3.0T MRI is widely used for cardiac exams. However, failed ECG triggering may occur due to T-wave elevation in higher magnetic field. Low voltage in ECG and complex QRS form are also frequent causes of triggering failure. These downsides may impair the image quality and the diagnostic performance (1). Peripheral pulse gating (PPG) image acquisition has been reported as alternative approach. Detection of active inflammation is important issue in cardiovascular disease. The usefulness of diffusion weighted image (DWI) for detection of acute edema has been established in cerebrovascular field as well as T2-weighted image (2) (3). However, there is a barrier to obtain DWI in cardiac exams due to constant and frequent motion. Recently, DWI using Single-Shot Turbo Spin Echo (ssTSE) has developed with short acquisition time and less distortion artifact (4). In this study, we evaluated the feasibility of optimal gating method and imaging parameters for ssTSE-DWI.

[Subjects] Five male healthy volunteers (age range: 30-40 years) were examined with 3.0T MR unit (Ingenia, Philips Healthcare). [Hypotheses] To determine feasible parameters to perform simple and robust cardiac DWI, we hypothesized as follows; 1) Maximum signal at limited readout time enhance DWI quality. 2) Postponement of readout time may improve signal collection. 3) The best readout time should be when cardiac motion is at rest. 4) In ECG gating, setting PPG approach may contribute due to detection of pulse which usually shifts behind ECG-R wave. Since detection of the R wave is slower than an ECG synchronization method as for the PPG synchronization method, we are thought to be able to postpone indication time (Fig.1). 5) Efficiency of signal collection in readout time depends on Refocusing Flip Angle (RFA) and SENSE-factor. [Scan protocols] Image parameters: b-value=150s/mm², slice thickness=6mm, NSA=4, pixel size=3.75X3.75mm², TE=33msec, TR=4beat, acquisition time=2m30s. [Visual score analysis] Three experienced readers evaluated image quality. Five points scale (1=non- evaluative, 5=excellent quality) in terms of uniformity and articulacy was employed for visual analysis. Comparison of image quality for RFA, SENSE-factor, PPG, and ECG gating methods were done. We changed 80 degrees, 120 degrees, 160 degrees, SENSE-factor with 2.0, 3.0, 4.0, 5.0 for imaging evaluated RFA to optimize an imaging parameters of cardiac ssTSE-DWI. The image quality evaluation method performed cine imaging of a healthy volunteer and observed standstill time (diastolic phase) of the heart and confirmed optimal trigger delay. [Statistical analysis] Statistical analysis was carried out with Wilcoxon analyses and Steel-Dwass analyses, and judged the difference as significant at p<0.05.

The PPG synchronization method was significantly higher in visual scoring than that of the ECG synchronization method (p=0.02) (Fig.2). RFA tended to be higher in images with higher visual score (Fig.3). A significantly higher image quality was detected in SENSE-factor 4.0 compared with SENSE-factor 2.0 (p=0.005). A significantly higher image quality was detected in SENSE-factor 4.0 compared with SENSE-factor 5.0 (p=0.05) (Fig.4).

In this preliminary study, visualization of cardiac DWI succeeded by using the parameters we suggested. The parameters of setting RFA value needed to be overcome because the higher RFA was not available under the ECG triggering. When optimal triggering point was set on diastolic phase by ECG, higher RFA was no longer available due to the following R wave. In contrast, triggering point by PPG was delayed approximately 100msec than ECG triggering. To obtain better signal intensity ratio, setting higher RFA was still available by PPG due to sufficient interval. Higher RFA also enabled to reduce the signal from blood pool. Among higher RFA values (120 degrees, 160 degrees, 180 degrees) though higher value tended to show further improving of visual scoring but did not reach the statistical significance. This may be because higher RFA prolonged shot time, and the images became blurring by motion artifact. As for SENSE-factor, ssTSE-DWI using SENSE-factor 4.0 brought about the best image quality. This was because higher factor enabled to shorten acquisition time which may cause artifacts by cardiac motion (Fig.5). This study has several limitations. 1) This was the preliminary trial for normal volunteers whose heart rate under 60bpm. The feasibility study for various heart rates will be necessary. 2) b-value was set 150s/mm² in this study. Higher b-value will be needed to detect abnormal signals, and to compare conventional image sequences (Gd-enhancement, or T2-black blood). Although under those limitations, this was a first trial to visualize cardiac DWI by using such novel approach.

In this study, visualizing cardiac DWI was feasible under above parameters. And the results showed a possibility to detect abnormal signal from myocardium by non-contrast MRI.