摘要:

Two‐dimensional diffusion coefficient maps (images) of a carefully controlled diffusion phantom have been measured by a new diffusion imaging sequence using a 0.6‐T whole‐body nuclear magnetic resonance (NMR) scanner having a gradient field strength of 2.5 mT/m. The free induction decay (FID) data for the diffusion coefficient images was collected by varying the duration of the readout gradient in the conventional two‐dimensional Fourier imaging sequence. The experimental results obtained by the proposed NMR diffusion measurement technique indicate a close agreement with other previous measurements. The selection of optimum spin‐echo time for maximum signal‐to‐noise ratio (SNR) in diffusion imaging is studied and also experimentally confirmed. Finally, a preclinical study with human volunteers has been performed and results are presented.

ISSN:0094-2405    

DOI:10.1118/1.595850

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Anatomical structures that are displaced periodically during respiration are repeated as ghosts in magnetic resonance (MR) images. These ghosts can be suppressed in many ways: the averaging of multiple sets of data, respiratory gating, deliberate positioning of ghosts, and respiratory ordering of phase encoding. Each method has a unique mechanism, which is described in detail. A theoretical investigation has been conducted into the effects that the methods have on the point spread function of a moving point. Data acquired in Fourier imaging are actually in the spatial frequency domain, so that respiratory motion can be regarded as a function of spatial frequency. The four methods above modify this functional dependence in different ways, allowing a unified comparison. Motion artifact suppression imposes additional constraints on image acquisition, which can prolong the imaging time. A technique has been developed that keeps the imaging time short by using the configuration of the subject to regulate the timing of image acquisition.

ISSN:0094-2405    

DOI:10.1118/1.595851

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Receiver coil response is a major cause of nonuniformities in magnetic resonance images. The spatial dependence of the sensitivity and phase of single‐saddle receiver coils has been investigated quantitatively by calculating theH1field and comparing the results with measurements of a uniform phantom. Agreement between the measurements and calculations is excellent. A method is developed which corrects for both the nonuniform sensitivity and the phase shifts introduced by receiver coils.

ISSN:0094-2405    

DOI:10.1118/1.595967

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

As with other digital imaging systems in heavy medical use, it is desirable with magnetic resonance imaging (MRI) to obtain extensive, rigorous system performance measures from a small set of images of one or two relatively simple test objects. Digital analysis of the parallel square rod (PSR) test object introduces digital image system self‐evaluation to MRI and extends automated image evaluation to include rigorous measures throughout the imaging volume rather than just average measures over the image. Precise comparisons with theory and between systems can be performed as well as quality control and corrections for nonuniformities. The PSR test object consists of an 18×18×36 cm rectangular acrylic container enclosing 60 parallel square acrylic rods running the entire length. The inter‐rod space is filled with a liquid or gel that produces strong, tissuelike signals in MRI and high contrast relative to the rods for computed tomography (CT). For profiles of slice thickness and separation, the rods are tilted in the test object to intersect the image plane at a 45° angle when the test object sides are parallel and perpendicular to the image plane. The test object itself is rotated 6–12° about its major axis so that the sides of the rods make a small angle to the rows and columns of pixels. This allows digital sampling at finer spacing than the pixels for determination of edge response functions. Over the 25–49 blocks in each slice of the imaged volume, maxima, minima, mean values, variances, and ratios currently are reported for the following variables: signal‐to‐noise ratio and sensitivity, linear and nonlinear image distortion, full width at half maximum (FWHM) resolution of the point spread function (PSF), slice separation, and slice thickness. These performance values at each rod or edge are displayed as gray scale functional images. Individual rod values are recorded and plotted as histograms and profiles. Results of the automated analysis for MRI system examples are in good agreement with expectations from theory and more manual tests.

ISSN:0094-2405    

DOI:10.1118/1.595804

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Monte Carlo simulation was applied to study the histories of photon interactions in a soft‐tissue‐equivalent medium under diagnostic imaging conditions. We examined the dependence on incident x‐ray energy and phantom thickness of the basic properties of photon scattering, including the probabilities of occurrence of the various interaction processes, and the frequency distributions of scattering events. We investigated the properties of scattered radiation for monoenergetic incident x rays, which provide a basis for deriving the physical properties of scattered radiation for any polyenergetic incident beam. We also included four incident x‐ray beams with broad spectra; these represented the incident x rays typically used for diagnostic imaging.

ISSN:0094-2405    

DOI:10.1118/1.595805

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

A model of scatter is developed from a signal processing approach. The scattering process is viewed as a nonlinear filter (NLF), which transforms a two‐dimensional signal representing phantom thickness into a two‐dimensional signal of scattered radiation. The NLF point spread function (PSF) is derived from a single scattering model, using the principles of Compton scattering and x‐ray attenuation. The PSF is characterized by three approximations: a constant geometric shape, a volume that depends on the phantom thickness, and a width that depends on the phantom‐to‐detector distance. This leads to a closed form expression for the scatter‐to‐primary ratio as a function of phantom thickness, field size, photon energy, source‐to‐phantom distance, and phantom‐to‐detector distance. The NLF model is compared with previously reported measurements using constant thickness phantoms, and discrepancies are discussed. The good agreement found between the NLF model and measured data shows that the functional dependence of scatter on the above parameters, previously only explained in terms of empirical models or Monte Carlo simulations, can be incorporated into a signal processing model.

ISSN:0094-2405    

DOI:10.1118/1.595806

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

It is shown that predictions can be made of scatter‐to‐primary ratios for a variety of mammographic configurations. The different configurations can be produced by changes in source–detector distance; source–phantom distance; air gap; photon energy; phantom composition, thickness, and cross‐sectional area. A detailed analysis of the effect of a grid is also considered. The predictions are not computationally intensive. Experimental verifications of the predictions for varying phantom cross‐sectional dimensions, source–detector distance, and air gaps with and without a grid have been carried out. Variations with phantom thickness were not considered in this paper. Detailed comparisons between experiment and theory indicate that scatter‐to‐primary ratios were predicted within one standard deviation and the coefficient of variation is within 3% for most of the data points with the worst case coefficient of variation about 6%. Measurements of grid secondary transmission have also been compared to theoretical predictions and agreement is within the coefficient of variation of about ±15%.

ISSN:0094-2405    

DOI:10.1118/1.595807

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Single‐frame images obtained with image intensifier (II)–TV digital systems contain a large amount of structure mottle. In the present study, we examined several II–TV digital systems by use of Wiener spectral analysis and noted considerable variation of the structure mottle over the wide spatial frequency range. We found that the structure mottle in these systems may originate in the input phosphor, the output phosphor, and/or the electronic components, and that the Wiener spectra of structure mottle seem to depend on the specific combination of these components. The results of observer performance studies indicated that structure mottle can significantly decrease the detection of low‐contrast objects in a single‐frame image when the exposure incident on the II is greater than approximately 0.1 mR. In addition, we showed that the structure mottle can be removed by subtraction of a uniformly exposed mask. This simple procedure will improve the quality of radiologic images obtained with II–TV digital systems. Note, however, that the structure mottle is largely eliminated by subtraction in digital subtraction angiography (DSA) images.

ISSN:0094-2405    

DOI:10.1118/1.595808

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The improved appearance of digital radiographs filtered to improve local contrast and sharpen edges has not increased acceptance of these images by radiologists. Furthermore, many radiologists assert that correct diagnosis is not improved with these filtered images. This study was designed to test this assertion for digital subtraction angiograms (DSA) of renal images. Four experiments are described. First, phantom studies identified filters and their parameters thought likely to be acceptable and useful in diagnosing renal images formed by DSA. Second, these filters and parameters were then tested on medical images to assess their acceptance by radiologists. Third, display modes of windowing, positive/negative presentation, and magnification were varied for filtered and unfiltered images to assess preferences of radiologists. Fourth, filtered and unfiltered magnified images were used to test improved diagnosis. In the final experiment, 148 images from 33 renal studies (15 normal, 18 abnormal) were magnified, gray level windowed, and filtered. Diagnosis was not improved by the two edge sharpening filters tested.

ISSN:0094-2405    

DOI:10.1118/1.595809

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

An empirically derived logit function is proposed to describe the modulation transfer function of x‐ray screen–film systems. Simple and exact weighted linear regression analyses are performed to characterize the function and to describe the data within experimental accuracy. Calculations comparing the proposed function and the complementary error function (probit) representations of the modulation transfer function are given and discussed.

ISSN:0094-2405    

DOI:10.1118/1.595810

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY