Improved MR thermometry for laser interstitial thermotherapy

Abstract

Objectives: To develop, test and evaluate improved 2D and 3D protocols for proton resonance frequency shift magnetic resonance temperature imaging (MRTI) of laser interstitial thermal therapy (LITT). The objective was to develop improved MRTI protocols in terms of temperature measurement precision and volume coverage compared to the 2D MRTI protocol currently used with a commercially available LITT system. Methods: Four different 2D protocols and four different 3D protocols were investigated. The 2D protocols used multi-echo readouts to prolong the total MR sampling time and hence the MRTI precision, without prolonging the total acquisition time. The 3D protocols provided volumetric thermometry by acquiring a slab of 12 contiguous slices in the same acquisition time as the 2D protocols. The study only considered readily available pulse sequences (Cartesian 2D and 3D gradient recalled echo and echo planar imaging [EPI]) and methods (partial Fourier and parallel imaging) to ensure wide availability and rapid clinical implementation across vendors and field strengths. In vivo volunteer studies were performed to investigate and compare MRTI precision and image quality. Phantom experiments with LITT heating were performed to investigate and compare MRTI precision and accuracy. Different coil setups were used in the in vivo studies to assess precision differences between using local (such as flex and head coils) and non-local (i.e., body coil) receive coils. Studies were performed at both 1.5 T and 3 T. Results: The improved 2D protocols provide up to a factor of two improvement in the MRTI precision in the same acquisition time, compared to the currently used clinical protocol. The 3D echo planar imaging protocols provide comparable precision as the currently used 2D clinical protocol, but over a substantially larger field of view, without increasing the acquisition time. As expected, local receive coils perform substantially better than the body coil, and 3 T provides better MRTI accuracy and precision than 1.5 T. 3D data can be zero-filled interpolated in all three dimensions (as opposed to just two dimensions for 2D data), reducing partial volume effects and measuring higher maximum temperature rises. Conclusions: With the presented protocols substantially improved MRTI precision (for 2D imaging) or greatly improved field of view coverage (for 3D imaging) can be achieved in the same acquisition time as the currently used protocol. Only widely available pulse sequences and acquisition methods were investigated, which should ensure quick translation to the clinic. Lasers Surg. Med. 51:286-300, 2019.

Fiber Bragg Grating Sensors for Performance Evaluation of Fast Magnetic Resonance Thermometry on Synthetic Phantom

“…Indeed, was found to be around 0.4 °C where the highest temperature gradient is measured and ~0.2 °C in the regions not subjected to temperature increase. In [ 14 ], similar values ranging from 0.2 to 0.4 °C are found. In this case, the temperature precision is measured as temporal standard deviation in a non-heated ROI and for eight different protocols including 2D GRE and 3D EPI during phantom laser therapy.…”

“…They found out that the GRE-EPI sequence is more appropriate for temperature monitoring in the laser ablation panorama in terms of higher precision and accuracy (MOD of 0.2 °C and LOA of 3.7/−3.2 °C for the GRE-EPI sequence versus 0.3 °C and 3.5/−2.9 °C for FLASH sequence) using two fiber-optic temperature probes. In [ 14 ], the accuracy of the set of 2D GRE and 3D EPI protocols was calculated also inserting a single fiber optic sensor as reference temperature. Comparison results, this time, were expressed as RMSE LR and mean difference (0.5 °C, and <0.3 °C, respectively) for three 3D EPI protocols.…”

“…Currently, MRTI is largely employed during thermal treatments such as laser ablation and focused ultrasound [ 10 , 13 ]. Laser ablation is especially well-suited for MR guidance since the thin fiber optic applicator does not create large susceptibility artifacts in the MR images, and a relatively small field of view is sufficient to monitor the localized heating around the fiber [ 14 ]. The efficacy of MRTI-guidance during a thermal therapy depends on: (1) Excellent spatial (1–3 mm) and temporal resolution (1 s or less); (2) robustness to artifacts; (3) accuracy of temperature values (better than 1–2 °C) [ 15 ].…”

Elementwise approach for simulating transcranial MRI-guided focused ultrasound thermal ablation

“…In the others, a multiecho readout was performed (TR: 28.0 ms; TE: 3.1/7.8/12.5/17.2/21.9 ms; flip angle: 30°; receiver bandwidth: ±35.71 kHz; field of view: 28 cm; slice thickness: 3 mm; matrix: 256 × 128). The higher readout bandwidth of the multiecho sequence reduced spatial distortions in the frequency-encoding direction with minimal loss of SNR [ 20 ].…”

Clinical Performance and Future Potential of Magnetic Resonance Thermometry in Hyperthermia

“…It can be seen that in recent years more studies have satisfied these minimum requirements. A total of 8 out of those 13 studies are feasibility or comparison studies (marked in grey in Table 5 ), which can be grouped into having investigated: Hardware: MR-HIFU for different treatment location applications [ 45 , 47 ] Thermometry method: MR spectroscopy to measure absolute temperature [ 64 ] Sequences and parameters for MRT [ 43 , 56 , 57 , 70 ] Performance of MRT at different anatomical sites [ 71 ] …”