Digital breast tomosynthesis has become accepted in clinical use. It is important to physically evaluate a system to ensure that it is working at full performance. Non-linear reconstruction processing is proposed to improve interpretation of clinical images by enhancing the minute contrasts of breast tissue while suppressing metal artifacts. Because existing measuring methods assume a linear system, physical evaluation applied to images reconstructed with non-linear processing may result in unnatural values. We investigated the influence of different reconstruction methods on physical evaluations. We suggest using images reconstructed by back projection processing without a filter to ensure the device performance directly.
In our previous stereoscopic image for medical use research, we reported that observers found it is easier to identify target objects in stereoscopic images than in two dimensional images, however, we found that mental and visual fatigue levels are equivalent in viewing the stereoscopic and the two dimensional images. We reported that a number of users dislike the sensations accompanying stereoscopic vision. Hence, we studied personal variation of stereoscopic visibility and the training effect for the stereoscopic visibility in this research. Simulated images, in which prepared calcifications were arranged at parallactic angles between ±2° to ±15° at object heights from 40 to 80mm, were displayed on a stereoscopic 3D display. Seven observers were selected to judge the achievement of stereoscopic vision (stereopsis) and their visibility was determined. The observers were asked to point the stereoscopic cursor of the 3D mammography viewer at the simulated calcifications and the accuracy rates were determined. Subsequently, re-examination was implemented after 3D visual training for 15 to 20 minutes per day for two weeks, and the visibility and accuracy rates were measured again. We found individual differences in the parallactic angles at which stereopsis was realized. Moreover, the parallactic angles of stereopsis widened through training and the average visibility improved from 69% to 84% as the result of training. Furthermore, the average accuracy rates improved from 53% to 60% the accuracy of depth commands improved. This suggests that observers who are weak in stereoscopic vision can be trained to be better at stereoscopic viewing.
A stereoscopic viewing technology is expected to improve diagnostic performance in terms of reading efficiency by
adding one more dimension to the conventional 2D images. Although a stereoscopic technology has been applied to
many different field including TV, movies and medical applications, physiological fatigue through reading stereoscopic
radiographs has been concerned although no established physiological fatigue data have been provided. In this study,
we measured the α-amylase concentration in saliva, heart rates and normalized tissue hemoglobin index (nTHI) in blood
of frontal area to estimate physiological fatigue through reading both stereoscopic radiographs and the conventional 2D
radiographs. In addition, subjective assessments were also performed.
As a result, the pupil contraction occurred just after the reading of the stereoscopic images, but the subjective
assessments regarding visual fatigue were nearly identical for the reading the conventional 2D and stereoscopic
radiographs. The α-amylase concentration and the nTHI continued to decline while examinees read both 2D and
stereoscopic images, which reflected the result of subjective assessment that almost half of the examinees reported to feel
sleepy after reading. The subjective assessments regarding brain fatigue showed that there were little differences
between 2D and stereoscopic reading.
In summary, this study shows that the physiological fatigue caused by stereoscopic reading is equivalent to the
conventional 2D reading including ocular fatigue and burden imposed on brain.
To increase the detection performance of breast cancers in mammograms, we need to improve shape delineation of
micro calcifications and tumors. We accomplished this by developing a direct-conversion mammography system with an
optical reading method and a new dual a-Se layer detector. The system achieved both small pixel size (50 micrometer)
and a high Detective Quantum Efficiency (DQE) realized by 100 % of fill factor and noise reduction. We evaluated
image quality performance and determined the best exposure conditions.
We measured DQE and Modulation Transfer Function(MTF) according to the IEC62220-1-2. High DQE was maintained
at a low radiation dosage, indicating that the optical reading method accompanies low noises. Response of MTF was
maintained at up to the Nyquist frequency of 10 cyc/mm, which corresponds to 50 micrometer pixel size.
To determine the best exposure conditions, we measured Contrast to Noise Ratio (CNR) and visually evaluated images
of a resected breast under conditions of MoMo, MoRh, and WRh. There were occasional disagreements between the
exposure conditions for achieving the maximum CNR and those for the best image graded by the visual evaluation. This
was probably because CNR measurement does not measure effects of scattered X-ray. The images verified the
improvement in detection and delineation performance of micro calcifications and tumors.
For Computed Radiography (CR) systems that use a columnar phosphor plate (CPP) and a powder phosphor plate (PPP), we designed the systems to obtain the best image quality. To determine the optimum phosphor layer thickness for each phosphor plate, the relationship between the intensity and spatial spread of photo-stimulated luminescence (PSL), and the phosphor layer thickness of the phosphor plate is quantitatively clarified. Next, to determine the stimulation light intensity, we measured PSL, modulation transfer function (MTF) and detective quantum efficiency (DQE) by varying the stimulation light intensity, using the determined optimum phosphor layer thickness. We also investigated the noise components of each phosphor plate. Results show that, compared to the PPP, the CPP is more favorable in allowing thicker phosphor layer without reduction in MTF. As the result of the relationship between the layer thickness and the PSL, noise analysis, it was confirmed that the CPP could detect PSL in the deep region of the phosphor layer without reducing the intensity of PSL. This suggests that in comparison to the PPP, the CPP can make efficient use of X-ray information, thereby promising to enhance image quality and to reduce exposure dose.
The purpose of this study is to determine the relative effect of MTF, DQE, and pixel size on the shape of
microcalcifications in mammography. Two original images were obtained by a) scanning the film that accompanies an
RMI-156 phantom at a resolution of 25μm per pixel, b) creating an image with various shapes on a computer. Simulated
images were then obtained by changing MTF, adding noise to simulate DQE effects, and changing the resolution of the
original images. These images were visually evaluated to determine the recognition of the shape. In the evaluation of
400μm microcalcifications on the RMI-156 phantom, we found that shape recognition is maintained with a pixel size of
50μm or less regardless of MTF. However, at resolutions over 50μm, recognition was insufficient even when MTF was
increased. Adding noise decreased visibility but did not affect shape recognition. The same results were obtained using
computer-created shapes. The effect of pixel size on the recognition of the shape of microcalcifications was shown to be
greater compared to MTF and DQE. It was also found that increasing MTF does not compensate for information lost
because of enlarged pixel size.
In X-ray-to-light conversion digital radiography, we compared the image quality of a system in which photodetection is done from the X-ray incident surface (hereafter referred to as a front exposure system) and a system in which photodetection is done from the back side opposite the X-ray incident surface (hereafter referred to as a back exposure system). Modulation transfer function (MTF) and detective quantum efficiency (DQE) measurements were performed using the method IEC prescribes. Both MTF and DQE were higher with the front exposure system than with the back exposure system, with the former delivering better image quality. This difference can be accounted for by differences in the distribution of absorbed X-ray doses in the phosphor layer, the readout efficiency, which varies as a function of depth in the phosphor layer, and depth-dependent blurs of light. Furthermore, we determined changes in image quality incurred by varying the quality of X-rays, the thickness of the phosphor layer and the crystal structure of phosphors. The advantage of the front exposure system becomes more pronounced with decreasing X-ray tube voltage, increasing phosphor layer thickness, and the use of phosphors in powder form.
The concept proposed by EUREF which determines the AGD between upper limit corresponding to the acceptable level
of AGD and lower limit corresponding to lower limit of image quality was applied to CR Mammography, and the
resulting object thickness tracking and tube voltage tracking were determined.
EUREF specifies threshold contrast visibility for a 5 cm of PMMA. In accordance with this definition, the lower limit of
CNR for a 5 cm of PMMA was determined by measuring the CNR at the lower limit of AGD where threshold contrast
visibility was just acceptable. The obtained lower limit of CNR was then multiplied by the object thickness
compensation factor to estimate the lower limits of CNR for all object thicknesses. AGDs are now determined for each
lower limit of CNR to obtain thickness tracking and tube voltage tracking characteristics. Although the limited range of
examined target/filter combinations and tube voltages should be taken into consideration, these tracking characteristics
for constant CNR differ in their profiles from those of a screen-film system with AEC for providing constant optical
density. Among our findings, we found that a lower AGD is achieved while maintaining CNR for a thick object when
using a combination of target/filter and tube voltage that generates higher X-ray energy compared to the combination
given in the EUREF's typical spectra per PMMA thickness (Mo/Rh 32 kV at PMMA 4cm and Rh/Rh 28 kV at PMMA
6 cm, for example).
We also found that the thickness tracking characteristics for constant S value behaves similarly to constant CNR under
certain conditions of the target/filter combination and tube voltage.
We performed an image quality simulation for the line scan system, which realizes a compact and high-speed Computed Radiography (CR) system.
The line scan system uses a line light source and a linear CCD sensor. In this system, the emitted light must be efficiently focused onto the CCD sensor to detect the emitted light as much as possible. To realize the effective light detection, we analyzed the spread of the light in the photostimulable phosphor layer. We also estimated the image quality based on X-ray absorption, the amount of emitted light, light collecting efficiency and electric noise. It clarified the image quality is affected strongly by such factors as the spread of the PSL, the size of photo diodes of the CCD sensor and electric noise.
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