摘要:

We are developing a technique for determination of the three‐dimensional (3‐D) structure of vascular objects from two radiographic projection images acquired at arbitrary and unknown relative orientations. No separate calibration steps are required with this method, which exploits an inherent redundancy of biplane imaging to extract the imaging geometry as well as the 3‐D locations of eight or more object points. The theoretical basis of this technique has been described previously. In this paper, we review the method from the perspective of linear algebra and describe an improvement, not heretofore reported, that reduces the method's sensitivity to experimental error. We then examine the feasibility and inherent accuracy of this approach by computer simulation of biplane imaging experiments. The precision with which 3‐D object structure may be retrieved, together with the dependence of precision on the actual imaging geometry and errors in various measured quantities, is studied in detail. Our simulation studies show that the method is not only feasible but potentially accurate, typically determining object‐point configurations with root‐mean‐square (RMS) error on the order of 1 to 2 mm. The method is also quite fast, requiring approximately one second of CPU time on a VAX 11/750 computer (0.6 MIPS).

ISSN:0094-2405    

DOI:10.1118/1.596462

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

A curve fitting technique combined with an angulated slit image has been developed for the measurement of the presampling modulation transfer function (MTF) of film digitizers. The noisy line spread functions (LSFs) acquired from an angulated slit image are fitted using a combination of two functions by means of a nonlinear least‐square fitting technique. The parameters in the model function for each LSF are obtained by minimizing the residual root mean square (RMS), and then averaged over all the LSF fittings. The resulting analytical function is representative of the continuous presampled LSF. We have found that a combination of Gaussian and exponential functions provides a good fit to the LSFs obtained with film digitizers. The corresponding analytical Fourier transformation of the model function yields the presampling MTF, without Nyquist frequency limitation. Measurements of spatial resolution properties using this method were performed for two laser scanners and an optical drum scanner.

ISSN:0094-2405    

DOI:10.1118/1.596463

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

This paper deals with methods of reducing the total time required to acquire the projection data for a set of contiguous computed tomography (CT) images. Normally during the acquisition of a set of slices, the patient is held stationary during data collection and translated to the next axial location during an interscan delay. We demonstrate using computer simulations and scans of volunteers on a modified scanner how acceptable image quality is achieved if the patient translation time is overlapped with data acquisition. If the concurrent patient translation is ignored, structured artifacts significantly degrade resulting reconstructions. We present a number of weighting schemes for use with the conventional convolution/backprojection algorithm to reduce the structured artifacts through the use of projection modulation using the data from individual and multiple slices. We compare the methods with respect to structured artifacts, noise, resolution and to patient motion. Review of preliminary results by a panel of radiologists indicates that the residual image degradation is tolerable for selected applications when it is critical to acquire more slices in a patient breathing cycle than is possible with conventional scanning.

ISSN:0094-2405    

DOI:10.1118/1.596464

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Many applications in radiography require, or would benefit from, the ability to translate,i.e. move, an optical image in the detector plane. In this paper, we describe the design and characterization of a prism‐based optical image translator for insertion into existing XRII‐video imaging systems. A pair of prisms rotatable about the optical axis form a very compact in‐line optical image translator for installation in the parallel light path between an x‐ray image intensifier and its video camera. Rotation of the prisms translates the XRII optical image on the camera target. With the addition of x‐ray and light collimators to limit the image to a single video line, x‐ray streak images may be acquired. By rotating an object in the x‐ray beam during a streak, a complete computed tomography (CT) data set may be acquired. This image translator can translate an image anywhere in the focal plane of a 50‐mm‐output lens within a 40‐mm‐diam circle. The prisms have an aperture of 50 mm, permitting an optical speed of F/2 with a 50‐mm output lens. The design is insensitive to angular alignment errors. This image translator is achromatic, since the spectral width of the output phosphors of image intensifiers is sufficient to introduce blurring in a nonachromatic design. A prism‐based image translator introduces image distortion, since the prisms do not operate at minimum deviation. The distortion is less than 4% over all parts of a typical detector area, and less than 1% in the central region of the image. The prisms were found not to degrade the modulation‐transfer function of an optical system.

ISSN:0094-2405    

DOI:10.1118/1.596550

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Sources of variations in the measurement of half‐value layer (HVL) in film–screen mammography are evaluated. Two principal sources are the energy responses of ionization chambers and the elemental compositions of the 1100 aluminum‐alloy attenuation materials. Methods in measuring the thickness of attenuation materials are also an important potential source of error. These factors as well as the influence of geometry of measurement, position of measurement in the field, and precision of radiation output are examined. Recommendations for consistency in measurement are provided.

ISSN:0094-2405    

DOI:10.1118/1.596444

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Values of the effective dose equivalent, HE, per unit entrance skin dose (+backscatter) are presented for twelve common radiological examinations. These HEdata are derived from organ doses calculated for standard man using Monte Carlo techniques and are based on a specific beam size. The resultant HEdata are presented as functions of x‐ray kVp, beam filtration (mm Al) and the projection (anterior–posterior (AP); posterior–anterior (PA); lateral; or oblique) used. In addition, two methods are presented which permit the computation of HEvalues for standard man for radiological examinations, which employ different field sizes.

ISSN:0094-2405    

DOI:10.1118/1.596445

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Subsecond fast low‐angle shot (FLASH) magnetic resonance imaging (MRI) allows single shot studies of the human heart within measuring times of about 100–300 ms depending on the data matrix. In contrast to conventional FLASH MRI subsecond applications acquire dataduringthe approach to steady state. A detailed analysis of the saturation behavior of the signal is given for the ideal case of a rectangular slice profile. In a second step, realistic slice profiles assuming Gaussian‐shaped excitation pulses were taken into account by means of a numerical solution of the Bloch equations. It turns out that the signal strength and the resulting image intensity is considerably higher than may be expected from steady‐state considerations. Correspondingly optimized flip angles depend on the number of phase‐encoding steps. Assuming longT1relaxation times as, for example, encountered in muscle and brain tissue and repetition times of 5 ms or less, optimum flip angles are 12°–16°. The use of even higher flip angles (≥20°) causes heavily distorted slice profiles and a dynamic increase of the effective slice thickness. Flip angles of the order of the Ernst angle (6°) correspond to steady‐state conditions and lead to considerable signal losses. The theoretical results are confirmed by subsecond FLASH MRI studies of the human heart using a 2.0 T whole‐body system (Siemens Magnetom).

ISSN:0094-2405    

DOI:10.1118/1.596452

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

A comparison of two methods of scatter compensation in single photon emission computed tomography (SPECT) imaging is made on the basis of improvements in quantification. The methods, scatter‐window subtraction and constrained deconvolution of an average point source response function (PSRF), are described; the theoretical basis of each method is also briefly assessed. Improvements in relative quantification offered by each method are measured by examining ratios of counts in hot cylinders to counts in a slightly radioactive background. The cylinder/background concentration ratio was varied by a factor of five; the sizes of the cylinders remained constant. Keeping the diameter of the cylinders constant allowed for an assessment of the effect of concentration differences, without the conflicting effect of variation in the size of the hot source. Results showed that while both scatter‐window subtraction and constrained deconvolution offer quantification improvements in the final image, the method of prereconstruction deconvolution of a planar PSRF from each planar projection is substantially more successful than either scatter‐window subtraction or other methods of implementing the deconvolution procedure.

ISSN:0094-2405    

DOI:10.1118/1.596453

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

A mathematical model is used to represent the vascular bed of the left coronary circulation by an arterial, a capillary and a venous compartment. The model is first adjusted so as to reproduce arterial hemodynamics known from measurements during normal perfusion. Additionally, measurements under coronary sinus occlusion are used to assess the venous section of the model. While the calculated phasic shapes of epicardial flows are seen to agree with measurements, intramyocardial flows, which are inaccessable to measurement, are predicted from the model. The model is run under stepwise changes of coronary perfusion pressure and coronary artery resistance for both the normal state and coronary sinus occlusion. Intramyocardial flow between capillaries and veins, being the main determinant for a possible therapeutic effect of coronary sinus interventions, is estimated.

ISSN:0094-2405    

DOI:10.1118/1.596454

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Dose rates along the transverse axes of125I model 6702,125I model 6711 and192Ir 0.2‐mm steel sources for interstitial brachytherapy have been measured in a solid‐water phantom for distances up to 10 cm using LiF thermoluminescent dosimeters (TLDs). Specific dose rate constants, the dose rates in water per unit source strength 1 cm along the perpendicular bisector of the source, are determined to be 0.90±0.03, 0.85±0.03, and 1.09±0.03 cGy h−1U−1for125I model 6702,125I model 6711 and192Ir 0.2‐mm steel sources, respectively (1 U=unit of air kerma strength =1 μGy m2h−1=1 cGy cm2h−1). In older and obsolete units of source strength (i.e., mCi apparent), these are 1.14±0.03, 1.08±0.03, and 4.59±0.15 cGy h−1mCi−1(apparent). Currently accepted values of specific dose rate constant for125I sources are up to 20% higher than our measured values which are in good agreement with the results of our Monte Carlo simulations. But for192Ir there is good agreement between our measured value of the specific dose rate constant and currently accepted values. The radial dose function for125I model 6702 is found to be consistently larger than that for125I model 6711, with an increasing difference as the distance from the source increases. Our measured values for the radial dose function for125I sources are in good agreement with the results of our Monte Carlo simulation as well as the measured values of Schelletal. [Int. J. Radiat. Oncol. Biol. Phys.13, 795–799 (1987)] for model 6702 and Lingetal. [Int. J. Radiat. Oncol. Biol. Phys.9, 1747–1752 (1983)] for model 6711. However, some of the recently reported Monte Carlo values of the radial dose function for125I sources are significantly larger than measured values; up to 18% at a distance of 5 cm. Our measured radial dose function for the192Ir seed is in good agreement with our Monte Carlo calculated values, and with both our earlier data for the high activity192Ir source of a remote afterloader and recommended values by Meisbergeretal. [Radiol.90, 953–957 (1968)].

ISSN:0094-2405    

DOI:10.1118/1.596584

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY