Japanese Journal of Radiological Technology

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In positron emission tomography (PET), the large number of lines of responses in three-dimensional (3D) acquisition mode creates a high volume of sinogram data and increases reconstruction time in iterative reconstruction. We tried to decrease sinogram data volume by reducing the number of views using angular compression and then evaluated the accuracy of this mashed mode. Three methods were compared, conventional mode (CONV), X2 mashed mode (X2: two adjacent projection angles are added together), and X4 mashed mode (X4: four adjacent angles added). A point source of <18>^F was used to measure spatial resolution. A hot spot phantom made of 6 hot spheres (10-38 mm in diameter) within water of 20 cm in diameter was scanned to evaluate the recovery coefficient (RC). A lung-heart-liver phantom made of homogenous radioactive myocardium, a spherical hot mass in the lung (10 mm in diameter), and background activity in the liver was scanned to evaluate the homogeneity of the myocardial wall. The quality of the reconstructed images was evaluated in terms of the normalized mean square error (NMSE), Bull's eye map, profile curve, and peak value of the spherical hot mass. The reconstruction times of X2 and X4 were one-half and one-quarter, respectively, of that of CONV. In terms of spatial resolution, FWHM of CONV, X2, and X4 were, 4.26, 4.33, and 4.48 (mm) at the center, 4.81, 5.68, and 8.73 tangentially, and 8.01, 8.19, and 8.27 radially at R=200 mm, respectively. RC was similar for all methods. The NMSE values of X2 and X4 compared with CONV were 0.0003 and 0.0014, respectively. In the hot mass, these methods showed almost the same profile curves, although the peak value of X4 was only -1.95% less than that of CONV. Although the result of spatial resolution of X4 was slightly degraded, image quality and physical performance were good. Therefore, the X4 mashed mode used with angular compression was considered clinically useful.

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In order to apply digital watermarking to medical imaging, it is required to find a trade-off between strength of watermark embedding and deterioration of image quality. In this study, watermarks were embedded in 4 types of modality images to determine the correlation among the watermarking strength, robustness against image processing, and image deterioration due to embedding. The results demonstrated that watermarks which were embedded by the least significant bit insertion method became unable to be detected and recognized on image processing even if the watermarks were embedded with such strength that could cause image deterioration. On the other hand, watermarks embedded by the Discrete Cosine. Transform were clearly detected and recognized even after image processing regardless of the embedding strength. The maximum level of embedding strength that will not affect diagnosis differed depending on the type of modality It is expected that embedding the patient information together with the facility information as watermarks will help maintain the patient information, prevent mix-ups of the images, and identify the test performing facilities. The concurrent use of watermarking less resistant to image processing makes it possible to detect whether any image processing has been performed or not.

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In radiotherapy, dose distribution and calculation of dose monitor units (DMU) are generally performed by a radiation treatment planning system using CT images. Therefore, differences in calculation can arise as a result of the quality of the CT image data. The quality of CT images involves contrast resolution, resolution, noise, slice thickness, and other factors. Among these factors, we examined noise and slice thicknesses Results demonstrated that, even if noise increased, CT value did not change, and, therefore, did not influence DMU. Examination of slice thickness showed that, when the radiation field was rectangular, it was not influenced by slice thickness. However, when a multi-leaf colhrnator (MLC) was used, if slice thickness was thicker than the size of the MLC, a difference arose in the position of the MLC, and, therefore, some difference arose in dose. Therefore, slice thickness should be thinner than the size of the MLC.

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Various clustering methods are used in cluster analyses, with each clustering method demonstrating unique advantages. Therefore, it is important to make the best use of the advantages each method provides. We have recognized that it is necessary in the evaluation of X-ray images to classify observers quantitatively according to visual characteristics (grouping of observers) and have clustered observers using the UPGMA method, which is one of the clustering methods. We found that the observers were clustered into two different groups, one with radiologist-like characteristics and the other with medical physicist-like characteristics. Furthermore, we suggested that the group with radiologist-like characteristics was suitable for QC of X-ray images. However, it is doubtful whether the UPGMA method is most suitable for the grouping of observers. In this work we clustered observers using various clustering methods and examined the most suitable method for the evaluation of X-ray images. The results showed that the ward method was least suitable for the grouping of observers, and they were distinctly grouped into two different categories by using a further method.

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