Hairong Shi

Tissue-mimicking agar/gelatin materials for use in heterogeneous elastography phantoms

Five 9 cm x 9 cm x 9 cm phantoms, each with a 2-cm-diameter cylindrical inclusion, were produced with various dry-weight concentrations of agar and gelatin. Elastic contrasts ranged from 1.5 to 4.6, and values of the storage modulus (real part of the complex Youngs modulus) were all in the soft tissue range. Additives assured immunity from bacterial invasion and can produce tissue-mimicking ultrasound and NMR properties. Monitoring of strain ratios over a 7 to 10 month period indicated that the mechanical properties of the phantoms were stable, allowing about 1 month for the phantom to reach chemical equilibrium. The only dependable method for determining the storage moduli of the inclusions is to make measurements on samples excised from the phantoms. If it is desired to produce and accurately characterize a phantom with small inclusions with other shapes, such as an array of small spheres, an auxiliary phantom with the geometry of the cylindrical inclusion phantoms or the equivalent should be made at the same time using the same materials. The elastic contrast can then be determined using samples excised from the auxiliary phantom. A small increase of about 10% in volume of the cylindrical inclusions occurred-a tolerable increase. Interestingly, the smallest increase (about 5%) occurred in the phantom with the largest elastic contrast.

Two-dimensional multi-level strain estimation for discontinuous tissue

A large number of the strain estimation methods presented in the literature are based on the assumption of tissue continuity that establishes a continuous displacement field. However, in certain locations in the body such as the arteries in vivo scanning may produce displacement fields that are discontinuous between the two walls of the artery. Many of the displacement or strain estimators fail when the displacement fields are discontinuous. In this paper, we present a new 2D multi-level motion or displacement tracking method for accurate estimation of the strain in these situations. The final high-resolution displacement estimate is obtained using two processing steps. The first step involves an estimation of a coarse displacement estimate utilizing B-mode or envelope signals. To reduce computational time, the coarse displacement estimates are obtained starting from down-sampled B-mode pre- and post-compression image pairs using a pyramidal processing approach. The coarse displacement estimate obtained from the B-mode data is used to guide the final 2D cross-correlation computations on radio-frequency (RF) data. Results from finite element simulations and in vivo experimental data demonstrate the feasibility of this approach for imaging tissue with discontinuous displacement fields.

Preliminary in vivo atherosclerotic carotid plaque characterization using the accumulated axial strain and relative lateral shift strain indices

In this paper, we explore two parameters or strain indices related to plaque deformation during the cardiac cycle, namely, the maximum accumulated axial strain in plaque and the relative lateral shifts between plaque and vessel wall under in vivo clinical ultrasound imaging conditions for possible identification of vulnerable plaque. These strain indices enable differentiation between calcified and lipidic plaque tissue utilizing a new perspective based on the stiffness and mobility of the plaque. In addition, they also provide the ability to distinguish between softer plaques that undergo large deformations during the cardiac cycle when compared to stiffer plaque tissue. Soft plaques that undergo large deformations over the cardiac cycle are more prone to rupture and to release micro-emboli into the cerebral bloodstream. The ability to identify vulnerable plaque, prone to rupture, would significantly enhance the clinical utility of this method for screening patients. We present preliminary in vivo results obtained from ultrasound radio frequency data collected over 16 atherosclerotic plaque patients before these patients undergo a carotid endarterectomy procedure. Our preliminary in vivo results indicate that the maximum accumulated axial strain over a cardiac cycle and the maximum relative lateral shift or displacement of the plaque are useful strain indices that provide differentiation between soft and calcified plaques.

Elastographic imaging of thermal lesions in liver in-vivo using diaphragmatic stimuli.

Radiofrequency or microwave ablations are interstitial focal ablative therapies that can be used in a percutaneous fashion for treating tumors in the liver, kidney, and prostate. These modalities provide in situ destruction of tumors. We present a method for in-vivo elastographic visualization of the ablated regions in the liver during and after thermal therapy. In-vivo elastographic imaging uses compressions of the liver due to movement of the diaphragm during the respiratory cycle. Elastography of the liver and other abdominal organs has not been attempted previously due to the difficulty in providing controlled compressions. Gating of the data acquisition to the respiratory waveform would provide access to data where the compression increments are similar in both magnitude and direction, thereby enabling reproducible imaging of the thermal lesion or tumor. Comparison of elastograms with gross-pathology of ablated tissue illustrates the correspondence between elastographic image features and pathology. Ultrasound is routinely used to guide the rf ablation procedure, so the same imaging system could be used for elastographic imaging. Since the technique utilizes physiological motion of the diaphragm due to respiration, it may also be employed in the visualization of cancerous tumors in the liver.

Spherical lesion phantoms for testing the performance of elastography systems

A set of three cubic one-litre phantoms containing spherical simulated lesions was produced for use in comparing lesion detection performance of different elastography systems. The materials employed are known to be stable in heterogeneous configurations regarding geometry and elastic contrast identical with (storage modulus of lesion material) / (storage modulus of background material), and regarding ultrasound and NMR properties. The materials mimic soft tissues in terms of elastic, ultrasound and NMR properties. Each phantom has only one value of elastic contrast (3.3, 4.6 or 5.5) and contains arrays of 1.6 mm, 2 mm, 3 mm and 4 mm diameter spherical simulated lesions. All the spheres of a given diameter are arranged in a regular array with coplanar centres. Elastograms of an array made with ultrasound allow determination of the depth range over which lesions of that diameter and elastic contrast can be detected. Two phantoms are made from agar-plus-gelatin-based materials, and one is made from oil-in-gelatin dispersions. The methods for producing the phantoms are described in detail. Lesion detection performances for two ultrasound systems, both operating at about 7.5 MHz and focused at about 5 cm, were quantified with distinctions between the two systems demonstrated. Neither system was capable of detecting any of the 1.6 mm lesions. Phantoms such as these should be useful in research labs that are refining hardware and/or software for elastography.

Spatial-angular compounding for elastography using beam steering on linear array transducers

Spatial-angular compounding is a new technique that enables the reduction of noise artifacts in ultrasound elastography. Under this method, compounded elastograms are obtained from a spatially weighted average of local strain estimated from radio frequency (rf) echo signals acquired at different insonification angles. In previous work, the acquisition of the rf signals was performed through the lateral translation of a phased-array transducer. Clinical applications of angular compounding would, however, require the utilization of beam steering on linear-array transducers to obtain angular data sets, which is more efficient than translating phased-array transducers. In this article, we investigate the performance of angular compounding for elastography by using beam steering on a linear-array transducer. Quantitative experimental results demonstrate that spatial angular compounding provides significant improvement in both the elastographic signal-to-noise ratio and the contrast-to-noise ratio. For the linear array transducer used in this study, the optimum angular increment is around 1.5 degrees-3.75 degrees, and the maximum angle that can be used in angular compounding should not exceed 10 degrees.

In vivo attenuation and equivalent scatterer size parameters for atherosclerotic carotid plaque: Preliminary results

We have previously reported on the equivalent scatterer size, attenuation coefficient, and axial strain properties of atherosclerotic plaque ex vivo. Since plaque structure and composition may be damaged during a carotid endarterectomy procedure, characterization of in vivo properties of atherosclerotic plaque is essential. The relatively shallow depth of the carotid artery and plaque enables non-invasive evaluation of carotid plaque utilizing high frequency linear-array transducers. We investigate the ability of the attenuation coefficient and equivalent scatterer size parameters to differentiate between calcified, and lipidic plaque tissue. Softer plaques especially lipid rich and those with a thin fibrous cap are more prone to rupture and can be classified as unstable or vulnerable plaque. Preliminary results were obtained from 10 human patients whose carotid artery was scanned in vivo to evaluate atherosclerotic plaque prior to a carotid endarterectomy procedure. Our results indicate that the equivalent scatterer size obtained using Farans scattering theory for calcified regions are in the 120-180 microm range while softer regions have larger equivalent scatterer size distribution in the 280-470 microm range. The attenuation coefficient for calcified regions as expected is significantly higher than that for softer regions. In the frequency bandwidth ranging from 2.5 to 7.5 MHz, the attenuation coefficient for calcified regions lies between 1.4 and 2.5 dB/cm/MHz, while that for softer regions lies between 0.3 and 1.3 dB/cm/MHz.

Anthropomorphic phantoms for assessment of strain imaging methods involving saline-infused sonohysterography.

Two anthropomorphic uterine phantoms were developed that allow assessment and comparison of strain imaging systems adapted for use with saline-infused sonohysterography (SIS). Tissue-mimicking (TM) materials consist of dispersions of safflower oil in gelatin. TM fibroids are stiffer than the TM myometrium/cervix, and TM polyps are softer. The first uterine phantom has 3-mm-diameter TM fibroids distributed randomly in TM myometrium. The second uterine phantom has a 5-mm and 8-mm spherical TM fibroid, in addition to a 5-mm spherical and a 12.5-mm-long (medicine capsule-shaped) TM endometrial polyp protruding into the endometrial cavity; also, a 10-mm spherical TM fibroid projects from the serosal surface. Strain images using the first phantom show the stiffer 3-mm TM fibroids in the myometrium. Results from the second uterine phantom show that, as expected, parts of inclusions projecting into the uterine cavity will appear very stiff, whether they are stiff or soft. Results from both phantoms show that although there is a five-fold difference in the Youngs moduli values, there is not a significant difference in the strain in the transition from the TM myometrium to the TM fat. These phantoms allow for realistic comparison and evolution of SIS strain imaging techniques and can aid clinical personnel to develop skills for SIS strain imaging.

Ultrasonic Attenuation Estimation in Small Plaque Samples Using a Power Difference Method

Many studies have shown that atherosclerosis changes the ultrasonic attenuation properties of the vessel wall and plaque. Accurate estimation of the attenuation coefficient slope could therefore provide an early indication of atherosclerosis and the differentiation between low, mild and highly-attenuating plaque within the vessel. However, the traditional reference phantom method that fits the power spectrum in a region of interest fails to accurately estimate the attenuation coefficient for small irregular shaped ex-vivo plaque specimens. This discrepancy was primarily due to partial volume effects and the unknown backscatter coefficient of the plaque sample. We have developed a method based on the reference-phantom method that utilizes the difference in the acoustic power above and below the sample to accurately compute values of the attenuation coefficient ex vivo. Our results demonstrate that this approach overcomes the two drawbacks mentioned earlier and provides accurate estimates of the attenuation coefficient slope for small excised tissue samples.

Correction for simultaneous catheter eccentricity and tilt in intravascular elastography.

Intravascular elastography can provide significant new information about the elastic properties of vascular tissue and plaque, useful for the diagnosis of disease and appropriate selection of interventional methods. Knowledge of the plaque composition, vulnerability and its elastic properties can assist the clinician in selecting appropriate interventional techniques. However, several noise sources have to be addressed to obtain quality intravascular elastograms. Misalignment of the vessel lumen and the ultrasound beam can produce erroneous strain estimates in elastography. Errors in the strain estimate are introduced due to the eccentricity and tilt of the intravascular transducer within the vessel lumen. Previous work in this area has provided theoretical expressions for the correction of eccentricity and tilt errors when they occur independent of each other. However, under most imaging conditions, both eccentricity and tilt errors are simultaneously present. In this paper, we extend the theoretical correction factor by accounting for the influence of both of these errors occurring simultaneously in the positioning of the catheter within the vessel lumen.