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Circle of Willis at CT Angiography: Dose Reduction and Image Quality—Reducing Tube Voltage and Increasing Tube Current Settings¹

¹ From the Department of Radiology, University Medical Center Utrecht, Heidelberglaan 100, 3584 CX Utrecht, the Netherlands. Received July 22, 2005; revision requested September 23; revision received March 26, 2006; accepted May 2; final version accepted August 7. Address correspondence to A.W. (e-mail: annetwaaijer{at}gmail.com).

Purpose: To prospectively assess the effects of lower tube voltage and various effective tube currents on image quality for computed tomographic (CT) angiography of the circle of Willis.

Materials and Methods: Institutional review board approval was obtained. Patients or family provided written informed consent. Signal-to-noise ratios (SNRs) were determined in a head phantom for various effective tube currents with tube voltages of 90, 120, and 140 kVp. Patients were referred for CT angiography because of acute subarachnoid hemorrhage (n = 20) or family history of cerebral aneurysms (n = 20). In each group, 10 patients were scanned with 120 kVp and 200 mAs_eff and 10 were scanned with 90 kVp and 330 mAs_eff (CT dose index volumes, 27.2 mGy and 20.6 mGy, respectively). CT numbers were measured in the internal carotid artery at the T junction and compared with a t test. Two radiologists used a five-point scale to subjectively score arterial enhancement, depiction of small arterial detail, image noise, venous contamination, and interference of subarachnoid blood. Mann-Whitney U test was used for statistical analysis.

Results: In the phantom, SNR² was proportional to effective tube current and CT dose index volume. With an identical effective tube current, SNR² was lower at 90 kVp than at 120 or 140 kVp. With identical CT dose index volume, tube voltage of 90 kVp resulted in a 45%–52% increase of SNR² compared with SNR² at 120 kVp. In patients, mean attenuation in the internal carotid artery T junction was higher with 90 kVp (340 HU) than with 120 kVp (252 HU, P < .001). Although dose at 90 kVp was 30% lower than dose at 120 kVp, scores for arterial enhancement and depiction of small arterial detail were higher at 90 kVp than at 120 kVp (4.0 vs 3.2 and 3.6 vs 3.1, respectively; P < .005).

Conclusion: In head phantoms, lower tube voltage improved SNR at equal radiation doses. For CT angiography of the circle of Willis, this translated into superior image quality at 90 kVp.

© RSNA, 2007

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