Resonance Imaging Thermometry Research Articles

Assessing critical temperature dose areas in the kidney by magnetic resonance imaging thermometry in an ex vivo Holmium:YAG laser lithotripsy model

PurposeWe aimed to assess critical temperature areas in the kidney parenchyma using magnetic resonance thermometry (MRT) in an ex vivo Holmium:YAG laser lithotripsy model.MethodsThermal effects of Ho:YAG laser irradiation of 14 W and 30 W were investigated in the calyx and renal pelvis of an ex vivo kidney with different laser application times (tL) followed by a delay time (tD) of tL/tD = 5/5 s, 5/10 s, 10/5 s, 10/10 s, and 20/0 s, with irrigation rates of 10, 30, 50, 70, and 100 ml/min. Using MRT, the size of the area was determined in which the thermal dose as measured by the Cumulative Equivalent Minutes (CEM43) method exceeded a value of 120 min.ResultsIn the calyx, CEM43 never exceeded 120 min for flow rates ≥ 70 ml/min at 14 W, and longer tL (10 s vs. 5 s) lead to exponentially lower thermal affection of tissue (3.6 vs. 21.9 mm2). Similarly at 30 W and ≥ 70 ml/min CEM43 was below 120 min. Interestingly, at irrigation rates of 10 ml/min, tL = 10 s and tD = 10 s CEM43 were observed > 120 min in an area of 84.4 mm2 and 49.1 mm2 at tD = 5 s. Here, tL = 5 s revealed relevant thermal affection of 29.1 mm2 at 10 ml/min.ConclusionWe demonstrate that critical temperature dose areas in the kidney parenchyma were associated with high laser power and application times, a low irrigation rate, and anatomical volume of the targeted calyx.

Analysis of cavitation artifacts in Magnetic Resonance Imaging Thermometry during laser ablation monitoring.

Magnetic Resonance Thermometry Imaging (MRTI) holds great potential in laser ablation (LA) monitoring. It provides the real-time multidimensional visualization of the treatment effect inside the body, thus enabling accurate intraoperative prediction of the thermal damage induced. Despite its great potential., thermal maps obtained with MRTI may be affected by numerous artifacts. Among the sources of error producing artifacts in the images., the cavitation phenomena which could occur in the tissue during LA induces dipole-structured artifacts. In this work., an analysis of the cavitation artifacts occurring during LA in a gelatin phantom in terms of symmetry in space and symmetry of temperature values was performed. Results of 2 Wand 4 W laser power were compared finding higher symmetry for the 2 W case in terms of both dimensions of artifact-lobes and difference in temperature values extracted in specular pixels in the image. This preliminary investigation of artifact features may provide a step forward in the identification of the best strategy to correct and avoid artifact occurrence during thermal therapy monitoring. Clinical Relevance- This work presents an analysis of cavitation artifacts in MRTI from LA which must be corrected to avoid error in the prediction of thermal damage during LA monitoring.

Exposure to Long Magnetic Resonance Imaging Thermometry Does Not Cause Significant DNA Double-Strand Breaks on CF-1 Mice

Exposure to Long Magnetic Resonance Imaging Thermometry Does Not Cause Significant DNA Double-Strand Breaks on CF-1 Mice

On the optimization of imaging parameters for magnetic resonance imaging thermometry using magnetic microparticles

Magnetic Resonance Imaging thermometry is an extremely useful technique which allows one to determine, noninvasively, the temperature deep in the tissue in two or three dimensions. Many methods of MR thermometry have been developed, including those that rely on the intrinsic MR properties of tissue and those which depend on the addition of contrast agents injected into the tissue to create temperature dependent MR images. One such method is to introduce magnetic particles whose magnetization’s temperature dependence influences the MR properties of the surrounding tissue and obtain temperature from calibrated intensity changes of T2* weighted MR images. One limitation of this method is the temperature resolution which is determined by the rate of change of the magnetization with temperature. One can change the MR response either through varying the particles properties or finding the MR scan parameters which maximize the image contrast due to T2* weighting of images. In this work we calculate the MR signal strength, using known values of T1 and T2* relaxation times for agarose gel phantoms with embedded magnetic particles, and compared this with the temperature dependent intensity of experimental MR images. We seek to optimize the change in signal intensity with temperature by varying the selectable MR scanner parameters: echo time, repetition time, and flip angle. Based on comparison with experimental data we find that the change in signal with temperature can be significantly increased (by as much as 100%) through the appropriate choice of MR scan parameters.

Magnetic resonance thermometry using a GdIII-based contrast agent.

The complexes described here serve as contrast agents for magnetic resonance imaging thermometry. The complexes differentially enhance contrast between 275 and 325 K. The basis of the temperature response of the fluorinated contrast complex is the modulation of water exchange caused by trifluoromethyl groups that can be chemically controlled.

Structural, magnetic and toxicity studies of ferrite particles employed as contrast agents for magnetic resonance imaging thermometry

We carried out comprehensive structural, magnetic and toxicity studies of Mg1−xZnxFe2O4 and Cu1−xZnxFe2O4 ferrites that we plan to use as contrast agents for magnetic resonance-imaging thermometry. Samples were prepared by a standard ceramic technique. Their structural properties were investigated with an x-ray diffraction system, and data was analyzed using the Rietveld method that allowed us to determine cations distribution. Magnetic properties of MgZn and CuZn ferrites were studied using a superconducting quantum interference device magnetometer, a vibrating sample magnetometer, and Mössbauer spectrometer. Magnetometry measurements were cross-correlated with XRD results and compared with magnetic properties determined by Mössbauer spectroscopy, allowing us to understand the magnetic structure of studied ferrites. The temperature dependent magnetic measurements were used as guidance to select the most suitable MgZn and CuZn ferrites compositions for MRI thermometry near human body temperature. The inflammation studies indicated that MgZn ferrites are more suitable from the toxicology point of view for MRI thermometry.

Nano-sized ferrite particles for magnetic resonance imaging thermometry

Recently, we reported the application of magnetic particles as temperature sensors for use in magnetic resonance imaging thermometry (tMRI). In this novel method, the brightness of the magnetic resonance (MR) image changes with temperature due to a temperature-dependent local magnetic field inhomogeneity caused by the dipolar field of the magnetic particles. Ferrites are new and promising compounds for tMRI applications because of their biocompatibility and because their magnetic properties can be varied by changing composition. Earlier studies used micrometer sized ferrite particles in a proof-of-concept demonstration. However, such large particles cannot be administered intravenously for in-vivo use. In this report, we establish the use of nanoscale ferrite particles as temperature sensors for tMRI. Scanning transmission electron microscopy and X-ray diffraction demonstrate the synthesis of 210 nm Co0.3Zn0.7Fe2O4 clusters comprised of 10–30 nm crystallites. Temperature-dependent magnetization measurements reveal a Curie temperature of around 275 K. We conducted nuclear magnetic resonance (NMR) and MRI studies of samples with different concentrations of ferrite nanoparticles suspended in agar gel. The relative MR image intensity shows a near-linear temperature dependence. At concentrations as low as 0.12 g/L these ferrite nanoparticles provide sufficient image contrast to determine temperature changes with accuracy of ±1.0 K at 310 K, bolstering the potential viability of this material for bio-medical applications.

Development of Ferrite-Based Temperature Sensors for Magnetic Resonance Imaging: A Study of Cu1−xZnxFe2O4

We investigate the use of Cu1-x Zn x Fe2O4 ferrites (0.60 < x < 0.76) as potential sensors for magnetic- resonance-imaging thermometry. Samples are prepared by a standard ceramic technique. Their structural and magnetic properties are determined using x-ray diffraction, scanning electron microscopy, super-conducting quantum-interference device magnetometry, and Mossbauer and 3-T nuclear-magnetic-resonance spectroscopies. We use the mass magnetization of powdered ferrites and transverse relaxivity r*2 of water protons in Ringer's-solution-based agar gels with embedded micron-sized particles to determine the best composition for magnetic-resonance-imaging (MRI) temperature sensors in the (280-323)-K range. A preclinical 3-T MRI scanner is employed to acquire T*2 weighted temperature-dependent images. The brightness of the MRI images is cross-correlated with the temperature of the phantoms, which allows for a temperature determination with approximately 1°C accuracy. We determine that the composition of 0.65 < x < 0.70 is the most suitable for MRI thermometry near human body temperature.

Laser Interstitial Thermal Therapy Technology, Physics of Magnetic Resonance Imaging Thermometry, and Technical Considerations for Proper Catheter Placement During Magnetic Resonance Imaging-Guided Laser Interstitial Thermal Therapy.

MRg-LITT, magnetic resonance-guided laser-induced thermal therapyPAD, precision aiming device.

A simple method for determining the coagulation threshold temperature of transparent tissue-mimicking thermal therapy gel phantoms: Validated by magnetic resonance imaging thermometry.

Tissue-mimicking thermal therapy phantoms that coagulate at specific temperatures are valuable tools for developing and evaluating treatment strategies related to thermal therapy. Here, the authors propose a simple and efficient method for determining the coagulation threshold temperature of transparent thermal therapy gel phantoms. The authors used a previously published gel phantom recipe with 2% (w/v) of bovine serum albumin as the temperature-sensitive protein. Using the programmable heating settings of a polymerase chain reaction (PCR) machine, the authors heated 50 μl gel samples to various temperatures for 3 min and then imaged them using the BioRad Gel Doc system to determine the coagulation temperature using an opacity quantification method. The estimated coagulation temperatures were then validated for gel phantoms prepared with different pH levels using high-intensity focused ultrasound (HIFU) heating and magnetic resonance imaging (MRI) thermometry methods on a clinical MR-HIFU system. The PCR heating method produced consistent and reproducible coagulation of gel samples in precise correlation with the set incubation temperatures. The resulting coagulation threshold temperatures for gel phantoms of varying pH levels were found to be 44.1 ± 0.1, 53.4 ± 0.9, and 60.3 ± 0.9 °C for pH levels of 4.25, 4.50, and 4.75, respectively. This corresponded well with the coagulation threshold temperatures determined by MR-thermometry, with coagulation defined as a 95% decrease in T2 relaxation time, which were estimated at 53.6 ± 1.9 and 62.9 ± 2.4 °C for a pH of 4.50 and 4.75, respectively. The opacity quantification method provides a fast and reproducible estimate of the coagulation threshold temperature of transparent temperature-sensitive gel phantoms. The temperatures determined using this method were well within the range of temperatures estimated using MR-thermometry. Due to the specific heating capabilities of the PCR machine, and the robust determination of coagulation threshold temperatures based on the statistically significant increase in the opacity of gel samples, coagulation temperatures can be determined more precisely and with less variability compared to MRI-based methods.

Magnetic Resonance Image Guided Transurethral Ultrasound Prostate Ablation: A Preclinical Safety and Feasibility Study with 28-Day Followup

We determine the safety and feasibility of magnetic resonance image guided transurethral ultrasound prostate ablation using active temperature feedback control in a preclinical canine model with 28-day followup. After a long acclimatization period we performed ultrasound treatment in 8 subjects using the magnetic resonance image guided TULSA-PRO™ transurethral ultrasound prostate ablation system. Comprehensive examinations and observations were done before and throughout the 28-day followup, including assessment of clinically significant treatment related adverse events. In addition to gross pathology evaluation, extensive histopathological analysis was done to assess cell kill inside and outside the prostate. We evaluated prostate conformal heating by comparing the spatial difference between the treatment plan and the 55C isotherm measured on magnetic resonance imaging thermometry acquired during treatment. These findings were confirmed on contrast enhanced magnetic resonance imaging immediately after treatment and at 28 days. Clinically there were no adverse events in any of the 8 subjects throughout the 28-day followup. All subjects had normal urinary and bowel function. Gross necropsy and histology confirmed that the intended thermal cell kill was confined to the prostate. No surrounding tissue was damaged, including the rectum and the external urinary sphincter. Conformal heating was achieved with an average -0.9 mm accuracy and 0.9 mm precision. Contrast enhanced magnetic resonance imaging and histological analysis confirmed tissue ablation in targeted areas of the prostate. Urethral tissue was spared from thermal damage. Magnetic resonance image guided transurethral ultrasound is a safe, feasible procedure for accurate and precise conformal thermal ablation of prostate tissue, as demonstrated in a preclinical model with 28-day followup.

MR-Guided High-Intensity Focused Ultrasound: Current Status of an Emerging Technology

The concept of ideal tumor surgery is to remove the neoplastic tissue without damaging adjacent normal structures. High-intensity focused ultrasound (HIFU) was developed in the 1940s as a viable thermal tissue ablation approach. In clinical practice, HIFU has been applied to treat a variety of solid benign and malignant lesions, including pancreas, liver, prostate, and breast carcinomas, soft tissue sarcomas, and uterine fibroids. More recently, magnetic resonance guidance has been applied for treatment monitoring during focused ultrasound procedures (magnetic resonance-guided focused ultrasound, MRgFUS). Intraoperative magnetic resonance imaging provides the best possible tumor extension and dynamic control of energy deposition using real-time magnetic resonance imaging thermometry. We introduce the fundamental principles and clinical indications of the MRgFUS technique; we also report different treatment options and personal outcomes.

Measurement of temperature changes in cooling dead rats using magnetic resonance thermometry

Magnetic resonance imaging thermometry has been introduced as a technique for measurement of temperature changes in cooling dead rats. Rat pelvic magnetic resonance images were acquired sequentially more than 2 h after euthanasia by halothane overdose. A series of temperature difference maps in cooling dead rats was obtained with calculating imaging phase changes induced by the water proton frequency shift caused by temperature changes. Different cooling processes were monitored by the temperature difference maps in the rats. Magnetic resonance imaging thermometry applied in the study of laboratory animals could theoretically reproduce a variety of causes of death with different environmental conditions. Outcomes from experimental animal studies could be translated into a temperature-based time of death estimation in forensics.

Thermally-triggered ‘off–on–off’ response of gadolinium-hydrogel–lipid hybrid nanoparticles defines a customizable temperature window for non-invasive magnetic resonance imaging thermometry

For effective and safe thermotherapy, real-time, accurate, three-dimensional tissue thermometry is required. Magnetic resonance imaging (MRI)-based thermometry in combination with current temperature responsive contrast agents only provides an ‘off–on’ signal at a certain temperature, not indicating temperature increases beyond the desired therapeutic levels. To overcome this limitation, a novel Gd-chelated hydrogel–lipid hybrid nanoparticle (HLN) formulation was developed that provides an ‘off–on–off’ signal defining a thermometric window for MR thermometry. Novel thermally responsive poly(N-isopropylacrylamide-co-acrylamide) (NIPAM–co-AM) hydrogel nanoparticles (<15nm) with bisallylamidodiethylenetriaminetriacetic acid, a novel crosslinker with Gd3+ chelation functionality, were synthesized. The Gd-hydrogel nanoparticles were encapsulated in a solid lipid nanoparticle matrix that prevented T1-weighted contrast signal enhancement. Melting of the matrix lipid freed the Gd-hydrogel nanoparticles into the bulk water and an ‘off–on’ contrast signal enhancement occurred. As the temperature was further increased to temperatures greater than, the volume phase transition temperature of the hydrogel nanoparticles, they collapsed and provided an ‘on-off’ signal diminution. Both the ‘off–on’ and the ‘on–off’ transition temperature could be tailored by changing the lipid matrix and altering the NIPAM/AM ratio in the hydrogel, respectively. This allowed MRI thermometry of different temperature windows using the Gd-HLN system.

Full-field and real-time surface plasmon resonance imaging thermometry

The feasibility of surface plasmon resonance (SPR) imaging thermometry is tested as a potential tool for full-field and real-time temperature field mapping for thermally transient liquid mediums. Using the well-known Kretschmann's analysis [Physik 241, 313 (1971)]. parametric examinations are performed to delineate the effects of important optical properties, including seven different prism materials with different refractive index values and seven different measured dielectric constants for thin gold (Au) films (approximately 47.5 nm in thickness), on the temperature dependence of SPR reflectance intensity variations. Furthermore, a laboratory-implemented real-time SPR thermometry system demonstrates the full-field mapping capabilities for transient temperature field developments in the near-wall region when a hot water droplet (80 degrees C) contacts the Au metal surface (20 degrees C) and spreads either in an air- or in a water-surrounded environment.

Prostate Tissue Analysis Immediately Following Magnetic Resonance Imaging Guided Transurethral Ultrasound Thermal Therapy

Preclinical experiments were performed in an acute canine model to analyze the spatial pattern of thermal damage generated in the prostate gland following treatment with a prototype magnetic resonance imaging guided transurethral ultrasound heating system. In particular the boundary of tissue coagulation was analyzed to quantify the treatment margin resulting from this technology. A heating device incorporating a planar 20 x 3.5 mm transducer operated at 9.1 MHz was used to deliver ultrasound energy to targeted regions in the prostate gland in 7 animals monitored with magnetic resonance imaging thermometry during heating. The animals were sacrificed approximately 45 minutes after treatment. The thermal damage pattern was evaluated using contrast enhanced magnetic resonance imaging, vital tissue staining, and whole mount hematoxylin and eosin stained histological sections. An image warping technique enabled quantitative comparison of these data. Regions of thermal fixation, coagulative necrosis and hemorrhage were observed in the treated prostate glands. The extent of the necrotic region was relatively insensitive to vessel cooling effects. Metabolic enzyme functionality coincided with tissue outside of the treatment area. At the edge of the thermal damage pattern the transition from coagulative necrosis to no visible damage occurred within 3 mm or less. The narrow extent of the thermal margin suggests that tissue sparing outside of the prostate could be an advantage of this treatment. Histological measurements showed a high level of spatial accuracy, useful for developing accurate control techniques for directional transurethral ultrasound thermal therapy in the treatment of prostate diseases.

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.