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J Korean Med Assoc > Volume 54(12); 2011 > Article
Park and Jung: CT radiation dose and radiation reduction strategies


There has been a recent increase in attention focused on the potential risk of radiation-induced carcinogenesis from diagnostic radiology, with a particular emphasis on computed tomography (CT). After the rapid adoption of multidetector CT (MDCT), radiation doses from CT are now the single largest source of diagnostic radiation exposure to patients, and the carcinogenesis risk from diagnostic CT radiation dose exposure can no longer be ignored by physicians. To understand the exposure risk and monitor radiation dose exposure, an understanding and interest in CT dose reports is necessary. Almost all MDCTs now show and allow storage of the volume CT dose index (CTDIvol), dose length product (DLP), and effective dose estimations on dose reports, which are essential to assess patient radiation exposure and risks. To decrease these radiation exposure risks, the principles of justification and optimization should be followed. Justification means that the examination must be medically indicated and useful. Optimization means that the imaging should be performed using doses that are as low as reasonably achievable (ALARA), consistent with the diagnostic task. Optimization includes understanding and changing CT protocols to perform the same diagnostic task with the minimal amount of radiation exposure while maintaining diagnostic accuracy. Physicians and radiologists must be aware of the radiation risks associated with CT exams, and understand and implement the principles for patient radiation dose reduction.


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Figure 1
Radiation dose difference between general X-ray (A) and computed tomography (CT) (B). General X-ray radiography is projectional passing from one side of the body to the other in a single direction, which results in the highest radiation dose at the entry site of the beam and the least at the exit site of the beam (A). On the other hand, due to the 360 degree rotational nature of CT, radiation enters the body uniformly causing the highest radiation dose near the skin and the least radiation dose at the center of the body (B).
Figure 2
(A) The typical bell curve appearance of computed tomography (CT) radiation dose along the Z axis in a single CT scan slice is depicted. (B) The combined CT radiation dose and resultant multiple scan average dose (MSAD) along the Z axis for multiple CT scan slices is depicted.
Figure 3
This dose report was generated on a LightSpeed VCT scanner (GE Healthcare, Milwaukee, WI, USA) during a 4 phase dynamic liver computed tomography (CT) in a 49-year-old man. Note the volume CT dose index (CTDIvol) and dose length product (DLP). Dose reports from this GE scanner include scanning type, scan range, CTDIvol, and DLP. Series 200 shows the radiation dose occurring during contrast bolus tracking. In this exam, a scout image was taken, followed by nonenhanced imaging, bolus tracking, arterial, portal, and delayed phase imaging with a total 4 phases. In this case, the portal phase scan range included the pelvis, which explains the relatively larger scan range and higher DLP. The total DLP for this patient is estimated as 1,033.89 mGy×cm.
Figure 4
This dose report was generated on a SOMATOM Sensation 64 (Siemens Healthcare, Forchheim, Germany) computed tomography (CT) scanner during a 4 phase dynamic liver CT. Note the volume CT dose index (CTDIvol) and dose length product (DLP). Dose reports from this scanner include kV, mAs/reference mAs, CTDIvol, DLP, and tube rotation time. This dose report shows separate premonitoring and monitoring radiation doses in regard to bolus tracking. In this exam, a scout image was taken, followed by nonenhanced imaging, bolus tracking, arterial, portal, and delayed phase imaging with a total 4 phases. For nonenhanced liver CT, tube current modulation using CareDose 4D was used at 100 kVP with a reference mAs of 180. 120 kVp was used on arterial to delayed phase enhanced images. In this case, the portal phase scan range included the pelvis, which explains the relatively larger scan range and higher DLP. Portal phase CTDIvol was 14.15 mGy, DLP was 787 mGy×cm, and total DLP was 2,096 mGy×cm. TI, time per rotation; cSL, collimated slice.


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