320-row CT scanning: reduction in tube current parallels reduction in radiation exposure?

Nowadays, multiple studies involving over several thousands of patients have established that CT angiography is a highly accurate noninvasive approach for delineation of the presence and severity of coronary atherosclerosis [1–35]. With its high negative predictive value cardiac CT is optimally suited for the evaluation of patients with a low or intermediate risk of coronary disease, allowing the non-invasive exclusion of coronary disease at relatively low cost and risk [36–48]. However, the appropriate radiation dose remains an important issue in cardiac CT. On one hand, a too low radiation dose may result in a high level of image noise and therefore in non-evaluable images. On the other hand, using higher radiation exposure levels may put patients at unnecessary risk of radiation damage [49–58]. Effective strategies to reduce radiation dose, such as prospective gating, ECG-correlated modulation of the tube current, and tube voltage below 100 kV, are becoming more and more available [59–65]. Leschka et al. [61] showed that adjustment of the scan length of CT coronary angiography using the images from calcium scoring instead of the scout was associated with a 16% reduction in radiation dose of dual-source CT coronary angiography. In a large multicenter study of coronary CT angiography in patients with excellent heart rate control, Labounty et al. [62] reported that the use of minimal padding (i.e. additional surrounding X-ray beam on time), was associated with a substantial reduction in radiation dose (from mean 5.7 mSv to mean 2.0 mSv) together with preserved image interpretability. Rogalla et al. [63] showed that the anterior-posterior diameter adapted tube current in dynamic volume CT coronary angiography provided a new simple and practical approach to keep image quality constant by accounting for differences in patient size. Maintaining a constant image quality in CT, independent of patient body habitus, significantly contributed to a substantially improved diagnostic image quality together with a reduced radiation dose for the patient. Blankstein et al. [64] investigated the effective radiation dose and image quality of 100 kV versus 120 kV tube voltage among patients referred for cardiac dual source CT imaging in 294 consecutive patients. They convincingly demonstrated that use of low kV resulted in a substantial reduction of radiation dose without compromising image quality. The effective radiation dose for the 100 and 120 kV scans was 8.5 and 15.4 mSv, respectively. In the recently published PROTECTION II trial, Hausleiter et al. [65] studied 400 non-obese patients undergoing CT angiography with either 100 or 120 kV CT angiography. The study specifically examined the impact of a reduction in tube voltage to 100 kV using 64-slice CT angiography systems from three different manufacturers. It was demonstrated that a further 31% reduction in radiation exposure could be obtained with 100 kV tube voltage settings while image quality was preserved. In the current issue of the International Journal of Cardiovascular Imaging, Zhang et al. [66] prospectively evaluated image quality parameters, contrast volume and radiation dose at the 100 kV tube voltage setting during CT coronary angiography. Interestingly, the authors used a 320-row CT scanner. The authors studied 107 consecutive patients with a heart rate