International Atomic Energy Agency Patient Dose Management - Equipment & Physical Factors L 5

International Atomic Energy Agency

Patient Dose Management -Patient Dose Management -Equipment & Physical FactorsEquipment & Physical Factors

L 5

Lecture 5: Patient dose management 2Radiation Protection in Cardiology

Educational ObjectivesEducational 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


Physical factors and challenges to Physical factors and challenges to radiation managementradiation management

To create image some x rays must interact with tissues while others completely penetrate through the patient.

Spatially uniform beam enters patient

X rays interact in patient, rendering beam non-uniform

Non-uniform beam exits patient, pattern of non-uniformity is the

image

Reproduced with permission from Wagner LK and Archer BR. Minimizing Risks from Fluoroscopic Radiation, R. M. Partnership, Houston, TX 2004.


Because image production requires that beam interact differentially in tissues, beam entering patient must be of

much greater intensity than that exiting the patient.

Beam entering patient typically ~100x more intense than exit beam

As beam penetrates patient, x rays interact in tissue causing biological changes

Only a small percentage (typically ~1%) penetrate through to create the image.




Lesson:Entrance skin tissue receives highest dose of x rays and

is at greatest risk for injury.

Beam entering patient typically ~100x more intense than exit beam in average size

patient

As beam penetrates patient x rays interact in tissue causing biological changes

Only a small percentage (typically ~1%) penetrate through to create the image.




X-ray intensity decreases rapidly with distance from source; conversely, intensity increases rapidly with closer distances to source.

1 unit of intensity4 units of

intensity16 units of intensity64 units of

intensity


70 cm35 cm17.5 cm

8.8 cm

Reproduced with permission from Wagner LK, Houston, TX 2004.


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.



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.


2 units of intensity4 units of

intensity16 units of intensity64 units of

intensity



Physical factors and challenges to radiation Physical factors and challenges to radiation managementmanagement

All other conditions unchanged, moving image receptor toward patient lowers radiation output rate and lowers skin dose rate.

4 units of intensity

ImageReceptor


ImageReceptor

ImageReceptor






ImageReceptor


ImageReceptor

ImageReceptor


Lesson: Keep the image intensifier as close to the patient as is practicable for the procedure.



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.



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.



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



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.



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



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 cm20 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




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?


International Atomic Energy Agency

100 cm80 cm

Dose rate: 20 – 40 mGyt/min

Thick Oblique vs Thin PA geometryThick Oblique vs Thin PA geometry

100 cm

50 cm

Dose rate: ~250 mGyt/min

40 cm


A word about collimationA word about collimation

What does collimation do?

Collimation confines the x-ray beam to an area of the users choice.




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 in-room personnel

4. Reduces potential overlap of fields when beam is reoriented



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



Lesson: Reorienting the beam distributes dose to other skin sites and reduces risk to single skin site.




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.





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.



Dose rate dependence on image receptor

field-of-view or magnification mode.


INTENSIFIER Field-of-view (FOV)

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


• 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.



Unnecessary body mass in beamUnnecessary body mass in beam

Reproduced from Wagner – Archer, Minimizing Risks from Fluoroscopic X Rays, 3rd ed, Houston, TX, R. M. Partnership, 2000

Reproduced with permission from Vañó et al, Brit J Radiol 1998, 71,

510-516

Unnecessary body parts in direct radiation fieldUnnecessary body parts in direct radiation field


Wagner and Archer. Minimizing Risks from Fluoroscopic X Rays. 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.



Big problem!

Lessons:

1. Output increases because arm is in beam.

2. Arm receives intense rate because it is so close to source.

Arm positioning – important and not easy!



Reproduced with permission from MacKenzie, Brit J Ca 1965; 19, 1 - 8

Reproduced with permission from Vañó, Br J Radiol 1998; 71, 510 - 516.

Reproduced with permission from Granel et al, Ann Dermatol Venereol 1998; 125; 405 - 407

Examples of Injury when Female Examples of Injury when Female Breast is Exposed to Direct BeamBreast is Exposed to Direct Beam


Lesson Learned:

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



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

Design of fluoroscopic equipment for Design of fluoroscopic equipment for proper radiation controlproper radiation control


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Photon Energy (keV)

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Beam energy: In general, every x-ray system produces a range of energies as depicted in the diagram below:

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



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:

Middle energy x rays are retained for best compromise on image quality and dose

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Photon Energy (keV)

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Low-contrast high energy x rays are reduced by lower kVp

Filtration reduces poorly penetrating low energy x rays



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:

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Photon Energy (keV)

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kVpkVp



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.


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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.

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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.





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.




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.




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




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




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.




Design of fluoroscopic equipment for proper radiation Design of fluoroscopic equipment for proper radiation controlcontrol

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.



30 images in 1 second

Continuous fluoroscopy

X rays

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.

Continuous stream of x rays produces blurred images in each frame

Images



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

Pulsed fluoroscopy, no dose reduction

Images

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).

X rays


Reproduced with permission fromWagner LK and Archer BR. Minimizing Risks from Fluoroscopic Radiation, R. M. Partnership, Houston, TX 2004.



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.



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




Pulsed fluoroscopy at 7.5 images per second with only 25% the dose

Pulsed fluoroscopy, dose reduction at 7.5 pulses per second

Images

X rays

Average 7.5 images in 1

second



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




Design of fluoroscopic equipment for proper radiation controlDesign 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.



Quantum Noise 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.




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!



32

Why does the five-minute timer exist? Why does the five-minute timer exist?

No dose monitoring devicesNo dose monitoring devices


Deterministic Risks to Skin

AJR 2001; 173: 3-20

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International Atomic Energy Agency Patient Dose Management - Equipment & Physical Factors L 5