Giảm thiểu các rủi ro trong việc chụp XQuang Minimizing risks

¬ Appropriate credentials and training for physicians performing fluoroscopy ¬ Operators be trained and understand system operation, and implications of radiation exposure for each mode

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Minimizing Risks from Fluoroscopic X-rays

Brent K Stewart, PhD, DABMP Professor, Radiology and Medical Education Director, Diagnostic Physics

a copy of this lecture may be found at:

http://courses.washington.edu/radxphys/PhysicsCourse04-05.html

Brent K Stewart, PhD, DABMP

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Introduction

¬ On September 9th, 1994, the FDA issued an advisory for facilities that use fluoroscopy for invasive procedures Recommendations…

¬ Appropriate credentials and training for physicians performing fluoroscopy

¬ Operators be trained and understand system operation, and implications of radiation exposure for each mode of operation

¬ Physicians be educated in assessing risks and benefits on a case-by-case basis for patients

¬ Patients be counseled regarding the symptoms and risks of large radiation exposures

¬ Physicians justify and limit use of high dose rate modes of operation

Who can perform Fluoroscopy and Associated Radiography?

¬ Most states have regulations regarding the operation of radiation producing equipment and these regulations vary from state to state

¬ In some states, it may be illegal for an untrained person to operate

an x-ray machine even under the direct orders of a physician

¬ However, the fact is that many physicians who use fluoroscopy have essentially no training in this area

Washington State Law

¬ WAC 246-225-020

¬ Operators shall be adequately instructed in safe operating procedures and shall be able to demonstrate competence

¬ A medical x-ray machine operator shall be licensed, certified or registered by the department as either:

¬a licensed health care practitioner

¬a certified diagnostic or therapeutic RT

¬a registered x-ray technician

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What should an operator know?

¬ How to operate the machine

¬ How to properly position the patient

¬ How to minimize the use of radiation

¬ How to properly use shielding devices and personnel monitoring devices

¬ How the radiation is distributed in the room

¬ How to control the factors that optimize image quality (kVp, mA etc.)

¬ How to control factors that reduce radiation levels (collimation)

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What an operator should know

¬ Two professionals trained in specific aspects of fluoroscopy arethe radiological technologist and medical physicist

¬ Nurses or physician assistants should be trained in its safe and proper operation if asked to operate x-ray equipment

¬ Physician is ultimately responsible for assuring that the x-rays are safely and properly applied and that appropriate radiation protection measures are followed

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Skin Injuries

¬ During the application of x-rays, the patient has no sensation of temperature rise in the skin, even if the patient is fully conscious and even for all but the most massive doses of radiation

¬ Small doses from modern equipment might induce cancer, but the frequency of induction would be too low to detect a direct relationship with x-rays

¬ Chronic exposure to low doses can also result in gradual erosionof tissue

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Potential effects in skin from fluoroscopy

c.f Wagner and Archer.

c.f Wagner and Archer Minimizing Risks from Fluoroscopic X-rays 1996.

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c.f Koenig TR, et al Skin Injuries from Fluoroscopically Guided Procedures:

Part 1, Characteristics of Radiation Injury AJR 2001, 177, pp 3-11.

Three weeks post rf cardiac catheter ablation Ischemic dermal necrosis 5 months post procedure

Exposed to 20 minutes fluoro with elbow

20-25 cm from focal spot Note circular pattern coinciding with x-ray beamport

Suggesting that the 18 Gy threshold was passed during the procedure

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c.f Koenig TR, et al.Skin Injuries from Fluoroscopically Guided Procedures:

Deep ulceration with exposure of the humerus at 6.5 months post-procedure

Some radiation ulcers never heal completely, but break down intermittently Progression

of the ulcer may ensue and can be extensive, exposing deep tissues such as tendons, muscles or bones.

c.f Koenig TR, et al Skin Injuries from Fluoroscopically Guided Procedures:

Three transjugular intrahepatic portosystemic shunt placements within a week

Injuries that are advanced to this stage require surgical excision and grafting

Non-healing deep tissue necrotic ulcer with processes of vertebra at 22 mos.

At 23 months, musculocutaneous skin grafting was performed Disfigurement is permanent.

Radiation Injuries of the Skin

same area if the skin

the same area of the skin

¬ Documentation of certain conditions will help physicians if future procedures are needed

¬ A careful record identifying the location of the exposed skin will alert

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Controlling Image Quality, Dose and Dose Rate

¬ The following ten factors are the principal determinants of image quality, radiation dose rate and total radiation dose to the patient and to personnel during fluoroscopy “the Ten Commandments”

¬ patient size

¬ tube current (mA) and kVp

¬ proximity of the x-ray tube to the patient

¬ proximity of the II to the patient

¬ image magnification

¬ x-ray field collimation and use of a grid

¬ shielding and position of personnel relative to patient and equipment

¬ beam-on time

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Commandment #1:

Patient Size

¬ Keep in mind that dose rates are greater and dose accumulates quicker for larger patients

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Commandment #2:

Tube Current (mA)

¬ Keep the tube current as low as possible

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Commandment #3:

Tube Kilovoltage (kVp)

¬ Keep the kVp as high as possible to achieve the appropriate compromise between image quality and low patient dose

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Commandment #4:

Proximity of X-ray tube to Patient

¬ Keep the x-ray tube at the maximal “reasonable” distance from the patient

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Commandment #5:

Proximity of the Image Intensifier to the Patient

¬ Keep the image intensifier as close to the patient as possible

¬ To optimize image quality and reduce radiation dose

¬ Optimize image quality ⇒distortion of anatomy and image blur decreases

¬ Radiation Dose decrease ⇒x-ray intensity required to produce a bright image (automatic brightness control) decreases

Commandment #6:

Image Magnification

¬ Don’t overuse the magnification mode of operation

¬ Magnification can be achieved in 2 ways:

¬magnification option on the image intensifier

¬geometric magnification

(#6) Magnification

¬ Magnification options of the image intensifier

¬ This is achieved by making the x-ray field smaller and displaying the smaller field over the full viewing area of the monitor

¬ The mode of least magnification (largest field) usually delivers the lowest dose rate

¬ Sometimes the dose rate does not change with magnification but frequently, the dose rate increases with magnification

¬ To optimize overall radiation management, use the lowest level

of magnification consistent with the goals of the procedure and reduce the irradiated volume of the patient by employing narrow collimation

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(#6) Magnification

¬ Geometric Magnification

¬ Achieved by increasing the distance between the patient and the image intensifier (contrary to dose reduction method)

¬ Geometric magnification can be used with isocentric systems

¬ Dose typically increases with the square of the magnification

¬i.e., if magnification increases by 2x, dose rate goes up by 4x

¬ Maximum dose rates in this configuration may exceed 10 R/min (legal entrance exposure limit)

¬this is because compliance dose rates are tested under conditions of least geometric magnification (patient closest to image intensifier)

¬ Again, the minimum magnification consistent with the goals of the procedure should be used to manage radiation properly

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Commandment #7:

The Grid

¬ Remove the grid during procedures on small patients, thin body parts or when the image intensifier cannot be placed close to the patient

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Commandment #8:

X-Ray field Collimation

¬ Always use tight collimation

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Commandment #9:

Distance and Shielding

¬ Personnel must wear protective aprons, use shielding, monitor their doses, and know how to position themselves and the imaging equipment for minimum dose

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(#9) Shielding and Distance

¬ The principal source of radiation for the patient is the x-ray tube

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¬ The principal source of radiation for the operator and other personnel is scatter from the patient

by which personnel can reduce dose to themselves is by using shielding and properly positioning themselves relative to the patient and the fluoroscopic equipment

positioned behind a radiation barrier must wear a lead apron during a procedure

¬ Lead aprons

¬ lead equivalency: 0.25 mm to 0.50 mm

¬ 0.25 mm: absorbs > 90% of scatter

¬ 0.35 -0.50 mm: absorbs 95 -99% of scatter (but heavier)

¬ Lead aprons should be properly stored on a hanger when not in use

¬ Aprons should be checked annually for holes, cracks or other forms

of deterioration

x-ray

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¬ Aprons do not protect the thyroid gland or the eyes

¬ Thyroid shields and leaded glass can be used

¬ Leaded glass attenuates 30%-70% depending on the content of lead in glass

¬ Protective gloves of 0.5 mm lead of greater should be worn if hands are going to be near the primary beam (false sense of protection)

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Protection of Physician’s Hands

¬ Dermal atrophy of the forearm and hands were observed in physician who performed fluoroscopy for years

¬ Convinced some physicians to wear special radiation-attenuating surgical gloves or hand shields

¬ Such devices are not likely to protect hands if placed fully into the beam

¬ The automatic brightness control (ABC) detect the reduction in brightness due to the attenuation by the gloves and boost the radiation output to penetrate the “protective” gear

¬ Protective hand gear can be relied on only to protect against radiation outside the field of view of the ABC

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¬ To protect hands during fluoroscopy, it is recommended:

¬ Keep hands out of and away from the x-ray field when the beam

is on unless physician control of invasive devices is requires for patient care during fluoroscopy

¬ Work on the exit-beam side of the patient whenever possible

¬x-ray tube should be below table for vertical orientations

¬for oblique and lateral projections, stand on the side of the patient where the image intensifier is located

¬for adult abdomen, exit radiation is only about 1% the intensity of the entrance radiation

¬extra care must be exercised in situations where physician must work on the x-ray tube side of the patient

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¬ To protect hands during fluoroscopy, it is recommended to:

¬ wear a ring badge to measure your hand exposure monthly

¬ring monitors dose only at the base of the finger

¬dose at the finger tips may be significantly higher

c.f Wagner and Archer Minimizing Risks from Fluoroscopic X-rays – Supplement 1 1997.

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fluoroscopic procedures are required to wear a radiation monitoring device, usually a film badge

to 10% of the occupational annual limit (50 mGy or 5000 mrem) need a radiation badge

wear their badges anteriorly on their collar outside of lead apron

the radiation safety office (RSO)

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(#9) Distance

¬ Radiation Dose to personnel can be significantly reduced by increasing their distance from the radiation source

¬ Inverse-square law: the dose rate drops significantly as the distance from the source increases

(#9) Distance

9 ft 2

4 ft 2

1 ft 2

2 ft

4 ft

6 ft

Given:

Exposure Rate at 2 ft

= 90 mR/hr.

Exposure Rate at 4 ft = (90 mR/hr)(2ft/4ft)2 = 22.5 mR/hr

Exposure Rate at 6 ft = (90 mR/hr)(2ft/6ft)2 = 10 mR/hr

1 ft

2 ft

3 ft

2

1 1 2

D

D E

E = ⎜⎜ ⎝ ⎛ ⎟⎟ ⎠ ⎞

1 m

x-ray

About 0.1% of patient entrance radiation exposure reaches 1 meter from patient

The NCRP recommends that personnel stand at least 2 meters from the x-ray tube, whenever possible (6 feet = 1.82 m)

(#9) Radiation at 1 Meter From Patient

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(#9) C-Arm Fluoroscopy-Shielding

¬ With the C-arm oriented vertically, the x-ray tube should be located beneath the patient with the II above

¬ In a lateral or oblique orientation, the x-ray tube should be positioned opposite the area where the operator and other personnel are working

¬ In other words, the operator and II should be located on the same side of the patient

¬ This orientation takes advantage of the patient as a protective shield

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(#9) The Separator Device (or Spacer Cone)

patient’s skin is at least a specified fixed distance from the X-ray source

the intense beam emerging from the x-ray source is too close to the patient’s skin

designed with removable spacers

minimum distance can be as short as 20 cm (potentially dangerous)

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(#9) The Separator Device (or Spacer Cone)

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Commandment #10: Beam On-Time

¬ Keep beam-on time to an absolute minimum! -The Golden Rule

¬ Control over beam on-time is almost always the most important aspect of radiation management

¬ It is essential practice to disengage fluoroscopic exposure whenthe image on the monitor is not being used

¬ Absentmindedly leaving the x-rays on while viewing other factors associated with the procedure, such as direct observation of the patient or communication with other personnel in the room, must be strictly avoided

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to remind the operator audibly of each 5-minute time interval and to allow the technologist to keep track of the total amount of fluoro time for the exam

Fluoroscopic Timer

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Good vs Bad Geometry: Patient Dose and the

Position of the Fluoroscope

Good vs Bad Geometry: Patient Dose and the

Position of the Fluoroscope

c.f Wagner and Archer Minimizing Risks from

Good vs Bad Geometry: Summary

impact on dose to a patient’s skin

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