Tissue temperature monitoring with multiple gradient-echo imaging sequences

Effectiveness of fat suppression methods and influence on proton-resonance frequency shift (PRFS) MR thermometry

PURPOSE

To evaluate the effectiveness of fat suppression techniques experimentally and illustrate their influence on the accuracy of PRFS MR-thermometry.

METHODS

The residual magnitudes of the main fat peaks are measured using a water-fat decomposition algorithm in an oil phantom and in vivo in swine bone marrow, either with spectral fat saturation (FS), water excitation (WE) or fast water excitation (FWE), as implemented on 1.5 T whole-body clinical MRIs. Thermometry experiments in tissue-mimicking oil-water phantoms (10 and 30 % fat) allow determining temperature errors in PRFS MR-thermometry with no fat suppression, FS and WE, compared against reference fiber optic thermometry.

RESULTS

WE attenuates the signal of the main methylene fat peak more than FS (2 % and 22 % amplitude attenuation in the oil phantom, respectively), while the olefinic and glycerol peaks surrounding the water peak remain unaltered with both FS and WE. Within the 37 °C to 60 °C temperature range explored, FS and WE strongly attenuate temperature errors compared to PRFS without fat suppression. The residual fat signal after FS and WE leads to errors in PRFS thermometry, that increase with the fat content and oscillate with TE and temperature. In our tests limited to a single MR provider, fat suppression with WE appears to suppress fat signal more effectively.

CONCLUSIONS

We propose a protocol to quantify the remaining fraction of each spectral fat peak after fat suppression. In PRFS thermometry, despite spectral fat suppression, the remnant fat signal leads to temperature underestimation or overestimation depending on TE, fat fraction and temperature range. Fat suppression techniques should be evaluated specifically for quantitative MRI methods such as PRFS thermometry.

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.

Improved MR temperature imaging at 0.5 T using view-sharing accelerated multiecho thermometry for MR-guided laser interstitial thermal therapy

The aim of the current study was to improve temperature-monitoring precision using multiecho proton resonance frequency shift-based thermometry with view-sharing acceleration for MR-guided laser interstitial thermal therapy (MRgLITT) on a 0.5-T low-field MR system. Both precision and speed of the temperature measurement for clinical MRgLITT treatments suffer at low field, due to reduced image signal-to-noise ratio (SNR), decreased temperature-induced phase changes, and limited RF receiver channels. In this work, a bipolar multiecho gradient-recalled echo sequence with a temperature-to-noise ratio optimal weighted echo combination is applied to improve the temperature precision. A view-sharing-based approach is utilized to accelerate signal acquisitions while preserving image SNRs. The method was evaluated using ex vivo (pork and pig brain) LITT heating experiments and in vivo (human brain) nonheating experiments on a high-performance 0.5-T scanner. In terms of results, (1) after echo combination, multiecho thermometry (i.e., ~7.5-40.5 ms, 7 TEs) provides ~1.5-1.9 times higher temperature precision than the no echo combination case (i.e., TE7 = 40.5 ms) within the same readout bandwidth. Additionally, echo registration is necessary for the bipolar multiecho sequence; (2) for a threefold acceleration, the view-sharing approach with variable-density subsampling shows around 1.8 times lower temperature errors than the GRAPPA method. Particularly for view-sharing, variable-density subsampling performs better than Interleave subsampling; and (3) ex vivo heating and in vivo nonheating experiments demonstrated that the temperature accuracy was less than 0.5 ° C $$ {}^{{}^{\circ}}\mathrm{C} $$ and that the temperature precision was less than 0.6 ° C $$ {}^{{}^{\circ}}\mathrm{C} $$ using the proposed 0.5-T thermometry. It was concluded that view-sharing accelerated multiecho thermometry is a practical temperature measurement approach for MRgLITT at 0.5 T.

Iron-based coupling media for MRI-guided ultrasound surgery

PURPOSE

In this study, we examine the effects of a recently developed, iron-based coupling medium (IBCM) on guidance magnetic resonance (MR) scans during transcranial, magnetic-resonance-guided, focused ultrasound surgery (tMRgFUS) procedures. More specifically, this study tests the hypotheses that the use of the IBCM will (a) not adversely affect image quality, (b) remove aliasing from small field-of-view scans, and (c) decouple image quality from the motion state of the coupling fluid.

METHODS

An IBCM, whose chemical synthesis and characterization are reported elsewhere, was used as a coupling medium during tMRgFUS procedures on gel phantoms. Guidance magnetization-prepared rapid-gradient-echo (MP-RAGE), TSE, and GRE scans were acquired with fields of view of 28 and 18 cm. Experiments were repeated with the IBCM in several distinct flow states. GRE scans were used to estimate temperature time courses as a gel target was insonated. IBCM performance was measured by computing (i) the root mean square difference (RMSD) of TSE and GRE pixel values acquired using water and the IBCM, relative to the use of water; (ii) through-time temperature uncertainty for GRE scans; and (iii) Bland-Altman analysis of the temperature time courses. Finally, guidance TSE and GRE scans of a human volunteer were acquired during a separate sham tMRgFUS procedure. As a control, all experiments were repeated using a water coupling medium.

RESULTS

Use of the IBCM reduced RMSD in TSE scans by a factor of 4 or more for all fields of view and nonstationary motion states, but did not reduce RMSD estimates in MP-RAGE scans. With the coupling media in a stationary state, the IBCM altered estimates of temperature uncertainty relative to the use of water by less than 0.2°C. However, with a high flow state, the IBCM reduced temperature uncertainties by the statistically significant amounts (at the 0.01 level) of 0.5°C (28 cm field of view) and 5°C (18 cm field of view). Bland-Altman analyses found a 0.1°C ± 0.5°C difference between temperature estimates acquired using water and the IBCM as coupling media. Finally, scans of a human volunteer using the IBCM indicate more conspicuous grey/white matter contrast, a reduction in aliasing, and a less than 0.2°C change in temperature uncertainty.

CONCLUSIONS

The use of an IBCM during tMRgFUS procedures does not adversely affect image quality for TSE and GRE scans, can decouple image quality from the motion state of the coupling fluid, and can remove aliasing from scans where the field of view is set to be much smaller than the spatial extent of the coupling fluid.

Assessment of radio-frequency heating of a parallel transmit coil in a phantom using multi-echo proton resonance frequency shift thermometry

We propose a workflow for validating parallel transmission (pTx) radio-frequency (RF) magnetic field heating patterns using Proton-Resonance Frequency shift (PRF)-based MR thermometry. Electromagnetic (EM) and thermal simulations of a 7 T 8-channel dipole coil were done using commercially available software (Sim4Life) to assess RF heating. The fabrication method for a phantom with electrical properties matched to human tissue is also described, along with methods for its electrical and thermal characterisation. Energy was deposited to specific transmit channels, whilst acquiring 3D PRF data using a pair of interleaved RF shim transmit modes. A multi-echo readout and pre-scan stabilisation protocol were used for increased sensitivity and to correct for measurement-to-measurement instabilities. The electrical properties of the phantom were found to be within 10% of the intended values. Adoption of a 14-min stabilisation scan gave sufficient suppression of any evolving background spatial variation in the B₀ field to achieve <0.001 °C/mm thermometry drift over 10 min of subsequent scanning. Using two RF shim transmit modes enabled full phantom coverage and combining multiple echo times enabled a 13-54% improvement in the RMSE sensitivity to temperature changes. Combining multiple echoes reduced the peak RMSE by 45% and visually reduced measurement-to-measurement instabilities. A reference fibre optic probe showed temperature deviations from the PRF-estimated temperature to be smaller than 0.5 °C. Given the importance of RF safety in pTx applications, this workflow enables accurate validation of RF heating simulations with minimal additional hardware requirements.

Volumetric MRI thermometry using a three-dimensional stack-of-stars echo-planar imaging pulse sequence

PURPOSE

To measure temperature over a large brain volume with fine spatiotemporal resolution.

METHODS

A three-dimensional stack-of-stars echo-planar imaging sequence combining echo-planar imaging and radial sampling with golden angle spacing was implemented at 3T for proton resonance frequency-shift temperature imaging. The sequence acquires a 188x188x43 image matrix with 1.5x1.5x2.75 mm3 spatial resolution. Temperature maps were reconstructed using sensitivity encoding (SENSE) image reconstruction followed by the image domain hybrid method, and using the k-space hybrid method. In vivo temperature maps were acquired without heating to measure temperature precision in the brain, and in a phantom during high-intensity focused ultrasound sonication.

RESULTS

In vivo temperature standard deviation was less than 1°C at dynamic scan times down to 0.75 s. For a given frame rate, scanning at a minimum repetition time (TR) with minimum acceleration yielded the lowest standard deviation. With frame rates around 3 s, the scan was tolerant to a small number of receive coils, and temperature standard deviation was 48% higher than a standard two-dimensional Fourier transform temperature mapping scan, but provided whole-brain coverage. Phantom temperature maps with no visible aliasing were produced for dynamic scan times as short as 0.38 s. k-Space hybrid reconstructions were more tolerant to acceleration.

CONCLUSION

Three-dimensional stack-of-stars echo-planar imaging temperature mapping provides volumetric brain coverage and fine spatiotemporal resolution. Magn Reson Med 79:2003-2013, 2018. © 2017 International Society for Magnetic Resonance in Medicine.

Specialized volumetric thermometry for improved guidance of MRgFUS in brain

PURPOSE

MR thermometry is critical for safe and effective transcranial focused ultrasound. The current single-slice MR thermometry sequence cannot achieve all desired treatment monitoring requirements. We propose an approach in which the imaging requirements of different aspects of treatment monitoring are met by optimizing multiple sequences.

METHODS

Imaging requirements were determined for three stages of MR-guided focused ultrasound brain treatment: 1) focal spot localization, 2) focal spot monitoring, and 3) background monitoring. Multiple-echo spiral thermometry sequences were optimized for each set of requirements and then validated with in vivo signal-to-noise ratio measurements and with phantom heating experiments.

RESULTS

Each of the proposed sequences obtained better precision than the current two-dimensional Fourier transform (2DFT) thermometry sequence. Five-slice focal spot localization achieved two-fold better resolution with 1.9-fold better precision but two-fold longer acquisition compared to 2DFT. Five-slice focal monitoring achieved 2.1-fold better precision with similar speed but 12% larger voxels than 2DFT. Full-brain background monitoring was demonstrated in both axial (7.1 s) and sagittal (11.4 s) orientations. Phantom heating time curves were consistent across all sequences after correcting for resolution.

CONCLUSION

Multiple-echo spiral imaging significantly improves MR thermometry efficiency, enabling multiple-slice monitoring. Optimizing multiple specialized sequences provides better performance than can be achieved by any single sequence. Magn Reson Med 78:508-517, 2017. © 2016 International Society for Magnetic Resonance in Medicine.

Improved MRI thermometry with multiple-echo spirals

PURPOSE

Low-bandwidth PRF shift thermometry is used to guide HIFU ablation treatments. Low sampling bandwidth is needed for high signal-to-noise ratio with short acquisition times, but can lead to off-resonance artifacts. In this work, improved multiple-echo thermometry is presented that allows for high bandwidth and reduced artifacts. It is also demonstrated with spiral sampling, to improve the trade-off between resolution, speed, and measurement precision.

METHODS

Four multiple-echo thermometry sequences were tested in vivo, one using two-dimensional Fourier transform (2DFT) sampling and three using spirals. The spiral sequences were individually optimized for resolution, for speed, and for precision. Multifrequency reconstruction was used to correct for off-resonance spiral artifacts. Additionally, two different multiecho temperature reconstructions were compared.

RESULTS

Weighted combination of per-echo phase differences gave significantly better precision than least squares off-resonance estimation. Multiple-echo 2DFT sequence obtained precision similar to single-echo 2DFT, while greatly increasing sampling bandwidth. The multiecho spiral acquisitions achieved 2× better resolution, 2.9× better uncertainty, or 3.4× faster acquisition time, without negatively impacting the other two design parameters as compared to single-echo 2DFT.

CONCLUSION

Multiecho spiral thermometry greatly improves the capabilities of temperature monitoring, and could improve transcranial treatment monitoring capabilities. Magn Reson Med 76:747-756, 2016. © 2015 Wiley Periodicals, Inc.

Accelerated model-based proton resonance frequency shift temperature mapping using echo-based GRAPPA reconstruction

PURPOSE

To develop an acceleration method for MR temperature estimation using model-based proton resonance frequency (PRF) shift method.

MATERIALS AND METHODS

Images of 16 different echo times (TE) were acquired in one RF excitation using a multi-echo gradient-recalled echo (GRE) sequence. Fully sampled k-space data were retrospectively under-sampled at a net reduction factor between two and three using the proposed under-sampling strategy. K-spaces of three different TEs were combined together to perform the proposed reconstruction method called Echo-based GRAPPA. Ex vivo goose liver cooling experiment and in vivo breast imaging experiment were performed to investigate the accuracy of Echo-based GRAPPA. Conventional GRAPPA reconstruction was implemented for comparison using the same sampling pattern.

RESULTS

The goose liver imaging experiment shows that the reconstruction-induced temperature RMSE of a selected region of interest (ROI) is less than 1.4 °C for Echo-based GRAPPA at a net reduction factor of 2.3. The breast imaging experiment shows that the mean temperature error of water-fat mixed ROIs is 2.3 °C at a net reduction factor of 2.7. Conventional GRAPPA shows larger temperature RMSE than Echo-based GRAPPA.

CONCLUSION

The proposed method can accelerate the MR temperature estimation using model-based PRF at a net reduction factor between two and three with a reconstruction-induced temperature error less than 3°C in water-fat mixed ROIs.

Non-invasive temperature mapping using temperature-responsive water saturation shift referencing (T-WASSR) MRI

We present a non-invasive MRI approach for assessing the water proton resonance frequency (PRF) shifts associated with changes in temperature. This method is based on water saturation shift referencing (WASSR), a method first developed for assessing B0 field inhomogeneity. Temperature-induced water PRF shifts were determined by estimating the frequency of the minimum intensity of the water direct saturation spectrum at each temperature using Lorentzian line-shape fitting. The change in temperature was then calculated from the difference in water PRF shifts between temperatures. Optimal acquisition parameters were first estimated using simulations and later confirmed experimentally. Results in vitro and in vivo showed that the temperature changes measured using the temperature-responsive WASSR (T-WASSR) were in good agreement with those obtained with MR spectroscopy or phase-mapping-based water PRF measurement methods,. In addition, the feasibility of temperature mapping in fat-containing tissue is demonstrated in vitro. In conclusion, the T-WASSR approach provides an alternative for non-invasive temperature mapping by MRI, especially suitable for temperature measurements in fat-containing tissues.

Multiparametric fat-water separation method for fast chemical-shift imaging guidance of thermal therapies

PURPOSE

A k-means-based classification algorithm is investigated to assess suitability for rapidly separating and classifying fat/water spectral peaks from a fast chemical shift imaging technique for magnetic resonance temperature imaging. Algorithm testing is performed in simulated mathematical phantoms and agar gel phantoms containing mixed fat/water regions.

METHODS

Proton resonance frequencies (PRFs), apparent spin-spin relaxation (T2*) times, and T1-weighted (T1-W) amplitude values were calculated for each voxel using a single-peak autoregressive moving average (ARMA) signal model. These parameters were then used as criteria for k-means sorting, with the results used to determine PRF ranges of each chemical species cluster for further classification. To detect the presence of secondary chemical species, spectral parameters were recalculated when needed using a two-peak ARMA signal model during the subsequent classification steps. Mathematical phantom simulations involved the modulation of signal-to-noise ratios (SNR), maximum PRF shift (MPS) values, analysis window sizes, and frequency expansion factor sizes in order to characterize the algorithm performance across a variety of conditions. In agar, images were collected on a 1.5T clinical MR scanner using acquisition parameters close to simulation, and algorithm performance was assessed by comparing classification results to manually segmented maps of the fat/water regions.

RESULTS

Performance was characterized quantitatively using the Dice Similarity Coefficient (DSC), sensitivity, and specificity. The simulated mathematical phantom experiments demonstrated good fat/water separation depending on conditions, specifically high SNR, moderate MPS value, small analysis window size, and low but nonzero frequency expansion factor size. Physical phantom results demonstrated good identification for both water (0.997 ± 0.001, 0.999 ± 0.001, and 0.986 ± 0.001 for DSC, sensitivity, and specificity, respectively) and fat (0.763 ± 0.006, 0.980 ± 0.004, and 0.941 ± 0.002 for DSC, sensitivity, and specificity, respectively). Temperature uncertainties, based on PRF uncertainties from a 5 × 5-voxel ROI, were 0.342 and 0.351°C for pure and mixed fat/water regions, respectively. Algorithm speed was tested using 25 × 25-voxel and whole image ROIs containing both fat and water, resulting in average processing times per acquisition of 2.00 ± 0.07 s and 146 ± 1 s, respectively, using uncompiled MATLAB scripts running on a shared CPU server with eight Intel Xeon(TM) E5640 quad-core processors (2.66 GHz, 12 MB cache) and 12 GB RAM.

CONCLUSIONS

Results from both the mathematical and physical phantom suggest the k-means-based classification algorithm could be useful for rapid, dynamic imaging in an ROI for thermal interventions. Successful separation of fat/water information would aid in reducing errors from the nontemperature sensitive fat PRF, as well as potentially facilitate using fat as an internal reference for PRF shift thermometry when appropriate. Additionally, the T1-W or R2* signals may be used for monitoring temperature in surrounding adipose tissue.

Multipathway sequences for MR thermometry

MR-based thermometry is a valuable adjunct to thermal ablation therapies as it helps to determine when lethal doses are reached at the target and whether surrounding tissues are safe from damage. When the targeted lesion is mobile, MR data can further be used for motion-tracking purposes. The present work introduces pulse sequence modifications that enable significant improvements in terms of both temperature-to-noise-ratio properties and target-tracking abilities. Instead of sampling a single magnetization pathway as in typical MR thermometry sequences, the pulse-sequence design introduced here involves sampling at least one additional pathway. Image reconstruction changes associated with the proposed sampling scheme are also described. The method was implemented on two commonly used MR thermometry sequences: the gradient-echo and the interleaved echo-planar imaging sequences. Data from the extra pathway enabled temperature-to-noise-ratio improvements by up to 35%, without increasing scan time. Potentially of greater significance is that the sampled pathways featured very different contrast for blood vessels, facilitating their detection and use as internal landmarks for tracking purposes. Through improved temperature-to-noise-ratio and lesion-tracking abilities, the proposed pulse-sequence design may facilitate the use of MR-monitored thermal ablations as an effective treatment option even in mobile organs such as the liver and kidneys.

An internal reference model-based PRF temperature mapping method with Cramer-Rao lower bound noise performance analysis

The conventional phase difference method for MR thermometry suffers from disturbances caused by the presence of lipid protons, motion-induced error, and field drift. A signal model is presented with multi-echo gradient echo (GRE) sequence using a fat signal as an internal reference to overcome these problems. The internal reference signal model is fit to the water and fat signals by the extended Prony algorithm and the Levenberg-Marquardt algorithm to estimate the chemical shifts between water and fat which contain temperature information. A noise analysis of the signal model was conducted using the Cramer-Rao lower bound to evaluate the noise performance of various algorithms, the effects of imaging parameters, and the influence of the water:fat signal ratio in a sample on the temperature estimate. Comparison of the calculated temperature map and thermocouple temperature measurements shows that the maximum temperature estimation error is 0.614 degrees C, with a standard deviation of 0.06 degrees C, confirming the feasibility of this model-based temperature mapping method. The influence of sample water:fat signal ratio on the accuracy of the temperature estimate is evaluated in a water-fat mixed phantom experiment with an optimal ratio of approximately 0.66:1.

Validation of fast MR thermometry at 1.5 T with gradient-echo echo planar imaging sequences: phantom and clinical feasibility studies

The purpose of this work was to validate in phantom studies and demonstrate the clinical feasibility of MR proton resonance frequency thermometry at 1.5 T with segmented gradient-echo echo planar imaging (GRE-EPI) sequences during liver tumour radiofrequency (RF) ablation. Classical GRE acquisitions and segmented GRE-EPI acquisitions were performed at 1.5 T during simultaneous RF heating with an MR-compatible RF electrode placed in an agar gel phantom. Temperature increments were calculated and compared with four optical temperature probe measurements using Bland- Altman analysis. In a preliminary clinical feasibility study, the rapid GRE-EPI sequence (echo train length = 13) was used for MR temperature monitoring of RF ablation of liver tumours in three patient procedures. For phantom experiments, the Bland-Altman mean of differences between MR and optical probe temperature measurements was <0.4 degrees C, and the 95% limits of agreement value was <1.4 degrees C. For the in vivo studies, respiratory-triggered GRE-EPI acquisitions yielded a temperature accuracy of 1.3 +/- 0.4 degrees C (acquisition time = 0.6 s/image, spatial coverage of three slices/respiratory cycle). MR proton resonance frequency thermometry at 1.5 T yields precise and accurate measurements of temperature increment with both classical GRE and rapid GRE-EPI sequences. Rapid GRE-EPI sequences minimize intra-scan motion effects and can be used for MR thermometry during RF ablation in moving organs.

In vitro and in vivo targeting of hollow gold nanoshells directed at epidermal growth factor receptor for photothermal ablation therapy

Laser-induced phototherapy is a new therapeutic use of electromagnetic radiation for cancer treatment. The use of targeted plasmonic gold nanoparticles can reduce the laser energy necessary for selective tumor cell destruction. However, the ability for targeted delivery of the currently used gold nanoparticles to tumor cells is limited. Here, we describe a new class of molecular specific photothermal coupling agents based on hollow gold nanoshells (HAuNS; average diameter, approximately 30 nm) covalently attached to monoclonal antibody directed at epidermal growth factor receptor (EGFR). The resulting anti-EGFR-HAuNS exhibited excellent colloidal stability and efficient photothermal effect in the near-infrared region. EGFR-mediated selective uptake of anti-EGFR-HAuNS in EGFR-positive A431 tumor cells but not IgG-HAuNS control was shown in vitro by imaging scattered light from the nanoshells. Irradiation of A431 cells treated with anti-EGFR-HAuNS with near-infrared laser resulted in selective destruction of these cells. In contrast, cells treated with anti-EGFR-HAuNS alone, laser alone, or IgG-HAuNS plus laser did not show observable effect on cell viability. Using 111In-labeled HAuNS, we showed that anti-EGFR-HAuNS could be delivered to EGFR-positive tumors at 6.8% ID/g, and the microscopic image of excised tumor with scattering signal from nanoshells confirmed preferential delivery to A431 tumor of anti-EGFR-HAuNS compared with IgG-HAuNS. The absence of silica core, the relatively small particle size and high tumor uptake, and the absence of cytotoxic surfactant required to stabilize other gold nanoparticles suggest that immuno-HAuNS have the potential to extend to in vivo molecular therapy.

Echo combination to reduce proton resonance frequency (PRF) thermometry errors from fat

PURPOSE

To validate echo combination as a means to reduce errors caused by fat in temperature measurements with the proton resonance frequency (PRF) shift method.

MATERIALS AND METHODS

Computer simulations were performed to study the behavior of temperature measurement errors introduced by fat as a function of echo time. Error reduction by combining temperature images acquired at different echo times was investigated. For experimental verification, three echoes were acquired in a refocused gradient echo acquisition. Temperature images were reconstructed with the PRF shift method for the three echoes and then combined in a weighted average. Temperature measurement errors in the combined image and the individual echoes were compared for pure water and different fractions of fat in a computer simulation and for a phantom containing a homogenous mixture with 20% fat in an MR experiment.

RESULTS

In both simulation and MR measurement, the presence of fat caused severe temperature underestimation or overestimation in the individual echoes. The errors were substantially reduced after echo combination. Residual errors were about 0.3 degrees C for 10% fat and 1 degrees C for 20% fat.

CONCLUSION

Echo combination substantially reduces temperature measurement errors caused by small fractions of fat. This technique then eliminates the need for fat suppression in tissues such as the liver.

Dynamic chemical shift imaging for image-guided thermal therapy: analysis of feasibility and potential

A fast chemical shift imaging (CSI) technique based on a multiple gradient-recalled acquisition using a small number of echoes with intentional aliasing of the reference lipid peak is studied to determine its feasibility for temperature monitoring. Simulations were implemented to find parameters where the lipid and water peaks can be measured using a Fourier-based peak fitting approach as well as using an innovative autoregressive moving average technique. A phantom consisting of 50% mayonnaise/50% lemon juice was calibrated to temperature and compared to literature values. A porcine kidney was treated ex vivo with an external laser and imaged with the CSI technique with comparisons to temperature readings from a fluoroptic monitoring system and complex phase difference (CPD) calculations. To demonstrate the technique in vivo, a Balb/c mouse with a CT26 xenograft in the subcutaneous lower back was treated using gold-coated, silica-core nanoshells heated with an 808 nm interstitial laser. Compared to standard CPD techniques using a two-dimensional fast spoiled gradient recalled echo, this technique maintains spatiotemporal resolution, has high signal-to-noise ratio and accuracy over a wide range of T2* tissue values, can separate water and lipid signals, and additionally can use the lipid peak, when present, as an internal reference.

Temperature mapping considerations in the breast with line scan echo planar spectroscopic imaging

A line-scan echo planar spectroscopic imaging (LSEPSI) sequence was used to serially acquire spectra from 4,096 voxels every 6.4 s throughout the breasts of nine female subjects in vivo. Data from the serial acquisitions were analyzed to determine the potential of the technique to characterize temperature changes using either the water frequency alone or the water-methylene frequency difference. Fluctuations of the apparent temperature change under these conditions of no heating were smallest using the water-methylene frequency difference, most probably due to a substantial reduction of motion effects both within and without the imaged plane. The approach offers considerable advantages over other methods for temperature change monitoring in the breast with magnetic resonance but suffers from some limitations, including the unavailability of lipid and water resonances in some voxels as well as a surprisingly large distribution of water-methylene frequency differences, which may preclude absolute temperature measurement.

3-D modeling of the thermal coagulation necrosis induced by an interstitial ultrasonic transducer

This letter describes a temperature-varying attenuation approach for preoperative planning of high intensity ultrasound interstitial targeted therapy. Such approach is mainly aimed at the treatment of primary liver cancer for which a precise lesion control must be achieved. It is shown through simulation that the shape and size of the resulting necrotic volume is significantly different from the one obtained when this tissue property is considered constant in time.

Correction of proton resonance frequency shift temperature maps for magnetic field disturbances using fat signal

PURPOSE

To improve the immunity of the proton resonance frequency shift (PRFS) method of MRI temperature mapping against magnetic field disturbances. Since PRFS is a phase-sensitive method, it misinterprets magnetic field disturbances as artifact temperature changes. If not corrected, the resulting temperature artifacts can completely obscure the true temperature estimation, especially if the temperature elevations are small.

MATERIALS AND METHODS

Since the fat protons experience the same magnetic field disturbances as the water protons, but no temperature-related frequency shift, the fat signal has been used for correcting PRFS temperature maps for the disturbances. A simple correction method is proposed that has either better compensation capability than the phase correction methods previously reported or higher spatial and temporal resolution than the spectroscopic correction methods previously reported. The evaluated method is based on the utilization of several gradient and spin echoes acquired within one repetition interval with water- and fat-selective scans.

RESULTS

In a series of phantom experiments, the improved method is shown to enable the reconstruction of accurate temperature maps in spite of interscan motion, suboptimal fat-water separation, and a wide range of magnetic field disturbances.

CONCLUSION

Our approach can be used for the guidance of thermal therapies involving tissues containing fat or surrounded by fat.

Magnetic resonance temperature imaging for focused ultrasound surgery: a review

Magnetic resonance temperature imaging (MRTI) is an enabling technology that has recently demonstrated the potential to bring the emerging minimally invasive image-guided thermal therapy procedures, such as radiofrequency, microwave, laser, ultrasound, and cryosurgery, into the clinical setting with a level of safety and efficacy not previously possible. By coupling the wealth of soft tissue contrast mechanisms available with magnetic resonance imaging with its intrinsic temperature sensitivity, magnetic resonance imaging is in a unique position to provide image-guided treatment planning and verification and quantitative or qualitative feedback during treatment delivery, heightening of the control the physician has over the method, and enhancement of the ability to deliver conformal treatments. The basic principles behind MRTI technology and its application to minimally invasive thermal therapy during ultrasound thermal therapy delivery are reviewed in this study.

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.

MR imaging–guided breast ablative therapy

The integration of imaging and thermal therapy can provide a minimally invasive or even noninvasive alternative to breast surgery for small tumors. Ongoing trials seek to show safety and efficacy for laser, radiofrequency, microwave, cryoablation, and focused ultrasound surgery. To be successful, these therapies must achieve equivalent or even greater efficacy as surgical outcomes and must demonstrate total ablation of the dominant lesion with negative margins, while sparing normal tissue beyond the target tissue. Procedures have been validated by histopathology subsequent to resection.

Intraoperative magnetic resonance: the future of surgery

Intraoperative magnetic resonance imaging (iMRI) is a new development in medicine that bridges the specialties of surgery and radiology. Deficiencies in the visualization of anatomical architecture and the perception of tumour boundaries in conventional open surgery have led to the integration of imaging within surgery. The superior soft tissue and multiplanar imaging features of magnetic resonance (MR) make this imaging modality superior to that of alternatives. The unique properties of MR to detect heat change and perfusion, and diffusion characteristics of tissue enhance the usefulness of this medium. Concurrent developments in computer aided image guidance and thermoablative technology, herald the era of minimally invasive tumour ablation. Applications have been developed for areas such as neurosurgery, general surgery, gynaecology and urology.

Nuclear magnetic resonance spectroscopy and imaging in animal research

Nuclear magnetic resonance (NMR) spectroscopy and imaging can be used to investigate, noninvasively, a wide range of biological processes in systems as diverse as protein solutions, single cells, isolated perfused organs, and tissues in vivo. It is also possible to combine different NMR techniques enabling metabolic, anatomical, and physiological information to be obtained in the same experiment. This review provides a simple overview of the basic principles of NMR and outlines both the advantages and disadvantages of NMR spectroscopy and imaging. A few examples of potential applications of NMR spectroscopy and imaging are presented, which demonstrate the range of questions that can be asked using these techniques. The potential impact of using NMR techniques in a biomedical research program on the total number of animals required for specific investigations, as well as the number of animals used in research, are discussed. The article concludes with a personal perspective on the impact of continuing improvements in NMR technology for future applications in animal research.

Potential heating effect in the gravid uterus during MR HASTE imaging

Our purpose was to evaluate if temperature changes occur in maternal or fetal tissues during HASTE imaging.

METHODS

Pregnant pigs were scanned with the HASTE technique, and temperatures were measured with phase maps and temperature probes inserted into the amniotic fluid and fetal brain.

RESULTS

Fiberoptic probes showed that no heating occurred in fetal tissues or amniotic fluid during HASTE imaging.

CONCLUSION

Our current HASTE protocols do not deposit a significant amount of heat in the gravid uterus. J. Magn. Reson. Imaging 2001;13:856-861.

MR imaging-guided focused ultrasound surgery of fibroadenomas in the breast: a feasibility study

PURPOSE

To test the feasibility of noninvasive magnetic resonance (MR) imaging-guided focused ultrasound surgery (FUS) of benign fibroadenomas in the breast.

MATERIALS AND METHODS

Eleven fibroadenomas in nine patients under local anesthesia were treated with MR imaging-guided FUS. Based on a T2-weighted definition of target volumes, sequential sonications were delivered to treat the entire target. Temperature-sensitive phase-difference-based MR imaging was performed during each sonication to monitor focus localization and tissue temperature changes. After the procedure, T2-weighted and contrast material-enhanced T1-weighted MR imaging were performed to evaluate immediate and long-term effects.

RESULTS

Thermal imaging sequences were improved over the treatment period, with 82% (279 of 342) of the hot spots visible in the last seven treatments. The MR imager was used to measure temperature elevation (12.8 degrees -49.9 degrees C) from these treatments. Eight of the 11 lesions treated demonstrated complete or partial lack of contrast material uptake on posttherapy T1-weighted images. Three lesions showed no marked decrease of contrast material uptake. This lack of effective treatment was most likely due to a lower acoustic power and/or patient movement that caused misregistration. No adverse effects were detected, except for one case of transient edema in the pectoralis muscle 2 days after therapy.

CONCLUSION

MR imaging-guided FUS can be performed to noninvasively coagulate benign breast fibroadenomas.

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.

Simultaneous measurements of temperature and pH in vivo using NMR in conjunction with TmDOTP5-

NMR techniques for temperature and pH measurements have attracted increasing interest in recent years, motivated in part by the growing importance of medical hyperthermia for the treatment of cancer. The chemical shifts of thulium 1, 4, 7, 10-tetraazacyclododecane-1, 4, 7, 10-tetrakis(methylene phosphonate) (TmDOTP5-) have been studied as a function of temperature and pH. The results demonstrate that TmDOTP5- resonance shifts are highly sensitive to temperature (approximately 1.0 ppm/degrees C) and pH (approximately 3.2 ppm/pH unit) at clinically relevant field strengths. A new method is presented which utilizes two magnetically non-equivalent protons in TmDOTP5- for simultaneous NMR measurements of both temperature and pH. The difference in the chemical shift values of pairs of 1H resonances provides a temperature sensitivity of about 1.6 ppm/ degrees C. The technique is demonstrated in live rats undergoing ultrasound-induced hyperthermia therapy.

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.

Magnetic resonance temperature imaging for guidance of thermotherapy

Continuous thermometry during a hyperthermic procedure may help to correct for local differences in heat conduction and energy absorption, and thus allow optimization of the thermal therapy. Noninvasive, three-dimensional mapping of temperature changes is feasible with magnetic resonance (MR) and may be based on the relaxation time T(1), the diffusion coefficient (D), or proton resonance frequency (PRF) of tissue water. The use of temperature-sensitive contrast agents and proton spectroscopic imaging can provide absolute temperature measurements. The principles and performance of these methods are reviewed in this paper. The excellent linearity and near-independence with respect to tissue type, together with good temperature sensitivity, make PRF-based temperature MRI the preferred choice for many applications at mid to high field strength (>/= 1 T). The PRF methods employ radiofrequency spoiled gradient-echo imaging methods. A standard deviation of less than 1 degrees C, for a temporal resolution below 1 second and a spatial resolution of about 2 mm, is feasible for a single slice for immobile tissues. Corrections should be made for temperature-induced susceptibility effects in the PRF method. If spin-echo methods are preferred, for example when field homogeneity is poor due to small ferromagnetic parts in the needle, the D- and T(1)-based methods may give better results. The sensitivity of the D method is higher that that of the T(1) methods provided that motion artifacts are avoided and the trace of D is evaluated. Fat suppression is necessary for most tissues when T(1), D, or PRF methods are employed. The latter three methods require excellent registration to correct for displacements between scans.

Perturbations in hyperthermia temperature distributions associated with counter-current flow: numerical simulations and empirical verification

Two numerical techniques are used to calculate the effect of large vessel counter-current flow on hyperthermic temperature distributions. One is based on the Navier-Stokes equation for steady-state flow, and the second employs a convective-type boundary condition at the interface of the vessel walls. Steady-state temperature fields were calculated for two energy absorption rate distributions (ARD) in a cylindrical tissue model having two pairs of counter-current vessels (one pair with equal diameter vessels and another pair with unequal diameters). The first assumed a uniform ARD throughout cylinder; the second ARD was calculated for a tissue cylinder inside an existing four antenna Radiofrequency (RF) array. A tissue equivalent phantom was constructed to verify the numerical calculations. Temperatures induced with the RF array were measured using a noninvasive magnetic resonance imaging technique based on the chemical shift of water. Temperatures calculated using the two numerical techniques are in good agreement with the measured data. The results show: 1) the convective-type boundary condition technique reduces computation time by a factor of ten when compared to the fully conjugated method with little quantitative difference (approximately 0.3 degree C) in the numerical accuracy and 2) the use of noninvasive magnetic resonance imaging (thermal imaging) to quantitatively access the temperature perturbations near large vessels is feasible using the chemical shift technique.

Temperature mapping using the water proton chemical shift: self-referenced method with echo-planar spectroscopic imaging

An echo-planar spectroscopic imaging method of temperature mapping is proposed. This method is sufficiently faster than the so-called 3D magnetic resonance spectroscopic imaging (3D-MRSI) method and does not require image subtractions, unlike the conventional phase mapping method when an internal reference signal is detectable. The water proton chemical shift measured by using the tissue lipid as an internal reference clearly visualized the temperature change in a porcine liver sample in vitro. It was also demonstrated that the internally referenced echo-planar spectroscopic imaging method could markedly reduce a temperature error caused by a simple, translational motion between scans compared with the phase-mapping method.

Teleradiology in the operating room of the future

Recent advances in magnetic resonance imaging (MRI) are rapidly making this modality the imaging method of choice for image-guided neurosurgical operations. However, to be ready for its prime time in the operating room (OR), utilization of MRI in the OR requires development of better techniques for image-guided navigation, as well as interactive real-time teleradiologic methods that will allow tele-collaboration between the surgeon and the radiologist. This presentation describes our work in progress toward achievement of teleradiology in the OR.

Transrectal ultrasound applicator for prostate heating monitored using MRI thermometry

PURPOSE

For potential localized hyperthermia treatment of tumors within the prostate, an ultrasound applicator consisting entirely of nonmagnetic materials for use with magnetic resonance imaging (MRI) has been developed and tested on muscle tissue ex vivo and in vivo.

METHODS AND MATERIALS

A partial-cylindrical intracavitary transducer consisting of 16 elements in a 4 x 4 pattern was constructed. It produced a radially propagating acoustic pressure field. Each element of this array (1.5 x 0.75 cm), operating at 1.5 MHz, could be separately powered to produce a desired energy deposition pattern within a target volume. Spatial and temporal temperature elevations were determined using the temperature-dependent proton resonant frequency (PRF) shift and phase subtraction of MR images acquired during ultrasonic heating. Four rabbits were exposed to the ultrasound to raise the local tissue temperature to 45 degrees C for 25 minutes. Six experiments compared thermocouple temperature results to PRF shift temperature results.

RESULTS

The tests showed that the multi-element ultrasound applicator was MRI-compatible and allowed imaging during sonication. The induced temperature distribution could be controlled by monitoring the RF power to each transducer element. Therapeutic temperature elevations were easily achieved in vivo at power levels that were about 16% of the maximum system power. From the six thermocouple experiments, comparison between the thermocouple temperature and the PRF temperature yielded an average error of 0.34+/-0.36 degrees C.

CONCLUSIONS

The MRI-compatible intracavitary applicator and driving system was able to control the ultrasound field and temperature pattern in vivo. MRI thermometry using the PRF shift can provide adequate temperature accuracy and stability for controlling the temperature distribution.