Kagayaki Kuroda

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. Magn Reson Med 43:220–225, 2000.

An inverse method to optimize heating conditions in RF-capacitive hyperthermia

An inverse method to directly optimize the electrode configuration (positions, sizes, and driving voltages) for radio frequency (RF) capacitive hyperthermia was proposed. The main algorithm, based on the two-dimensional finite element method (2-D-FEM) solution of Laplace and bio-heat transfer equations, iteratively modified the individual boundary potentials around an object thereby making a calculated temperature distribution approach a target temperature distribution. A penalty function governed continuity and smoothness among the boundary potentials so that the optimized boundary potentials became attainable for two plate electrodes. Case simulations demonstrated the viability of the algorithm. For instance, in a computed tomography (CT)-based human abdomen model which had deep-and shallow-seated tumors, the optimized electrodes produced a temperature distribution suitable for heating the tumors; the average temperature differences between the tumors and normal tissues were 3.5/spl deg/C for the deep-seated and 7.6/spl deg/C for the shallow-seated tumors within 600 s of heating. A drawback with the present algorithm is that the choice of penalty coefficient and modification of the boundary potentials to coincide with the use of two plate electrodes are carried out manually. These procedures would be automated.

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/°C for the in vitro studies and −0.0089 ppm/°C for the in vivo studies. The error range was estimated to be ± 3°C and ± 4.5 °C, respectively. Color‐coded temperature maps were compared with macroscopic lesion sizes of the specimen. Regions defined using a 20°C elevation in the initial images following hot saline injection (around 55°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. J. Magn. Reson. Imaging 2000;12:330–338.

Effects of blood perfusion rate on the optimization of RF-capacitive hyperthermia

The effects of the blood perfusion rate on the optimization of heating conditions in radio-frequency capacitive hyperthermia were examined using numerical simulations. When the blood perfusion rate in the tumor was smaller than approximately one-half that of normal tissues, optimal selective heating of the tumor was obtained.

Real-time monitoring and analysis of MR-guided laser ablation in an open-configuration MR system

Our goal was to investigate whether an open 0.5T MR-system with integrated frameless stereotactic guidance tools can provide sufficient intraoperative monitoring of interstitial laser therapy (ILT). Temperature-sensitive T1-weighted Fast- Spin-Echo (FSE)- or Spoiled Gradient-Echo sequences (SPGR) were applied and various image processing techniques (pixel- subtraction, phase mapping, optical flow computation) developed in order to control the thermal energy deposition during ILT in patients with brain- and liver tumors. While images from T1-weighted FSE- or SPGR sequences were acquired within 5 - 13 seconds, ILT lasted 2 to 26 minutes. Pixel subtraction or optical flow computation of T1-weighted images was performed within less than 110 msec. Alternating, phase- mapping of real- and imaginary components of SPGR sequences was performed within 220 msec. Pixel subtraction of T1- weighted images identified thermal changes in liver and brain tumors but could not evaluate the temperature values as chemical-shift based imaging, which was however, more affected by susceptibility effects and motion. Optical flow computation displayed the predicted course of thermal changes and revealed that the rate of heat deposition can be anisotropic, which may be related to heterogeneous tumor structure and/or vascularization.

Non-uniform grid finite-difference time-domain method for the simulation of electromagnetic distributions

To solve the memory requirement problem of the FDTD method some previous studies presented subgridding methods that employ two different grid sizes to divide the calculation space. With small structures or on the boundary of different materials a small grid size is used, otherwise a large grid size is used. There are two types of subgridding: the mesh refinement algorithm (MRA) and the variable step size method (VSSM). The MRA makes two runs resulting in a longer computing time. The VSSM varies the step from area to area; this limits its applicability. Both methods use a coarse grid as the boundary of the refinement grid; because the coarse grid is not accurate this affected the refinement result. As a development of the VSSM, the article presents a non-uniform method that can freely use any grid size in the computation area only that the grids are integer times each other. The non-uniform method uses the adjacent grid as the boundary of a smaller grid size, this increases the calculation accuracy. A simulation study of a human thigh section in microwave hyperthermia was examined by this method.

Navigation technique for MR-endoscope system using a wireless accelerometer-based remote control device

The MR-endoscope system can perform magnetic resonance (MR) imaging during endoscopy and show the images obtained by using endoscope and MR. The MR-endoscope system can acquire a high-spatial resolution MR image with an intraluminal radiofrequency (RF) coil, and the navigation system shows the scopes location and orientation inside the human body and indicates MR images with a scope view. In order to conveniently perform an endoscopy and MR procedure, the design of the user interface is very important because it provides useful information. In this study, we propose a navigation system using a wireless accelerometer-based controller with Bluetooth technology and a navigation technique to set the intraluminal RF coil using the navigation system. The feasibility of using this wireless controller in the MR shield room was validated via phantom examinations of the influence on MR procedures and navigation accuracy. In vitro examinations using an isolated porcine stomach demonstrated the effectiveness of the navigation technique using a wireless remote-control device.

Theoretical and experimental studies on the transurethral applicator for thermal treatment of benign prostatic hyperplasia

This paper deals with the heating characteristics of dipole transurethral applicators for thermal treatment of BPH. A new mathematical model that describing the near field of intracavitary applicators was presented. The temperature field which considered the applicators surface cooling was calculated with the above model by finite difference method. The simulation results of SAR and temperature distributions were examined by experiments. Based on the comparison between the theoretical and experimental results, some discussions to make the BPH treatments fit best to the patients situation are given.