Samantha L. Lipman

B-Mode and Acoustic Radiation Force Impulse (ARFI) Imaging of Prostate Zonal Anatomy Comparison with 3T T2-Weighted MR Imaging

Prostate cancer (PCa) is the most common non-cutaneous malignancy among men in the United States and the second leading cause of cancer-related death. Multi-parametric magnetic resonance imaging (mpMRI) has gained recent popularity to characterize PCa. Acoustic Radiation Force Impulse (ARFI) imaging has the potential to aid PCa diagnosis and management by using tissue stiffness to evaluate prostate zonal anatomy and lesions. MR and B-mode/ARFI in vivo imaging datasets were compared with one another and with gross pathology measurements made immediately after radical prostatectomy. Images were manually segmented in 3D Slicer to delineate the central gland (CG) and prostate capsule, and 3D models were rendered to evaluate zonal anatomy dimensions and volumes. Both imaging modalities showed good correlation between estimated organ volume and gross pathologic weights. Ultrasound and MR total prostate volumes were well correlated (R2 = 0.77), but B-mode images yielded prostate volumes that were larger (16.82% ± 22.45%) than MR images, due to overestimation of the lateral dimension (18.4% ± 13.9%), with less significant differences in the other dimensions (7.4% ± 17.6%, anterior-to-posterior, and −10.8% ± 13.9%, apex-to-base). ARFI and MR CG volumes were also well correlated (R2 = 0.85). CG volume differences were attributed to ARFI underestimation of the apex-to-base axis (−28.8% ± 9.4%) and ARFI overestimation of the lateral dimension (21.5% ± 14.3%). B-mode/ARFI imaging yielded prostate volumes and dimensions that were well correlated with MR T2-weighted image (T2WI) estimates, with biases in the lateral dimension due to poor contrast caused by extraprostatic fat. B-mode combined with ARFI imaging is a promising low-cost, portable, real-time modality that can complement mpMRI for PCa diagnosis, treatment planning, and management.

Evaluating the Improvement in Shear Wave Speed Image Quality Using Multidimensional Directional Filters in the Presence of Reflection Artifacts

Shear waves propagating through interfaces where there is a change in stiffness cause reflected waves that can lead to artifacts in shear wave speed (SWS) reconstructions. Two-dimensional (2-D) directional filters are commonly used to reduce in-plane reflected waves; however, SWS artifacts arise from both inand out-of-imaging-plane reflected waves. Herein, we introduce 3-D shear wave reconstruction methods as an extension of the previous 2-D estimation methods and quantify the reduction in image artifacts through the use of volumetric SWS monitoring and 4-D-directional filters. A Gaussian acoustic radiation force impulse excitation was simulated in phantoms with Youngs modulus (E) of 3 kPa and a 5-mm spherical lesion with E = 6, 12, or 18.75 kPa. The 2-D-, 3-D-, and 4-D-directional filters were applied to the displacement profiles to reduce inand out-of-plane reflected wave artifacts. Contrast-to-noise ratio and SWS bias within the lesion were calculated for each reconstructed SWS image to evaluate the image quality. For 2-D SWS image reconstructions, the 3-D-directional filters showed greater improvements in image quality than the 2-D filters, and the 4-D-directional filters showed marginal improvement over the 3-D filters. Although 4-D-directional filters can further reduce the impact of large magnitude out-of-plane reflection artifacts in SWS images, computational overhead and transducer costs to acquire 3-D data may outweigh the modest improvements in image quality. The 4-D-directional filters have the largest impact in reducing reflection artifacts in 3-D SWS volumes.

Comparison of concurrently acquired in vivo 3D ARFI and SWEI images of the prostate

In the prostate, ARFI and SWEI imaging methods have reported that cancer and other pathologies as being stiffer than the surrounding tissue. A three-dimensional in vivo prostatic imaging system capable of concurrently acquiring ARFI and SWEI data was developed to compare the information available in the two image types. Data were acquired in a calibrated CIRS phantom to analyze the contrast, contrast to noise ratio (CNR), and resolution between the ARFI and SWEI images; SWEI images provided improved contrast and CNR, but lower spatial resolution than ARFI images. Challenges and potential artifacts in both the ARFI and SWEI images have been identified and reduced by viewing coronal sections and maximum value SWEI images, resulting in high correlation between the normalized ARFI displacement magnitude and the estimated shear wave speed. We have demonstrated that this combined ARFI and SWEI imaging system can image the anatomy of the prostate.

Improving the accuracy of shear wave speed reconstructions using 4D directional filters in the presence of reflection artifacts

Reflected waves from stiffness boundaries can lead to artifacts in shear wave speed (SWS) reconstructions. 2D directional filters are commonly used with planar imaging systems to reduce in-plane reflected waves; however SWS artifacts arise from both in and out-of imaging plane reflected waves. Herein, we quantify the reduction in image artifacts afforded by the use of volumetric SWS monitoring and 4D directional filters. A Gaussian acoustic radiation force impulse was simulated in a phantom with a Youngs modulus (E) of 3 kPa with a 5 mm spherical lesion with E = 6, 12 or 18.75 kPa. 2D, 3D, and 4D directional filters were applied to the displacement profiles to reduce in and out-of-plane reflected wave artifacts. SWS images were reconstructed and RMS error and CNR were calculated for each image to evaluate the image accuracy and quality. Applying 3D directional filters as compared to 2D led to larger improvements in image accuracy and quality than the improvements seen using 4D directional filters over 3D. This improvement in image accuracy is significant because the processing of these data could be performed on displacement data from a traditional 1D linear array with reasonable computational time and resources. Although 4D directional filters can further reduce the impact of large magnitude out-of-plane reflection artifacts in SWS images, computational overhead and transducer costs may outweigh the modest improvements in image quality.

Eliminating speckle noise with three-dimensional single-track-location shear wave elasticity imaging (STL-SWEI)

Conventional multiple track location shear wave elasticity imaging (MTL-SWEI) is a powerful tool for noninvasively estimating tissue elasticity. The resolution and noise of MTL-SWEI systems, however, are limited by ultrasound speckle. Single track location SWEI (STL-SWEI) is a novel variant which fixes the position of the tracking beam and modulates the push location to effectively cancel out the effects of speckle bias. We present a 3D STL-SWEI system which provides full suppression of lateral and elevation speckle bias for high resolution 3D elasticity imaging, and requires no inherent spatial filtering to make precise measurements of shear wave speed. The system is demonstrated in a uniform elasticity phantom.

Methodology to register prostate B-mode and ARFI images to MR and histology

Acoustic Radiation Force Impulse (ARFI) imaging is being developed for guiding needle biopsy and focal therapy of Prostate cancer (PCa). In vivo ARFI images portray internal structures in the prostate with higher contrast than matched B-mode images. Given the heterogeneity of the prostate and the poor visualization provided by B-mode, another gold standard for determining what is being visualized in ARFI images is necessary. In this study, we present image registration techniques that facilitate correlation of in vivo ARFI, B-mode ultrasound (US), and Magnetic Resonance (MR) images obtained prior to radical prostatectomy with whole mount histology data. Pathology and structures were identified and segmented in the different datasets. The segmented datasets were used to form 3D mesh models of the prostate and the node and element information was extrapolated into a 3D image matrix of equivalent size for all modalities. Non-rigid registration of the different models was performed and the registered images were evaluated for co-localization of confirmed pathology. The methodology was validated using simulated prostate anatomy and finite-element techniques and found to improve the average displacement of registration markers by 76% in the MR simulation and 58% in the US simulation. When implemented on the patient data, the registration methodology was found to simplify multi-modality image comparison and analysis. Confirmed pathology was found to align with similarly suspicious regions in both ARFI and MR images. With its improved anatomical visualization over traditional B-mode imaging, ARFI holds promise for providing targeted image guidance of prostate focal therapy and needle biopsy.

Impact of Acoustic Radiation Force Excitation Geometry on Shear Wave Dispersion and Attenuation Estimates

Shear wave elasticity imaging (SWEI) characterizes the mechanical properties of human tissues to differentiate healthy from diseased tissue. Commercial scanners tend to reconstruct shear wave speeds for a region of interest using time-of-flight methods reporting a single shear wave speed (or elastic modulus) to the end user under the assumptions that tissue is elastic and shear wave speeds are not dependent on the frequency content of the shear waves. Human tissues, however, are known to be viscoelastic, resulting in dispersion and attenuation. Shear wave spectroscopy and spectral methods have been previously reported in the literature to quantify shear wave dispersion and attenuation, commonly making an assumption that the acoustic radiation force excitation acts as a cylindrical source with a known geometric shear wave amplitude decay. This work quantifies the bias in shear dispersion and attenuation estimates associated with making this cylindrical wave assumption when applied to shear wave sources with finite depth extents, as commonly occurs with realistic focal geometries, in elastic and viscoelastic media. Bias is quantified using analytically derived shear wave data and shear wave data generated using finite-element method models. Shear wave dispersion and attenuation bias (up to 15% for dispersion and 41% for attenuation) is greater for more tightly focused acoustic radiation force sources with smaller depths of field relative to their lateral extent (height-to-width ratios <16). Dispersion and attenuation errors associated with assuming a cylindrical geometric shear wave decay in SWEI can be appreciable and should be considered when analyzing the viscoelastic properties of tissues with acoustic radiation force source distributions with limited depths of field.

Thee-Dimensional Single-Track-Location Shear Wave Elasticity Imaging

Conventional multiple-track-location shear wave elasticity imaging (MTL-SWEI) is a powerful tool for noninvasively estimating tissue elasticity. The resolution and noise levels of MTL-SWEI systems, however, are limited by ultrasound speckle. Single-track-location SWEI (STL-SWEI) is a novel variant which fixes the position of the tracking beam and modulates the push location to effectively cancel out the effects of speckle-induced bias. We present here a 3-D STL-SWEI system, which provides full suppression of lateral and elevation speckle bias for high-resolution volumetric elasticity imaging, and requires no spatial smoothing to make accurate measurements of shear wave speed. We demonstrate and analyze the system’s performance in homogeneous and layered elasticity phantoms.

On the feasibility of estimating ultrasonic shear wave attenuation using amplitude-based methods

Human tissues are known to be viscoelastic (VE), a type of material described by two behaviors: dispersion and attenuation. Several methods have been developed to measure shear wave dispersion, but quantifying shear wave attenuation (α) has been less successful, particularly using amplitude-based methods. Shear wave displacement amplitudes are dependent on several factors including geometric spreading, shear attenuation, and bias in ultrasonic displacement amplitude estimation. Recent developments using 3D single track location (STL) shear wave imaging have significantly reduced the error in effective tracking location arising from speckle bias, preserving the relative amplitudes of the shear wave. Using a cylindrically symmetric source, the shear wave can be modeled as a cylindrical wave, whose geometric decay is 1/√r. Shear wave displacements were generated from an analytic model and Gaussian white noise was added to the temporal shear wave velocity profiles to mimic jitter (25 dB SNR). One hundred realizations of noise were combined to model different radial trajectories from the excitation in the lateral-elevation plane of a volumetric acquisition. The time-domain Fourier Transform (FT) of these profiles was computed and shear attenuation as a function of frequency was estimated from the amplitude decay after correcting for geometric spreading. The results are compared with shear attenuation estimation obtained using the spectral spreading 2D-FT attenuation method. Root-mean-square-error (RMSE) compared to the analytic solution was calculated for both methods over the frequency range of 100 to 300 Hz. Both methods perform comparably with a large spatial extent (>40 mm); however the 2D-FT methods exhibit considerable bias with smaller window sizes.