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Dec 2000

Volume 27, Issue 12, pp. 2641-2830

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POINT/COUNTERPOINT: The Ph.D. degree is a handicap in the job market for clinical medical physicists

J. Daniel Bourland, David S. Marsden, and William R. Hendee, Moderator

Med. Phys. 27, 2641 (2000); http://dx.doi.org/10.1118/1.1328385 (3 pages)

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Abstract Unavailable
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01.40.-d Education
87.00.00 Biological and medical physics

DIAGNOSTIC IMAGING PHYSICS: A statistical methodology for mammographic density detection

John J. Heine and Robert P. Velthuizen

Med. Phys. 27, 2644 (2000); http://dx.doi.org/10.1118/1.1323981 (8 pages) | Cited 15 times

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A statistical methodology is presented based on a chi-square probability analysis that allows the automated discrimination of radiolucent tissue (fat) from radiographic densities (fibroglandular tissue) in digitized mammograms. The method is based on earlier work developed at this facility that shows mammograms may be considered as evolving from a linear filtering operation where a random input field is passed through a 1/f filtering process. The filtering process is reversible which allows the solution of the input field with knowledge obtained from the raw image (the output). The input field solution is analogous to a prewhitening technique or deconvolution. This field contains all the information of the raw image in a much simplified format that can be approximated and analyzed with parametric methods. In the work presented here evidence indicates that there are two random events occurring in the input field with differing variances: (1) one relating to fat tissue with the smaller variance, and (2) the second relating to all other tissue with the larger variance. A statistical comparison of the variances is made by scanning the image with a small search window. A relaxation method allows for making a reliable estimate of the smaller variance which is considered as the global reference. If a local variance deviates significantly from the reference variance, based on chi-square analysis, it is labeled as nonfat; otherwise it is labeled as fat. This statistical test procedure results in a region by region continuous labeling of fat and nonfat tissue across the image. In the work presented here, the emphasis is on the methodology development with supporting preliminary results that are very encouraging. It is widely accepted that mammographic density is a breast cancer risk factor. An important application of this work is to incorporate density-based risk analysis into the ongoing statistical-based detection work developed at this facility. Additional applications include risk analysis dependent on either percentages or total amounts of fat or dense tissue. This work may be considered as the initial step in introducing many of the known breast cancer risk factors into the actual image data analysis. © 2000 American Association of Physicists in Medicine.
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87.59.E- Mammography
02.50.Cw Probability theory
87.57.N- Image analysis
42.30.Va Image forming and processing

DIAGNOSTIC IMAGING PHYSICS: Phototimer setup for CR imaging

Emmanuel G. Christodoulou, Mitchell M. Goodsitt, Heang-Ping Chan, and Thomas W. Hepburn

Med. Phys. 27, 2652 (2000); http://dx.doi.org/10.1118/1.1319522 (7 pages) | Cited 11 times

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A study was performed to investigate the feasibility of using the standard deviation (σ) of the pixel values in a computed radiography (CR) image and a measure of the median incident exposure on the imaging plate (IP) as parameters for setting up phototimers in a CR system. Slabs of Lucite™ 4-, 6-, and 8-in.-thick were imaged with a CR system at 70, 90, and 125 kVp at various mA s values both with grid and without grid. Incident IP exposures were measured with an ionization chamber. Images were analyzed on a workstation. The σ’s in the “flat field” images were found to be approximately related to the mean incident exposure E by the relationship: σ∝E−1/2, indicating the quantum-noise-limited operation of the system. Derived relationships between the reading sensitivity of the (IP) reader (S number) and σ can be used to obtain images with a specific noise level. At our institution, where a 400 speed screen–film system is used for general radiography and 200 speed for chest radiography, radiologists generally find CR image quality acceptable when σ⩽11 (S⩽400) for general radiography (50–90 kVp), and σ⩽8 (S⩽200) for chest radiography (125 kVp). However, factors other than the amount of x-ray quanta that form the useful image, such as the image processing mode and the amount of collimation, may affect both the sensitivity value and the image quality. © 2000 American Association of Physicists in Medicine.
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87.59.B- Radiography
85.60.Ha Photomultipliers; phototubes and photocathodes
29.40.Cs Gas-filled counters: ionization chambers, proportional, and avalanche counters

DIAGNOSTIC IMAGING PHYSICS: A dose reduction x-ray beam positioning system for high-speed multislice CT scanners

Thomas L. Toth, Neil B. Bromberg, Tin-Su Pan, Jerry Rabe, Steven J. Woloschek, Jianying Li, and George E. Seidenschnur

Med. Phys. 27, 2659 (2000); http://dx.doi.org/10.1118/1.1323983 (10 pages) | Cited 13 times

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The introduction of multislice CT scanners and the associated dose increase compared to single and dual slice scanners has concerned many radiologists, health and medical physicists, as well as members of the regulatory community. Since multislice CT scanners are inherently post-patient collimated, they are less dose efficient than single slice CT scanners, which use prepatient collimation. The x-ray beam must be wide enough in the Z axis so that the beam remains on the detector in spite of typical movements due to thermal and mechanical flexing. We describe the x-ray beam tracking system that is employed on a GE LightSpeed QX/i® scanner to substantially reduce the multislice dose. The tracking system stabilizes the beam on the detector allowing a narrower x-ray exposure profile compared to the x-ray exposure profile without tracking. The tracking system measures the position of the beam every few milliseconds and continually repositions a source aperture to hold a narrow beam fixed on the detector. Using a standard LightSpeed QX/i® source collimator and segmented detector, dose reductions of up to 40% were measured when tracking was employed. We also show that tracking has the potential to provide a dose efficiency approaching single slice scanners. © 2000 American Association of Physicists in Medicine.
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87.59.bd Computed radiography
87.63.-d Non-ionizing radiation equipment and techniques

DIAGNOSTIC IMAGING PHYSICS: Image quality evaluation of a desktop computed radiography system

Kenneth A. Fetterly and Nicholas J. Hangiandreou

Med. Phys. 27, 2669 (2000); http://dx.doi.org/10.1118/1.1326449 (11 pages) | Cited 19 times

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The modulation transfer function (MTF), noise power spectrum (NPS), and detective quantum efficiency (DQE) of the Lumisys ACR-2000 desktop computed radiography (CR) reader were measured and compared to equivalent measurements acquired from a Fuji AC-3 CR system. The one-dimensional (1D) MTF was measured from an image of a sharp edge and the 1D NPS was derived from a 2D NPS measured from a uniform field exposure. The energy dependent ideal input signal to noise ratio of the incident x-ray beams was estimated using published x-ray spectra and attenuation coefficients. Measurements were acquired using Agfa, Fuji, and Kodak storage phosphor plates and it was concluded that use of the Fuji plates resulted in the highest system DQE for the ACR-2000. The DQE was measured using exposures of 0.10, 1.0, and 10.0 mR from 70 and 120 kVp x-ray beams filtered with aluminum. The DQE of the Lumisys ACR-2000 was lower than that of the Fuji AC-3. © 2000 American Association of Physicists in Medicine.
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87.59.bf Digital radiography
87.63.-d Non-ionizing radiation equipment and techniques

DIAGNOSTIC IMAGING PHYSICS: Modification and benchmarking of MCNP for low-energy tungsten spectra

J. R. Mercier, D. T. Kopp, W. D. McDavid, S. B. Dove, J. L. Lancaster, and D. M. Tucker

Med. Phys. 27, 2680 (2000); http://dx.doi.org/10.1118/1.1288397 (8 pages) | Cited 18 times

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The MCNP Monte Carlo radiation transport code was modified for diagnostic medical physics applications. In particular, the modified code was thoroughly benchmarked for the production of polychromatic tungsten x-ray spectra in the 30–150 kV range. Validating the modified code for coupled electron–photon transport with benchmark spectra was supplemented with independent electron-only and photon-only transport benchmarks. Major revisions to the code included the proper treatment of characteristic K x-ray production and scoring, new impact ionization cross sections, and new bremsstrahlung cross sections. Minor revisions included updated photon cross sections, electron–electron bremsstrahlung production, and K x-ray yield. The modified MCNP code is benchmarked to electron backscatter factors, x-ray spectra production, and primary and scatter photon transport. © 2000 American Association of Physicists in Medicine.
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02.70.Rr General statistical methods
87.59.-e X-ray imaging

NUCLEAR MEDICINE PHYSICS: An improved method for rapid objective measurement of gamma camera resolution

Trish A. Hander, Jack L. Lancaster, William McDavid, and David T. Kopp

Med. Phys. 27, 2688 (2000); http://dx.doi.org/10.1118/1.1328083 (5 pages)

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An improved method for an easy, rapid measurement of the intrinsic spatial resolution of a gamma camera is presented. A simplified model was previously developed based solely on mean and standard deviation measurements taken from a region of interest in bar pattern images. This led to an estimate of the modulation transfer function and the full width at half maximum (FWHM) of a line spread function (LSF). The improved method involved expanding the simplified model to incorporate input modulation, square wave input versus the assumed sinusoidal input, aperture (pixel) size, and scatter from the plastic in the bar pattern. The input square wave modulation was calculated to be unity for the typical bar patterns used for gamma camera quality control assessment. For the typical range of bar sizes available, in combination with the typical resolving capabilities of gamma cameras, the sinusoidal approximation of the bar pattern was found to be valid (<1% contribution to the measured resolution from higher harmonic frequencies present in a square wave input with an effective input modulation greater than unity by a factor of 4/π). The aperture correction factor was calculated for numerous bar and pixel size combinations. Applying the aperture correction factor results in an improvement in the accuracy of the calculated FWHM values, especially for large apertures (pixel sizes). For a camera with a specified FWHM value of 3.5 mm, the simplified model predicts values ranging from 3.2 to 4.1 mm, when the acquisition matrix varies from 1282 to 5122. When the expanded model is used with the aperture correction applied, this range was reduced to 3.6–3.9 mm. The scatter correction further improved the calculated FWHM (from 3.6 to 3.8 mm). It is suggested that the expanded model should be used when more accurate measurements are desired, such as in acceptance testing. © 2000 American Association of Physicists in Medicine.
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87.57.U- Nuclear medicine imaging
87.63.-d Non-ionizing radiation equipment and techniques
87.56.-v Radiation therapy equipment

OPTICAL IMAGING: Contrast-detail analysis for detection and characterization with near-infrared diffuse tomography

Brian W. Pogue, Claire Willscher, Troy O. McBride, Ulf L. Österberg, and Keith D. Paulsen

Med. Phys. 27, 2693 (2000); http://dx.doi.org/10.1118/1.1323984 (8 pages) | Cited 32 times

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Near-infrared (NIR) diffuse tomography is emerging as a medical imaging modality for obtaining information related to tissue hemoglobin concentration and oxygen saturation and may be used for characterizing diseased tissues such as breast cancer. The optimal methodology for NIR image reconstruction remains an ongoing research problem with several new approaches being demonstrated in recent years. However, a comparison of reconstruction methods is problematic because tools for the objective assessment of image quality have yet to be clearly defined for this type of nonlinear reconstruction problem. Contrast-detail analysis has become an accepted assessment tool to quantify x-ray mammography image quality, and in this study it has been applied to a prototype NIR diffuse tomography system that is being evaluated for breast cancer characterization. The minimum detectable levels of contrast have been defined for different sizes of objects, and the minimum contrasts which can be accurately reconstructed have also been determined for the same object sizes. In general, objects 8 mm and larger in diameter can be accurately reconstructed and detected for most absorption contrasts which are observed in human tissues (i.e., greater than 1% contrast in absorption). Objects as small as 2 mm can be detected with high contrast (i.e., near 100%), but cannot be accurately reconstructed. Within the size range of 2 mm to 8 mm, there is an inverse correlation between contrast and detail size which is characteristic of the total noise in the system. This analysis provides an objective method for assessing detection and characterization limits and can be applied to future improvements in hardware system architecture as well as reconstruction algorithms. © 2000 American Association of Physicists in Medicine.
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87.63.Hg Thermography
87.57.N- Image analysis

RADIATION THERAPY PHYSICS: Testing of the stability of intensity modulated beams generated with dynamic multileaf collimation, applied to the MM50 Racetrack Microtron

M. L. P. Dirkx and B. J. M. Heijmen

Med. Phys. 27, 2701 (2000); http://dx.doi.org/10.1118/1.1326450 (7 pages) | Cited 6 times

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Recently, we have published a method for the calculation of required leaf trajectories to generate optimized intensity modulated x-ray beams by means of dynamic multileaf collimation [Phys. Med. Biol. 43, 1171–1184 (1998)]. For the MM50 Racetrack Microtron it has been demonstrated that the dosimetric accuracy of this method, in combination with the dose calculation algorithm of the Cadplan 3D treatment planning system, is adequate for a clinical application (within 2% or 0.2 cm). Prior to initiating patient treatment with dynamic multileaf collimation (DMLC), tests have been performed to investigate the stability of DMLC fields generated at the MM50, (i) in time, (ii) subject to gantry rotation and (iii) in case of treatment interrupts, e.g., caused by an error detected by the treatment machine. The stability of relative dose profiles, normalized to a reference point in a relatively flat part of the modulated beam profile, was assessed from measurements with an electronic portal imaging device (EPID), with a linear diode array attached to the collimator and with film. The dose in the reference point was monitored using an ionization chamber. Tests were performed for several intensity modulated fields using 10 and 25 MV photon beams. Based on film measurements for sweeping 0.1 cm leaf gaps it was concluded that in an 80 days period the variation in leaf positioning was within 0.05 cm, without requiring any recalibration. For a uniform 10×10 cm2 field, realized dynamically by a scanning 0.4×10 cm2 slit beam, a maximum variation in slit width of 0.01 cm was derived from ionization chamber measurements, both in time and for gantry rotation. For a clinical example, the dose in the reference point reproduced within 0.2% (1 SD) over a period of 100 days. Apart from regions with very large dose gradients, variations in the relative beam profiles measured with the EPID were generally less than 1% (1 SD). For different gantry angles the dose profiles also reproduced within 1%, showing that gravity has a negligible influence. No significant deviations between uninterrupted and interrupted treatments could be observed, indicating that the effects of acceleration and deceleration of the leaves are negligible and that a DMLC treatment can be finished correctly after a treatment interrupt. Our previous and present studies have demonstrated that the dosimetric accuracy and stability of intensity modulated beams, generated at the MM50 by means of dynamic multileaf collimation, are adequate for clinical use. Patient treatment using dynamic multileaf collimation has been started in our clinic. © 2000 American Association of Physicists in Medicine.
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87.56.J- Collimation
87.53.Bn Dosimetry/exposure assessment
29.20.dg Cyclotrons
29.40.Cs Gas-filled counters: ionization chambers, proportional, and avalanche counters
87.55.-x Treatment strategy
07.85.Fv X- and γ-ray sources, mirrors, gratings, and detectors

RADIATION THERAPY PHYSICS: Monte Carlo and experimental investigations of multileaf collimated electron beams for modulated electron radiation therapy

Michael C. Lee, Steve B. Jiang, and C.-M. Ma

Med. Phys. 27, 2708 (2000); http://dx.doi.org/10.1118/1.1328082 (11 pages) | Cited 33 times

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Modulated electron radiation therapy (MERT) has been proposed as a means of delivering conformal dose to shallow tumors while sparing distal structures and surrounding tissues. Conventional systems for electron beam collimation are labor and time intensive in their construction and are therefore inadequate for use in the sequential delivery of multiple complex fields required by MERT. This study investigates two proposed methods of electron beam collimation: the use of existing photon multileaf collimators (MLC) in a helium atmosphere to reduce in-air electron scatter, and a MLC specifically designed for electron beam collimation. Monte Carlo simulations of a Varian Clinac 2100C were performed using the EGS4/BEAM system and dose calculations performed with the MCDOSE code. Dose penumbras from fields collimated by photon MLCs both with air and with helium at 6, 12, and 20 MeV at a range of SSDs from 70 to 90 cm were examined. Significant improvements were observed for the helium based system. Simulations were also performed on an electron specific MLC located at the level of the last scraper of a 25×25 cm2 applicator. A number of leaf materials, thicknesses, end shapes, and widths were simulated to determine optimal construction parameters. The results demonstrated that tungsten leaves 15 mm thick and 5 mm wide with unfocused ends would provide sufficient collimation for MERT fields. A prototype electron MLC was constructed and comparisons between film measurements and simulation demonstrate the validity of the Monte Carlo model. Further simulations of dose penumbras demonstrate that such an electron MLC would provide improvements over the helium filled photon MLC at all energies, and improvements in the 90–10 penumbra of 12% to 45% at 20 MeV and 6 MeV, respectively. These improvements were also seen in isodose curves when a complex field shape was simulated. It is thus concluded that an MLC specific for electron beam collimation is required for MERT. © 2000 American Association of Physicists in Medicine.
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87.53.Bn Dosimetry/exposure assessment
87.55.K- Monte Carlo methods
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