Geir Ultveit Haugen

Diverging Wave Volumetric Imaging Using Subaperture Beamforming

Several clinical settings could benefit from 3-D high frame rate (HFR) imaging and, in particular, HFR 3-D tissue Doppler imaging (TDI). To date, the proposed methodologies are based mostly on experimental ultrasound platforms, making their translation to clinical systems nontrivial as these have additional hardware constraints. In particular, clinically used 2-D matrix array transducers rely on subaperture (SAP) beamforming to limit cabling between the ultrasound probe and the back-end console. Therefore, this paper is aimed at assessing the feasibility of HFR 3-D TDI using diverging waves (DWs) on a clinical transducer with SAP beamforming limitations. Simulation studies showed that the combination of a single DW transmission with SAP beamforming results in severe imaging artifacts due to grating lobes and reduced penetration. Interestingly, a promising tradeoff between image quality and frame rate was achieved for scan sequences with a moderate number of transmit beams. In particular, a sparse sequence with nine transmissions showed good imaging performance for an imaging sector of 70° × 70° at volume rates of approximately 600 Hz. Subsequently, this sequence was implemented in a clinical system and TDI was recorded in vivo on healthy subjects. Velocity curves were extracted and compared against conventional TDI (i.e., with focused transmit beams). The results showed similar velocities between both beamforming approaches, with a cross-correlation of 0.90 ± 0.11 between the traces of each mode. Overall, this paper indicates that HFR 3-D TDI is feasible in systems with clinical 2-D matrix arrays, despite the limitations of SAP beamforming.

High frame rate 3D tissue velocity imaging using sub-aperture beamforming: A pilot study in vivo

Implementation of 3D high frame rate (HFR) tissue Doppler imaging (TDI) typically requires a fully wired matrix probe. However, such probes remain impractical for use in a clinical setting. Therefore, clinical matrix arrays rely on sub-aperture (SAP) beamforming. This makes diverging wave (DW) imaging challenging as side- and grating-lobes arise from the simultaneous reconstruction of image lines with considerably different orientations. We have previously shown in computer simulations that these difficulties could be mitigated by using a sparse transmit sequence. The present study looked at the implementation of this sequence in a clinical system. 3D TDI was acquired in vivo at 610 vol/s and compared against conventional TDI (i.e. focused transmissions). The velocity curves obtained from both methods were similar (cross correlation = 0.90 ± 0.11) and the full left ventricle could be imaged at HFR in a single acquisition using the 3D DW sequence. Overall, this study supports the feasibility of HFR 3D TDI in a clinical system, despite the limitations of SAP beamforming.

Temperature monitoring by channel data delays: Feasibility based on estimated delays magnitude for cardiac ablation

&NA; Ultrasound thermometry is based on measuring tissue temperature by its impact on ultrasound wave propagation. This study focuses on the use of transducer array channel data (not beamformed) and examines how a layer of increased velocity (heat induced) affects the travel‐times of the ultrasound backscatter signal. Based on geometric considerations, a new equation was derived for the change in time delay as a function of temperature change. The resulting expression provides insight into the key factors that link change in temperature to change in travel time. It shows that velocity enters in combination with heating geometry: complementary information is needed to compute velocity from the changes in travel time. Using the bio‐heat equation as a second source of information in the derived expressions, the feasibility of monitoring the temperature increase during cardiac ablation therapy using channel data was investigated. For an intra‐cardiac (ICE) probe, using this “time delay error approach” would not be feasible, while for a trans‐esophageal array transducer (TEE) transducer it might be feasible. HighlightsTemperature‐induced change of sound speed affects the arrival time of channel data.Delays are function of the equivalent sound speed and beam forming parameters.Delays are evaluated for two probes, ICE and TEE, monitoring RF ablation in‐silico.Monitoring RF‐ablation via delays on the aperture may be possible with TEE probe.The method demonstrates the importance of considering the beam forming parameters.