Question 2 How are computed tomography dose index (CTDI) and CTDI_w measured?

Answer 2

CTDI is defined as the integrated absorbed dose along the z-axis of the CTDI phantom, normalized by the total beam collimation. The integrated radiation exposure is measured by a 100-mm pencil ion chamber and then converted to the absorbed dose to the phantom. The measured CTDI value is labeled as CTDI₁₀₀, where “100” represents the length of the ion chamber in mm. CTDI₁₀₀ is measured at both center and peripheral positions in the phantom; the ion chamber can be inserted into holes at those locations (the holes are usually plugged with pins made of the same acrylic material). The weighted average of the peripheral and central dose values (CTDI_w) is calculated as:

Bauhs JA, Vrieze TJ, Primak AN, Bruesewitz MR, McCollough CH. CT dosimetry: comparison of measurement techniques and devices. Radiographics. 2008;28:245–253.

McCollough C, Cody D, Edyvean S, et al. The Measurement, Reporting, and Management of Radiation Dose in CT. AAPM Report No. 96; January, 2008.

McNitt-Gray M. AAPM/RSNA physics tutorial for residents: radiation dose in CT. Radiographics. 2002;22: 1541–1553.

Question 4 Is computed tomography dose index volume (CTDI_vol) the same as patient dose?

Answer 4

Because CTDI^vol is defined as the volume-averaged radiation dose to a standard-sized cylindrical phantom, it is not equivalent to the radiation dose received by a patient. CTDI_vol is used for three main applications: (a) to quantify a CT scanner’s radiation exposure output, (b) to perform scanner quality control, and (c) to compare exposure between makes or models for similar protocol settings or between different protocols on the same scanner.

Patient-specific dose can be estimated based on CTDI_vol and other acquisition parameters such as tube potential, tube current, and scan coverage and patient-specific information such as patient weight, age, sex, and location relative to the x-ray tube. Monte Carlo simulation is often used to estimate patient-specific dose based on CT images or realistic patient models.

McNitt-Gray M. AAPM/RSNA physics tutorial for residents: radiation dose in CT. Radiographics. 2002;22: 1541–1553.

Question 6 What are the radiation risks associated with computed tomography (CT) examinations?

Answer 6

Radiation-related risks can be deterministic (e.g., skin burn, hair loss), stochastic (e.g., cancer, genetic mutation), or both. Deterministic effects are very rare with diagnostic imaging procedures using ionizing radiation and are usually due to a malfunction or operator error. In a properly performed CT study, the skin entrance dose is generally much less than 2 Gy, which is near the dose threshold to induce transient erythema.

Effective dose is often used to assess the risk of stochastic effects, including carcinogenesis. The typical effective dose from a CT study is well under 50 mSv. According to a position statement on the radiation risks for medical imaging procedures from the American Association of Physicists in Medicine (AAPM), “Risks of medical imaging at effective doses below 50 mSv for single procedures or 100 mSv for multiple procedures over short periods are too low to be detectable and may be nonexistent. Predictions of hypothetical cancer incidence and deaths in patient populations exposed to such low doses are highly speculative and should be discouraged.”

Even though the risk from a single CT examination is minimal or may be unknown, it is still prudent to follow the “as low as reasonably achievable” (ALARA) rule, especially for pediatric patients.

Bushberg JT, Seibert JA, Leidholdt EM, Boone JM. Chapter 20 Radiation Biology, in the Essential Physics of Medical Imaging. 3rd ed. Philadelphia, PA: Lippincott Williams & Wilkins; 2012:751–836.

Hall EJ, Giaccia AJ. Radiation carcinogenesis. In: Hall EJ, Giaccia AJ, eds. Radiobiology for the Radiologist. 7th ed. Philadelphia, PA: Lippincott Williams & Wilkins; 2006:135–153.

Sadetzki S, Mandelzweig L. Childhood exposure to external ionizing radiation and solid cancer risk. Br J Cancer. 2009;100:1021–1025.

Question 8 How much does computed tomography (CT) contribute to the population dose among diagnostic medical imaging procedures?

Answer 8

Because of its superior image quality, fast scan time, and relative ease of operation, CT use has increased more quickly than the use of any other diagnostic imaging modality. According to one statistic, more than 80 million CT examinations were performed in the United States in 2008. From 1990 to 2006, CT use increased more than 10% each year. This rapid increase in CT use has also contributed to an overall increase in population dose (the annual collective effective dose divided by the size of the U.S. population) from the medical use of radiation. In 2009, CT contributed approximately 50% of the medical-related population dose from diagnostic imaging procedures.

Brenner DJ, Hall EJ. Computed tomography: an increasing source of radiation exposure. N Engl J Med. 2007;357(22):2277–2284.

Brenner DJ, Shuryak I, Einstein AJ. Impact of reduced patient life expectancy on potential cancer risks from radiologic imaging. Radiology. 2011;261(1):193–198.

Question 10 What are the common techniques used to reduce the computed tomography (CT) radiation dose?

Answer 10

Common techniques available from most CT vendors to reduce or optimize the CT radiation dose include (a) automatic tube current modulation (also called automatic exposure control [AEC]) and (b) patient size specific protocols.

Patient attenuation to x-ray photons varies along the longitudinal direction and within the imaging plane (e.g., different anteroposterior [AP] vs lateral patient thickness). CT scanners should be able to modulate the tube current (and hence the dose) automatically based on attenuation changes. Attenuation changes depend on the anatomy being imaged and total attenuation through that anatomy (e.g., larger lateral attenuation requires greater output than AP).

Patient size specific protocols are commonly used to reduce the dose for small patients, especially for the pediatric patient population. Pediatric patients are more vulnerable to radiation damage because some of their organs may still be under development; they also have a longer life span ahead of them during which any damage from radiation can be expressed. It is imperative, therefore, to reduce the radiation dose to this patient population, as proposed by the Image Gently campaign. The radiation dose delivered by CT examination to the pediatric patient should be based on either the age or the weight of the child. Some vendors even provide organ-based tube current modulation to reduce the dose to the gonads and/or breasts.

McCollough C, Cody D, Edyvean S, et al. The Measurement, Reporting, and Management of Radiation Dose in CT. AAPM Report No. 96; January, 2008.

McNitt-Gray M. AAPM/RSNA physics tutorial for residents: radiation dose in CT. Radiographics. 2002;22: 1541–1553.

Question 12 How does the dose distribution inside a patient differ between a computed tomography (CT) scan and a general x-ray radiograph?

Answer 12

A typical CT scan consists of multiple gantry rotations around the target region of the body with ~1,000 x-ray projections every 360°, whereas a general x-ray radiograph is a single-projection acquisition (i.e., the tube does not rotate around the target region). Because the body region is irradiated 360° during a CT scan, the dose distribution is more uniform around the target region inside the patient; the dose distribution is not uniform from a general x-ray radiograph, with the highest dose occurring at the body surface closest to the x-ray tube.

McNitt-Gray M. AAPM/RSNA physics tutorial for residents: radiation dose in CT. Radiographics. 2002;22: 1541–1553.