Real-time automatic temperature regulation during in vivo MRI-guided laser-induced thermotherapy (MR-LITT).

Precise control of tissue temperature during Laser-Induced Thermotherapy (LITT) procedures has the potential to improve the clinical efficiency and safety of such minimally invasive therapies. We present a method to automatically regulate in vivo the temperature increase during LITT using real-time rapid volumetric Magnetic Resonance thermometry (8 slices acquired every second, with an in-plane resolution of 1.4 mmx1.4 mm and a slice thickness of 3 mm) using the proton-resonance frequency (PRF) shift technique. The laser output power is adjusted every second using a feedback control algorithm (proportional-integral-derivative controller) to force maximal tissue temperature in the targeted region to follow a predefined temperature-time profile. The root-mean-square of the difference between the target temperature and the measured temperature ranged between 0.5 °C and 1.4 °C, for temperature increases between + 5 °C to + 30 °C above body temperature and a long heating duration (up to 15 min), showing excellent accuracy and stability of the method. These results were obtained on a 1.5 T clinical MRI scanner, showing a potential immediate clinical application of such a temperature controller during MR-guided LITT.

2D multislice MP2RAGE sequence for fast T1 mapping at 7 T: Application to mouse imaging and MR thermometry.

PURPOSE: To develop a 2D radial multislice MP2RAGE sequence for fast and reliable T1 mapping at 7 T in mice and for MR thermometry. METHODS: The 2D-MP2RAGE sequence was performed with the following parameters: TI1 -TI2 -MP2RAGETR = 1000-3000-9000 ms. The multiple dead times within the sequence were used for interleaved multislice acquisition, enabling one to acquire six slices in 9 seconds. The excitation pulse shape, inversion selectivity, and interslice gap were optimized. In vitro comparison with the inversion-recovery sequence was performed. The T1 variations with temperature were measured on tubes with T1 ranging from 800 ms to 2000 ms. The sequence was used to acquire T1 maps continuously during 30 minutes on the brain and abdomen of healthy mice. RESULTS: A three-lobe cardinal sine excitation pulse, combined with an inversion slice thickness and an interslice gap of respectively 150% and 50% of the imaging slice thickness, led to a SD and bias of the T1 measurements below 1% and 2%, respectively. A linear dependence of T1 with temperature was measured between 10°C and 60°C. In vivo, less than 1% variation was measured between successive T1 maps in the mouse brain. In the abdomen, no obvious in-plane motion artifacts were observed but respiratory motion in the slice dimension led to 6% T1 underestimation. CONCLUSION: The multislice MP2RAGE sequence could be used for fast whole-body T1 mapping and MR thermometry. Its reconstruction method would enable on-the-fly reconstruction.

Radial MP2RAGE sequence for rapid 3D T1 mapping of mouse abdomen: application to hepatic metastases.

OBJECTIVES: The T1 longitudinal recovery time is regarded as a biomarker of cancer treatment efficiency. In this scope, the Magnetization Prepared 2 RApid Gradient Echo (MP2RAGE) sequence relevantly complies with fast 3D T1 mapping. Nevertheless, with its Cartesian encoding scheme, it is very sensitive to respiratory motion. Consequently, a radial encoding scheme was implemented for the detection and T1 measurement of hepatic metastases in mice at 7T. METHODS: A 3D radial encoding scheme was developed using a golden angle distribution for the k-space trajectories. As in that case, each projection contributes to the image contrast, the signal equations had to be modified. Phantoms containing increasing gadoteridol concentrations were used to determine the accuracy of the sequence in vitro. Healthy mice were repetitively scanned to assess the reproducibility of the T1 values. The growth of hepatic metastases was monitored. Undersampling robustness was also evaluated. RESULTS: The accuracy of the T1 values obtained with the radial MP2RAGE sequence was > 90% compared to the Inversion-Recovery sequence. The motion robustness of this new sequence also enabled repeatable T1 measurements on abdominal organs. Hepatic metastases of less than 1-mm diameter were easily detected and T1 heterogeneities within the metastasis and between the metastases within the same animal were measured. With a twofold acceleration factor using undersampling, high-quality 3D T1 abdominal maps were achieved in 9 min. CONCLUSIONS: The radial MP2RAGE sequence could be used for fast 3D T1 mapping, to detect and characterize metastases in regions subjected to respiratory motion. KEY POINTS: • The Cartesian encoding of the MP2RAGE sequence was modified to a radial encoding. The modified sequence enabled accurate T 1 measurements on phantoms and on abdominal organs of mice. • Hepatic metastases were easily detected due to high contrast. Heterogeneity in T 1 was measured within the metastases and between each metastasis within the same animal. • As implementation of this sequence does not require specific hardware, we expect that it could be readily available for clinical practice in humans.

Compressed-Sensing MP2RAGE sequence: Application to the detection of brain metastases in mice at 7T.

PURPOSE: To develop a Compressed Sensing (CS)-MP2RAGE sequence to drastically shorten acquisition duration and then detect and measure the T1 of brain metastases in mice at 7 T. METHODS: The encoding trajectory of the standard Cartesian MP2RAGE sequence has been modified (1) to obtain a variable density Poisson disk under-sampling distribution along the ky -kz plane, and (2) to sample the central part of the k-space exactly at TI1 and TI2 inversion times. In a prospective study, the accuracy of the T1 measurements was evaluated on phantoms containing increasing concentrations of gadolinium. The CS acceleration factors were increased to evaluate their influence on the contrast and T1 measurements of brain metastases in vivo. Finally, the 3D T1 maps were acquired with at 4-fold increased spatial resolution. The volumes and T1 values of the metastases were measured while using CS to reduce scan time. RESULTS: The implementation of the CS-encoding trajectory did not affect the T1 measurements in vitro. Accelerating the acquisition by a factor of 2 did not alter the contrast or the T1 values of the brain metastases. 3D T1 maps could be obtained in < 1 min using a CS factor of 6. Increasing the spatial resolution enabled more accurately measurement of the metastasis volumes while maintaining an acquisition duration below 5 min. CONCLUSION: The CS-MP2RAGE sequence could be of great interest in oncology to either rapidly obtain mouse brain 3D T1 maps or to increase the spatial resolution with no penalty on the scan duration.