Automated breast volume scanner (ABVS) is an ultrasound imaging modality used in breast cancer screening. It has high sensitivity but limited specificity as it is hard to discriminate between benign and malignant lesions by echogenic properties. Specificity might be improved by shear strain imaging as malignant lesions, firmly bonded to its host tissue, show different shear patterns compared to benign lesions, often loosely bonded. Therefore, 3-D quasi-static elastography was implemented in an ABVS-like system. Plane wave instead of conventional focused transmissions were used to reduce scan times within a single breath hold. A 3-D strain tensor was obtained and shear strains were reconstructed in phantoms containing firmly and loosely bonded lesions. Experiments were also simulated in finite-element models (FEMs). Experimental results, confirmed by FEM-results, indicated that loosely bonded lesions showed increased maximal shear strains (~2.5%) and different shear patterns compared to firmly bonded lesions (~0.9%). To conclude, we successfully implemented 3-D elastography in an ABVS-like system to assess lesion bonding by shear strain imaging.

3-D Single Breath-Hold Shear Strain Estimation for Improved Breast Lesion Detection and Classification in Automated Volumetric Ultrasound Scanners

In elastography, conventional linear array (CLA)-based RF data acquisition provides more accurate displacement measurements in the direction of beam propagation (axial direction) when compared to the perpendicular direction (lateral). Obtaining good quality lateral displacement estimates in ultrasound (US) elastography will lead to several benefits such as obtaining accurate inverse solutions, improving shear strain elastogram quality, getting good quality poroelastograms, and obtaining reliable rotation elastograms. For accomplishing high-precision lateral displacement estimation (LDE), one of the popular methods is by interpolating additional A-lines in between neighboring RF A-lines. We describe a method wherein true RF A-lines (not interpolated) are acquired and augmented at subpitch locations using CLA transducer, instead of interpolating the data, and using this new frame data for further image formation and/or processing to yield better lateral resolution and LDE. We demonstrate the proposed method by translating the US beam of CLA transducer in subpitch range by the following two approaches: 1) actuator-assisted beam translation and 2) electronic translation of subaperture of a CLA by activating odd and even number of consecutive elements sequentially, referred to as electronic beam translation. The performances of the different methods were studied through simulations and experiments on phantoms. The results demonstrate that these methods yield better quality LDE compared to those obtained from interpolation of RF A-lines. These methods may provide affordable ways to obtain subpitch precision LDE using CLA.

Strategies to Obtain Subpitch Precision in Lateral Motion Estimation in Ultrasound Elastography

Regularized spectral log difference technique for ultrasonic attenuation imaging

Towards quantitative quasi-static ultrasound elastography using a reference layer for liver imaging application: A preliminary assessment.

Tissue motion estimation is an essential step for ultrasound elastography. Our previous study has shown that the affine-model-based optical flow (OF) method outperforms the normalized cross-correlation-based block matching (BM) method in motion estimation. However, the quality of lateral estimation using OF is still low due to inherent limitation of ultrasound imaging. BM-based spatial angular compounding (SAC) has been developed to obtain better motion estimation. In this paper, OF-based SAC (OF-SAC) is proposed to further improve the performance of lateral (and axial) estimation, and it is compared with BM-based SAC (BM-SAC). Plane wave as well as focused wave is transmitted in both simulations and phantom experiments on a linear array. In order to compare the performance quantitatively, the root-mean-square error (RMSE) of axial/lateral displacement and strain, and signal-to-noise ratio (SNR) and contrast-to-noise ratio (CNR) of axial/lateral strain are used as the evaluation criteria in the simulations. In the phantom experiments, the SNR and CNR are used to assess the quality of axial/lateral strain. The results show that for both OF and BM, SAC improves the performance of motion estimation, regardless of using plane or focused wave transmission. More importantly, OF-SAC is shown to outperform BM-SAC with lower RMSE, higher SNR, and higher CNR. In addition, preliminary in vivo experiments on the carotid artery of a healthy human subject also prove the superiority of OF-SAC. These results suggest that OF-SAC is preferred for both axial and lateral motion estimation to BM-SAC.

Spatial Angular Compounding With Affine-Model-Based Optical Flow for Improvement of Motion Estimation

The perception of stiffness and slipperiness of a breast mass on palpation is used by physicians to assess the level of suspicion of a lesion as being malignant or benign. However, most current ultrasound elastography imaging methods provide only stiffness-related information. There is no existing approach that provides information about the local rigid body rotation undergone by only a loosely bonded, asymmetrically oriented lesion subjected to a small quasi-static compression. The inherent poor lateral resolution in ultrasound imaging poses a limitation in estimating the local rigid body rotation. Several techniques have been reported in the literature to improve the lateral resolution in ultrasound imaging, and among them is spatial compounding. In this study, we explore the feasibility of obtaining better-quality rotation elastograms with spatial compounding through simulations using Field II and experiments on tissue-mimicking phantoms. The phantom was subjected to axial compression (∼1%–2%) from the top, and the angular axial and lateral displacement estimates were obtained using a multilevel 2-D displacement tracking algorithm at different insonification angles. A rotation elastogram (RE) was obtained by taking half of the difference between the lateral gradient of the axial displacement estimates and the axial gradient of the lateral displacement estimates. Contrast-to-noise ratio was used to quantify the improvements in quality of RE. Contrast-to-noise ratio values were calculated by varying the maximum steering angle and the incremental angle, and its effects on RE quality were evaluated. Both simulation and experimental results corroborated and indicated a significant improvement in the quality of RE using compounding technique.

Spatial Compounding Technique to Obtain Rotation Elastogram: A Feasibility Study.

During manual palpation of breast masses, the perception of its stiffness and slipperiness are the two commonly used information by the physician. In order to reliably and quantitatively obtain this information several non-invasive elastography techniques have been developed that seek to provide an image of the underlying mechanical properties, mostly stiffness-related. Very few approaches have visualized the "slip" at the lesion-background boundary that only occurs for a loosely-bonded benign lesion. It has been shown that axial-shear strain distribution provides information about underlying slip. One such feature, referred to as "fill-in" was interpreted as a surrogate of the rotation undergone by an asymmetrically-oriented-loosely bonded-benign-lesion under quasi-static compression. However, imaging and direct visualization of the rotation itself has not been addressed yet. In order to accomplish this, the quality of lateral displacement estimation needs to be improved. In this simulation study, we utilize spatial compounding approach and assess the feasibility to obtain good quality rotation elastogram. The angular axial and lateral displacement estimates were obtained at different insonification angles from a phantom containing an elliptical inclusion oriented at 45°, subjected to 1% compression from the top. A multilevel 2D-block matching algorithm was used for displacement tracking and 2D-least square compounding of angular axial and lateral displacement estimates was employed. By varying the maximum steering angle and incremental angle, the improvement in the lateral motion tracking accuracy and its effects on the quality of rotational elastogram were evaluated. Results demonstrate significantly-improved rotation elastogram using this technique.

Rotation elastogram: a novel method to visualize local rigid body rotation under quasi-static compression

The feasibility of using normalized cumulative difference attenuation (NCDA) map for tracking the spatial and temporal evolution of temperature during microwave hyperthermia experiment on in-vitro phantoms is explored in this study. The NCDA maps were estimated from the beamformed ultrasound radio frequency (RF) data using a regularized log spectral difference (RLSD) technique. The NCDA maps were estimated at different time instants for the entire period of the experiment. The contour maps of the NCDA and the ground truth temperature map, obtained using an infra-red(IR) thermal camera corresponding to the ultrasound imaging plane, showed that NCDA was able to locate the axial and lateral co-ordinates of the hotspot with the error of < 1.5 mm axially and < 0.1 mm laterally. The error in the estimated hotspot area was less than 8 %. This preliminary in-vitro study suggests that NCDA maps estimated using RLSD may have potential in evaluating the spatio-temporal evolution of temperature and may help in the development of ultrasound-based image-guided temperature monitoring system for microwave hyperthermia.

Ultrasound-Based Regularized Log Spectral Difference Method For Monitoring Microwave Hyperthermia

The study involves development of ultrasound-based regularized Nakagami imaging (RNI) that improves the quality of Nakagami image. The feasibility of RNI to image the spatial and temporal evolution of hotspot during microwave (MW) hyperthermia experiment was explored on in-vitro polyacrylamide gel (PAG)-agar based phantoms. The normalized cumulative differential regularized Nakagami (NCDRN) maps were estimated from the envelope of beamformed ultrasound radiofrequency (RF) data using proposed RNI technique. The NCDRN maps were estimated at different time instants for the entire duration of the experiment. The experiments were carried out on phantoms at power level of 12 W fed to the microwave antenna. The contour maps of the NCDRN and the ground truth temperature map, obtained using an infra-red (IR) thermal camera corresponding to ultrasound imaging plane, showed that NCDRN was able to locate the axial and lateral co-ordinates of the hotspot with an error of < 1.5 mm axially and < 0.4 mm laterally. This preliminary in-vitro study demonstrates that NCDRN maps estimated using the regularized Nakagami imaging may have potential in imaging and evaluating the spatio-temporal evolution of hotspot and may help in the development of ultrasound-based image guided hotspot monitoring system for microwave hyperthermia.

Regularized Nakagami Ultrasound Imaging for Microwave Hyperthermia monitoring

Microwave heating is a well-known technique for delivering hyperthermia treatment to biological tissues. Temperature elevation during microwave tissue heating and feedback for real-time power control is commonly done using a limited number of invasive thermometry probes. Although ultrasound-based monitoring in real-time has been investigated for use with different ablation modalities, like radio frequency (RF) electrode, High Intensity Focused Ultrasound, its use in microwave hyperthermia is little reported. In this simulation study, we focused on estimating the spatio-temporal distribution of temperature from the estimates of attenuation of pulse-echo ultrasound RF data. The ultrasound simulation was performed using the convolution model. The temperature maps estimated using an algorithm based on tracking the changes in ultrasound attenuation is able to detect the spatial location of the hotspot with considerable precision. However, the current algorithm underestimates the isothermal regions and suffers from temperature dependent errors.

AliArshad Kothawala

Papers

Spatial Compounding Technique to Obtain Rotation Elastogram: A Feasibility Study.

Rotation elastogram: a novel method to visualize local rigid body rotation under quasi-static compression

Ultrasound-Based Regularized Log Spectral Difference Method For Monitoring Microwave Hyperthermia

Regularized Nakagami Ultrasound Imaging for Microwave Hyperthermia monitoring

A time domain method to monitor temperature in microwave hyperthermia using ultrasound attenuation