This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. MR-guided cryoablation is a promising minimally invasive therapy for prostate tumors that are soliatary and unilateral. Temperature mapping within the frozen tissue with MRI would be preferable as it would provide maps throughout the frozen area, not just at discrete thermocouple locations. Tissue R2* is a particularly promising MR parameter to quantify and relate to temperature as it appears to be relatively linear over the temperature range of interest. The purpose of this work was optimize our R2* mapping by using a PR sequence, reducing the earliest echo time to 0.1 ms, and carefully measuring and compensating for eddy currents. Methods and Discussion: Half-RF pulse excitation is used to achieve an ultra-short echo times. Both B0 and linear eddy currents induced by the selective gradients and readout gradients are measured first. This is achieved by linearly fitting the phases of signals collected with various gradient amplitudes at several off-isocenter positions. At each time point, signal phases from the same slice are first linearly fit with respect to gradient amplitude. The linear coefficients are then fit with respect to the slice position. The resultant constant terms now correspond to the phases accumulated by B0 eddy currents, while the linear coefficients corresponds to k-space trajectory traversed by the applied gradient and linear eddy currents, respectively, both normalized by the selective gradient amplitude. For excitation, linear eddy currents are corrected by pre-compensating the RF pulse, while B0 eddy currents are corrected by varying the RF phase during excitation. Eddy currents due to readout gradients are corrected during image reconstruction. To calculate the R2*, several sets of data are acquired with different TEs for the first echo. These relatively short TE intervals are designed to accurately quantify short T2*, the third echo have similar point-spread-function (PSF) for a given T2* as the first echo, and are therefore used for better calculation of R2* for tissues with longer T2*. ISMRM abstracts:
Aimi ng Lu, Bruce L. Daniel, Kim B. Pauly. Improvements in R2* Mapping During In Vivo Cryoablation.
Aimi ng Lu, Bruce L. Daniel, Kim B. Pauly. A Simple Approach to Measure and Correct for B0 and Linear Eddy Currents.
Aimi ng Lu, Bruce L. Daniel, Kim B. Pauly. Improved Slice Excitation for Ultra-short TE Imaging with B0 and Linear Eddy Current Correction. PI: Kim B. Pauly Grants: NIH CA092061, P41 RR009784
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