Effect of edge-preserving adaptive image filter on low-contrast detectability in CT systems: application of ROC analysis.

OBJECTIVE: For the multislice CT (MSCT) systems with a larger number of detector rows, it is essential to employ dose-reduction techniques. As reported in previous studies, edge-preserving adaptive image filters, which selectively eliminate only the noise elements that are increased when the radiation dose is reduced without affecting the sharpness of images, have been developed. In the present study, we employed receiver operating characteristic (ROC) analysis to assess the effects of the quantum denoising system (QDS), which is an edge-preserving adaptive filter that we have developed, on low-contrast resolution, and to evaluate to what degree the radiation dose can be reduced while maintaining acceptable low-contrast resolution. MATERIALS AND METHODS: The low-contrast phantoms (Catphan 412) were scanned at various tube current settings, and ROC analysis was then performed for the groups of images obtained with/without the use of QDS at each tube current to determine whether or not a target could be identified. The tube current settings for which the area under the ROC curve (Az value) was approximately 0.7 were determined for both groups of images with/without the use of QDS. Then, the radiation dose reduction ratio when QDS was used was calculated by converting the determined tube current to the radiation dose. RESULTS: The use of the QDS edge-preserving adaptive image filter allowed the radiation dose to be reduced by up to 38%. CONCLUSION: The QDS was found to be useful for reducing the radiation dose without affecting the low-contrast resolution in MSCT studies.

Development of variable pitch factor scanning for multislice computed tomography.

RATIONALE AND OBJECTIVES: The latest multislice computed tomography (MSCT) scanners permit the chest and abdomen to be scanned continuously. However, conventionally, it has been necessary to perform scanning twice using different pitch factors for the cardiac and abdominal regions. We have developed a new scanning technique known as variable pitch factor scanning, in which the table speed is changed during scanning to obtain continuous images from the heart to the abdomen in a single scan, and have evaluated its physical characteristics. MATERIALS AND METHODS: A bead phantom, a comb phantom, and a gold wire placed at an angle were scanned using a 64-row MSCT scanner. The variation in the spatial resolution and continuity of images in the body axis direction because of changes in the pitch factor were evaluated. RESULTS: Because reconstruction taking the cone angle into consideration was employed, the spatial resolution in the body axis direction was unchanged and the continuity of images in the body axis direction was maintained at a certain level even when the pitch factor was changed. CONCLUSION: Variable pitch factor scanning is a useful technique for obtaining continuous images from the heart to the abdomen in a single scan.

Fluctuation in measurements of pulmonary nodule under tidal volume ventilation on four-dimensional computed tomography: preliminary results.

The present study aimed to assess the feasibility of four-dimensional (4D) chest computed tomography (CT) under tidal volume ventilation and the impact of respiratory motion on quantitative analysis of CT measurements. Forty-four pulmonary nodules in patients with metastatic disease were evaluated. CT examinations were performed using a 256 multidetector-row CT (MDCT) unit. Volume data were obtained from the lower lung fields (128 mm) above the diaphragm during dynamic CT acquisition. The CT parameters used were 120 kV, 100 or 150 mA, 0.5 s(-1), and 0.5 mm collimation. Image data were reconstructed every 0.1 s during one respiratory cycle by a 180 degrees reconstruction algorithm for four independent fractions of the respiratory cycle. Pulmonary nodules were measured along their longest and shortest axes using electronic calipers. Automated volumetry was assessed using commercially available software. The diameters of long and short axes in each frame were 9.0-9.6 mm and 7.1-7.5 mm, respectively. There was fluctuation of the long axis diameters in the third fraction. The mean volume in each fraction ranged from 365 to 394 mm(3). Statistically significant fluctuation was also found in the third fraction. 4D-CT under tidal volume ventilation is feasible to determine diameter or volume of the pulmonary nodule.

Improvement in image quality of noncontrast head images in multidetector-row CT by volume helical scanning with a three-dimensional denoising filter.

PURPOSE: The aim of this study was to improve the contrast-to-noise ratio on noncontrast head computed tomography (CT) images, which are crucial for assessing patients with acute ischemic stroke. We applied a technique combining volume helical scanning with a three-dimensional (3D) denoising filter. MATERIALS AND METHODS: We scanned phantoms for low-contrast resolutions and helical/cone-beam artifacts as well as stroke patients using a 16-row multidetector-row CT (MDCT) unit. Volume helical scans with 1-mm collimation and nonhelical scans with 8-mm thickness were performed. From the 1-mm thick volume data, 8-mm thick contiguous images were generated before and after applying a 3D denoising filter. RESULTS: On images stacked from volume data, the contrast-to-noise ratio was significantly improved by the 3D denoising filter and was nearly the same as that on nonhelical images. On stacked volume images, artifacts due to the cone beam and the helical scan were increased with larger helical pitches, but bone-related streak artifacts in the posterior fossa and underneath the calvarium were reduced when compared with nonhelical images. CONCLUSION: Volume helical scan with a 3D denoising filter effectively improves image quality in noncontrast head MDCT images.

Improvement of spatial resolution in the longitudinal direction for isotropic imaging in helical CT.

Experiments were conducted to confirm the isotropic spatial resolution of multislice CT with a 0.5 mm slice thickness. Isotropic spatial resolution means that the spatial resolution in the transaxial plane (X-Y plane) and that in the longitudinal direction (Z direction) are equivalent. To obtain point spread function (PSF) values in the X-Y-Z directions, three-dimensional voxel data were obtained by helical scanning of a bead phantom. The modulation transfer function (MTF) values were then obtained by three-dimensional Fourier transform of the PSF. Evaluation of the spatial resolution in the X-Y-Z directions by the MTF values showed that the spatial resolution in the Z direction does not depend on the reconstruction kernel used. It was also found that the spatial resolution in the Z direction, as compared with that in the X-Y plane, is superior with the standard kernel for the abdomen and is inferior with the high-definition kernel for the ears/bones. By performing sharpening filter processing in the Z direction with a high-definition kernel, comparable spatial resolution could be obtained in the X-Y-Z directions. It was confirmed that adjusting the spatial resolution in the Z direction with the reconstruction kernel used is an effective method for isotropic imaging.

New method of evaluating edge-preserving adaptive filters for computed tomography (CT): digital phantom method.

To evaluate the characteristics of edge-preserving adaptive filters for selectively eliminating noise without affecting resolution in low-dose scanning, we have developed a digital phantom image and evaluated noise statistical values, noise characteristics, and resolution characteristics. The results confirmed that edge-preserving adaptive filters function as smoothing filters in low-contrast regions containing noise, permitting the density resolution to be improved, while the strength of the smoothing filter is reduced to maintain spatial resolution in high-contrast regions containing small structures. It has therefore been confirmed that edge-preserving adaptive filters function as filters for selectively eliminating only the noise elements that are increased when the exposure dose is reduced and that such filters are effective for improving image quality. Using such digital phantom images, images acquired using conditions that are difficult to set in actual CT scanning can be obtained and images specifically for the evaluation target can easily be generated. In addition, the noise level, frequency distribution of the noise, and resolution characteristics of the objects present in the input image can be freely set. It is concluded that evaluation of processing using a digital phantom image is effective for evaluating image processing.

Simulator-assisted setting of scan protocols for X-ray CT: development and clinical usefulness of the Scan Plan Simulator.

To assist in the selection of complicated computed tomography (CT) scan protocols and to obtain stable image SD values, prototype "Scan Plan Simulator" software with the following functions was developed and evaluated. 1) The image SD value that will be obtained in actual scanning is estimated by entering the patient's body size and scan protocol, after which a simulated image is displayed so that the estimated image SD value can be checked. The exposure dose can also be estimated in the Scan Plan Simulator. 2) The appropriate tube current is automatically set by entering the required image SD value. We evaluated the accuracy of the Scan Plan Simulator by comparing the simulation results with the image SD values and exposure dose obtained in actual scanning and assessed the usefulness of this method using Monte Carlo simulation based on body thickness data obtained in clinical examinations. The results showed that the Scan Plan Simulator not only stabilizes image SD by minimizing the effects of body size but also permits the exposure dose to be reduced by optimizing tube current.

[Reduction of radiation exposure in CT perfusion study using a quantum de-noising filter].

CT perfusion study requires repeatedly scanning the same part of the patient's head, resulting in an increase of local radiation exposure. The purpose of this study was to assess the feasibility of the ultra-low-dose technique with a quantum de-noising filter. The newly developed quantum de-noising filter selectively reduced noise by two-thirds, while maintaining spatial resolution. The low-dose protocol using the quantum de-noising filter and slow rotation speed accomplished a 68% reduction in the local radiation dose compared with the previous standard protocol. The quantum de-noising filter is considered to be useful to reduce radiation exposure and to improve image quality in CT perfusion study.