Hiroyasu Ishii

Photoacoustic imaging system for peripheral small-vessel imaging based on clinical ultrasound technology

One of the features of photoacoustic (PA) imaging is small-vessel visualization realized without injection of a contrast agent or exposure to X-rays. For carrying out clinical studies in this field, a prototype PA imaging system has been developed. The PA imaging system utilizes a technological platform of FUJIFILM’s clinical ultrasound (US) imaging system mounting many-core MPU for enhancing the image quality of US B-mode and US Doppler mode, which can be superposed onto PA images. By evaluating the PA and US Doppler images of the prototyped system, the applicability of the prototype system to small-vessel visualization has been discussed. The light source for PA imaging was on a compact cart of a US unit and emitted 750 nm wavelength laser pulses. The laser light was transferred to illumination optics in a handheld US transducer, which was connected to the US unit. Obtained PA rf data is reconstructed into PA images in the US unit. 3D images were obtained by scanning a mechanical stage, which the transducer is attached to. Several peripheral parts such as fingers, palms and wrists were observed by PA and US Doppler imaging. As for small arteries, US Doppler images were able to visualize the bow-shaped artery in the tip of the finger. Though PA images cannot distinguish arteries and veins, it could visualize smaller vessels and showed good resolution and vascular connectivity, resulting in a complementary image for the US Doppler images. Therefore, superposed images of the PA, US B-mode and US Doppler can visualize from large to small vessels without a contrast agent, which should be a differentiating feature of US/PA combined technology from other clinical vascular imaging modalities.

Quantitative analysis for peripheral vascularity assessment based on clinical photoacoustic and ultrasound images

Photoacoustic (PA) imaging technology is expected to be applied to clinical assessment for peripheral vascularity. We started a clinical evaluation with the prototype PA imaging system we recently developed. Prototype PA imaging system was composed with in-house Q-switched Alexandrite laser system which emits short-pulsed laser with 750 nm wavelength, handheld ultrasound transducer where illumination optics were integrated and signal processing for PA image reconstruction implemented in the clinical ultrasound (US) system. For the purpose of quantitative assessment of PA images, an image analyzing function has been developed and applied to clinical PA images. In this analyzing function, vascularity derived from PA signal intensity ranged for prescribed threshold was defined as a numerical index of vessel fulfillment and calculated for the prescribed region of interest (ROI). Skin surface was automatically detected by utilizing B-mode image acquired simultaneously with PA image. Skinsurface position is utilized to place the ROI objectively while avoiding unwanted signals such as artifacts which were imposed due to melanin pigment in the epidermal layer which absorbs laser emission and generates strong PA signals. Multiple images were available to support the scanned image set for 3D viewing. PA images for several fingers of patients with systemic sclerosis (SSc) were quantitatively assessed. Since the artifact region is trimmed off in PA images, the visibility of vessels with rather low PA signal intensity on the 3D projection image was enhanced and the reliability of the quantitative analysis was improved.

Feasibility study of the sub-pixel scanning method for single-exposure x-ray refraction imaging by Talbot-Lau interferometer using an a-Se direct conversion type FPD

We propose a Sub-Pixel Scanning (SPS) method for imaging the refraction of X-rays by an object with a single exposure. The key feature of our SPS method is that the area where the G2 grating masks the self-image of the G1 grating cyclically changes along the several adjoining sub-pixels by means of G2 rotation with a predetermined angle to the selfimage. Thereby, periodically intensity-modulated signal can be acquired in a single exposure. This intensity-modulated signal nearly corresponds to that from the each phase step signal in the phase stepping method. Accordingly, a refraction image with the pixel size corresponding to the width of a sub-pixel set can be constructed by calculating the phase shift in each sub-pixel data set. An experimental equipment was designed and constructed using an amorphous Selenium (a-Se) direct conversion type Flat Panel Detector (FPD), to demonstrate the feasibility of the SPS method. An image acquired by SPS method was comparable to that of the image by phase stepping method with multiple exposure, where the imaging conditions, i.e. total imaging exposure dose and image resolution, are the same. We expect SPS method to contribute to the development of the practical imaging system in clinical diagnostic use.

Fabrication of high aspect grating using bonded substrate for X-ray refraction imaging by Talbot-Lau interferometer

In order to improve the image quality of X-ray refraction images using a Talbot-Lau interferometer, we have been attempting to fabricate gratings with high aspect ratio. In our attempt, deep grooves of grating structure were channeled on a Si substrate bonded by Au diffusion bonding method, and the grooves were filled with Au where the Au layer used for the bonding Si substrate was acting as a seed layer of Au electroplating. From the results of a visibility measurement and a cross sectional SEM image, it was confirmed that the grooves with a pitch of 5.8 μm and a depth of 100 μm could be successfully filled with Au over a large area of 72×80 mm2. Using this grating, the X-ray refraction images for the cartilage of a knee joint of a livestock pig could be obtained where SPS method was employed for the single-shot image acquisition.