Imran N. Ahmad, Kevin A. Wunderle, and Frederick A. Heupler, Jr.
BACKGROUND
Radiation-induced injury is a well-recognized risk associated with invasive procedures in cardiac catheterization laboratories. As the number and complexity of cases in catheterization laboratories increase and technologic advancements of fluoroscopic imaging equipment continue, so does the risk for radiation-induced injuries to patients and laboratory staff. Proper understanding of fluoroscopic imaging equipment and imaging parameters is essential to limiting these risks.
RADIATION PRODUCTION
A general understanding of fluoroscopic equipment and radiation production in the catheterization laboratory provides a foundation for appreciating radiation safety. A fluoroscope is essentially a dynamic x-ray machine capable of real-time imaging. Fluoroscopes used for cardiac catheterizations are typically in a C-arm configuration with the x-ray tube located below the patient table and the image receptor above (Fig. 44.1). There are four main components to a general x-ray imaging system: an x-ray tube, an image receptor, a generator, and an operating console.
FIGURE 44.1 Cardiac catheterization laboratory with biplane imaging capabilities. 1, Radiation dose display; 2, Bedside operation console; 3, Flat panel detector; 4, X-ray tube; 5, C-arm A-P camera; 6, C-arm lateral; 7, Wireless foot pedal; 8, Leaded skirt; 9, Moveable acrylic shield; 10, Monitors and digital flat panel screens; 11, Bedside FFR IVUS operation console.
X-Ray Tube
The x-ray tube consists of a rotating anode and generally a multifilament cathode situated inside an evacuated glass tube. The x-ray tube is immersed in oil for efficient cooling and placed inside a lead housing to limit unwanted radiation. The filament consists of a thin tungsten wire through which a high electric current is passed heating the wire white hot. This thermal energy is sufficient to allow some electrons to overcome the electron-binding energy and provide essentially unbound electrons around the filament. This is often referred to as an electron cloud. When the system is energized, these electrons are accelerated toward and collide with the positively charged tungsten target (anode). The larger the filament, the more electrons that can be generated at one time; however, the focal spot on the anode also increases in size resulting in increased geometric blurring that degrades resolution.
All operators of radiographic equipment should understand three commonly used units displayed on all modern fluoroscopes. The milliamperes (mA) is the first unit, and it is related to the number of electrons traveling across the anode–cathode gap per second. The base unit ampere is defined as the number of coulombs per second (charge per unit time), hence a current. This is exactly what occurs when the electrons flow across the anode–cathode gap. The coulomb (C) is a unit of charge; the charge of one electron is 1.60 × 10-19 C. The next unit is related to the first but fundamentally different; it is the milliampere × seconds (mAs). The mAs is the total number of electrons traveling across the anode–cathode gap for a given exposure. The mAs is obtained by multiplying the first unit, mA, by time in seconds (s), which yields a measure of total charge (coulombs/second × seconds = coulombs). The third unit, peak kilovoltage (kVp), is equal to the applied voltage across the anode–cathode gap, which also represents the highest potential photon energy in the x-ray beam. The base units of voltage are joules/coulomb (energy per unit charge). The electrons, being charged particles, are accelerated across the gap due to the applied voltage, and therefore they gain kinetic energy. This kinetic energy is converted to electromagnetic radiation when the electrons interact with the tungsten target, primarily producing heat in the form of infrared photons, while a small fraction are converted to x-ray photons. The x-ray photons are generated isotropically, that is, with the same intensity in all directions. However, only those x-rays traveling through a small solid angle toward the x-ray tube window are used for imaging purposes. Lead shielding surrounding the x-ray tube is used to attenuate most of the unwanted x-rays; however, leakage radiation (radiation penetrating the x-ray tube housing) can be as high as 880 μGy/h (100 mR/h) at a distance of 1 m, limited by state and federal regulations, though it is commonly well below this value. In terms of affecting the output of the x-ray tube, increases in mA (or mAs) increase the radiation output linearly. As a rule of thumb, increasing kVp from kVp1 to kVp2 increases the output of the system by the ratio of (kVp2/kVp1)2. Therefore, in general, increases in kVp result in a larger increase of x-ray tube output as compared to a comparable increase in mA (or mAs). Increasing kVp not only increases the radiation output of the x-ray tube but also increases the effective energy of the x-ray beam resulting in a more penetrating beam.
Image Receptor
There are two types of image receptors used in fluoroscopes: image intensifiers and digital flat panel detectors. Most state of the art fluoroscopic systems dedicated for cardiac catheterization employ digital flat panel detectors, which are required for 3-D volume imaging. Regardless of the type, the image receptor captures and converts x-rays transmitted through the patient to a signal (analog or digital) for display. It is also involved in regulating the output of the x-ray tube via a feedback loop that governs the generator and x-ray tube output.
Generator
The generator is the component responsible for supplying the electrical power to the x-ray tube. Through step-up or step-down transformers, it supplies a low current and high voltage to the anode and a high current and low voltage to the cathode, respectively. There are a variety of types of generators; however, most modern fluoroscopes use a high-frequency generator, which supplies a very stable voltage and current to the x-ray tube.
Operating Console
The operating console, in conjunction with the system software, is the interface with the fluoroscope. Commonly, there is a primary operating console that provides display and control of many of the fluoroscopic operating parameters and communicates with both the Hospital Information System (HIS) and the Picture Archiving and Communication System (PACS). There is also typically a bedside console that controls most of the fluoroscopy and acquisition functions, mechanical operations such as rotation and angulation of the C-arm, and image display properties.
RADIATION DOSE ESTIMATION AND DISPLAYS
Understanding radiation dose can be complicated. There are an inordinate number of radiation quantities, some of which are rather abstruse. Compounding this problem, there is a traditional and a System International (SI) set of radiation units. It is important to understand the fundamental radiation quantities and their units, some of which are displayed on modern fluoroscopes (Table 44.1).
TABLE
44.1 Radiation Dose Quantities
aQuantities commonly found on modern fluoroscopes
Radiation quantities commonly displayed on modern fluoroscopes are the air kerma rate at the reference plane 
a,r, air kerma at the reference plane Ka,r, and air kerma area product PKA commonly referred to as dose-area product (DAP) (Figs. 44.2 and 44.3). The usual dose units for a,r are mGy/min, for Ka,r are mGy, and for PKA are mGy cm2 or μGy m2. It is important to appreciate what these quantities are and what they are not. First, none of these quantities directly represents the patient’s peak skin dose Dskin,max, which is a quantity that is often sought to predict potential cutaneous injury. a,r and Ka,r are defined in air at a reference plane, often referred to as the interventional reference plane (IRP). The International Electrotechnical Commission (IEC) standards establish the IRP at a fixed distance 15 cm below the isocenter of the C-arm irrespective of the table height (Fig. 44.4). The assumption is that the center of the patient is at isocenter of the C-arm and that the patient is approximately 30 cm thick. Therefore, the IRP and skin entrance are both assumed to be 15 cm below isocenter. Ka,r and PKA are cumulative quantities for the entire procedure. They include radiations delivered at all projections and locations along the cranial–caudal direction (z). There are multiple reasons why Ka,r is not a surrogate for Dskin,max:
FIGURE 44.2 The displayed air kerma rate at the reference plane in this coronary cineangiogram is 71 mGy/min and the air kerma area product, commonly referred to as DAP, is 5,935.8 μGym2.
FIGURE 44.3 The air kerma at the reference plane, commonly referred to cumulative dose, is 1,260 mGy and the air kerma area product, commonly referred to as DAP, is 6,090.4 μGym2.
FIGURE 44.4 The x-ray focal spot is denoted on the fluoroscope by a small red dot on the x-ray tube housing (black circle). The source to image distance is the distance between the x-ray focal spot and the image receptor (A). The source to skin entrance distance is the distance between the x-ray focal spot and the table (B). The IRP is located at 15 cm below the isocenter of the C-arm, (C).
1. The IRP used for the Ka,r and the plane of the skin entrance are rarely the same; they may differ, depending on the projection (AP vs. vlateral), the size of the patient, or the height of the table.
2. Ka,r includes all radiations emitted from the x-ray tube. Many procedures use multiple projections at various locations along the z-axis that spreads the radiation delivered to the patient over a greater anatomical region generally reducing Dskin,max (as long as the fields of view do not overlap).
3. Ka,r by definition assumes that the medium of interaction is air. However, the absorption characteristics of skin and thereby Dskin,max are different from air. Furthermore, Dskin,max includes backscattered radiation as well, while air kerma does not.
The intensity of radiation follows the inverse square law that is the amount of radiation received drops by the inverse square of the distance (d) from the source, 1/d2. This is true for both patients and operators. Below are two clinical scenarios that exemplify limitations in air kerma as a proxy for peak skin dose.
Scenario 1
A small patient (~20 cm thick in the AP dimension) undergoes a cardiac catheterization. The physician performing the procedure is tall and performs the procedure with a source to image distance of 120 cm, source to skin entrance distance of 100 cm, and the image receptor lowered to the patient’s chest. For calculation purposes, let us assume there were two discrete nonoverlapping fields that equally split a total Ka,r of 4 Gy, the IRP is 60 cm from the x-ray tube focal spot, and that all imaging was performed in the straight AP projection (Fig. 44.5).
FIGURE 44.5 In Scenario 1, a relatively small patient 20 cm thick in the AP dimension undergoes catheterization. The source to image distance is 120 cm and the source to skin entrance distance is 100 cm. The IRP is approximately 60 cm above the x-ray tube. Because the radiation intensity drops as the inverse square of the distance from the x-ray source, the air kerma at the reference plane in this example grossly overestimates the air kerma at skin entrance.
Taking into account the inverse square law, the incident air kerma at skin entrance location Ka,i is less than the displayed Ka,r by (60/100)2 × Ka,r = 0.36 × Ka,r. Furthermore, because the radiation was split between two discrete fields, each field actually receives only half of the total air kerma, 0.36 × 0.5 × Ka,r = 0.18 × Ka,r. In other words, the actual air kerma at the skin entrance is 18% of the displayed air kerma at the reference plane, the Ka,r is a gross overestimation in this scenario. To obtain an estimated Dskin,max, the backscatter factor and tissue to air differences must be accounted for which will increase the Ka,i by about 30% to 50% depending on the x-ray beam characteristics, bringing the displayed value slightly closer to Dskin,max, but still off by approximately 75%.
Scenario 2
A morbidly obese patient (~50 cm thick in lateral dimension) undergoes a cardiac catheterization. Standard size collimator covers are removed and replaced with short collimator covers allowing for a source to skin entrance distance of 30 cm. The procedure requires only one unique lateral view (Fig. 44.6).
FIGURE 44.6 In Scenario 2, a relatively large patient 50 cm thick in the lateral dimension undergoes catheterization. The source to image distance is 90 cm and the source to skin entrance distance is 30 cm. The IRP is approximately 60 cm above the x-ray tube. Because the radiation intensity drops as the inverse square of the distance from the x-ray source, the air kerma at the reference plane in this example grossly underestimates the air kerma at skin entrance.
The Ka,r and Ka,i are different by a factor of (60/30)2 = 4. In other words, the Ka,i is four times higher than the Ka,r displayed; the Ka,r is a gross underestimate in this scenario. Again, accounting for backscatter and tissue versus air differences, the actual Dskin,max is greater than the displayed value by approximately 4.5 times.
IONIZING RADIATION AND RADIATION EFFECTS
Radiation-induced detriment can be broken down into two discrete categories, deterministic (nonstochastic) and stochastic.
Deterministic Risk (Nonstochastic)
Deterministic effects are those that have a threshold, below which a given effect is not expressed and above which the severity of the expression increases with increased radiation dose beyond the threshold. Primary examples of deterministic effects are skin related erythema, epilation, and necrosis (Table 44.2).
TABLE
44.2 Skin Effects from Various Radiation Doses
Most deterministic effects are described for acute single exposures. However, patients will often undergo repeated interventions. How the skin reacts to multiple exposures is rather complex and greatly depends on the length of time between irradiations. Although beyond the scope of this book, there are formulized models that originated with radiation therapy patients for approximating single-dose estimations from multiple irradiations within a limited span of time.
The primary radiation quantity associated with deterministic effects is the Dskin,max. However, Dskin,max is not a quantity that is currently displayed on any fluoroscopic equipment; the closest quantity available is Ka,r, but it should be well understood that these are not the same quantity, and they can differ substantially even though they have the same unit of measure (Gy). Dskin,max may be obtained directly by employing radiochromic films that can be calibrated to provide a dose estimate or by using smaller dosimeters such as thermoluminescent dosimeters (TLDs), placed on the patient’s skin at the location where Dskin,max is most likely to occur. Based on Ka,r though, patient education material should be made available to patients about the potential skin effects of radiation.
Generally speaking, there are no limits on the amount of radiation a patient can receive for diagnostic or interventional fluoroscopically guided procedures. It is assumed that any use of radiation is offset by the potential benefit to the patient otherwise the procedure would not be performed. The one exception is a quasi-threshold established by the Joint Commission (JC) that defines a sentinel event for >15 Gy of cumulative skin entrance dose to a single field. Our catheterization laboratory has a specific protocol to deal with escalating doses of patient radiation dose (Fig. 44.7). There is a great deal of ambiguity surrounding this sentinel event, not the least of which is what constitutes “cumulative.” The JC itself has suggested using 6 to 12 months as a period of time to include in the cumulative dose; however, they have not declared an official time period. If a patient has had multiple prolonged fluoroscopically guided procedures within a time period of 6 months or less, it is best to consult with a medical physicist regarding the likelihood of skin injury with a subsequent fluoroscopic procedure and determination of a combined dose for determining the classification of a sentinel event.
FIGURE 44.7 Protocol for increasing doses of cumulative radiation in the cardiac catheterization laboratory. (Courtesy Cleveland Clinic.)
Stochastic Risk
Stochastic effects are characterized by the absence of a threshold for expression. They are primarily governed by a statistical risk with the probability of onset increasing with increased radiation dose. There is an all or nothing expression of the effect. Although the likelihood increases with increased dose, the severity of the effect is unrelated to dose.
The primary radiation quantity associated with stochastic effects is the effective dose (ED), commonly expressed in millisieverts (mSv). Table 44.3 outlines typical EDs associated with various procedures that utilize ionizing radiation. There is no fluoroscopic equipment currently available that displays an ED. The closest related quantity is the PKA. Again, PKA is not the ED and other factors must be taken into account to obtain the ED. For some specific procedure types, there are conversion factors that have been derived to convert PKA to an estimated ED.
TABLE
44.3 Typical ED Estimates for Various Procedures that Utilize Ionizing Radiation
The primary example of a stochastic effect is carcinogenesis. Although there is much debate surrounding exposure to low levels of ionizing radiation, the currently accepted model is the linear no-threshold (LNT) model of carcinogenesis. According to the most recent reports available, there is approximately a 10% increased risk of cancer incidence and approximately a 5% increase in cancer death per 1,000 mSv of ED above natural occurrence for adults. Pediatric lifetime risk is elevated by as high as a factor of 3 or 4 compared with adults depending on various factors. Pediatric risk is higher because on average children will live longer providing a longer potential period to clinically express any radiation-induced effects.
RADIATION MANAGEMENT
The ALARA principle states that radiation exposure to the patient and operator should be kept as low as reasonably achievable to minimize both the deterministic and stochastic effects, essentially limiting the amount of radiation used to that which is absolutely necessary under reasonable conditions. In terms of operator and staff, the major contributor of exposure is scattered radiation from the patient and other objects in the path of the x-ray beam. Table 44.4outlines some of the key components to a procedure that minimize patient radiation exposure.
TABLE
44.4 Minimizing Patient Radiation Exposure in the Cardiac Catheterization Laboratory
Patient Radiation Management
Of the many specific parameters that affect patient dose, seven deserve special attention:
1. Patient distance to the radiation source and distance to image receptor
2. Path length and approach of the x-ray beam
3. Dose and frame rate settings
4. Fluoroscopy time and the number of acquisitions
5. Use of magnification
6. Collimation
7. Grid use
Patient Distance to the Radiation Source and Distance to the Image Receptor
One of the easiest ways to reduce patient dose is to keep the image receptor as close to the patient as possible. Because of the inverse square law even a 10 cm gap can mean a 40% or greater difference in the dose to the patient. Ideally, the patient should be as far away from the x-ray source as possible; however, once we reach the surface of the image receptor, it does not serve much purpose to increase both the patient and image receptor distances from the source.
Path Length and Approach of the X-Ray Beam
Lateral, oblique, or other projections that extend the x-ray path length through the patient increase the dose in two ways (as compared to a PA projection). First, the more tissue in the path length, the more attenuation of the x-ray beam, which results in increased radiation output by the fluoroscopic system. As a general rule, approximately 25% of the x-ray beam is attenuated for each 1 cm of soft tissue traversed. This does not mean that after 4 cm of soft tissue, there are zero photons left because entering the second centimeter of tissue was only 75% of the original beam. Therefore, after 4 cm of soft tissue, there is approximately (0.75)4 = 0.32 of the original beam. Likewise, if we increase the path length through the patient by 4 cm, the system needs to increase its output by approximately 68% to compensate. Second, the longer the path length, the closer the skin entrance is to the x-ray source, which increases the radiation dose by the inverse square law.
Dose and Frame Rate Settings
The machine settings make a significant difference in the dose/dose rate delivered to the patient. Pulsed fluoroscopy can provide dose rates 25% to 50% lower than continuous fluoroscopy. Generally, 7.5 to 10 pulses per second fluoroscopy is recommended for coronary angiographic procedures. If the pulse rate is slower, temporal lag produces a stuttering effect in the image, whereas a higher pulse rate results in unnecessarily high dose rates. The same concept applies to acquisition frame rates, the lower the frame rate, the lower the dose rate however the greater the temporal lag. Almost all fluoroscopic systems have the ability to choose between two or three different dose delivery modes. The dose rate mode can change the dose rate by 30% to 60%. The operator should choose the lowest dose mode available to start any procedure and then increase to a higher dose mode if there is too much noise in the image. Remember, the goal is to be able to perform the task at hand with the worst image quality that will allow you to successfully complete the task. Better image quality almost always results from higher radiation dose.
Fluoroscopy Time and Number of Acquisitions
Although it should be obvious, reducing the fluoroscopy time and the length and quantity of acquisitions will limit the amount of radiation delivered to the patient. Many systems now have the capability of recording fluoroscopic imaging sequences, which may allow for a reduction in acquisitions, thereby reducing patient dose. All modern fluoroscopic systems provide a last image hold feature allowing for the operator to stop x-ray production but still see the last image acquired. This feature can drastically reduce radiation doses if well utilized.
Use of Magnification
Magnification modes significantly increase the radiation dose; the greater the magnification, the greater the dose. The difference in dose rate between a 7 inches (~22 cm diagonal) field of view and a 5 inches (~16 cm diagonal) field of view can be as high as a factor of 2. The operator should always use the largest field of view possible while still being able to visualize the necessary anatomy and instrumentation.
Collimation
Although collimation does not generally affect the region of skin remaining in the field of view, it does reduce the overall amount of radiation absorbed by reducing the total quantity of tissue irradiated. Collimation reduces the PKA, which is the product of the Ka,r and the area of the field. The PKA is the closest related machine generated parameter to the ED. Therefore, collimation does not significantly affect deterministic effects; however, it does affect stochastic effects.
Grid Use
All fluoroscopic systems are provided with a grid on the face of the image receptor. The purpose of a grid is to remove scatter radiation, increase the contrast, and improve image quality. However, the down side of a grid is that it may absorb both scatter radiation and some primary radiation, which requires an increased output of the x-ray tube, thus increasing the patient dose. For petite patients and especially pediatric patients, it may be feasible to remove the grid, which will decrease the radiation dose to the patient; however, image contrast will suffer.
Operator Radiation Management
It is vitally important to first understand that the radiation dose to the operator is directly proportional to the radiation dose of the patient. In fact, the patient is the primary source of radiation exposure to the personnel in the lab. As a rule of thumb, the air kerma rate at 1 m from the patient is equal to 1/1,000th of the air kerma rate at the patient’s skin entrance. Therefore, all the methods used to decrease the radiation exposure to the patient will in effect also reduce the exposure to the operator.
In addition, there are ways an operator can further reduce his or her exposure. The cardinal principles of radiation protection are time, distance, acquisition mode, and shielding (Table 44.5). Scatter radiation to the operator is maximized in the LAO views and minimized in the RAO views when the operator is standing on the right side of a supine patient. By the inverse square law, the intensity of radiation decreases by the square of the distance from the source. Therefore, if we increase our distance from 1 m from the patient to 2 m from the patient, the air kerma rate will be approximately 1/4,000th that of the patient’s entrance exposure (it will decrease by a factor of 4). An operator can move farther away from the x-ray tube and patient by taking advantage of the catheter length, particularly during extremity cases and power injections.
TABLE
44.5 Minimizing Operator Radiation Exposure in the Cardiac Catheterization Laboratory
In general, shielding is a must in the catheterization laboratory. This includes a moveable acrylic shield, table skirt, leaded apron, thyroid collar, and leaded glasses. Lead aprons and thyroid collars are very effective at reducing radiation exposure. Generally speaking, the operator is protected from 95% or more of the incident scattered radiation by the lead protective apparel. In addition, floor-standing shields are particularly useful in cases that utilize biplane angiography to cover radiation scatter from the lateral x-ray tube. The operator must carefully avoid inserting his hands in the direct beam of the x-ray tube. Leaded gloves may increase the dose to the patient and operator if they are introduced into the direct x-ray beam. It is also of utmost importance that all staff present during fluoroscopically guided procedures wear dosimeters. Dosimeters are the only way that radiation management has to monitor the radiation doses to operators and staff.
REGULATORY RECOMMENDATIONS
As stated earlier, ED (mSv) reflects stochastic risk in wholebody equivalents by summing the weighted doses to each organ or tissue irradiated. The whole-body dose limits for radiation workers recommended by the National Council on Radiation Protection and Measurements (NCRP) are 50 mSv annually and the cumulative ED limit is 10 mSv times the operator’s age in years (Table 44.6). These recommendations have been adopted by the Nuclear Regulatory Commission and all US states as annual occupational exposure limits. The NCRP in part bases its recommendations on scientific data from United Nations Commission on the Effects of Ionizing Radiation (UNCEIR) and the National Academy of Sciences in the Biological Effects of Ionizing Radiation (BIER). Many of the US regulations are derived from NCRP reports and the recommended dose limits have been adopted into law. These recommendations are meant to ensure the cancer risk in radiation workers is that of workers in “safe” industries.
TABLE
44.6 Dose Limits Recommended by the NCRP
Personal dosimeters are used to monitor the ED with either one or two badges. Recommendations on how to wear radiation badges have been published by the Society for Cardiovascular Angiography and Interventions. The badges most commonly used are either TLDs or optical stimulated luminescence (OSL) dosimeters. Lithium fluoride crystals in the TLDs when heated emit light in direct proportion to the amount of radiation absorbed. Aluminum oxide crystals in the OSL dosimeters emit light after being stimulated by laser in direct proportion to the radiation absorbed.
The natural background radiation from cosmic rays, rocks and soil, radon gas, and ingested food is on average 3.6 mSv per year in the United States. Given that the average chest x-ray results in 0.04 mSv, background radiation produces an ED that is equivalent to 90 chest x-rays per year. The average coronary interventional procedure can produce anywhere from 7 to 20 mSv.
Skin injury risk is predictable and patient radiation exposure is monitored. If patient radiation doses approach concerning levels, the responsible physician must weigh the risks and benefits and continue only in cases when the potential benefit outweighs the potential risk.
CONCLUSION
Basic rules of radiation safety require the operator to minimize radiation exposure to the patient, him or herself, and to other personnel in the x-ray suite while obtaining adequate images for patient carIt is obviously important for the operator to understand the main principles of radiation production, protection, and radiation-induced injury.
SUGGESTED READINGS
Edward L. Nickoloff, Keith J. Strauss, Bruce T. Austin, et al. Cardiac Catheterization Equipment Performance. College Park: American Association of Physicists in Medicine (AAPM); 2001. Report 70.
Stephen Balter, Marvin Rosenstein, Cindy L. O’Brien, et al. Radiation Dose Management For Fluoroscopically-Guided Interventional Medical Procedures. Bethesda: National Council on Radiation Protection & Measurements (NCRP); Report 168.
Balter S, Hopewell J W, Miller DL, et al. Fluoroscopically guided interventional procedures: a review of radiation effects on patients’ skin and hair. Radiology. 254(2):326–341.
Balter S, Moses J. Managing patient dose in interventional cardiology. Catheter Cardiovasc Interv. 2007;70(2):244–249.
Budoff M, Gupta M. Radiation exposure from cardiac imaging procedures: do the risks outweigh the benefits? J Am Coll Cardiol 2010;56(9):712–714.
Bushong SC. Radiologic Science for Technologists: Physics, Biology, and Protection. 8th ed. St. Louis: Mosby; 2004.
Chambers E, Fetterly K, Holzer R, et al. Radiation safety program for the cardiac catheterization laboratory. Catheter Cardiovasc Interv. 2011;77:546–556.
Chen J, Einstein A, Fazel R, et al. Cumulative exposure to ionizing radiation from diagnostic and therapeutic cardiac imaging procedures: a population-based analysis. J Am Coll Cardiol. 2010;56(9):702–711.
Fazel R, Krumholz HM, Wang Y, et al. Exposure to low-dose ionizing radiation from medical imaging procedures. N Engl J Med. 2009;361(9):849–857.
Gerber TC, Carr JJ, Arai AE, et al. Ionizing radiation in cardiac imaging: a science advisory from the American Heart Association Committee on Cardiac Imaging of the Council on Clinical Cardiology and Committee on Cardiovascular Imaging and Intervention of the Council on Cardiovascular Radiology and Intervention. Circulation. 2009;119(7):1056–1065.
Hirshfeld J W, Balter S, Brinker JA, et al. American College of Cardiology Foundation, American Heart Association, HRS, SCAI, American College of Physicians Task Force on Clinical Competence and Training. ACCF/AHA/HRS/SCAI clinical competence statement on physician knowledge to optimize patient safety and image quality in fluoroscopically guided invasive cardiovascular procedures: a report of the American College of Cardiology Foundation/American Heart Association/American College of Physicians Task Force on Clinical Competence and Training. Circulation. 2005;111(4):511–532.
Jaco J W, Miller DL. Measuring and monitoring radiation dose during fluoroscopically guided procedures. Tech Vasc Interv Radiol. 2010;13(3):188–193.
Mettler F, Huda W, Yoshizumi T, et al. Effective doses in radiology and diagnostic nuclear medicine: a catalog. Radiology. 2008;248(1):254.
Mettler FA, Thomadsen BR, Bhargavan M, et al. Medical radiation exposure in the U.S. in 2006: preliminary results. Health Phys. 2008;95(5):502–507.
Miller DL, Balter S, Schueler BA, et al. Clinical radiation management for fluoroscopically guided interventional procedures. Radiology. 2010;257(2):321–332.
QUESTIONS AND ANSWERS
Questions
1. Which of these statements is TRUE regarding radiation dose terminology?
a. Dose-area product (DAP) reflects the total radiation delivered to the patient.
b. Skin dose is measured in units of Sieverts (Sv).
c. Kerma reflects the biologic impact of different radiation types.
d. Fluoroscopy time is a good measure of radiation dose.
2. What is the greatest source of radiation exposure to the operator?
a. Flat-panel detector close to the operator
b. Tube leakage
c. Scattered radiation from the patient
d. RAO camera angles
3. Which one of the following reduces air kerma radiation dose at the reference plane?
a. Collimation
b. Increasing the source to image distance
c. Changing the field of view from 22 to 16 cm
d. Lowering the flat-panel detector to the patient
4. Which of the following reduces DAP?
a. Collimation
b. Lowering the table height
c. Changing x-ray orientation from PA to AP
d. Increasing the table height
5. Which of the following statements regarding stochastic and deterministic risks is TRUE?
a. Deterministic risk is the major radiation hazard posed to operators.
b. For the same amount of radiation exposure, the stochastic risk is higher in older operators than younger operators.
c. A stochastic effect such as cancer cannot be predicted from a threshold level of radiation exposure.
d. A deterministic effect such as skin injury cannot be predicted from a threshold level of radiation exposure.
6. What is the maximal recommended ED per year for an operator in the cardiac catheterization laboratory?
a. 5 mSv
b. 10 mSv
c. 50 mSv
d. 100 mSv
7. Which one of the following is TRUE regarding SCAI recommendations regarding personal dosimeter badges in the cardiac catheterization laboratory?
a. Wear one badge inside the thyroid collar and one badge at the waist outside the apron.
b. Wear one badge outside the thyroid collar and one badge at the waist outside the apron.
c. Wear another operator’s badge if you forget your own.
d. Wear one badge outside the thyroid collar.
8. Which of the following statements is FALSE regarding radiation-induced skin injury?
a. Peak skin dose is difficult to estimate because of varying distance between the x-ray tube and the patient’s back during a procedure.
b. Radiation skin injury can manifest weeks to months after a procedure.
c. By the inverse square law, lowering the table height will decrease the skin dose (Gyt).
d. There are multiple ways to estimate skin dose including the use of radiochromic films.
9. X-rays are generated at the:
a. Generator
b. Cathode
c. Anode
d. Collimator
10. Which statement below is TRUE?
a. Kar is equal to the patient’s skin dose.
b. Dskinmax is displayed on the monitors in the catheterization laboratory.
c. Of the displayed radiation quantities, K a,r is closest related to deterministic effects.
d. Of the displayed radiation quantities, PKA is closest related to stochastic effects.
e. Both C and D
Answers
1. Answer A: Skin dose is estimated in units of Gray (Gy). Radiation from different sources can cause different degrees of biologic damage for the same amount of energy absorbed and this is accounted for by the equivalent dose. The ED is the sum of equivalent doses to each organ exposed to radiation and reflects stochastic risk. Fluoros-copy time is not a good measure of radiation dose.
2. Answer C: The greatest source of radiation exposure to the operator is scattered radiation from the patient. The lead housing around the x-ray tube is meant to prevent radiation leakage. With respect to camera angles, LAO views with the x-ray tube close to the operator on the right side of the patient are associated with higher operator exposure than RAO views due to the inverse square law, which states that radiation dose decreases the inverse square of the distance from the source.
3. Answer D: Collimation reduces scatter radiation and improves image quality. It does not reduce air kerma radiation dose. Increasing the source to image receptor distance and magnification of the field of view both increase air kerma dose. Lowering the flat-panel detector does decrease the air kerma dose.
4. Answer A: Collimation reduces the area of tissue irradiated without a change in the radiation beam intensity and therefore reduces DAP. DAP remains the same irrespective of table height or x-ray orientation and camera angle.
5. Answer C: Deterministic risk is the major hazard posed to patients from radiation-induced skin injury and can be directly correlated with peak skin dose measurements. Younger operators theoretically have a higher stochastic risk for the same amount of radiation due to the longer potential to express that risk. There is no threshold level of radiation exposure with which a stochastic effect is guaranteed to occur, although the probability increases with higher levels of exposure.
6. Answer C: The whole-body dose limit for radiation workers is 50 mSv annually.
7. Answer D: The recommendations are if two badges are worn, one should be outside the thyroid collar and the second at waist level under the apron. If one badge is worn, it should be outside the thyroid collar. You should never wear another operator’s badge.
8. Answer C: Lowering the table height will increase the skin dose as the radiation source is closer to the skin.
9. Answer C: The cathode is made up of tungsten filaments that become white hot from the current. The electrons travel from the filaments and strike the rotating anode, releasing the x-rays used for imaging.
10. Answer E: There is no fluoroscope available that displays peak skin dose measurements and air kerma is just an estimate.