Yuji Ohkawa

Indirect flat-panel detector with avalanche gain: fundamental feasibility investigation for SHARP-AMFPI (scintillator HARP active matrix flat panel imager).

An indirect flat-panel imager (FPI) with avalanche gain is being investigated for low-dose x-ray imaging. It is made by optically coupling a structured x-ray scintillator CsI(Tl) to an amorphous selenium (a-Se) avalanche photoconductor called HARP (high-gain avalanche rushing photoconductor). The final electronic image is read out using an active matrix array of thin film transistors (TFT). We call the proposed detector SHARP-AMFPI (scintillator HARP active matrix flat panel imager). The advantage of the SHARP-AMFPI is its programmable gain, which can be turned on during low dose fluoroscopy to overcome electronic noise, and turned off during high dose radiography to avoid pixel saturation. The purpose of this paper is to investigate the important design considerations for SHARP-AMFPI such as avalanche gain, which depends on both the thickness dSe and the applied electric field ESe of the HARP layer. To determine the optimal design parameter and operational conditions for HARP, we measured the ESe dependence of both avalanche gain and optical quantum efficiency of an 8μm HARP layer. The results were used in a physical model of HARP as well as a linear cascaded model of the FPI to determine the following x-ray imaging properties in both the avalanche and nonavalanche modes as a function of ESe: (1) total gain (which is the product of avalanche gain and optical quantum efficiency); (2) linearity; (3) dynamic range; (4) gain nonuniformity resulting from thickness nonuniformity; and (5) effects of direct x-ray interaction in HARP. Our results showed that a HARP layer thickness of 8μm can provide adequate avalanche gain and sufficient dynamic range for x-ray imaging applications to permit quantum limited operation over the range of exposures needed for radiography and fluoroscopy.

Photoconductive properties of organic films based on porphine complex evaluated with image pickup tubes

We have fabricated two types of organic photoconductive films; a layered structure of tetra(4-methoxyphenyl) porphine cobalt complex (Co–TPP) and bathocuproine (BCP) (target A), and another of Co–TPP, tris-8-hydroxyquinoline aluminum (Alq3) and BCP (target B). The photoconductive properties of each film have been measured using image pickup tubes. The dark current was drastically reduced in target B compared with that in target A at the same applied electric field. The external quantum efficiency reached 20% in target B, which is twenty times higher than that in target A. Image pickup of a test chart from target B at standard TV operation was also demonstrated. High resolution and excellent tone sufficient for television use has been obtained.

Image Pickup from Zinc Phthalocyanine/Bathocuproine Double-Layer Film Using Pickup Tube

We have demonstrated image pickup from zinc phthalocyanine/bathocuproine double-layer film incorporated into a pickup tube for both standard television systems and high-definition television (HDTV) systems. A limiting resolution of more than 800 television lines was obtained, which is sufficient for HDTV. The peak external quantum efficiency was 14.7% under 620 nm irradiation. The results indicate that organic molecules have great potential for use in imaging devices.

Development of FOP-HARP imaging device

The high-gain avalanche rushing amorphous photoconductor (HARP) camera tube achieves ultrahigh-sensitivity by using the avalanche multiplication. The applications of this tube extend beyond broadcasting into other fields. It is attracting a great deal of attention especially for radiation diagnosis, such as synchrotron radiation microangiography, because it can obtain high-resolution and high-contrast images with a low dose of radiation. However, in the present system, a fluorescent screen and the photoconductive film of the HARP tube are connected optically by a lens-coupling method, and low light throughput remains a big problem. To improve the light throughput by using a fiber-coupling method, we applied a fiber-optic plate (FOP) to the substrate of a HARP tube. The FOP consists of three types of glass that have differing hardnesses and elastic coefficients that make it difficult to flatten the FOP surface enough to form the HARP film. We thus introduced a new mechanical polishing method and succeeded in realizing avalanche multiplication in the FOP-HARP tube. The results of shooting experiments by applying the FOP-HARP to the microangiography showed that a spatial resolution of over 20 line pairs/mm was obtained. Moreover, rat femoral arteries of 150-200 μm in diameter could be visualized as motion pictures with a one-fourth lower concentration of contrast material than that needed for ordinary microangiography. Another potential application of the FOP-HARP is an ultrahigh-sensitivity nearinfrared (NIR) image sensor made by fiber-coupling with an image intensifier (I.I.). The image sensor provides highquality images and should be a powerful tool for NIR imaging.

An indirect flat-panel detector with avalanche gain for low dose x-ray imaging : SAPHIRE (Scintillator Avalanche Photoconductor with High Resolution Emitter readout)

An indirect flat-imager with programmable avalanche gain and field emitter array (FEA) readout is being investigated for low-dose x-ray imaging with high resolution. It is made by optically coupling a structured x-ray scintillator CsI (Tl) to an amorphous selenium (a-Se) avalanche photoconductor called HARP (high-gain avalanche rushing photoconductor). The charge image created by HARP is read out by electron beams generated by the FEA. The proposed detector is called SAPHIRE (Scintillator Avalanche Photoconductor with HIgh Resolution Emitter readout). The avalanche gain of HARP depends on both a-Se thickness and applied electric field ESe. At ESe of > 80 V/μm, the avalanche gain can enhance the signal at low dose (e.g. fluoroscopy) and make the detector x-ray quantum noise limited down to a single x-ray photon. At high exposure (e.g. radiography), the avalanche gain can be turned off by decreasing ESe to < 70 V/μm. In this paper the imaging characteristics of the FEA readout method, including the spatial resolution and noise, were investigated experimentally using a prototype optical HARP-FEA image sensor. The potential x-ray imaging performance of SAPHIRE, especially the aspect of programmable gain to ensure wide dynamic range and x-ray quantum noise limited performance at the lowest exposure in fluoroscopy, was investigated.

Improvement photoelectric conversion efficiency of red light in HARP film

We enhanced the photoelectric conversion efficiency of red light in a 15-&mgr;m-thick HARP film without deteriorating image pick-up characteristics or reliability. To achieve a higher photoelectric conversion efficiency for red light, we designed a new film structure with an increased amount of doped Te, which has a narrower band gap than that of a-Se. The thickness of the LiF-doped layer for trapping holes was increased from that of the conventional red-extended HARP film in order to weaken the internal field that would otherwise be enhanced by trapped electrons in extra doped Te. The new red-extended HARP film achieved a photoelectric conversion efficiency for red light of about 22.5% at a wavelength of 620 nm, which is twice that of the conventional red-extended film. We confirmed an improvement in signal to shot noise ratio of 3 dB and a dramatic improvement in color reproduction when we experimented with an HDTV camera with a camera tube incorporating the new film.

Improvement of the quantum efficiency of the new super-HARP image sensor

The New Super-HARP image sensor, which relies on avalanche multiplication in a photoconductive film made mainly of amorphous selenium, is ultra-high sensitive. The sensor has already ben used to film very dark scenes, however in such a situation, shot noise due to the quantum characteristics of light becomes a serious problem. Increasing the quantum efficiency of the image sensors can reduce shot noise. The quantum efficiency of the New Super-HARP sensor has been improved to obtain an even better picture. To increase the quantum efficiency for green incident-light two improvements have been made to the amorphous selenium film. The first is doping the film with a suitable amount of tellurium on its incident-light side. The sensor is reducing the lithium- fluoride-doped layer to about 60 percent of the thickness of the conventional film. The improved version of the New Super-HARP Film has higher quantum efficiency. Its quantum efficiency at a wavelength of 540 nm was evaluated to be double that of the conventional film. Shot noise is reduced by three dB, that is, the S/N is improved by three dB.

A novel device improves myopic visual acuity through pupil constriction during far accommodation

P2-e48 A novel device improves myopic visual acuity through pupil constriction during far accommodation Hidenori Horie1,2,3, Kenji Yuda4, Eiichi Ookawa5, Satoshi Hisahara5, Hiroshi Uozato6, Hiroko Horie2, Satomi Nakajima2, Kenkichi Tanioka7, Yuji Ohkawa7, Tomoki Matubara7, Wolfram Tetzlaff8 1 Brain & Oral Sciences, Kanagawa Dental College, Kanagawa, Japan; 2 TechnoMaster Co Ltd., Yokohama, Japan; 3 Adv. Inst. for Biol. Sci., Waseda University, Tokyo, Japan; 4 Kikuna Yuda Eye Clinic, Yokohama, Japan; 5 Healthcare Business Co., Matsushita Electric Ind. Co. Ltd., Yokohama, Japan; 6 Department of Ophthalmology & Visual Sciences, Kitasato University, Kanagawa, Japan; 7 NHK Sci. and Technical Res. Lab., Tokyo, Japan; 8 ICORD, University of British Columbia, Vancouver, Canada