Tsutomu Yamakawa

Initial evaluation of linear and spatially oriented planar images from a new dental panoramic system based on tomosynthesis

OBJECTIVE The purpose of this study was to describe a newly developed dental panoramic system based on the tomosynthesis method and to validate the accuracy of linear and spatially oriented planar images. STUDY DESIGN An original robotic mechanism incorporating a new high-speed cadmium-telluride (CdTe) semiconductor detector was fabricated to acquire panoramic images (raw data). The shift-and-add tomosynthesis method was applied to facilitate changes in the depth of the panoramic imaging layer. Using the texture mapping method, planar and spatially oriented images were reconstructed along a custom curved imaging plane. Using a custom phantom and dry skulls, the accuracy of selected linear measurements was evaluated. RESULTS Preliminary measurements demonstrated acceptable linear accuracy in reconstructed panoramic images with variations <5%. CONCLUSIONS This preliminary investigation demonstrates that dental panoramic images acquired by a novel robotic mechanism and CdTe detector using a tomosynthesis method provides planar and spatially oriented images with an image quality that may be acceptable for dental practice.

Simulation study on a photon counting X-ray CT system

In an x-ray CT system the energy fluence of x-rays is commonly measured with a scintillation detector in the data acquisition. Thus it is impossible to consider the shape of the energy spectrum of x-rays incident to the detector. If the data related to the energy spectrum could be measured, it would be possible to decrease the beam-hardening of x-rays and also enhance the contrast of lesions. The aim of this study is to evaluate the effectiveness of a photon counting x-ray CT with simulations. In these simulations we calculated the photons incident to the detector with a Monte Carlo method. And we multiplied a predefined energy weighting functions to the calculated energy spectra and integrated them in the whole energy range. For predefined energy weighting functions we selected five types of functions including the function which simulated the conventional data acquisition. The results of simulations showed that our proposed method could increase the low density contrast and reduce the beam-hardening effect

Development of mammography system using CdTe photon counting detector for the exposure dose reduction

We propose a new mammography system using a cadmium telluride (CdTe) photon-counting detector for exposure dose reduction. In contrast to conventional mammography, this system uses high-energy X-rays. This study evaluates the usefulness of this system in terms of the absorbed dose distribution and contrast-to-noise ratio (CNR) at acrylic step using a Monte Carlo simulation. In addition, we created a prototype system that uses a CdTe detector and automatic movement stage. For various conditions, we measured the properties and evaluated the quality of images produced by the system. The simulation result for a tube voltage of 40 kV and tungsten/barium (W/Ba) as a target/filter shows that the surface dose was reduced more than 60% compared to that under conventional conditions. The CNR of our proposal system also became higher than that under conventional conditions. The point at which the CNRs coincide for 4 cm polymethyl methacrylate (PMMA) at the 2-mm-thick step corresponds to a dose reduction of 30%, and these differences increased with increasing phantom thickness. To improve the image quality, we determined the problematic aspects of the scanning system. The results of this study indicate that, by using a higher X-ray energy than in conventional mammography, it is possible to obtain a significant exposure dose reduction without loss of image quality. Further, the image quality of the prototype system can be improved by optimizing the balance between the shift-and-add operation and the output of the X-ray tube. In future work, we will further examine these improvement points.

Accuracy of linear attenuation coefficients measured with a photon counting CT system

The purpose of our research is to develop a photon counting CT system with a semiconductor detector. The photon counting CT system enables us to decompose materials, and this information is useful to diagnose diseases. We developed a new CdTe detector that was able to measure x-rays with the count rate of 107 counts/sec/mm2 with four energy windows. The size of a CdTe detector module was 8×8 mm2, and that of a pixel was 0.2 × 0.2 mm2. The thickness of the detector was 1 mm. The active area of our detector was 8 × 144 mm2. We evaluated the performance of the photon counting CT images in terms of the accuracy of reconstructed linear attenuation coefficients. In addition, we compared these reconstructed images with those acquired with the energy integration detector with the CdTe semiconductor detector. The results showed that our detector could reconstruct linear attenuation coefficients with the error ratio of less than 3 % compared with the theoretical value.

Advantages of Energy-Binned Photon Counting Detector

We developed an energy-binned photon counting (EBPC) detector with a CdTe crystal, in which we can measure photons with four energy windows. The data acquired with the detector have information about the energy of x-rays that pass through the object, so that we can make a desired image by adding EBPC images after multiplying an appropriate weight to an image acquired with each energy window. With this energy weighting image we can improve the image contrast and reduce the beam hardening effects. This paper compared two data acquisition methods: one is an energy integration method and the other is the EBPC method. To compare these two methods we conducted Monte Carlo simulations and experiments with our newly developed EBPC detector. The results showed that the EBPC detector could improve the dynamic rage of acquired images.

Simulation Study on an Energy-Modulated X-ray CT

In an X-ray CT system the energy fluence of X-rays is commonly measured with scintillation detectors in the data acquisition. To enhance the contrast in low contrast media we proposed a new concept of X-ray CT system. This technique utilized the information for an energy spectrum of X-rays. That is, we applied more than two voltages to an X-ray tube and obtained data measured with different energy spectra. Without increasing the irradiation dosage of X-rays, the contrast enhancement was carried out with arithmetic between these reconstructed images. The aim of this study is to clarify the validity of such an energy-modulated X-ray CT with simulations.

A proposed new image display method with high contrast-to-noise ratio using energy resolved photon-counting mammography with a CdTe series detector

In this study, we propose a new image display method to obtain high contrast-to-noise ratio (CNR) using energy resolved photon-counting mammography (ERPCM) with a cadmium telluride (CdTe) series detector manufactured by JOB CORPOLATION. The CdTe series detector can detect high-energy photons with high sensitivity, enabling users to image with high-energy X-rays. Using this detector, it is possible to reduce the dose given to a patient while increasing the CNR. First, the spectrum was divided into three bins and their corresponding linear attenuation coefficients were calculated from input and output photon numbers. Further, absorption vector length (AVL) and average absorption length (AAL) were calculated from the linear attenuation coefficients and from thicknesses of objects after beam-hardening correction. We further compared the CNR between ERPCM and general mammography images under the constant average glandular dose (AGD). We imaged an acrylic plate (1 mm thick) on RMI-156 phantom, determined regions of interest (ROIs) on an acrylic plate and background, and calculated the CNR. Our ERPCM generated two types of images: an AVL image and an AAL image. AMULET Innovality manufactured by FUJIFILM generated an integrated image. MicroDose SI manufactured by Philips generated a count image and removed electrical noise by the photon-counting technique. The four images, in order of decreasing CNR, were the AAL image, AVL image, MicroDose image, and AMULET image. The proposed method using ERPCM generated an image with higher CNR than images using general mammography under the constant AGD.

Identification of breast tissue using the x-ray image measured with an energy-resolved cadmium telluride series detector based on photon-counting technique.

We have been developing a new mammography device with a cadmium-telluride series energy-resolved photon-counting X-ray detector. Using a photon-counting technique, we examined the sensitivity of the system for differentiating the composition of breast tissue and detecting breast tumors. To differentiate breast tissues, we prepared surgically resected specimens fixed in formalin, consisting of adipose, mammary-gland, and tumor tissues. In order to obtain the values of certain effective atomic numbers, we prepared phantoms with 0%, 50% and 100% simulated mammary-gland tissue. In our imaging system, the X-ray spectrum penetrating the object was measured using three energy bins, and the products of linear attenuation coefficients and thicknesses for the three bins were calculated. These linear attenuation coefficients were properly corrected for beam hardening and normalized, to ignore the thickness. These calculations were applied for each pixel, and the gravity point per ROI (region of interest) was plotted on scatterplots to examine their distribution. Adiposetissue values were similar to known values; however, mammary-gland values were distant from expected values. In most specimens, the tumor points were focused; however, in some specimens, it was difficult to distinguish between tumor and mammary-gland tissues given their close linear attenuation coefficients. Mammary-gland tissues may have been influenced by formalin, given its tubular structure.

Development of energy-resolved photon-counting mammography with a cadmium telluride series detector to reduce radiation exposure and increase contrast-to-noise ratio using the high-energy X-rays.

A new energy-resolved photon-counting mammography (ERPCM) device with a cadmium telluride (CdTe) series detector (JOB Corporation, Japan) is currently being developed. The CdTe series detector can detect higher-energy photons with high sensitivity, enabling the use of high-energy X-rays for imaging. Our previous research, in which we compared ERPCM using high-energy X-rays (tube voltage 50 kV) with general mammography using low-energy X-rays (tube voltage about 30 kV), reported that ERPCM had a higher CNR (contrast-to-noise ratio) than general mammography. The purpose of this study was to examine the magnitude of the CNR using a simulation and ERPCM; especially we would like to examine the CNR when the tube voltage of higher than 50 kV was adopted. In the comparison of the CNRs, It was necessary to pay attention to equalizing the average glandular dose (AGD). Using the simulation and ERPCM, we compared the CNR between images taken at 50 kV and 75 kV under a constant AGD. The simulation phantom was composed of 50% mammary gland and 50% adipose tissue, and contained tumor regions. The thickness of the simulation phantom was varied. We put an acrylic plate (1 mm thickness) on an RMI-156 phantom. Furthermore, we placed the thicker acrylic plate (10, 20, 30, 40 mm) on the 156 phantom and 1 mm-thick acrylic plate to simulate thicker breast. Based on the results from the simulation, in the phantom thickness of 80 mm, the CNR of image taken by 75kV got extremely closer to that taken by 50kV. The advantage of the image taken at 75 kV for the thicker breast was also confirmed in ERPCM.

Detection of microcalcifications and tumor tissue in mammography using a CdTe-series photon-counting detector

In this study, we proposed a method for detecting microcalcifications and tumor tissue using a cadmium telluride (CdTe) series linear detector. The CdTe series detector was used as an energy resolved photon-counting (hereafter referred to as the photon-counting) mammography detector. The CdTe series linear detector and two types of phantom were designed using a MATLAB simulation. Each phantom consisted of mammary gland and adipose tissue. One phantom contained microcalcifications and the other contained tumor tissue. We varied the size of these structures and the mammary gland composition. We divided the spectrum of an x-ray, which is transmitted to each phantom, into three energy bins and calculated the corresponding linear attenuation coefficients from the numbers of input and output photons. Subsequently, the absorption vector length that expresses the amount of absorption was calculated. When the material composition was different between objects, for example mammary gland and microcalcifications, the absorption vector length was also different. We compared each absorption vector length and tried to detect the microcalcifications and tumor tissue. However, as the size of microcalcifications and tumor tissue decreased and/or the mammary gland content rate increased, there was difficulty in distinguishing them. The microcalcifications and tumor tissue despite the reduction in size or increase in mammary gland content rate can be distinguished by increasing the x-ray dosage. Therefore, it is necessary to find a condition under which a low exposure dose is optimally balanced with high detection sensitivity. It is a new method to indicate the image using photon counting technology.