Chapter 5: Reducing Radiation Exposure
External
Exposure Protection
X-ray machines do not produce internal radiation
exposure like radioactive materials. There are three basic protection
methods for external sources of radiation: Minimizing exposure time, maximizing distance from the X-ray tube, and the utilization of shielding.
Minimizing Exposure Time: Reduce "Beam-on-time"
Radiation exposure during fluoroscopy is directly proportional to the length of time the unit is activated. Reductions can be realized by:
- Not exposing patient while not viewing the TV image;
- Pre-planning images. An example would be to ensure correct patient positioning before imaging to
eliminate unnecessary "panning;"
- Avoiding redundant views;
- Operator awareness of the 5-minute time notifications.
Fluoroscopy’s real-time imaging capabilities are invaluable for guiding procedures or observing dynamic
functions. However, there is no advantage over conventional X-ray techniques when viewing static images.
Use of Last-Image-Hold features, when available, allows static images to be viewed without
continuously exposing patient and operator to radiation.
Human eye integration time or recognition time of a fluoroscopy image is approximately 0.2 seconds. Therefore, short "looks" usually accomplish the same as a continuous exposure. Prolonged observation
will not improve the image brightness or resolution (Seifert 1996).
Maximize Distance
A small increase in the operator's distance from the patient can significantly
reduce the operator's exposure. Standing one step further away from the patient can cut
the physician's exposure rate by a factor of 4 (AAPM 1998) (Figure 5-1). You
should periodically self-evaluate you personal technique to identify whether opportunities to increase distance exist.
Figure 5-1: Benefit of Increasing Distance
Courtesy of Sorenson, 2000.
In percutaneous transluminal techniques, using the
femoral approach rather than the brachial approach yields distance benefits to the
operator (Figure 5-2).
Figure 5-2: Influence of Technique
Courtesy of Sorenson, 2000.
Substantial increases in operator distance may be
realized through remote fluoroscopy activation whenever automated contrast injectors are used.
Many procedures require staff to intermittently interact with the patient near the fluoroscopy system. The
operator can reduce staff exposure by delaying fluoroscopy until these activities are completed
and/or by alerting these personnel when imaging; especially during high dose rate modes like
cineangiography (Figure 5-3).
Figure 5-3: Benefit of Alerting Staff
Courtesy of Sorenson, 2000.
Room Lighting
Provisions should be made to eliminate extraneous light that can interfere with the fluoroscopic
examination. Room lighting should be dim to enhance visualization of the image. Excessive
light can decrease the ability of the eye to resolve detail. Measures taken to improve detail often involve
increasing patient/staff exposure.
X-ray Tube Position
Fluoroscopy examinations have the smallest operator exposure when the X-ray tube
is underneath the examination table (Figure 5-3). Whenever possible, the operator should
avoid the X-ray tube side of the table
when imaging oblique or lateral images.
Figure 5-3: Benefit of Under-Table Position
Courtesy of Sorenson, 2000.
Note: The benefit is exaggerated-some operator dose occurs on the
I-I side.
I-I to Patient Air Gap
The operator must be aware of the
X-ray tube-to-patient distance. Positions closer can lead to extremely high patient exposures due to
Inverse-Square-Law effects (case study). Minimizing the air gap between the
I-I and the patient typically ensures that this distance is maintained. Use
of the separator or spacer cone can prevent serious effects. The spacer cone is a spacer attached to the tube housing
designed to keep the patient at a reasonable distance from the x-ray source. This is done
specifically to avoid the high skin-dose rates that can be encountered near the tube port.
Spacer cones protect patients from extremely high local exposures by making it physically
impossible to get too close to the X-ray source (inverse-square law effects). For some X-ray machines, the spacer cone is designed to be
removable (Figure 5-3) in order to provide more flexibility in positioning for some special surgical procedures (e.g.,
portable C-arms). There is a risk of very high dose rates to the skin surface when it is removed.
Figure 5-3: Removable Spacer Cone
Courtesy of Rauch, 2000
Reduce Air Gaps
Keeping the I-I as close to patient’s surface as possible significantly
reduces patient and operator exposures (Figure 5-4). The I-I will intercept the primary beam earlier and allow less
scatter to operator and staff. In addition, The Automatic Brightness Control (ABC) system would not need to compensate for the increased X-ray tube
to I-I distance caused by the air gap. The presence of an air gap will always increase patient/operator radiation
exposure and decrease image quality.
Figure 5-4: Benefit of Reducing the Air Gap (I-I Close to Patient)
Courtesy of Sorenson, 2000.
Care should be taken whenever the image view angle is changed during the procedure
(e.g, changing from an ANT to a steep LAO). The I-I is often moved away from the patient while changing X-ray
tube position. Large air gaps can result if the table or I-I height remains
unadjusted.
I-I to Patient Distance Example:
After changing views, a 10-cm air gap between I-I and patient is inadvertently
maintained. What is the increase in radiation exposure to a 20-cm thick patient positioned with the table 30 cm away from the X-ray source, assuming the ABC
compensates by increasing mA only?
Note: mA only adjustments on ABC systems are reasonably
common.
Solution:
Assuming the air gap could have been eliminated by moving the I-I closer, and that the
brightness loss follows the inverse square law:
The brightness level with the air gap is only 69% of the zero air gap brightness.
The ABC system compensates for brightness loss by producing 31% more X-rays. The exposure rate to the patient and staff is subsequently increased by
31%.
Reducing air gaps between patient and I-I also reduces image blur. Blurring of the image is caused by
geometric magnification caused by air gaps. Gaps between patient and I-I enhance geometric magnification. The objects will appear larger with increasing gap size. However, note that image edges are more
fuzzy (Figure 5-4). The degree
of "fuzziness" will increase with increasing air gap.
Figure 5-4: I-I distance and Image Blur
Courtesy of Sorenson, 2000.
Courtesy of Sorenson, 2000.
Minimize Use of Magnification
Use of magnification modes significantly increases radiation exposure to patient, operator, and staff (See
Chapter 3). Magnification modes should be employed only when the increased resolution of fine
detail is necessary.
Collimate the Primary Beam
Collimating the primary beam to view only tissue regions of interest reduces unnecessary tissue
exposure and improves the patient’s overall benefit-to-risk ratio. Optimal collimation also reduces
image noise caused by scatter radiation originating from outside the region of interest (See Chapter
3). A good rule of thumb is that fluoroscopy images should not be totally "round" when collimators
are available for use, the collimator edges should always be visible in the
image.
Use Alternate Projections
Continuous exposure of the patient with the same projection (point of
X-ray beam entry) can cause very high skin dose to small areas. Thus, if
the point of X-ray beam (projection) entry can be changed, the skin may be
spared from the harmful effects of radiation. While this is an effective
protection method, care must be exercised to utilize this method intelligently
since longer beam paths through the patient can cause higher patient and worker
dose.
Steeply angled oblique images (e.g., LAO 50 with 30 cranial tilt) are typically associated with
increased radiation exposure since: X-rays must pass through more tissue before reaching I-I. ABC compensates for X-ray loss caused
by increased attenuation by generating more X-rays; Steep oblique angles are typically associated with increased X-ray tube to
I-I distances. The ABC compensates for brightness loss caused by inverse square law effects by generating more X-rays. Oblique views may bring the X-ray tube closer to the operator side of the table, increasing radiation
exposure from scatter.
Operator exposure from different projections.
When possible, use alternate views (e.g., ANT, LAO with no tilt) when similar information can be
obtained (Figure 5-5). The physician can reduce personal exposure by re-locating himself when oblique views are
taken. For example, dose rates can be reduced by a factor of 5 when the physician stands on the
I-I side of the table (versus X-ray tube side) during a lateral projection (AAPM 1998).
Figure 5-5: Physician Exposure for a variety of Projections
Courtesy of Sorenson, 2000.
Projections with
the X-ray tube neutral or tilted-away from the operator are highlighted blue, while those tilted towards the operator are in
red. Note the decrease seen between the LAO 40 views. The caudal tilt causes the tube to be more tilted away from the
operator.
Optimizing X-ray Tube Voltage
Selection of an adequate kVp value will allow sufficient X-ray penetration while reducing the patient’s dose
rate. In general, the highest kVp should be used which is consistent with the degree of contrast
required (high kVp decreases image contrast).
Henry Ford Hospital has many resources available (e.g., Staff Radiologists, Medical Physicists) to assist
the operator in optimizing the fluoroscopy image while minimizing patient exposure.
Use of Radiation Shields
Use of radiation shielding is highly effective in intercepting and reducing exposure from scattered
radiation (Figure 5-6).
The operator can realize radiation exposure reductions of more than 90 percent through the correct use of
any of the following shielding options. Shields are most effective when placed as near to the radiation
scatter source as possible (i.e., close to patient).
Many fluoroscopy systems contain side-table drapes or similar types of lead shielding. Use of these items
can significantly reduce operator exposures. Many operators have had little difficulty incorporating their
use, even during procedures requiring multiple re-positioning of the system.
Figure 5-6: Benefit of Hanging Shield
Courtesy of Sorenson, 2000.
Ceiling-mounted lead acrylic face
shields should be used whenever these units are available, especially during cardiac procedures.
Correct positioning is obtained when the operator can view the patient, especially the beam
entrance location, through the shield.
Portable radiation shields can also be employed to reduce exposure. Situations where these can be used
include shielding nearby personnel who remain stationary during the procedure.
Use of Personal Protective Equipment
Use of leaded garments substantially reduces radiation exposure by protecting specific body regions. Many fluoroscopy users
would exceed regulatory limits should lead aprons not be worn. Operator and nearby
staff (within 2 meters) are required to wear lead aprons whenever fluoroscopes are operated at
Henry Ford Hospital. Due to the poor material qualities of Leaded
garments, proper storage is essential to protect against damage (Figure 5-6).
Whenever leaded apron are required, they must be supplied and paid for by your
employer (Henry Ford Health System)
Figure 5-6: Properly Stored Leaded Garments
Courtesy of Sorenson, 2000.
Courtesy of Sorenson, 2000.
Lead aprons do not stop all the x-rays. Typically at least a 80% reduction in radiation exposure is obtained
by wearing a lead apron (Figure 5-7). It should be noted that the apron's effectiveness is reduced when more
penetrating radiation is employed (e.g., the ABC boost's kVp for thick patients). Two piece lead apron systems are recommended
for most users since they provide "wrap-around protection" and distribute weight more evenly on the user.
Some aprons contain an internal frame that distributes some of the weight from
the shoulders onto the hips much like a backpack frame. So called "light" aprons should be scrutinized to ensure that adequate levels of shielding are provided.
State of Michigan law requires the use of 0.5 mm lead equivalent aprons.
Figure 5-7: Lead Apron Protection Efficiency
Courtesy of Sorenson, 2000.
Note that higher tube voltages sharply reduces the shielding benefits
of lead aprons. Higher tube voltages will occur when imaging large patients or thick body portions. Also note that
light aprons (0.25 to 0.35 mm Pb) provide less protection compared to the recommended 0.5 mm thickness.
Thyroid shields provide similar levels of protection to the individual’s neck region. Thyroid shield use is
required for operators who use fluoroscopy extensively during their practice.
Optically clear lead glasses are available that can reduce the operator's eye exposure by 85-90% (Siefert
1996). However, due to the relatively high threshold for cataract development, leaded glasses are only
recommended for personnel with very high fluoroscopy work loads (e.g., busy Radiology and
Cardiology Interventionists). Glasses selected should be "wrap-around" in design to protect the eye lens
from side angle exposures. Leaded glasses also provide the additional benefit of providing splash
protection. Progressive style lenses for bifocal prescriptions are available
from a limited number of manufacturers.
The latex leaded gloves provide extremely limited protection. Standard (0.5 mm
lead equivalent) leaded gloves provide useful protection to the user’s hands.
However, trade-offs associated with use of 0.5 mm leaded gloves include loss in tactile feel, increased
encumbrance and sterility. For these reasons, use of leaded gloves is left
to the operator’s discretion. To minimize radiation exposure to the hands, the operator should:
- Avoid placing his hands in the primary beam at all times;
- Place hands only on top of the patient. Hands should never be placed underneath the patient or
table top during imaging;
- Consider using leaded gloves if hand placement within the X-ray beam is necessary or positioned
nearby for extended periods of time.
Radiation Monitoring-Dosimeter Badges
Unlike many workplace hazards, radiation is imperceptible to human senses. Therefore, monitoring of personnel exposed to radiation is performed using a radiation dosimeter or "badge." Monitoring is useful to identify both equipment problems and opportunities for improving individual technique (ensuring
radiation doses are ALARA). Monitoring also documents the level of occupational exposure.
The requirements for dosimetry has been determined by the Radiation Safety Committee
for each work area. These specify the types of dosimeters issued as well
as the collection frequency.
Some workers are issued a single whole body badge (black figure icon). This
whole body dosimeter should be worn on the collar outside of any protective equipment worn (lead aprons). Readings from this position provide an
estimate of the radiation exposure to the eyes. Dose estimates to the individual’s whole body are
made using the appropriate algorithm. Other workers are issued multiple dosimeters. These are designed to be
worn as shown (Figure 5-8):
Figure 5-8: Protective Devices
Lieto and Jackson, 2000.
Ring badge and Sterility
Infection Control has evaluated the use of ring badges in surgical
arenas. For open surgical theaters, ring badges are contraindicated.
Catheter procedures may be performed with ring badges.
Dosimetry Practices
In order to provide an accurate estimate of personal risk, radiation badges are to be used at all times
when working with radiation. It is also important to turn in the radiation badges on time. The accuracy of
the readings depends on the timely processing of the dosimeter with the corresponding control dosimeters.
Absent dosimeters are taken very seriously by the institution.
Reports of which individuals have failed to properly return dosimeters (who did
not report the loss of the dosimeter to the RSO) are sent to: the Radiation
Safety Committee; the Department chairs; the Hospital Medical Executive
Committee; and the Board of the institution. To avoid this negative attention, turn your
dosimeter in on time and promptly report the loss of a dosimeter to the
Radiation Safety Office. A new dosimeter will be issued at no cost and
your good name will be preserved.
The Radiation Safety Officer (RSO) reviews dosimetry records on a monthly basis. Investigations of any
exposure exceeding the established standards are performed to determine whether corrective action can
eliminate or reduce exposures for all concerned. The circumstances surrounding most cases of excessive
radiation exposures are often readily mitigated.
Radiation reports are provided annually to all monitored personnel employed or practicing at
Henry Ford Hospital. In addition, monthly reporting of radiation exposure is available for highly exposed
fluoroscopy users. Individuals can access their personal records at any time, and written dose
estimates are provided upon request.
ALARA Philosophy
Regulatory dose limits
should be viewed as the maximum tolerable levels. Since stochastic radiation effects, such as
carcinogenesis, can not be ruled-out at low levels of exposure, it is prudent to minimize radiation exposure
whenever possible. This concept leads to the As-Low-As-Reasonably-Achievable (ALARA) philosophy.
Simply stated, the ALARA philosophy requires that all reasonable measures to reduce radiation
exposure be taken. Typically, the operator defines what is reasonable. The principles discussed in
this manual are intended to assist the operator in evaluating what constitutes ALARA for his/her
fluoroscopy usage.
The Henry Ford Hospital administration is committed to ensuring that radiation exposure to its medical
staff and employees is kept ALARA. Full attainment of this goal is not possible without the co-operation
of all medical users of radiation devices.
Henry Ford Hospital Radiation Safety Program
Hospital administration has authorized the Radiation Safety Committee (RSC) to oversee all uses of
radiation. The RSC is composed of physicians, physicists, and other professionals who have extensive
experience dealing with radiation protection matters. The committee appoints a qualified expert
(Medical Health Physicist) to administer the day-to-day activities of the
Radiation Safety Office.
Summary of Fluoroscopy Safety
- Keep beam ON-time to an absolute minimum!
- Always use tight collimation!
- Do not overuse the magnification mode.
- Keep the image intensifier as close to the patient as possible, and the
tube as far away from the patient as possible.
- Keep the kVp as high as possible considering the patient dose versus image
quality.
- Keep tube current (mA) as low as possible.
- Minimize room lighting to optimize image viewing.
- Do not overuse the high dose rate.
- Personnel must wear protective aprons, use shielding, monitor doses and know how to position themselves and the machines for minimum dose.
- Change projections angle for long procedures to minimize local skin doses.
- Remember that the X-ray output, patient dose, and area scatter levels increase for
larger patients.
End of Manual!
Acknowledgements:
We wish to
recognize the significant efforts of Scott Sorenson who gave a
substantial portion of this course material to the Medical Physics world to
support his public health efforts in fluoroscopy. Please also recognize
the work of Ralph Lieto, Phil Rauch and Laura Smith. Any errors in this training
module are the sole responsibility of Alan Jackson and Donald Peck of the HFH
Radiation Safety Office.
Fluoroscopy
Operator Course Examination
Select from one of the following exams, but do not repeat any exam you
have tested with previously:
12/01 more to come . . .
