摘要:

The unique abilities of magnetic resonance imaging (MRI) to provide detailed images of blood flow in the body, and without resorting to the injection of contrast agents, has provided the stimulus for the keen interest in this subject. In this paper we discuss a technique aimed at providing quantitative information on the distribution of velocity of blood flow across the lumen of a vessel. The technique is designed to reduce the signal from stationary material allowing for a much more accurate determination of the signal from moving material. A particular feature is that the physical phenomenon sensitized is time‐of‐flight displacements or “fresh spin inflow,” and the technique does not rely upon phase‐sensitive modulations. Further this technique produces a steady‐state spatial distribution of magnetization, and hence a signal, which reflects both the time at which material enters the selected slice and how long the material remains in the slice. In this way, a single image will provide the information necessary to extract information on the velocities of interest.

ISSN:0094-2405    

DOI:10.1118/1.596108

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The definition and mapping of oblique planes by magnetic resonance imaging (MRI) requires the simultaneous application of two or three orthogonal gradients to define the desired intermediate direction of the frequency encoding or “readout” gradient. Each of the three main gradient coils produces different patterns of eddy currents. Consequently, the application of dephasing and rephasing lobes of these gradients will produce echoes at slightly different times for each gradient. If two or three gradients are applied simultaneously to create an arbitrary view direction, the resulting echo will therefore be shifted in time and considerably reduced in intensity. In this article, we present an analysis of the behavior of the magnetization in a typical two‐dimensional Fourier transform pulse sequence for the imaging of oblique slices. The theoretical displacements in time and reduction in intensity of the echo amplitudes are calculated and compared to the experimental behavior. We show that, in spite of this phenomenon, the final image suffers only marginally in signal‐to‐noise ratio, provided the slice width is small compared to the field of view. This is due to the fact that there always exists a cycle in the sequence in which the phase‐encoding gradient almost completely compensates for the above described effect.

ISSN:0094-2405    

DOI:10.1118/1.596109

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The use of a convolution‐filtering method to estimate the scatter distribution in images acquired with a digital subtraction angiography (DSA) imaging system has been studied. Investigation of more than 175 convolution kernels applied to images of anthropomorphic head, chest, and pelvic phantoms using 15‐, 25‐, and 36‐cm fields of view (digitized onto a 512×512 pixel image matrix) showed that two‐dimensional exponential kernels with a full width at half maximum (FWHM) of 50–150 pixels best reproduced the scatter fields within these images with a root‐mean‐square percentage error from 4% to 8%. A two‐dimensional exponential kernel with a FWHM of 75 pixels in each dimension applied to ten different anatomic presentations and fields of view, resulted in an average root‐mean‐square percentage error of 6.6% for the ten cases studied. The method should be implementable using an array of small lead beam stops placed in the field of only a single mask image and the above described convolution kernel applied to both mask and postopacification images. The mask beam‐stop data are used to scale both mask and postopacification convolution‐filtered images. This scaled, convolution‐filtered image is then subtracted from the original image to produce a largely scatter‐corrected image.

ISSN:0094-2405    

DOI:10.1118/1.596126

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

An efficient method has been developed to speed up the computational time required for tissue separation after the dual‐energy scan in computerized tomography and digital radiography. This method consists of two steps. First, an isotransmission line method is used to replace the iterative algorithm for solving the two nonlinear equations, which is computationally noisy and sensitive. Second, two tables, one for the aluminum component and the other for the plastic component, are derived from calibration data in order to replace calculation by a table lookup. Results from this method are compared with the conventional method.

ISSN:0094-2405    

DOI:10.1118/1.596110

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

A fourth‐generation, computed tomographic (CT) scanner, equipped for prospectively gated cardiac imaging, was modified to control the scan data acquisition by using knowledge of the location of redundant rays in the sinogram. In conventional prospective gating, a computer monitors the electrocardiogram (ECG) and calculates when to initiate the next scan in a gated series in order to acquire all 360° of projection data for a desired phase of the cardiac cycle. However, in each scan of a series, every projection ray is measured twice (when the positions of the source and detector are reversed). Redundant‐ray prospective gating takes advantage of this information to improve the efficiency of data acquisition. Using a heart phantom “beating” at 90 min−1, images of all phases of the cardiac cycle with 100‐ms temporal resolution were obtained in four scans with redundant‐ray gating; whereas a four‐scan series with conventional prospective gating yielded worse images of 170‐ms resolution.

ISSN:0094-2405    

DOI:10.1118/1.596134

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Texts in diagnostic radiography give rules for adjustment of exposure parameters to compensate for the effect of temperature on the efficiency of film–screen combinations. These rules are appropriate for traditional phosphors, such as calcium tungstate (CaWO4) and lead barium sulfate (BaSO4:Pb) but may not be applicable to newer screens in current use such as those of the rare‐earth type. The effect of temperature on x‐ray induced luminescence from modern and traditional screens has been compared over a temperature range of ∼−130 to ∼60 °C and has been found to differ markedly. Typically, the luminescent emission from rare‐earth phosphors varies by less than 10% over this temperature range compared to the 60% change in efficiency for conventional screens over the same temperature range. One type of modern screen, however, shows a continuous decrease in luminescent efficiency as the temperature is lowered. Qualitative physical explanations for these effects are presented.

ISSN:0094-2405    

DOI:10.1118/1.596070

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The purpose of this study was to design an improved flattening filter for a Therac 20 medical linear accelerator. Profiles of the 18‐MV x‐ray beam produced by this accelerator measured along the diagonal of a 40×40 cm field at a depth of 5 cm were measured, and it was found that there were regions near the corners of the field where the dose was 109% of the central axis dose. An iterative algorithm for designing flattening filters was developed which required, as input, precise measurements of the following data: the unflattened primary beam profile, the fraction of the beam due to contamination radiation arising from interactions of primary photons with the flattening filter and the collimator assemblies, and the attenuation of the primary photons in water and lead as a function of angle from the central axis of the beam. A new flattening filter was designed and profiles of the beam were measured at a number of depths. These measurements showed that the beam was flattened to within ±1% out to 24 cm along the diagonal of a 40×40 cm field at a depth of 5 cm.

ISSN:0094-2405    

DOI:10.1118/1.596071

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The polarity effect was investigated for three different commercially available plane‐parallel ionization chambers: the Memorial Pipe chamber, the Victoreen/Nuclear Associates model 30‐329 chamber manufactured by PTW, Frieburg, and the Capintec PS‐033 thin‐window ionization chamber. The primary study was the polarity effect versus depth below the phantom surface for 6‐, 10‐, 18‐, and 24‐MV x‐ray beams, and 9‐ and 22‐MeV electron beams. The polarity effect in the region of nonelectronic equilibrium that exists at the interface of two dissimilar materials, polystyrene and aluminum, was investigated as well as the effects of field size. For the group of plane‐parallel ionization chambers that we studied, we found a polarity effect of only 1%–2% for electron beams at the depth ofdmax. At depths greater thandmax, the polarity effect for electrons increased and was as high as 4.5% for some chambers. When used in the buildup region of high‐energy photon beams, these same chambers exhibited up to a 30% difference in collected charge between one polarity and the other. This effect and its relationship to physical chamber characteristics is discussed.

ISSN:0094-2405    

DOI:10.1118/1.596072

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The AAPM Task Group dosimetry protocol is ambiguous regarding the replacement correction factorsPreplto be applied to electron measurements with parallel‐plate chambers. By intercomparison with a cylindrical chamber whosePreplvalues atdmaxmay be calculated from Task Group 21, thePreplvalues for a PTW/Markus parallel‐plate chamber have been determined in the range of mean incident energies of 5–11 MeV. ThePreplvalues for this chamber are found to differ significantly from unity, if one assumes that the cylindrical chamber values are valid.

ISSN:0094-2405    

DOI:10.1118/1.596073

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

A projection operator technique is used to derive an equation for local tissue tracer contentQ(t), assuming linear multicompartmental kinetics for tracer utilization. The resulting equation has the form (d/dt) Q(t)=FCa(t) −ΛQ(t)−∫0tdτ [ξ(τ)Ca(t−τ)−ψ(τ)Q(t−τ)], whereFandCa(t) denote local blood flow and concentration of tracer, respectively. Tissue complexity is contained within the new parameters Λ, ξ(t), and ψ(t), where the time dependent coefficients are expressed as sums of exponentials. Two simple applications are considered: tissue heterogeneity and internal trapping of tracer. The relationship to effective single compartmental analysis, as is used for local cerebral blow flow determination, is evaluated.

ISSN:0094-2405    

DOI:10.1118/1.596135

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY