Towards 70 Kvp Protocols For Dynamic CT Cardiac Perfusion: A Preclinical Feasibility Study

Introduction
Recent studies in cardiac imaging are focused towards an integrated assessment of CAD, including the calculation of myocardial perfusion obtained by dynamic CT. It has been reported that this technique offers a high, nearly comparable diagnostic accuracy to that of coronary angiography and Fractional Flow Reserve (FFR) measurement [AM Huber, et al.(2003) Radiology]. However, dynamic CT scanning remains associated with relatively high radiation doses, typically over 10 mSv for these type of acquisitions. The aim of this study is to investigate the feasibility of a dedicated low tube voltage (70 kVp) scan protocol for dynamic CT cardiac perfusion (CTP) and to report the impact on radiation dose, contrast agent dose and obtained perfusion metrics. This preclinical study was perfomed on a minipig model.
Methods
Prospective gated dynamic perfusion scans (22 repeated one-beat passes) over a 140 mm volume were performed on an adult minipig with standard 100 kVp and reduced 70 kVp (Revolution CT, GE Healthcare). With the 70 kVp protocol, the contrast concentration was reduced from 320 mg I/mL down to 200 mg I/mL. Acquired images were treated offline by postprocessing software (3DSlicer and simpleITK). Two 4.5 mL regions of interest were defined in the myocard (MC) and left ventricle cavity (LVC). Following figures of merit were considered by semi-quantitative approach: maximum enhancement (HUmax) and standard deviation (SD) in the LVC and MC, the upslope and myocardial blood flow (MBF). Effective dose (E) and dose to the female breast tissue were calculated (CTExpov1.7.1).
Results
With the (70 kV, 200 mg I/mL) protocol, the maximum enhancement in the blood and myocard were 450 HU and 65 HU, compared to 290 HU and 45 HU with (100 kV, 320 mg I/mL). Despite their difference in scan protocol and contrast injection settings, both protocols exhibit a similar profile of contrast uptake. Image noise in the 70 kV protocol was largely increased. Compared to the 100 kV protocol, the mean SD in both LV and MC increased from 12 to 29 HU. Despite this, the observed mean upslope's in the LV, with 3.76 HU/s for the (70 kV, 200 mg I/mL) protocol and 2.17 HU/s for the (100 kV, 320 mg I/mL) protocol, resulted in comparable estimated MBF's of 0.75 and 0.67 mL/g/min respectively. Radiation dose by CTDIvol and effective dose, were reduced from 7.02 mGy (1.9 mSv) down to 1.61 mGy (0.5 mSv) per pass. Female breast dose reduced from 3.2 mSv to 0.7 mSv per pass.
Conclusions
Reducing radiation dose and contrast agent dose are important aspects towards a safe implementation of this new technique, in particular for younger patients and patients with renal insufficiency. Initial data suggests that a 70 kV CTP protocol allows myocard perfusion evaluation at significantly reduced radiation dose per pass (74%) and contrast medium dose (38%). Further post processing is currently carried out to assess the perfusion metrics in a 17-Segment model.