Yasutoshi Ishihara

Method and apparatus for non-invasive measurement of temperature distribution within target body using nuclear magnetic resonance imaging

A non-invasive measurement of a temperature distribution within a target body using a nuclear magnetic resonance imaging, capable of realizing a high speed and a high precision measurement, and accounting for a displacement of the target body during the measurement. The chemical shift data from the target body at each voxel in an imaging target region on the target body are collected with and without a temperature change of the target body, a difference between the chemical shift data collected with the temperature change and the chemical shift data collected without the temperature change at each voxel, and a temperature distribution image is constructed and displayed according to the difference calculated. The chemical shift data are preferably collected by using a phase mapping imaging sequence.

Human brain glucose metabolism mapping using multislice 2D 1H‐13C correlation HSQC spectroscopy

A method for multivolume 2D 1H‐13C correlation spectroscopy, multislice heteronuclear single quantum coherence (HSQC), is proposed. This permits human brain metabolism from glucose to amino acids to be followed using a 2‐T whole‐body scanner. The modifications from the conventional HSQC are that the 180°(13C) and 180°(1H) pulses are separated in time in the preparation period and that the 180°(13C) pulse is applied at 1/(4JCH) before the 90°(1H) polarization transfer (PT) pulse. The preparation (echo) time can be set longer than 1/(2JCH) so that, even in a whole‐body system, slice‐selective pulses and gradients can be applied. Another modification is that the 90°(1H) reverse PT pulses after the creation of 2IzSz are used as multislice pulses. The time‐course of glutamate C4 could be followed with 15‐min temporal resolution from the HSQC spectra obtained from the brains of volunteers after the oral administration of glucose C1, and the maximum S/N was 3. Magn Reson Med 43:525–533, 2000.

3D localized 1H-13C heteronuclear single-quantum coherence correlation spectroscopy in vivo

A method for spatially three‐dimensional (3D) localized two‐dimensional (2D) 1H‐13C correlation spectroscopy, localized HSQC, is proposed. This method has the following special feature in the preparation period. The 180°(13C) and 180°(1H) pulses are separated in time, and the 180°(13C) pulse is applied at 1/(4 1JCH) before the 90°(1H) polarization transfer pulse. The preparation (echo) period 2τ can then be set substantially longer than 1/(2 1JCH), so that even in a whole‐body system, slice‐selective 90°(1H) pulses and gradient pulses can be applied in that period. The localization capabilities of this method were confirmed in a phantom experiment. The 3D localized 2D 1H‐13C correlation spectra from a monkey brain in vivo were obtained after [1‐13C]glucose injection, and amino acid metabolism was detected; that is, [4‐13C]glutamate appeared immediately after the injection, followed by the appearance of [2‐13C]glutamate, [3‐13C]glutamate, and [4‐13C]glutamine. Magn Reson Med 43:200–210, 2000.

Hyperthermia applicator based on a reentrant cavity for localized head and neck tumors

A new applicator based on a reentrant cavity is proposed for treating localized tumors such as those of the head and neck. In order to effectively heat the localized tumor without causing dissipation of heat into the surrounding normal tissues, the electric field must be localized over the target region. Although a small applicator may produce an appropriate localized electric field, the higher resonant frequency due to downsizing of the equipment results in very poor heating distribution; this occurs due to the changes in electric permittivity and conductivity consequent upon an increase in the resonant frequency. In this article, we introduce a method for reducing the resonant frequency by inserting a dielectric material into the applicator; the efficacy of this method has been determined by calculating the electromagnetic field and heating distribution with the help of the finite element method. By using the proposed applicator, a reduction in the resonant frequency and localized heating over spherical regions 100 mm in diameter can be achieved.

Temperature monitoring of internal body heating induced by decoupling pulses in animal 13C-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 (13C‐MRS), is proposed. The method is based on the temperature dependence of the water proton chemical shift (−0.01 ppm/°C) combined with phase mapping. Using this method, temperature changes were measured in rats (n = 4) employing practical 1H‐decoupled 13C‐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 = 128 × 32, spatial resolution = 0.8 × 1.5 mm). Measurement error was 0.18°C (SD) for 1D acquisition and 0.39°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.

Evaluation of magnetic nanoparticle samples made from biocompatible ferucarbotran by time-correlation magnetic particle imaging reconstruction method

BackgroundMolecular imaging using magnetic nanoparticles (MNPs)—magnetic particle imaging (MPI)—has attracted interest for the early diagnosis of cancer and cardiovascular disease. However, because a steep local magnetic field distribution is required to obtain a defined image, sophisticated hardware is required. Therefore, it is desirable to realize excellent image quality even with low-performance hardware. In this study, the spatial resolution of MPI was evaluated using an image reconstruction method based on the correlation information of the magnetization signal in a time domain and by applying MNP samples made from biocompatible ferucarbotran that have adjusted particle diameters.MethodsThe magnetization characteristics and particle diameters of four types of MNP samples made from ferucarbotran were evaluated. A numerical analysis based on our proposed method that calculates the image intensity from correlation information between the magnetization signal generated from MNPs and the system function was attempted, and the obtained image quality was compared with that using the prototype in terms of image resolution and image artifacts.ResultsMNP samples obtained by adjusting ferucarbotran showed superior properties to conventional ferucarbotran samples, and numerical analysis showed that the same image quality could be obtained using a gradient magnetic field generator with 0.6 times the performance. However, because image blurring was included theoretically by the proposed method, an algorithm will be required to improve performance.ConclusionsMNP samples obtained by adjusting ferucarbotran showed magnetizing properties superior to conventional ferucarbotran samples, and by using such samples, comparable image quality (spatial resolution) could be obtained with a lower gradient magnetic field intensity.

Heating applicator based on reentrant cavity with optimized local heating characteristics

Purpose: A reentrant-cavity-based applicator can produce a concentrated electric field between reentrant electrodes for localized heating. However, this field is inadequate for treating early small tumors localized in the head and neck. In order to safely heat such well-localized lesions, the electric field distribution should be more localized. Materials and methods: In order to achieve localized heating, four parameters of the reentrant cavity (applicator height, outer diameter, reentrant diameter, and reentrant gap size), which influence the distribution of the electric field produced in the reentrant gap, are optimized using the Taguchi method. The variation in the heating characteristics affected by the size of the heating object is estimated using the signal-to-noise ratio (SNR) index. In this study, the electromagnetic field distributions in a cylindrical phantom and an oblate sphere phantom are analyzed by the three-dimensional finite element method, and the full width at half height (FWHH) of the specific absorption rate (SAR) distribution in the reentrant gap is evaluated. Results: It is shown that the optimized applicator yields both the maximum SNR and minimum mean FWHH, and the sizes of the heating region in the phantom expressed using the averaged FWHH values of the SAR distribution are 60 and 80 mm along the radial and long-axis directions of the applicator, respectively. Conclusions: A heating region can be robustly and optimally localized by using the Taguchi method and considering the variation in the size of the heating object.

Sensitivity improvement of a molecular imaging technique based on magnetic nanoparticles

Magnetic particle imaging (MPI) using the nonlinear interaction between internally administered magnetic nanoparticles (MNPs) and electromagnetic waves irradiated from outside of the body has attracted attention for the early diagnosis of diseases such as cancer. In MPI, the local magnetic field distribution is scanned, and the magnetization signal from MNPs inside an object region is detected. However, the signal sensitivity and image resolution are degraded by interference from the magnetization signal generated by MNPs that exist outside of the desired region, owing to nonlinear responses. Earlier, we proposed an image reconstruction method for suppressing the interference component while emphasizing the signal component using the property of the higher harmonic components generated by the MNPs. However, edge areas in the reconstructed image were emphasized excessively owing to the high-pass-filter effect of this method. Here, we propose a new method based on correlation information between the observed signal and a system function. We performed a numerical analysis and found that, although the image was somewhat blurred, the detection sensitivity can clearly be improved without the inverse-matrix operation used in conventional image reconstruction.

Resolution improvement of the molecular imaging technique based on magnetic nanoparticles

Magnetic particle imaging (MPI) based on the nonlinear interaction between internally administered magnetic nanoparticles and electromagnetic waves that externally irradiate the body has attracted attention for the early diagnosis of diseases such as cancer. In MPI, the local magnetic field distribution is scanned, and the magnetization signals are detected from the magnetic nanoparticles inside a target region. However, interference of the magnetization signals generated from the magnetic nanoparticles outside a target region due to nonlinear responses results in the degradation of image resolution. In this study, we clearly show that the degradation of image resolution is a result of the presence of even harmonics in the magnetization response of magnetic nanoparticles. We propose a new image reconstruction method for reducing these even harmonics and a correction method for suppressing the interference of the signals. This is achieved by taking into account the difference between the saturated waveform of the magnetization signal detected from the magnetic nanoparticles outside a target region and that detected from the magnetic nanoparticles inside a target region. In this study, we perform numerical analyses to prove that the image resolution in the molecular imaging technique can be improved by using our proposed image reconstruction method, which is based on the abovementioned ideas. Furthermore, a fundamental system is constructed and the numerical analyses are experimentally validated by using magnetic nanoparticles with a diameter of ~20 nm.

Non-invasive temperature measurement by using phase changes in electromagnetic waves in a cavity resonator

Purpose: To improve the efficacy of hyperthermia treatment, a novel method of non-invasive measurement of changes in body temperature is proposed. The proposed method is based on phase changes with temperature in electromagnetic waves in a heating applicator and the temperature dependence of the dielectric constant. An image of the temperature change inside a body is reconstructed by applying a computed tomography algorithm. This method can be combined easily with a heating applicator based on a cavity resonator and can be used to treat cancer effectively while non-invasively monitoring the heating effect. In this paper the phase change distributions of electromagnetic waves with temperature changes are measured experimentally, and the accuracy of reconstruction is discussed. Materials and methods: The phase change distribution is reconstructed by using a prototype system with a rectangular aluminum cavity resonator that can be rotated 360° around an axis of rotation. To make measurements without disturbing the electromagnetic field distribution, an optical electric field sensor is used. The phase change distribution is reconstructed from 4-projection data by using a simple back-projection algorithm. Results: The paper demonstrates that the phase change distribution can be reconstructed. The difference between phase changes obtained experimentally and by numerical analysis is about 20% and is related mainly to the limited signal detection sensitivity of electromagnetic waves. Conclusions: A temperature change inside an object can be reconstructed from the measured phase changes in a cavity resonator.