Yoshihiko Hayakawa

Sens-A-Ray characteristics with variations in beam quality***

This study examined the relationship between x-ray beam quality and image characteristics with the use of the Sens-A-Ray (Regam Medical Systems, Sundsvall, Sweden). To evaluate the x-ray response, properties of the image sensor, dark current and statistical noise, sensitivity, and contrast resolution were measured. Tube voltage, tube current, and exposure time were varied. Exposures were determined with the use of an ionization chamber. Pixel values beneath a lead strip were used to calculate dark current effects. Contrast was measured with pixel values from the images of a step-wedge. Dark current noise increased in proportion to exposure time. Although the sensitivity at each tube voltage was linear and proportional to the exposure, the gradient was inversely proportional to increased tube voltage. After subtraction of the dark current and statistical noise, the gradient of the sensitivity at low tube voltage remained steeper than at high tube voltages. Hence, sensitivity decreased with increased tube voltage. Sensitivity gradients at each different tube current were similar. Irrespective of exposure, contrast resolution between adjoining step-wedge steps > or = 9 mm were too small to distinguish at 50 kVp. The gray levels for each step below 12 mm in thickness were distinguishable with 90 kVp. Increased dark current noise consequent to long exposure times narrowed the available gray scale range. Lower tube voltage resulted in higher sensitivity with larger gradients than higher tube voltages.

Absorbed doses with intraoral radiography: Function of various technical parameters

Both the maxillary incisor and mandibular molar regions of a Rando phantom were radiographed using specific technical parameters. The parotid gland and thyroid gland doses were measured using thermoluminescence dosimetry. Tube voltage, total filtration, mAs, collimation, and cone length were varied while keeping the other factors constant. Increased tube voltage or total filtration generally resulted in a slightly increased absorbed dose. Absorbed dose was decreased as the beam was more tightly collimated or the cone length was increased.

Development of qualitative near infrared vascular imaging system with tuned aperture computed tomography

We developed a novel system for imaging and qualitative analysis of surface vessels using near infrared (NIR) radiation with tuned aperture computed tomography (TACT®), even if the NIR cannot transmit through thick regions. NIR-sensitive CCD camera was surrounded by sixty light emitting diodes (alternating wavelengths of 700 nm and 810 nm), and could only detect the NIR from the subcutaneous tissue. We obtained multiple near infrared projections of surface vessels at each wavelength in accordance with the optical aperture theory within one second. Then, we created tomograms using the TACT program, and determined the venous oxygenation index (VOI), which reflected the oxygen saturation level, calculated from the image signals at each wavelength. This system produced thinner NIR tomograms under 0.5 mm. The change in VOI after load test calculated from NIR tomograms was more sensitive than that from NIR images without tomography. Our novel system makes it possible to non-invasively obtain NIR tomograms and accurately analyze changes in oxygen saturation.

Tuned-Aperture Computed Tomography (TACT®) to assess impacted teeth using 2D slices and 3D pseudo-holograms

The purpose of this clinical study was to compare the diagnostic information yield of interactive TACT 3D pseudo-holograms, 2D TACT slices and conventional dento-alveolar images for the imaging and localization of impacted maxillary incisor teeth.

Dose Response of Two Storage-Phosphor Systems for Intra-Oral Radiography

We evaluated the dose response of two storage-phosphor systems, DenOptix and Digora, for intra-oral radiography. Dose response curves were generated using uniform flat field exposure. The dynamic range of both systems was assessed. Root mean square (RMS) granularity curves were also generated to compare noise levels at different exposures. For DenOptix, curves were obtained by activating and inactivating the automatic range control (ARC) setting. For Digora, curves were generated at different calibration exposures. Moreover, tube voltage and tube current dependencies in both systems were examined.In the dose response curve of the DenOptix with the activated ARC setting, the pixel value was kept almost constant regardless of exposure. The dose response curve of the Digora was changed by the different calibration exposures. The curves at relatively large calibration exposures had inflection points and the pixel value gradually decreased at exposures higher than the inflection points. These results clearly show that the DenOptix with the activated ARC setting, as well as the Digora, can produce images with identical density, regardless of exposure. The dynamic range of both storage-phosphor systems was significantly greater than that of CCD-based systems. With both DenOptix and Digora, higher exposure decreased relative noise levels. The sensitivity of both systems increased with higher tube voltage, although tube current dependency was not observed. Both systems allow for a dose reduction due to a high tube voltage.

Absorbed doses reduced by the use of rare-earth intensifying screens in rotational panoramic radiography with constant potential generator

Absorbed doses modified by the use of rare-earth intensifying screens were measured with rotational panoramic radiography. Equipments with the constant potential, Veraview (J.Morita) and PM 2002 CC (Planmeca) was utilized. The optimum exposure settings at each tube voltage were determined by radiography of a Rando® phantom. Absorbed doses to parotid and thyroid glands were measured under the optimum exposure settings by using the Rando® phantom and thermoluminescence dosimeters.The sensitivity of the rare-earth intesifying screen system increased as the tube voltage bacame higher. Absorbed doses decreased steeply as the tube voltage became higher. The highly sensitive system of the LANEX REGULAR and T-Mat G (Eastman Kodak) reduced the thyroid gland dose to a half at 70 kV in comparison with the regular system of the PX-III (Kasei Optonics) and X-Omat S (Eastman Kodak). This high sensitivity, however, made it impossible to set the optimum tube current at above 70 kV. The another rare-earth system, the TRIMAX 16 and TRIMAX XUD (3M) had a lower sensitivity than the regular system of the PX-III and X-Omat S at below 80 kV. But they had equal sensitivity at the highest tube voltage of 80 kV.