ISSN:0094-2405    

DOI:10.1118/1.597086

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

ISSN:0094-2405    

DOI:10.1118/1.597059

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The general solution to the Bloch equation in the rotating frame which includes the effects of relaxation during a constant amplitude, off‐resonance rf field has been developed. The solution is used to monitor the transient approach to steady‐state saturation levels during off‐resonance irradiation applied continuously or in pulses to systems with arbitrary relaxation rates. The time course of the magnetization during amplitude modulated rf pulses is followed with the general solution and the transverse relaxation time dependence of typical slice profiles is examined. The calculations serve to illustrate the generality of the solution. A thorough discussion of various computational concerns is provided.

ISSN:0094-2405    

DOI:10.1118/1.597063

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Contrast agents based on the lanthanide elements gadolinium and holmium have recently been developed for magnetic resonance imaging (MRI). Because of the increased atomic number of these elements relative to iodine, these new compounds, used as x‐ray contrast agents, may yield higher radiographic contrast, and hence improved x‐ray image quality, relative to conventional iodinated compounds, for clinically useful x‐ray spectra. This possibility has been investigated, in independent experimental and theoretical studies, for two x‐ray imaging systems: a digital radiographic system, using an x‐ray image intensifier (XRII) and charge‐coupled device (CCD) detector; and a conventional screen/film system, using a Lanex Regular screen. Iodine, gadolinium, and holmium contrast agents were investigated over a wide range of concentration‐thickness products (0.1–0.6 M cm) and diagnostic x‐ray spectra (60–120 kVp). A simple theoretical model of x‐ray detector response predicts the experimental radiographic contrast measurements with a mean absolute error of 8.0% for the XRII/CCD system and 5.9% for the screen/film system, and shows that the radiographic contrast for these two systems is representative of all XRII and screen/film systems. An index of image quality is defined, and its dependence on radiographic contrast, x‐ray fluence per unit dose, and detective quantum efficiency (DQE) is shown. Theoretical values of the index, predicted by our model, are then used to compare the performance of the three contrast agents for the two systems investigated. In general, iodine performance decreases steadily with increasing kVp, gadolinium performance has a broad maximum near 85 kVp, and gadolinium outperforms holmium. Gadolinium outperforms iodine for spectra above (and vice versa below) about 72 kVp, depending slightly on spectrum filtration, object thickness, and detector type. Thus, raising the kVp to shorten exposure times or reduce x‐ray tube heat loading results in a loss of image quality with iodine, but not with gadolinium. Similarly, beam‐hardening artifacts in performing video densitometry with iodine would be reduced with gadolinium. Gadolinium‐based contrast agents are thus shown to offer several practical advantages over conventional iodinated contrast agents.

ISSN:0094-2405    

DOI:10.1118/1.597134

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

A method for estimation of the absolute renal activity within background activity in renal planar scintigraphy will be presented. This method corrects for oversubtraction of background activity with consideration of background activity of the kidney volume, and employs the depth‐independent buildup factor (DIBF) method for the correction of attenuation and scatter of photons. This method requires the depth and thickness of the organ and the thickness of background for a background activity correction. The transmission factor (TF) for a volume source is derived from integrating TF for a thin source over the thickness of the organ for the DIBF method. To validate this method, phantom studies with various uniform background activity concentrations were performed and the data were compared with conventional background subtraction that do not consider the organ's volume. The results showed that with the conventional background subtraction method at all depth activity was underestimated with errors of 5%–30% for organ/background concentration ratios of 5–40, while this method estimated the true count rate with errors of less than 5%. More accurate quantifications of renal functions such as renal uptake, glomerular filtration rate (GFR) and renogram may be obtained by this proposed method on the planar images.

ISSN:0094-2405    

DOI:10.1118/1.597057

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The dynamic range of the gray level of a digital image is limited by the noise it contains. Two statistical methods called “runs test” and “join‐count statistic” are used to measure the noise level in a digital image. A residual image is formed by subtracting an original image from its smoothed version. Theoretically, the noise level in the residual image should be identical to that in the original image. The noise level is determined by examining each bit plane of the residual image individually starting from the least significant bit up to the bit plane whose statistic does not show a random pattern. Images from three digital modalities: computerized tomography, magnetic resonance, and computed radiography are used to evaluate the gray‐level dynamic range. Both methods are easy to implement and fast to perform.

ISSN:0094-2405    

DOI:10.1118/1.597058

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The quality of the reconstructed images in Digital Tomosynthesis is often limited by the presence of artifacts due to blur from planes other than the fulcrum plane. A technique has been developed for the separation and subsequent removal of unrelated structures from the reconstructed plane. The method involves the reconstruction of the blur originating in user‐selected “noisy” planes as it appears on the plane of interest. This is achieved by projecting the reconstructed images of the selected plane on the image formation plane for all viewing angles, and subsequently, synthesizing its blurred image on the plane of interest. There are no restrictions as to the identity of the planes to be removed. Reproduction of the noise is performed using the tomosynthesis algorithm itself, thus the technique can be modified to suit any reconstruction algorithm. The proposed technique was implemented on the Multiple Projection Algorithm and was experimentally evaluated using a radiotherapy simulator unit.

ISSN:0094-2405    

DOI:10.1118/1.597060

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

A high quality film‐duplication system was developed in order to improve the image quality of duplicated radiographs and to recover improperly exposed films. The system consists of a laser film digitizer, a laser film printer, a workstation, and a magneto‐optical disk. Radiographs are digitized by the laser digitizer, processed by the computer for image enhancement, and then printed on a film by the laser printer. A nonlinear density‐correction technique is employed in recovering improperly exposed radiographs using the H&D curve of the screen‐film system. Using the new duplication system in our department, the average recovery rate was over 80% for chest and abdominal films rejected due to over‐ or underexposure. The basic imaging properties of the duplication system were compared with those of a Computed Radiography (CR) system and a conventional screen‐film system. For low spatial frequencies, the MTF of the CR system is superior to that of the digital duplication system; however, for high spatial frequencies, the MTF of the duplication system is superior. The noise in the duplication system is about half of that in the CR system.

ISSN:0094-2405    

DOI:10.1118/1.597061

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

A scatter‐glare correction technique for x‐ray images acquired with an antiscatter grid was developed. In the technique, the scatter‐glare image was estimated from exposure conditions and subtracted from the acquired image. The basic procedure in the estimation of the scatter‐glare image is convolution filtering; however, the novel aspects of the technique are as follows: (1) To estimate the scatter‐glare intensity, a formula that does not include the term of object thickness was used. With this formula, the correction can be performed, even for nonuniform phantoms; (2) To estimate the scatter‐glare distribution, the experimental scatter‐glare point spread function (PSF) was directly used as a convolution kernel. Although the shape of the PSF changed slightly for water thicknesses of 5–25 cm, we applied the PSF measured at a water thickness of 15 cm to the correction experiments. For the stepped water phantom (10–20 cm), scatter‐glare estimation produced an average error of 10%, with respect to the lead bar data. Furthermore, the improvement of image quality and quantitative accuracy resulting from the correction was examined.

ISSN:0094-2405    

DOI:10.1118/1.597062

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The segmentation and three‐dimensional representation of the liver from a computed tomography (CT) scan is an important step in many medical applications, such as in the surgical planning for a living‐donor liver transplant and in the automatic detection and documentation of pathological states. A method is being developed to automatically extract liver structure from abdominal CT scans usingaprioriinformation about liver morphology and digital image‐processing techniques. Segmentation is performed sequentially image‐by‐image (slice‐by‐slice), starting with a reference image in which the liver occupies almost the entire right half of the abdomen cross section. Image processing techniques include gray‐level thresholding, Gaussian smoothing, and eight‐point connectivity tracking. For each case, the shape, size, and pixel density distribution of the liver are recorded for each CT image and used in the processing of other CT images. Extracted boundaries of the liver are smoothed using mathematical morphology techniques and B‐splines. Computer‐determined boundaries were compared with those drawn by a radiologist. The boundary descriptions from the two methods were in agreement, and the calculated areas were within 10%.

ISSN:0094-2405    

DOI:10.1118/1.597064

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY