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We wanted to evaluate the effect of the number of diffusion-sensitizing gradient directions on the image quality for evaluating myocardial anisotropy and fiber tracking by using

The DT-MR images, using a SENSE-based echoplanar imaging technique, were acquired from ten excised porcine hearts by using a 3T MR scanner. With a b-value of 800 s/mm^{2}, the diffusion tensor images were obtained for 6, 15 and 32 diffusion-sensitizing gradient directions at the midventricular level. The number of tracked fibers, the fractional anisotropy (FA), and the length of the tracked fibers were measured for the quantitative analysis. Two radiologists assessed the image quality of the fiber tractography for the qualitative analysis.

By increasing the number of diffusion-sensitizing gradient directions from 6 to 15, and then to 32, the FA and standard deviation were significantly reduced (

The image quality of

It is well known that the three-dimensional organization of the myocardial fiber structure is a key determinant of ventricular torsion, strain and stress (

Diffusion tensor MR imaging (DT-MRI) has provided a means for rapidly and noninvasively evaluating the cardiac geometry and the three-dimensional fiber structure (

Theoretically, a higher number of diffusion-sensitizing gradient directions yields more accurate diffusion tensor information (

Our study was performed on the basis of the guidelines set both by the U.S. National Institute of Health and the recommendations of the Committee on Animal Research at our institution. The study protocol was approved by the local Institutional Review Committee on Animal Care. The heart specimens were obtained from a separate study. Ten pigs (n = 10, 30-35 kg) were subjected to 90 minutes of occlusion of the left anterior descending coronary artery, and this was followed by 90 minutes of reperfusion. After performing a left lateral thoracotomy along the fifth intercostal space, the left anterior descending coronary artery was isolated distal to the first diagonal branch, and a snare loop made with 4-0 silk was placed in a slender plastic tube. Occlusion or reperfusion of the left anterior descending coronary artery was produced by simply fastening or releasing the snare loop. This procedure was previously described in detail (

The DT-MRI were acquired from the ten excised hearts by using a Philips 3T Achieva scanner (Philips Medical System, Best, Netherlands) and an eight-channel head coil. The formalin-fixed porcine heart specimens were suspended in a cylinder filled with formalin to avoid tissue-air susceptibility artifacts. After the scout images were acquired on the four-chamber and two-chamber views, the short axis images of the left ventricle were obtained at the midventricular level for DT-MRI.

The sensitivity encoding (SENSE)-based echo-planar imaging technique was applied to shorten the image acquisition time and to alleviate any image distortion caused by susceptibility artifacts from the single-shot echoplanar imaging sequences. The SENSE factor was chosen to be 2.4 in this study as a trade off between image distortion due to the high echo-planar imaging factor and the SENSE artifacts caused by a high SENSE factor, as was previously suggested (

With a b-value of 800 s/mm^{2}, the diffusion tensor images were obtained, respectively, for 6, 15 and 32 diffusion gradient directions at the same midventricular level of each specimen. The imaging parameters were as follows: TE = 55 ms, TR = 5000 ms, number of slices = 5, slice thickness = 1.13 mm, slice gap = 0 mm and the number of excitations = 1.

Three-dimensional reconstruction of the myocardial fibers was done by using the commercially available PRIDE software package (Philips Medical Systems, Best, Netherlands). For each of the short-axis slices, the right ventricles were removed in the images and then excluded from the analysis. The numbers of tracked fibers, the fractional anisotropy (FA) and the length of the tracked fibers in the left ventricle were measured using the PRIDE software by an experienced cardiac radiologist who has 12 years of experience with quantitative analysis. The fiber distribution was investigated with the FA magnitude and directional thresholds set at 0.15 and 40°, respectively (

The image quality of the fiber tractography was assessed by two radiologists for qualitative analysis. They scored each image on a 3-point scale: 1 (poor), 2 (moderate) and 3 (good). The definition of each rating score is as follows: 'poor' was visualization of the myocardium with frequent discontinuity except for in the infarcted myocardium; 'moderate' was visualization of the myocardium with partial discontinuity except for in the infarcted myocardium; 'excellent' was visualization of myocardial fibers without or with minimal discontinuity except in the infarcted myocardium. The average of the scores for each image was used for qualitative analysis. For statistical analysis among the three paired data sets, all quantitative and qualitative comparisons were analyzed by using Friedmann's test and the Wilcoxon-signed rank test for post-hoc consideration.

The results of the quantitative analysis are summarized in

Our result showed that higher numbers of diffusion-sensitizing gradient directions can provide more detailed information about the myocardial fiber structure on

To the best our current knowledge, no consensus exists for the optimal number of diffusion-sensitizing gradient directions for DT-MRI. Jones (

This is a first report on using a higher number of diffusion-sensitizing gradient directions for cardiac DT-MRI. Most studies have used low diffusion resolution with employing 6 diffusion-sensitizing gradient directions for cardiac DT-MRI. Wu et al. (

Although the advantages of MRI at 3T over the 1.5T systems have not been established for the use of cardiac DT-MRI, a better depiction of fiber tracts at 3T compared with 1.5T has been demonstrated with performing high spatial resolution DT-MRI in brain (

To the best of our knowledge, there is no consensus concerning the optimum method for evaluating the quality of DT-MRI. Naganawa et al. (

Without loss of generality, the DTI-derived FA index (

The present study has some limitations. First, we have no direct histopathological correlations to validate our findings. However, it is difficult to precisely quantify the numbers and the length of myocardial fibers on histopathological exam. Second, our study was not performed by using

From this study, we conclude that the image quality of

Fiber tractographies of oblique view (

Color tensor fiber tractographies as based on different number of diffusion gradient directions. (

Schematic cross section of heart at midventricular level (

Quantitative Analysis of Fractional Anisotropy and Fiber Tracking According to Number of Diffusion Gradient Directions

Note.-No. of directions = number of diffusion gradient directions No. of fibers = number of tracked fibers, FA = fractional anisotropy SD = standard deviation

Qualitative Analysis According to Different Number of Diffusion Gradient Directions

Note.-No. of directions = number of diffusion gradient directions, FA = fractional anisotropy, SD = standard deviation, FT quality = average score for images of fiber tractography by two radiologists, vs. = versus