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Title: Fundamental x-ray interaction limits in diagnostic imaging detectors: Spatial resolution

Abstract

The practice of diagnostic x-ray imaging has been transformed with the emergence of digital detector technology. Although digital systems offer many practical advantages over conventional film-based systems, their spatial resolution performance can be a limitation. The authors present a Monte Carlo study to determine fundamental resolution limits caused by x-ray interactions in four converter materials: Amorphous silicon (a-Si), amorphous selenium, cesium iodide, and lead iodide. The ''x-ray interaction'' modulation transfer function (MTF) was determined for each material and compared in terms of the 50% MTF spatial frequency and Wagner's effective aperture for incident photon energies between 10 and 150 keV and various converter thicknesses. Several conclusions can be drawn from their Monte Carlo study. (i) In low-Z (a-Si) converters, reabsorption of Compton scatter x rays limits spatial resolution with a sharp MTF drop at very low spatial frequencies (<0.3 cycles/mm), especially above 60 keV; while in high-Z materials, reabsorption of characteristic x rays plays a dominant role, resulting in a mid-frequency (1-5 cycles/mm) MTF drop. (ii) Coherent scatter plays a minor role in the x-ray interaction MTF. (iii) The spread of energy due to secondary electron (e.g., photoelectrons) transport is significant only at very high spatial frequencies. (iv) Unlike themore » spread of optical light in phosphors, the spread of absorbed energy from x-ray interactions does not significantly degrade spatial resolution as converter thickness is increased. (v) The effective aperture results reported here represent fundamental spatial resolution limits of the materials tested and serve as target benchmarks for the design and development of future digital x-ray detectors.« less

Authors:
; ;  [1];  [2];  [3];  [4];  [2]
  1. Imaging Research Laboratories, Robarts Research Institute, P.O. Box 5015, London, Ontario N6A 5K8 (Canada)
  2. (Canada) and Department of Medical Biophysics, University of Western Ontario, London, Ontario N6A 3K7 (Canada)
  3. (Canada) and London Regional Cancer Program, London Health Sciences Centre, London, Ontario N6A 4L6 (Canada)
  4. (Canada)
Publication Date:
OSTI Identifier:
21120850
Resource Type:
Journal Article
Resource Relation:
Journal Name: Medical Physics; Journal Volume: 35; Journal Issue: 7; Other Information: DOI: 10.1118/1.2924219; (c) 2008 American Association of Physicists in Medicine; Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
62 RADIOLOGY AND NUCLEAR MEDICINE; BIOMEDICAL RADIOGRAPHY; CESIUM IODIDES; COMPTON EFFECT; DIGITAL SYSTEMS; IMAGE PROCESSING; IMAGES; LEAD IODIDES; MONTE CARLO METHOD; SEMICONDUCTOR MATERIALS; SPATIAL RESOLUTION; TRANSFER FUNCTIONS; X RADIATION

Citation Formats

Hajdok, G., Battista, J. J., Cunningham, I. A., London Regional Cancer Program, London Health Sciences Centre, London, Ontario N6A 4L6, Departments of Medical Biophysics and Oncology, University of Western Ontario, London, Ontario N6A 3K7, Imaging Research Laboratories, Robarts Research Institute, P.O. Box 5015, London, Ontario N6A 5K8, and Departments of Diagnostic Radiology and Nuclear Medicine, London Health Sciences Centre, London, Ontario N6A 5W9. Fundamental x-ray interaction limits in diagnostic imaging detectors: Spatial resolution. United States: N. p., 2008. Web. doi:10.1118/1.2924219.
Hajdok, G., Battista, J. J., Cunningham, I. A., London Regional Cancer Program, London Health Sciences Centre, London, Ontario N6A 4L6, Departments of Medical Biophysics and Oncology, University of Western Ontario, London, Ontario N6A 3K7, Imaging Research Laboratories, Robarts Research Institute, P.O. Box 5015, London, Ontario N6A 5K8, & Departments of Diagnostic Radiology and Nuclear Medicine, London Health Sciences Centre, London, Ontario N6A 5W9. Fundamental x-ray interaction limits in diagnostic imaging detectors: Spatial resolution. United States. doi:10.1118/1.2924219.
Hajdok, G., Battista, J. J., Cunningham, I. A., London Regional Cancer Program, London Health Sciences Centre, London, Ontario N6A 4L6, Departments of Medical Biophysics and Oncology, University of Western Ontario, London, Ontario N6A 3K7, Imaging Research Laboratories, Robarts Research Institute, P.O. Box 5015, London, Ontario N6A 5K8, and Departments of Diagnostic Radiology and Nuclear Medicine, London Health Sciences Centre, London, Ontario N6A 5W9. 2008. "Fundamental x-ray interaction limits in diagnostic imaging detectors: Spatial resolution". United States. doi:10.1118/1.2924219.
@article{osti_21120850,
title = {Fundamental x-ray interaction limits in diagnostic imaging detectors: Spatial resolution},
author = {Hajdok, G. and Battista, J. J. and Cunningham, I. A. and London Regional Cancer Program, London Health Sciences Centre, London, Ontario N6A 4L6 and Departments of Medical Biophysics and Oncology, University of Western Ontario, London, Ontario N6A 3K7 and Imaging Research Laboratories, Robarts Research Institute, P.O. Box 5015, London, Ontario N6A 5K8 and Departments of Diagnostic Radiology and Nuclear Medicine, London Health Sciences Centre, London, Ontario N6A 5W9},
abstractNote = {The practice of diagnostic x-ray imaging has been transformed with the emergence of digital detector technology. Although digital systems offer many practical advantages over conventional film-based systems, their spatial resolution performance can be a limitation. The authors present a Monte Carlo study to determine fundamental resolution limits caused by x-ray interactions in four converter materials: Amorphous silicon (a-Si), amorphous selenium, cesium iodide, and lead iodide. The ''x-ray interaction'' modulation transfer function (MTF) was determined for each material and compared in terms of the 50% MTF spatial frequency and Wagner's effective aperture for incident photon energies between 10 and 150 keV and various converter thicknesses. Several conclusions can be drawn from their Monte Carlo study. (i) In low-Z (a-Si) converters, reabsorption of Compton scatter x rays limits spatial resolution with a sharp MTF drop at very low spatial frequencies (<0.3 cycles/mm), especially above 60 keV; while in high-Z materials, reabsorption of characteristic x rays plays a dominant role, resulting in a mid-frequency (1-5 cycles/mm) MTF drop. (ii) Coherent scatter plays a minor role in the x-ray interaction MTF. (iii) The spread of energy due to secondary electron (e.g., photoelectrons) transport is significant only at very high spatial frequencies. (iv) Unlike the spread of optical light in phosphors, the spread of absorbed energy from x-ray interactions does not significantly degrade spatial resolution as converter thickness is increased. (v) The effective aperture results reported here represent fundamental spatial resolution limits of the materials tested and serve as target benchmarks for the design and development of future digital x-ray detectors.},
doi = {10.1118/1.2924219},
journal = {Medical Physics},
number = 7,
volume = 35,
place = {United States},
year = 2008,
month = 7
}
  • A frequency-dependent x-ray Swank factor based on the ''x-ray interaction'' modulation transfer function and normalized noise power spectrum is determined from a Monte Carlo analysis. This factor was calculated in four converter materials: amorphous silicon (a-Si), amorphous selenium (a-Se), cesium iodide (CsI), and lead iodide (PbI{sub 2}) for incident photon energies between 10 and 150 keV and various converter thicknesses. When scaled by the quantum efficiency, the x-ray Swank factor describes the best possible detective quantum efficiency (DQE) a detector can have. As such, this x-ray interaction DQE provides a target performance benchmark. It is expressed as a function ofmore » (Fourier-based) spatial frequency and takes into consideration signal and noise correlations introduced by reabsorption of Compton scatter and photoelectric characteristic emissions. It is shown that the x-ray Swank factor is largely insensitive to converter thickness for quantum efficiency values greater than 0.5. Thus, while most of the tabulated values correspond to thick converters with a quantum efficiency of 0.99, they are appropriate to use for many detectors in current use. A simple expression for the x-ray interaction DQE of digital detectors (including noise aliasing) is derived in terms of the quantum efficiency, x-ray Swank factor, detector element size, and fill factor. Good agreement is shown with DQE curves published by other investigators for each converter material, and the conditions required to achieve this ideal performance are discussed. For high-resolution imaging applications, the x-ray Swank factor indicates: (i) a-Si should only be used at low-energy (e.g., mammography); (ii) a-Se has the most promise for any application below 100 keV; and (iii) while quantum efficiency may be increased at energies just above the K edge in CsI and PbI{sub 2}, this benefit is offset by a substantial drop in the x-ray Swank factor, particularly at high spatial frequencies.« less
  • This article investigates the limitations on the formation of focused ion beam images from secondary electrons. We use the notion of the information content of an image to account for the effects of resolution, contrast, and signal-to-noise ratio and show that there is a competition between the rate at which small features are sputtered away by the primary beam and the rate of collection of secondary electrons. We find that for small features, sputtering is the limit to imaging resolution, and that for extended small features (e.g., layered structures), rearrangement, redeposition, and differential sputtering rates may limit the resolution inmore » some cases. {copyright} {ital 1996 American Vacuum Society}« less
  • Image guided radiation therapy solutions based on megavoltage computed tomography (MVCT) involve the extension of electronic portal imaging devices (EPIDs) from their traditional role of weekly localization imaging and planar dose mapping to volumetric imaging for 3D setup and dose verification. To sustain the potential advantages of MVCT, EPIDs are required to provide improved levels of portal image quality. Therefore, it is vital that the performance of EPIDs in clinical use is maintained at an optimal level through regular and rigorous quality assurance (QA). Traditionally, portal imaging QA has been carried out by imaging calibrated line-pair and contrast resolution phantomsmore » and obtaining arbitrarily defined QA indices that are usually dependent on imaging conditions and merely indicate relative trends in imaging performance. They are not adequately sensitive to all aspects of image quality unlike fundamental imaging metrics such as the modulation transfer function (MTF), noise power spectrum (NPS), and detective quantum efficiency (DQE) that are widely used to characterize detector performance in radiographic imaging and would be ideal for QA purposes. However, due to the difficulty of performing conventional MTF measurements, they have not been used for routine clinical QA. The authors present a simple and quick QA methodology based on obtaining the MTF, NPS, and DQE of a megavoltage imager by imaging standard open fields and a bar-pattern QA phantom containing 2 mm thick tungsten line-pair bar resolution targets. Our bar-pattern based MTF measurement features a novel zero-frequency normalization scheme that eliminates normalization errors typically associated with traditional bar-pattern measurements at megavoltage x-ray energies. The bar-pattern QA phantom and open-field images are used in conjunction with an automated image analysis algorithm that quickly computes the MTF, NPS, and DQE of an EPID system. Our approach combines the fundamental advantages of linear systems metrics such as robustness, sensitivity across the full spatial frequency range of interest, and normalization to imaging conditions (magnification, system gain settings, and exposure), with the simplicity, ease, and speed of traditional phantom imaging. The algorithm was analyzed for accuracy and sensitivity by comparing with a commercial portal imaging QA method (PIPSPRO, Standard Imaging, Middleton, WI) on both first-generation lens-coupled and modern a-Si flat-panel based clinical EPID systems. The bar-pattern based QA measurements were found to be far more sensitive to even small levels of degradation in spatial resolution and noise. The bar-pattern based QA methodology offers a comprehensive image quality assessment tool suitable for both commissioning and routine EPID QA.« less