Saving Dose in Triple-Rule-Out CT Examination Using a High-Pitch Dual Spiral Technique

Objective
High radiation doses remain a drawback of current triple-rule-out computed tomography (CT) protocols. With dual source CT, a new highpitch dual spiral technique offers the possibility to acquire an Electrocardiography (ECG)-gated-synchronized dataset of the whole chest in less than 1 second. The aim of this study was to compare the dose of such a protocol to a standard, nongated chest scan and to a conventional, retrospectively ECG-gated triple-rule-out protocol. Also, the efficacy and dose of this dual spiral protocol was to be compared in patients examined with this high-pitch technique and matched controls scanned with the conventional technique.

Materials and Methods
An anthropomorphic Alderson Rando phantom was equipped with thermoluminescent detectors and scanned with the highpitch protocol (Siemens Somatom Definition Flash; 2 x 120 kVp, 426 mAseff, 128 x 0.6 mm collimation, pitch 3.2), the nongated chest scan (same scanner; 120 kVp, 160 mAseff, 128 x 0.6 mm, pitch 1.2; equivalent Computed Tomography Dose Index (CTDI) of 7.12 mGy), and the conventional gating technique (Siemens Somatom Definition; 120 kVp, 560 mAseff with ECG pulsing interval at 30 %–70 % of the R-R cycle, 64 x 0.6 mm, pitch 0.3). Noise was measured in air, central and peripheral soft tissue of the phantom. Conversion factors were determined based on the measured dose and the dose-length products of the scanner. The protocol was then applied with ethics committee approval in 31 patients suffering from acute chest pain. The 120 mL of contrast material (Ultravist 370, Bayer Schering Pharma) was applied at 5 mL/s. Dose was calculated based on the doselength products and the conversion factor. Image quality was assessed by 2 readers for aorta, pulmonary arteries, and coronary arteries. The results were compared with matched controls scanned with the conventional ECG gating technique and non-ECG gated thorax scans.

Results
The dose determined with thermoluminescent dosimeters measurements amounted to 2.65, 2.68, and 19.27 mSv, respectively, for the dual spiral technique, the standard chest scan, and the conventional retrospective technique. There was no significant difference in image noise. Respective conversion factors were 0.0186, 0.0188, and 0.0180 mSv/mGy _ cm. In the patient examinations, dose was 4.08 ± 0.81 mSv with the high-pitch protocol compared with 20.4 ± 5.3 mSv in the matched controls with the conventional technique, and 4.40 ± 0.83 mSv for the non-ECG gated thorax scan. Scan times were 0.7 ± 0.1 seconds for the high-pitch scan and 15 ± 3 seconds for the conventional chest pain scan. Aorta and pulmonary arteries were depicted in diagnostic quality in both groups. About 84.7 % of coronary artery segments were rated as diagnostic in the high-pitch exams (95.4 % below 65 bpm and only 72.8 % in higher heart rates), whereas 92.9 % were diagnostic with the conventional approach.

Conclusion
The high-pitch dual spiral technique requires only about onefifth of the dose of conventional ECG gated triple-rule-out protocols, thus matching that of a standard nongated chest scan. With less than 1 second, the scan time is very short. This protocol can be recommended for patients with unclear chest pain with rhythmic heart rates below 65 bpm.

Full text: Invest Radiol. 2010 Feb;45(2):64-71

Authors: Sommer WH, Schenzle JC, Becker CR, Nikolaou K, Graser A, Michalski G, Neumaier K, Reiser MF, Johnson TR.