A.R. Skovoroda

Internal displacement and strain imaging using ultrasonic speckle tracking

Previous ultrasound speckle tracking methods have been extended, permitting measurement of internal displacement and strain fields over a wide dynamic range of tissue motion. The markedly increased dynamic range of this approach should lead to enhanced contrast resolution in strain and elasticity images. Results of experiments on gelatin-based, tissue equivalent phantoms show the capabilities of the method.<<ETX>>

Tissue elasticity reconstruction based on ultrasonic displacement and strain images

A method is presented to reconstruct the elastic modulus of soft tissue based on ultrasonic displacement and strain images. Incompressible and compressible media are considered separately. Problems arising with this method, as well as applications to real measurements on gel-based, tissue equivalent phantoms, are given. Results show that artifacts present in strain images can be greatly reduced using a hybrid reconstruction procedure based on numerical solution of the partial differential equations describing mechanical equilibrium of a deformed medium.<<ETX>>

Biophysical Bases of Elasticity Imaging

Elasticity imaging is based on two processes. The first is the evaluation of the mechanical response of a stressed tissue using imaging modalities, e.g. ultrasound, magnetic resonance imaging (MRI), computed tomography (CT) scans and Doppler ultrasound. The second step is depiction of the elastic properties of internal tissue structures by mathematical solution of the inverse mechanical problem. The evaluation of elastic properties of tissues has the potential for being an important diagnostic tool in the detection of cancer as well as other injuries and diseases. The success of breast self-examination in conjunction with mammography for detection and continuous monitoring of lesions has resulted in early diagnosis and institution of therapy. Self-examination is based on the manually palpable texture difference of the lesion relative to adjacent tissue and, as such, is limited to lesions located relatively near the skin surface and increased lesion hardness with respect to the surrounding tissue. Imaging of tissue “hardness” should allow more sensitive detection of abnormal structures deeper within tissue. Tissue hardness can actually be quantified in terms of the tissue elastic moduli and may provide good contrast between normal and abnormal tissues based on the large relative variation in shear (or Young’s) elastic modulus.

Strain rate imaging using two-dimensional speckle tracking

Strain rate images (SRI) of the beating heart have been proposed to identify non-contracting regions of myocardium. Initial attempts used spatial derivatives of tissue velocity (Doppler) signals. Here, an alternate method is proposed based on two-dimensional phase-sensitive speckle tracking applied to very high frame rate, real-time images. This processing can produce high resolution maps of the time derivative of the strain magnitude (i.e., square root of the strain intensity). Such images complement traditional tissue velocity images (TVI), providing a more complete description of cardiac mechanics. To test the proposed approach, SRI were both simulated and measured on a thick-walled, cylindrical, tissue-equivalent phantom modeling cardiac deformations. Real-time ultrasound images were captured during periodic phantom deformation, where the period was matched to the data capture rate of a commercial scanner mimicking high frame rate imaging of the heart. Simulation results show that SRI with spatial resolution between 1 and 2 mm are possible with an array system operating at 5 MHz. Moreover, these images are virtually free of angle-dependent artifacts present in TVI and simple strain rate maps derived from these images. Measured results clearly show that phantom regions of low deformation, which are difficult to identify on tissue velocity-derived SRI, are readily apparent with SRI generated from two-dimensional phase-sensitive speckle tracking.

Theoretical analysis and verification of ultrasound displacement and strain imaging

Evaluation of internal displacement and strain distributions in tissue under externally applied forces is a necessary step in elasticity imaging. To obtain a quantitative image of the elastic modulus, strain and displacement fields must be measured with reasonable accuracy and inverted based on an accurate theoretical model of soft tissue mechanics. In this paper, results of measured internal strain and displacement fields from gel-based phantoms are compared with theoretical predictions of a linear elastic model. In addition, some aspects of elasticity reconstruction based on measured displacement and strain fields are discussed.<<ETX>>

Lateral displacement estimation using tissue incompressibility

Using the incompressibility property of soft tissue, lateral displacements can be reconstructed from axial strain measurements. Results of simulations and experiments on gelatin-based tissue equivalent phantoms are compared with theoretical displacements, as well as estimates derived from traditional speckle tracking. Incompressibility processing greatly improves the accuracy and signal-to-noise ratio (SNR) of lateral displacement measurements compared with more traditional speckle tracking.

Displacement and strain imaging of coronary arteries with intraluminal ultrasound

Tissue elasticity can be estimated from displacement and strain images acquired under controlled deformation. We extend this approach for coronary arteries, deformed and imaged by an integrated angioplasty balloon and ultrasonic imaging probe. Because the lumen cross section of a severely occluded artery is not circular, we have also developed a technique to perform all motion computations in the reference frame of the lumens geometric center. This coordinate system is independent of the imaging catheter and consequently referencing to this frame removes artifacts associated with probe motion within the balloon during deformation. Displacements and strains estimated by phase-sensitive correlation-based speckle tracking were used to distinguish arterial plaques in simulated coronary arteries of differing elastic moduli: hard, soft, and homogenous. We have also applied these methods to images of a homogeneous gelatin phantom collected with the integrated probe. The maximum phantom displacement was about 40 pm, and the maximum radial normal strain was about 4% (absolute value). The spatial dependence of these quantities shows good agreement with theoretically predicted values.

Measuring the elastic modulus of small tissue samples

Independent measurements of the elastic modulus (Youngs modulus) of tissue are a necessary step in turning elasticity imaging into a clinical tool. A system capable of measuring the elastic modulus of small tissue samples was developed. The system tolerates the constraints of biological tissue, such as limited sample size (≤1.5 cm3) and imperfections in sample geometry. A known deformation is applied to the tissue sample while simultaneously measuring the resulting force. These measurements are then converted to an elastic modulus, where the conversion uses prior calibration of the system with plastisol samples of known Youngs modulus. Accurate measurements have been obtained from 10 to 80 kPa, covering a wide range of tissue modulus values. In addition, the performance of the system was further investigated using finite element analysis. Finally, preliminary elasticity measurements on canine kidney samples are presented and discussed.

Ultrasonic displacement and strain imaging of coronary arteries with a catheter array

Tissue elasticity can be estimated from displacement and strain images acquired under controlled deformation. We extend this approach for coronary arteries, deformed and imaged by an integrated angioplasty balloon and ultrasonic imaging probe. Because the lumen cross section of a severely occluded artery is generally not circular, we have also developed a technique to perform all motion computations in the reference frame of the lumens geometric center. This coordinate system is independent of the imaging catheter, and consequently referencing to this frame removes artifacts associated with probe motion within the balloon during deformation. Displacements and strains estimated by phase-sensitive correlation-based speckle tracking were used to distinguish arterial plaques in simulated coronary arteries of differing elastic moduli: hard, soft, and homogenous. We have also applied these methods to images of a homogeneous gelatin phantom collected with the integrated probe. The spatial dependence of these quantities shows good agreement with theoretically predicted values.

Magnetic‐resonance imaging techniques for detection of elasticity variation

The relative success of manual palpation in the detection of breast cancer would suggest that a method for remote palpation resulting in a measurement of tissue elasticity could provide a diagnostic tool for detecting cancerous lesions deeper within the breast. This presumption is based in part on the excellent contrast between neoplastic and normal tissue due to the large (orders of magnitude) relative variation in the shear elastic modulus. By comparison, the bulk deformational modulus maintains the same value to within 20% for most soft tissues. A specific method of magnetic-resonance imaging (MRI) which measures tissue displacements has been used in experiments with a phantom containing regions of increased Youngs modulus as a demonstration. The spatial modulation of magnetization technique uses the displacement of a spatial grid pattern caused by spin saturation to track regional motion. Mathematical reconstruction of the distribution of elastic moduli is shown for select examples. Any modality, e.g., MRI, ultrasound, etc., which can detect local tissue motion with sufficient spatial resolution can be used and therefore the results presented here should give an indication of the utility of such motion tracking techniques to future measurement of tissue elasticity.