Accurate field mapping in the presence of B₀ inhomogeneities, applied to MR thermometry

Abstract

Purpose To describe how B0 inhomogeneities can cause errors in proton resonance frequency (PRF) shift thermometry, and to correct for these errors. Methods With PRF thermometry, measured phase shifts are converted into temperature measurements through the use of a scaling factor proportional to the echo time, TE. However, B0 inhomogeneities can deform, spread, and translate MR echoes, potentially making the ‘true’ echo time vary spatially within the imaged object and take on values that differ from the prescribed TE value. Acquisition and reconstruction methods able to avoid or correct for such errors are presented. Results Tests were performed in a gel phantom during sonication, and temperature measurements were made with proper shimming as well as with intentionally-introduced B0 inhomogeneities. Errors caused by B0 inhomogeneities were observed, described, and corrected by the proposed methods. No statistical difference was found between the corrected results and the reference results obtained with proper shimming, while errors by more than 10% in temperature elevation were corrected for. The approach was also applied to an abdominal in vivo dataset. Conclusion Field variations induce errors in measured field values, which can be detected and corrected. The approach was validated for a PRF thermometry application.

Improved PRF‐based MR thermometry using k‐space energy spectrum analysis

“…The previous method from Mei et al 21 was implemented and compared with the results obtained here, to test their robustness to noise. White noise with zero mean and Gaussian distribution was added to the data obtained as described previously, to create eight separate scenarios corresponding to different signal‐to‐noise ratio (SNR) values.…”

“…The PRF thermometry sequences often use a low readout bandwidth to help optimize the temperature‐to‐noise ratio, 20 but a low readout bandwidth also increases the relative importance of susceptibility effects. If such susceptibility effects can distort the effective value of TE, then they can affect the proportionality constant in PRF thermometry and therefore affect the measured PRF temperature values 18,21 . The goal of the present work was to detect and compensate for these effects and achieve more accurate PRF thermometry results.…”

MRI for Noninvasive Thermometry

“…Differential measurement makes the method sensitive to all non-temperature related effects that alter the local magnetic field between scans. These include a) drift in the static field [146], b) patient motion [16,63,147], c) local or global changes in magnetic susceptibility (e.g. due to tissue swelling [148], fat [46,161] air [149], movement of interstitial applicators [43] or injection of contrast agent [150]), which can be altered by heating itself [49].…”

MRI in radiation oncology: Underserved needs

“…System sources include gradient nonlinearity and B 0 field inhomogeneity. Although several correction methods have been investigated and implemented [eg, see ], resulting in residual distortion of approximately 0.5 mm as in the case of small field of view imaging of the prostate , there remains a need to improve the spatial accuracy of the MR data beyond the conventionally accepted limit of a 35‐ to 40‐cm field of view centered at the magnet isocenter. Correction of patient‐induced geometric distortion (eg, from B 0 effects) still remains challenging and is further complicated due to the fact that it is difficult to quantitatively evaluate the effectiveness of distortion correction methods in individual patients.…”