[Proposal and verification of presampled MTF measurement by simple analysis using the edge method]

Effect of beam quality and readout direction in the edge profile on the modulation transfer function of photostimulable phosphor systems via the edge method

Purpose: The objective of this study was to investigate the effects of (i) the difference in the beam qualities on the presampled modulation transfer function (MTF) using the edge method and (ii) the readout direction of the edge profile for the photostimulable phosphor (PSP) system. Approach: The International Electrotechnical Commission (IEC) defined a technique using the "radiation qualities based on a phantom made up of an aluminum added filter" (RQA). A general radiographic system with a tube voltage of 50 kV and 9.7 mm of additional aluminum filtration was used to conform the x-ray to the IEC-specified beam quality definition RQA3. Additionally, we employed two different beams with tube voltages of 60 and 70 kV using a dental x-ray unit. The MTF was measured in the readout direction from low-to-high exposure regions and vice versa with respect to the scanning and subscanning directions of the PSP system. Results: The difference in the 50%MTF value for all directions between 60 and 70 kV averaged less than 0.05 . The 50%MTF of RQA3 was on average 0.2 lower than the value for 60 kV for each direction. For all beam qualities, no difference was observed between the MTFs measured in the readout direction from the low-to-high exposure regions and vice versa. Conclusions: The MTFs, measured using the dental x-ray unit, were unaffected by the tube voltage, and they were slightly higher than those measured using the RQA3. Furthermore, the MTF was unaffected by the differences in the readout directions of the edge profile.

Effect of readout direction in the edge profile on the modulation transfer function of computed radiographic systems by use of the edge method

We investigated the effect of the readout direction of the edge profile obtained by the edge method on the presampled modulation transfer function (MTF) in various computed radiographic (CR) systems. There were no differences in the MTFs derived from two edge profiles in the sub-scanning direction of four CR systems used in this study. On the other hand, the MTFs measured at a readout direction from the low (edge) to the high (direct exposure) exposure region were higher than those measured at a readout direction from the high to the low exposure region in the laser-beam scanning direction for three of the four CR systems. Although this phenomenon depends on the CR system, it is important to understand and indicate both MTFs at the two edge profiles in the laser-beam scanning direction for accurate assessment of the resolution property.

[Method of measuring modulation transfer function in computed tomography using slit method with air gap phantom]

There are several methods for measuring modulation transfer function (MTF) in computed tomography (CT) images. The aluminum slit method, scanning a phantom consisting of a thin aluminum foil sandwiched by flat plastic slabs, is a standard method for measuring field of view (FOV) in clinical CT scan. But this method requires extreme caution when handling metal foil of high precision. Therefore, we devised a more simple method named air gap slit (AS) method. This new technique is based on the aluminum slit method but use air gap instead of metal foil between phantoms. The MTF was calculated from a reversed profile curve of air slit which indicated minimum CT number. The aim of this study was to investigate a possibility of AS method evaluating MTF. We investigated fluctuation of MTF and FOV in clinical CT scan compared with the aluminum slit method. The result showed that the fluctuation of MTF was caused by statistics noise and is more affected by a bone kernel than standard kernel when reconstructing. Also, the MTF value in AS method was slightly higher than in aluminum slit method and did not correspond with. AS method is a useful method for measurement of MTF in clinical CT scan. When we use this method, we have to take into consideration the noise influence of data.

Digital magnification mammography with matched incident exposure: physical imaging properties and detectability of simulated microcalcifications

Our purpose was to evaluate the usefulness of digital magnification mammography with matched incident exposure by investigating the physical imaging properties and doing an observer performance test. A computed radiography system and a mammographic unit were used in this study. Contact and magnification radiographies of 1.2-1.8 in combination with focal spot sizes of 0.1 mm without grid and 0.3 mm with grid were performed. Physical imaging properties, namely, scatter fraction, total modulation transfer function (MTF) including the presampled MTF and the MTF of focal spot size, and Wiener spectrum (WS), were measured. Detail visibility was evaluated by use of free-response receiver operating characteristic analysis of the detectability of simulated microcalcifications. Scatter fractions decreased considerably as the magnification factor increased without grid technique. In the grid technique, scatter fractions for all magnification techniques were comparable. The total MTFs of magnification techniques with a focal spot size of 0.1 mm improved significantly compared with the conventional contact technique. However, the improvement of the total MTFs of magnification techniques with the combination of 0.3 mm focal spot size was small. The WSs degraded with an increase of the magnification factor compared with the contact technique due to the maintained exposure incident on the object. The observer performance test indicated that the 1.8 magnification technique with the 0.1 mm focal spot size provided higher detectability than did the contact technique. Digital magnification mammography under the same incident exposure conditions improved the detectability of microcalcifications.

[Study of appropriate dosing in consideration of image quality and patient dose on the digital radiography]

Recently about 90% of radiographs have been taken by the digital radiographic system in Japan, but the exposure dose of the patients are about ten-times different among the systems. We understood it by a surveytaken in 2007. We studied the visual evaluation with varying exposure doses using the image phantom of the lumber AP, lumber lateral and hip AP. Additionally we measured quantum efficiency (DQE) of the digital systems. We also studied the exposure index (EI) of IEC standard to see whether it is able to be the sensitivity index among the digital systems. DQE in 1.0 cycle/mm of CR, FPD (GOS), FPD (CsI, a-Se) became 0.2-0.25, 0.3, 0.5, respectively. Our results display that the dose reduction is relative to DQE. The visual evaluation results also show that dose reduction is possible among the systems. From these results, we are able to reduce the exposure dose of the patients at the clinical site. We also suggest that we manage the exposure dose using the E.I of the IEC standard.

[Influence of angle-measurement error on pre-sampled MTF and proposal of an optimal technique of angle measurement]

The presampled modulation transfer function (MTF) is recognized as the established metric for characterizing the resolution performance of a digital imaging system. In the past, the three general approaches for assessing the presampled MTF were using the angulated slit, angulated edge, and angulated square-wave test pattern all of which are tilted slightly against the column direction of the detector. In all methods, it is important to determine the exact angle of the respective test devices. In this study, we examined the influence of angle-measurement error in three test devices and the optimal technique of angle measurement. These results demonstrated that the influences of angle-measurement error in each method were equal. We also investigated three angle-measurement techniques using trace of objects, Hough transfer, and comparative observation of synthetic profiles. These results suggested that the technique using synthetic profiles was the most optimal technique in the angle measurement. Through use of the technique, angle-measurement error was completely overcome. This technique will contribute to improved accuracy of presampled MTF measurements.