Evaluation of x-ray effective focal spot size dependency on x-ray exposure settings using edge response analysis.

The effective focal spot size of x-ray tubes is one of the major factors that substantially affect the resultant x-ray images, and it is known to be dependent on the x-ray exposure setting used. This study aims to evaluate the relationship between the effective focal spot size and the tube current and voltage and assess its reproducibility among several x-ray tubes. The evaluation was performed using edge response analysis, in which a 1-mm thick tungsten edge was projected onto a flat panel detector with a magnification factor of 2. The edge image was then differentiated to obtain the line spread function, followed by a detector blur-removing process through Fourier analysis to obtain the true focus profile. The resultant focal spot size increased as the tube current increased, whereas it decreased as the tube voltage increased, as expected. The rate of change was similar along the width and the length directions, while the small focus changed more significantly than the large focus. The reproducibility among four x-ray tubes of the same model was excellent as the maximum variation < 20%. In conclusion, the edge response method can provide useful information on the x-ray focal spot relationship with the x-ray exposure settings used, as well as its reproducibility among several x-ray tubes.

Measurement of the x-ray effective focal spot size with edge response analysis using digital detectors.

Purpose: For the focal spot measurement of x-ray tubes, we propose a practical method in which only a metal edge and a digital detector are used, together with a process of removing detector blur inherently associated. Approach: The evaluation was made through the optical transfer function (OTF) measurements using the edge response of a 1-mm-thick tungsten plate. First, we made the acquisition of a geometrically magnified edge response, which consists of focal spot penumbra and detector blur, followed by the acquisition of nonmagnified edge response, which includes only detector blur. Then the detector blur was removed by taking the ratio of the two OTFs. Finally, the focal spot profile was obtained by the inverse Fourier transform of this ratio. Results: Resultant full widths at the half-maximum of a small focus profile were 0.529 ± 0.005 mm for the proposed method and 0.527 ± 0.020 mm for the conventional slit method with film, indicating excellent agreement between both methods. Comparing between results obtained using two flat panel detectors with different pixel pitches (0.143 and 0.175 mm) confirmed no differences with these variations. Conclusion: Through the whole study, the accuracy and the practicality of the proposed method were demonstrated, indicating a possibility of the method to be widely used to evaluate the effective focal spot size and profile of x-ray systems.

Image Format Conversion to DICOM and Lookup Table Conversion to Presentation Value of the Japanese Society of Radiological Technology (JSRT) Standard Digital Image Database.

Japanese Society of Radiological Technology (JSRT) standard digital image database contains many useful cases of chest X-ray images, and has been used in many state-of-the-art researches. However, the pixel values of all the images are simply digitized as relative density values by utilizing a scanned film digitizer. As a result, the pixel values are completely different from the standardized display system input value of digital imaging and communications in medicine (DICOM), called presentation value (P-value), which can maintain a visual consistency when observing images using different display luminance. Therefore, we converted all the images from JSRT standard digital image database to DICOM format followed by the conversion of the pixel values to P-value using an original program developed by ourselves. Consequently, JSRT standard digital image database has been modified so that the visual consistency of images is maintained among different luminance displays.

[Investigation of an Optimum Determination Method of Exposure Index for Posterior-Anterior Chest Radiographs Using an Auto Exposure Control].

Fifty posterior-anterior chest radiographs taken using an auto exposure control were evaluated in order to find an optimum determination method of the exposure index (EI). Four types of the relevant image regions were tested: (a) full image, (b) central 25% area, (c) full image excluding direct x-ray area, and (d) pulmonary area only, whereas four types of the value of interest (VOI) were adopted to each relevant image region: mean, median, mode, and middle. When the target EI was determined as the average of the 50 images, the deviation index (DI) was within ±1.0 only if pulmonary area was selected as the relevant image region, with the VOI of mean, median, and middle. This result strongly suggests that pulmonary area should be selected as the relevant image region of an EI when an auto exposure control is used.