摘要:

On‐line electronic portal imaging devices are beginning to come into clinical service in support of radiotherapy. A variety of technologies are being explored to provide real‐time or near real‐time images of patient anatomy within x‐ray fields during treatment on linear accelerators. The availability of these devices makes it feasible to verify treatment portals with much greater frequency and clarity than with film. This article reviews the physics of high‐energy imaging and describes the operation principles of the electronic portal imaging devices that are under development or are beginning to be used clinically.

ISSN:0269-3127    

DOI:10.1118/1.596878

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The edge‐spread function that is expected when imaging a sharp edge in a highly scattering medium using a time‐of‐flight imaging system has been investigated. Experimental results have been compared with computer simulations. The effect of scattering is to broaden the image of the edge, accompanied by an offset of the midpoint (50% transmission) of the edge. Small random errors in the measurement of the time of flight results in an increase in the offset of the edge position at very short times of flight. The offset in the midpoint of the edge is evidence of the nonlinear nature imaging process, in view of which we conclude that nonlinear algorithms will be necessary to maximize the image information available from measured intensities.

ISSN:0269-3127    

DOI:10.1118/1.596875

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Basis material decomposition represents dual‐energy x‐ray attenuation measurements in terms of the attenuation coefficients or thicknesses of two standard materials which, when combined, produce attenuation equivalent to the object being measured. In tomographic imaging, the reconstructed attenuation coefficient is calculated in terms of the attenuation coefficients of the basis materials, while in projection imaging, the thicknesses of two materials can be specified in terms of the basis materials. This analysis shows that basis material decomposition is exact in a dual‐monoenergetic system, but for broad spectra, x‐ray beam hardening introduces a bias into quantitative measurements. The error is small enough that it can be ignored when dual‐energy imaging is used primarily to enhance the contrast of one material over another. The magnitude of the error in quantitative measurements depends on the details of the specific application including the energy of the x‐ray beam, and the composition and thickness of the materials included in the object. The magnitude of the error for dual‐energy bone densitometry has been analyzed using a first‐order propagation of error analysis and the calculations verified by computer simulation. This analysis shows that the magnitude of the systematic error can be as high as 3% for 1 g/cm2of bone mineral when aluminum and acrylic basis materials are used for the calibration. This systematic error is eliminated when the basis materials are the same as the materials that are being quantified (i.e., bone mineral and water).

ISSN:0269-3127    

DOI:10.1118/1.596889

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

A simulation study was performed to evaluate a new set of calibration standards for estimating the fat content of the body via dual‐photon absorptiometry (DPA) and dual‐energy x‐ray absorptiometry (DXA). The standards, proposed by Nord and Payne [presented at the 2nd meeting of The Bath Conference on Bone Mineral Measurement (1990)] consist of stearic acid (100% fat) and 0.6% NaCl in water (100% lean). They were compared with other standards consisting of average composition adipose/muscle tissues and fatty adipose / lean muscle tissues. Source and detector properties of a Gd‐153 DPA system and three commercial DXA systems were modeled. For each system and calibration set, rms errors in the calculated fat contents of simulated tissues having fat mass percentages that ranged from about 4%–44% and thicknesses that ranged from 5–20 cm were determined. Beam hardening errors for the systems were evaluated as was a calibration technique employed by one of the manufacturers to correct for such errors. In general, the smallest rms errors (2% or less when the calibration standards and tissues were of equal thickness) were obtained with the average adipose/muscle standards. Equivalent results were obtained with standards consisting of stearic acid and 0.8% NaCl. The latter is a higher salt content than proposed by Nord and Payne and results from differences in the x‐ray attenuation coefficients that were employed in calculating the fat equivalence of water. Other, more convenient standards, such as lucite and water may be employed by using appropriate fat equivalences (∼69% for lucite and ∼10% for water). Beam hardening errors for the DXA systems are considerable, and the simulated correction technique was shown to be effective.

ISSN:0269-3127    

DOI:10.1118/1.596890

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Currently, routine tests of fluoroscopic image quality in common use are highly subjective. As part of an effort to develop more quantitative routine tests of fluoroscopic image quality, a method was developed to quickly and easily measure Wiener spectra (WS) of TV‐viewed fluoroscopic systems that considers both spatial and temporal noise correlations. A PC‐mounted frame grabber captures images at the TV frame rate to form a three‐dimensional (3‐D) array of pixels. Scans of a “two‐dimensional slit” are then synthesized from which a one‐dimensional central section of a 3‐D WS is calculated. To avoid errors due to coarse (8‐bit) quantization, a video amplifier is used to expand a portion of the signal to the full digitizer range. A reference signal (2 mm of aluminum) is then used to normalize image contrast. Ensemble averages of 250 spectra were obtained in ∼1 min, including all processing. Results are presented to demonstrate reproducibility, sensitivity, and behavior of the WS. The eventual goal of this work is to use this method in conjunction with measurements of an MTF to calculate fundamental descriptors of image quality, such as SNR and NEQ.

ISSN:0269-3127    

DOI:10.1118/1.596891

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The measurement of modulation transfer function for radiographic screen‐film systems depends critically upon a proper linearization of the measured line spread function. This is normally done by photographic photometry (i.e., using the measured density versus log exposure relationship to transform the density line spread function into an exposure line spread function). It has been long appreciated that this procedure may fail for asymmetrical dual screen systems that use film with emulsion coated on both sides of the support. The advent of asymmetrical and near‐zero crossover films that can be used with highly asymmetric screen pairs has prompted a reinvestigation of these concerns about the definition and measurement of modulation transfer function. For such cases, it is useful to define the contrast transfer function, which is a function of exposure and spatial frequency. When normalized by its zero frequency value the contrast transfer function can serve as the “effective MTF” for low‐contrast input signals in such systems. In the limit of symmetrical systems this quantity approaches the conventionally measured MTF. The utility of this approach is demonstrated by applying it to a commercially available asymmetrical screen‐film combination.

ISSN:0269-3127    

DOI:10.1118/1.596892

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

ISSN:0269-3127    

DOI:10.1118/1.596893

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Proton NMR spectroscopy was applied to quantitate the measurement of total body water/fat distributionsinvivo. A special MR protocol was developed to excite a thick slab of tissue and display the magnitude NMR spectrum of the collected response signal. Very short echo time TE (8 ms) and long repetition time TR (4000 ms) were used to minimize relaxation damping of the signal intensities. The spectrum was then decomposed into individual lines and proton densities of different species were calculated. Proton density information was converted into weight percentage data using stoichiometrical and physiological information. The technique was validated using phantoms that contained different mixtures of water/maize oil. A high‐resolution NMR spectrum of maize oil samples was used to determine the stoichiometric information. The test results showed good agreement with the known composition of the phantom within the whole range of water content (0–100%). This method is very fast since no phase encoding of data is required. Preliminary results for monkeys show promising potential in clinical applications.

ISSN:0269-3127    

DOI:10.1118/1.596935

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Previous efforts concerning lesion extraction in radiologic images indicated that autoassociative memory models can be a valuable tool in automated lesion detection. Preliminary results are expanded to resolve the technical problems of image registration and magnification. Instead of operating on selected portions of the MR images, each entire image matrix is operated upon as image vector comprising all stacked columns of the matrix. Spin density weighted images (TR=3000 ms and TE=40 ms) of 42 normal subjects were remapped and standardized with respect to location and magnification. All image vectors were orthonormalized to span a linear manifold. Standardized abnormal image vectors were then tested by the stored autoassociative memory and the abnormalities (novelties) were extracted by application of an autocorrelation matrix to the input vector. The autocorrelation matrix is computed using image vectors from normal subjects. The lesions (multiple sclerosis and tumors) are then identified as the orthogonal component to the linear manifold spanned by the basis vectors of the normal brain scans. Lesion extraction has been achieved with the intention of quantitating and staging diseased parenchyma after automated edge detection.

ISSN:0269-3127    

DOI:10.1118/1.596853

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Quantitative MR phase imaging is frequently used to measure spin velocities. A potential difficulty may arise, however, when in‐plane phase images are acquired of a vessel carrying laminar flow, for which the fluid velocity profile is parabolic. In that case, depending on the flow velocity (v), the vessel diameter (D), and the chosen MR slice thickness (ST), a spin velocity gradient will be present to some extent within each intraluminal voxel. The resulting intravoxel phase dispersion may be expected to affect the net pixel phase value, and hence compromise the assumed linear correlation between phase shift and velocity. In this study, the effects of alterations ofv,D, and ST on the apparent image phase are investigated for the case of laminar flow directed parallel to the sequence read gradient. A theoretical model is developed and the conclusions experimentally tested using a flow phantom. The data demonstrate that when quantitating in‐plane phase‐flow images, significant velocity underestimations may occur when the net flow‐induced phase shifts are small and the MR slice thickness is an appreciable fraction of the vessel diameter.

ISSN:0269-3127    

DOI:10.1118/1.596854

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY