A precise and fast temperature mapping using water proton chemical shift

Intraoperative magnetic resonance imaging and magnetic resonance imaging-guided therapy for brain tumors

Since their introduction into surgical practice in the mid 1990s, intraoperative MRI systems have evolved into essential, routinely used tools for the surgical treatment of brain tumors in many centers. Clear delineation of the lesion, "under-the-surface" vision, and the possibility of obtaining real-time feedback on the extent of resection and the position of residual tumor tissue (which may change during surgery due to "brain-shift") are the main strengths of this method. High-performance computing has further extended the capabilities of intraoperative MRI systems, opening the way for using multimodal information and 3D anatomical reconstructions, which can be updated in "near real time." MRI sensitivity to thermal changes has also opened the way for innovative, minimally invasive (LASER ablations) as well as noninvasive therapeutic approaches for brain tumors (focused ultrasound). Although we have not used intraoperative MRI in clinical applications sufficiently long to assess long-term outcomes, this method clearly enhances the ability of the neurosurgeon to navigate the surgical field with greater accuracy, to avoid critical anatomic structures with greater efficacy, and to reduce the overall invasiveness of the surgery itself.

Optimization of electromagnetic phased-arrays for hyperthermia via magnetic resonance temperature estimation

A technique for the optimization of electromagnetic annular phased arrays (APAs) for therapeutic hyperthermia has been developed and implemented. The controllable inputs are the amplitudes and phases of the driving signals of each element of the array. Magnetic resonance imaging (MRI) is used to estimate noninvasively the temperature distribution based on the temperature dependence of the proton resonance frequency (PRF). A parametric model of the dynamics that couple the control inputs to the resultant temperature elevations is developed based on physical considerations. The unknown parameters of this model are estimated during a pretreatment identification phase and can be continuously updated as new measurement data become available. Based on the parametric model, a controller automatically chooses optimal phases and amplitudes of the driving signals of the APA. An advantage of this approach to optimizing the APA is that no a priori information is required, eliminating the need for patient-specific computational modeling and optimization. Additionally, this approach represents a first step toward employing temperature feedback to make the optimization of the APA robust with respect to modeling errors and physiological changes. The ability of the controller to choose therapeutically beneficial driving amplitudes and phases is demonstrated via simulation. Experimental results are presented which demonstrate the ability of the controller to choose optimal phases for the APA using only information from magnetic resonance thermometry (MRT).

Multiplanar MR temperature-sensitive imaging of cerebral thermal treatment using interstitial ultrasound applicators in a canine model

PURPOSE

To study the feasibility of an interleaved gradient-echo, echo-planar imaging (iGE-EPI) sequence for multiplanar magnetic resonance temperature imaging (MRTI) to monitor intracerebral thermal treatment three-dimensionally using multielement ultrasound applicators.

MATERIALS AND METHODS

Transmissible venereal tumor (TVT) fragments were injected into the right cerebral hemisphere of five dogs. Guided by MRI, an interstitial ultrasound applicator was inserted into the tumor or normal brain tissue. The iGE-EPI sequence was used to estimate temperature changes by computing the complex phase-difference induced by temperature-dependent shifts in the proton resonance frequency of water. The thermal dose maps were updated every 6-8 seconds for five to seven image planes during treatment. The results of MRTI were compared with those of post-treatment MRI and histologic analysis.

RESULTS

The multiplanar MRTI monitored temperature and thermal dose distributions in tumor and normal brain tissue over the entire user-defined treatment volume. The ultrasound applicators produced contiguous areas of coagulative necrosis, resulting in 1.5-4.0 cm(3) volumes of tissue necrosis. MRTI-based assessments of thermal-dose distributions were consistent with the results of post-treatment MRI and histologic analysis.

CONCLUSION

Multiplanar MRTI is feasible for measuring necrosing thermal doses during intracerebral thermal delivery by interstitial ultrasound applicators.

Proton magnetic resonance spectroscopy in the brain: report of AAPM MR Task Group #9

AAPM Magnetic Resonance Task Group #9 on proton magnetic resonance spectroscopy (MRS) in the brain was formed to provide a reference document for acquiring and processing proton (1H) MRS acquired from brain tissue. MRS is becoming a common adjunct to magnetic resonance imaging (MRI), especially for the differential diagnosis of tumors in the brain. Even though MR imaging is an offshoot of MR spectroscopy, clinical medical physicists familiar with MRI may not be familiar with many of the common practical issues regarding MRS. Numerous research laboratories perform in vivo MRS on other magnetic nuclei, such as 31P, 13C, and 19F. However, most commercial MR scanners are generally only capable of spectroscopy using the signals from protons. Therefore this paper is of limited scope, giving an overview of technical issues that are important to clinical proton MRS, discussing some common clinical MRS problems, and suggesting how they might be resolved. Some fundamental issues covered in this paper are common to many forms of magnetic resonance spectroscopy and are written as an introduction for the reader to these methods. These topics include shimming, eddy currents, spatial localization, solvent saturation, and post-processing methods. The document also provides an extensive review of the literature to guide the practicing medical physicist to resources that may be useful for dealing with issues not covered in the current article.

Automatic control of hyperthermic therapy based on real-time Fourier analysis of MR temperature maps

Local hyperthermia is increasingly being used for therapeutic purposes, such as tumor ablation. Heat conduction and energy absorption in vivo during the hyperthermic procedure are largely unknown, thus making feedback temperature control highly desirable. Here, a general method for temperature control based on Fourier transformation (FT) of the bio-heat equation is presented, taking into account heat diffusion (D) and energy absorption (alpha) together with temperature distribution derived from rapid, continuous MR temperature mapping. The main advantages of the new method are: 1) the spatial distribution of heat deposition and conduction over the full region of interest (ROI) is taken into account, and 2) the high speed resulting from the use of fast FT (FFT) of temperature maps allows rapid feedback coupling. Initial tests based on MRI-guided focused ultrasound (FUS) demonstrated that high-quality temperature regulation can be obtained even for erroneous values of D and alpha, so long as their relative error remained in the same range. Performance of the automated control procedure was validated ex vivo and in vivo on rabbit thigh using moderate FUS heating. During the procedure, the standard deviation (SD) of the temperature remained in the range of temperature noise obtained by MRI, indicative of the performance of the regulation algorithm.

The use of quantitative temperature images to predict the optimal power for focused ultrasound surgery: in vivo verification in rabbit muscle and brain

In this study, we investigated the use of MRI-derived thermal imaging for determining the exposure parameters for focused ultrasound (FUS) surgery. Since the temperature rise induced by a FUS beam scales linearly with power, the temperature maps acquired during subthreshold sonications can be used to determine the power necessary to produce thermal tissue damage with a desired size. Thermal images acquired during multiple sonications delivered at different locations in rabbit thigh muscle and brain tissue in vivo were analyzed to test this hypothesis. First, the linearity of the induced temperature rise with the acoustic power was tested. Next, the temperature maps acquired during preliminary low power sonications were scaled up until the estimated size of the tissue damage was equal to the tissue damage size of subsequent high power sonications. A threshold thermal dose was used to estimate the onset of thermal damage. The predicted power (based on amount of scaling required to reach the target size) was then compared to the true high power value. Overall, the temperature rise varied linearly with power (slope of deltaThigh/deltaTlow vs Power(high)/Power(low) = 0.97, 0.93 for pairs of sonications at each location in brain, muscle). The predicted power matched the true high power in the brain sonications (slope = 1.04). The predicted power underestimated the true high power in the muscle sonications (slope = 0.87). This under-prediction was due to a deviation from linearity in those cases where tissue damage was detected in subsequent MR images (slope of deltaThigh/deltaTlow vs Power(high)/Power(low) = 1.02, 0.84 for no tissue damage, tissue damage). The source of this deviation was not clear from these experiments. Even with this underestimation of the power, this method will be useful because it will allow an estimate of the proper power to use during FUS surgery without exact knowledge of the tissue parameters.

Magnetic resonance imaging-guided focused ultrasound thermal therapy in experimental animal models: correlation of ablation volumes with pathology in rabbit muscle and VX2 tumors

PURPOSE

To further investigate the use of magnetic resonance-guided focused ultrasound therapy (MRgFUS) as a noninvasive alternative to surgery in the local control of soft-tissue tumors by ablating prescribed volumes of VX2 rabbit tumors and comparing with ablation of normal tissue volumes.

MATERIALS AND METHODS

Small, ellipsoidal ablations at shallow depth were created using 5- to 15-second sonication pulses at radio frequency (RF) powers of 50-125 W using a spherical, air-backed transducer operating at 1.463 MHz under MR guidance in a 1.5-T clinical scanner.

RESULTS

Excellent correlation was observed between prescribed treatment volumes, MR thermal dosimetry, post-treatment verification MRI, and histopathology. Multifocal ablations of VX2 tumors in rabbits at depths of up to 2.5 cm resulted in complete ablation of the prescribed treatment volume.

CONCLUSION

MRgFUS is an effective technique for treating tumors in vivo. Techniques developed for treatments in homogeneous tissue volumes are applicable in the more complicated tumor environment if MR temperature feedback is available to modify treatment delivery parameters.

Novel laser system and laser irradiation method reduced the risk of carbonization during laser interstitial thermotherapy: assessed by MR temperature measurement

BACKGROUND AND OBJECTIVE

To establish laser interstitial thermotherapy (LITT) for intracranial tumors, the authors investigated a method to regulate localized temperature generated by interstitial laser irradiation using magnetic resonance (MR) temperature mapping.

STUDY DESIGN/MATERIALS AND METHODS

A diode laser system and six different types of optical-fiber system were developed for LITT. The characteristics of temperature profiles produced by each laser-fiber system were investigated with MR temperature measurement (the water proton chemical technique), and differences in the temperature profile induced by two laser-irradiation methods (continuous and intermittent) were observed.

RESULTS

All fiber systems with the exception of the diffuse-projection fiber system, created a spherical temperature profile. Carbonization sometimes occurred around the bare-end fiber tip upon high power laser irradiation. The diffuse-projection fiber system produced a cylindrical temperature distribution, and the temperature profile showed a more gradual temperature elevation than the bare-end fiber. No carbonization occurred at the tip of the diffuse-projection fiber system. In addition, the utilization of the intermittent irradiation method also increased temperature gradually. Fiber-system modification and intermittent irradiation reduced laser-beam intensity and the risk of carbonization.

CONCLUSION

The use of a diffuse-projection fiber system which intermittently transmits a reduced intensity laser beam is an effective tool to regulate temperature during LITT using MR temperature measurement.

Thermal effects of focused ultrasound energy on bone tissue

The effects of focused ultrasound (US) at therapeutic acoustic power levels were studied in vivo on the bone-muscle interface in rabbit thighs. The purpose of this study was to provide direction in establishing safety guidelines for treating tissue masses using focused US on or near bone. A positioning device was used to manipulate a focused US transducer (1.5 MHz) in a magnetic resonance imaging (MRI) scanner. This system was used to sonicate the femurs of 10 rabbits at acoustic power levels of 26, 39, 52 and 65 W for 10 s. The rabbits were euthanized either 4 h or 28 days after the sonications and the bone samples were harvested for histology examinations. In the femurs studied, acoustic power levels from 39 to 65 W resulted in soft tissue damage characterized grossly by coagulated tissue and bone damage depicted by yellow discoloration. Histologic examination of lesions from sonications from 39 to 65 W demonstrated that osteocyte damage and necrosis, characterized by pyknotic cells and empty lacunae, occurred within the ablation area extending through the bone. The follow-up MR images demonstrated an increase in the amount of damage in the femurs at 28 days posttreatment in comparison to images taken immediately after treatment. Focused US directed at the femur caused immediate significant thermal damage to bone in the form of osteocyte necrosis extending through the (approximately) 1 cm bone in this study. The results suggest that, when focused US energy is directed at or near bone-muscle interfaces, precautions should be taken to avoid thermal damage to the bone that can compromise its strength for extended periods.

Significant precision improvement for temperature mapping

MR temperature measurements are important for applications such as the evaluation of thermal therapies and radiofrequency (RF) coil heating effects. In this work the spherical mean value (SMV) method has been applied to significantly improve the precision of MR temperature mapping in a homogeneous gel phantom. Temperature-increase maps of the phantom were obtained with three-dimensional (3D) MR phase difference mapping after heating with the RF coil. The temperature-increase distribution in most regions in the phantom is a harmonic function with the mean value property. Based on this property, the precision of temperature-increase maps was improved up to sixfold with the SMV method. Comparison of this method with conventional smoothing, further precision improvement, and the in vivo application of the SMV method are discussed.

Noninvasive MR imaging-guided focal opening of the blood-brain barrier in rabbits

PURPOSE

To determine if focused ultrasound beams can be used to locally open the blood-brain barrier without damage to surrounding brain tissue and if magnetic resonance (MR) imaging can be used to monitor this procedure.

MATERIALS AND METHODS

The brains of 18 rabbits were sonicated (pulsed sonication) in four to six locations, with temporal peak acoustic power ranging from 0.2 to 11.5 W. Prior to each sonication, a bolus of ultrasonographic (US) contrast agent was injected into the ear vein of the rabbit. A series of fast or spoiled gradient-echo MR images were obtained during the sonications to monitor the temperature elevation and potential tissue changes. Contrast material-enhanced MR images obtained minutes after sonications and repeated 1-48 hours later were used to depict blood-brain barrier opening. Whole brain histologic evaluation was performed.

RESULTS

Opening of the blood-brain barrier was confirmed with detection of MR imaging contrast agent at the targeted locations. The lowest power levels used produced blood-brain barrier opening without damage to the surrounding neurons. Contrast enhancement correlated with the focal signal intensity changes in the magnitude fast spoiled gradient-echo MR images.

CONCLUSION

The blood-brain barrier can be consistently opened with focused ultrasound exposures in the presence of a US contrast agent. MR imaging signal intensity changes may be useful in the detection of blood-brain barrier opening during sonication.

MRI monitoring of the thermal ablation of tissue: effects of long exposure times

MRI-derived thermometry based on the temperature-dependence of the proton resonant frequency (PRF) is extremely sensitive to changes in tissue unrelated to temperature changes, including tissue swelling. This study investigated the maximum amount of time that this phase-subtraction-based method can be used to accurately monitor temperature changes in vivo. Long-duration focused ultrasound sonications were delivered in rabbit thigh muscle with a phased-array transducer, and the time that tissue swelling began was monitored. Tissue swelling began to occur at about one minute. The temperature correlated well with an implanted thermocouple up to this time. After this time, severe artifacts in the phase-difference maps were observed. The thermal dose model predicted the extent of tissue damage well for subsequent one minute sonications. These results will have implications for MRI guidance of thermal therapies with long exposure times.

Temperature quantification using the proton frequency shift technique: In vitro and in vivo validation in an open 0.5 tesla interventional MR scanner during RF ablation

Open magnetic resonance (MR) scanners allow MR-guided targeting of tumors, as well as temperature monitoring of radio frequency (RF) ablation. The proton frequency shift (PFS) technique, an accurate and fast imaging method for temperature quantification, was used to synthesize thermal maps after RF ablation in an open 0.5 T MR system under ex vivo and in vivo conditions. Calibration experiments with 1.5% agarose gel yielded a chemical shift factor of 0.011 +/- 0.001 ppm/ degrees C (r2 = 0.96). Three gradient echo (GRE) pulse sequences were tested for thermal mapping by comparison with fiberoptic thermometer (Luxtron Model 760) readings. Temperature uncertainty decreased from high to low bandwidths (BW): +/-5.9 degrees C at BW = 15.6 kHz, +/-1.4 degrees C at BW = 3.9 kHz, and +/-0.8 degrees C at BW = 2.5 kHz. In vitro experiments (N = 9) in the paraspinal muscle yielded a chemical shift factor of 0.008 +/- 0.001 ppm/ degrees C. Temperature uncertainty was determined as +/-2.7 degrees C (BW = 3.9 kHz, TE = 19.3 msec). The same experiments carried out in the paraspinal muscle (N = 9) of a fully anesthetized pig resulted in a temperature uncertainty of +/-4.3 degrees C (BW = 3.9 kHz, TE = 19.3 msec), which is higher than it is in vitro conditions (P < 0.15). Quantitative temperature monitoring of RF ablation is feasible in a 0.5 T open-configured MR scanner under ex vivo and in vivo conditions using the PFS technique.

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.

High-precision mapping of the magnetic field utilizing the harmonic function mean value property

The spatial distributions of the static magnetic field components and MR phase maps in space with homogeneous magnetic susceptibility are shown to be harmonic functions satisfying Laplace's equation. A mean value property is derived and experimentally confirmed on phase maps: the mean value on a spherical surface in space is equal to the value at the center of the sphere. Based on this property, a method is implemented for significantly improving the precision of MR phase or field mapping. Three-dimensional mappings of the static magnetic field with a precision of 10(-11) approximately 10(-12) T are obtained in phantoms by a 1.5-T clinical MR scanner, with about three-orders-of-magnitude precision improvement over the conventional phase mapping technique. In vivo application of the method is also demonstrated on human leg phase maps.

MR monitoring of focused ultrasound surgery in a breast tissue model in vivo

The objective of this study was to investigate MRI methods for monitoring focused ultrasound surgery (FUS) of breast tumors. To this end, the mammary glands of sheep were used as tissue model. The tissue was treated in vivo with numerous single sonications which covered extended target volumes by employing a scanning technique. The ultrasound focus position was controlled by online temperature mapping based on the temperature dependence of the relaxation time T(1). This approach proved to be reliable and offers thus an alternative to proton resonance frequency methods, whose application is hampered in fatty tissues. FUS-induced tissue changes were visible on T(2)- as well as on pre- and post-contrast T(1)-weighted images. According to our initial experience, noninvasive MRI-guided FUS of breast tumors is feasible.

Assessment of thermal tissue ablation with MR elastography

An important part of thermal ablation therapy is the assessment of the spatial extent of tissue coagulation. In this work, the mechanical properties of thermally-ablated tissue were quantitatively evaluated using magnetic resonance elastography (MRE). This study shows that the mechanical properties of focused ultrasound ablated tissue are significantly different from normal tissue, and the difference can be imaged and measured using MRE. Repeated experiments revealed a reproducible pattern of tissue mechanical property change during thermal ablation in ex vivo bovine muscle. This pattern may reflect changes in intrinsic tissue structure and could be used to evaluate tissue coagulation during thermal ablation therapy. Magn Reson Med 45:80-87, 2001.

Apoptosis in ultrasound-produced threshold lesions in the rabbit brain

Focused ultrasound (US) surgery has been used to induce high temperature elevations in tissue to coagulate the proteins and kill the tissue. The introduction of noninvasive online temperature monitoring has made it possible to induce well-controlled thermal exposures. In this study, we used magnetic resonance imaging (MRI) thermometry to monitor thermal exposures near the threshold of tissue damage, and then investigated if apoptosis was induced. Rabbit brains were sonicated with an eight-sector phased array to create a large region of uniform temperature elevation at the end of a 30-s sonication. Histological examination demonstrated that apoptosis was induced in some cells. At 4 h after the sonications, the apoptotic cells constituted 9 +/- 7% of identifiable cells. By 48 h after the sonications, the number of apoptotic cells had increased up to 17 +/- 9%. The impact of this finding for therapy needs to be explored further.

On-line correction and visualization of motion during MRI-controlled hyperthermia

Displacement of tissue during MRI-controlled hyperthermia therapy can cause significant problems. Errors in calculated temperature may result from motion-related image artifacts and inter-image object displacement, leading to incorrect spatial temperature reference. Here, cyclic navigator echoes were incorporated in rapid gradient-echo MRI sequences, used for temperature mapping based on the proton resonance frequency. On-line evaluation of navigator information was used in three ways. First, motion artifacts were minimized in echo-shifted (TE > TR) gradient-echo images using the phase information of the navigator echo. Second, navigator profiles were matched for a quantitative evaluation of displacement. Together with a novel processing method, this information was employed to correct the reference temperature maps, thereby avoiding persistence of motion-related temperature errors throughout the hyperthermic period. Third, on-line visualization of displacement, together with temperature maps and thermal dose images, was developed, allowing physician intervention at all times. Examples are given of on-line corrections during hyperthermia procedures with focused ultrasound and radiofrequency heat sources. Magn Reson Med 45:128-137, 2001.

Comparison of thermal damage calculated using magnetic resonance thermometry, with magnetic resonance imaging post-treatment and histology, after interstitial microwave thermal therapy of rabbit brain

Clinical application of high-temperature thermal therapy as a treatment for solid tumours requires an accurate and close to real-time method for assessing tissue damage. Imaging methods that detect structural changes during heating may underestimate the extent of thermal damage. This is due to the occurrence of delayed damage manifested at tissue locations exposed to temperatures lower than those required to cause immediate structural changes. An alternative approach is to measure temperature and then calculate the expected damage based on the temperature history at each tissue location. Magnetic resonance (MR) imaging methods now allow temperature maps of the target and surrounding tissues to be generated in almost real-time. The aim of this work was to evaluate whether thermal damage zones calculated on the basis of MR thermometry maps measured during heating correspond to actual tissue damage as measured after treatment by histological methods and MR imaging. Four male rabbits were treated with high-temperature thermal therapy delivered in the brain by a single microwave antenna operating at 915 MHz. MR scanning was performed before, during and after treatment in a 1.5 T whole-body scanner. Temperature maps were produced using the proton resonance frequency (PRF) shift method of MR thermometry. In addition, conventional T1-weighted and T2-weighted spin-echo images were acquired after treatment. Thermal damage zones corresponding to cell death, microvascular blood flow stasis and protein coagulation were calculated using an Arrhenius analysis of the MR temperature/time course data. The calculated zones were compared with the lesions seen on histopathological examination of the brains which were removed within 6-8 h of treatment. The results showed that calculated damage zones based on MR thermometry agreed well with areas of damage as assessed using histology after heating was completed. The data suggest that real-time calculations of final expected thermal damage based on an Arrhenius analysis of MR temperature data may provide a useful method of real-time monitoring of thermal therapy when combined with conventional T2-weighted images taken after treatment.

MR-based temperature monitoring for hot saline injection therapy

We applied magnetic resonance (MR) phase mapping methods to monitor the thermal frequency shift of water in order to study temperature changes from percutaneous hot saline injection therapy (PSIT) using in vitro swine livers and in vivo rabbit livers. The thermal coefficients calculated from the shifts of the water frequency with thermocouple based temperature measurements were -0.0085 +/- 0.0019 ppm/ degrees C for the in vitro studies and -0.0089 ppm/ degrees C for the in vivo studies. The error range was estimated to be +/- 3 degrees C and +/- 4.5 degrees C, respectively. Color-coded temperature maps were compared with macroscopic lesion sizes of the specimen. Regions defined using a 20 degrees C elevation in the initial images following hot saline injection (around 55 degrees C in absolute temperature) closely correlated with visible coagulation in size. We conclude that MR temperature monitoring of PSIT is quite feasible and may be helpful in expanding the clinical use of this thermal therapeutic tool for liver tumors.

Usefulness of MR imaging-derived thermometry and dosimetry in determining the threshold for tissue damage induced by thermal surgery in rabbits

PURPOSE

To investigate in vivo the feasibility of using magnetic resonance (MR) imaging-derived temperature and thermal dose measurements to find the threshold of thermal tissue damage.

MATERIALS AND METHODS

Sonications were delivered in rabbit thigh muscles at varying powers. Temperature-sensitive MR images obtained during the sonications were used to estimate the temperature and thermal dose. The temperature, thermal dose, and applied power were then correlated to the occurrence of tissue damage observed on postsonication images. An eight-element phased-array transducer was used to produce spatially flat temperature profiles that allowed for averaging to reduce the effects of noise and the voxel size.

RESULTS

The occurrence of tissue damage correlated well with the MR imaging-derived temperature and thermal dose measurements but not with the applied power. Tissue damage occurred at all locations with temperatures greater than 50.4 degrees C and thermal doses greater than 31.2 equivalent minutes at 43.0 degrees C. No tissue damage occurred when these values were less than 47.2 degrees C and 4.3 equivalent minutes.

CONCLUSION

MR imaging thermometry and dosimetry provide an index to predict the threshold for tissue damage in vivo. This index offers improved online control over minimally invasive thermal treatments and should allow for more accurate target volume coagulation.

Magnetic resonance image-guided thermal ablations

Magnetic resonance imaging (MRI)-based monitoring has been shown in recent years to enhance the effectiveness of minimally or noninvasive thermal therapy techniques, such as laser, radiofrequency, microwave, ultrasound, and cryosurgery. MRI's unique soft-tissue contrast and ability to image in three dimensions and in any orientation make it extremely useful for treatment planning and probe localization. The temperature sensitivity of several intrinsic parameters enables MRI to visualize and quantify the progress of ongoing thermal treatment. MRI is sensitive to thermally induced tissue changes resulting from the therapies, giving the physician a method to determine the success or failure of the treatment. These methods of using MRI for planning, guiding, and monitoring thermal therapies are reviewed.

MRI detection of the thermal effects of focused ultrasound on the brain

This study tested the hypothesis that MRI thermometry can be correlated with the different degrees of tissue damage observed after focused ultrasound (US) exposure of brain. The brains of 6 rabbits were sonicated to calibrate the MRI proton resonant shift with temperature. In addition, 13 rabbits were sonicated at acoustic powers ranging from 3.5 to 17.5 W. The experiments were performed in a 1.5-T MRI scanner with the temperature-sensitive phase imaging used during the sonications of 4-5 different locations in each rabbit. MR images were obtained 2 h and 2 days after the sonications, depending on when the animals were sacrificed. Whole brain histologic evaluation was performed by sectioning the brain and performing a microscopic investigation. The MRI-derived temperature elevation was found to correlate well with the degree of tissue damage. In addition to the common histology findings, apoptotic cells were observed in the lesions. The T1-weighted contrast enhanced and T2-weighted scans both detected the brain damage. The applied acoustic power did not correlate well with the degree of damage. As a conclusion, the results showed that the measurement of temperature elevations by MRI during sonications can improve the accuracy and safety of clinical US brain surgery.

Temperature monitoring of internal body heating induced by decoupling pulses in animal (13)C-MRS experiments

A temperature monitoring method to promote safety with regard to tissue heating induced by RF irradiation during MRI procedures, especially carbon-13 magnetic resonance spectroscopy ((13)C-MRS), is proposed. The method is based on the temperature dependence of the water proton chemical shift (-0.01 ppm/ degrees C) combined with phase mapping. Using this method, temperature changes were measured in rats (n = 4) employing practical (1)H-decoupled (13)C-MRS pulse sequences for 1D projections (TR = 1000 ms, acquisition time = 15 ms, matrix = 256, spatial resolution = 0.2 mm) and 2D images (TR = 1500 ms, acquisition time = 840 ms, matrix = 128x32, spatial resolution = 0.8x1.5 mm). Measurement error was 0.18 degrees C (SD) for 1D acquisition and 0.39 degrees C (SD) for 2D acquisition, demonstrating the feasibility of this temperature mapping method. Further studies should be conducted in human subjects to monitor patient safety and to optimize the pulse sequences employed. Magn Reson Med 43:796-803, 2000.

Magnetic resonance imaging of thermal coagulation effects in a phantom for calibrating thermal therapy devices

A material has been developed and tested that permanently records thermal response patterns from heating devices. The material consists of a mixture of polyacrylamide and 18% w/w bovine serum albumin. Thermal denaturation is complete when the local temperature exceeds 70 degrees C, causing a large reduction in the T2 of the material. Three-dimensional distributions of "thermal damage" can be assessed using standard magnetic resonance imaging sequences. The material works well with microwave heating devices and is adaptable for use with ultrasound, radio-frequency, or laser heating devices. Suggested uses include characterizing heating devices prior to treatment and developing new clinical applications for thermal therapies.

Imaging prostate cancer invasion with multi-nuclear magnetic resonance methods: the Metabolic Boyden Chamber

The physiological milieu within solid tumors can influence invasion and metastasis. To determine the impact of the physiological environment and cellular metabolism on cancer cell invasion, it is necessary to measure invasion during well-controlled modulation of the physiological environment. Recently, we demonstrated that magnetic resonance imaging can be used to monitor cancer cell invasion into a Matrigel layer [Artemov D, Pilatus U, Chou S, Mori N, Nelson JB, and Bhujwalla ZM (1999). Dynamics of prostate cancer cell invasion studied in vitro by NMR microscopy. Mag Res Med 42, 277-282.]. Here we have developed an invasion assay ("Metabolic Boyden Chamber") that combines this capability with the properties of our isolated cell perfusion system. Long-term experiments can be performed to determine invasion as well as cellular metabolism under controlled environmental conditions. To characterize the assay, we performed experiments with prostate cancer cell lines preselected for different invasive characteristics. The results showed invasion into, and degradation of the Matrigel layer, by the highly invasive/metastatic line (MatLyLu), whereas no significant changes were observed for the less invasive/metastatic cell line (DU-145). With this assay, invasion and metabolism was measured dynamically, together with oxygen tensions within the cellular environment and within the Matrigel layer. Such a system can be used to identify physiological and metabolic characteristics that promote invasion, and evaluate therapeutic interventions to inhibit invasion.

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.

Hyperthermia by MR-guided focused ultrasound: accurate temperature control based on fast MRI and a physical model of local energy deposition and heat conduction

Temperature regulation in MR-guided focused ultrasound requires rapid MR temperature mapping and automatic feedback control of the ultrasound output. Here, a regulation method is proposed based on a physical model of local energy deposition and heat conduction. The real-time evaluation of local temperature gradients from temperature maps is an essential element of the control system. Each time a new image is available, ultrasound power is adjusted on-the-fly in order to obtain the desired evolution of the focal point temperature. In vitro and in vivo performance indicated fast and accurate control of temperature and a large tolerance of errors in initial estimates of ultrasound absorption and heat conduction. When using correct estimates for the physical parameters of the model, focal point temperature was controlled within the measurement noise limit. Initial errors in absorption and diffusion parameters are compensated for exponentially with a user-defined response time, which is suggested to be on the order of 10 sec.

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.

MRI phase mapping of temperature distributions induced in food by microwave heating

A range of temperature-sensitive MRI parameters of water (T2, T1, diffusion coefficient, and chemical shift) were evaluated to map in three dimensions the non-uniform temperature distributions induced by microwave heating in both model and real food systems. Phase mapping was found to be the most robust method, and evaluations of possible experimental errors were based on semi-quantitative studies of homogeneous and heterogeneous systems. The MRI protocol provides complementary phase and magnitude data, which are related to the sample temperature and structural heterogeneity, respectively. Used together, they relate the temperature changes to the differential thermal properties of the various components within a heterogeneous sample. The potential applications of this technique to microwave and other forms of heating is discussed.

Reliability of water proton chemical shift temperature calibration for focused ultrasound ablation therapy

Our purpose in this work was to assess the reliability of the calibration coefficient for magnetic resonance water proton chemical shift temperature mapping. Over a six month period, the calibration coefficient was measured 15 times in several different phantoms. A highly linear relationship between water proton chemical shift and temperature change was found. The average temperature calibration coefficient determined from all studies was 0.009+/-0.001 ppm/degrees C. Four of the 15 studies were conducted on the same day using the same phantom. The average temperature calibration coefficient of these four studies was 0.0096+/-0.0001 ppm/degrees C.

Signal formation in echo-shifted sequences

Echo shifted (ES) gradient-recalled echo sequences (whose TE > TR) have found important applications in fMRI and MR thermometry since their introduction. The technique increases T *(2) weighting in BOLD imaging and temperature change sensitivity in phase-based temperature imaging compared to FLASH sequences. Yet, inconsistent observations have previously been reported when variants of this technique were used in various MRI experiments. Previous understanding of the ES sequences (the "FLASH-like" postulation) was not sufficient to explain these observations. This work provides an in-depth study on the various ES sequences. It was found that there are two types of ES sequences: ES-FLASH that spoils coherent transverse magnetization and ES-GRE, which is based on SSFP signals. A signal expression was derived for the clinically popular TR-periodic ES-GRE sequence with one echo shift. This signal expression reduced to the "FLASH-like" postulation under certain conditions. The new knowledge about the echo formation mechanism in ES sequences helps explain the experimental observations previously reported. Moreover, the resonance offset angle based analysis demonstrates an elegant methodology to analyze short TR imaging sequences. Magn Reson Med 42:864-875, 1999.

Simplified treatment planning for interstitial laser thermotherapy by disregarding light transport: a numerical study

BACKGROUND AND OBJECTIVE

The objective was to investigate the effect of light transport on the temperature distribution and the coagulated volume under conditions relevant to interstitial laser thermotherapy (ILT) of tumors in the human liver.

STUDY DESIGN/MATERIALS AND METHODS

Temperature distributions and coagulated volumes produced with a diffusing laser fiber or a conductive heat source, at equal output power, were numerically calculated for tissue with different optical penetration depths. Four irradiation times (5, 10, 20, and 30 min) were studied. A three-dimensional finite-element model was used to calculate the temperature distribution during heating with four conductive heat sources (no light emission). Results were compared with measured temperature distributions during laser irradiation in a gel phantom with known optical properties.

RESULTS

Numerical calculations showed that the influence of light transport on the coagulated volume was negligible in tissue with optical penetration depths below 3-4 mm at all studied irradiation times. The phantom experiment indicated good agreement with the calculated temperature distribution, both with a single diffusing laser fiber and with four fibers.

CONCLUSION

Light transport influences coagulated volumes only slightly under conditions presented in this work, which is relevant to ILT of tumors in the human liver.

Magnetic resonance imaging-guided focused ultrasound synovectomy

OBJECTIVE

To investigate the feasibility of magnetic resonance imaging (MRI)-guided high power focused ultrasound (FUS) to perform synovectomy noninvasively.

METHODS

Five New Zealand white male rabbit knees with experimentally induced arthritis underwent MRI-guided thermal surgery by high power (60 W/10 s) sonication. Evidence of tissue coagulation was monitored during the procedure and confirmed by gross and microscopic evaluation and MRI.

RESULTS

Partial synovectomy was performed in five animals. Necrotized synovial tissue was observed on gross and microscopic evaluation. Visible signal intensity alterations including high signal intensity on T2-weighted (T2W) images and lack of contrast-enhancement on T1-weighted (T1W) post-contrast, post-sonication images were characteristic and reproducible.

CONCLUSION

Our results demonstrate the ability of high power sonication to destroy synovial tissue in vivo.

Thermal dosimetry of a focused ultrasound beam in vivo by magnetic resonance imaging

Magnetic resonance imaging (MRI) thermometry has been utilized for in vivo evaluation of thermal exposure induced by a focused ultrasound beam. A simulation study of the focused ultrasound beam was conducted to select imaging parameters for reducing the error due to the spatial and temporal averaging of MRI. Temperature imaging based on the proton resonance frequency shift was utilized to obtain the temperature distribution during sonication in the skeletal muscle of eight rabbits. MRI-derived temperature information was then used to calculate the thermal dose distribution induced by the sonication and to estimate the coagulated tissue volume. The tissue changes were also evaluated directly by taking the T2-weighted and the contrast agent enhanced T1-weighted MR images. Errors in the temperature and thermal dose measurements were found to be minimal using the following parameters: slice thickness = 3 mm, voxel dimension = 0.6 mm, and scan time per image = 3.4 s. The estimated dimensions of the coagulated tissue volume were in good agreement with the tissue damages seen on the contrast agent enhanced T1-weighted images. The tissue damage seen on the histology was closely matched to the ones seen on the T2-weighted images. This study showed that MRI thermometry has significant potential for both monitoring the thermal exposure and evaluating the tissue damage. This would allow real-time control of the sonication parameters to optimize clinical treatments.

Fast lipid-suppressed MR temperature mapping with echo-shifted gradient-echo imaging and spectral-spatial excitation

The water proton resonance frequency (PRF) is temperature dependent and can thus be used for magnetic resonance (MR) thermometry. Since lipid proton resonance frequencies do not depend on temperature, fat suppression is essential for PRF-based temperature mapping. The efficacy of echo-shifted (TE > TR) gradient-echo imaging with spectral-spatial excitation is demonstrated, resulting in accurate and rapid, lipid-suppressed, MR thermometry. The method was validated on phantoms, fatty duck liver, and rat thigh, demonstrating improvements in both the speed and precision of temperature mapping. Heating of a rat thigh with focused ultrasound was monitored in vivo with an accuracy of 0.37 degree C and a time resolution of 438 msec.

An MRI calorimetry technique to measure tissue ultrasound absorption

High-intensity focused ultrasound (US) surgery guided by magnetic resonance imaging (MRI) is a very promising form of minimally invasive thermal therapy. To apply this technique optimally, the interaction mechanisms of high-intensity US with tissue need to be better understood, in particular, the variation of ultrasound absorption with frequency and temperature. However, agreement on the value of measured tissue US absorption is poor, largely because of intrinsic experimental complications of prior investigations. A new approach toward measuring tissue US absorption, based on a form of MRI calorimetry, is proposed here, which allows non-invasive energy measurement through spatial temperature mapping with MRI. A modified two-dimensional spoiled gradient-echo sequence has been implemented to map temperature based on proton resonance frequency (PRF) shift. Validation experiments show excellent agreement of MRI measured energy with that delivered by a calibrated source. MRI calorimetry of US heating of tissue-mimicking polyethylene glycerol material has been performed. Using a hydrophone measurement of the incident US field, its US absorption coefficient was measured as 0.032 cm-1. As this approach can be applied over a range of frequencies, tissues, and temperatures, it should provide a much improved means of measuring absolute tissue US absorption coefficients to improve US therapy planning, future transducer design, and US dosimetry models.

Temperature Measurement Using Echo-Shifted FLASH at Low Field for Interventional MRI

Temperature Measurement Using Echo-Shifted FLASH at Low Field for Interventional MRI. Yiu-Cho Chung, Jeffrey L. Duerk, Ajit Shankaranarayanan, Monika Hampke, Elmar M. Merkle, and Jonathan S. Lewin. (Article was originally published in the Journal of Magnetic Resonance Imaging, Volume 9, No. 1, 1999). In this article, some of the references were printed with the incorrect journal name. Here is the corrected list of references for this article.

Determination of the optimal delay between sonications during focused ultrasound surgery in rabbits by using MR imaging to monitor thermal buildup in vivo

PURPOSE

To use magnetic resonance (MR) imaging to monitor thermal buildup and its effects in treated tissues during sequentially delivered sonications in vivo to optimize the intersonication delay for any set of ultrasound and tissue parameters.

MATERIALS AND METHODS

Sequential sonications were delivered next to each other in both thighs in 10 male New Zealand white rabbits. The time between sonications was 11-60 seconds. Phase-difference MR imaging was used to monitor temperature rise, which was used to estimate the thermal dose delivered to the tissue. T2-weighted and contrast agent-enhanced T1-weighted imaging were used to gauge the extent of tissue coagulation.

RESULTS

With a short intersonication delay (11-40 seconds), the estimated temperature rise and the extent of tissue coagulation increased dramatically in subsequent sonications. However, when the delay was long (50-60 seconds), the size and shape of the destroyed tissue with subsequent sonications was uniform, and the temperature buildup was substantially lower.

CONCLUSION

MR imaging can be used to monitor thermal buildup and its effects due to sequential, neighboring sonications in vivo to produce evenly shaped regions of tissue coagulation. The temperature information obtained from the monitoring can be used to optimize the intersonication delay for any set of ultrasound and tissue parameters.

Quantitative MR temperature monitoring of high-intensity focused ultrasound therapy

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

Accuracy of MR phase mapping for temperature monitoring during interstitial laser coagulation (ILC) in the liver at rest and simulated respiration

The chemical shift or proton-resonance frequency (phase mapping) can be used to measure temperature changes. As a subtraction technique, it requires scans at exactly the same location, making it prone to respiration-induced artifacts. The accuracy of magnetic resonance (MR) phase mapping for temperature monitoring of interstitial laser coagulation (ILC) was therefore investigated in two ex vivo models with simulated respiration. MR temperatures were calibrated to interstitially measured temperature. Gradual cooling of a homogenous medium (gel) was monitored for four starting temperatures (room temperature, 40 degrees C, 50 degrees C, and 60 degrees C) during 30 min. Temperature increases were measured during ILC in ex vivo porcine liver with Nd:YAG for 6 min with 5 Watt. Experiments were performed at rest and with simulated respiratory motion (both n = 5). In liver, accuracy did not decrease with respiration simulation (P = 0.32), whereas a significant decline was found in the gel model (P = 0.002). In all experiments a small drift over time was observed between temperature determined with MR and thermoprobes. Correction for temperature-independent phase-shift at a reference location did not enhance agreement. Temperatures could be determined correctly by MR in the moving liver within a range of +/-3.5 degrees C after 6 min of laser application (95% confidence interval), justifying further pre-clinical studies.

Heat-source orientation and geometry dependence in proton-resonance frequency shift magnetic resonance thermometry

The proton-resonance frequency (PRF) shift method of thermometry has become a promising tool for magnetic resonance image-guided thermal therapies. Although the PRF thermal coefficient has recently been shown to be independent of tissue type when measured ex vivo, significant discrepancy remains on its value for tissues measured in vivo under a variety of experimental conditions. The authors identify a potential source of variation in the PRF thermal coefficient that arises from temperature-induced changes in the volume magnetic susceptibility of tissue and is dependent on the orientation and geometry of the heat-delivery device and its associated heat pattern. This study demonstrates that spatial variations in the apparent PRF thermal coefficient could lead to errors of up to +/-30% in the magnetic resonance estimated temperature change if this effect is ignored.

Real-time control of focused ultrasound heating based on rapid MR thermometry

RATIONALE AND OBJECTIVES

Real-time control of the heating procedure is essential for hyperthermia applications of focused ultrasound (FUS). The objective of this study is to demonstrate the feasibility of MRI-controlled FUS.

METHODS

An automatic control system was developed using a dedicated interface between the MR system control computer and the FUS wave generator. Two algorithms were used to regulate FUS power to maintain the focal point temperature at a desired level.

RESULTS

Automatic control of FUS power level was demonstrated ex vivo at three target temperature levels (increase of 5 degrees C, 10 degrees C, and 30 degrees C above room temperature) during 30-minute hyperthermic periods. Preliminary in vivo results on rat leg muscle confirm that necrosis estimate, calculated on-line during FUS sonication, allows prediction of tissue damage. CONCLUSIONS. The feasibility of fully automatic FUS control based on MRI thermometry has been demonstrated.