摘要:

This study investigates factors associated with the imaging of a patient using a high‐energy radiotherapy treatment beam. Both single‐stage (e.g., solid‐state detector) and two‐stage (e.g., scintillation screen plus TV) systems are considered. First an expression is derived that relates dose at the buildup depth in the object to the structure of the object, the scatter‐to‐primary signal‐variance ratio and the differential‐signal‐to‐noise ratio in the image. Second the number of bits required to digitize the image is derived. Third the effect of scattered radiation is investigated for photon counting, photopeak, and Compton detector types. Fourth the effect of noise in the detection process is considered. Finally, the relationship between x‐ray source size, detector aperture, and image magnification is derived. The optimum magnification for given source size and detector aperture is discussed in terms of the system transfer function. The study indicates that at a primary beam energy of 2 MeV, a dose of 10−3cGy is required to detect reliably the presence of a bone section of area 10×10 mm and thickness 4 mm in 250 mm of soft tissue. For this example, it is also estimated that a digitization accuracy of 10 bits is required. The calculations indicate that for a Compton detector, the scatter‐to‐primary signal‐variance ratio drops from a value of around 30% at the exit surface of the object to 5% at a distance of 80 cm from the object with a consequent small reduction in the dose required to form the image.

ISSN:0094-2405    

DOI:10.1118/1.596735

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

In the analysis of x‐ray system performance, the log‐signal function, or negative logarithm of the relative detector signal, and the analogously defined log‐variance function, are of central importance. These are smooth, monotonic functions of object thickness, which are nonlinear for nonmonoenergetic x‐ray source spectra. If we assume a dual‐energy decomposition of the object into two basis materials, then they can be written as analytic functionsf(x,y) andf*(x,y), respectively, of the component thicknesses (x,y) of the object. In this paper, we analytically develop the Taylor series of these functions, prove that they converge everywhere, and parametrize their coefficients via suitable central spectral moments of the basis‐material attenuation coefficients. We then show how the lower‐order moments can be used to construct, in closed form, smooth, monotonic, second‐order (conic) surface functions which closely approximatef(x,y) andf*(x,y) over the entire feasible domain. A simplified construction, based on using appropriate asymptotic values of the basis‐material attenuation coefficients to match the asymptotic behavior of these functions, is also given. The inclusion of image components withK‐edge absorption spectra, such as iodine, is done without effort. Extension of the results to the construction of similar (virtually exact) third‐order (cubic) surface approximations is straightforward. As an illustration of the broad applicability of this approach, we extend our analysis to the construction of similar approximations to the inverse (decomposition) functions for an arbitrary dual‐energy system, and investigate their numerical accuracy for a model dual‐kVp system. We conclude that this extended analysis provides an accurate description of the system behavior in terms of a small number of physically meaningful parameters. This parametrization permits greater physical insight into the system behavior, while at the same time simplifying its mathematical description, and similarly facilitates the analysis of various measures of imaging performance via either analytic or numerical methods.

ISSN:0094-2405    

DOI:10.1118/1.596744

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The use of characteristic‐line radiation from rare‐earth targets bombarded by high‐energy (up to 1 MeV) electron beams has been evaluated as an x‐ray source for dual energyK‐edge subtraction imaging of the human coronary arteries. Two characteristic‐line x‐ray sources, one using the splitKα1andKα2lines of lanthanum excited by a high‐energy electron beam and the other using theKαlines of barium and cerium, were studied. A Monte Carlo electron–photon simulation was used to calculate x‐ray spectra and energy deposition profiles from targets of these elements bombarded by electrons in the energy range 140 keV to 1 MeV. A general dual‐energy imaging model was developed that used these calculated source spectra to numerically investigate the dependence of the subtraction image signal‐to‐noise ratio on such factors as the ratio ofK‐line to x‐ray continuum yield, continuum spectral shape, x‐ray filtering, and detector response. A signal averaging technique for enhancing the signal‐to‐noise ratio was also evaluated. The results of these calculations were used to identify an optimum electron beam, target, filter, and detector configuration. A compact electron accelerator capable of providing the required electron beam parameters was designed. Calculations indicate that under ideal conditions the optimized system would be capable of imaging 2 mg/cm2of iodine contrast agent in 20 g/cm2of tissue with a signal‐to‐noise ratio of 5, a detector pixel size of 0.25 mm2, and a total image acquisition time of 10 ms. These parameters are consistent with those needed to image the human coronary arteries after an intravenous injection of iodine contrast agent. These capabilities, along with the relatively modest hardware requirements of this system, make it attractive as an x‐ray source for dual energy transvenous coronary angiography.

ISSN:0094-2405    

DOI:10.1118/1.596648

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Fluoroscopic contrast resolution is commonly determined at a specified kVp by imaging a test object comprised of targets where contrast decreases gradually and sequentially. Threshold contrast or contrast resolution is the contrast of the lowest contrast target that can be perceived. This approach suffers from two problems. First, test object contrast is specified at a x‐ray tube voltage that is not always obtainable in practice. Second, the small change in contrast between adjacent targets contributes to observer variability making consistent and reproducible contrast threshold determinations difficult. Described is a contrast resolution test tool that eliminates or reduces these problems. The novel target arrangement allows one to quickly and easily specify the contrast resolution of a fluoroscopic imaging chain to a precision of ≂0.5%. Tables of target contrast versus x‐ray tube potential are developed that permit one to employ the test object for contrast resolution determination over the normal range of tube potentials encountered on clinical units.

ISSN:0094-2405    

DOI:10.1118/1.596605

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

A data‐acquisition system designed for x‐ray medical imaging utilizes a segmented high‐purity germanium (HPGe) detector array with 2‐mm wide and 6‐mm thick elements. The detectors are contained within a liquid‐nitrogen cryostat designed to minimize heat losses. The 50‐ns pulse‐shaping time of the preamplifier electronics is selected as the shortest time constant compatible with the 50‐ns charge collection time of the detector. This provides the detection system with the fastest count‐rate capabilities and immunity from microphonics, with moderate energy resolution performance. A theoretical analysis of the preamplifier electronics shows that its noise performance is limited primarily by its input capacitance, and is independent of detector leakage current up to approximately 100 nA. The system experimentally demonstrates count rates exceeding 1 million counts per second per element with an energy resolution of 7 keV for the 60‐keV gamma ray photon from241Am. The results demonstrate the performance of a data acquisition system utilizing HPGe detector systems which would be suitable for dual‐energy imaging as well as systems offering simultaneous x‐ray transmission and radionuclide emission imaging. Key words: high‐purity germanium detectors, data acquisition electronics, x‐ray radiography, radionuclide imaging

ISSN:0094-2405    

DOI:10.1118/1.596606

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

CT scanning in spiral geometry is achieved by continuously transporting the patient through the gantry in synchrony with continuous data acquisition over a multitude of 360‐deg scans. Data for reconstruction of images in planar geometry are estimated from the spiral data by interpolation. The influence of spiral scanning on image quality is investigated. Most of the standard physical performance parameters, e.g., spatial resolution, image uniformity, and contrast, are not affected; results differ for pixel noise and slice sensitivity profiles. For linear interpolation, pixel noise is expected to be reduced by a factor of 0.82; reduction factors of 0.81 to 0.83 were measured. Slice sensitivity profiles are changed as a function of table feedd, measured in millimeters per 360‐deg scan; they are smoothed as the original profile is convolved with the object motion function. The motion function is derived for linear interpolation that constitutes a triangle with a base line width of 2dand a maximal height equal to 1/d. Calculations of both the full width at half‐maximum and the shape of the profiles were in good agreement with experimental results. The effect of the widened profiles, in particular of their extended tail ends, on image quality is demonstrated in phantom measurements.

ISSN:0094-2405    

DOI:10.1118/1.596607

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The feasibility of using the time correlated associated particle technique forinvivo14‐MeV neutron activation analysis has been investigated. Gamma rays following neutron inelastic scattering with nitrogen, carbon, and oxygen have been measured with a 12.5×10‐cm NaI(Tl) detector. The results have been scaled to a proposed facility comprising four such detectors past which the subject would be scanned. Based on counting statistics, the precision of estimation of these elements has been determined to be 2.1%, 1.0%, and 1.1%, respectively for experimental measurements on a sample containing physiological concentrations of the major body elements. The average body dose level would be restricted to 0.3 mSv.

ISSN:0094-2405    

DOI:10.1118/1.596714

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The voltage‐ripple dependence relationship on x‐ray energy‐spectral values (energy fluence per unit interval of photon energy) and exposures at 70‐kV peak were obtained theoretically by using the semiempirical formula of emission spectra given by Birch and Marshall [Phys. Med. Biol.24, 505 (1979)]. The calculations were performed with and without various thicknesses of aluminum. As the ripple increases, the energy‐spectral values decrease as expected. When the ripple is large, however, energy‐spectral values (per mAs) take the minimum values; therefore, the exposure (per mAs) also reaches the minimum value for the unsaturating current modes, contrary to expectation. The reasons for this phenomenon were clarified. Exposures clearly take the minimum value in 2‐pulse units. This phenomenon was experimentally verified.

ISSN:0094-2405    

DOI:10.1118/1.596736

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Characteristics of Mg2SiO4(Tb) thermoluminescent dosimeters (TLD) were ascertained preparatory to measuring doses from diagnostic x‐ray examinations received by Adult Health Study participants. These detectors are small, relatively sensitive to low‐dose x rays, and are appropriate for precise dosimetry. Extensive calibration is necessary for precisely determining doses according to their thermoluminescent intensities. Their sensitivities were investigated by dose, according to x‐ray tube voltage, and by exposure direction, to obtain directional dependence. Dosimeter sensitivity lessened due to the fading effect and diminution of the planchet. However, these adverse effects can be avoided by storing the dosimeters at least 1.5 h and by using fresh silver‐plated planchets. Thus the TLDs, for which sensitivities were determined in this study, will be useful in subsequent diagnostic x‐ray dosimetry.

ISSN:0094-2405    

DOI:10.1118/1.596608

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Several x‐ray spectra, including those of the ISO reference radiations, were measured by 2‐hp Ge detectors. Measurements were carried out in different experimental conditions with regard to detector size, beam collimation, and SDD. A stripping procedure to improve the spectrum analysis was developed on the basis of a detailed evaluation (by means of a Monte Carlo method) of the detector's spurious effects. These effects includeK‐photon escape, Compton photon escape, electron escape, and collimation effect. The stripping procedure also allows us to determine directly the spectra of possible scattered radiation reaching the detector in addition to the primary beam. When the primary beam is heavily filtered, the leakage radiation from the x‐ray tube housing scattered onto the detector may not be negligible even when the x‐ray tube is provided with appreciable shielding. Possible practical consequences of these effects are discussed. The results obtained on the ISO x‐ray spectra are in agreement with previous determinations. The advantage of the present procedure is its more immediate applicability to Ge detectors of any size and with different beam collimation conditions.

ISSN:0094-2405    

DOI:10.1118/1.596742

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY