Investigation of measurement accuracy of factors used for detective quantum efficiency measurement in digital radiography.

In the detective quantum efficiency (DQE) evaluation of detectors for digital radiography (DR) systems, physical image quality indices such as modulation transfer function (MTF) and normalized noise power spectrum (NNPS) need to be accurately measured to obtain highly accurate DQE evaluations. However, there is a risk of errors in these measurements. In this study, we focused on error factors that should be considered in measurements using clinical DR systems. We compared the incident photon numbers indicated in IEC 62220-1 with those estimated using a Monte Carlo simulation based on X-ray energy spectra measured employing four DR systems. For NNPS, influences of X-ray intensity non-uniformity, tube voltage and aluminum purity were investigated. The effects of geometric magnifications on MTF accuracy were also examined using a tungsten edge plate at distances of 50, 100 and 150 mm from the detector surface at a source-image receptor distance of 2000 mm. The photon numbers in IEC 62220-1 coincided with our estimates of values, with error rates below 2.5%. Tube voltage errors of approximately ±5 kV caused NNPS errors of within 1.0%. The X-ray intensity non-uniformity caused NNPS errors of up to 2.0% at the anode side. Aluminum purity did not affect the measurement accuracy. The maximum MTF reductions caused by geometric magnifications were 3.67% for 1.0-mm X-ray focus and 1.83% for 0.6-mm X-ray focus.

[Method of segmenting inferior horns of lateral ventricles using active contour models].

Recent research has suggested that the measurement of regional atrophy in the structure of the medial temporal lobe is a promising way to discriminate Alzheimer-type dementia patients from healthy control subjects. There are some reports that the inferior horns of the lateral ventricles are expanded by atrophying the structure of the medial temporal lobe. We developed a technique to automatically detect the region of the inferior horns of the lateral ventricles by gray-level thresholding and morphological processing. However, there were some incorrect regions in this method. Accordingly, we proposed a technique for which active contour models (ACM) were used. Our ACM incorporates the improved edge-based image and the external constraint to improve convergence and to reduce its dependence on initial estimation. In this study, we present the details of an algorithm that traces the contours of the inferior horns of the lateral ventricles and its performance relative to manual methods. The average degree of correspondence between the extract region and manual trace was measured in 30 inferior horns of 15 subjects. The average degree of correspondence of the proposed method was about 4% higher than that of the conventional method. These results suggest that the proposed method is more accurate than the conventional method.

[Development of automated measurement method for medial temporal lobe on CT images].

Recent research has suggested that the measurement of regional atrophy in the structure of the medial temporal lobe is a promising way to discriminate Alzheimer-type dementia patients from healthy control subjects. The purpose of this study was to develop a technique to measure the medial temporal lobe automatically in axial CT images. Linear measurements of width of the inferior horns of the lateral ventricles, width of the medial temporal lobe, and the interuncal distance were performed. Area measurements of the inferior horns of the lateral ventricles were also performed. In the algorithm for the automatic measurement of the medial temporal lobe, brain contour, and sagittal plane were detected first, and the inferior horns of the lateral ventricles. Our method was applied to ten patients clearly without cerebral hemorrhage or infarct. The rates of accuracy of automated detection were 93% with the linear measurements and 75% with the area of the inferior horns. The rates were improved to 100% with the variable function of the threshold value. We suggest that this automated measurement method is both objective and simple enough to be used in routine clinical applications.

[Modulation transfer function measurements on magnetic resonance imaging: use of the edge spread function].

A convenient, accurate method to measure the modulation transfer function (MTF) of magnetic resonance(MR) images is discussed. To avoid any distortion of the edge spread function(ESF) which is inadvertently produced by the magnitude operator, the MTF was calculated from the ESF obtained from the salad-oil/water interface. Our MTF findings closely correlated with the MTF findings as calculated by an alternate method (Steckner et al.). Our method was only adapted to some specific sequences and some specific encoding directions, however, MTF was successfully obtained by the magnitude operator without any errors. In our method, the production and arrangement of phantoms were also easier to control. Our new method is therefore considered to be useful for evaluating the resolution property of MR images from various institutions.