# Requirements and Hardware Limitations of High-Frame-Rate 3-D Ultrasound Imaging Systems
## 4 Results and Discussion  
Table 3 summarizes the system performance assessed for the five experiments. The estimated FRMAX values are bold highlighted. All metrics that correspond to the achievement of the expected maximum limit (bottleneck) are highlighted in orange. The results show that, compared to the star topology, the ring topology allows the achievement of higher FRMAX in all cases, with increments up to 620% (see the row at bottom) for DW1 imaging sequences. However, the increments are not homogeneous throughout the experiments. The minimum differences were found when a small number of channels was processed (Experiment I): in this case, the star topology, especially for the DW3 and DW5 imaging sequences, turns out to be almost as efficient as the ring one. The  
Figure 3: Virtual source steering angles set during the transmission of diverging waves. Left panel: in Experiments I and II, involving 2-D imaging sequences, the virtual sources were maintained over the imaging xz-plane; only the central source (dot) was used in DW1, two additional lateral virtual sources (diamonds) were used in DW3, and all the five virtual sources (dot, diamonds and crosses) were used in DW5. Right panel: in experiment III IV and V, only the central source (dot) was used for the scan sequence DW1; for the scan sequence DW5, the five sources (dot and crosses) were steered on both the xz and yz angle to ensure an homogeneous insonification of both lateral directions. results also confirm that the FR\({}_{\text{MAX}}\) differences are, in general, significantly reduced when compounding is applied. These results can be explained as follows. As suggested by the orange-highlighted values, for the star configuration, B\({}_{\text{MC}}\) is, in most cases, close to the upper limit (\(\approx\)1.8 GB/s), while the beamformer output rate is much lower than the achievable B\({}_{\text{BF}}\) value (\(\approx\)630 MSPS). This means that, in general, the bottleneck was represented by the rate of the SRIO link between the FE boards and the MC board, rather than by the beamformer. Conversely, for the ring configuration, B\({}_{\text{BF}}\) is always closer to (and B\({}_{\text{MC}}\) farther from) the upper limit, especially when compounding was applied. This means that the bottleneck was not related to the rate of the SRIO link anymore, i.e., FR\({}_{\text{MAX}}\) could be increased up to the value permitted by the beamforming rate. Only in the tests of Experiment I that involved compounding (DW3 and DW5), B\({}_{\text{BF}}\) achieved values close to 630 MSPS for both topologies: correspondingly, the frame rates turned out to be similar, especially in DW5 mode. It is worth noting that, in the ring topology, for all DW1 cases, both B\({}_{\text{MC}}\) and B\({}_{\text{BF}}\) are lower than the theoretical upper limits. This means that the bottleneck is elsewhere. Although this limit could not be quantitatively and systematically assessed, a reasonable hypothesis is that, for PRF values beyond FR\({}_{\text{MAX}}\), the DSP cannot sustain the requested working load. The comparison between the results of Experiment II and Experiment I confirm that the N\({}_{\text{CH}}\) increase (from 128 to 256) linearly affects FR\({}_{\text{MAX}}\) for the star (all values are nearly halved) but not for the ring topology. This is consistent with the observation that a two-fold increase in N\({}_{\text{CH}}\) corresponds to the doubling of the required B\({}_{\text{MC}}\) in the star topology, but not in the ring topology. The results of Experiment III confirm that the performance linearly depends on the number of reconstructed lines. Indeed, doubling the number of lines (up to 192), as needed in bi-plane imaging, resulted in a halved FR\({}_{\text{MAX}}\). The results of Experiment IV highlight that relaxing the requirements on the memories by reducing the number of read/write cycles, does not impact the performance of the star topology, which performs as in Experiment III.