The purpose of our research is to make clear the mechanism that a reader (physician or radiological technologist) effectively identify abnormal findings in CT images of lung cancer screening by using with CAD system. A method guessing the 2X2 decision matrix between reader / CAD and reader / reader with CAD was investigated. We suppose the next scene to be it. At first, a reader judges whether abnormal findings per one patient per one CT image are present (1) or absent (0) without CAD results. The second, a reader judges whether abnormal findings are present (1) or absent (0) with CAD results. We expresses the correlation between diagnoses by a reader and CAD system for abnormal cases and for normal cases by following formula using phi correlation coefficient:φ=(cd-ab)/√(a+c)(b+d)(b+c)(a+d). a,b,c,d: 2X2 decision matrix parameters. If TPR1=(a+c)/n, TPR2=(b+c)/n and TPR3=(a+b+c)/n for abnormal cases, TPR3=TPR1+TPR2 - TPR1×TRR2 - φ√TPR1(1-TPR1)TPR2(1-TPR2). Therefore, a=n (TPR3 - TPR1), b=n (TPR3 - TPR2), c=n (TPR1 + TPR2 -TPR3), d=n (1.0 - TPR3). This theory was applied for the experimental data. The 41 students interpreted the same CT images [no training]. A second interpretation was performed after they had been instructed on how to interpret CT images [training], and third was assisted by a virtual CAD [training + CAD]. The mechanism that makes up for a good point of a reader and a CAD with CAD in interpreting CT images was theoretically and experimentally investigated. We concluded that a method guessing the decision matrix (2X2) between a reader and a CAD decided the "presence" or "absence" of abnormal findings explain the improvement mechanism of diagnostic performance with CAD system.
The conventional respiratory-gated CT scan technique includes anatomic motion induced artifacts due to the low temporal resolution. They are a significant source of error in radiotherapy treatment planning for the thorax and upper abdomen. Temporal resolution and image quality are important factors to minimize planning target volume margin due to the respiratory motion. To achieve high temporal resolution and high signal-to-noise ratio, we developed a respiratory gated segment reconstruction algorithm and adapted it to Feldkamp-Davis-Kress algorithm (FDK) with a 256-detector row CT. The 256-detector row CT could scan approximately 100 mm in the cranio-caudal direction with 0.5 mm slice thickness in one rotation. Data acquisition for the RS-FDK relies on the assistance of the respiratory sensing system by a cine scan mode (table remains stationary). We evaluated RS-FDK in phantom study with the 256-detector row CT and compared it with full scan (FS-FDK) and HS-FDK results with regard to volume accuracy and image noise, and finally adapted the RS-FDK to an animal study. The RS-FDK gave a more accurate volume than the others and it had the same signal-to-noise ratio as the FS-FDK. In the animal study, the RS-FDK visualized the clearest edges of the liver and pulmonary vessels of all the algorithms. In conclusion, the RS-FDK algorithm has a capability of high temporal resolution and high signal-to-noise ratio. Therefore it will be useful when combined with new radiotherapy techniques including image guided radiation therapy (IGRT) and 4D radiation therapy.
The increasing number of CT images to be interpreted in mass screening requires radiologists to interpret a huge number of CT images, and the capacity for screening has therefore been limited by the capacity to process images. To remedy this situation we considered paramedical staff, especially radiological technologists, as "potential screeners," and investigated their capacity to detect abnormalities in CT images of lung cancer screening with and without the assistance of a computer-aided diagnosis (CAD) system. We then compared their performances with those of physicians. A set of 100 slices of thoracic CT images from 100 cases ( 73 abnormal and 27 normal), one slice per case, was interpreted by 43 paramedical college students. A second interpretation by the students was performed after they had been instructed on how to interpret CT images, and a third interpretation was assisted by a virtual CAD system. We calculated the areas under the ROC curve (Az values) for both students and physicians. For the first set of interpretations, the Az values of 40% out of students placed the Az values within the range of Az values of the physicians, which varied from 0.870 to 0.964. For the second set of interpretations after the students had been instructed on CT image interpretation, the students' rate was 86%, and for the third set of virtual CAD-assisted interpretations it was 95%. The performance of paramedical college students in detecting abnormalities from thoracic CT images proved to be sufficient to qualify them as "potential screeners."
We have developed a four-dimensional CT (4D CT) using continuous rotation of cone-beam x-ray. The maximum
nominal beam width of the 4D CT is 128 mm at the center of rotation in the longitudinal direction. In
order to obtain appropriate estimations of exposure dose, detailed single-slice dose profi les perpendicular to the
rotation axis including scattered radiation were measured in PMMA cylindrical phantoms, which were cylindrical
lucite phantoms of 160 mm and 320 mm diameter and 900 mm length. Dose profi les were measured with
a pin photodiode detector at the center and a peripheral point of 10 mm depth. A pin silicon photodiode sensor
with 3 × 3 × 3 mm sensitive region was used as an x-ray detector, which was scanned along longitudinal direction
in the phantom for beam widths of 20, 42, 74, 106 and 138 mm. The dose profi les had long tails caused
by scattered radiation more than 200 mm out of the beam width edge. The exposure dose covered 95 % was
distributed along about 360 mm length at the center and about 310 mm at the periphery, which was independent
of the beam width. Before the advent of multi-detector CT, CTDI100 was used to approximate integral dose for
clinical scan conditions. However, for 4D CT employing a variable beam width, the standard CTDI was not a
good estimation. This work was carried out to establish a method of the dose measurements including scattered
radiation for cone-beam CT such as 4D CT. In order to perform the dose assessment including scattered radiation,
dose measured length should be recommended to measure integral dose over beam widths plus at least 230
mm, which covered 95 % total exposure dose.
In this paper we present two methods of evaluating the effectiveness of double check (by two radiologists or by a CAD system and a radiologist): One method uses ROC analysis and the other uses the phi correlation coefficient (φ). We used the first method to evaluate the effectiveness of two radiologists conducting double check through discussion (i.e. the radiologists confer; conference system). We used the second method to evaluate the effectiveness of double check in which Reader 2 makes a final assessment by referring to the assessment of Reader 1 (reference system). It is suggested that double check conducted by two radiologists through discussion may not be so effective; however, double check in which Reader 2 makes a final assessment by referring to the assessment or Reader 1 may be very effective. In addition, we discuss problems that may occur in relation to Reader 2 deciding whether to adopt the assessment of Reader 1, and practical models of double check by a CAD system and a radiologist. Continued research is necessary to establish a double check system that improves diagnostic accuracy in practical situations, i.e. it is unknown if assessments are correct.
We have developed a prototype of 4-dimensional (4D) CT-scanner that employs continuous rotation of cone-beam. Because a cone-beam scan along a circle orbit did not collect a complete set of data to make rigid reconstruction of volume (3D image), it might bring disadvantages or artifacts. To examine effects of the cone-beam data collection on image quality, we have evaluated basic performances of the prototype and compared them to those of a state-of-the-art multi-detector (MD) CT-scanner. As the results image characteristics such as noise, uniformity, high contrast and low contrast detectability of 4D CT were independent of z-coordinate, and comparable to those of MD CT. The transverse spatial resolution of 4D CT was independent of z-coordinate, and showed slightly better performance than that of MD CT, while the longitudinal spatial resolution of 4D CT was the same as the transverse one, and much better than that of MD CT in the present scan conditions. Isotropic resolving power of 0.5mm was achieved for 4D CT. A Feldkamp artifact was observed in distortion measurement though its clinical meaning has not been clarified. Exposure dose measured with CT dose index (CTDI) for 4D CT was comparable to that for MD CT. As a whole our first model of 4D CT-scanner was successful to take a volume data of 10cm long along longitudinal direction in a single rotation scan with comparable image quality and exposure dose to the state-of-the-art MD CT-scanner.
We have developed a method to remove the noise from the cone beam CT image and consider the reduction of a patient's dose. In diagnostic medicine, cone beam CT increases a patient' exposure dose. The X-ray CT image is degraded by the noise that is called quantum mottle, and the noise becomes so remarkable with decreasing patient' dose. It is known that the image signal can be separated from the noise by measuring the Lipschitz exponents of the image singularities from the evolution of wavelet transform modulus maxima across scales. We identify the singularities of 2-D projections by computing the wavelet transform modulus sum (WTMS) in the direction which is indicated by the phase of wavelet transform. Our preliminary results show the validity of the method based on 2-D WTMS for removing quantum mottle from 2-D projection. And it shows the possibility that the patient's dose can be reduced by this method.
KEYWORDS: Modulation transfer functions, Sensors, Signal to noise ratio, Image processing, Monte Carlo methods, Computed tomography, Image quality, X-rays, Imaging systems, X-ray computed tomography
We have examined the characteristic of a flat-panel detector used for our cone-beam CT scanner. We calculated detection efficiency, the modulation transfer function and the noise characteristic of flat-panel detector by the Monte Carlo method, and compared the results with experiment data. From these data, we estimate the relation between the patient dose and signal-to-noise ratio of projection data of CT image. Furthermore, to reduce the patient dose, we have examined the effect of removing the noise from projection degraded by quantum mottle, by the wavelet analysis. Our preliminary results show that de-noising of projection data with wavelet analysis has an effect to reduce the patient dose to less than 1/10, without decreasing the quality of CT image.
An active matrix flat-panel imager (FPI) is a good candidate for the 2-dimensional detector of cone beam CT (CBCT), because it has a wider dynamic range and less geometrical distortion than video-fluoroscopic system so far employed. However the performance of FPI-based CBCT has not been sufficiently examined yet. The aim of this work is to examine the performance of CBCT using a FPI with several phantoms. An X-ray tube, a phantom and a FPI were aligned on an experimental table. The FPI was PaxScan2520 provided by Varian Medical Systems. It has an active area of approximately 180x240mm and the pixel size is 127 micrometer. CsI is used as a scintillator. The phantom was rotated with 1-degree steps while 360 projection frames (1408x1888 active pixels each frame) were collected. 2x2 pixels were combined into a single pixel to reduce noise. 512x512x512 voxels were reconstructed with the Feldkamp method. The comparison was made between reconstructed images with or without scatter rejecting grid. The uniformity and linearity of reconstruction value was drastically improved with the grid. Scatter rejection using a thin-vane collimator was also examined, and it showed more effective than the grid.
Cone beam CT has a capability of 3-dimensional imaging of large volumes with isotropic resolutions, and has a potentiality of 4-dimensional imaging (dynamic volume imaging) because cone beam CT acquires a large volume data with one rotation of X-ray tube-detector pair. However, one of the potential drawbacks with cone beam CT is larger amount of scattered X-rays. These X-rays may enhance the noise in reconstructed images, and thus affect low contrast detectability. The aim of this work was to estimate scatter fractions and effects of scatter on image noise, and was to seek methods of improving image quality in cone beam CT. First we derived a relationship between the noises in reconstructed image and in X-ray intensity measurement. Then we estimated scatter to primary ratios in X-ray measurements, using a Monte Carlo simulation. From these we estimated image noise in clinical relevant conditions. The results showed that scatter radiation made a substantial contribution to the image noise. However focused collimators improved it, because they decreased scatter radiation drastically while keeping the primary radiation nearly the same level. A conventional grid also improved image noise though the improvement was less than that of focused collimators.
A synchrotron light source dedicated to medical applications is designed at NIRS. The synchrotron ring accelerates electrons up to 1.8 GeV and stores them with about 400 mA, and is equipped with two superconducting multipole wigglers to generate sufficient photon flux for medical diagnoses. One of the most interesting applications for us is monochromatic x-ray computed tomography (CT). It plays an important role in advancing heavy ion radiotherapy of cancers which is being performed at NIRS. The radiotherapy is carried out based on a treatment planning which is a protocol for irradiation of the heavy ion beam on a target to maximize dose distribution and a biological advantage. The treatment planning converts CT-number of tissues along paths of the heavy ion beam to electron densities in order to calculated end-of-range of the heavy ion beam. The conventional x-ray CT introduces uncertainty into the CT- numbers due to beam hardening effect. While the monochromatic x-ray makes the CT-scan free from the beam hardening effect. Furthermore, dual energy x-ray CT-scans give the electron density directly without conversion from the CT-number. We focus on the x-ray CT using two monochromatic x-rays, and outline its beamline and the compact ring.
We have been developing a computerized scheme to restore cone-beam CT images that are degraded by scatter radiation. In reconstructing 3D image, the cone-beam CT scanner has some advantages over helical CT scanner in shorter reconstruction time and in higher spatial resolution. However, because large amount of scatter radiation is included in 2D projection data, the quality of reconstructed image is worse and cupping artifacts appear in CT images. We have tried to improve the quality of reconstructed images using the multiresolution analysis. To achieve this, the projection data was decomposed into several signals with different spatial resolution and then reconstructed from these decomposed signals by multiplying weighting factors. Moreover, using these weighting factors, we have designed the optimal filter to reconstruct the CT images without any extra computing time. Our results showed that the CT images were enhanced the contrast and improved the quality.
KEYWORDS: 3D image reconstruction, Sensors, Photons, 3D image processing, CT reconstruction, Optical filtering, 3D scanning, Monte Carlo methods, X-ray computed tomography, Image filtering
We have calculated the intensity distribution of scatter radiation as well as primary radiation on a two-dimensional detector plane in a cone-beam x-ray three-dimensional CT scanner, using Monte Carlo method. From the two-dimensional projections including the scatter radiation arising in cylindrical water phantoms for various conditions, we have reconstructed 3D images by cone-beam reconstruction techniques and have evaluated the effects of scatter on reconstructed images. The effects of wedge filter, which make the primary x-ray intensity uniform on a detector plane, on the scatter intensity distribution have been examined. With the filter, the scatter to primary ratio at the center of the projection reduce by half compared to that without the filter and cupping artifacts in reconstructed image are eliminated. Furthermore, the usage of the filter decreases in absorbed dose of about 60% to without filter.
KEYWORDS: 3D image processing, X-rays, 3D scanning, Sensors, Data acquisition, 3D acquisition, X-ray computed tomography, 3D image enhancement, X-ray imaging, X-ray detectors
In order to acquire 3D data of high contrast objects such as bone, lung and vessels enhanced by contrast media for use in 3D image processing, we have developed a 3D CT-scanner using cone beam x ray. The 3D CT-scanner consists of a gantry and a patient couch. The gantry consists of an x-ray tube designed for cone beam CT and a large area two-dimensional detector mounted on a single frame and rotated around an object in 12 seconds. The large area detector consists of a fluorescent plate and a charge coupled device video camera. The size of detection area was 600 mm X 450 mm capable of covering the total chest. While an x-ray tube was rotated around an object, pulsed x ray was exposed 30 times a second and 360 projected images were collected in a 12 second scan. A 256 X 256 X 256 matrix image (1.25 mm X 1.25 mm X 1.25 mm voxel) was reconstructed by a high-speed reconstruction engine. Reconstruction time was approximately 6 minutes. Cylindrical water phantoms, anesthetized rabbits with or without contrast media, and a Japanese macaque were scanned with the 3D CT-scanner. The results seem promising because they show high spatial resolution in three directions, though there existed several point to be improved. Possible improvements are discussed.
An x-ray image detection system consisting of a fluorescent screen optically coupled to a CCD camera can produce a large uniform image. In this system, it is difficult to produce a noiseless image because of loss in the optical system. In order to improve the efficiency of light collection, we have developed a new screen with an interference filter. The angular distribution of the light from the new screen was improved, and the amount of light collected by the CCD was increased. The spatial resolution of the new screen was a little lower than that of the conventional screen. In this paper, we report a new application of an interference filter to a fluorescent screen, and report the performance.
KEYWORDS: Image processing, 3D image processing, 3D image reconstruction, Digital signal processing, X-ray computed tomography, 3D acquisition, Parallel processing, Parallel computing, Data processing, X-rays
Computer tomography using cone beam x ray generates a great amount of image data. Reconstruction of the three-dimensional (3D) image requires considerable processing power. We have developed a parallel computer utilizing up to 64 processors that perform the 3D reconstruction algorithm very quickly. The maximum processing power is 3.2 GFLOPS. Data acquisition is done by a video camera at video rate. In order to accommodate the large amount of image data, we also developed a frame buffer. The buffer can be used as a shared memory for all parallel processors.
Scatter to primary radiation ratios at detector positions were calculated for spherical water phantoms in the cone-beam x-ray 3D CT scanner using the Monte Carlo simulation method. In the Monte Carlo simulation, 1 X 106 photons of cone-beamed 120-kV polyenergetic x-rays with the filter of 4 mm Al have been traced individually as these photons interact within the phantoms with diameters 15 cm and 20 cm. The decrease in CT number due to scatter was found to be about 100. In the process of image reconstruction, the effect of scatter was isolated by applying our new calculation technique. The scatter reduces CT numbers in the whole region of images and the intensity of scattered rays is nearly uniform, so that rounded- off image is reconstructed.
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