Fat-referenced MR thermometry in the breast and prostate using IDEAL

Real-time multislice MR-thermometry of the prostate: Assessment of feasibility, accuracy and sources of biases in patients

PURPOSE

The primary purpose of this study was to evaluate the accuracy of an MR-thermometry sequence for monitoring prostate temperature. The secondary purposes were to analyze clinical and technical factors that may affect accuracy and testing the method in a realistic setting, with MR-guided Laser ablation on an ex vivo muscle sample.

MATERIALS AND METHODS

An ex vivo muscle sample was subjected to Laser ablation while using a two-dimensional multislice segmented echo planar imaging sequence for MR thermometry. The MR thermometry measurements were compared with invasive sensor temperature readings to assess accuracy. Subsequently, 56 men with a median age of 70 years (age range: 53-84 years) who underwent prostate MRI examinations at 1.5- (n = 27) or 3 T (n = 24) were prospectively included. For each patient, the proportion of 'noisy voxels' (i.e., those with a temporal standard deviation of temperature [SD(T)] > 2 °C) in the prostate was calculated. The impact of clinical and technical factors on the proportion of noisy voxels was also examined.

RESULTS

MR-thermometry showed excellent correlation with invasive sensors during MR-guided Laser ablation on the ex vivo muscle sample. The median proportion of noisy voxels per patient in the entire cohort was 1 % (Q1, 0.2; Q3, 4.9; range: 0-90.4). No significant differences in median proportion of noisy voxels were observed between examinations performed at 1.5 T and those at 3 T (P = 0.89 before and after adjustment). No clinical or technical factors significantly influenced the proportion of noisy voxels.

CONCLUSION

Two-dimensional real time multislice MR-thermometry is feasible and accurate for monitoring prostate temperature in patients.

Technical advances in motion-robust MR thermometry

Proton resonance frequency shift (PRFS) MR thermometry is the most common method used in clinical thermal treatments because of its fast acquisition and high sensitivity to temperature. However, motion is the biggest obstacle in PRFS MR thermometry for monitoring thermal treatment in moving organs. This challenge arises because of the introduction of phase errors into the PRFS calculation through multiple methods, such as image misregistration, susceptibility changes in the magnetic field, and intraframe motion during MRI acquisition. Various approaches for motion correction have been developed for real-time, motion-robust, and volumetric MR thermometry. However, current technologies have inherent trade-offs among volume coverage, processing time, and temperature accuracy. These tradeoffs should be considered and chosen according to the thermal treatment application. In hyperthermia treatment, precise temperature measurements are of increased importance rather than the requirement for exceedingly high temporal resolution. In contrast, ablation procedures require robust temporal resolution to accurately capture a rapid temperature rise. This paper presents a comprehensive review of current cutting-edge MRI techniques for motion-robust MR thermometry, and recommends which techniques are better suited for each thermal treatment. We expect that this study will help discern the selection of motion-robust MR thermometry strategies and inspire the development of motion-robust volumetric MR thermometry for practical use in clinics.

Volumetric hyperthermia delivery using the ExAblate Body MR-guided focused ultrasound system

OBJECTIVES

To investigate image-guided volumetric hyperthermia strategies using the ExAblate Body MR-guided focused ultrasound ablation system, involving mechanical transducer movement and sector-vortex beamforming.

MATERIALS AND METHODS

Acoustic and thermal simulations were performed to investigate volumetric hyperthermia using mechanical transducer movement combined with sector-vortex beamforming, specifically for the ExAblate Body transducer. The system control in the ExAblate Body system was modified to achieve fast transducer movement and MR thermometry-based hyperthermia control, mechanical transducer movements and electronic sector-vortex beamforming were combined to optimize hyperthermia delivery. The experimental validation was performed using a tissue-mimicking phantom.

RESULTS

The developed simulation framework allowed for a parametric study with varying numbers of heating spots, sonication durations, and transducer movement times to evaluate the hyperthermia characteristics for mechanical transducer movement and sector-vortex beamforming. Hyperthermic patterns involving 2-4 sequential focal spots were analyzed. To demonstrate the feasibility of volumetric hyperthermia in the system, a tissue-mimicking phantom was sonicated with two distinct spots through mechanical transducer movement and sector-vortex beamforming. During hyperthermia, the average values of Tmax, T10, Tavg, T90, and Tmin over 200 s were measured within a circular ROI with a diameter of 10 pixels. These values were found to be 8.6, 7.9, 6.6, 5.2, and 4.5 °C, respectively, compared to the baseline temperature.

CONCLUSIONS

This study demonstrated the volumetric hyperthermia capabilities of the ExAblate Body system. The simulation framework developed in this study allowed for the evaluation of hyperthermia characteristics that could be implemented with the ExAblate MRgFUS system.

MR Thermometry during Transcranial MR Imaging-Guided Focused Ultrasound Procedures: A Review

Interest in transcranial MR imaging-guided focused ultrasound procedures has recently grown. These incisionless procedures enable precise focal ablation of brain tissue using real-time monitoring by MR thermometry. This article will provide an updated review on clinically applicable technical underpinnings and considerations of proton resonance frequency MR thermometry, the most common clinically used MR thermometry sequence.

Validation of a drift-corrected 3D MR temperature imaging sequence for breast MR-guided focused ultrasound treatments

Real-time temperature monitoring is critical to the success of thermally ablative therapies. This work validates a 3D thermometry sequence with k-space field drift correction designed for use in magnetic resonance-guided focused ultrasound treatments for breast cancer. Fiberoptic probes were embedded in tissue-mimicking phantoms, and temperature change measurements from the probes were compared with the magnetic resonance temperature imaging measurements following heating with focused ultrasound. Precision and accuracy of measurements were also evaluated in free-breathing healthy volunteers (N = 3) under a non-heating condition. MR temperature measurements agreed closely with those of fiberoptic probes, with a 95% confidence interval of measurement difference from -2.0 °C to 1.4 °C. Field drift-corrected measurements in vivo had a precision of 1.1 ± 0.7 °C and were accurate within 1.3 ± 0.9 °C across the three volunteers. The field drift correction method improved precision and accuracy by an average of 46 and 42%, respectively, when compared to the uncorrected data. This temperature imaging sequence can provide accurate measurements of temperature change in aqueous tissues in the breast and support the use of this sequence in clinical investigations of focused ultrasound treatments for breast cancer.

Detection of erosions and fat metaplasia of the sacroiliac joints in patients with suspected sacroiliitis using a chemical shift-encoded sequence (IDEAL-IQ)

PURPOSE

To evaluate the performance of a chemical shift-encoded sequence called IDEAL-IQ for detecting sacroiliac joint (SIJ) erosions and fat metaplasia compared to T1-weighted fast spin echo (T1 FSE) using qualitative and quantitative analysis.

METHOD

Thirty-four patients with suspicion of sacroiliitis who underwent both MRI and CT were included. Each SIJ was divided into four quadrants for analysis. For qualitative analysis, the diagnostic performance of IDEAL-IQ and T1 FSE for erosions were compared by the McNemar test, using CT as the gold standard. Cochran's Q and McNemar tests were used to determine differences in structural changes detected by different imaging methods. For quantitative analysis, two-sample t test and receiver operating characteristic (ROC) analysis were used for the analysis of histogram parameters of proton density fat fraction (PDFF).

RESULTS

Diagnostic sensitivity and accuracy of IDEAL-IQ were greater than T1 FSE for erosions (all P < 0.05). IDEAL-IQ and CT detected more erosions than T1 FSE (all P < 0.05). IDEAL-IQ did not statistically significantly differ from T1 FSE for the detection of fat metaplasia (P = 0.678). All histogram parameters were different between groups with and without fat metaplasia (all P < 0.05) and could distinguish the two groups (all P < 0.05). PDFF_75th was the most effective histogram parameter.

CONCLUSION

IDEAL-IQ detects SIJ erosions with better accuracy than T1 FSE and is similar to T1 FSE for detection of fat metaplasia, enabling further quantitative analysis of the latter via histogram analysis.

Multi-echo gradient echo pulse sequences: which is best for PRFS MR thermometry guided hyperthermia?

PURPOSE

MR thermometry (MRT) enables noninvasive temperature monitoring during hyperthermia treatments. MRT is already clinically applied for hyperthermia treatments in the abdomen and extremities, and devices for the head are under development. In order to optimally exploit MRT in all anatomical regions, the best sequence setup and post-processing must be selected, and the accuracy needs to be demonstrated.

METHODS

MRT performance of the traditionally used double-echo gradient-echo sequence (DE-GRE, 2 echoes, 2D) was compared to multi-echo sequences: a 2D fast gradient-echo (ME-FGRE, 11 echoes) and a 3D fast gradient-echo sequence (3D-ME-FGRE, 11 echoes). The different methods were assessed on a 1.5 T MR scanner (GE Healthcare) using a phantom cooling down from 59 °C to 34 °C and unheated brains of 10 volunteers. In-plane motion of volunteers was compensated by rigid body image registration. For the ME sequences, the off-resonance frequency was calculated using a multi-peak fitting tool. To correct for B0 drift, the internal body fat was selected automatically using water/fat density maps.

RESULTS

The accuracy of the best performing 3D-ME-FGRE sequence was 0.20 °C in phantom (in the clinical temperature range) and 0.75 °C in volunteers, compared to DE-GRE values of 0.37 °C and 1.96 °C, respectively.

CONCLUSION

For hyperthermia applications, where accuracy is more important than resolution or scan-time, the 3D-ME-FGRE sequence is deemed the most promising candidate. Beyond its convincing MRT performance, the ME nature enables automatic selection of internal body fat for B0 drift correction, an important feature for clinical application.

Phase-independent thermometry by Z-spectrum MR imaging

PURPOSE

Z-spectrum imaging, defined as the consecutive collection of images after saturating over a range of frequency offsets, has been recently proposed as a method to measure the fat-water fraction by the simultaneous detection of fat and water resonances. By incorporating a binomial pulse irradiated at each offset before the readout, the spectral selectivity of the sequence can be further amplified, making it possible to monitor the subtle proton resonance frequency shift that follows a change in temperature.

METHODS

We tested the hypothesis in aqueous and cream phantoms and in healthy mice, all under thermal challenge. The binomial module consisted of 2 sinc-shaped pulses of opposite phase separated by a delay. Such a delay served to spread out off-resonance spins, with the resulting excitation profile being a periodic function of the delay and the chemical shift.

RESULTS

During heating experiments, the water resonance shifted downfield, and by fitting the curve to a sine function it was possible to quantify the change in temperature. Results from Z-spectrum imaging correlated linearly with data from conventional MRI techniques like T₁ mapping and phase differences from spoiled GRE.

CONCLUSION

Because the measurement is performed solely on magnitude images, the technique is independent of phase artifacts and is therefore applicable in mixed tissues (e.g., fat). We showed that Z-spectrum imaging can deliver reliable temperature change measurement in both muscular and fatty tissues.

Heat Modulation of Intrinsic MR Contrasts for Tumor Characterization

(1) Background: The longitudinal relaxation time (T1), transverse relaxation time (T2), water proton chemical shift (CS), and apparent diffusion coefficient (ADC) are MR quantities that change with temperature. In this work, we investigate heat-induced intrinsic MR contrast types to add salient information to conventional MR imaging to improve tumor characterization. (2) Methods: Imaging tests were performed in vivo using different rat tumor models. The rats were cooled/heated to steady-state temperatures from 26–36 °C and quantitative measurements of T1, T2, and ADC were obtained. Temperature maps were measured using the proton resonance frequency shift (PRFS) method during the heating and cooling cycles. (3) Results: All tissue samples show repeatable relaxation parameter measurement over a range of 26–36 °C. Most notably, we observed a more than 3.3% change in T1/°C in breast adenocarcinoma tumors compared to a 1% change in benign breast fibroadenoma lesions. In addition, we note distinct values of T2/°C change for rat prostate carcinoma cells compared to benign tissue. (4) Conclusion: These findings suggest the possibility of improving MR imaging visualization and characterization of tissue with heat-induced contrast types. Specifically, these results suggest that the temporal thermal responses of heat-sensitive MR imaging contrast mechanisms in different tissue types contain information for improved (i) characterization of tumor/tissue boundaries for diagnostic and therapy purposes, and (ii) characterization of salient behavior of tissues, e.g., malignant versus benign tumors.

Sonication strategies toward volumetric ultrasound hyperthermia treatment using the ExAblate body MRgFUS system

PURPOSE

The ExAblate body MRgFUS system requires advanced beamforming strategies for volumetric hyperthermia. This study aims to develop and evaluate electronic beam steering, multi-focal patterns, and sector vortex beamforming approaches in conjunction with partial array activation using an acoustic and biothermal simulation framework along with phantom experiments.

METHODS

The simulation framework was developed to calculate the 3D acoustic intensity and temperature distribution resulting from various beamforming and scanning strategies. A treatment cell electronically sweeping a single focus was implemented and evaluated in phantom experiments. The acoustic and thermal focal size of vortex beam propagation was quantified according to the vortex modes, number of active array elements, and focal depth.

RESULTS

Turning off a percentage of the outer array to increase the f-number increased the focal size with a decrease in focal gain. 60% active elements allowed generating a sonication cell with an off-axis of 10 mm. The vortex mode number 4 with 60% active elements resulted in a larger heating volume than using the full array. Volumetric hyperthermia in the phantom was evaluated with the vortex mode 4 and respectively performed with 100% and 80% active elements. MR thermometry demonstrated that the volumes were found to be 18.8 and 29.7 cm³, respectively, with 80% array activation producing 1.58 times larger volume than the full array.

CONCLUSIONS

This study demonstrated that both electronic beam steering and sector vortex beamforming approaches in conjunction with partial array activation could generate large volume heating for HT delivery using the ExAblate body array.

Proton resonance frequency-based thermometry for aqueous and adipose tissues

PURPOSE

The proton resonance frequency (PRF)-based thermometry uses heating-induced phase variations to reconstruct magnetic resonance (MR) temperature maps. However, the measurements of the phase differences may be corrupted by the presence of fat due to its phase being insensitive to heat. The work aims to reconstruct the PRF-based temperature maps for tissues containing fat.

METHODS

This work proposes a PRF-based method that eliminates the fat's phase contribution by estimating the temperature-insensitive fat vector. A vector in a complex domain represents a given voxel's magnetization from an acquired, complex MR image. In this method, a circle was fit to a time series of vectors acquired from a heated region during a heating experiment. The circle center served as the fat vector, which was then subtracted from the acquired vectors, leaving only the temperature-sensitive vectors for thermal mapping. This work was verified with the gel phantoms of 10%, 15%, and 20% fat content and the ex vivo phantom of porcine abdomen tissue during water-bath heating. It was also tested with an ex vivo porcine tissue during focused ultrasound (FUS) heating.

RESULTS

A good agreement was found between the temperature measurements obtained from the proposed method and the optical fiber temperature probe in the verification experiments. In the gel phantoms, the linear regression provided a slope of 0.992 and an R² of 0.994. The Bland-Altman analysis gave a bias of 0.49°C and a 95% confidence interval of ±1.60°C. In the ex vivo tissue, the results of the linear regression and Bland-Altman methods provided a slope of 0.979, an intercept of 0.353, an R² of 0.947, and a 95% confidence interval of ±3.26°C with a bias of -0.14°C. In FUS tests, a temperature discrepancy of up to 28% was observed between the proposed and conventional PRF methods in ex vivo tissues containing fat.

CONCLUSIONS

The proposed PRF-based method can improve the accuracy of the temperature measurements in tissues with fat, such as breast, abdomen, prostate, and bone marrow.

Preliminary analysis of interaction of the fat fraction in the sacroiliac joint among sex, age, and body mass index in a normal Chinese population

Objective

Iterative decomposition of water and fat with echo asymmetry and least-squares estimation-iron quantification (IDEAL-IQ) is a noninvasive and objective method used to quantitatively measure fat content. Although this technique has been used in the entire abdomen, IDEAL-IQ findings in the sacroiliac joint (SIJ) have rarely been reported. This preclinical study was performed to quantify the amount of fat in the SIJ in healthy volunteers by IDEAL-IQ.

Methods

From April to November 2017, 60 healthy volunteers with low back pain were included in this retrospective study. The participants were allocated into groups by age (15–30, 31–50, and ≥51 years), sex (male and female), and body mass index (BMI) (<18.5, 18.5–23.9, and ≥24.0 kg/m²). The iliac-side (Fi) and sacral-side (Fs) fat fractions were obtained in all groups. Two- and three-factor multivariate analyses were performed to analyze the effects of sex, age, and BMI on the Fi and Fs.

Results

The interaction among sex, age, and BMI had no statistically significant effect on the dependent variable. Both Fi and Fs were significantly influenced by age. Fs was significantly influenced by sex.

Conclusion

The IDEAL-IQ sequence can be used to quantitatively assess the SIJ fat content in healthy volunteers.

Contactless Thermometry by MRI and MRS: Advanced Methods for Thermotherapy and Biomaterials

Control of temperature variation is of primordial importance in particular areas of biomedicine. In this context, medical treatments such as hyperthermia and cryotherapy, and also the development and use of hydrogel-based biomaterials, are of particular concern. To enable accurate temperature measurement without perturbing or even destroying the biological tissue or material to be monitored, contactless thermometry methods are preferred. Among these, the most suitable are based on magnetic resonance imaging and spectroscopy (MRI, MRS). Here, we address the latest developments in this field as well as their current and anticipated practical applications. We highlight recent progress aimed at rendering MR thermometry faster and more reproducible, versatile, and sophisticated and provide our perspective on how these new techniques broaden the range of applications in medical treatments and biomaterial development by enabling insight into finer details of thermal behavior. Thus, these methods facilitate optimization of clinical and industrial heating and cooling protocols.

Phantom Validation of DCE-MRI Magnitude and Phase-Based Vascular Input Function Measurements

Accurate, patient-specific measurement of arterial input functions (AIF) may improve model-based analysis of vascular permeability. This study investigated factors affecting AIF measurements from magnetic resonance imaging (MRI) magnitude (AIF_MAGN) and phase (AIF_PHA) signals, and compared them against computed tomography (CT) (AIF_CT), under controlled conditions relevant to clinical protocols using a multimodality flow phantom. The flow phantom was applied at flip angles of 20° and 30°, flow rates (3-7.5 mL/s), and peak bolus concentrations (0.5-10 mM), for in-plane and through-plane flow. Spatial 3D-FLASH signal and variable flip angle T1 profiles were measured to investigate in-flow and radiofrequency-related biases, and magnitude- and phase-derived Gd-DTPA concentrations were compared. MRI AIF performance was tested against AIF_CT via Pearson correlation analysis. AIF_MAGN was sensitive to imaging orientation, spatial location, flip angle, and flow rate, and it grossly underestimated AIF_CT peak concentrations. Conversion to Gd-DTPA concentration using T1 taken at the same orientation and flow rate as the dynamic contrast-enhanced acquisition improved AIF_MAGN accuracy; yet, AIF_MAGN metrics remained variable and significantly reduced from AIF_CT at concentrations above 2.5 mM. AIF_PHA performed equivalently within 1 mM to AIF_CT across all tested conditions. AIF_PHA, but not AIF_MAGN, reported equivalent measurements to AIF_CT across the range of tested conditions. AIF_PHA showed superior robustness.

Simultaneous fat-referenced proton resonance frequency shift thermometry and MR elastography for the monitoring of thermal ablations

PURPOSE

Simultaneous fat-referenced proton resonance frequency shift (FRPRFS) thermometry combined with MR elastography (MRE) is proposed, to continuously monitor thermal ablations for all types of soft tissues, including fat-containing tissues. Fat-referenced proton resonance frequency shift thermometry makes it possible to measure temperature even in the water fraction of fat-containing tissues while enabling local field-drift correction. Magnetic resonance elastography allows measuring the mechanical properties of tissues that are related to tissue structural damage.

METHODS

A gradient-echo MR sequence framework was proposed that combines the need for multiple TE acquisitions for the water-fat separation of FRPRFS, and the need for multiple MRE phase offsets for elastogram reconstructions. Feasibility was first assessed in a fat-containing gelatin phantom undergoing moderate heating by a hot water circulation system. Subsequently, high intensity focused ultrasound heating was conducted in porcine muscle tissue ex vivo (N = 4; 2 samples, 2 locations/sample).

RESULTS

Both FRPRFS temperature maps and elastograms were updated every 4.1 seconds. In the gelatin phantom, FRPRFS was in good agreement with optical fiber thermometry (average difference 1.2 ± 1°C). In ex vivo high-intensity focused ultrasound experiments on muscle tissue, the shear modulus was found to decrease significantly by 34.3% ± 7.7% (experiment 1, sample 1), 17.9% ± 10.0% (experiment 2, sample 1), 55.1% ± 8.7% (experiment 3, sample 2), and 34.7% ± 8.4% (experiment 4, sample 2) as a result of temperature increase (ΔT = 22.5°C ± 4.2°C, 14.0°C ± 2.8°C, 14.7°C ± 3.7°C, and 14.5°C ± 3.0°C, respectively).

CONCLUSION

This study demonstrated the feasibility of monitoring thermal ablations with FRPRFS thermometry together with MRE, even in fat-containing tissues. The acquisition time is similar to non-FRPRFS thermometry combined with MRE.

A variable flip angle golden-angle-ordered 3D stack-of-radial MRI technique for simultaneous proton resonant frequency shift and T1 -based thermometry

PURPOSE

To develop and evaluate a variable-flip-angle golden-angle-ordered 3D stack-of-radial MRI technique for simultaneous proton resonance frequency shift (PRF) and T₁ -based thermometry in aqueous and adipose tissues, respectively.

METHODS

The proposed technique acquires multiecho radial k-space data in segments with alternating flip angles to measure 3D temperature maps dynamically on the basis of PRF and T₁ . A sliding-window k-space weighted image contrast filter is used to increase temporal resolution. PRF is measured in aqueous tissues and T₁ in adipose tissues using fat/water masks. The accuracy for T₁ quantification was evaluated in a reference T₁ /T₂ phantom. In vivo nonheating experiments were conducted in healthy subjects to evaluate the stability of PRF and T₁ in the brain, prostate, and breast. The proposed technique was used to monitor high-intensity focused ultrasound (HIFU) ablation in ex vivo porcine fat/muscle tissues and compared to temperature probe readings.

RESULTS

The proposed technique achieved 3D coverage with 1.1-mm to 1.3-mm in-plane resolution and 2-s to 5-s temporal resolution. During 20 to 30 min of nonheating in vivo scans, the temporal coefficient of variation for T₁ was <5% in the brain, prostate, and breast fatty tissues, while the standard deviation of relative PRF temperature change was within 3°C in aqueous tissues. During ex vivo HIFU ablation, the temperatures measured by PRF and T₁ were consistent with temperature probe readings, with an absolute mean difference within 2°C.

CONCLUSION

The proposed technique achieves simultaneous PRF and T₁ -based dynamic 3D MR temperature mapping in aqueous and adipose tissues. It may be used to improve MRI-guided thermal procedures.

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.

A phase-cycled temperature-sensitive fast spin echo sequence with conductivity bias correction for monitoring of mild RF hyperthermia with PRFS

OBJECTIVE

Mild hyperthermia (HT) treatments are generally monitored by phase-referenced proton resonance frequency shift calculations. A novel phase and thus temperature-sensitive fast spin echo (TFSE) sequence is introduced and compared to the double echo gradient echo (DEGRE) sequence.

THEORY AND METHODS

For a proton resonance frequency shift (PRFS)-sensitive TFSE sequence, a phase cycling method is applied to separate even from odd echoes. This method compensates for conductivity change-induced bias in temperature mapping as does the DEGRE sequence. Both sequences were alternately applied during a phantom heating experiment using the clinical setup for deep radio frequency HT (RF-HT). The B₀ drift-corrected temperature values in a region of interest around temperature probes are compared to the temperature probe data and further evaluated in Bland-Altman plots. The stability of both methods was also tested within the thighs of three volunteers at a constant temperature using the subcutaneous fat layer for B₀-drift correction.

RESULTS

During the phantom heating experiment, on average TFSE temperature maps achieved double temperature-to-noise ratio (TNR) efficiency in comparison with DEGRE temperature maps. In-vivo images of the thighs exhibit stable temperature readings of ± 1 °C over 25 min of scanning in three volunteers for both methods. On average, the TNR efficiency improved by around 25% for in vivo data.

CONCLUSION

A novel TFSE method has been adapted to monitor temperature during mild HT.

Multi-echo MR thermometry using iterative separation of baseline water and fat images

PURPOSE

To perform multi-echo water/fat separated proton resonance frequency (PRF)-shift temperature mapping.

METHODS

State-of-the-art, iterative multi-echo water/fat separation algorithms produce high-quality water and fat images in the absence of heating but are not suitable for real-time imaging due to their long compute times and potential errors in heated regions. Existing fat-referenced PRF-shift temperature reconstruction methods partially address these limitations but do not address motion or large time-varying and spatially inhomogeneous B₀ shifts. We describe a model-based temperature reconstruction method that overcomes these limitations by fitting a library of separated water and fat images measured before heating directly to multi-echo data measured during heating, while accounting for the PRF shift with temperature.

RESULTS

Simulations in a mixed water/fat phantom with focal heating showed that the proposed algorithm reconstructed more accurate temperature maps in mixed tissues compared to a fat-referenced thermometry method. In a porcine phantom experiment with focused ultrasound heating at 1.5 Tesla, temperature maps were accurate to within 1^∘ C of fiber optic probe temperature measurements and were calculated in 0.47 s per time point. Free-breathing breast and liver imaging experiments demonstrated motion and off-resonance compensation. The algorithm can also accurately reconstruct water/fat separated temperature maps from a single echo during heating.

CONCLUSIONS

The proposed model-based water/fat separated algorithm produces accurate PRF-shift temperature maps in mixed water and fat tissues in the presence of spatiotemporally varying off-resonance and motion.

Dual-step iterative temperature estimation method for accurate and precise fat-referenced PRFS temperature imaging

PURPOSE

The aim of this study was to propose dual-step iterative temperature estimation (DITE) of a fat-referenced proton resonance frequency shift (PRFS) method to improve both the accuracy and precision of temperature estimations in fat-containing tissues.

METHODS

A fat-water signal model with multiple fat peaks was used to simultaneously estimate the temperature, fat/water intensity and <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msubsup><mml:mtext>T</mml:mtext> <mml:mrow><mml:mn>2</mml:mn></mml:mrow> <mml:mrow><mml:mrow/> <mml:mo>∗</mml:mo></mml:mrow> </mml:msubsup> </mml:math> , and field offset. In DITE, model fitting was implemented with alternating 2-step minimizations. The estimated temperature map was smoothed between the 2-step minimizations, which is considered to be the most important step for improving the temperature precision. The performance of DITE was evaluated with a Monte Carlo simulation, fat/water phantoms, and ex vivo brown adipose tissue experiments and then compared with the performance of previous fat-referenced proton resonance frequency shift methods.

RESULTS

In fat/water phantom experiment with a smooth temperature profile, the temperatures estimated by DITE are consistent with the thermometer results and present a better accuracy and precision than those of previous fat-referenced proton resonance frequency shift methods. In the brown adipose tissue heating experiment, the average mean error, SD, and RMS error were -0.08ºC, 0.46ºC, and 0.56ºC, respectively, over all of the measurements within the region of interest.

CONCLUSION

Our proposed DITE method improves both the accuracy and precision of temperature measurements in tissues with fat fractions between 20% and 80% under smooth distribution of the temperature profile and represents a simple fat-referenced thermometry method.

Efficacy of 3D VIBE Dixon fat quantification for differentiating clear-cell from non-clear-cell renal cell carcinoma

AIM

To assess the efficacy of three-dimensional (3D) volumetric interpolated breath-hold examination (VIBE) magnetic resonance imaging (MRI) with Dixon quantification for differentiating clear-cell from non-clear-cell types of renal cell carcinoma (RCC).

MATERIALS AND METHODS

The 3D VIBE Dixon renal MRI examinations of 44 patients with 45 histologically confirmed RCCs was analysed. The fat fractions and signal intensity indexes (SI_index) of the solid portions of clear-cell and non-clear-cell RCCs were measured and compared using Student's t-test and receiver operating characteristic (ROC) curves. The agreement of measurements among observers was evaluated by the intraclass correlation coefficient (ICC), and Bland-Altman plots.

RESULTS

The mean values of fat fraction (13.16±7.16%) and SI_index (22.64±15.7%) in clear-cell RCCs were significantly higher than that in non-clear-cell RCCs (7.7±2% and 7.9±4.8%; p<0.001, respectively). With the area under the ROC curve (AUC) of the fat fraction at 0.811, 75% (95% CI: 55.1-89.43%) sensitivity and 76.5% (95% CI: 50.1-93.2%) specificity for diagnosing clear-cell RCC were obtained at a cut-off fat fraction value of 8.9%. With a cut-off value of 8.89%, the diagnostic sensitivity and specificity were 85.7% (95% CI: 67.3-96%) and 70.6% (95% CI: 44-89.7%), respectively. The AUC of the SI_index was 0.870 (0.766-0.973). ICC and Bland-Altman plots show excellent agreement of the tumour fat fraction and SI_index measurement between the two observers.

CONCLUSION

Intracellular lipid content analysis using the 3D Dixon technique can help to differentiate clear-cell from non-clear-cell RCCs.

Referenced MR thermometry using three-echo phase-based fat water separation method

A three-point image reconstruction method for internally referenced MR thermometry was developed. The technique exploits the fact that temperature-induced changes in the water resonance frequency are small relative to the chemical shift difference between water and fat signals. This property enabled the use of small angle approximations to derive an analytic phase-based fat-water separation method for MR thermometry. Ethylene glycol and cream cool-down experiments were performed to validate measurement technique. Over a cool-down temperature range of 20 °C, maximum deviation between probe and MR measurement (averaged over 1.3 cm³ region surrounding probe) was 0.6 °C and 1.1 °C for ethylene glycol and cream samples, respectively.

In Vivo Assessment of Neurodegeneration in Type C Niemann-Pick Disease by IDEAL-IQ

Objective

To noninvasively assess the neurodegenerative changes in the brain of patients with Niemann-Pick type C (NPC) disease by measuring the lesion tissue with the iterative decomposition of water and fat with echo asymmetry and least square estimation-iron quantification (IDEAL-IQ).

Materials and Methods

Routine brain MRI, IDEAL-IQ and ¹H-proton magnetic resonance spectroscopy (¹H-MRS, served as control) were performed on 12 patients with type C Niemann-Pick disease (4 males and 8 females; age range, 15–61 years; mean age, 36 years) and 20 healthy subjects (10 males and 10 females; age range, 20–65 years; mean age, 38 years). The regions with lesion and the normal appearing regions (NARs) of patients were measured and analyzed based on the fat/water signal intensity on IDEAL-IQ and the lipid peak on ¹H-MRS.

Results

Niemann-Pick type C patients showed a higher fat/water signal intensity ratio with IDEAL-IQ on T2 hyperintensity lesions and NARs (3.7–4.9%, p < 0.05 and 1.8–3.0%, p < 0.05, respectively), as compared to healthy controls (HCs) (1.2–2.3%). After treatment, the fat/water signal intensity ratio decreased (2.2–3.4%), but remained higher than in the HCs (p < 0.05). The results of the ¹H-MRS measurements showed increased lipid peaks in the same lesion regions, and the micro-lipid storage disorder of NARs in NPC patients was detectable by IDEAL-IQ instead of ¹H-MRS.

Conclusion

The findings of this study suggested that IDEAL-IQ may be useful as a noninvasive and objective method in the evaluation of patients with NPC; additionally, IDEAL-IQ can be used to quantitatively measure the brain parenchymal adipose content and monitor patient follow-up after treatment of NPC.

Quantification of fat infiltration in the sacroiliac joints with ankylosing spondylitis using IDEAL sequence

AIM

To quantitatively assess fat infiltration in the sacroiliac joints (SIJs) of patients with ankylosing spondylitis (AS) by measuring the fat/water signal ratios of periarticular bone marrow with iterative decomposition of water and fat with echo asymmetry and least square estimation (IDEAL).

MATERIALS AND METHODS

Routine SIJ magnetic resonance imaging (MRI) and IDEAL were performed on 40 patients with AS and 30 healthy subjects. The fat infiltration regions (FIRs) and normal-appearing regions (NARs) of patients were measured based on the fat/water signal intensity on IDEAL.

RESULTS

AS patients had higher fat/water signal ratios on FIRs and NARs (65.4-85.4%, p<0.05, and 44.1-70.7%, p<0.05, respectively) compared to healthy controls (38.3-43.3%). After treatment, the fat/water signal ratios of FIRs and NARs decreased (42.1-53.7% and 41.5-50.3%, respectively), but they remained higher than in the healthy controls (p<0.05). The fat infiltration was detected more effectively with a fat fraction map of the IDEAL sequence (95%) than other sequences, including the T1-weighted sequence (65%), and the fat/water signal ratios of the sacrum and ilium between the left and right sides of SIJs were approximately the same.

CONCLUSION

The findings of this study suggest that IDEAL may be useful as a quantitative and objective method for evaluating the fat infiltration in the periarticular bone marrow of SIJs with AS; additionally, the sensitivity of IDEAL is better than that of routine sequences in detecting micro-fat infiltration of SIJs, and IDEAL can be used to quantitatively measure the adipose content and monitor patient follow-up after AS treatment.

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.

Time-multiplexed two-channel capacitive radiofrequency hyperthermia with nanoparticle mediation

BACKGROUND

Capacitive radiofrequency (RF) hyperthermia suffers from excessive temperature rise near the electrodes and poorly localized heat transfer to the deep-seated tumor region even though it is known to have potential to cure ill-conditioned tumors. To better localize heat transfer to the deep-seated target region in which electrical conductivity is elevated by nanoparticle mediation, two-channel capacitive RF heating has been tried on a phantom.

METHODS

We made a tissue-mimicking phantom consisting of two compartments, a tumor-tissue-mimicking insert against uniform background agarose. The tumor-tissue-mimicking insert was made to have higher electrical conductivity than the normal-tissue-mimicking background by applying magnetic nanoparticle suspension to the insert. Two electrode pairs were attached on the phantom surface by equal-angle separation to apply RF electric field to the phantom. To better localize heat transfer to the tumor-tissue-mimicking insert, RF power with a frequency of 26 MHz was delivered to the two channels in a time-multiplexed way. To monitor the temperature rise inside the phantom, MR thermometry was performed at a 3T MRI intermittently during the RF heating. Finite-difference-time-domain (FDTD) electromagnetic and thermal simulations on the phantom model were also performed to verify the experimental results.

RESULTS

As compared to the one-channel RF heating, the two-channel RF heating with time-multiplexed driving improved the spatial localization of heat transfer to the tumor-tissue-mimicking region in both the simulation and experiment. The two-channel RF heating also reduced the temperature rise near the electrodes significantly.

CONCLUSIONS

Time-multiplexed two-channel capacitive RF heating has the capability to better localize heat transfer to the nanoparticle-mediated tumor region which has higher electrical conductivity than the background normal tissues.

Influence of water and fat heterogeneity on fat-referenced MR thermometry

PURPOSE

To investigate the effect of the aqueous and fatty tissue magnetic susceptibility distribution on absolute and relative temperature measurements as obtained directly from the water/fat (w/f) frequency difference.

METHODS

Absolute thermometry was investigated using spherical phantoms filled with pork and margarine, which were scanned in three orthogonal orientations. To evaluate relative fat referencing, multigradient echo scans were acquired before and after heating pork tissue via high-intensity focused ultrasound (HIFU). Simulations were performed to estimate the errors that can be expected in human breast tissue.

RESULTS

The sphere experiment showed susceptibility-related errors of 8.4 °C and 0.2 °C for pork and margarine, respectively. For relative fat referencing measurements, fat showed pronounced phase changes of opposite polarity to aqueous tissue. The apparent mean temperature for a numerical breast model assumed to be 37 °C was 47.2 ± 21.6 °C. Simulations of relative fat referencing for a HIFU sonication (ΔT = 29.7 °C) yielded a maximum temperature error of 6.6 °C compared with 2.5 °C without fat referencing.

CONCLUSION

Variations in the observed frequency difference between water and fat are largely due to variations in the w/f spatial distribution. This effect may lead to considerable errors in absolute MR thermometry. Additionally, fat referencing may exacerbate rather than correct for proton resonance frequency shift-temperature measurement errors.

Validation of MR thermometry: method for temperature probe sensor registration accuracy in head and neck phantoms

PURPOSE

Magnetic resonance thermometry (MRT) is an attractive means to non-invasively monitor in vivo temperature during head and neck hyperthermia treatments because it can provide multi-dimensional temperature information with high spatial resolution over large regions of interest. However, validation of MRT measurements in a head and neck clinical set-up is crucial to ensure the temperature maps are accurate. Here we demonstrate a unique approach for temperature probe sensor localisation in head and neck hyperthermia test phantoms.

METHODS

We characterise the proton resonance frequency shift temperature coefficient and validate MRT measurements in an oil-gel phantom by applying a combination of MR imaging and 3D spline fitting for accurate probe localisation. We also investigate how uncertainties in both the probe localisation and the proton resonance frequency shift (PRFS) thermal coefficient affect the registration of fibre-optic reference temperature probe and MRT readings.

RESULTS

The method provides a two-fold advantage of sensor localisation and PRFS thermal coefficient calibration. We provide experimental data for two distinct head and neck phantoms showing the significance of this method as it mitigates temperature probe localisation errors and thereby increases accuracy of MRT validation results.

CONCLUSIONS

The techniques presented here may be used to simplify calibration experiments that use an interstitial heating device, or any heating method that provides rapid and spatially localised heat distributions. Overall, the experimental verification of the data registration and PRFS thermal coefficient calibration technique provides a useful benchmarking method to maximise MRT accuracy in any similar context.

Measurement of SAR-induced temperature increase in a phantom and in vivo with comparison to numerical simulation

PURPOSE

To compare numerically simulated and experimentally measured temperature increase due to specific energy absorption rate from radiofrequency fields.

METHODS

Temperature increase induced in both a phantom and in the human forearm when driving an adjacent circular surface coil was mapped using the proton resonance frequency shift technique of magnetic resonance thermography. The phantom and forearm were also modeled from magnetic resonance image data, and both specific energy absorption rate and temperature change as induced by the same coil were simulated numerically.

RESULTS

The simulated and measured temperature increase distributions were generally in good agreement for the phantom. The relative distributions for the human forearm were very similar, with the simulations giving maximum temperature increase about 25% higher than measured.

CONCLUSION

Although a number of parameters and uncertainties are involved, it should be possible to use numerical simulations to produce reasonably accurate and conservative estimates of temperature distribution to ensure safety in magnetic resonance imaging. R01 EB006563