## 5 Discussion  
In this study, a methodology to find the anatomical relevant space for functional cardiac imaging has been described. This method was used to compute the parameters for a conical scan and to find an optimal MLT scan sequence to further speed up the acquisition process. In particular, an average opening angle of 19.7\({}^{\circ}\) and a mean thickness of 19\({}^{\circ}\) for the conical scan were found, resulting in 48.9% less transmit events per volume and, therefore, a gain in frame rate of about 2. In comparison, simply reducing the pyramidal FOV of current 3D systems to the epicardium in end-diastole would only result in a reduction of 25.6% in the number of transmit events or a gain in frame rate of about 1.3. This is expected, given the significant size of the inner cone, which would be subtracted in the presented approach. Clearly, these values depend on the threshold \(T\) defining the amount of myocardial coverage that should be retained. The lower the coverage, the higher the gain in frame rate but the more myocardium that remains outside the FOV and can therefore not be analyzed for function. Thus, this threshold should be chosen with caution and in consideration of the specific application. Although \(T\) was considered fixed in this study, it could also be chosen dynamic. Hereby, one would accommodate  
Figure 4: Two-way beam profiles of a 10 MLT system on a C-plane at 65 mm, when transmitting at one direction (i.e., \(\theta_{i}\)) and receiving in all other MLT directions (i.e., \(\theta_{1}\),\(\theta_{i}\),\(\theta_{j}\)). Tukey apodization (\(\alpha=0.5\)) was used in transmit and receive. The first subplot can be seen as the single line transmit signal (i.e., the ideal case), and the remaining ones as noise (i.e., cross-talk). This information was used to evaluate Equation (1) which quantifies the cross-talk energy level of the system. Figure 5: Cross-talk energy level of the different tested setups for the estimated ‘mean’ conical FOV. The tested systems were: 8 MLT, 9 MLT, 10 MLT and 12 MLT, all with transmit/receive Tukey apodization (\(\alpha=0.5\)). The cross-talk cut-off was defined as \(-30\) dB for an acceptable B-mode image quality. for the fact that the myocardial wall thickness changes during the cardiac cycle, which implies that the FOV is expected to be dynamic as well. If \(T\) is constant, this implies that the line density would have to be variable during the cardiac cycle in order to keep the acquisition time of a single frame constant. Alternatively, line density is kept fixed implying that the \(T\) would dynamically need adjustments to keep the total scan time for a given anatomic volume the same. In this study, the \(T\) value was arbitrarily set fixed to 85% but could be modified as required. Nevertheless, this arbitrary threshold allowed to provide initial insight into the characteristics of the conical FOV to be encountered in a clinical setting. It should be mentioned that the ring-shape template matching used to determine the opening angles of the conical volume might not be ideal given the left ventricular geometry. As such, an elliptic template might be more adequate but as this also complicates the MLT setup, this extension remains to be investigated in future work and was considered outside the scope of this initial feasibility study on anatomical volumetric imaging. In this proof-of-concept study, the segmentation was performed at the end-diastole phase only to obtain the largest epicardial mask possible as an upper limit of the outer cone to limit the FOV. However, this could imply that the inner cone, of the field of view was underestimated as the endocardial border moves inwards during systole. Ideally, the segmentation is therefore envisioned to be dynamic throughout the cardiac cycle in order to adjust the FOV dynamically as well.