In-vivo cDTI is challenging due to cardiac motion and the intrinsically low SNR of the technique. However, our previous study showed that cDTI using the DW-STEAM sequence at 3T can be performed in-vivo in healthy volunteers with good reproducibility
. We now show that this technique can be performed apparently successfully in patients with HCM and have assessed its interstudy reproducibility and the variation of quantitative measures around the myocardium.
FA had a global value of 0.613 ± 0.044 and analysis by slice and wall yielded similar results with no statistical differences. The interstudy reproducibility of global FA was very good at 7.2%, with values ranging from 6.8% to 9.7% for slices and walls. This very robust reproducibility suggests that the values have clinical validity. MD had a global value of 0.750 ± 0.154 × 10-3 mm2/s and was significantly higher in the septum (0.784 ± 0.188 × 10-3 mm2/s, p < 0.001). The interstudy reproducibility of global MD was reasonable at 18.6%, with values ranging from 15.4% to 30.9% for slices and walls. The interstudy reproducibility of MD was significantly less good than for FA (p = 0.003).
One possible interpretation of the finding of higher MD in the septum might be that it resulted from the commonly found septal hypertrophy in HCM in association with an enlarged interstitial space from disarray or fibrosis. However, considerable caution must be observed in the interpretation of this result because of known and unknown technical issues. One issue might be that the SNR across the LV is not homogeneous due to the decreasing sensitivity of the receiver surface coils with depth (the signal being higher the closer to the coil elements). It is well documented in diffusion tensor imaging that FA and MD values are biased by SNR due to the Rician noise distribution and due to the fact that only magnitude images are used to derive the diffusion tensor parameters
. Therefore, higher SNR may lead to higher values of MD. The evaluation of the SNR in these images is beyond the scope of this paper, but will be the subject of future work. Another possible confounding issue is sensitivity to the difference in myocardial position between the 2 successive cardiac cycles used for imaging, which might affect different regions of the LV in a different manner and create artefactual MD differences across the LV. A further potential issue is mis-registration of the regions of interest used to segment the LV, with influence of right ventricular papillary muscles affecting the septal measurements. Finally, there will also be unidentified technical issues which could have a bearing on the results. The relative contribution of each of these issues is currently unknown, and in-depth analysis of larger numbers of patients and healthy volunteers will be necessary to determine whether the increased MD is an actual clinical finding or caused by limitations of the present cDTI technique.
Analysis of HA was divided into 3 transmural layers because of prior findings. The global value for endocardial HA was 38.9° ± 8.1°, and SD of the difference between the measurement for the 2 scans was 4.8°. For the mesocardium, the global value was 3.5° ± 6.9° with an interstudy SD of 3.4°. For the epicardium, the global value was −34.3° ± 7.6° with an interstudy SD of 2.9°. These findings indicate very good reproducibility between scans of HA in all 3 layers and suggest clinical validity. CoVs were not calculated because their magnitude is a function of the coordinate angles chosen, and in the current system were incalculable in the mesocardial layer as the mean helical angle approximates to zero as myocyte orientation rotates from a left-handed to right-handed helix creating unstable values by division. Significant differences in HA were found between the reference lateral wall and the septum in all 3 transmural layers (ranging from 8.5° to −10.4°, all p < 0.001). Significant differences also affected the inferior wall in the endo and mesocardium (p < 0.001), and the anterior wall in the epicardium (p = 0.004). One possible interpretation of the finding of different HA values in the septum is disturbance from myocardial disarray. However, once again this interpretation needs to be regarded with considerable caution. In this study the myocardium was segmented transmurally into 3 equal thickness layers. This does not necessarily represent the real left-handed, circular and right-handed HA structure of the myocardium and therefore averaging within these 3 layers is bound to happen. Although this should not largely affect the reproducibility, it could influence the LV wall comparison due to the varying myocardial thickness per wall segment. The segmentation might be subject-dependent due to the fact that the papillary muscles and RV wall are visually excluded and might differ between initial and repeat scans, affecting the values due to mis-registration of the region of interest, and also across patients. The design, implementation, optimisation and comparison of segmentation algorithms to extract robust and meaningful quantitative HA data will be a very important task for the translation of cardiac DTI into routine, and is the subject of ongoing research. On the other hand, HA distribution maps appeared healthy in the majority of cases. We hypothesise that this might be due to the fact that the patient cohort scanned for this study was composed of stable HCM patients. There is a possibility that this is the reason why the HA maps appear normal, and that more severe forms of HCM might correlate with disturbed HA distribution maps.
There is relatively little previous in-vivo work on the absolute values of FA and MD. In healthy human subjects we reported an MD value of 0.8 ± 0.2 × 10-3 mm2/s using a similar breath hold DW-STEAM protocol,
 and Reese et al.
 reported an MD of 0.9 ± 0.3 × 10-3 mm2/s, which are both within one standard deviation of the global value of 0.750 ± 0.154 × 10-3 mm2/s found in the current study of HCM patients. Tseng et al. performed in-vivo DTI in 5 patients with HCM and 5 volunteers
. They measured FA at 0.78 in the free wall and 0.56 in the septum in HCM patients (p = 0.03 for the difference) and 0.78 and 0.72 respectively in normal subjects (p = 0.15 for the difference). In our current study, we measured slightly lower FA at 0.614 ± 0.041 × 10-3 mm2/s in the lateral wall and 0.610 ± 0.059 × 10-3 mm2/s in the septum, with no significant difference between our values (p = 0.77). Further studies will be needed to compare measures, standardise and optimise the methods and further validate results. It should be noted that the quantitative results in this study are obtained from acquisitions at end-systole. We might expect differences between in-vivo and ex-vivo data, and also between fresh and fixed ex-vivo data, and finally also between data acquired at different cardiac phases.
The general limitations of this study are the following: The modest in-plane resolution of 2.7 mm might lead to volume averaging of the blood pool in endocardial layers or the RV papillary muscles in epicardial layers, which could bias the FA, MD and HA values in these areas; Data acquisition was performed at end-systole, which is considered a phase when the diffusion acquisition is not affected by strain,
 and although the systolic pause in HCM is longer than in healthy volunteers, motion artifacts may still have occurred related to the optimal positioning of the acquisition window; The time between the 2 scans in the study might have resulted in alteration in the myocardium due to disease progression, but this is unlikely given the slow rate of progression and the lack of any clinical events in the intervening period; This is a preliminary study on a small sample size of only 10 HCM patients, and further work with a larger number of patients is required, which will include quantitative regional comparisons with analysis of LGE; The application of the current technique to patients with limited breathold capability or frequent ectopy remains to be determined, but would be more challenging.