Japanese Journal of Radiological Technology

We proposed a method for practical estimation of the presampling modulation transfer function(MTF) of a computed radiography (CR) system by using the MTFs of an imaging plate and the sampling aperture. TheMTFs of three imaging plates (GP-25,ST-VN, and RP-1S)with different photostimulable phosphors weremeasured by using direct fluorescence (the light emitted instantaneously by x-ray exposure), and the presamplingMTFs were estimated from these imaging plate MTFs and the sampling aperture MTF. Our results indicated that for imaging plate RP-1S the measured presampling MTF was significantly superior to the estimated presampling MTF at any spatial frequency. This was because the estimated presampling MTF wasdegraded by the diffusion of direct fluorescence in the protective layer of the imaging plate's surface. Therefore, when the presampling MTF of the imaging plate with a thick protective layer is estimated, correction forthe thickness of the protective layer should be carried out. However, the estimated presampling MTF of imaging plates with a thin protective layer were almost the same as the measured presampling MTF, except in the high spatial frequency range. Therefore, we consider this estimation method to be useful and practical, because the spatial resolution property of a CR system can be obtained simply from the imaging plate MTF measured with direct fluorescence.

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For the quantitative accuracy of simultaneous emission/transmission (SET)acquisition using the mask technique, we confirmed the contamination of photons that spilled over from the transmission source into the emission data, and evaluated the influence of decay correction with different half-lives of the transmission source and emission source. First, to confirm the contamination of photons from the transmission source,SET scans were performed with a cold phantom and blank status in the field of the gantry. Second, to evaluate the quantitative accuracy of the PET values obtained by the decay correction, serial scanning with SET and conventional scanning were carried out with the same frame time. All of the data were reconstructed with the decay correction based on the same scan start time, and counts of ROI on the reconstructed images were evaluated. For SET acquisition, contamination of photons from the transmission source into the emission data due to the scattered photons, which were from the object and spillover fraction through the mask window, was shown. The intensity of this contamination was dependent on the activity of the transmission source. Quantitative accuracy was affected by the decay correction of the SET data. The effect was emphasized by increasing the emission activity or reducing the transmission activity. It is suggested that the effect of attenuation correction is based on the transmission data, which are subtracted from the emission data.These characteristics should be carefully considered for the quantitative estimation of SET acquisition.

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In recent years, as gradient magnetic resonance imaging (MRI)systems have become larger and faster, the influence of low-frequency alternating magnetic fields on the human body during rapid scanning have become significant in terms of patient health. It has become more important to monitor time-variant magnetic fields (dB/dt). From this point of view, we measured the maximum dB/dt for clinical scanning using a self-made search coil and storage oscilloscope that conformed to international standard IEC 60601-2-33(1995). The results were almost the same as the calculated values. In this brief report, we introduce to clinical operators the method of measuring maximum dB/dt.

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We evaluated image quality and necessary patient exposure when using the CXDI-11 (Canon Inc.)flat panel detector(FPD). This detector, which consists of a rare earth fluorescent screen(Gd_2O_2S: Tb)and amorphous silicon sensor, was compared with the FCR-5000(Fuji Film Medical Co. Ltd.)CR and the UR3/HGM2 (Fuji Film Medical Co.Ltd.)film-screen (F/S)combination. Comparisons of both physical imaging characteristics and clinical image quality were carried out. The final MTF of the FPD was found to be similar to or better than those of the CR and F/S systems. For identical exposures, the overall Wiener spectrum of the FPD was found to be slightly poorer than that of the F/S combination. The NEQ of the FPD was found to be similar to or better than those of the CR and F/S systems. Comparison of chest images showed that the FPD produced images with quality comparable to or higher than those of the CR system. Similarly,evaluation of abdominal and bone images using a 5-scale method showed that the FPD produced images with quality comparable to or higher than those of the CR system. As with CR, the x-ray quantum mottle in FPD images becomes noticeable at low exposures. Clinical images were therefore taken with a 30% increase in exposure, giving a Wiener spectrum for the FPD images similar to that obtained with the F/S images. This is probably not a significant increase in exposure, given the improvement in image quality and increased ease of use provided by the CXDI system. Further, future improvements in hardware and imageprocessing may allow images to be taken with the same exposure used for F/S images. Our evaluation of both image quality and x-ray exposure has therefore indicated the value of the FPD in the clinical environment.

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