Correction of susceptibility-induced GRE phase shift for accurate PRFS thermometry proximal to cryoablation iceball

The susceptibility contrast between frozen and unfrozen tissue disturbs the local magnetic field in the proximity of the ice-ball during cryotherapy. This effect should be corrected for in real time to allow PRFS-based monitoring of near-zero temperatures during intervention

A pilot study for clinical feasibility of the near-harmonic 2D referenceless PRFS thermometry in liver under free breathing using MR-guided LITT ablation data

The conventional implementations of proton resonance frequency shift (PRFS) magnetic resonance thermometry (MRT) require the subtraction of single or multiple temporal references, a motion sensitive critical feature. A pilot study was conducted here to investigate the clinical feasibility of near-harmonic two-dimensional (2D) referenceless PRFS MRT, using patient data from MR-guided laser ablation of liver malignancies

PRFS-Based MR Thermometry Versus an Alternative T1 Magnitude Method – Comparative Performance Predicting Thermally Induced Necrosis in Hepatic Tumor Ablation

<div><p>Objective</p><p>To compare the accuracy of a semi-quantitative proton resonance frequency shift (PRFS) thermal mapping interface and an alternative qualitative T1 thermometry model in predicting tissue necrosis in an established routine setting of MRI-guided laser ablation in the human liver.</p> <p>Materials and Methods</p><p>34 cases of PRFS-guided (GRE) laser ablation were retrospectively matched with 34 cases from an earlier patient population of 73 individuals being monitored through T1 magnitude image evaluation (FLASH 2D). The model-specific real-time estimation of necrotizing thermal impact (above 54 °C zone and T1 signal loss, respectively) was correlated in size with the resulting necrosis as shown by lack of enhancement on the first-day contrast exam (T1). Matched groups were compared using the Mann-Whitney test.</p> <p>Results</p><p>Online PRFS guidance was available in 33 of 34 cases. Positive size correlation between calculated impact zone and contrast defect at first day was evident in both groups (p < 0.0004). The predictive error estimating necrosis was median 21 % (range 1 % - 52 %) in the PRFS group and 61 % (range 22 - 84 %) in the T1 magnitude group. Differences in estimating lethal impact were significant (p = 0.004), whereas the real extent of therapy-induced necrosis showed no significant difference (p > 0.28) between the two groups.</p> <p>Conclusion</p><p>PRFS thermometry is feasible in a clinical setting of thermal hepatic tumor ablation. As an interference-free MR-tool for online therapy monitoring its accuracy to predict tissue necrosis is superior to a competing model of thermally induced alteration of the T1 magnitude signal.</p> </div

T1 magnitude thermal imaging in a dual applicator ablation of hepatic colorectal carcinoma metastasis.

<p>Preablation T1 (A), mandrin placement (B), peak temperature with T1 signal loss (T1 FLASH 2D: TE 4,8 ms, TR 100 ms, BW 260 Hz/pixel, flip angle 70°, slice thickness 5 mm, fat saturation) (C), necrotic contrast defect on 24 h CEMR (D).</p

Reference-free PRFS MR-thermometry using near-harmonic 2-D reconstruction of the background phase

Proton resonance frequency shift (PRFS) MR thermometry (MRT) is the generally preferred method for monitoring thermal ablation, typically implemented with gradient-echo (GRE) sequences. Standard PRFS MRT is based on the subtraction of a temporal reference phase map and is, therefore, intrinsically sensitive to tissue motion (including deformation) and to external perturbation of the magnetic field. Reference-free (or reference-less) PRFS MRT has been previously described by Rieke and was based on a 2-D polynomial fit performed on phase data from outside the heated region, to estimate the background phase inside the region of interest. While their approach was undeniably a fundamental progress in terms of robustness against tissue motion and magnetic perturbations, the underlying mathematical formalism requires a thick unheated border and may be subject to numerical instabilities with high order polynomials. A novel method of reference-free PRFS MRT is described here, using a physically consistent formalism, which exploits mathematical properties of the magnetic field in a homogeneous or near-homogeneous medium. The present implementation requires as input the MR GRE phase values along a thin, nearly-closed and unheated border. This is a 2-D restriction of a classic Dirichlet problem, working on a slice per slice basis. The method has been validated experimentally by comparison with the “ground truth” data, considered to be the standard PRFS method for static ex vivo tissue. “Zero measurement” of the gradient-echo phase baseline was performed in healthy volunteer liver with rapid acquisition (300 ms/image). In vivo data acquired in sheep liver during MR-guided high intensity focused ultrasound (MRgHIFU) sonication were post-processed as proof of applicability in a therapeutic scenario. Bland and Altman mean absolute difference between the novel method and the “ground truth” thermometry in ex vivo static tissue ranged between 0.069 °C and 0.968 °C, compared to the inherent “white” noise SD of 0.23 °C. The accuracy and precision of the novel method in volunteer liver were found to be on average 0.13 °C and respectively 0.65 °C while the inherent “white” noise SD was on average 0.51 °C. The method was successfully applied to large ROIs, up to 6.2 cm inner diameter, and the computing time per slice was systematically less than 100 ms using C++. The current limitations of reference-free PRFS thermometry originate mainly from the need to provide a nearly-closed border, where the MR phase is artifact-free and the tissue is unheated, plus the potential need to reposition that border during breathing to track the motion of the anatomic zone being monitored.A reference-free PRFS thermometry method based on the theoretical framework of harmonic functions is described and evaluated here. The computing time is compatible with online monitoring during local thermotherapy. The current reference-free MRT approach expands the workflow flexibility, eliminates the need for respiratory triggers, enables higher temporal resolution, and is insensitive to unique-event motion of tissue

Box blot analysis of predictive errors for PRFS and T1 magnitude thermal monitoring.

<p>Values (parts per hundred) of over- and underestimation were evaluated in the PRFS group (range 1 % - 52 %), of underestimation alone in the T1 magnitude group (range 22 % - 84 %). The predictive error is almost threefold the amount with the older T1 magnitude method (median 61 % as compared with 21 %). </p

MR-guided therapy control.

<p>Example case (A) of an initially T1-hypointense recurrent tumor (arrowheads) at the margin of an older inhomogeneously hyperintense ablation zone. Phase difference image derived from subtracting a non-heating reference image (B) and thermal map with color-coded pixels in a quadrate ROI at peak temperature as being displayed on screen during the procedure (C), fast GRE sequences at 1.5 T Magnetom Avanto, Siemens Healthcare, Erlangen, Germany; TE 12 ms, TR 970 ms, BW 260 Hz/pixel, flip angle 65°, field of view (FOV) 320 mm, matrix size 128x128, slice thickness of 3 mm, fat suppression. Color-codes of phase image and online ROI are unequal. The last picture (D) shows the ablation-induced necrosis (arrowheads) demarcated as a lack of extracellular contrast uptake at portal venous phase on 24 h CEMR.</p

Matched-pair analysis of PRFS and T1 magnitude thermal monitoring.

<p>Comparative display of 2D size correlation (cm²) between above 54 °C isothermal zone and necrosis in PRFS group (dark column) on the one hand and T1 signal loss and necrosis in the T1 magnitude group (light column) on the other hand. The estimated necrosis (thermometry) is delineated in parts per hundred, with the resulting necrosis (24h CEMR) representing 100 %. Positive correlation of estimated and ensued impact zones was statistically significant for both groups (p < 0.0004). Overestimation (average 2.2 % in 2 cases) only was found when using PRFS thermometry; peak correlation with the approved necrosis in the T1 magnitude group was 78 %.</p