• Section 10 Doses and Risks Associated with Diagnostic Radiology, Interventional Radiology/Cardiology, and Nuclear Medicine Answers and Feedback

  • Section 10 Doses and Risks Associated with Diagnostic Radiology, Interventional Radiology/Cardiology, and Nuclear Medicine Answers and Feedback 

    1. ANSWER: D
    FEEDBACK: Transient erythema is evident in hours, and the main wave of erythema occurs after 10 days. Epilation occurs after about 3 weeks. Ulceration and depigmentation are late effects due to damage to the dermis. Pain would be secondary to extremely high doses. The correct option is D. 

    2. ANSWER: C
    FEEDBACK: The thyroid is most sensitive to induction by radiation of both benign nodules and malignant tumors. None of the other tissues shows such an incidence of benign tumors due to x-rays. The correct option is C. 

    3. ANSWER: A
    FEEDBACK: Prior to 8 weeks after conception, the fetal thyroid does not take up iodine, so hypothyroidism will not occur in the fetus even from a therapeutic dose. Option A is true. The total body dose to the conceptus from the administration of 20 mCi is less than 7 rad (70 mGy), which is too low to cause death. Option B is false. The red marrow dose to the mother from the administration of 20 mCi is about 0.09 Sv. The absorbed dose to maternal organs other than the thyroid is typically not more than 0.1212 Sv from a 20-mCi administration. The ICRP risk of all cancers from an acute dose of radiation is about 8%/Sv and for leukemia about 1%/Sv. Both C and D are false.

    FEEDBACK: The patient attenuates a fraction of the incident x-rays. This fractional reduction in x-ray intensity is essentially independent of the patient’s position between the x-ray source and image receptor. Therefore, the automatic brightness control response is also independent of patient position. Changing the source-to-skin distance from 20 inches to 12 inches can increase the entrance exposure rate by a factor of 2.8. Option A is true. Many modern fluoroscopy systems have the capability to freeze the last image on the monitor after the x-ray exposure is terminated. This allows the physician to study the last image and plan the next task without radiation exposure to the patient. Option B is true. During pulsed fluoroscopy, the x-ray beam is emitted as a series of short pulses rather than continuously. For continuous fluoroscopy, the video display is a constant 30 frames per second. At reduced frame rates, substantial dose reduction can be achieved. Images can be acquired at 15 frames per second rather than the usual 30 frames per second. Dose reduction is 50%. However, manufacturers may increase the radiation level per frame to achieve a more pleasing visual appearance, and the dose reduction may be only 25%. Nevertheless, pulsed fluoroscopy offers reduced exposure rate and improved image quality compared with continuous fluoroscopy (sampling time for each frame is less with pulsed fluoroscopy). Pulsed fluoroscopy can be performed at lower frame rates (10, 7.5, or 3 frames per second). Low frame rates adversely affect the ability to display rapidly moving structures. Option C is true. The design of the fluoroscopy system may incorporate operator-selectable filtration. Substantial reductions in skin dose, particularly for large patients, can be achieved by inserting appropriate metal filters (aluminum, copper, or other materials) into the x-ray beam at the collimator. Filtration reduces skin dose by preferentially removing low-energy x-rays, which generally do not penetrate the patient to contribute to the image. Option D is true. The presence of a grid improves contrast by absorbing scattered x-rays. However, the dose to the patient is increased by a factor of two or more. For pediatric cases, the removal of the grid reduces the dose, with little degradation of image quality. Grids should be used with discretion when fluoroscopic studies are performed on children. These systems should have the capability for easy removal and reintroduction of the grid. Option E is true. In most interventional fluoroscopic procedures, the x-ray beam is directed toward a particular anatomic region for the bulk of the fluoroscopic time. Some reduction in maximum skin dose can be achieved by periodically rotating the fluoroscope to image the anatomy of interest from a different direction. This method tends to spread the entry dose over a broader area, thereby reducing the maximum skin dose. Option F is true. A magnified image (reduced field of view) typically results in higher dose to the patient. There are two ways to magnify the image, geometric and electronic. The geometric method usually positions the patient closer to the x-ray source but can also be achieved by moving the image receptor farther away from the patient. The technique factors are driven higher for electronic magnification because a smaller area of the image receptor is utilized for image formation. Each method increases the entrance exposure rate. Options G and H are false. 

    5. ANSWER: D
    FEEDBACK: The use of protective clothing (aprons and thyroid collars) in addition to ceiling-mounted lead glass shields is well-documented. The transmission of x-rays through radiation-attenuating surgical gloves is typically stated as 60%–70% at 80 kVp. In clinical practice, this value is higher (less attenuating) because of beam geometry differences. By comparison, devices made with 0.5-mm lead-equivalent material have a transmission of 2%. Radiation-attenuating surgical gloves offer minimal protection of the operator’s hands. The instantaneous dose from scatter radiation is reduced somewhat when the hands covered with one layer of glove material are located near the radiation field. However, the total time near the radiation field depends on the speed at which the procedure is performed, as well as the distance from the imaged anatomy when the x-ray beam is activated. The increased thickness of these gloves reduces dexterity and therefore can increase procedure time. The automatic brightness control system in fluoroscopy increases the radiation output to penetrate the glove when the hand is present in the beam. This can be confirmed by noting that anatomy is seen even though the glove is present. The dose to the hand is comparable to that when the radiation-attenuating glove is not present. The following statements are from “Teaching Radiation Safety to Invasive Fluoroscopists” by S. Balter, PhD:

    Radiation-attenuating surgical gloves are available. They offer minimal protection to the operator’s hands. (There is a real myth around that these gloves eliminate hand exposure.) The automatic exposure control system often increases radiation output to “penetrate” the gloves. Anatomy is seen through 2, 4, or 6 layers of glove material. The operator’s hand is covered by a single layer of glove material.

    The cost of radiation-attenuating surgical gloves and the minimal dose reduction do not justify the use of these devices in a risk-versus-benefit radiation protection analysis. Radiation-attenuating surgical gloves provide a false sense of protection and are not recommended. The correct option is D. 

    6. ANSWER: C
    FEEDBACK: On September 30, 1994, the FDA issued a Public Health Advisory regarding radiation-induced skin injuries from fluoroscopic procedures. These injuries are usually delayed, so the physician cannot discern damage by observing the patient immediately after the treatment. The radiation dose required to cause skin injury is typically 3 Gy for erythema (onset 1–2 days after exposure) and temporary epilation (onset 2 – 3 weeks after exposure). Additional fluoroscopy time above the threshold dose increases the severity of the skin injury: a dose of 6 Gy for main erythema (onset 10 days after exposure) and a dose of 15 –20 Gy for moist desquamation (onset 4 weeks after exposure). Vascular damage is expected for skin doses above 20 Gy. The procedures of concern are primarily interventional procedures during which fluoroscopy is used to guide instruments. The dose rate to the skin from the direct beam is typically between 1 and 3 rad/min (10–30 mGy/min) for continuous fluoroscopy. The maximum exposure rate permitted by the FDA is 10 R/min. The dose rate is usually reduced if pulsed fluoroscopy is employed. For units with a high-level fluoroscopy mode of operation, the maximum exposure rate to the skin from the direct beam is 20 R/min. Risk of skin injury is associated with prolonged fluoroscopy time. The time required to deliver a threshold dose for temporary epilation is typically greater than 1.5 hours in fluoroscopy (0.03 Gy/min). At maximum exposure rate, the time to deliver this dose to a single skin site is 30 minutes in normal mode and 15 minutes in high-level fluoroscopy mode. Large patients are more susceptible to skin injury because automatic brightness control automatically changes (without operator intervention) the technique factors to increase the x-ray output. The correct option is C. 

    7. ANSWER: D
    FEEDBACK: Since ultrasound is nonionizing radiation, no radiation dose is delivered to the fetus. Scattered radiation to the fetus from a head CT examination is less than 0.01 mGy. The entrance exposure for a KUB radiograph is usually about 300 mR. By using a dose conversion factor of 0.2 rad/R for the fetus (depends on beam quality, patient size, and field size), the dose per radiograph is estimated to be 60 mrad (0.6 mGy). The chest radiographic examination is a very low-dose procedure requiring only an entrance exposure of 10–40 mR (0.1–0.4 mGy) for the posteroanterior projection and slightly more for the lateral projection. Scattered radiation to the fetus would be considerably lower. The correct option is D. 

    8. ANSWER: B
    FEEDBACK: For a declared pregnancy, the dose limit to the fetus is 500 mrem (5 mSv). For fluoroscopy, portable radiography, and nuclear medicine imaging, the dose to the conceptus from occupational exposure to the mother will very likely be less than 5 mSv if proper radiation protection practices are followed. Radioiodine treatments with I-131 sodium iodide are considered to place the fetus at higher probability of exceeding 5 mSv and are potentially very hazardous in terms of uncontrolled release of the radioactive material. The fetal thyroid takes up radioiodine after age 12 weeks. The correct option is B. 

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