Over the past decade, several methods have been proposed to image tissue elasticity based on imaging methods collectively called elastography. While progress in developing these systems has been rapid, the basic understanding of tissue properties to interpret elastography images is generally lacking. To address this limitation, we developed a system to measure the Young's modulus of small soft tissue specimens. This system was designed to accommodate biological soft tissue constraints such as sample size, geometry imperfection and heterogeneity. The measurement technique consists of indenting an unconfined small block of tissue while measuring the resulting force. We show that the measured force-displacement slope of such a geometry can be transformed to the tissue Young's modulus via a conversion factor related to the sample's geometry and boundary conditions using finite element analysis. We also demonstrate another measurement technique for tissue elasticity based on quasi-static magnetic resonance elastography in which a tissue specimen encased in a gelatine-agarose block undergoes cyclical compression with resulting displacements measured using a phase contrast MRI technique. The tissue Young's modulus is then reconstructed from the measured displacements using an inversion technique. Finally, preliminary elasticity measurement results of various breast tissues are presented and discussed.

MR elastography (MRE) is an MRI modality that is increasingly being used to image tissue elasticity throughout the body. One MRE technique that has received a great deal of attention is based on visualizing shear waves, which reveal stiffness by virtue of their local wavelength. However, the shape of propagating shear waves can also provide valuable information about the nonlinear stressstrain behavior of tissue. Here an experiment is proposed that allows the observation of nonlinear wave propagation based on spatial-temporal phase contrast images. A theoretical description of the wave propagation was developed that reflects typical MRE excitation, which involves excitation modes both parallel and perpendicular to B 0 . Based on this model, it is shown that both odd and even higher harmonics are produced with their amplitudes dependent on the details of the actuator, imaging geometry, and the nonlinear tissue properties. In recent years, MR elastography (MRE) has received considerable interest as a novel modality that is capable of imaging tissue elasticity in vivo. Originally based on ultrasound (US) imaging, elastography applications in MRI have made rapid progress. The highly-resolved soft-tissue contrast gained by MRI, combined with the shear modulus as a sensitive elasticity parameter, have led several investigators to employ MRE in a number of clinical applications, including the prostate (1,2), head (3-6), skeletal muscle (7-9), and breast (10 -12). MRE is based on the detection of spin phase contrast arising from oscillatory motion in the presence of phase-locked magnetic field gradients (13). This allows the propagation of acoustic shear waves to be imaged along the direction of the applied motion-encoding gradients. To date, tissue stiffness has been examined in MRE in terms of the shear modulus as a linear elastic material property. The linear shear modulus can be analyzed by means of local wave speeds based on local frequency estimators (14,15), from inverse solutions of the Navier equations (11,16), or an iterative refinement of displacements (17,18). However, using realtime US, recently demonstrated that low-frequency (100 Hz) transverse waves can exhibit nonlinear propagation effects while traveling through agarose gel. Although such thermo-reversible gels have been found to be linear elastic under small static deformations, shear waves cause third-order nonlinear effects due to the high particle deflection speed relative to the low shear wave propagation speed. One can visualize the nonlinear effects by creating higher harmonic frequency components of the fundamental shear vibration, whose intensity ratios, shock speeds, and total amplitudes are sensitive to both applied strain components and the inherent nonlinear stress-strain function of the material (22). These nonlinear parameters may provide new information regarding tissue type, as previous rheologic experiments have shown that most tissues exhibit nonlinear constitutive properties (23). The goal of this study was to introduce a methodolo...

The authors' system is accurate for performing MR-guided needle localizations for both medial and lateral approaches. Factors that increased the uncorrected needle placement error included small lesion size, fatty breast density, and tissue shift in the z plane.

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The ability to control the shape of thermal coagulation was investigated for various interstitial heating applicators incorporating planar transducers and device rotation. Magnetic-resonance-compatible interstitial ultrasound applicators were constructed and the effects of ultrasound power, frequency, scan rate and heating time on lesion radius were studied in heating experiments in excised liver tissue. Continuous thermal lesions were generated by scanning heating applicators over a 180 angular sector. The region of thermal coagulation was restricted to the prescribed sector. Lesion radius increased with acoustic power and heating time and decreased with increasing frequency. The relationship between the temperature distribution generated by the applicator and the resulting thermal lesion was assessed with MRI. Analysis of MR temperature maps revealed that the temperature distribution could be measured accurately within 2 mm from the surface of the applicator, and the boundary of thermal coagulation was defined by a temperature of 54 +/- 12 degrees C. Calculations of temperature distributions indicated that slower scan rates can overcome the tendency of perfusion to reduce the radius of thermal lesion. This applicator design and delivery strategy make conformal interstitial heating possible.

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ABSTRACT:Harmonic MR elastography (MRE) monitors the propagation of acoustic waves in tissues in the audio regime. Oscillatory motions with large amplitudes can induce nonlinear wave propagation effects resulting in harmonics that evolve over space. In order to understand these effects, knowledge of the motions of applied mechanical motion is needed to rule out the presence of harmonic motion arising from the mechanical source. We propose a simple technique to measure the spectral content of mechanical excitation based on the use of a set of detection coils mounted on the elastography excitation system. The motion of these coils causes a small signal to be induced from the applied static magnetic field of the MRI system. A detailed analysis shows that quantitative assessment of excitations is possible with correct geometrical arrangement of the detector coils. However, it shows that nonlinear effects can also occur depending on the alignment of the detection coils with respect to Bo. The system is easy to operate and allows for the time resolved observation of the actuator motion for each experimental setup. The system is intended to be used before and after MRE experiments to determine excitation spectral content and repeatability. We demonstrate its use in a onedimensional elastography experiment and show that this information is an essential prerequisite for studying material nonlinear elastic properties using MRE.