Raj Kumar Parajuli

Tissue Viscoelasticity Measurement System by Simultaneous Multiple-Frequency Excitation

Tissue elasticity measurements by an ultrasonic wave are a promising technique for the qualitative diagnosis of tumors and liver diseases. The viscoelastic characteristics in soft tissue can be quantitatively evaluated by considering the frequency dependence of the velocity of the shear wave propagating in the tissue. To improve the reliability of the in vivo viscoelasticity measurement, we propose a novel elasticity imaging method using continuous vibration wave excitation, which was realized by developing a three dimensional ultrasonic (3D US) wave Doppler measurement system with multiple-frequency excitation. In vivo experiments on the brachial muscle were carried out in order to demonstrate the validity and effectiveness of the developed system. The experimental results show that this system can successfully measure the velocity of a shear wave propagating through a muscle layer. This system has the potential to obtain viscoelastic information from a target with high repeatability and reliability.

Shear Wave Velocity Estimation by Virtual Sensing Array Spectrum Analysis

Tissue displacement by continuous shear wave propagation successfully gives local wavelength, which depends on the local velocity of the shear wave. Our goal for velocity distribution estimation in future clinical applications is to achieve less than 10% error and less than 5 mm spatial resolution. However, this has been difficult to achieve owing to the presence of multiple shear waves generated by complicated propagation such as reflection, scattering, and diffraction. In this paper, we propose a velocity estimation method based on wave number spectrum analysis from the limited small area of the displacement distributions. To increase the spatial resolution, we derive a deconvolution-based displacement estimation method for high-frequency excitation. The effectiveness of this method is demonstrated through numerical simulations and experiments with agar phantom. Even if a large reflected wave from a boundary exists, it is found that this method achieved the velocity estimation error of 10% and spatial resolution of 4.5 mm at a wavelength of 2.2 mm.

Characterization of nonlinearity of shear elasticity using local velocity mapping

The emerging technology of ultrasonic imaging of the soft tissue strain and elasticity, which aims at providing information about the mechanical properties of the tissues, has become a peer research issue since the 1990s. An elasticity imaging method using continuous shear wave excitation (CSWE) is expected to be a safe and quantitative technique. We have already proposed a local velocity mapping method for shear elasticity by reconstructing the small phase modulation components of the harmonic distortion in CSWE. In this paper, we propose a simple model of static hysteresis as the nonlinearity of shear elasticity. This model is based on the presence of harmonic phase modulation components in shear wave propagation. By using this model, we attempt to characterize the static hysteresis of shear elasticity. The relationship between the texture patterns of the local velocity map and the nonlinearity of the medium, which is measured by a rheometer, shows that the proposed model can be adopted for the imaging of the nonlinearity of shear elasticity.

Evaluation of Radio-Photoluminescence Spectra of Copper-Doped Phosphate Glass Dosimeter Irradiated with Ionized Particles

A radio-photoluminescence (RPL) dosimeter with a copper-ion luminescent center was fabricated to evaluate its response in ionized particle detection. A focused proton microbeam with varying energies up to 3 MeV and heavy ions of 490 MeV osmium (Os) were employed along with X-rays to evaluate its performance in micrometer-scale radiation monitoring. The response to ionized particles was evaluated under focused proton beam irradiation where the peak wavelength differed from that obtained under X-ray irradiation. Two peaks were observed under Os irradiation where the secondary-generated particles and photons have a significant effect on the dosimeter. The results suggest that the fabricated RPL dosimeter with copper luminescence center could be used to estimate the irradiation effect of primary ionized particles separately from the effects of secondary particles, photons, and environmental background radiation.

Imaging of monochromatic beams by measuring secondary electron bremsstrahlung for carbon-ion therapy using a pinhole x-ray camera

A feasibility study on the imaging of monochromatic carbon-ion beams for carbon-ion therapy was performed. The evaluation was based on Monte Carlo simulations and beam-irradiation experiments, using a pinhole x-ray camera, which measured secondary electron bremsstrahlung (SEB). The simulation results indicated that the trajectories of the carbon-ion beams with injection energies of 278, 249 and 218 MeV/u in a water phantom, were clearly imaged by measuring the SEB with energies from 30 to 60 keV, using a pinhole camera. The Bragg-peak positions for these three injection energies were located at the positions where the ratios of the counts of SEB acquisitions to the maximum counts were approximately 0.23, 0.26 and 0.29, respectively. Moreover, we experimentally demonstrated that it was possible to identify the Bragg-peak positons, at the positions where the ratios coincided with the simulation results. However, the estimated Bragg-peak positions for the injection energies of 278 and 249 MeV/u were slightly deeper than the expected positions. In conclusion, for both the simulations and experiments, we found that the 25 mm shifts in the Bragg-peak positions can be observed by this method.

In situ ion-beam-induced luminescence analysis for evaluating a micrometer-scale radio-photoluminescence glass dosimeter

Micrometer-scale responses of radio-photoluminescence (RPL) glass dosimeters to focused ionized particle radiation were evaluated by combining ion-beam-induced luminescence (IBIL) and proton beam writing (PBW) using a 3 MeV focused proton microbeam. RPL phosphate glass dosimeters doped with ionic Ag or Cu activators at concentrations of 0.2 and 0.1% were fabricated, and their scintillation intensities were evaluated by IBIL spectroscopy under a PBW micropatterning condition. Compared with the Ag-doped dosimeter, the Cu-doped dosimeter was more tolerant of the radiation, while the peak intensity of its luminescence was lower, under the precise dose control of the proton microprobe. Proton-irradiated areas were successfully recorded using these dosimeters and their RPL centers were visualized under 375 nm ultraviolet light. The reproduction of the irradiated region by post-RPL imaging suggests that precise estimation of irradiation dose using microdosimeters can be accomplished by optimizing RPL glass dosimeters for various proton microprobe applications in organic material analysis and in micrometer-scale material modifications.

Fabrication and evaluation of flexible Mach–Zehnder waveguide structure embedded in a poly(dimethylsiloxane) thin film using a proton microbeam

A flexible Mach–Zehnder (MZ) optical waveguide was fabricated in a poly(dimethylsiloxane) (PDMS) film by proton beam writing (PBW). A focused 750 keV proton microbeam was used to fabricate a 40 × 20 mm2 MZ optical waveguide structure with a width of 8 µm embedded in a PDMS film for the single-mode light propagation of infrared (IR) laser light. The structure was measured by ion-beam-induced luminescence (IBIL) analysis and the beam fluence was optimized according to the IBIL intensity obtained from the waveguide structure. The entire structure of the MZ waveguide functioned well, confirmed by observing the near-field pattern (NFP) with a tunable IR laser (1.55 µm) for different PDMS film conditions. The optical throughput measurements for different sample configurations were obtained under continuous mechanical stress and a relatively low optical loss was observed at an inclination angle of 16°. Our results suggest that the MZ waveguide can be used for optical interlink connections under continuous mechanical stress.