Patient size and radiation exposure in thoracic, pelvic, and abdominal CT examinations performed with automatic exposure control.

Objective: The purpose of this study was to investigate the effect of patient size on the amount of radiation used to perform CT examinations of the chest, abdomen, and pelvis with automatic exposure control and on the corresponding patient doses.

Materials And Methods: Ninety-one patients underwent CT of the chest, abdomen, and pelvis with a 64-MDCT scanner with automatic exposure control in the x, y, and z planes (noise index, 11.5; tube rotation speed, 1 second; maximal x-ray tube capacity, 800 mA; slice thickness, 5 mm; slice interval, 5 mm; table speed, 40 mm/rotation; pitch, 1; tube voltage, 120 kVp). Volume CT dose index was obtained from the scanner console at the completion of each examination. The volume CT dose index and a dosimetry calculator were used to determine the organ dose in a 70-kg patient. Patient organ doses were obtained by correction of the calculator organ doses by factors that accounted for size variations in the lung and abdomen among patients and the corresponding regions in the phantom.

Results: The average volume CT dose index for a 60-kg patient was approximately 11 mGy, which increased to approximately 22 mGy for an 80-kg patient and to approximately 33 mGy for a 100-kg patient. The corresponding average liver doses for 60-kg patients was approximately 16 mGy, which increased to approximately 25 mGy for 80-kg patients and to approximately 34 mGy for 100-kg patients. For this patient cohort, the median doses to the colon, stomach, and liver were approximately 25 mGy; to the bladder, 31 mGy; and to the red bone marrow, 16 mGy. The 90th percentile organ doses were generally three to four times that of the corresponding 10th percentile organ doses.

Conclusion: For body CT examinations performed with automatic exposure control, the radiation used to perform examinations of 100-kg patients is approximately three times that for a 60-kg patient and results in organ doses that are generally twice as high as those in a 60-kg patient.