摘要:

Coherent scattering of x‐ray photons leads to the phenomenon of x‐ray diffraction, which is widely used for determining atomic structure in materials science. A technique [x‐ray diffraction computed tomography (CT)] is described, analogous to conventional CT, in which the x‐ray diffraction properties of a stack of two‐dimensional object sections may be imaged. The technique has been investigated using a first generation (single pencil beam) CT scanner to measure small angle coherent scatter, in addition to the customary transmitted radiation. Diffraction data from a standard CT performance phantom obtained with this new technique and with an x‐ray diffractometer are compared. The agreement is satisfactory bearing in mind the poor momentum resolution of our apparatus. The dose and sensitivity of x‐ray diffraction CT are compared with those of conventional transmission CT. Diffraction patterns of some biological tissues and plastics presented in a companion paper indicate the potential of x‐ray diffraction CT for tissue discrimination and material characterization. Finally, possibilities for refinement of the technique by improving the momentum resolution are discussed.

ISSN:0269-3127    

DOI:10.1118/1.596063

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

X‐ray diffraction allows the investigation of the atomic or molecular structure of materials. The combination of diffractometry with computerized tomography enables spatially resolved imaging of the diffraction properties of extended objects as described in more detail in a companion article [Hardinget al., Med. Phys.14, 515 (1987)]. We present measured diffraction patterns of some plastics and several biological materials, which allow further optimization of our method and the selection of suitable application areas.

ISSN:0269-3127    

DOI:10.1118/1.596143

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

In this paper we describe a technique for determining the modulation transfer function (MTF) of an imaging system from an experimentally obtained edge profile. The technique includes an exact correction for the frequency passband of the finite‐element differentiation required to obtain the line spread function from the edge spread function. This correction has been ignored by investigators in the past and is required whenever finite‐element differentiation is used rather than analytic differentiation of a model fitted to the edge response data. The magnitude of the MTF correction is ∼11% atf=fc/2 and ∼57% atf=fc, wherefc=fs/2 is the maximum frequency reproducible without aliasing with a sampling rate offs. The correction is performed in the spatial frequency domain by multiplying the uncorrected MTF by 1/sinc(πf/2fc). A computer simulation is presented to demonstrate the effect and the correction procedure. An experimental MTF of an x‐ray image intensifier system obtained using this technique is found to be consistent with an MTF obtained using a bar pattern test phantom.

ISSN:0269-3127    

DOI:10.1118/1.596064

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

We have investigated the application of computer‐based methods to the detection of microcalcifications in digital mammograms. The computer detection system is based on a difference‐image technique in which a signal‐suppressed image is subtracted from a signal‐enhanced image to remove the structured background in a mammogram. Signal‐extraction techniques adapted to the known physical characteristics of microcalcifications are then used to isolate microcalcifications from the remaining noise background. We employ Monte Carlo methods to generate simulated clusters of microcalcifications that are superimposed on normal mammographic backgrounds. This allows quantitative evaluation of detection accuracy of the computer method and the dependence of this accuracy on the physical characteristics of the microcalcifications. Our present computer method can achieve a true‐positive cluster detection rate of approximately 80% at a false‐positive detection rate of one cluster per image. The potential application of such a computer‐aided system to mammographic interpretation is demonstrated by its ability to detect microcalcifications in clinical mammograms.

ISSN:0269-3127    

DOI:10.1118/1.596065

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

We developed an iterative deconvolution technique to determine the size of a “blurred” vessel in a digital subtraction angiographic (DSA) image by taking into account the unsharpness of the DSA system. Initially, a region of interest over a small segment of the contrast‐filled vessel was selected in a DSA image, and the center line of the opacified vessel was determined by polynomial curve fitting of the locations of the peak pixel values along the vessel image. The blurred image profile was then obtained from pixel values across the vessel in a direction perpendicular to the center line. This measured profile was compared iteratively with a calculated profile for various size vessels, which was obtained from a cylindrical vessel model and from the line spread function, until the root‐mean‐square difference between the two profiles was minimized. The size of a cylindrical vessel yielding the matched profile was considered the best estimate of the unknown vessel size. Studies with a blood vessel phantom indicated that vessels larger than 0.5 mm could be measured with an accuracy and precision of approximately 0.1 mm, which is about 1/3 of the pixel size used in our DSA system. Details of our approach and some clinical vessel images with and without simulated stenotic lesions are presented.

ISSN:0269-3127    

DOI:10.1118/1.596066

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The errors associated with derivation of cerebral blood flow values by the xenon‐enhanced computed tomography (CT) method have been evaluated as a function of tissue heterogeneity and CT noise. The results of this study indicate that CT noise introduces large errors in the derived flow value when data for a single, unprocessed voxel are used for this purpose. CT noise increases the derived flow values in a systematic way. Tissue heterogeneity results in a systematic error which lowers the derived flow values. Errors due to both parameters are computed for typical and extreme conditions.

ISSN:0269-3127    

DOI:10.1118/1.596067

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

A technique for performing accurate bone mineral content (BMC) determination, using dual‐energy scanning‐slit fluorography, has been developed. X‐ray scatter and veiling glare are suppressed using a multiple scanning‐slit device interfaced to a conventional digital subtraction angiography system. The technique employs a pair of calibration step wedges, one tissue and one bone equivalent. The proper use of the step wedges obviates the need for accurate kVp calibration or absolute calibration of the system's response to BMC. Phantom studies intended to simulate vertebral BMC measurement indicate that the technique is capable of measuring BMC with less than 30 mg/cm2inaccuracy over a wide range of BMC.

ISSN:0269-3127    

DOI:10.1118/1.596068

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Estimation of collision kerma at a geometric point arising from scattered photons is a potentially important application of Monte Carlo simulation, especially in the presence of steep flux gradients. We examine the usual method of extracting point‐kerma estimates from randomly generated photon trajectories which consists of tallying the energy lost by photon collisions occurring in the vicinity of the point of interest. Several other methods derived from the equivalence of track length per unit volume and flux are evaluated as to accuracy and efficiency. Finally, a next‐flight estimator is discussed in which the expected contribution of each simulated photon collision to kerma at the point of interest is calculated regardless of proximity of the collision to the point. All of these techniques are shown to involve a trade‐off between statistical precision and spatial resolution: increasing the number of contributing collisions requires averaging kerma over a larger volume. Based upon both analytic models and realistic Monte Carlo simulations, use of next‐flight and track‐length estimators is shown to improve simulation efficiencies by factors of 2 to 20 compared to analog scoring. Practical guidelines as to choice of estimator and successful implementation are presented.

ISSN:0269-3127    

DOI:10.1118/1.596069

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Clinical electron beams consist of primary electrons, primary bremsstrahlung generated in the regular photon and electron collimator system determining the composite beam, and some short‐range contaminant photon and electron scatter arising from the lower parts of the standard or regular electron applicator. Any beam‐shaping insert placed inside the applicator causes some extra (“contaminant”) bremsstrahlung and electron scatter. The new dose calculation model is based on separate treatment of these components. For the calculation of the primary electron dose we use experimentally determined electron scatter functions and differential electron scatter functions. The primary bremsstrahlung is treated as an unflattened but otherwise regular x‐ray beam. The contaminant components arising from the rim area of the regular electron collimator and from beam‐shaping inserts are considered separately. The behavior of the in‐air ionization profiles is described using the concepts of effective electron source position and effective electron source diameter. The model has been tested for several electron energies.

ISSN:0269-3127    

DOI:10.1118/1.596144

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

A model based on an approximation called the partial fluence approximation is presented for the calculation of dose distributions in the vicinity of medium interfaces in photon beams. The predictions of the model are compared with dose distributions measured in layered phantoms consisting of aluminum and polystyrene, for photon beams ranging in energy from60Co to 24 MV.

ISSN:0269-3127    

DOI:10.1118/1.596023

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY