Shyam Bharat

Three-Dimensional Electrode Displacement Elastography Using the Siemens C7F2 fourSight Four-Dimensional Ultrasound Transducer

Because ablation therapy alters the elastic modulus of tissues, emerging strain imaging methods may enable clinicians for the first time to have readily available, cost-effective, real-time guidance to identify the location and boundaries of thermal lesions. Electrode displacement elastography is a method of strain imaging tailored specifically to ultrasound-guided electrode-based ablative therapies (e.g., radio-frequency ablation). Here tissue deformation is achieved by applying minute perturbations to the unconstrained end of the treatment electrode, resulting in localized motion around the end of the electrode embedded in tissue. In this article, we present a method for three-dimensional (3D) elastographic reconstruction from volumetric data acquired using the C7F2 fourSight four-dimensional ultrasound transducer, provided by Siemens Medical Solutions USA, Inc. (Issaquah, WA, USA). Lesion reconstruction is demonstrated for a spherical inclusion centered in a tissue-mimicking phantom, which simulates a thermal lesion embedded in a normal tissue background. Elastographic reconstruction is also performed for a thermal lesion created in vitro in canine liver using radio-frequency ablation. Postprocessing is done on the acquired raw radio-frequency data to form surface-rendered 3D elastograms of the inclusion. Elastographic volume estimates of the inclusion compare reasonably well with the actual known inclusion volume, with 3D electrode displacement elastography slightly underestimating the true inclusion volume.

Young's Modulus Reconstruction for Radio-Frequency Ablation Electrode-Induced Displacement Fields: A Feasibility Study

Radio-frequency (RF) ablation is a minimally invasive treatment for tumors in various abdominal organs. It is effective if good tumor localization and intraprocedural monitoring can be done. In this paper, we investigate the feasibility of using an ultrasound-based Youngs modulus reconstruction algorithm to image an ablated region whose stiffness is elevated due to tissue coagulation. To obtain controllable tissue deformations for abdominal organs during and/or intermediately after the RF ablation, the proposed modulus imaging method is specifically designed for using tissue deformation fields induced by the RF electrode. We have developed a new scheme under which the reconstruction problem is simplified to a 2-D problem. Based on this scheme, an iterative Youngs modulus reconstruction technique with edge-preserving regularization was developed to estimate the Youngs modulus distribution. The method was tested in experiments using a tissue-mimicking phantom and on ex vivo bovine liver tissues. Our preliminary results suggest that high contrast modulus images can be successfully reconstructed. In both experiments, the geometries of the reconstructed modulus images of thermal ablation zones match well with the phantom design and the gross pathology image, respectively.

Radio-frequency ablation electrode displacement elastography: A phantom study

This article describes the evaluation of a novel method of tissue displacement for use in the elastographic visualization of radio-frequency (rf) ablation-induced lesions. The method involves use of the radio-frequency ablation electrode as a displacement device, which provides localized compression in the region of interest. This displacement mechanism offers the advantage of easy in vivo implementation since problems such as excessive lateral and elevational displacements present when using external compression are reduced with this approach. The method was tested on a single-inclusion tissue-mimicking phantom containing a radio-frequency ablation electrode rigidly attached to the inclusion center. Full-frame rf echo signals were acquired from the phantom before and after electrode displacements ranging from 0.05 to 0.2 mm. One-dimensional cross-correlation analysis between pre- and postcompression signals was used to measure tissue displacements, and strains were determined by computing the gradient of the displacement. The strain contrast, contrast-to-noise ratio, and signal-to-noise ratio were estimated from the resulting strain images. Comparisons are drawn between the elastographically measured dimensions and those known a priori for the single-inclusion phantom. Electrode displacement elastography was found to slightly underestimate the inclusion dimensions. The method was also tested on a second tissue-mimicking phantom and on in vitro rf-ablated lesions in canine liver tissue. The results validate previous in vivo findings that electrode displacement elastography is an effective method for monitoring rf ablation.

Electrode displacement strain imaging of thermally-ablated liver tissue in an in vivo animal model.

PURPOSE Percutaneous thermal ablation is increasingly being used to destroy hepatic tumors in situ. The success of ablative techniques is highly dependent on adequate ablation zone monitoring, and ultrasound-based strain imaging could become a convenient and cost-effective means to delineate ablation zone boundaries. This study investigates in vivo electrode displacement-based strain imaging for monitoring hepatic ablation procedures that are difficult to perform with conventional elastography. METHODS a In our method, minute displacements (less than a millimeter) are applied to the unconstrained end of the ablation electrode, resulting in localized tissue deformation within the ablation zone that provides the mechanical stimuli required for strain imaging. This article presents electrode displacement strain images of radiofrequency ablation zones created in porcine liver in vivo (n = 13). RESULTS Cross-sectional area measurements from strain images of these ablation zones were obtained using manual and automated segmentation. Area measurements from strain images were highly correlated with areas measured on histopathology images, quantitated using linear regression (R = 0.894, P < 0.001 and R = 0.828, P < 0.001, respectively). CONCLUSIONS This study further demonstrates that electrode displacement elastography is capable of providing high-contrast images using widely available commercial ultrasound systems which may potentially be used to assess the extent of thermal ablation zones.

Characterizing the compression-dependent viscoelastic properties of human hepatic pathologies using dynamic compression testing

Recent advances in elastography have provided several imaging modalities capable of quantifying the elasticity of tissue, an intrinsic tissue property. This information is useful for determining tumour margins and may also be useful for diagnosing specific tumour types. In this study, we used dynamic compression testing to quantify the viscoelastic properties of 16 human hepatic primary and secondary malignancies and their corresponding background tissue obtained following surgical resection. Two additional backgrounds were also tested. An analysis of the background tissue showed that F4-graded fibrotic liver tissue was significantly stiffer than F0-graded tissue, with a modulus contrast of 4:1. Steatotic liver tissue was slightly stiffer than normal liver tissue, but not significantly so. The tumour-to-background storage modulus contrast of hepatocellular carcinomas, a primary tumour, was approximately 1:1, and the contrast decreased with increasing fibrosis grade of the background tissue. Ramp testing showed that the background stiffness increased faster than the malignant tissue. Conversely, secondary tumours were typically much stiffer than the surrounding background, with a tumour-to-background contrast of 10:1 for colon metastases and 10:1 for cholangiocarcinomas. Ramp testing showed that colon metastases stiffened faster than their corresponding backgrounds. These data have provided insights into the mechanical properties of specific tumour types, which may prove beneficial as the use of quantitative stiffness imaging increases.

Radiofrequency electrode vibration-induced shear wave imaging for tissue modulus estimation: a simulation study.

Quasi-static electrode displacement elastography, used for in-vivo imaging of radiofrequency ablation-induced lesions in abdominal organs such as the liver and kidney, is extended in this paper to dynamic vibrational perturbations of the ablation electrode. Propagation of the resulting shear waves into adjoining regions of tissue can be tracked and the shear wave velocity used to quantify the shear (and thereby Youngs) modulus of tissue. The algorithm used utilizes the time-to-peak displacement data (obtained from finite element analyses) to calculate the speed of shear wave propagation in the material. The simulation results presented illustrate the feasibility of estimating the Youngs modulus of tissue and is promising for characterizing the stiffness of radiofrequency-ablated thermal lesions and surrounding normal tissue.

Contrast-transfer improvement for electrode displacement elastography

Electrode displacement elastography is a strain imaging method that can be used for in-vivo imaging of radiofrequency ablation-induced lesions in abdominal organs such as the liver and kidney. In this technique, tissue motion or deformation is introduced by displacing the same electrode used to create the lesion. Minute displacements (on the order of a fraction of a millimetre) are applied to the thermal lesion through the electrode, resulting in localized tissue deformation. Ultrasound echo signals acquired before and after the electrode-induced displacements are then utilized to generate strain images. However, these local strains depend on the modulus distribution of the tissue region being imaged. Therefore, a quantitative evaluation of the conversion efficiency from modulus contrast to strain contrast in electrode-displacement elastograms is warranted. The contrast-transfer efficiency is defined as the ratio (in dB) of the observed elastographic strain contrast and the underlying true modulus contrast. It represents a measure of the efficiency with which elastograms depict the underlying modulus distribution in tissue. In this paper, we develop a contrast-transfer efficiency formalism for electrode displacement elastography (referred to as contrast-transfer improvement). Changes in the contrast-transfer improvement as a function of the underlying true modulus contrast and the depth of the inclusion in the simulated phantom are studied. We present finite element analyses obtained using a two-dimensional mechanical deformation and tissue motion model. The results obtained using finite element analyses are corroborated using experimental analysis and an ultrasound simulation program so as to incorporate noise artifacts.

In vivo ultrasound electrode displacement strain imaging

The incidence of primary and secondary liver tumors has increased significantly over the last two decades. Due to poor liver function and/or distribution of lesions, surgery is often not an option for affected patients. An alternative to surgical resection, percutaneous radiofrequency ablation is often used to thermally destroy the tumor in situ. Radiofrequency ablation is evolving into one of the more popular minimally-invasive treatments for hepatic tumors. Monitoring the treated region is an important factor in the success of radiofrequency ablation, and ultrasound elastography could become a convenient, cost-effective means to delineate the thermal lesion boundaries for clinical personnel during the procedure. This study assesses “electrode displacement” strain imaging for monitoring abdominal radiofrequency ablation procedures. We present results utilizing a novel approach of displacing the ablation electrode itself to introduce the mechanical stimuli required for strain imaging. Utilizing a Siemens Antares clinical ultrasound scanner equipped with a research interface, ultrasound radiofrequency data were acquired immediately following radiofrequency ablation of porcine liver. The porcine liver was excised following the procedure, and the dimensions of the thermal lesion in the imaging plane measured by slicing though the lesion. Strain images of the lesions were produced offline using axial guidance based block-matching and multi-level motion tracking algorithms. The area of the lesion on the strain image was compared to the area of the lesion in pathology images obtained from eight separate lesions. The estimated linear correlation coefficient between the pathology image and the strain image was r = 0.961 (p ≪ .001) for manual segmentation using 4 observers. The area of the lesion in the strain image slightly underestimates the area of the lesion in the pathology image for all slices, agreeing with earlier ex-vivo experiments.

P4F-1 Contrast-Transfer Improvement for Electrode Displacement Elastography

Electrode displacement elastography is a strain imaging method that can be used for in-vivo imaging of radiofrequency ablation-induced lesions in abdominal organs such as the liver and kidney. In this technique, tissue motion or deformation is introduced by displacing the same electrode used to create the lesion. Minute displacements (on the order of a fraction of a millimeter) are applied to the thermal lesion through the electrode, resulting in localized tissue deformation. Ultrasound echo signals acquired before and after the electrode-induced displacements are then utilized to generate strain images. However, these local strains depend on the modulus distribution of the tissue region being imaged. Therefore, a quantitative evaluation of the conversion efficiency from modulus contrast to strain contrast in electrode-displacement elastograms is warranted. The contrast-transfer efficiency is defined as the ratio (in dB) of the observed elastographic strain contrast and the underlying true modulus contrast. It represents a measure of the efficiency with which elastograms depict the underlying modulus distribution in tissue. In this paper we develop a contrast-transfer efficiency formalism for electrode displacement elastography (referred to as contrast-transfer improvement). Changes in the contrast-transfer improvement as a function of the underlying true modulus contrast and the depth of the inclusion in a simulated phantom are studied. We present finite element analyses obtained using a 2-D mechanical deformation and tissue motion model. The results obtained using finite element analyses are corroborated using experimental analysis and an ultrasound simulation program so as to incorporate noise artifacts.

TU‐E‐201C‐05: Electrode Displacement Strain Imaging for Monitoring In‐Vivo Ablative Therapies

Purpose: Percutaneous RF ablation is evolving into an accepted minimally‐invasive treatment for hepatic tumors. Monitoring and delineating the treated region is essential for its success. Ultrasoundelastography can become a convenient and cost‐effective means to delineate thermal lesion boundaries. This study assesses electrode displacement based strain imaging for monitoring abdominal RF ablation procedures that are difficult to monitor with conventional elastography Method and Materials: Thirteen RFablated regions were created in‐vivo in pig liver.Radiofrequency echo signal data for strain imaging were acquired using a Siemens Antares clinical scanner immediately following RF ablation procedures. Small displacements were applied to the unconstrained end of the ablationelectrode in‐vivo, resulting in localized tissue deformation. Strain images were then compared to gross‐pathology images of the same lesion along the two‐dimensional imaging plane. Gross‐pathology images were obtained by fixing the excised thermal lesion and slicing though the lesion, utilizing marks on the liver surface denoting the imaging plane and visual inspection of the electrode track. Results: Cross‐sectional area measurements of the thermal lesion obtained from the strain images were derived using both manual and automated segmentation. Areas were compared with cross‐sectional area measurements from gross pathology images. Area measurements from strain images were highly correlated to areas measured on gross‐pathology, where the linear correlation coefficients were R = 0.894, P < 0.001 and R = 0.828, P < 0.001, for the manual and automated segmentation, respectively. Conclusions:Electrode displacement based strain imaging provides high contrast between ablated and normal liver tissue, allowing for clear delineation of the thermal ablation zone. This complements clinical ultrasound imaging, the preferred modality for real‐time guidance for the placement of the RF needle into the tumor, allowing multiple imaging tasks to be performed with a single ultrasound machine. Supported by NIH‐NCI grants R01CA112192‐03 and R01CA112192‐S103.