A signal/noise analysis of quasi-static MR elastography.

In quasi-static magnetic resonance elastography, strain images of a tissue or material undergoing deformation are produced. In this paper, the signal/noise (S/N) ratio [SNR] of elastographic strain images, as measured by a phase-contrast technique, is analyzed. Experiments are conducted to illustrate how diffusion-mediated signal attenuation limits maximum strain SNR in small displacement cases, while the imaging point-spread function limits large displacement cases. A simple theoretical treatment agrees well with experiments and shows how an optimal displacement encoding moment can be predicted for a given experimental set of parameters to achieve a maximum strain SNR. A further experiment demonstrates how the limitation on strain SNR posed by the imaging point-spread function may potentially be overcome.

Interstitial ultrasound heating applicator for MR-guided thermal therapy.

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.