Masahiko Ohtaka

A Current-Mode Detector for Unfolding X-ray Energy Distribution

To turn the advantage of energy measurement in x-ray transmission diagnosis into practice, we propose a novel detector for the estimation of x-ray energy distribution. This detector consists of several segment detectors arrayed in the direction of x-ray incidence. Each segment detector measures x-rays as current. With unfolding measured currents, the x-ray energy distribution is obtained. The practical application of this detector was verified by estimating the iodine thickness in an acryl phantom.

Unfolding Method with X-ray Path Length-Dependant Response Functions for Computed Tomography Using X-ray Energy Information

The computed tomography (CT) values obtained by the energy subtraction method with a transXend detector, which measured X-rays as current and gave the corresponding X-ray energy information, show the disadvantage that the CT values are dependent on the thickness of a homogeneous phantom. In order to obtain constant CT values for a uniform material, a new unfolding method is proposed using variable response functions of the transXend detector according to the X-ray path length in the phantom. The CT values measured using the new unfolding method are discussed with respect to the energy range used in the unfolding process, the number of segment detectors, and the substrate of the segment detectors.

Measurement of effective atomic numbers using energy-resolved computed tomography

For ion radiation therapy, the measurement of effective atomic numbers, Zeff, is necessary to know the material distribution in a human body; the range of ions entering the human body is influenced by the material distribution along their paths. Zeff, however, cannot be measured at hospitals because monochromatic X-rays with different energies are necessary and are used only at synchrotron facilities. To make Zeff measurements at hand, we propose energy-resolved computed tomography (CT) using a “transXend detector”. By assigning two narrow energy ranges in the unfolding process of the data obtained by the transXend detector, Zeff for acrylic and aluminum can be estimated by energy-resolved CT. The estimated Zeff are compared with those obtained by dual-energy and monochromatic X-ray CT.

Low-dose exposure energy-resolved X-ray computed tomography using a contrast agent with a high-energy K-edge

X-ray computed tomography (CT) with iodine contrast agent is widely employed to locate cancers. However, this method has shortcomings such as high-radiation dose exposure, iodine side effects, and a beam hardening effect. We have been working on the energy-resolved CT measurement method using a novel X-ray detection system, the “transXend” detector, which measures X-rays as electric currents and gives the energy distribution of incident X-rays after analysis. In the present study, we propose a method for low-dose exposure CT that involves the combination of the energy-resolved CT method, which is free from the beam hardening effect, and a harmless contrast agent with high-energy K-edge absorption, such as gold nanoparticles expected as a future contrast agent. Comparisons of radiation dose exposures as functions of aluminum filter thickness at the exit aperture of an X-ray tube and the K-edge energies of contrast agents are described.

Using energy-resolved X-ray computed tomography with a current mode detector to distinguish materials

In conventional X-ray computed tomography (CT), X-rays are measured as electric current. Materials inside a subject are described by the linear attenuation coefficients averaged by the energy spectrum of the X-rays. A CT image cannot distinguish materials such as iodine and calcium, because the linear attenuation coefficient is not inherent to a material, but the product of X-ray mass attenuation coefficient and the density of the material. Materials such as iodine and calcium can be distinguished using an energy-resolved CT technique, with a current-mode detector system, using segment detectors aligned in the direction of X-ray incidence: the energy-resolved CT images are reconstructed by the X-rays with the energy of interest, by unfolding electric currents measured by the segment detectors.

Low dose exposure diagnosis with a transXend detector aiming for iodine-marked cancer detection

The energy resolved computed tomography (CT), which had advantage over conventional CT (twofold higher CT value for iodine contrast agent and being free from beam hardening effect), was shown practical by employing the transXend detector: it measured X-rays as electric current and gave energy distribution of incident X-rays after analysis. This article shows a new application of the transXend detector for estimating the thicknesses of acrylic, iodine, and aluminum in a phantom. For this purpose, the responses of the segment detectors in the transXend detector are changed intentionally with inserting filters. With previously obtained two-dimensional maps for acrylic–iodine and acrylic–aluminum thicknesses, which are shown by the ratios of electric currents measured by the segment detectors, the thickness of materials on the path of the X-rays are obtained by a transmission measurement.

Using X-ray Energy Information in CT Measurement of a Phantom with an Al Region

X-ray computed tomography (CT) using X-ray energy information has been studied by the present authors on acrylic phantoms containing an iodine contrast medium. To observe a human body, however, it is necessary to consider the bone. In this paper, CT measurements were made on a phantom with regions of both iodine and aluminum, which was used as a substitute for the bone. A filtered back-projection method and a maximum likelihood-expectation maximization method were employed for image reconstruction.

Advantages of Response Function Change in a transXend Detector with Various Scintillators as Substrates of Segment Detectors

To enable practical computed tomography (CT) that uses the energy information of X-rays, the “transXend detector” was developed to provide energy information about incident X-rays by measuring them as an electric current. The transXend detector requires a spectrum survey method in the unfolding process for measuring the energy distribution of incident X-rays, when the response functions of segment detectors have nearly the same behavior. When employing various scintillators with different effective atomic numbers and densities as the substrates of the segment detectors, better convergence is obtained in the unfolding process by using only one initial guess spectrum. Additionally, less dose exposure is possible when using the transXend detector with various segment detectors, compared with the transXend detector that consists of the segment detectors with the same substrate.

Energy-resolved computed tomography measurements of iron solution and adipose as a simulation for estimating the iron concentration in the human liver

To measure the iron concentration in the human liver, which comprises soft tissue and adipose as well as iron, energy-resolved computed tomography (CT) is applied using a transXend detector. The transXend detector measures X-rays as electric current and gives an energy distribution after analysis. Energy-resolved CT clearly separates iron solution from acrylic, a substitute for soft tissue, whereas conventional current-measurement CT cannot. Using CT values obtained in two different energy ranges, a two-dimensional map of iron, adipose and soft tissue is plotted. With this map, the components in the liver can be identified.

Computed tomography reconstruction from two transmission measurements for iodine-marked cancer detection

The authors invented the transXend detector, which measures X-rays as electric currents, and then gives the energy distribution of the X-rays after an unfolding process. In a previous paper, it was shown that the material thickness distributions can be estimated with the transXend detector by using reference points plotted from the electric current ratios, such as the I 2/I 1 − I 3/I 1 graph, where Ii denotes the electric current measured by the i-th segment of the transXend detector. In this paper, the tomographic images of iodine, aluminum, and the acrylic those surround the other two materials are reconstructed from their material thickness distributions, which are estimated from two X-ray incidence directions. The X-ray event ratios are also used to estimate the material thickness distributions.