MRI-temperature mapping during ultrasound prostate ablation using fat for phase estimation

Measurement of temperature dependent changes in bone marrow using a rapid chemical shift imaging technique

PURPOSE

To provide quantitative temperature monitoring for thermal therapies in bone marrow by measuring temperature-dependent signal changes in the bone marrow of ex vivo canine femurs heated with a 980-nm laser at 1.5T and 3.0T.

MATERIALS AND METHODS

Using a multi-gradient echo (≤ 16) acquisition and signal modeling with the Stieglitz-McBride algorithm, the temperature sensitivity coefficients (TSC, ppm/°C) of water and multiple lipid components' proton resonance frequency (PRF) values are measured at high spatiotemporal resolutions (1.6 × 1.6 × 4 mm(3) , ≤ 5 seconds). Responses in R(2) * and amplitudes of each peak were also measured as a function of temperature simultaneously.

RESULTS

Calibrations demonstrate that lipid signal may be used to compensate for B(0) errors to provide accurate temperature readings (<1.0°C). Over a temperature range of 17.2-57.2°C, the TSCs after correction to a bulk methylene reference are -0.87 × 10(-2) ± 4.7 × 10(-4) ppm/°C and -0.87 × 10(-2) ± 4.0 × 10(-4) ppm/°C for 1.5T and 3.0T, respectively.

CONCLUSION

Overall, we demonstrate that accurate and precise temperature measurements can be made in bone marrow. In addition, the relationship of R(2) * and signal amplitudes with respect to temperature are shown to differ significantly where conformal changes are predicted by Arrhenius rate model analysis.

Referenceless MR thermometry for monitoring thermal ablation in the prostate

Referenceless proton resonance frequency (PRF) shift thermometry provides a means to measure temperature changes during minimally invasive thermotherapy that is inherently robust to motion and tissue displacement. However, if the referenceless method is used to determine temperature changes during prostate ablation, phase gaps between water and fat in image regions used to determine the background phase can confound the phase estimation. We demonstrate an extension to referenceless thermometry which eliminates this problem by allowing background phase estimation in the presence of phase discontinuities between aqueous and fatty tissue. In this method, images are acquired with a multiecho sequence and binary water and fat maps are generated from a Dixon reconstruction. For the background phase estimation, water and fat regions are treated separately and the phase offset between the two tissue types is determined. The method is demonstrated feasibile in phantoms and during in vivo thermal ablation of canine prostate.