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Dec 1997

Volume 24, Issue 12, pp. 1819-2059

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Digital radiology using active matrix readout of amorphous selenium: Theoretical analysis of detective quantum efficiency

Wei Zhao and J. A. Rowlands

Med. Phys. 24, 1819 (1997); http://dx.doi.org/10.1118/1.598097 (15 pages) | Cited 97 times

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A flat-panel x-ray imaging detector using a layer of amorphous selenium (a-Se) for direct conversion of x rays (to charge) and an active matrix for self-scanned readout is being investigated for digital radiology. A theoretical analysis of the spatial frequency dependent detective quantum efficiency (DQE(f )) of the self-scanned a-Se detector is performed based on a model of signal and noise propagation in a cascaded imaging system. Because of the high intrinsic resolution of a-Se and the pixelated active matrix readout method, such detectors are inherently undersampled and aliasing is present. The presampling modulation transfer function (MTF) and aliased noise power spectrum (NPS) of the detector were used in the analysis of DQE(f ). It is proven that the aliased NPS for the self-scanned a-Se detectors is white. Since the shape of DQE(f ) is determined by the ratio of MTF squared and the NPS, the shape of DQE(f ) follows the square of the presampling MTF of the detector as a result of the white NPS. The analysis also shows that DQE(0) is proportional to the pixel fill factor, i.e., the fraction of each pixel area used for image charge collection. The DQE analysis is applied to detector parameters for three x-ray imaging applications: mammography, chest radiography, and fluoroscopy. The effects of pixel fill factor, imaging geometry (i.e., incident angle of x rays), and various sources of electronic noise on the detector DQE(f ) are discussed. Strategies for maximizing detector DQE for each x-ray imaging application are proposed. © 1997 American Association of Physicists in Medicine.
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87.59.bf Digital radiography
87.63.-d Non-ionizing radiation equipment and techniques

Digital radiology using active matrix readout of amorphous selenium: Construction and evaluation of a prototype real-time detector

Wei Zhao, Ira Blevis, Stephen Germann, J. A. Rowlands, David Waechter, and Zhongshou Huang

Med. Phys. 24, 1834 (1997); http://dx.doi.org/10.1118/1.598098 (10 pages) | Cited 58 times

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The goal of the present work is to develop a large area, flat-panel solid-state detector for both digital radiography and fluoroscopy. The proposed detector employs a photoconductive layer of amorphous selenium (a-Se) to convert x rays into charge. The charge image formed by the a-Se layer is electronically read out in situ using a two dimensional array of thin film transistors (TFTs), or active matrix. Since the active matrix readout is capable of producing x-ray images in real-time, it can potentially be applied in both radiography and fluoroscopy. In this paper, the imaging performance of this concept is investigated using a prototype x-ray imaging detector. The designs for the active matrix, the peripheral electronic circuits, and the image acquisition system are described. Measurements of x-ray imaging properties of the prototype detector, i.e., x-ray sensitivity, presampling modulation transfer function (MTF), and noise power spectrum (NPS), were performed, and from which the spatial frequency dependent detective quantum efficiency (DQE) of the prototype was derived. The experimental results are in agreement with the results of our theoretical analysis. The factors affecting the imaging performance and methods of improvement in the future are discussed. © 1997 American Association of Physicists in Medicine.
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87.59.bf Digital radiography
87.63.-d Non-ionizing radiation equipment and techniques

A multiresolution approach for contour extraction from brain images

Hamid Soltanian-Zadeh and Joe P. Windham

Med. Phys. 24, 1844 (1997); http://dx.doi.org/10.1118/1.598099 (10 pages) | Cited 12 times

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Many image registration methods use head surface, brain surface, or inner/outer surface of the skull to estimate rotation and translation parameters. The inner surface of the skull is also used for intracranial volume segmentation which is considered the first step in segmentation and analysis of brain images. The surface is usually characterized by a set of edge or contour points extracted from cross-sectional images. Automatic extraction of contour points is complicated by discontinuity of edges in the back of the eyes and ears and sometimes by a previous surgery or an inadequate field of view. We have developed an automated method for contour extraction that connects discontinuities using a multiresolution pyramid. Steps of the method are: (1) Contour points are found by an edge-tracking algorithm; (2) A multiresolution pyramid of contour points is constructed; (3) Contour points of reduced images are found; (4) From the continuous contour found at the lowest resolution, contour points at a higher resolution are found; (5) Step 4 is repeated until contour points at the highest resolution (original image) are found. The method runs fast and has been successful in extracting contours from MRI and CT images. We illustrate the method and its performance using MRI and CT images of the human brain. © 1997 American Association of Physicists in Medicine.
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87.61.-c Magnetic resonance imaging
07.05.Pj Image processing
87.59.bd Computed radiography

Determination of 3D positions of pacemaker leads from biplane angiographic sequences

Kenneth R. Hoffmann, Benjamin B. Williams, Jacqueline Esthappan, Shiuh-Yung J. Chen, John D. Carroll, Hajime Harauchi, Vince Doerr, G. Neal Kay, Allen Eberhardt, and Mary Overland

Med. Phys. 24, 1854 (1997); http://dx.doi.org/10.1118/1.598158 (9 pages) | Cited 10 times

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In vitro and in vivo analyses of stress on pacemaker leads and their components during the heart cycle have become especially important because of incidences of failure of some of these mechanical components. For stress analyses, the three-dimensional (3D) position, shape, and motion of the pacemaker leads must be known accurately at each time point during the cardiac cycle. We have developed a method for determination of the in vivo 3D positions of pacemaker leads during the entire heart cycle. Sequences of biplane images of patients with pacemakers were obtained at 30 frames/s for each projection. The sequences usually included at least two heart cycles. After patient imaging, biplane images of a calibration object were obtained from which the biplane imaging geometry was determined. The centerlines of the leads and unique, identifiable points on the attached electrodes were indicated manually for all acquired images. Temporal interpolation of the lead and electrode data was performed so that the temporal nonsynchronicity of the image acquisition was overcome. Epipolar lines, generated from the calculated geometry, were employed to identify corresponding points along the leads in the pairs of biplane images for each time point. The 3D positions of the lead and electrodes were calculated from the known geometry and from the identified corresponding points in the images. Using multiple image sets obtained with the calibration object at various orientations, the precision of the calculated rotation matrix and of the translation vector defining the imaging geometry was found to be approximately 0.7° and 1%, respectively. The 3D positions were reproducible to within 2 mm, with the error lying primarily along the axis between the focal spot and the imaging plane. Using data obtained by temporally downsampling to 15 frames/s, the interpolated data were found to lie within approximately 2 mm of the true position for most of the heart cycle. These results indicate that, with this technique, one can reliably determine pacemaker lead positions throughout the heart cycle, and thereby it will provide the basis for stress analysis on pacemaker leads. © 1997 American Association of Physicists in Medicine.
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87.59.Dj Angiography
07.05.Pj Image processing

Molybdenum, rhodium, and tungsten anode spectral models using interpolating polynomials with application to mammography

John M. Boone, Thomas R. Fewell, and Robert J. Jennings

Med. Phys. 24, 1863 (1997); http://dx.doi.org/10.1118/1.598100 (12 pages) | Cited 143 times

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Computer simulation is a convenient and frequently used tool in the study of x-ray mammography, for the design of novel detector systems, the evaluation of dose deposition, x-ray technique optimization, and other applications. An important component in the simulation process is the accurate computer-generation of x-ray spectra. A computer model for the generation of x-ray spectra in the mammographic energy range from 18 kV to 40 kV has been developed. The proposed model requires no assumptions concerning the physics of x-ray production in an x-ray tube, but rather makes use of x-ray spectra recently measured experimentally in the laboratories of the Center for Devices and Radiological Health. Using x-ray spectra measured for molybdenum, rhodium, and tungsten anode x-ray tubes at 13 different kV’s (18, 20, 22,…,42 kV), a spectral model using interpolating polynomials was developed. At each energy in the spectrum, the x-ray photon fluence was fit using 2, 3, or 4 term (depending on the energy) polynomials as a function of the applied tube voltage (kV). Using the polynomial fit coefficients determined at each 0.5 keV interval in the x-ray spectrum, accurate x-ray spectra can be generated for any arbitrary kV between 18 and 40 kV. Each anode material (Mo, Rh, W) uses a different set of polynomial coefficients. The molybdenum anode spectral model using interpolating polynomials is given the acronym MASMIP, and the rhodium and tungsten spectral models are called RASMIP and TASMIP, respectively. It is shown that the mean differences in photon fluence calculated over the energy channels and over the kV range from 20 to 40 kV were −0.073% (σ=1.58%) for MASMIP, −0.145% (σ=1.263%) for RASMIP, and 0.611% (σ=2.07%) for TASMIP. The polynomial coefficients for all three models are given in an Appendix. A short C subroutine which uses the polynomial coefficients and generates x-ray spectra based on the proposed model is available on the World Wide Web at http://www.aip.org/epaps/epaps.html.© 1997 American Association of Physicists in Medicine.
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87.59.E- Mammography
02.10.De Algebraic structures and number theory
02.60.Ed Interpolation; curve fitting
07.05.Tp Computer modeling and simulation

The accuracy and reproducibility of a global method to correct for geometric image distortion in the x-ray imaging chain

Ed Gronenschild

Med. Phys. 24, 1875 (1997); http://dx.doi.org/10.1118/1.598101 (14 pages) | Cited 41 times

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A method to correct for geometric image distortion in the x-ray imaging chain, so-called dewarping, has been developed. A global two-dimensional polynomial model of which the degree is optimized is used. The performance of the method has been tested in a number of experiments using images of a plate with a 1 cm spaced wire grid put against the input screen of the x-ray image intensifier (14/17/27 cm). Both offline cine film and online video images were analyzed. The accuracy of the dewarp method was derived from the acquired images and from computer-simulated distorted images. The robustness and reproducibility of the dewarp method was evaluated by means of imaging the grid in various random orientations. Three parameters describing the behavior of the algorithm were considered. One is the reproducibility of the location of a dewarped position. The second parameter is the reproducibility of the distance between two adjacent dewarped positions as a measure of the reproducibility of the size of an object under investigation. The third parameter is the reproducibility of the pixel size in the plane of the calibration plate. The major results are: the reproducibility of the location of a dewarped position was 0.01–0.04 mm for cine film and 0.04–0.07 mm for video images. The coefficient of variation of the distance between two dewarped positions was 0.04%–0.11% for cine film and 0.15%–0.18% for video images. The dewarp algorithm turned out to be fast and accurate and the distortion was removed over the whole image field down to a low random residual level. It was found that a random orientation of the grid did not affect the assessment of the distortion nor its correction. The dewarp method proved to be intrinsically robust and highly reliable. Time instability of the imaging chain was the main source of variability in the dewarp results. © 1997 American Association of Physicists in Medicine.
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87.59.bf Digital radiography
07.05.Pj Image processing

Is the indicator dilution theory really the adequate base of many blood flow measurement techniques?

P.-A. Doriot, P.-A. Dorsaz, L. Dorsaz, and W. J. Rutishauser

Med. Phys. 24, 1889 (1997); http://dx.doi.org/10.1118/1.598102 (10 pages) | Cited 6 times

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The indicator dilution theory is the underlying model of many blood flow measurement techniques used daily in hospitals, for instance in cardiac catheterization laboratories. The basic version of this theory applies to a “stationary” flow system with one inlet and one outlet, into which a small amount M of indicator is injected “suddenly” at time t=0 at the inlet. The quintessence of the theory consists in three equations, which themselves result from some apparently simple assumptions about the considered flow systems. The first equation states that the (constant) flow Q through the system can be calculated by use of the known amount of indicator, M, and of the indicator concentration-time curve c(t) recorded at the outlet. The second one allows the calculation of the “mean transit time” t∗ of fluid and indicator particles through the system from the curve c(t). The third equation, V=Qt∗, yields the system volume V. It is generally believed that these three equations would be absolutely valid if the assumptions of the theory could be perfectly fulfilled. We show, by considering a simple model, that all three equations are actually incorrect for most flow systems when the detector used to record the curve c(t) is of the “trans-illumination” type, as is the case for instance in dye dilution methods and in many angiographic or CT techniques. A further consequence is that t∗, which is truly the “center of mass” of the concentration-time curve c(t), does not have the well known property of being the adequate parameter for flow determinations. Many flow measurement techniques thus appear to have no theoretical base. © 1997 American Association of Physicists in Medicine.
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87.19.U- Hemodynamics
87.19.Wx Pneumodyamics, respiration
87.10.-e General theory and mathematical aspects

Temperature monitoring of ultrasonically heated muscle with RARE chemical shift imaging

Robert V. Mulkern, Andrew H. Chung, Ferenc A. Jolesz, and Kullervo Hynynen

Med. Phys. 24, 1899 (1997); http://dx.doi.org/10.1118/1.598103 (8 pages) | Cited 8 times

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The ability to monitor tissue temperature in ultrasonically heated rabbit muscle is demonstrated using a chemical shift imaging approach based on the rapid acquisition with relaxation enhancement (RARE) fast imaging method [Hennig et al., Magn. Reson. Med. 3, 823–833 (1986)] applied in a line scan format. A three echo sequence with a 16 Hz spectral resolution with 64 ms echo readouts and 78 ms echo spacings is shown capable of measuring relevantly small water frequency shifts in phantoms. Applied to the in vivo model of ultrasonically heated rabbit muscle, water resonance frequencies at the ultrasonic focal point were found to be linearly related to temperature with a slope of −0.007 ± 0.001 ppm/°C (N=6 studies). Measurements of the frequency shift in unheated tissue located away from the ultrasonically heated tissue varied by approximately 0.011 ppm over the course of the experiments, leading to an estimated temperature accuracy of ±1.6 °C in vivo. © 1997 American Association of Physicists in Medicine.
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87.50.wp Therapeutic applications
87.61.-c Magnetic resonance imaging
87.19.Pp Biothermics and thermal processes in biology
87.50.C- Static and low-frequency electric and magnetic fields effects

Rapid measurement of Gd-DTPA extraction fraction in a dialysis system using echo-planar imaging

Eric R. Niendorf, Thomas M. Grist, Richard Frayne, Peter C. Brazy, and Giles E. Santyr

Med. Phys. 24, 1907 (1997); http://dx.doi.org/10.1118/1.598104 (7 pages) | Cited 11 times

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Gd-DTPA (Magnevist, Berlex, Wayne, NJ) extraction fractions (EF) have been measured for three dialysis filter types using an echo-planar imaging (EPI) Look-Locker T1 measurement technique under conditions of fast and slow flow. The mean EF measured in Fresenius (Bad Homburg, Germany) F3, F6, and F8 dialysis filters were 0.015±0.005, 0.053±0.004, and 0.084±0.003, respectively, under conditions of fast flow which provided complete refreshment of spins in the intervals between read-out pulse samples of the T1 relaxation recovery. Data acquisition and post-processing techniques were developed to extend the accuracy of the LL technique to systems with slow flow which did not provide complete refreshment of spins between samples of the T1 recovery. A multi-shot EPI LL interleaved acquisition of relaxation recovery space (IRRS) provided T1 measurement accuracy comparable to the refreshed system, ±10, but at the expense of increased scan times (factor of 2 or 3). Discarding the first few non-equilibrium relaxation recovery samples from the T1 fit allowed accurate T1 estimation (±10) with a single-shot EPI LL method under conditions of slow flow. These EPI LL EF measurement methods may provide useful techniques for evaluating renal function in vivo. © 1997 American Association of Physicists in Medicine.
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87.61.-c Magnetic resonance imaging

Enhanced cross-talk correction technique for simultaneous dual-isotope imaging: A TL-201/Tc-99m myocardial perfusion SPECT dog study

Karin Knešaurek and Josef Machac

Med. Phys. 24, 1914 (1997); http://dx.doi.org/10.1118/1.598105 (10 pages) | Cited 5 times

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A newly developed cross-talk correction method for simultaneous dual-isotope SPECT imaging was tested in a canine model. The method is based on the assumption that the transformations, which modify the primary energy window images into the scatter images as viewed in the other energy windows, are known. These transformations were found by measuring the point spread functions (PSFs) in two different energy windows for both isotopes in water. The dual-isotope correction method is described by two convolution equations which were applied in frequency space. The equations take into account the different spatial distributions of the primary and scatter cross-talk photons. The new enhancement of the method was in applying restoration filters to the resulting corrected images. Three separate studies were acquired in our dog study: two single-isotope and one dual-isotope study. The single isotope images were used as references. The contrast between the left ventricle cavity (LVC) and the myocardium was used in transaxial and short-axis slices as a parameter to evaluate results of dual-isotope correction method with restoration. The change in contrast in the dual-isotope corrected images in both energy windows, i.e., Tc-99m primary window (140 keV) and Tl-201 primary window (70 keV), was significant. The only exception was for the short-axis Tc-99m window images. The corrected 140 keV dual-isotope short-axis slice had the contrast of 0.60 vs 0.58, which was the value in the noncorrected dual-isotope short-axis slice. For dual-isotope 140 keV transaxial slice, the contrast changed from 0.72 to 0.82 after correction. In comparison, for single-isotope Tc-99m 140 keV transaxial slice, contrast changed from 0.62 to 0.84 after restoration correction. There was less change in contrast in the short-axis Tc-99m 140 keV slice, i.e., from 0.56 to 0.61. In the Tl-201 primary window for the transaxial slices the improvement of contrast was from 0.38 to 0.64, and for short-axis slices from 0.22 to 0.32 after correction. In the same 70 keV energy window for single-isotope Tl-201 images, contrast improved from 0.61 to 0.69 and from 0.35 to 0.38 for transaxial and short-axis slice, respectively, after applying restoration correction. In conclusion, the presented dual-isotope correction method with restoration improves the quality of the simultaneous rest Tl-201/stress Tc-99m sestamibi SPECT imaging. © 1997 American Association of Physicists in Medicine.
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87.57.uh Single photon emission computed tomography (SPECT)
07.05.Pj Image processing
87.19.rh Fluid transport and rheology
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Medical Physics is an official science journal of the AAPM and of the COMP/CCPM/IOMP
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Departments of Radiology, Radiation Oncology, Biophysics, Community and Public Health, Medical College of Wisconsin
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