ISSN:0094-2405    

DOI:10.1002/j.2473-4209.1996.tb00105.x

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

A model is presented in this paper to describe how the contrast of a reconstructed object and slice sensitivity profile are affected by (1) the table speed or helical pitch, (2) the x‐ray collimation, (3) the size of the object, (4) the alignment between the reconstructed slice and the object, (5) the distance of the object from the axis of rotation, and (6) the helical CT reconstruction algorithm employed. This contrast model is validated by both computer simulations and experiments. With this model, the contrast of a reconstructed object, slice sensitivity profile, and the longitudinal MTF can be accurately predicted. The optimal scan strategy and the point of diminishing returns can be determined prior to scanning. Several conclusions can be drawn from this model. First, overlapping reconstruction significantly improves overall scan contrast sensitivity of helical CT. Second, with a given x‐ray collimation, low pitch helical scans provide better longitudinal resolutions. Third, with a given volume coverage rate (i.e., a given table speed), narrow collimation high pitch helical scans provide better longitudinal resolutions than wide collimation low pitch ones and therefore are recommended for high‐contrast thin‐slice applications. A lesion conspicuity model is also established.

ISSN:0094-2405    

DOI:10.1118/1.597774

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The modulation transfer function (MTF) is a commonly used metric for defining the spatial resolution characteristics of imaging systems. While the MTF is defined in terms of how an imaging system demodulates the amplitude of a sinusoidal input, this approach has not been in general use to measure MTFs in the medical imaging community because producing sinusoidal x‐ray patterns is technically difficult. However, for optical systems such as charge coupled devices (CCD), which are rapidly becoming a part of many medical digital imaging systems, the direct measurement of modulation at discrete spatial frequencies using a sinusoidal test pattern is practical. A commercially available optical test pattern containing spatial frequencies ranging from 0.375 cycles/mm to 80 cycles/mm was used to determine the MTF of a CCD‐based optical system. These results were compared with the angulated slit method of Fujita [H. Fujita, D. Tsia, T. Itoh, K Doi, J. Morishita, K. Ueda, and A Ohtsuka, “A simple method for determining the modulation transfer function in digital radiography,” IEEE Trans. Medical Imaging11, 34–39 (1992)]. The use of a semi‐automated profile iterated reconstruction technique (PIRT) is introduced, where the shift factor between successive pixel rows (due to angulation) is optimized iteratively by least‐squares error analysis rather than by hand measurement of the slit angle. PIRT was used to find the slit angle for the Fujita technique and to find the sine‐pattern angle for the sine‐pattern technique. Computer simulation of PIRT for the case of the slit image (a line spread function) demonstrated that it produced a more accurate angle determination than “hand” measurement, and there is a significant difference between the errors in the two techniques (Wilcoxon Signed Rank Test,p<0.001). The sine‐pattern method and the Fujita slit method produced comparable MTF curves for the CCD camera evaluated.

ISSN:0094-2405    

DOI:10.1118/1.597840

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The suitability of a Y2O3:Eu3+scintillator for use in radiation detectors and medical image receptors was studied. Y2O3:Eu3+was used in the form of laboratory prepared screens of different coating thicknesses. The x‐ray luminescence efficiency of the screens was measured for tube voltages between 50–200 kVp and in both transmission and reflection modes of observation. The intrinsic x ray to light conversion efficiency (nc) and other parameters of the Y2O3:Eu3+phosphor material related to optical scattering, absorption, and reflection were determined. These were used in the calculation of the image transfer characteristics, MTF and zero frequency DQE, for various screen coating thicknesses and x‐ray tube voltages. The light emission spectrum of Y2O3:Eu3+was measured (narrow band peak at 613 nm) and its spectral compatibility to the spectral sensitivity of several commonly employed optical photon detectors was determined. The x‐ray luminescence efficiency varied with x‐ray tube voltage, attaining maximum value at about 80 kVp for all screen thicknesses. It also varied with coating thickness reaching 25 μW m−2/mR s−1and 18 μW m−2/mR s−1at 175 mg/cm2for reflection and transmission modes, respectively. The intrinsic x ray to light conversion efficiency and the image transfer characteristics were found to be comparable to several commercially used phosphors:nc=0.095, MTF0.05ranged between 10 and 25 line pairs per mm and peak values of DQE(0) varied between 0.33 and 0.14 in the coating thickness and kVp ranges useful for x‐ray imaging. Spectral compatibility to some red sensitive optical photon detectors was excellent (0.9 or better). Results indicated that Y2O3:Eu3+is a medium to high overall performance material that could be used in medical x‐ray detectors and image receptors.

ISSN:0094-2405    

DOI:10.1118/1.597769

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Stains have been used in optical microscopy to visualize the distribution and intensity of substances to which they are attached. Quantitative measures of optical density in the microscopic images can in principle be used to determine the amount of the stain. When multiple dyes are used to simultaneously visualize several substances to which they are specifically attached, quantification of each stain cannot be made using any single wavelength because attenuation from the several stain components contributes to the total optical density. Although various dyes used as optical stains are perceived as specific colors, they, in fact, have complex attenuation spectra. In this paper, we present a technique for multiple wavelength image acquisition and spectral decomposition based upon the Lambert–Beer absorption law. This algorithm is implemented based on the different spectral properties of the various stain components. By using images captured atNwavelengths,Ncomponents with different colors can be separated. This algorithm is applied to microscopy images of doubly and triply labeled prostate tissue sections. Possible applications are discussed.

ISSN:0094-2405    

DOI:10.1118/1.597841

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The presence of dense fibroglandular tissue within the breast is the most significant cause of failure to detect breast cancer with mammography. The dense tissue often produces a range of exposure which exceeds the useful dynamic range of film‐screen mammography. It has been shown that equalization radiography overcomes the latitude limitations of film‐screen imaging. Equalization compensates for regional variations in x‐ray transmission within the patient through spatial modulation of the entrance exposure. We have proposed rotary scanning equalization radiography (RSER), a scan‐rotate geometry for efficient equalization radiography. In RSER the image receptor is exposed by repeated scans of a source‐modulated fan beam. The fan beam is rotated with respect to the patient between scans. Numerical simulations and theoretical analysis have shown that the superposition of exposure from appropriately modulated fan beams at a variety of angles is an entrance exposure that effectively equalizes the film exposure. The design and characteristics of a prototype RSER imaging system are described. Anthropomorphic breast phantom images are used to determine the improvement in image contrast obtained with RSER, the expected tube loading, and the presence of artifacts. RSER increases the fraction of the breast imaged with high contrast (at least 90% of peak gradient) from 46% (conventional mammography) to 80%. Subjective examination of the phantom images show that RSER achieves image quality very similar to that of much less efficient equalization geometries with only 2.7 times greater tube loading than conventional mammography. As predicted by theoretical analysis of exposure artifacts in RSER, the prototype RSER system is relatively immune to artifacts. Exposure artifacts were demonstrated for extreme variations in x‐ray transmission within the patient. These results show that RSER is an efficient, practical means of overcoming the latitude limitations of film‐screen mammography, and improving the detection of breast cancer.

ISSN:0094-2405    

DOI:10.1118/1.597770

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The purpose of this technical report is to make available an up‐to‐date source of attenuation coefficient data to the medical physics community, and to compare these data with other more familiar sources. Data files from Lawrence Livermore National Laboratory (in Livermore, CA) were truncated to match the needs of the medical physics community, and an interpolation routine was written to calculate a continuous set of cross sections spanning energies from 1 keV to 50 MeV. Coefficient data are available for elementsZ=1 throughZ=100. Values for mass attenuation coefficients, mass‐energy‐transfer coefficients, and mass‐energy absorption coefficients are produced by a single computer subroutine. In addition to total interaction cross sections, the cross sections for the photoelectric, Rayleigh, Compton, pair, and some triplet interactions are also produced by this single program. The coefficients were compared to the 1970 data of Storm and Israel over the energy interval from 1 to 1000 keV; for elements 10, 20, 30, 40, 50, 60, 70, and 80, the average positive difference between the Storm and Israel coefficients and the coefficients reported here are 1.4%, 2.7%, and 2.6%, for the mass attenuation, mass energy‐transfer, and mass‐energy absorption coefficients, respectively. The 1969 data compilation of mass attenuation coefficients from McMasteretal. were also compared with the newer LLNL data. Over the energy region from 10 keV to 1000 keV, and from elementsZ=1 toZ=82 (inclusive), the overall average difference was 1.53% (σ=0.85%). While the overall average difference was small, there was larger variation (>5%) between cross sections for some elements. In addition to coefficient data, other useful data such as the density, atomic weight,K,L1,L2,L3,M, andNedges, and numerous characteristic emission energies are output by the program, depending on a single input variable. The computer source code, written in C, can be accessed and downloaded from the World Wide Web at:http://www.aip.org/epaps/epaps.html[E‐MPHSA‐23‐1997].

ISSN:0094-2405    

DOI:10.1118/1.597899

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

As research efforts for developing computer‐aided diagnosis (CAD) schemes of digitized mammograms increase and interscheme results are compared, the desire to establish an acceptable consistent reporting protocol of the distribution of abnormal characteristics is becoming an issue. “Mass contrast” is very important and frequently reported in current CAD studies. In this report, 100 verified mass regions were analyzed systematically using 6 different definitions of “mass contrast.” Measured variability in mass contrast was demonstrated by the distribution shift in this group of masses. The need for universally accepted and largely standardized descriptors of objects of interest is clearly demonstrated.

ISSN:0094-2405    

DOI:10.1118/1.597775

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Intensity modulation provides greatly increased control, leading to superior dose distributions with a potential for improved clinical outcome. It also allows us to compensate for deviations from the expected patterns in dose distributions caused by the lateral transport of radiation. This is important not only to produce more homogeneous dose distributions in the target volume but also, more importantly, to allow a reduction in the margins for the penumbra and a corresponding reduction in the volume of normal tissue irradiated. Potentially, this would permit escalation of doses to higher levels and further improve local control for the same or lower normal tissue complications. The intensity‐modulated treatment design process, regardless of the specific method used, involves the tracing of rays from the source of radiation through the target volume. Intensities of rays are adjusted iteratively in an attempt to produce a desired homogeneous dose within the target volume, while at the same time striving to maintain normal tissue exposure within the limits of tolerance. If the lateral transport of scattered radiation is ignored, as is commonly done because of the complexities of incorporating it, the resulting dose distribution within the target volume may be considerably different from the anticipated pattern. This would be particularly true if there are high gradients in fluence patterns within the field to shield a normal anatomic structure. Similarly, the lateral transport of radiation would lead to a dose deficit just inside the boundary of the planning target volume (PTV) and a dose excess just outside it. The conventional remedy to make up for the loss of dose near the boundaries, thereby ensuring complete coverage of the target volume, would be to employ a margin for the “penumbra.” We demonstrate that, with intensity modulation, we have an important new tool to improve target coverage, namely an appropriate increase in fluence just inside the boundary. Most suitably, a combination of increased fluence and a smaller than conventional margin should be employed. We have used an iterative scheme to compensate for lateral transport in the intensity modulation optimization process. In each iteration, the intensity distribution is first designed ignoring lateral transport. At the end of each iteration, the dose distribution is calculated using a pencil beam convolution method, thereby incorporating lateral transport and revealing the deviations from the anticipated dose distribution caused by lateral transport. In the next iteration, ray intensities are further adjusted to rectify the deviations. In general, only a few iterations are needed to adequately account for the lateral transport of radiation. We have applied this method to intensity‐modulated prostate treatment plans and demonstrate that this methodology allows the use of smaller margins, improves target dose homogeneity, and provides greater protection for normal tissues. We examine the variation of the magnitude of the gain from one patient to another. The methodology described in this paper has been introduced into routine clinical use.

ISSN:0094-2405    

DOI:10.1118/1.597848

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

An electron beam dose calculation algorithm has been developed which is based on a superposition of pregenerated Monte Carlo electron track kernels. Electrons are transported through media of varying density and atomic number using electron tracks produced in water. The perturbation of the electron fluence due to each material encountered by the electrons is explicitly accounted for by considering the effect of (i) varying stopping power, (ii) scattering power, and (iii) radiation yield. For each step of every electron track, these parameters affect the step length, the step direction, and the energy deposited in that step respectively. Dose distributions in both homogeneous water and nonwaterlike phantoms, and heterogeneous phantoms show consistent agreement with “standard” Monte Carlo results. For the same statistical uncertainty in broad beam geometries, this new calculation method uses a factor of 9 less computation time than a full Monte Carlo simulation.

ISSN:0094-2405    

DOI:10.1118/1.597842

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY