Osamu Tokuoka

Absolute measures of image quality for the Sens-A-Ray direct digital intraoral radiography system*

To study the noise characteristics of the Sens-A-Ray (Regam Medical Systems AB, Sundsvall, Sweden) system, 20 radiographs were obtained at each of three different exposure levels at 70 and 90 kVp with a homogeneous x-ray field. Exposures were measured with an ionization chamber. Noise power spectra were calculated over three areas within each radiograph, and ensemble averages were subsequently found from 60 data files at each exposure level. Noise equivalent quanta were calculated with the noise power spectra and modulation transfer function data from previous studies. Finally, the detective quantum efficiency was calculated by dividing the noise equivalent quanta by the estimated incident photon fluence at the different exposures. The system has a maximum detective quantum efficiency of approximately 0.030 at 70 kVp and 0.025 at 90 kVp. A broad maximum exists at approximately 2 cycles/mm, indicating that the signal-to-noise ratio is most favorable at this spatial frequency.

Linear or Curved Display of Digital Radiographs: Results of a "Beauty Contest"

CCD detectors used in direct digital intraoral radiography have a linear dose response. When images are displayed on computer monitors a linear display function is also most often employed. Dentists using digital radiography have pointed out that the visual characteristics of such images are different from those of film based images. The aim of the present work was to study the effects of curving the display function. For this purpose 30 digital radiographs were selected, and a computer program was constructed by which the gray level display could be manipulated. Dentists at Osaka University, Osaka, Japan, and Karolinska Institutet, Stockholm, Sweden, were asked to manipulate the display of each individual radiograph so that it exhibited what subjectively appeared to be the best possible characteristics for diagnostic work. The viewers were in agreement in their preference for a curved display function for digital intraoral radiographs. Effects of the curved display are analyzed and discussed. It is concluded that from a purely subjective point of view the dose response function of digital intraoral radiographs should be curved in such a way that contrast is increased in areas with low exposure and decreased in areas with high exposure. Whether or not this has an effect on the diagnostic performance may be studied employing, for example, example, ROC techniques.

Monte carlo calculations of mean bone marrow dose and energy imparted from CT head scanner

Because of the potential for high radiation doses in CT procedures, the object of this study was to estimate the mean bone marrow dose and the energy imparted from a 3rd generation CT head scanning procedure using Monte Carlo simulation. The simulated and calculated mean bone marrow dose (μGy) and energy imparted (mJ) per 1cm slice thickness scan are as follows:Adult 120kV,effective mAs 553mAsChild 120kV, effective mAs 213mAs173-376μGy, 16.0-18.3mJ (Western)67-136μGy, 5.5-6.5mJ (Western)110-208μGy, 16.0-18.3mJ (Japanese)64-120μGy, 5.5-6.5mJ (Japanese)