Patient Dose Management Equipment

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Patient Dose Management -Equipment Physical
Factors

• L 5

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Educational Objectives

• Physical factors challenge to dose management
• Understanding the role of operator in patient
  dose management
• How to manage patient dose using equipment
  factors

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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.
4
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.
5
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.
6
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.
1 unit of intensity
4 units of intensity
16 units of intensity
64 units of intensity
8.8 cm
70 cm
35 cm
17.5 cm
Reproduced with permission from Wagner LK,
Houston, TX 2004.
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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.
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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
Lesson Keep the x-ray tube at the practicable
maximum distance from the patient.
Reproduced with permission from Wagner LK,
Houston, TX 2004.
9
Physical factors and challenges to radiation
management
Physical factors and challenges to 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
Image Receptor
Reproduced with permission from Wagner LK,
Houston, TX 2004.
10
Physical factors and challenges to radiation
management
Physical factors and challenges to radiation
management
4 units of intensity
Image Receptor
Image Receptor
Lesson Keep the image intensifier as close to
the patient as is practicable for the procedure.
Reproduced with permission from Wagner LK,
Houston, TX 2004.
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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.
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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.
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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.
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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.
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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.
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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
20 cm
25 cm
30 cm
ESD 1 unit
ESD 2-3 units
ESD 4-6 units
ESD 8-12 units
Example 2 Gy
Example 4-6 Gy
Example 8-12 Gy
Example 16-24 Gy
Reproduced with permission from Wagner LK,
Houston, TX 2004.
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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.
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Thick Oblique vs Thin PA geometry
100 cm
Dose rate 20 40 mGyt/min
Dose rate 250 mGyt/min
80 cm
100 cm
50 cm
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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.
20
A word about collimation

• Why is narrowing the field-of-view beneficial?
• Reduces stochastic risk to patient by reducing
  volume of tissue at risk
• Reduces scatter radiation at image receptor to
  improve image contrast
• Reduces ambient radiation exposure to in-room
  personnel
• Reduces potential overlap of fields when beam is
  reoriented

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A word about collimation

• What collimation does not do
• It does NOT reduce dose to the exposed portion
  of patients skin

• In fact, dose at the skin entrance site
  increases, sometimes by a factor of 50 or so,
  depending on conditions.

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Physical factors and challenges to radiation
management
Physical factors and challenges to 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.
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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.
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Physical factors and challenges to radiation
management
Physical factors and challenges to 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.
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Dose rate dependence on image receptor
field-of-view or magnification mode.
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RELATIVE PATIENT ENTRANCE DOSE RATE FOR SOME UNITS
INTENSIFIER
Field-of-view (FOV)
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• 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.

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Unnecesary Body Mass in Field of View
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Unnecessary body parts in direct radiation field
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Reproduced with permission from Wagner LK and
Archer BR. Minimizing Risks from Fluoroscopic
Radiation, R. M. Partnership, Houston, TX 2004.
31
Arm positioning important and not easy!

• Lessons
• Output increases because arm is in beam.
• 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.
32
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.
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Lesson Learned

• Keep unnecessary body parts, especially arms and
  female breasts, out of the direct beam.

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Beam Energy Filter kVp
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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
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In general, every x-ray system produces a range
of energies as depicted in the diagram below
Beam energy
Low energy x rays high image contrast but high
skin dose
Middle energy x rays high contrast for iodine
and moderate skin dose
High energy x rays poor contrast and low dose
Reproduced with permission from Wagner LK,
Houston, TX 2004.
37
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
Beam energy
Low-contrast high energy x rays are reduced by
lower kVp
Middle energy x rays are retained for best
compromise on image quality and dose
Filtration reduces poorly penetrating low energy
x rays
Reproduced with permission from Wagner LK,
Houston, TX 2004.
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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
Reproduced with permission from Wagner LK,
Houston, TX 2004.
39
Design of fluoroscopic equipment for proper
radiation control
Physical factors and challenges to radiation
management
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.
Reproduced with permission from Wagner LK,
Houston, TX 2004.
40
Design of fluoroscopic equipment for proper
radiation control
Physical factors and challenges to 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.
Reproduced with permission from Wagner LK,
Houston, TX 2004.
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Design of fluoroscopic equipment for proper
radiation control
Physical factors and challenges to radiation
management
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.
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Design of fluoroscopic equipment for proper
radiation control
Physical factors and challenges to radiation
management
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.
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Design of fluoroscopic equipment for proper
radiation control
Physical factors and challenges to radiation
management
Fluoroscopic kVp

• Fluoroscopic kVp in modern systems is controlled
  by the system. The user might be able to
  influence the way the system works
• By selecting various dose rate selection options
• By selecting a kVp floor

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Design of fluoroscopic equipment for proper
radiation control
Physical factors and challenges to radiation
management
Lessons regarding kVp floor

• Available on a few machines
• Sets kVp below which system does not operate
• Unit usually operates at floor kVp unless
  regulatory dose rates are challenged due to poor
  beam penetration.
• If set too low, dose rates are always excessive
  because system always operates at maximum rates

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Design of fluoroscopic equipment for proper
radiation control
Physical factors and challenges to radiation
management
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.
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Pulsed Fluoroscopy
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Design of fluoroscopic equipment for proper
radiation control
Design of fluoroscopic equipment for proper
radiation control
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.
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Reproduced with permission from Wagner LK and
Archer BR. Minimizing Risks from Fluoroscopic
Radiation, R. M. Partnership, Houston, TX 2004.
49
Reproduced with permission fromWagner LK and
Archer BR. Minimizing Risks from Fluoroscopic
Radiation, R. M. Partnership, Houston, TX 2004.
50
Design of fluoroscopic equipment for proper
radiation control
Physical factors and challenges to radiation
management
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.
51
Reproduced with permission from Wagner LK and
Archer BR. Minimizing Risks from Fluoroscopic
Radiation, R. M. Partnership, Houston, TX 2004.
52
Reproduced with permission from Wagner LK and
Archer BR. Minimizing Risks from Fluoroscopic
Radiation, R. M. Partnership, Houston, TX 2004.
53
Reproduced with permission from Wagner LK,
Houston, TX 2004.
54
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.
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Quantum Noise Control
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Design of fluoroscopic equipment for proper
radiation control
Physical factors and challenges to radiation
management
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.
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Design of fluoroscopic equipment for proper
radiation control
Physical factors and challenges to radiation
management
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!
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No dose monitoring devices
Why does the five-minute timer exist?
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Deterministic Risks to Skin
AJR 2001 173 3-20