摘要:

Methods are reviewed for estimating the transverse relaxation timeT2and the pseudodensity (PD) from spin‐echo measurements acquired at an arbitrary set of echo times {TE i}. Least‐squares fitting is applied to the logarithmically processed signals for the case in which the weights are proportional to the inverse of the logarithmically transformed signal variances (the minimum variance case). General formulas are derived for the estimated noise levels in the PD andT2estimates due to the propagation of uncertainties in the original measurements. It is shown that theT2and PD estimates are anticorrelated. Additionally, an expression is derived for the variance in a synthetic spin‐echo signal subsequently formed from the PD andT2estimates. It is shown that under many circumstances a signal synthesized at some echo time can have a signal‐to‐noise ratio superior to that in a signal directly acquired at that time. Experimental measurements made on phantoms match the theoretical predictions to a high degree.

ISSN:0269-3127    

DOI:10.1118/1.595956

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Image segmentation algorithms based on hierarchical clustering have been developed for analysis ofT1andT2nuclear magnetic resonance images. Application of these algorithms to simultaneousT1–T2images of healthy volunteers extracted fundamental tissue types in the brain. These algorithms also were used both to identify the extent of the region of involvement of a subject with a history of a grade 3 astrocytoma of the right frontal lobe of the brain, and to characterize the tissue within the region of involvement. These results suggest that a simple segmentation algorithm can produce reasonable clustering of tissue types within the brain.

ISSN:0269-3127    

DOI:10.1118/1.595916

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Methods for accurate calculation of nuclear magnetic resonance relaxation times are presented. Data from inversion recovery sequences with different delay times were fitted to deriveT1information on water solutions of paramagnetic ions.T2values of the same samples were obtained by fitting signals from a multiecho sequence. According to the theory, for the field strength used (0.5 T),T1's andT2's are nearly equal and the enhancement of relaxation rates is found to be directly proportional to molar concentration. Tests performed suggest the opportunity of using the same approach in studies of tissues.

ISSN:0269-3127    

DOI:10.1118/1.595917

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

We investigated the effects of imaging and display conditions on the detectability of low‐contrast objects in digital subtraction angiographic (DSA) images. The test images were produced by superimposition of low‐contrast objects on a uniform noisy background obtained with a DSA system. We employed 18‐alternative forced‐choice (18‐AFC) experiments and predictions based on statistical decision theory to study the dependence of the threshold contrasts of the test objects on the object size, incident x‐ray exposure, display window width, and display medium. The results indicated that the threshold contrast decreased with increasing object size, and that the detectability of an object of a given size increased with increasing incident x‐ray exposure and decreasing width of the display window. We found that the signal‐to‐noise ratio (SNR) obtained from theperceivedstatistical decision theory model, which includes the observer's internal noise, can accurately predict the detectability of low‐contrast objects in DSA images. The threshold SNR corresponding to 50% correct detection in the 18‐AFC experiments had a constant value of 3.8, in agreement with results reported previously for screen–film systems. The theoretical model will be useful for prediction of the performance of a DSA system based on its physical characteristics, and for evaluation of the tradeoff between patient exposure and diagnostic accuracy for a given DSA unit.

ISSN:0269-3127    

DOI:10.1118/1.595918

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Results of an 18‐alternative forced‐choice experiment have shown that observers were capable of detecting a signal with a contrast of 1 in terms of 10‐bit data which were displayed on a CRT monitor with an 8‐bit video generator and a window width setting of 1024. We investigated the conditions under which 10‐bit signal data can be detected when displayed using an 8‐bit video generator. Results show that the 10‐bit digital quantum noise, which was approximately Gaussian distributed, can act as a carrier of the signal data, thus allowing a signal having a contrast of a fraction of a displayed grey level to be detected. We demonstrate the relationship between the rms value of the digital noise (obtainable with a clinical digital subtraction angiography system under various exposure levels), the number of bits available in the display video generator, and the “transmitted” signal contrast displayed on the CRT monitor.

ISSN:0269-3127    

DOI:10.1118/1.595919

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

A scatter correction algorithm for digitally acquired radiographs (SCADAR) is presented. SCADAR requires the acquisition of two digital images, taken at different object‐to‐detector distances. These two images are digitally magnification compensated, and subtracted. The primary component in the resulting difference image Δ is mathematically eliminated, and hence the Δ image is used as a measure of the local contribution of scattered radiation. A gray scale transformation is used to transform the Δ image to a scattered component image, which is then smoothed by Fourier filtering, using a matched filter to increase the signal‐to‐noise ratio. The smoothed scatter image is then subtracted from its corresponding original image, resulting in the corrected SCADAR image. Implementation of SCADAR can result in a large increase in image contrast, and a significant reduction in shading due to scattered radiation effects. The mathematical derivation of the algorithm is developed, and experimental verification is given for some of the principles used. Using experimental images acquired from scattering phantoms, the results of SCADAR on various image parameters such as contrast and detail signal‐to‐noise ratio are discussed.

ISSN:0269-3127    

DOI:10.1118/1.595920

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

A method based upon backprojection of projection images which allows six degrees of freedom in the selection of the plane to be tomosynthesized is described. Data can be collected with any type of classical tomographic apparatus, including linear, circular, or other complex motion unit, which has been modified by the addition of a digital imaging chain. A set of projection images can be acquired in a single sweep, although, in principle, they could be acquired simultaneously. In addition, C‐arms can also be used with no limitations of the selection of tomosynthesized planes.

ISSN:0269-3127    

DOI:10.1118/1.595873

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

We report the evaluation of a prototype dual‐energy implementation using rapid kVp switching on a clinical computed tomographic scanner. The method employs prereconstruction basis material decomposition of the dual‐energy projection data. Each dual‐energy scan can be processed into conventional single‐kVp images, basis material density images, and monoenergetic images. Phantom studies were carried out to qualitatively and quantitatively evaluate and validate the approach.

ISSN:0269-3127    

DOI:10.1118/1.595958

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

A prototype dual‐energy computed tomographic (CT) scanner (Siemens Somatom DR3) with rapid kVp switching and prereconstruction processing has been used to measure vertebral bone mineral density. With this approach misregistration and beam hardening inaccuracies can be reduced considerably. Basis material images of aluminum‐ and Lucite‐equivalent density enable measurements of bone mineral density that are nearly independent of the amount of marrow fat. To simulate variable marrow fat, alcohol–water mixtures were used as media in calibration standards. A section of dried trabecular bone was also scanned immersed in varying alcohol–water mixtures. In both simulations it was shown that the dual‐energy measurement is nearly independent of marrow composition whereas the single‐energy measurement would be strongly influenced by marrow fat. Dual‐energy CT was compared to dual‐photon absorptiometry (153Gd) for the measurement of bone mineral mass of ten excised human vertebrae. There was a high degree of correlation between the two measurements (r=0.97). Dual‐energy and single‐energy CT measurements on 17 patients with suspected metabolic bone disease strongly support the conclusion that the influence of fat can lead to significant errors in single‐energy determinations of the mineral density of trabecular bone.

ISSN:0269-3127    

DOI:10.1118/1.595951

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Quantitative studies performed with scintillation detectors often require corrections for lost data because of the finite resolving time of the detector. Methods that monitor losses by means of a reference source or pulser have unacceptably large statistical fluctuations associated with their correction factors. Analytic methods that model the detector as a paralyzable system require an accurate estimate of the system resolving time. Because the apparent resolving time depends on many variables, including the window setting, source distribution, and the amount of scattering material, significant errors can be introduced by relying on a resolving time obtained from phantom measurements. These problems can be overcome by curve‐fitting the data from a reference source to a paralyzable model in which the true total count rate in the selected window is estimated from the observed total rate. The resolving time becomes a free parameter in this method which is optimized to provide the best fit to the observed reference data. The fitted curve has the inherent accuracy of the reference source method with the precision associated with the observed total image count rate. Correction factors can be simply calculated from the ratio of the true reference source rate and the fitted curve. As a result, the statistical uncertainty of the data corrected by this method is not significantly increased.

ISSN:0269-3127    

DOI:10.1118/1.595874

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY