Scanning beyond anatomic limits of the thorax in chest CT: findings, radiation dose, and automatic tube current modulation.

OBJECTIVE Our objective was to determine additional radiation dose associated with scanning beyond the anatomic limits of the thorax in chest CT protocol and to assess the effect of z-axis modulation on the additional radiation dose associated with the scanning protocol. MATERIALS AND METHODS "Extra" images for routine chest CT were defined as those above lung apices (supraapical) and those caudal to the lowermost portion of lung parenchyma (infrapulmonary), including images obtained beyond the adrenal glands (infraadrenal). One hundred and forty-eight consecutive chest CT examinations (70 men, 78 women; age range, 15-90 years) performed September 13-25, 2003, were reviewed to determine the number of supraapical, infrapulmonary, and infraadrenal extra images. All examinations were performed using z-axis modulation (n = 70) or fixed tube current (n = 78). The CT dose index volume and dose-length product (DLP) values for the extra images were calculated. Two radiologists reviewed these extra images for pathologic findings. Student's t test was used to perform the statistical analysis. RESULTS One hundred forty-four (97%) examinations had supraapical extra images and 145 (98%) had infrapulmonary extra images. A total of 31 additional findings were observed in extra images. Most clinically important findings were identified in patients with a history of malignancy. With z-axis modulation, the mean DLP for supraapical and infrapulmonary extra images was 39.98 mGy x cm and 132.59 mGy x cm, respectively. With fixed tube current, the mean DLP for supraapical and infrapulmonary extra images was 30.31 mGy x cm and 95.91 mGy x cm, respectively. CONCLUSION A substantial number of extra images are acquired during chest CT that do not add clinically important information in patients with nonmalignant indications. The use of z-axis modulation increased radiation dose for the extra images.