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Patient Dose Management -Equipment & Physical Factors. L 5. 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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Patient dose management equipment physical factors

Patient Dose Management -Equipment & Physical Factors

L 5


Educational objectives

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

Lecture 5: Patient dose management


Patient dose management equipment physical factors

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.

Lecture 5: Patient dose management


Patient dose management equipment physical factors

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.

Lecture 5: Patient dose management


Patient dose management equipment physical factors

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.

Lecture 5: Patient dose management


Patient dose management equipment physical factors

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

17.5 cm

35 cm

70 cm

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

Lecture 5: Patient dose management


Patient dose management equipment physical factors

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.

Lecture 5: Patient dose management


Patient dose management equipment physical factors

2 units of intensity

4 units of intensity

16 units of intensity

64 units of intensity

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.

Lecture 5: Patient dose management


Patient dose management equipment physical factors

2 units of intensity

Image

Receptor

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.

Lecture 5: Patient dose management


Patient dose management equipment physical factors

2 units of intensity

Image

Receptor

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.

Lecture 5: Patient dose management


Patient dose management equipment physical factors

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.

Lecture 5: Patient dose management


Patient dose management equipment physical factors

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.

Lecture 5: Patient dose management


Patient dose management equipment physical factors

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.

Lecture 5: Patient dose management


Patient dose management equipment physical factors

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.

Lecture 5: Patient dose management


Patient dose management equipment physical factors

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.

Lecture 5: Patient dose management


Patient dose management equipment physical factors

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.

Lecture 5: Patient dose management


Patient dose management equipment physical factors

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.

Lecture 5: Patient dose management


Patient dose management equipment physical factors

40 cm

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


A word about collimation

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.

Lecture 5: Patient dose management


A word about collimation1

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

Lecture 5: Patient dose management


A word about collimation2

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.

Lecture 5: Patient dose management


Patient dose management equipment physical factors

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.

Lecture 5: Patient dose management


Patient dose management equipment physical factors

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.

Lecture 5: Patient dose management


Patient dose management equipment physical factors

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.

Lecture 5: Patient dose management


Patient dose management equipment physical factors

Dose rate dependence on image receptor

field-of-view or magnification mode.

Lecture 5: Patient dose management


Patient dose management equipment physical factors

12" (32 cm) 100

9" (22 cm) 200

6" (16 cm) 300

4.5" (11 cm) 400

RELATIVE PATIENT ENTRANCE DOSE RATE

FOR SOME UNITS

INTENSIFIER

Field-of-view (FOV)

Lecture 5: Patient dose management


Patient dose management equipment physical factors

  • 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.

Lecture 5: Patient dose management


Patient dose management equipment physical factors

Unnecesary

Body Mass

in Field of View

Lecture 5: Patient dose management


Unnecessary body parts in direct radiation field

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

Lecture 5: Patient dose management


Patient dose management equipment physical factors

Wagner and Archer. Minimizing Risks from Fluoroscopic X Rays.

Surgical flap

At 3 wks

At 6.5 mos

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.

Lecture 5: Patient dose management


Patient dose management equipment physical factors

Big problem!

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.

Lecture 5: Patient dose management


Patient dose management equipment physical factors

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.

Lecture 5: Patient dose management


Lesson learned

Lesson Learned:

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

Lecture 5: Patient dose management


Patient dose management equipment physical factors

Beam Energy

Filter & kVp

Lecture 5: Patient dose management


Patient dose management equipment physical factors

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

Lecture 5: Patient dose management


Patient dose management equipment physical factors

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.

Lecture 5: Patient dose management


Patient dose management equipment physical factors

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

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.

Lecture 5: Patient dose management


Patient dose management equipment physical factors

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.

Lecture 5: Patient dose management


Patient dose management equipment physical factors

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.

Lecture 5: Patient dose management


Patient dose management equipment physical factors

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.

Lecture 5: Patient dose management


Patient dose management equipment physical factors

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.

Lecture 5: Patient dose management


Patient dose management equipment physical factors

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.

Lecture 5: Patient dose management


Patient dose management equipment physical factors

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

Lecture 5: Patient dose management


Patient dose management equipment physical factors

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

Lecture 5: Patient dose management


Patient dose management equipment physical factors

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.

Lecture 5: Patient dose management


Patient dose management equipment physical factors

Pulsed Fluoroscopy

Lecture 5: Patient dose management


Patient dose management equipment physical factors

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.

Lecture 5: Patient dose management


Patient dose management equipment physical factors

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.

Lecture 5: Patient dose management


Patient dose management equipment physical factors

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.

Lecture 5: Patient dose management


Patient dose management equipment physical factors

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.

Lecture 5: Patient dose management


Patient dose management equipment physical factors

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.

Lecture 5: Patient dose management


Patient dose management equipment physical factors

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.

Lecture 5: Patient dose management


Patient dose management equipment physical factors

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.

Lecture 5: Patient dose management


Patient dose management equipment physical factors

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.

Lecture 5: Patient dose management


Patient dose management equipment physical factors

Quantum Noise

Control

Lecture 5: Patient dose management


Patient dose management equipment physical factors

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.

Lecture 5: Patient dose management


Patient dose management equipment physical factors

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!

Lecture 5: Patient dose management


No dose monitoring devices

No dose monitoring devices

Why does the five-minute timer exist?

Lecture 5: Patient dose management

32


Patient dose management equipment physical factors

Deterministic Risks to Skin

AJR 2001; 173: 3-20

Lecture 5: Patient dose management


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