Temperature monitoring of ultrasonically heated muscle with RARE chemical shift imaging

Estimation of the Proton Resonance Frequency Coefficient in Agar-based Phantoms

Aim

Agar-based phantoms are popular in high intensity focused ultrasound (HIFU) studies, with magnetic resonance imaging (MRI) preferred for guidance since it provides temperature monitoring by proton resonance frequency (PRF) shift magnetic resonance (MR) thermometry. MR thermometry monitoring depends on several factors, thus, herein, the PRF coefficient of agar phantoms was estimated.

Materials and Methods

Seven phantoms were developed with varied agar (2, 4, or 6% w/v) or constant agar (6% w/v) and varied silica concentrations (2, 4, 6, or 8% w/v) to assess the effect of the concentration on the PRF coefficient. Each phantom was sonicated using varied acoustical power for a 30 s duration in both a laboratory setting and inside a 3T MRI scanner. PRF coefficients were estimated through linear trends between phase shift acquired using gradient sequences and thermocouple-based temperatures changes.

Results

Linear regression (R 2 = 0.9707-0.9991) demonstrated a proportional dependency of phase shift with temperature change, resulting in PRF coefficients between -0.00336 ± 0.00029 and -0.00934 ± 0.00050 ppm/°C for the various phantom recipes. Weak negative linear correlations of the PRF coefficient were observed with increased agar. With silica concentrations, the negative linear correlation was strong. For all phantoms, calibrated PRF coefficients resulted in 1.01-3.01-fold higher temperature changes compared to the values calculated using a literature PRF coefficient.

Conclusions

Phantoms developed with a 6% w/v agar concentration and doped with 0%-8% w/v silica best resemble tissue PRF coefficients and should be preferred in HIFU studies. The estimated PRF coefficients can result in enhanced MR thermometry monitoring and evaluation of HIFU protocols.

Magnetic resonance thermometry and its biological applications - Physical principles and practical considerations

Most parameters that influence the magnetic resonance imaging (MRI) signal experience a temperature dependence. The fact that MRI can be used for non-invasive measurements of temperature and temperature change deep inside the human body has been known for over 30 years. Today, MR temperature imaging is widely used to monitor and evaluate thermal therapies such as radio frequency, microwave, laser, and focused ultrasound therapy. In this paper we cover the physical principles underlying the biological applications of MR temperature imaging and discuss practical considerations and remaining challenges. For biological tissue, the MR signal of interest comes mostly from hydrogen protons of water molecules but also from protons in, e.g., adipose tissue and various metabolites. Most of the discussed methods, such as those using the proton resonance frequency (PRF) shift, T₁, T₂, and diffusion only measure temperature change, but measurements of absolute temperatures are also possible using spectroscopic imaging methods (taking advantage of various metabolite signals as internal references) or various types of contrast agents. Currently, the PRF method is the most used clinically due to good sensitivity, excellent linearity with temperature, and because it is largely independent of tissue type. Because the PRF method does not work in adipose tissues, T₁- and T₂-based methods have recently gained interest for monitoring temperature change in areas with high fat content such as the breast and abdomen. Absolute temperature measurement methods using spectroscopic imaging and contrast agents often offer too low spatial and temporal resolution for accurate monitoring of ablative thermal procedures, but have shown great promise in monitoring the slower and usually less spatially localized temperature change observed during hyperthermia procedures. Much of the current research effort for ablative procedures is aimed at providing faster measurements, larger field-of-view coverage, simultaneous monitoring in aqueous and adipose tissues, and more motion-insensitive acquisitions for better precision measurements in organs such as the heart, liver, and kidneys. For hyperthermia applications, larger coverage, motion insensitivity, and simultaneous aqueous and adipose monitoring are also important, but great effort is also aimed at solving the problem of long-term field drift which gets interpreted as temperature change when using the PRF method.

Correlation distance dependence of the resonance frequency of intermolecular zero quantum coherences and its implication for MR thermometry

PURPOSE

Because the resonance frequency of water-fat intermolecular zero-quantum coherences (iZQCs) reflects the water-fat frequency separation at the microscopic scale, these frequencies have been proposed and used as a mean to obtain more accurate temperature information. The purpose of this work was to investigate the dependence of the water-fat iZQC resonance frequency on sample microstructure and on the specific choice of the correlation distance.

METHODS

The effect of water-fat susceptibility gradients on the water-methylene iZQC resonance frequency was first computed and then measured for different water-fat emulsions and for a mixture of porcine muscle and fat. Similar measurements were also performed for mixed heteronuclear spin systems.

RESULTS

A strong dependence of the iZQC resonance frequency on the sample microstructure and on the specific choice of the correlation distance was found for spin systems like water and fat that do not mix, but not for spin systems that mix at the molecular level.

CONCLUSIONS

Because water and fat spins do not mix at the molecular level, the water-fat iZQC resonance frequency and its temperature coefficient are not only affected by sample microstructure but also by the specific choice of the correlation distance. Magn Reson Med 79:1429-1438, 2018. © 2017 International Society for Magnetic Resonance in Medicine.

Non-invasive MR thermography using the water proton chemical shift

Among various proton magnetic resonance (MR) parameters, such as longitudinal relaxation time, transverse relaxation time, diffusion coefficient and chemical shift, the chemical shift of water protons is recognized as the most reliable indicator of temperature. The chemical shift is the only frequency-based parameter and is independent of the other parameters, which are measured based on the intensity of the MR signal. In this paper, the basic principle and the recent progress in imaging temperature by spectroscopic techniques using the water proton chemical shift are discussed. The advantages of spectroscopic imaging over phase mapping for measuring temperature are that the former can distinguish water resonance from other resonances, and that another resonance can be used as an internal reference to reduce the effects of external magnetic field instability, tissue susceptibility and inter-scan tissue movement or deformation. Methods utilizing various magnetic resonance spectroscopy (MRS) techniques, such as single voxel spectroscopy, conventional magnetic resonance spectroscopic imaging (MRSI), echo planar spectroscopic imaging (EPSI) and line scan echo planar spectroscopic imaging (LSEPSI) are discussed.

Quantitative MRI-based temperature mapping based on the proton resonant frequency shift: Review of validation studies

MRI-based temperature imaging that exploits the temperature-sensitive water proton resonant frequency shift is currently the only available method for reliable quantification of temperature changes in vivo. Extensive pre-clinical work has been performed to validate this method for guiding thermal therapies. That work has shown the method to be useful for all stages of the thermal therapy, from resolving heating below the threshold for damage to ensuring that the thermal exposure is sufficient within the target volume and protecting surrounding critical structures and to accurately predicting the extent of the ablated volume. In this paper, these validation studies will be reviewed. In addition, clinical studies that have shown this method feasible in human treatments will be overviewed.

Noninvasive monitoring of brain temperature during mild hypothermia

The main purpose of this study was to verify the feasibility of brain temperature mapping with high-spatial- and reduced-spectral-resolution magnetic resonance spectroscopic imaging (MRSI). A secondary goal was to determine the temperature coefficient of water chemical shift in the brain with and without internal spectral reference. The accuracy of the proposed MRSI method was verified using a water and vegetable oil phantom. Selective decrease of the brain temperature of pigs was induced by intranasal cooling. Temperature reductions between 2 degrees C and 4 degrees C were achieved within 20 min. The relative changes in temperature during the cooling process were monitored using MRSI. The reference temperature was measured with MR-compatible fiber-optic probes. Single-voxel (1)H MRS was used for measurement of absolute brain temperature at baseline and at the end of cooling. The temperature coefficient of the water chemical shift of brain tissue measured by MRSI without internal reference was -0.0192+/-0.0019 ppm/degrees C. The temperature coefficients of the water chemical shift relative to N-acetylaspartate, choline-containing compounds and creatine were -0.0096+/-0.0009, -0.0083+/-0.0007 and -0.0091+/-0.0011 ppm/degrees C, respectively. The results of this study indicate that MRSI with high spatial and reduced spectral resolutions is a reliable tool for monitoring long-term temperature changes in the brain.

k-space inherited parallel acquisition (KIPA): application on dynamic magnetic resonance imaging thermometry

In this study, a novel method for dynamic parallel image acquisition and reconstruction is presented. In this method, called k-space inherited parallel acquisition (KIPA), localized reconstruction coefficients are used to achieve higher reduction factors, and lower noise and artifact levels compared to that of generalized autocalibrating partially parallel acquisition (GRAPPA) reconstruction. In KIPA, the full k-space for the first frame and the partial k-space for later frames are required to reconstruct a whole series of images. Reconstruction coefficients calculated for different segments of k-space from the first frame data set are used to estimate missing k-space lines in corresponding k-space segments of other frames. The local determination of KIPA reconstruction coefficients is essential to adjusting them according to the local signal-to-noise ratio characteristics of k-space data. The proposed algorithm is applicable to dynamic imaging with arbitrary k-space sampling trajectories. Simulations of magnetic resonance thermometry using the KIPA method with a reduction factor of 6 and using dynamic imaging studies of human subjects with reduction factors of 4 and 6 have been performed to prove the feasibility of our method and to show apparent improvement in image quality in comparison with GRAPPA for dynamic imaging.

Temperature monitoring with line scan echo planar spectroscopic imaging

A new magnetic resonance imaging method, line scan echo planar spectroscopic imaging (LSEPSI), is shown capable of providing rapid, internally referenced temperature monitoring from water and fat chemical shifts.

METHODS

Orthogonal 90 degrees and 180 degrees slice selective RF pulses inclined by 45 degrees from the image plane solicit a spin echo from a tissue column. The echo is read by asymmetric sampling of 32 gradient echoes spaced 1.4-1.8 ms apart. Sixty-four adjacent columns are sequentially sampled in 4.2-6.4 s with 4,096 voxels sampled with voxel volumes of 0.08-0.13 cm3. Mixed mayonnaise/water phantoms were used to correlate LSEPSI-derived chemical shifts and thermocouple-based temperature measurements from 23 to 60 degrees C with a 1.5 T scanner. Measurement artifacts unrelated to temperature were investigated with the phantom, as was the feasibility of applying the sequence in human breast in vivo.

RESULTS

The correlation between LSEPSI and thermocouple-based temperature measurements in the phantom was excellent (r2>0.99). Field drifts affecting the temperature measurements using the water peak alone were corrected by using the water/lipid peak difference. The sequence had an average temperature resolution of 1.4 degrees C in the phantom. The frequency difference measurement reduced the sensitivity to artifacts related to temperature. Both water and lipid peaks were detectable throughout many locations in the breast, suggesting the applicability of LSEPSI in this organ.

DISCUSSION

T1-saturation losses occur in conventional and echo-planar based 2D CSI sequences using phase encoding methods with short TR periods. These losses are eliminated when individual columns are sampled in snapshot fashion with LSEPSI since the effective TR becomes the time between scans rather than excitations. T1 saturation can make small spectral peaks difficult to detect at high temperatures and generally lowers the signal-to-noise ratio of the spectra. The rapid acquisition and insensitivity to T1 saturation effects make LSEPSI an attractive technique for monitoring thermal therapies in breast using the internally referenced fat/water frequency separation.

Line scan echo planar spectroscopic imaging

A line scan echo planar spectroscopic imaging (LSEPSI) sequence is presented which can rapidly produce 2D chemical shift imaging (CSI) data with minimal relaxation weighting and motion-related artifacts. The technique is based on successive "snapshot" 1D CSI acquisitions of individual tissue columns, and avoids T(1) saturation problems associated with the short TR periods needed for very rapid scanning with either conventional or echo planar-based 2D CSI methods. Potential applications include rapid fat/water spectral quantitation in the abdomen and internally referenced temperature monitoring for interventional procedures.

Quantitative MR temperature monitoring of high-intensity focused ultrasound therapy

A new quantitative method has been developed for real-time mapping of temperature changes induced by high intensity focused ultrasound (HIFU). It is based on the temperature dependence of the T1 relaxation time and the equilibrium magnetization. To calibrate the temperature measurement, the functional relationship between T1 and temperature was examined in different samples of porcine muscle and fatty tissue. The method was validated by a comparison of calculated temperature maps with fiber-optic measurements in heated muscle tissue. The experiment showed that the accuracy of the MR method for temperature measurements is better than 1 degree C. Since the acquisition time of the employed MR sequence takes only 3 s per slice and the calculation of the temperature map can be performed within seconds, the imaging technique works nearly in real-time. The temperature measurement could be realized during HIFU showing no disturbances by ultrasound sonication. In comparison to other MR approaches, the advantages of the introduced method lie in a sufficient accuracy and time resolution combined with a reasonable robustness against motion as well as the feasibility for temperature monitoring in fatty tissues.