Uniform image quality achieved by tube current modulation using SD of attenuation in coronary CT angiography

OBJECTIVE. The objective of our study was to evaluate whether the SD of CT attenuation values obtained from unenhanced scans of the left atrium is a reliable parameter for the individual modulation of tube current to achieve uniform image quality in coronary CT angiography (CTA). MATERIALS AND METHODS. One hundred patients ( 59 men and 41 women) who were suspected to have coronary artery disease underwent coronary CTA using a 64-MDCT scanner. In addition to clinical studies, we also performed measurements on water phantoms. Tube current was modulated by the SD of the CT attenuation values measured from the left atrium on unenhanced images scanned at 300 mA. A modulation table was created from data obtained from the studies of water phantoms scanned at various tube currents. Other scanning parameters were identical to those used to obtain unenhanced and contrast-enhanced studies of the 100 patients. The SD values were measured from images scanned at an adjusted tube current, and the images of normal coronary and internal mammary arteries were graded. Radiation doses measured using the volume CT dose index ( CTDIvol) were compared between the SD of the CT attenuation values and the modulation parameters suggested by the manufacturer of the scanner. RESULTS. Image quality was rated as grade 3 ( low mottle) on a 4-grade scale by four observers for 92 - 94 of the 100 patients ( average, 92.5%). The mean SD value at an adjusted tube current was 12.1 H with an SD of 0.758 H ( target SD = 12 H). A radiation dose reduction of 9 - 45% was achieved in patients grouped by weight who weighed less than 70 kg, and a reduction of up to 71% was seen in individual cases. CONCLUSION. Modulating tube current using the SD of CT attenuation values from the left atrium is a highly reliable method of achieving uniform image quality in coronary CTA.