Naoki Wadamori

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.

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.

Photoacoustic depth profiling of a skin model for non-invasive glucose measurement

A measurable depth of the blood glucose level by using photoacoustic spectroscopy (PAS) was discussed experimentally in order to improve detection sensitivity. Since a measurable depth of the PAS depends on a modulation frequency of the chopped light irradiated to a sample, the relationship between modulation frequency and the thickness of a sample was evaluated. The photoacoustic detector used in these experiments consisted of an acoustic resonance pipe and an optical microphone, and a two-layer model composed of silicone sheets with the different optical absorption. The measurable depth was 2–3 mm with modulation frequency of 1000–2000 Hz. Furthermore, the reason for the measurable depth to be deeper than the thermal diffusion length of the sample was discussed theoretically. From these analyses of thermoelastic wave, the relation between the photoacoustic signal propagation and the measurable depth in a tissue was clarified.

Noninvasive Thermometry in a Reentrant Resonant Cavity Applicator

In order to improve the effect of hyperthermia treatment, a novel noninvasive temperature measurement method is proposed. The key mechanism of this thermometry is based on the phase change of the electrical field accompanying the temperature change inside the resonant cavity. First, the line-integrated data of this phase change distribution parallel to the electrical field are collected by rotating a cavity resonator. Then, the temperature change distribution within an object is reconstructed by the back-projection algorithm using these projection data. The proposed thermometric method may be able to monitor the temperature change distribution inside the body noninvasively without requiring a huge set-up as in the case of MRI. The basic algorithm of this method is described, and then the feasibility is indicated by the numerical analysis using the finite difference time domain (FDTD) method.

Miniaturization of photoacoustic cell for smart endoscope to improve sensitivity

Ultrathin endoscopes, such as transnasal endo-scopes, have been developed to alleviate discomfort during diagnosis and therapy. However, their application to optional diagnostics is limited since many optional diagnostic instruments are designed to fit through larger side channels. The aim of this study was to develop a smart endoscope that can obtain various diagnoses based on photoacoustic spectroscopy. The photoacoustic process comprises complex energy conversions involving optical, thermal, and elastic processes. This work focused on the scaling potential of photoacoustic sensors. Photoacoustic sensors with two different volumes were developed, and the amplitudes and frequency responses of the photoacoustic signals for silicone rubbers with six different Youngs moduli were investigated. The results showed that photoacoustic signals can be enhanced by reducing the volumes of the sensors. Embedding a miniaturized photoacoustic sensor in an endoscope was confirmed to improve the sensitivity.

Image Resolution and Sensitivity Improvements of a Molecular Imaging Technique Based on Magnetic Nanoparticles

The enhanced permeation and retention (EPR) effect (Matsumura & Maeda, 1986) caused by the leakage of internally administered nanoparticles from blood vessels and their accumulation in cancerous tissues can be used to diagnose cancer. Gleich and Weizenecker proposed the magnetic particle imaging (MPI) approach (Gleich & Weizenecker, 2005), whereby the positions of these magnetic nanoparticles (MNPs) accumulated in cancerous tissue can be detected by applying a local alternating magnetic field from a source positioned outside the body. In basic MPI, the local magnetic field distribution is scanned to encode the spatial information, and the magnetization signal with odd-order harmonics is detected from MNPs inside a selected region when an alternating magnetic field is applied to the MNPs. Furthermore, a fast data acquisition method by scanning spatial data along with a Lissajous trajectory was proposed (Gleich et al., 2008; Knopp et al., 2009), and real time image-data acquisition was achieved (Weizenecker et al., 2009). However, interference from the magnetization signal generated from the MNPs outside the selected region degraded the image resolution and signal sensitivity (signal-to-noise ratio). We proposed an image reconstruction method for reducing these interference signals mainly generated by even harmonics, and a correction method to suppress the interference signals (Kusayama & Ishihara, 2007; 2009; Ishihara & Kusayama, 2009). This was achieved by taking into account the difference between the saturated waveform of the magnetization signal detected from the MNPs outside the selected region and that detected from the MNPs inside the region. We performed 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 was constructed and the numerical analyses were experimentally validated using MNPs with diameters of 10–50 nm. The detection sensitivity and the resolution were improved by the use of methods in the case of locally distributed MNPs. However, a reconstructed image with the correct distribution of MNPs may not be obtained when the MNPs are distributed continuously. This is because the abovementioned proposed method acts as an intense high-pass filter against the reconstructed image (Ishihara & Kusayama, 2011).

Localized heating characteristics of hyperthermia using a reentrant cavity.

An applicator based on a reentrant cavity with excellent localized heating characteristics is proposed in order to treat deep tumours localized in the head and neck. Numerical and experimental analyses of the applicator, which produces an electromagnetic field distribution required for this type of localized heating, are performed. A simple and clear procedure for achieving localized heating requires applicator miniaturization. However, miniaturization causes an increase in the resonant frequency and leads to the degradation of the characteristics for heating deep tissue. Therefore, it is proposed that dielectrics be inserted into the applicator and the resonant frequency be reduced. From the results of the numerical analysis and experimental examination, it is shown that a deep region of 100–120 mm around the centre of a phantom can be heated.

Possible photoacoustic gas detection for a smart endoscope

Endogenous gas analysis is a potentially fast and convenient noninvasive diagnostic method for a variety of diseases. However, sampling and sample preparation are error-prone steps and must be optimized to achieve reliable results. A miniature photoacoustic system was developed to allow gas sampling using a smart endoscope. The photoacoustic system was demonstrated to have a 1% detection limit for CO2, which is too high. Many improvements, including modifying the structure of PA cell, must be considered for further development.

Behavior of long-period measurements using a small-sized photoacoustic cell for aqueous glucose monitoring

Reliable, noninvasive glucose-monitoring devices are not currently available. From the patients point of view, it is necessary that glucose-monitoring devices are portable as well as noninvasive. In photoacoustic spectroscopy (PAS), the PA signal induced by the irradiation of the sample with modulated light depends on the optical absorption coefficient of the sample. Unlike the sensitivity of conventional absorption spectroscopy, the sensitivity of PAS scales inversely with the dimensions. An external laser (wavelength of 1550 nm) and a PA cell with a volume of only 4.0 mm3 were used for monitoring a glucose solution contained in a special sample reservoir. We present PA measurements of glucose in aqueous solutions using a sample reservoir that is suitable for investigations of liquid samples, such as native capillary blood, by performing a long-period measurement.