Purpose: The purpose of this study was to detect two dimensional and sub-pixel displacement with high spatial resolution using an ultrasonic diagnostic apparatus. Conventional displacement detection methods assume neighborhood uniformity and cannot achieve both high spatial resolution and sub-pixel displacement detection. Subjects and Methods: A deep-learning network that utilizes ultrasound images and output displacement distribution was developed. The network structure was constructed by modifying FlowNet2, a widely used network for optical flow estimation, and a training dataset was developed using ultrasound image simulation. Detection accuracy and spatial resolution were evaluated via simulated ultrasound images, and the clinical usefulness was evaluated with ultrasound images of the liver exposed to high-intensity-focused ultrasound (HIFU). These results were compared to the Lucas-Kanade method, a conventional sub-pixel displacement detection method. Results and Discussion: For a displacement within ±40 μm (±0.6 pixels), a pixel size of 67 μm, and signal noise of 1％, the accuracy was above 0.5 μm and 0.2 μm, the precision was above 0.4 μm and 0.3 μm, and the spatial resolution was 1.1 mm and 0.8 mm for the lateral and axial displacements, respectively. These improvements were also observed in the experimental data. Visualization of the lateral displacement distribution, which determines the edge of the treated lesion using HIFU, was also realized. Conclusion: Two-dimensional and sub-pixel displacement detection with high spatial resolution was realized using a deep- learning methodology. The proposed method enabled the monitoring of small and local tissue deformations induced by HIFU exposure.

Purpose: The contrasts of flowing blood in in vitro experiments using porcine blood and in vivo measurements of human jugular veins were analyzed to demonstrate that the hemorheological property was dependent on the shear rate. Methods: Blood samples (45％ hematocrit) suspended in saline or plasma were compared with examine the difference in viscoelasticity. Ultrafast plane-wave imaging at an ultrasonic center frequency of 7.5 MHz was performed on different steady flows in a graphite-agar phantom. Also, in vivo measurement was performed in young, healthy subjects and patients with diabetes. A spatiotemporal matrix of beamformed radio-frequency data was used for the singular value decomposition (SVD) clutter filter. The clutter-filtered B-mode image was calculated as the amplitude envelope normalized at the first frame in the diastolic phase to evaluate contrast. The shear rate was estimated as the velocity gradient perpendicular to the lateral axis. Results: Although nonaggregated erythrocytes at a high shear rate exhibited a low echogenicity, the echogenicity in the plasma sample overall increased due to erythrocyte aggregation at a low shear rate. In addition, the frequency of detection of specular components, defined as components beyond twice the standard deviation of a contrast map obtained from a clutter-filtered B-mode image, increased in the porcine blood at a high shear rate and the venous blood in healthy subjects versus patients with diabetes. Conclusion: The possibility of characterizing hemorheological properties dependent on the shear rate and diabetes condition was indicated using ultrafast plane-wave imaging with an SVD-based clutter filter.

Purpose: The safety of acoustic radiation force impulse (ARFI) elastography, which applies higher acoustic power with a longer pulse duration (PD) than conventional diagnostic ultrasound, is yet to be verified. We assessed the ARFI-induced lung injury risk and its relationship with peak rarefactional pressure amplitude (PRPA) and mechanical index (MI). Methods: Eighteen and two rabbits were included in the ARFI (0.3-ms push pulses) and sham groups, respectively. A 5.2-MHz linear probe was applied to the subcostal area and aimed at both lungs through the liver for 30 ARFI emissions. The derated PRPA varied among the six ARFI groups－0.80 MPa, 1.13 MPa, 1.33 MPa, 1.70 MPa, 1.91 MPa, and 2.00 MPa, respectively. Results: The occurrence of lung hemorrhage and the mean lesion area among all samples in the seven groups were 0/6, 0/6, 1/6 (1.7 mm²), 4/6 (8.0 mm²), 4/6 (11.2 mm²), 5/6 (23.8 mm²), and 0/4 (sham), respectively. Logistic regression analysis showed that derated PRPA was significantly associated with lung injury occurrence (odds ratio: 207, p < 0.01), with the threshold estimated to be 1.1 MPa (MI, 0.5). Spearman’s rank correlation showed a positive correlation between derated PRPA and lesion area (r = 0.671, p < 0.01). Conclusion: This study demonstrated that the occurrence and severity of ARFI-induced lung hemorrhage increased with a rise in PRPA under clinical conditions in rabbits. This indicates a potential risk of lung injury due to ARFI elastography, especially when ARFI is unintentionally directed to the lungs during liver, heart, or breast examinations.