Developments in cardiovascular MR have made it possible to rapidly acquire data without the need for ECG gating. Moreover, since data is acquired rapidly and continuously at a high temporal resolution, it is possible to bin the acquired data into multiple cardiac phases (self-gating). A previous study by Likhite et al. reported that myocardial perfusion estimates using a multi-slice self-gated approach were similar to those using an ECG-gated acquisition at rest [1]. For application in longitudinal studies, it is of particular interest to look at the inter-study repeatability of this self-gated approach. Our recent work in [2] presents our findings on the inter-study repeatability of self-gated quantitative myocardial perfusion. This abstract summarizes the results published in [2].

Ten subjects (48 ± 12 years, eight males and two females) were imaged on a Siemens 3T Verio scanner. Subjects were scanned on 2 separate days using a rest–stress protocol with no ECG gating. The separation between the two scans was 9.5 ± 4.5 days. The perfusion scans were performed using an ungated saturation recovery prepared TurboFLASH pulse sequence with golden angle radial acquisition. The acquisition parameters for the scans were 24 rays per image, TR = 2.2 msec, TE = 1.2 msec, flip angle = 10°, resolution = 1.8 × 1.8 × 8 mm3 voxels. Four short-axis (SA) slices were acquired after a single saturation pulse with a saturation recovery time of ∼25 msec before the first slice. Gadoteridol 0.05 mmol/kg at a rate of 5 mL/sec was injected and ∼240 frames were acquired over a minute with shallow breathing and no ECG gating. This was followed 20 ± 5 minutes later by an injection of regadenoson to induce hyperemia. Contrast was injected ∼70 sec after regadenoson injection to ensure maximal stress and the scan protocol was repeated to acquire four slices at stress. Slices were acquired from base to apex. The images were self-gated (binned) into near systolic and near diastolic cardiac phases followed by deformable registration, similar to the methods in [2]. Figure 1 shows example images from scan 1 and scan 2 for 4 subjects.

The use of self-gating along with deformable registration gave near-systole and near-diastole datasets from a single scan. These two datasets were processed individually to quantify MBF at near-systole and near-diastole. The processing steps involved segmentation of the myocardium followed by the extraction of the arterial input function (AIF) and the tissue curves. The most basal diastolic slice, which had the smallest saturation recovery time, was used to obtain the AIF. The myocardium was segmented into 6 circumferential regions in the remaining three slices to obtain the tissue curves. The AIF and the tissue curves were converted to gadolinium concentration to correct for any signal saturation. Fermi-constrained deconvolution was used to estimate the myocardial blood flow (MBF). The myocardial perfusion reserve (MPR) was calculated as the ratio of the stress and rest MBF for each region.

A total of 180 segmental MBF and MPR values were obtained from each of the self-gated rest (near-systole), self-gated rest (near-diastole), self-gated stress (near-systole), and self-gated stress (near-diastole) at each visit. The interstudy reproducibility was represented by the coefficient of variation (CoV), calculated as the standard deviation of the within-subject difference between the two scans relative to the mean of the two scans and expressed as a percent. Bland–Altman analysis was performed to assess the agreement between studies using the technique for data with repeated measures

Figure 2 shows a Bland Altman plot comparing the MBF estimates between scan 1 and scan 2 at rest and stress using the self-gated near-systole dataset. Figure 3 shows a comparison of global MPR values between scan 1 and scan 2 for self-gated near systole and self-gated near-diastole. A CoV of 18.6%, indicating good interstudy repeatability, was obtained for territorial MPR estimates using the self-gated near-systole dataset. Less good interstudy repeatability of CoV = 46.2% was seen for the territorial MPR estimates from self-gated near-diastole data. Table 1 compares the results to previously published results on repeatability of quantitative myocardial perfusion [3-5]. All of the studies in table 1 made use of ECG-gated, breath-held acquisition. Three of the four studies reported results in a single mid-ventricular slice. The current study was free-breathing and self-gated and quantified perfusion in three slices.

While the self-gated diastole gave a larger CoV compared to self-gated systole due in part to more challenging registration, the repeatability of the multi-slice self-gated systole CMR was similar or better than published ECG-gated studies [3-5].