# Free-Breathing and Ungated Cardiac MRI Reconstruction Using a Deep Kernel Representation
## 5 Results
### Comparisons with Fully-Sampled Breath-Hold Images  
In this section, we show the results of the comparison between the reconstructed free-breathing images and the fully-sampled breath-hold images. For the data acquisition, except for the highly undersampled k-space data for all the slices from the five subjects, we also acquired the fully-sampled breath-hold data for all the corresponding slices using the clinical bSSFP sequence. Moreover, we also acquired the fully-sampled breath-hold images for one middle slice for each subject using the commercial FGRE sequence. In Figure 4, we showed the visual comparison of the reconstructed free-breathing images and the fully-sampled breath-hold images acquired using the bSSFP sequence. Figure 3: Comparison between the bSSFP Cartesian real-time images with acceleration factor 5 and the GRE spiral real-time images reconstructed using the deep kernel method. The three slices (one basal slice, one mid slice, and one apex slice) are from Subject #3. The first row shows the bSSFP Cartesian real-time images, and the second row is the GRE spiral real-time images. Figure 2: Showcase of the reconstruction of free-breathing and ungated cardiac MRI using the proposed deep kernel method. The eight slices are from Subject #2. The first row shows the end-diastolic phase, and the second row is the end-systolic phase. The slices shown in Figure 4 were from Subject #4. Figure 5 was the visual comparison of the reconstructed free-breathing images and the fully-sampled breath-hold images acquired using the FGRE sequence. We reconstructed the whole time series, and we randomly chose a cardiac cycle (from end-systolic to end-diastolic) for comparison with the breath-hold cine. Quantitative comparisons between the free-breathing images and the breath-hold images were performed based on image quality assessment by experts, LVEF, and RVEF. Figure 6 showed the results of consensus image quality scoring done in a blinded fashion by two medical experts. From the scores, we can see that the fully-sampled breath-hold images acquired using the bSSFP have the highest image quality. While we do not see a big difference in the image quality for the free-breathing images and the fully-sampled breath-hold images acquired using the FGRE sequence (average score of 2.8 and 2.9, respectively). Figure 4: Visual comparison of the reconstructed free-breathing images and the fully-sampled breath-hold images. End-systolic and end-diastolic phases from six slices are shown in the figure. The six slices are from Subject #4. We note that the free-breathing images are acquired using the GRE sequence and the fully-sampled breath-hold images are acquired using the bSSFP sequence. This results in different contrast between the two types of the images. Figure 5: Visual comparison of the reconstructed free-breathing images and the fully-sampled breath-hold images acquired using FGRE sequence. For each subject, we acquired fully-sampled breath-hold images for one middle slice using the FGRE sequence. End-systolic and end-diastolic phases from two slices from two subjects are shown in the figure. Table 2 gave the results of the LVEF and LVEF comparison. The LVEF and RVEF calculations for the fully-sampled images were done using commercial software (Circle cv42, Calgary, AB, Canada). Automatic segmentation was first performed for the fully-sampled images using the software, and we then manually corrected the segmentation. The LVEF and RVEF calculations for the free-breathing images were done using Matlab 2019a. We manually drew the contours for the free-breathing images to calculate the EF. Since bulk motions were involved in the data acquisition for Subject #1 and we did not have exactly matching breath-hold and free-breathing slices. Hence we skipped Subject #1 for LVEF and RVEF comparison. We also reported the corresponding left ventricular end-diastolic volume (LVEDV), left ventricular end-systolic volume (LVESV), right ventricular end-diastolic volume (RVEDV), and right ventricular end-systolic volume (RVESV).