摘要:

We used Wiener spectral analysis in order to investigate the different noise sources and the effects of various parameters such as pixel size, image intensifier (II) field size, and exposure level on the noise in an II–TV digital system. The digital Wiener spectra in terms of relative x‐ray intensity were determined directly from the digital noise data in terms of pixel values, by use of the characteristic curve of the imaging system. From averaged, subtracted, and/or combination images, the amount of structure mottle relative to the amount of quantum mottle was estimated. We found that a substantial amount of structure mottle was included in our II–TV digital subtraction angiography system, whereas the electronic noise of the TV system was quite small relative to the quantum and structure mottle. The effects of time jitter on the noise in single‐frame images (consisting of multiple video frames) and in subtracted and averaged images were also investigated.

ISSN:0269-3127    

DOI:10.1118/1.595937

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

A systematic investigation was conducted of how a number of parameters which alter the system modulation transfer function (MTF) influence the count‐dependent Metz filter. Since restoration filters are most effective at those frequencies where the object power spectrum dominates that of the noise, it was observed that parameters which significantly degrade the MTF at low spatial frequencies strongly influence the formation of the Metz filter. Thus the radionuclide imaged and the depth of the source in a scattering medium had the most influence. This is because they alter the relative amount of scattered radiation being imaged. For low‐energy photon emitters, the collimator employed and the distance from the collimator were found to have less of an influence but still to be significant. These cause alterations in the MTF which are more gradual, and hence are most pronounced at mid to high spatial frequencies. As long as adequate spatial sampling is employed, the Metz filter was determined to be independent of the exact size of the sampling bin width, to a first approximation. For planar and single photon emission computed tomographic (SPECT) imaging, it is shown that two‐dimensional filtering with the Metz filter optimized for the imaging conditions is able to deconvolve scatter and other causes of spatial resolution loss while diminishing noise, all in a balanced manner.

ISSN:0269-3127    

DOI:10.1118/1.595938

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Parameters related to the detective quantum efficiency (DQE) of several representative screens of different thicknesses, phosphor grain sizes, and optical properties were measured by the scintillation spectrum method, using monoenergetic x rays produced from x‐ray fluorescence. The experimental results, including those for spectral shape and average light energies (EA) emitted, are compared with conventional theories of the operation of screens. It was hoped that this would vindicate the theory of the effect of optical properties and so permit the simple calculation of all parameters related to DQE from standard x‐ray attenuation tables. Rather more substantial energy‐dependent deviations ofEAare found than was previously realized, which preliminary analysis suggests are due to both optical effects and photoelectron escape. We conclude that although DQE for a single energy can be calculated by simplified methods to within ±10%, the effective DQE when polyenergetic beams are used is much less accurately estimated and requires a fuller theoretical treatment.

ISSN:0269-3127    

DOI:10.1118/1.595939

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

An electronic scanning‐slit technology is under development which involves replacing paired fore and aft slits for scatter rejection with only one fore slit. As the slit scans across the portion of the patient to be imaged, pulsed x‐ray exposures produce images of the slit in successive positions, which are digitized and stored in computer memory. Software techniques are used for tracking the slit image, and discriminating against scatter and veiling glare lying outside the slit image. Such “electronic collimation” does not require synchronization between the slit‐scanning and detector readout, which makes it much simpler than alternative methods and potentially adaptable to any digital fluorography system. The performance characteristics of a prototype unit are described and images of a low‐contrast phantom are presented.

ISSN:0269-3127    

DOI:10.1118/1.595940

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

This paper describes several applications of magnetic resonance imaging (MRI) with time‐varying gradients within the framework of existing imaging systems. An alternative form of slice selection is shown where a time‐varying gradient is used, during the reception interval, to isolate the slice of interest (the reconstruction of the slice itself is treated by the conventional techniques). This system allows for precise control of the slice, thinner slices (less than 1 mm) and the ability to select multiple, closely packed slices using a single imaging sequence. All this is done by postprocessing so the slice(s) of interest can be selected after the measurements have been completed. It is also shown how time‐varying gradients can be used to generate the signals required for conventional projection‐reconstruction and two‐dimensional Fourier transform techniques (yet achieving higher resolution and using a resonant gradient system). In addition to cross‐sectional imaging, this same approach provides a simple system for projection imaging of the entire volume. All of these techniques constitute a good starting point for the exploitation of time‐varying gradients for fast, high‐resolution MRI with existing imaging systems.

ISSN:0269-3127    

DOI:10.1118/1.595941

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

We present a model of noise propagation from acquired magnetic resonance (MR) images toTR‐extrapolated synthetic images. This model assumes that images acquired at two repetition timesTR1andTR2are used to generate synthetic images at arbitrary repetition timesTR. The predictions of the model are compared with experimentally acquired phantom data, and show excellent agreement. The model is utilized in an analysis of two applications of MR image synthesis: scan time reduction and multiple‐image synthesis. Scan time is reduced by acquiring data at two short repetition times, and synthesizing at a longer repetition time, withTR1+TR2less thanTR. ForT1=800 ms, a reduction of 20% in scan time results in a 45% reduction in signal‐to‐noise ratio SNR, when compared to direct acquisition. Reducing scan time by much more than 20% produces large noise levels in the synthetic image, and is unlikely to be useful. In multiple‐image synthesis, images are synthesized at any repetition time in the range 0 toTR1+TR2, for contrast optimization. IfT1=800 ms, andTR1+TR2=2000 ms, the optimum combination ofTR1, TRresults in synthetic images whose SNR is at worst 22% less than the SNR of directly acquired images. For many values ofTR, the synthetic images have SNR superior to that obtainable by direct acquisition.

ISSN:0269-3127    

DOI:10.1118/1.595954

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

A new technique for detecting blood flow in magnetic resonance imaging is proposed. This technique is tailored to enhance areas containing flow while suppressing static and nonsignal areas with the objective of optimizing the contrast of vascularized tumors. Unlike flow phase imaging, in‐plane flow directionality (parallel versus antiparallel to applied flow gradient) is removed by the proposed method to reduce phase cancellation of flow signals. This signal loss is apt to occur in instances of complicated vascularity patterns consisting of many small vessels having multiple flow directions. The new flow‐enhanced imaging method is compared to flow phase imaging by computer simulation of simple objects containing many small vessels. The results indicate that flow‐enhanced imaging yields significantly greater detectability of regions of complicated small‐vessel patterns than phase imaging.

ISSN:0269-3127    

DOI:10.1118/1.595942

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

SimultaneousT1andT2images have been obtained using multiple‐echo self‐normalizing sequences. DifferentialT1andT2discrimination for two of these sequences, double saturation recovery and inversion–saturation recovery, have been explored in detail. The inversion–saturation recovery sequence exhibitsT1discrimination comparable to, and in many cases better than, that of optimalT1weighted conventional pulse sequences, but is poor forT2imaging. The double saturation recovery sequence yields comparable tissue discrimination forT1andT2imaging, but is poor compared to optimalT1weighted andT2weighted conventional pulse sequences. SimultaneousT1andT2images represent quantitative maps of intrinsic tissue properties, and hence form a natural basis for an automated image analysis and tissue classification system.

ISSN:0269-3127    

DOI:10.1118/1.595943

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The perturbation effect in parallel‐plate ionization chambers used for buildup measurements has been investigated. The fluence perturbation due to electrons emitted through the side walls are thoroughly investigated by measurements using film and extrapolation chambers and by calculations. The electron fluence varies both with side wall material and chamber geometry. In order to obtain a small perturbation effect, the chamber should have a large guard width compared to the electrode separation and the side walls should have as large an angle as possible with the central axis. The side wall should be of the same material as the rest of the chamber. The perturbation effect is also dependent on the electron contamination of the beam and angular distribution of the electron fluence. It is thus not possible to correct the perturbation effect in one parallel‐plate chamber with fixed plate separation with correction factors obtained with extrapolation chambers of other dimensions. In order to make accurate surface dose measurements extrapolation chambers are therefore strongly recommended in favor of fixed parallel‐plate chambers.

ISSN:0269-3127    

DOI:10.1118/1.595895

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

An analog of Fano's theorem for ionization in cavities is shown to hold for the stepwise representation of electron paths used in Monte Carlo computer models of electron transport. This brings to light an error in the distribution of electron paths and hence energy deposition which is induced by interrupting steps which cross the interface between media of different densities. The magnitude of the error depends on the shape of the cavity and its size relative to the electron path length in the cavity gas. In a typical calculation of a cylindrical chamber exposed to60Co radiation, if the electron step size is taken as 10% of the remaining path, then a 3% energy deficit in the cavity results. An algorithm for crossing an interface is described which does not produce this error.

ISSN:0269-3127    

DOI:10.1118/1.595896

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY