Improving thermal dose accuracy in magnetic resonance-guided focused ultrasound surgery: Long-term thermometry using a prior baseline as a reference

Abstract

Purpose-To investigate thermal dose volume (TDV) and non-perfused volume (NPV) of magnetic resonance-guided focused ultrasound (MRgFUS) treatments in patients with soft tissue tumors, and describe a method for MR thermal dosimetry using a baseline reference.Materials and Methods-Agreement between TDV and immediate post treatment NPV was evaluated from MRgFUS treatments of five patients with biopsy-proven desmoid tumors. Thermometry data (gradient echo, 3T) were analyzed over the entire course of the treatments to discern temperature errors in the standard approach. The technique searches previously acquired baseline images for a match using 2D normalized cross-correlation and a weighted mean of phase difference images. Thermal dose maps and TDVs were recalculated using the matched baseline and compared to NPV.Results-TDV and NPV showed between 47%-91% disagreement, using the standard immediate baseline method for calculating TDV. Long-term thermometry showed a nonlinear local temperature accrual, where peak additional temperature varied between 4-13°C (mean = 7.8°C) across patients. The prior baseline method could be implemented by finding a previously acquired matching baseline 61% ± 8% (mean ± SD) of the time. We found 7%-42% of the disagreement between TDV and NPV was due to errors in thermometry caused by heat accrual. For all patients, the prior baseline method increased the estimated treatment volume and reduced the discrepancies between TDV and NPV (P = 0.023).Conclusion-This study presents a mismatch between in-treatment and post treatment efficacy measures. The prior baseline approach accounts for local heating and improves the accuracy of thermal dose-predicted volume.Magnetic resonance-guided focused ultrasound (MRgFUS) has been used in clinical trials to ablate soft tissue tumors such as breast tumors, uterine fibroids, and prostate tumors. Some of these trials aimed to show safety and feasibility through partial tumor ablation. [1][2][3][4][5] More recently, patient studies have targeted total tumor volume ablation, including large uterine Total tumor ablation demands a highly accurate method to assess treatment efficacy. MR thermometry-derived thermal dose mapping is the primary method used to assess tumor ablation during an MRgFUS procedure. 9 MR-derived proton resonance frequency shift (PRF) thermometry provides a change in temperature, which is then used to calculate thermal dose; neither absolute temperature nor absolute dose are calculated using these methods. 10 Because these are not absolute measurements, the standard method does not account for local accumulation of heat over the course of a treatment, which can cause errors in thermometry, and thus, errors in the estimated thermal dose delivered to tissue (Fig. 1). 11,12 Underestimation of thermometry and thermal dose may result in unnecessary additional sonications of tissue that has already been ablated through sonication of adjacent areas; these extra sonications provide no benefit, while increasing risks to the patient, such ...

AAPM Task Group 241: A medical physicist’s guide to MRI‐guided focused ultrasound body systems

“…While this value defaults to 37℃, it can be altered by the user. However, when the baseline is determined from the previous sonication, the larger, predicted thermal dose volume was found to have better agreement with the NPV assessment 133 . In addition, it has been demonstrated that a significant amount of thermal dose accumulates during the cooling phase of the sonication and more accurate thermal dose measurements are achieved when both the heating and cooling phase of each sonication are used to calculate the cumulated thermal dose 133 .…”

Simultaneous fat‐referenced proton resonance frequency shift thermometry and MR elastography for the monitoring of thermal ablations

“…8 Thermal dose has also been shown to be poorly correlated to nonperfused volume, a reliable marker of postablation tissue damage, showing potential errors of MR thermometry due to long-term heat accrual. 9 Third, PRFS thermometry in fat-containing tissues may result in significant temperature errors depending on TE, fat fraction, and extent of temperature increase, because the temperature-dependent electron-screening constant in fat is negligible compared with the one of water protons. 10,11 Fat-referenced PRFS MR thermometry (FRPRFS) 12,13 using water-fat separation has been developed to tackle some of these limitations.…”

Prolonged heating in nontargeted tissue during MR‐guided focused ultrasound of bone tumors

“…13 A recent study in patients treated with MRgFUS showed the buildup of heat between ablations has a cumulative effect that contributes to a larger ablation volume than what was targeted. 14 Considering the high acoustic absorption of bone (14-22 dB/cm/MHz) compared with soft tissue (1-4 dB/cm/MHz), tissues adjacent to bone may be especially vulnerable to localized heat accumulation. 15 A number of studies have investigated ablative heating near bone via thermal response simulation models and in preclinical studies.…”

“…Underprediction of thermal dose may account for the discrepancy between thermal dose and contrastenhanced imaging showing posttreatment nonperfused volume (NPV) reported in treatments near bone. 14,23,24 Indeed, a preclinical study revealed that the ablation zone reached as far as 4 cm beyond the bone target with bone absorption, perfusion, angle of incidence, and shear waves playing a role in the extent of thermal dose in tissue adjacent to bone. 13 Figure 1 shows an example of our experience, where after MRgFUS treatment of a metastasis in the left iliac bone, the posttreatment nonperfused area extends 2 cm beyond the bone target, through the muscle tissue and into the fat layer, well beyond the prediction of lethal thermal dose ( Fig.…”