Feasibility of a radiation dose conserving CT protocol for myocardial function assessment

Abstract

Objective: Assessment of myocardial function can be performed at higher noise levels than necessary for coronary arterial evaluation. We evaluated image quality and radiation exposure of a dose-conserving function-only acquisition vs retrospectively electrocardiogram(ECG)-gated coronary CTA with automatic tube current modulation.

Methods: Of 26 patients who underwent clinically indicated coronary CTA for coronary and function evaluation, 13 (Group I) underwent prospectively ECG-triggered coronary CTA, followed by low-dose retrospectively ECG-gated scan for function (128-slice dual-source, 80 kVp; reference tube current, 100 mA; 8-mm-thick multiplanar reformatted reconstructions) performed either immediately (n = 6) or after 5- to 10-min delay for infarct assessment (n = 7). 13 corresponding controls (Group II) underwent retrospectively ECG-gated protocols (automatic tube potential selection with CARE kV/CARE Dose 4D; Siemens Healthcare, Forchheim, Germany) with aggressive dose modulation. Image quality assessment was performed on the six Group I subjects who underwent early post-contrast dedicated function scan and corresponding controls. Radiation exposure was based on dose-length product.

Results: Contrast-to-noise ratio (CNR) was preserved throughout the cardiac cycle in Group I and varied according to dose modulation in Group II. Visual image quality indices were similar during end systole but were better in Group II at end diastole. Although the total radiation exposure was equivalent in Group I and Group II (284 vs 280 mGy cm), the median radiation exposure associated with only the dedicated function scan was 138 mGy cm (interquartile range, 116-203 mGy cm).

Conclusion: A low-dose retrospective ECG-gated protocol permits assessment of myocardial function at a median radiation exposure of 138 mGy cm and offers more consistent multiphase CNR vs traditional ECG-modulation protocols. This is useful for pure functional evaluation or as an adjunct to single-phase scan modes.

Advances in knowledge: Radiation exposure can be limited with a tailored myocardial function CT protocol while maintaining preserved images.

Figure 1.

8-mm-thick multiplanar reformatted left ventricular short-axis reconstructions in a representative Group I patient at late diastole (a) and at end systole (b). Scan parameters: retrospective ECG gated, tube potential of 80 kVp and reference tube current of 100 mA; and dose–length product (DLP) = 74 mGy cm. Compare these to a body mass index and heart rate matched Group II patient at late diastole and end systole (c, d). Scan parameters: retrospective ECG gated with aggressive dose modulation [MinDose® (Siemens Healthcare, Forchheim, Germany), 4%], tube voltage of 100 kVp and current per automatic tube potential selection. Radiation exposure based on DLP was 135 mGy cm. Note the ease of endocardial border recognition at both late diastole and end systole with consistent image quality in the patients of Group I, as opposed to marked difference in image quality between systolic and diastolic phases in the patients of Group II.

Figure 2.

Contrast-to-noise ratio (CNR) according to cardiac phase in two representative patients, including a Group I patient undergoing low-dose dedicated function scan as described, and a body mass index and heart rate-matched patient of Group II undergoing conventional retrospective acquisition with tube current modulation. Note the relatively consistent CNR throughout the entire cardiac cycle for the patient of Group I vs patient of Group II. R-R, cycle length.