MR temperature measurement in liver tissue at 0.23 T with a steady‐state free precession sequence

Abstract

MRI can be used for monitoring temperature during a thermocoagulation treatment of tumors. The aim of this study was to demonstrate the suitability of a 3D steady-state free precession sequence (3D Fast Imaging with Steady-State Precession, 3D TrueFISP) for MR temperature measurement at 0.23 T, and to compare it to the spin-echo (SE) and spoiled 3D gradient-echo (3D GRE) sequences. The optimal flip angle for the TrueFISP sequence was calculated for the best temperature sensitivity in the image signal from liver tissue, and verified from the images acquired during the thermocoagulation of excised pig liver. The treatment of hepatic metastases by local heat delivery has drawn considerable interest over the last few years. The efficiency of thermoablation is improved if real-time monitoring of heat distribution is available during the treatment (1-3). At the moment, MRI is the only medical imaging modality that can provide noninvasive temperature information from tissue.Temperature monitoring with MRI is possible using the temperature sensitivity of the proton resonance frequency, i.e., chemical shift (4), diffusion (5), or longitudinal relaxation time T 1 and equilibrium magnetization M 0 (6 -8). The applicability of each method depends on the field strength of the MR scanner used, and on the target anatomy (9).Low-field open magnets are well suited for performing tumor ablations. They offer maximum access to the patient; dedicated tools for interventional MRI, such as needle tracking systems and in-room controls; less-intense needle artifacts; and lower-cost procedures. A disadvantage is that temperature-measuring methods utilizing the temperature sensitivity of the proton resonance frequency are difficult to use, as the change in chemical shift is directly proportional to field strength. For example, the sensitivity is 6.5 times less sensitive at 0.23 T than at 1.5 T.Motion artifacts hamper diffusion-based temperature measurement methods (10), making them impractical for use in the liver. With low-field open scanners, the easiest method for obtaining temperature data from the liver is to use the temperature dependence of T 1 and M 0 . Their temperature sensitivity is still high at lower magnetic field strengths, and they are easily measurable with standard sequences and hardware.The accuracy of temperature measurement with T 1 and M 0 depends strongly on the type of MR sequence used and on the choice of acquisition parameters, such as the repetition time (TR) and flip angle (␣). Temperature resolution may be affected by a factor of 5, depending on the type of sequence used for the measurement (11). The parameters need to be carefully chosen to optimize the temperature measurement while maintaining a reasonable temporal and spatial resolution.Spin-echo (SE) sequences, gradient-recalled acquisitions in the steady state (GRASS), and spoiled gradientrecalled acquisitions (SPGR) have been employed for monitoring temperature by using the temperature dependency of T 1 (7,8,11,12). A steady-state sequence (3D Fast Imagi...

Non-invasive 3d magnetic resonance thermal mapping: determination of the lesion size during laser-therapy in ex vivo tissues

“…While the slice saturation in the orthogonal slice is not really a problem for the PRF technique as long as the SNR remains adequate, it can be an issue for this technique based on T1. However, Germain et al [18] considered this potential problem as "insignificant". For instance in their study, the accuracy of sequence used during thermocoagulation procedures in the liver at 0.23 T, was estimated to be ±3.3 • C within a voxel volume of 37.5 mm 3 and an acquisition time of 18 s. In our study, at 0.2 T, a voxel volume of 15.6 mm 3 and an acquisition time of 10 s were obtained giving an accuracy of ±2.2 • C. Thanks to the imaging technique used in this study, quantitative temperature monitoring with a low field 0.2 T open MRI is sufficient as demonstrated by the correlation observed between lesions calculated from thermal mapping and macroscopic lesions.…”

MR imaging-guided laser ablation of osteoid osteomas with use of optical instrument guidance at 0.23�T

“…In our study we used open-configuration 0.23-T MRI scanner and a modified CBASS sequence to imagine treated area during laser ablation. As reported previously, the CBASS sequence was constructed to facilitate the detection of possible change in T1 relaxation time and related signal void caused by increasing temperature in tissue [31]. Although all the lesions were reliably identified for puncture, reliable signal change correlating to the change of temperature during treatment was not detected with the used thermal sequence.…”

“…The MRI guidance has been successfully applied to percutaneous bone biopsies and interstitial laser treatment guidance and monitoring in soft tissue tumors [23,24,25,26,27,28,29,30]. The thermal monitoring capability of a low-field MRI scanner has also been reported [31].…”

Determination of the Lesion Size in Laser-Induced Interstitial Thermal Therapy (LITT) using a Low-Field MRI

“…Using this equation, no T1 mapping is required during the treatment since the temperature is deduced directly from signal changes ∆S [7].…”