happens when
cranial tilt is employed?
Reproduced with permission from Wagner LK, Houston, TX 2004.
Radiation Protection in Cardiology
Lecture 5: Patient dose management
17
Thick Oblique vs Thin PA geometry
International Atomic Energy Agency
100 cm
40 cm
Dose rate:
20 – 40 mGyt/min
80 cm
Dose rate:
~250 mGyt/min
100 cm
50 cm
A word about collimation
What does collimation do?
Collimation confines the x-ray beam to an area
of the users choice.
Reproduced with permission from Wagner LK and Archer BR. Minimizing Risks from Fluoroscopic Radiation, R. M. Partnership, Houston, TX 2004.
Radiation Protection in Cardiology
Lecture 5: Patient dose management
19
A word about collimation
Why is narrowing the field-of-view beneficial?
1. Reduces stochastic risk to patient by
reducing volume of tissue at risk
2. Reduces scatter radiation at image receptor
to improve image contrast
3. Reduces ambient radiation exposure to inroom personnel
4. Reduces potential overlap of fields when
beam is reoriented
Radiation Protection in Cardiology
Lecture 5: Patient dose management
20
A word about collimation
What collimation does not do –
It does NOT reduce dose to the exposed
portion of patient’s skin
In fact, dose at the skin entrance
site increases, sometimes by a
factor of 50% or so, depending
on conditions.
Radiation Protection in Cardiology
Lecture 5: Patient dose management
21
Physical
factors
and
challenges
to
Physical factors and challenges to radiation management
radiation management
Lesson: Reorienting the beam distributes dose to other
skin sites and reduces risk to single skin site.
Reproduced with permission from Wagner LK, Houston, TX 2004.
Radiation Protection in Cardiology
Lecture 5: Patient dose management
22
Physical factors and challenges to
radiation management
Lesson: Reorienting the beam in small increments may
leave area of overlap in beam projections, resulting in
large accumulations for overlap area (red area). Good
collimation can reduce this effect.
Reproduced with permission from Wagner LK, Houston, TX 2004.
Radiation Protection in Cardiology
Lecture 5: Patient dose management
23
Physical
factors
and
challenges
to
Physical factors and challenges to radiation management
radiation management
Conclusion: Orientation of beam is usually
determined and fixed by clinical need. When
practical, reorientation of the beam to a new skin
site can lessen risk to skin. Overlapping areas
remaining after reorientation are still at high risk.
Good collimation reduces the overlap area.
Radiation Protection in Cardiology
Lecture 5: Patient dose management
24
Dose rate dependence
on image receptor
field-of-view or
magnification mode.
Radiation Protection in Cardiology
Lecture 5: Patient dose management
25
INTENSIFIER
Field-of-view (FOV)
Radiation Protection in Cardiology
RELATIVE PATIENT
ENTRANCE DOSE RATE
FOR SOME UNITS
12" (32 cm)
100
9" (22 cm)
200
6" (16 cm)
300
4.5" (11 cm)
400
Lecture 5: Patient dose management
26
• How input dose rate changes with
different FOVs depends on
machine design and must be
verified by a medical physicist to
properly incorporate use into
procedures.
• A typical rule is to use the least
magnification necessary for the
procedure, but this does not apply
to all machines.
Radiation Protection in Cardiology
Lecture 5: Patient dose management
27
Radiation Protection in Cardiology
Lecture 5: Patient dose management
28
Unnecessary body parts in direct radiation field
Unnecessary body mass in beam
Reproduced with
permission from
Vañó et al, Brit J
Radiol 1998, 71,
510-516
Reproduced from Wagner – Archer,
Minimizing Risks from Fluoroscopic X
Rays, 3rd ed, Houston, TX, R. M.
Partnership, 2000
Radiation Protection in Cardiology
Lecture 5: Patient dose management
29
Wagner and Archer. Minimizing Risks from Fluoroscopic X Rays.
At 3 wks
At 6.5 mos
Surgical flap
Following ablation procedure with arm in beam near port
and separator cone removed. About 20 minutes of
fluoroscopy.
Reproduced with permission from Wagner LK and Archer BR. Minimizing Risks from Fluoroscopic Radiation, R. M. Partnership, Houston, TX 2004.
Radiation Protection in Cardiology
Lecture 5: Patient dose management
30
Big problem!
Arm positioning –
important and not
easy!
Lessons:
1. Output increases because
arm is in beam.
2. Arm receives intense rate
because it is so close to
source.
Reproduced with permission from Wagner LK and Archer BR. Minimizing Risks from Fluoroscopic Radiation, R. M. Partnership, Houston, TX 2004.
Radiation Protection in Cardiology
Lecture 5: Patient dose management
31
Examples of Injury when Female
Breast is Exposed to Direct Beam
Reproduced with permission from MacKenzie, Brit
J Ca 1965; 19, 1 - 8
Reproduced with permission from Granel et al,
Ann Dermatol Venereol 1998; 125; 405 - 407
Reproduced with permission from Vañó, Br J
Radiol 1998; 71, 510 - 516.
Radiation Protection in Cardiology
Lecture 5: Patient dose management
32
Lesson Learned:
• Keep unnecessary body parts,
especially arms and female breasts,
out of the direct beam.
Radiation Protection in Cardiology
Lecture 5: Patient dose management
33
Radiation Protection in Cardiology
Lecture 5: Patient dose management
34
Design of fluoroscopic equipment for
proper radiation control
Beam energy:
X rays used in fluoroscopy systems have a
spectrum of energies that can be controlled to
manipulate image quality.
How a system manipulates the spectrum
depends on how the system is designed.
Some systems permit the operator to select
filtration schemes
Radiation Protection in Cardiology
Lecture 5: Patient dose management
35
Beam energy: In general, every x-ray system produces a range
of energies as depicted in the diagram below:
1
Relative intensity
0.8
0.6
0.4
0.2
0
0
10
20
30
40
50
60
70
80
90
Photon Energy (keV)
Low energy x rays: Middle energy x rays:
high image contrast high contrast for iodine
but high skin dose and moderate skin
dose
High energy x
rays: poor
contrast and
low dose
Reproduced with permission from Wagner LK, Houston, TX 2004.
Radiation Protection in Cardiology
Lecture 5: Patient dose management
36
Beam energy:
The goal is to shape the beam energy spectrum for the
best contrast at the lowest dose. An improved spectrum
with 0.2 mm Copper filtration is depicted by the dashes:
1
Relative intensity
0.8
0.6
Low-contrast high
energy x rays are
reduced by lower kVp
0.4
0.2
0
0
10
20
30
40
50
60
70
80
90
Photon Energy (keV)
Filtration reduces
poorly penetrating low
energy x rays
Middle energy x rays are
retained for best compromise
on image quality and dose
Reproduced with permission from Wagner LK, Houston, TX 2004.
Radiation Protection in Cardiology
Lecture 5: Patient dose management
37
kVp
Beam energy: kVp controls the high-energy end of the spectrum and is
usually adjusted by the system according to patient size and
imaging needs:
1
Relative intensity
0.8
0.6
0.4
0.2
0
0
10
20
30
40
50
60
70
80
90
Photon Energy (keV)
Reproduced with permission from Wagner LK, Houston, TX 2004.
Radiation Protection in Cardiology
Lecture 5: Patient dose management
38
Design
of fluoroscopic
equipmenttofor
Physical
factors and challenges
radiation
management
proper
radiation
control
Beam energy: Filtration controls the low-energy end of the spectrum. Some
systems have a fixed filter that is not adjustable; others have
a set of filters that are used under differing imaging schemes.
1
Relative intensity
0.8
0.6
0.4
0.2
0
0
10
20
30
40
50
60
70
80
90
Photon Energy (keV)
Reproduced with permission from Wagner LK, Houston, TX 2004.
Radiation Protection in Cardiology
Lecture 5: Patient dose management
39
Design
of fluoroscopic
equipmenttofor
Physical
factors and challenges
proper
radiation
control
radiation
management
Filters: The advantages of filters are that they can reduce skin
dose by enormous factors. (Factors of about 2 or more.)
The disadvantages are that they reduce overall beam
intensity and require heavy-duty x-ray tubes to produce
sufficient radiation outputs that can adequately penetrate
the filters.
Beam energy spectrum
before and after adding 0.2
mm of Cu filtration. Note
the reduced intensity and
change in energies. To
regain intensity tube
current must increase,
requiring special x-ray tube.
1
Relative intensity
0.8
0.6
0.4
0.2
0
0
10
20
30
40
50
60
Photon Energy (keV)
Radiation Protection in Cardiology
70
80
90
Reproduced with permission from Wagner LK, Houston, TX 2004.
Lecture 5: Patient dose management
40
Design
of fluoroscopic
equipmenttofor
Physical
factors and challenges
radiation
management
proper
radiation
control
If filters reduce intensity excessively, image quality is
compromised, usually in the form of increased motion blurring
or excessive quantum mottle.
Lesson: To use filters optimally, systems must be designed to
produce appropriate beam intensities with variable filter
options that depend on patient size and the imaging task.
Radiation Protection in Cardiology
Lecture 5: Patient dose management
41
Design
of fluoroscopic
equipmenttofor
Physical
factors and challenges
radiation
management
proper
radiation
control
Modern fluoroscopy systems employ special filtration to reduce
skin dose and, for detail cardiologic work, employ a set of filters
with varying properties that are switched by the system
according to imaging needs. Some schemes are selectable by the
user.
Conclusion: Users must establish protocols for use of
manufacturer supplied filter options that provide the best
compromise in patient dose and image quality for each machine
employed.
Radiation Protection in Cardiology
Lecture 5: Patient dose management
42
Design
of fluoroscopic
equipmenttofor
Physical
factors and challenges
radiation
management
proper
radiation
control
Fluoroscopic kVp:
Fluoroscopic kVp in modern systems is controlled by
the system. The user might be able to influence the
way the system works:
1. By selecting various dose rate selection options
1. By selecting a kVp floor
Radiation Protection in Cardiology
Lecture 5: Patient dose management
43
Design
of fluoroscopic
equipmenttofor
Physical
factors and challenges
radiation
management
proper
radiation
control
Lessons regarding kVp floor:
1. Available on a few machines
2. Sets kVp below which system does not operate
3. Unit usually operates at floor kVp unless
regulatory dose rates are challenged due to poor
beam penetration.
4. If set too low, dose rates are always excessive
because system always operates at maximum rates
Radiation Protection in Cardiology
Lecture 5: Patient dose management
44
Design
of fluoroscopic
equipmenttofor
Physical
factors and challenges
radiation
management
proper
radiation
control
The kVp floor:
Lesson:
Be sure kVp floor, if available, is set at
appropriately high value to assure system
operates at moderate to low dose rates.
Radiation Protection in Cardiology
Lecture 5: Patient dose management
45
Radiation Protection in Cardiology
Lecture 5: Patient dose management
46
Design
of fluoroscopic
for
Design
of fluoroscopic
equipment equipment
for proper radiation
control control
proper radiation
Understanding Variable Pulsed Fluoroscopy
Background: dynamic imaging captures many still
images every second and displays these still-frame
images in real-time succession to produce the
perception of motion. How these images are captured
and displayed can be manipulated to manage both dose
rate to the patient and dynamic image quality. Standard
imaging captures and displays 25 - 30 images per
second.
Radiation Protection in Cardiology
Lecture 5: Patient dose management
47
Continuous fluoroscopy
In conventional continuous-beam fluoroscopy there is an
inherent blurred appearance of motion because the exposure
time of each image lasts the full 1/30th of a second at 30 frames
per second.
Images
30 images in 1 second
X rays
Continuous stream of x rays produces blurred
images in each frame
Reproduced with permission from Wagner LK and Archer BR. Minimizing Risks from Fluoroscopic Radiation, R. M. Partnership, Houston, TX 2004.
Radiation Protection in Cardiology
Lecture 5: Patient dose management
48
Pulsed fluoroscopy, no dose reduction
Pulsed fluoroscopy produces sharp appearance of motion
because each of 30 images per second is captured in a pulse
or snapshot (e.g., 1/100th of a second).
Images
30 images in 1 second
X rays
Each x-ray pulse shown above has greater intensity
than continuous mode, but lasts for only 1/100th of a
second; no x rays are emitted between pulses; dose to
patient is same as that with continuous
Reproduced with permission fromWagner LK and Archer BR. Minimizing Risks from Fluoroscopic Radiation, R. M. Partnership, Houston, TX 2004.
Radiation Protection in Cardiology
Lecture 5: Patient dose management
49
Design
of
fluoroscopic
equipment
for
Physical factors and challenges to radiation management
proper radiation control
Pulsed imaging controls:
Displaying 25 – 30 picture frames per second is usually
adequate for the transition from frame to frame to
appear smooth.
This is important for entertainment purposes, but not
necessarily required for medical procedures.
Manipulation of frame rate can be used to produce
enormous savings in dose accumulation.
Radiation Protection in Cardiology
Lecture 5: Patient dose management
50
Pulsed fluoroscopy, dose reduction at 15 pulses per second
Sharp appearance of motion captured at 15 images per second
in pulsed mode. Dose per pulse is same, but only half as many
pulses are used, thus dose is reduced by 50%. The tradeoff is a
slightly choppy appearance in motion since only half as many
images are shown per second
Images
X rays
15 images in 1 second
Reproduced with permission from Wagner LK and Archer BR. Minimizing Risks from Fluoroscopic Radiation, R. M. Partnership, Houston, TX 2004.
Radiation Protection in Cardiology
Lecture 5: Patient dose management
51
Pulsed fluoroscopy, dose reduction at 7.5 pulses per second
Pulsed fluoroscopy at 7.5 images per second with
only 25% the dose
Images
X rays
Average 7.5
images in 1
second
Reproduced with permission from Wagner LK and Archer BR. Minimizing Risks from Fluoroscopic Radiation, R. M. Partnership, Houston, TX 2004.
Radiation Protection in Cardiology
Lecture 5: Patient dose management
52
Pulsed fluoroscopy, dose enhancement at 15 pulses per second
Dose per pulse is enhanced because pulse intensity and
duration is increased. Overall dose is enhanced.
Images
X rays
15 images in 1 second
Reproduced with permission from Wagner LK, Houston, TX 2004.
Radiation Protection in Cardiology
Lecture 5: Patient dose management
53
Design of fluoroscopic equipment for
proper radiation control
Design of fluoroscopic equipment for proper radiation control
Lesson: Variable pulsed fluoroscopy is an important
tool to manage radiation dose to patients but the
actual effect on dose can be to enhance, decrease or
maintain dose levels. The actual effect must be
measured by a qualified physicist so that variable
pulsed fluoroscopy can be properly employed.
Radiation Protection in Cardiology
Lecture 5: Patient dose management
54
Quantum Noise
Control
Radiation Protection in Cardiology
Lecture 5: Patient dose management
55
Design
of
fluoroscopic
equipment
for
Physical factors and challenges to radiation management
proper radiation control
Quantum noise controls:
Quantum noise controls control the clarity of the
image by changing the dose rate to the image
receptor. This requires that dose rate to the
patient be manipulated.
They come in two forms – conventional dose level
controls and high level controls. Conventional
level controls permit the adjustment of dose rates
only within the low-dose rate regulatory limits.
High level controls permit the adjustment of dose
rates beyond these limits.
Radiation Protection in Cardiology
Lecture 5: Patient dose management
56
Design
of
fluoroscopic
equipment
for
Physical factors and challenges to radiation management
proper radiation control
Quantum noise controls:
Lessons :
Adjust quantum noise options so that image quality is
adequate and not excessive for the task at hand.
Limit the use of high-level control to very brief episodes
when fine detail is required.
Overuse of high-level controls as a surrogate for
conventional fluoroscopy can be dangerous and can
result in very high dose accumulations and possible
severe injury in a matter of minutes!
Radiation Protection in Cardiology
Lecture 5: Patient dose management
57
No dose monitoring devices
Why does the five-minute timer exist?
Radiation Protection in Cardiology
Lecture 5: Patient dose management
58
32
Deterministic Risks to Skin
AJR 2001;
173: 3-20
Radiation Protection in Cardiology
Lecture 5: Patient dose management
59