Magnetic resonance thermometry using a GdIII-based contrast agent

The complexes described here serve as contrast agents for magnetic resonance imaging thermometry.

Clinical Performance and Future Potential of Magnetic Resonance Thermometry in Hyperthermia

Hyperthermia treatments in the clinic rely on accurate temperature measurements to guide treatments and evaluate clinical outcome. Currently, magnetic resonance thermometry (MRT) is the only clinical option to non-invasively measure 3D temperature distributions. In this review, we evaluate the status quo and emerging approaches in this evolving technology for replacing conventional dosimetry based on intraluminal or invasively placed probes. First, we define standardized MRT performance thresholds, aiming at facilitating transparency in this field when comparing MR temperature mapping performance for the various scenarios that hyperthermia is currently applied in the clinic. This is based upon our clinical experience of treating nearly 4000 patients with superficial and deep hyperthermia. Second, we perform a systematic literature review, assessing MRT performance in (I) clinical and (II) pre-clinical papers. From (I) we identify the current clinical status of MRT, including the problems faced and from (II) we extract promising new techniques with the potential to accelerate progress. From (I) we found that the basic requirements for MRT during hyperthermia in the clinic are largely met for regions without motion, for example extremities. In more challenging regions (abdomen and thorax), progress has been stagnating after the clinical introduction of MRT-guided hyperthermia over 20 years ago. One clear difficulty for advancement is that performance is not or not uniformly reported, but also that studies often omit important details regarding their approach. Motion was found to be the common main issue hindering accurate MRT. Based on (II), we reported and highlighted promising developments to tackle the issues resulting from motion (directly or indirectly), including new developments as well as optimization of already existing strategies. Combined, these may have the potential to facilitate improvement in MRT in the form of more stable and reliable measurements via better stability and accuracy.

Fiber Bragg Grating Sensors for Performance Evaluation of Fast Magnetic Resonance Thermometry on Synthetic Phantom

The increasing recognition of minimally invasive thermal treatment of tumors motivate the development of accurate thermometry approaches for guaranteeing the therapeutic efficacy and safety. Magnetic Resonance Thermometry Imaging (MRTI) is nowadays considered the gold-standard in thermometry for tumor thermal therapy, and assessment of its performances is required for clinical applications. This study evaluates the accuracy of fast MRTI on a synthetic phantom, using dense ultra-short Fiber Bragg Grating (FBG) array, as a reference. Fast MRTI is achieved with a multi-slice gradient-echo echo-planar imaging (GRE-EPI) sequence, allowing monitoring the temperature increase induced with a 980 nm laser source. The temperature distributions measured with 1 mm-spatial resolution with both FBGs and MRTI were compared. The root mean squared error (RMSE) value obtained by comparing temperature profiles showed a maximum error of 1.2 °C. The Bland-Altman analysis revealed a mean of difference of 0.1 °C and limits of agreement 1.5/−1.3 °C. FBG sensors allowed to extensively assess the performances of the GRE-EPI sequence, in addition to the information on the MRTI precision estimated by considering the signal-to-noise ratio of the images (0.4 °C). Overall, the results obtained for the GRE-EPI fully satisfy the accuracy (~2 °C) required for proper temperature monitoring during thermal therapies.

Magnetic Resonance Thermometry: An Emerging Three-Dimensional Temperature Diagnostic Technique

Magnetic Resonance Thermometry (MRT) is a developing diagnostic technique that leverages advanced medical technologies to accurately measure the temperature of a fluid flow within and around complex geometries. The full three-dimensional temperature field obtained by MRT can be used to analyze heat transfer characteristics and potentially investigate thermal boundary layers near arbitrarily complex surfaces. This technique requires neither optical nor physical accessibility, thereby enabling a wide range of engineering applications. This paper describes the current state of the art for MRT measurement, detailing turbulent water channel tests, materials selection, scanning parameters, data analysis of time-averaged temperature measurements, and uncertainty estimates. The purpose of this work was to evaluate and refine the MRT technique to increase the accuracy of temperature measurements and minimize the error in fully turbulent flow measurements. In the present study, a plate with a vertical cylinder extending from both of its sides was placed between two channels, and a diagonal hole was drilled through the cylinder from one side of the plate to the other. This enabled fluid from one channel to mix with the fluid in the other. This experiment studied the mixing of two fluids at different temperatures. The upstream temperatures of each fluid were measured with thermocouples. Both flows were fully turbulent, and the colder temperature channel had a Reynolds number of 11,800. Tests were run with four different fluid temperatures for calibration and to determine any temperature dependence of measurements. Three-dimensional temperature field measurements are reported and details about data processing and procedures to conduct the experiments are provided. This work resulted in several notable improvements to MRT experimental methods. The test section and water channel were designed to limit the effects of thermal expansion in the stereolithography materials used for manufacturing the complex internal flow geometry. Multiple echo scans were used to minimize the effects of magnetic field drift commonly observed in extended scanning periods in MRI systems. Data analysis techniques were used to quantify expansion effects for both hot and cold flow cases. To quantify measurement uncertainty, the standard deviation of the mean was calculated at each data point across different scan numbers and confidence intervals established using a student t-test. An improved data processing code was used to filter data resulting in increased measurement accuracy and reduced uncertainty to less than 1 °C for most of the domain. Future work will further refine the experimental techniques to improve scanning procedures, employ high conductivity ceramics and larger geometries with relevant applications, and simplify data processing methods to generate full-field flow temperature data.

Three-Dimensional Velocity and Temperature Field Measurements of Internal and External Turbine Blade Features Using Magnetic Resonance Thermometry

Magnetic resonance thermometry (MRT) is a maturing diagnostic tool used to measure three-dimensional temperature fields. It has a great potential for investigating fluid flows within complex geometries leveraging medical grade magnetic resonance imaging (MRI) equipment and software along with novel measurement techniques. The efficacy of the method in engineering applications increases when coupled with other well-established MRI-based techniques such as magnetic resonance velocimetry (MRV). In this study, a challenging geometry is presented with the direct application to a complex gas turbine blade cooling scheme. Turbulent external flow with a Reynolds number of 136,000 passes a hollowed NACA-0012 airfoil with internal cooling features. Inserts within the airfoil, fed by a second flow line with an average temperature difference of 30 K from the main flow and a temperature-dependent Reynolds number in excess of 1,800, produces a conjugate heat transfer scenario including impingement cooling on the inside surface of the airfoil. The airfoil cooling scheme also includes zonal recirculation, surface film cooling, and trailing edge ejection features. The entire airfoil surface is constructed of a stereolithography resin—Accura 60—with low thermal conductivity. The three-dimensional internal and external velocity field is measured using an MRV. The fluid temperature field is measured within and outside of the airfoil with an MRT, and the results are compared with a computational fluid dynamics (CFD) solution to assess the current state of the art for combined MRV/MRT techniques for investigating these complex internal and external flows. The accompanying CFD analysis provides a prediction of the velocity and temperature fields, allowing for errors in the MRT technique to be estimated.