Pedro Santos

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.

Acoustic output of multi-line transmit beamforming for fast cardiac imaging: a simulation study

Achieving higher frame rates in cardiac ultrasound could unveil short-lived myocardial events and lead to new insights on cardiac function. Multi-line transmit (MLT) beamforming (i.e., simultaneously transmitting multiple focused beams) is a potential approach to achieve this. However, two challenges come with it: first, it leads to cross-talk between the MLT beams, appearing as imaging artifacts, and second, it presents acoustic summation in the near field, where multiple MLT beams overlap. Although several studies have focused on the former, no studies have looked into the implications of the latter on acoustic safety. In this paper, the acoustic field of 4-MLT was simulated and compared with single-line transmit (SLT). The findings suggest that standard MLT does present potential concerns. Compared with SLT, it shows a 2-fold increase in mechanical index (MI) (from 1.0 to 2.3), a 6-fold increase in spatial-peak pulse-average intensity (Isppa) (from 99 to 576 W·cm-2) and a 12-fold increase in spatial-peak temporalaverage intensity (Ispta) (from 119 to 1407 mW·cm-2). Subsequently, modifications of the transmit pulse and delay line of MLT were studied. These modifications allowed for a change in the spatio-temporal distribution of the acoustic output, thereby significantly decreasing the safety indices (MI = 1.2, Isppa = 92 W·cm-2 and Ispta = 366 mW·cm-2). Accordingly, they help mitigate the concerns around MLT, reducing potential tradeoffs between acoustic safety and image quality.

A Comparison of the Performance of Different Multiline Transmit Setups for Fast Volumetric Cardiac Ultrasound

It was previously demonstrated in 2-D echocardiography that a proper multiline transmit (MLT) implementation can be used to increase frame rate while preserving image quality. Initial findings for extending MLT to 3-D showed that it might address the low spatiotemporal resolution of current volumetric ultrasound systems. However, to date, it remains unclear how much transmit/receive parallelization would be possible using a 3-D MLT system. Therefore, the aim of this paper was to contrast different MLT setups for 3-D imaging by computer simulation in order to determine an optimal tradeoff between the amount of parallelization of an MLT system and the corresponding signal-to-noise ratio of the resulting images. Hereto, the image quality of several MLT setups was estimated by quantifying their crosstalk energy level. The results showed that for the tested setups, 4MLT broad beams and 9MLT narrow beams with Tukey (α = 0.5) apodization in transmit and receive give the highest frame rate gain while maintaining an acceptable interbeam interference level. Moreover, although 16MLT narrow beams with Tukey/Tukey (α = 0.5) apodization did show more pronounced interbeam interference, its gain in frame rate might outweigh its predicted loss in image quality. As such both 9MLT and 16MLT narrow beams were tested experimentally. For both systems, four receive lines were reconstructed from each transmit beam. The contrast-to-noise ratio of these imaging strategies was quantified and compared with the image quality obtained with line-by-line scanning. Despite some expected loss in image quality, the resulting images of the parallelized systems were very competitive to the benchmark, while speeding up the acquisition process by a factor of 36 and 64, respectively.

Fast volumetric cardiac ultrasound: A comparison of different multi-line transmit setups by computer simulation

Current 3D cardiac ultrasound systems suffer from relatively low spatiotemporal resolution limiting their applicability in clinical practice. To overcome this limitation, we have previously demonstrated in both 2D and 3D, that a proper multi-line transmit (MLT) implementation can be used to increase frame rate while preserving image quality. However, to date a direct comparison of different MLT implementations for 3D imaging has not been performed. The aim of this study was therefore to determine the optimal trade-off between the amount of transmit parallelization of a MLT system and its corresponding SNR. Hereto, the image quality of several MLT systems was estimated by quantifying their transmit crosstalk using computer simulation. A SNR of -30dB was defined as a cut-off for an acceptable cross-talk level. The results showed that for the setups tested, a 16MLT combined with 4MLA (i.e. multi-line acquisition) system using a Tukey (α=0.5) apodization gave the highest gain in frame rate while keeping the expected B-mode image quality acceptable.

Safety of Multi-Line Transmit beam forming for fast cardiac imaging - a simulation study

Achieving higher frame rate is a hot topic in cardiac ultrasound. An attractive way to accomplish this is through Multi-Line Transmit (MLT) beam forming. However, transmitting into multiple directions simultaneously may pose additional safety concerns due to acoustic superposition of the MLT beams. In this study, acoustic MLT fields were compared against conventional Single-Line Transmit fields. MLT beam forming showed more than a two-fold increase in peak-compressional acoustic pressure (from 1.8 MPa to 4.6 MPa) and in the mechanical index (MI) (from 1.0 to 2.2), as well as a six-fold increase in averaged intensity. As this might bring safety issues, two modifications of standard MLT beam forming were proposed: i) transmitting the MLT beams with different pulse phases; and ii) reorganising the delay lines of the MLT transmit event. Results showed a decrease in acoustic parameters in the near field using either modification, with the latter leading to peak pressures of 2.3 MPa and an MI of 1.2. In conclusion, straightforward implementation of MLT does bring additional hazard, but small modifications of the transmit beam forming can make it safe for clinical use.

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.

Volumetric imaging of fast mechanical waves in the heart using a clinical ultrasound system

The detection and quantification of fast myocardial waves, such as the ones following mechanical activation and aortic valve closure (AVC), may reveal important clinical information about both systolic and diastolic cardiac function. Such waves have previously been observed using multiple high frame rate (HFR) 2-D recordings. However, making multiple 2D recordings lengthens the data acquisition process. Moreover, heart rate variability in combination with the short-lived nature of the studied events makes temporal alignment of these multiple 2D recordings challenging. Fast volumetric imaging would mitigate both problems. However, clinical matrix arrays hinder the straightforward implementation of diverging waves (DW), due to sub-aperture processing (SAP). In this paper, a 3×3 DW sequence implemented on a clinical system using a transesophageal echocardiographic SAP-based matrix array has been validated in-vitro. Subsequently, ultrafast 3D tissue Doppler, strain rate and tissue acceleration were recorded for 4 healthy volunteers at 610 vol/s and used to characterize fast myocardial events. Early myocardial ejection shortening was found in the basal and mid septal wall, as well as in the basal inferior wall. In the septal wall, its propagation was measured as 1.1±0.1 m/s. The wave following AVC was seen traveling both laterally and towards the apex at 4.2±1.0 m/s. These findings seem in agreement with previous reports based on 2D HFR imaging, therefore corroborating the suitability of the 3×3 DW sequence to detect fast myocardial events in 3D within a single heartbeat in a clinical setting.

High frame rate multi-plane echocardiography using multi-line transmit beamforming: First experimental findings

Given the limited spatiotemporal resolution of 3D echocardiography, simultaneous assessment of all ventricular myocardial segments can clinically be performed using multi-plane acquisitions (MP) — i.e. biplane (BP) or triplane (TP). However, the wider field of view of MP impairs spatiotemporal resolution, thus hindering the performance of e.g. speckle tracking. Multi Line Transmit (MLT) beam forming (i.e. simultaneously transmitting multiple focused beams) has been shown an appealing approach to improve temporal resolution without sacrificing image quality. At IUS 2016, an MLT sequence dedicated for MP imaging was presented and evaluated using computer simulations. The aim of the present study was to implement and validate this sequence on an experimental ultrasound system.

Volumetric imaging of fast mechanical waves in the heart using a clinical ultrasound system: A feasibility study

Fast mechanical waves following e.g. electromechanical activation and aortic valve closure (AVC) have been imaged using 2D diverging waves (DW). However, their full characterization requires multiple 2D recordings in subsequent heartbeats. Given the heart cycle variability and short-lived nature of these waves, temporal alignment is challenging. Moreover, clinical matrix arrays hinder the straightforward implementation of DW, due to sub-aperture (SAP) beam forming. We have recently proposed a sparse DW sequence capable of imaging a 70°x70° volume in a clinical system and reported its preliminary validation in vivo against 2D Tissue Doppler Imaging (TDI). Herein, the feasibility of measuring fast mechanical events in 3D in vivo was investigated.

Diverging wave compounding: Direct comparison of two popular approaches

Diverging wave imaging is the most prominent transmit sequence in fast cardiac imaging. Indeed, as this type of ultrasound transmit insonifies a significant part of the region of interest at once, less transmit events are required to reconstruct an entire image leading to an improved time resolution. However, the flipside of this approach is that image quality reduces in terms of spatial resolution, signal- and contrast-to-noise ratio. To mitigate these negative side effects, coherent spatial compounding is typically used. However, different investigators have proposed different compounding schemes and to date, these approaches have not directly been contrasted. The goal of the current work was, therefore, to compare two popular compounding approaches both by computer simulations and experimentally.