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Title: Modeling the performance of a photon counting x-ray detector for CT: Energy response and pulse pileup effects

Abstract

Purpose: Recently, photon counting x-ray detectors (PCXDs) with energy discrimination capabilities have been developed for potential use in clinical computed tomography (CT) scanners. These PCXDs have great potential to improve the quality of CT images due to the absence of electronic noise and weights applied to the counts and the additional spectral information. With high count rates encountered in clinical CT, however, coincident photons are recorded as one event with a higher or lower energy due to the finite speed of the PCXD. This phenomenon is called a ''pulse pileup event'' and results in both a loss of counts (called ''deadtime losses'') and distortion of the recorded energy spectrum. Even though the performance of PCXDs is being improved, it is essential to develop algorithmic methods based on accurate models of the properties of detectors to compensate for these effects. To date, only one PCXD (model DXMCT-1, DxRay, Inc., Northridge, CA) has been used for clinical CT studies. The aim of that study was to evaluate the agreement between data measured by DXMCT-1 and those predicted by analytical models for the energy response, the deadtime losses, and the distorted recorded spectrum caused by pulse pileup effects. Methods: An energy calibration wasmore » performed using {sup 99m}Tc (140 keV), {sup 57}Co (122 keV), and an x-ray beam obtained with four x-ray tube voltages (35, 50, 65, and 80 kVp). The DXMCT-1 was placed 150 mm from the x-ray focal spot; the count rates and the spectra were recorded at various tube current values from 10 to 500 {mu}A for a tube voltage of 80 kVp. Using these measurements, for each pulse height comparator we estimated three parameters describing the photon energy-pulse height curve, the detector deadtime {tau}, a coefficient k that relates the x-ray tube current I to an incident count rate a by a=kxI, and the incident spectrum. The mean pulse shape of all comparators was acquired in a separate study and was used in the model to estimate the distorted recorded spectrum. The agreement between data measured by the DXMCT-1 and those predicted by the models was quantified by the coefficient of variation (COV), i.e., the root mean square difference divided by the mean of the measurement. Results: Photon energy versus pulse height curves calculated with an analytical model and those measured using the DXMCT-1 were in agreement within 0.2% in terms of the COV. The COV between the output count rates measured and those predicted by analytical models was 2.5% for deadtime losses of up to 60%. The COVs between spectra measured and those predicted by the detector model were within 3.7%-7.2% with deadtime losses of 19%-46%. Conclusions: It has been demonstrated that the performance of the DXMCT-1 agreed exceptionally well with the analytical models regarding the energy response, the count rate, and the recorded spectrum with pulse pileup effects. These models will be useful in developing methods to compensate for these effects in PCXD-based clinical CT systems.« less

Authors:
; ; ; ; ; ; ; ;  [1]
  1. Division of Medical Imaging Physics, Russell H. Morgan Department of Radiology and Radiological Science, Johns Hopkins University School of Medicine, Baltimore, Maryland 21287 (United States)
Publication Date:
OSTI Identifier:
22096895
Resource Type:
Journal Article
Journal Name:
Medical Physics
Additional Journal Information:
Journal Volume: 38; Journal Issue: 2; Other Information: (c) 2011 American Association of Physicists in Medicine; Country of input: International Atomic Energy Agency (IAEA); Journal ID: ISSN 0094-2405
Country of Publication:
United States
Language:
English
Subject:
46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND TECHNOLOGY; CALIBRATION; CAT SCANNING; COBALT 57; ENERGY SPECTRA; KEV RANGE 100-1000; PERFORMANCE; PHOTONS; PULSE PILEUP; PULSE SHAPERS; RADIATION DETECTORS; SIMULATION; TECHNETIUM 99; X RADIATION; X-RAY DETECTION; X-RAY TUBES

Citation Formats

Taguchi, Katsuyuki, Zhang, Mengxi, Frey, Eric C., Xiaolan, Wang, Iwanczyk, Jan S., Nygard, Einar, Hartsough, Neal E., Tsui, Benjamin M. W., Barber, William C., DxRay, Inc., Northridge, California 91324, Interon, AS, N-1395 Hvalstad, DxRay, Inc., Northridge, California 91324, Division of Medical Imaging Physics, Russell H. Morgan Department of Radiology and Radiological Science, Johns Hopkins University School of Medicine, Baltimore, Maryland 21287, and DxRay, Inc., Northridge, California 91324. Modeling the performance of a photon counting x-ray detector for CT: Energy response and pulse pileup effects. United States: N. p., 2011. Web. doi:10.1118/1.3539602.
Taguchi, Katsuyuki, Zhang, Mengxi, Frey, Eric C., Xiaolan, Wang, Iwanczyk, Jan S., Nygard, Einar, Hartsough, Neal E., Tsui, Benjamin M. W., Barber, William C., DxRay, Inc., Northridge, California 91324, Interon, AS, N-1395 Hvalstad, DxRay, Inc., Northridge, California 91324, Division of Medical Imaging Physics, Russell H. Morgan Department of Radiology and Radiological Science, Johns Hopkins University School of Medicine, Baltimore, Maryland 21287, & DxRay, Inc., Northridge, California 91324. Modeling the performance of a photon counting x-ray detector for CT: Energy response and pulse pileup effects. United States. doi:10.1118/1.3539602.
Taguchi, Katsuyuki, Zhang, Mengxi, Frey, Eric C., Xiaolan, Wang, Iwanczyk, Jan S., Nygard, Einar, Hartsough, Neal E., Tsui, Benjamin M. W., Barber, William C., DxRay, Inc., Northridge, California 91324, Interon, AS, N-1395 Hvalstad, DxRay, Inc., Northridge, California 91324, Division of Medical Imaging Physics, Russell H. Morgan Department of Radiology and Radiological Science, Johns Hopkins University School of Medicine, Baltimore, Maryland 21287, and DxRay, Inc., Northridge, California 91324. Tue . "Modeling the performance of a photon counting x-ray detector for CT: Energy response and pulse pileup effects". United States. doi:10.1118/1.3539602.
@article{osti_22096895,
title = {Modeling the performance of a photon counting x-ray detector for CT: Energy response and pulse pileup effects},
author = {Taguchi, Katsuyuki and Zhang, Mengxi and Frey, Eric C. and Xiaolan, Wang and Iwanczyk, Jan S. and Nygard, Einar and Hartsough, Neal E. and Tsui, Benjamin M. W. and Barber, William C. and DxRay, Inc., Northridge, California 91324 and Interon, AS, N-1395 Hvalstad and DxRay, Inc., Northridge, California 91324 and Division of Medical Imaging Physics, Russell H. Morgan Department of Radiology and Radiological Science, Johns Hopkins University School of Medicine, Baltimore, Maryland 21287 and DxRay, Inc., Northridge, California 91324},
abstractNote = {Purpose: Recently, photon counting x-ray detectors (PCXDs) with energy discrimination capabilities have been developed for potential use in clinical computed tomography (CT) scanners. These PCXDs have great potential to improve the quality of CT images due to the absence of electronic noise and weights applied to the counts and the additional spectral information. With high count rates encountered in clinical CT, however, coincident photons are recorded as one event with a higher or lower energy due to the finite speed of the PCXD. This phenomenon is called a ''pulse pileup event'' and results in both a loss of counts (called ''deadtime losses'') and distortion of the recorded energy spectrum. Even though the performance of PCXDs is being improved, it is essential to develop algorithmic methods based on accurate models of the properties of detectors to compensate for these effects. To date, only one PCXD (model DXMCT-1, DxRay, Inc., Northridge, CA) has been used for clinical CT studies. The aim of that study was to evaluate the agreement between data measured by DXMCT-1 and those predicted by analytical models for the energy response, the deadtime losses, and the distorted recorded spectrum caused by pulse pileup effects. Methods: An energy calibration was performed using {sup 99m}Tc (140 keV), {sup 57}Co (122 keV), and an x-ray beam obtained with four x-ray tube voltages (35, 50, 65, and 80 kVp). The DXMCT-1 was placed 150 mm from the x-ray focal spot; the count rates and the spectra were recorded at various tube current values from 10 to 500 {mu}A for a tube voltage of 80 kVp. Using these measurements, for each pulse height comparator we estimated three parameters describing the photon energy-pulse height curve, the detector deadtime {tau}, a coefficient k that relates the x-ray tube current I to an incident count rate a by a=kxI, and the incident spectrum. The mean pulse shape of all comparators was acquired in a separate study and was used in the model to estimate the distorted recorded spectrum. The agreement between data measured by the DXMCT-1 and those predicted by the models was quantified by the coefficient of variation (COV), i.e., the root mean square difference divided by the mean of the measurement. Results: Photon energy versus pulse height curves calculated with an analytical model and those measured using the DXMCT-1 were in agreement within 0.2% in terms of the COV. The COV between the output count rates measured and those predicted by analytical models was 2.5% for deadtime losses of up to 60%. The COVs between spectra measured and those predicted by the detector model were within 3.7%-7.2% with deadtime losses of 19%-46%. Conclusions: It has been demonstrated that the performance of the DXMCT-1 agreed exceptionally well with the analytical models regarding the energy response, the count rate, and the recorded spectrum with pulse pileup effects. These models will be useful in developing methods to compensate for these effects in PCXD-based clinical CT systems.},
doi = {10.1118/1.3539602},
journal = {Medical Physics},
issn = {0094-2405},
number = 2,
volume = 38,
place = {United States},
year = {2011},
month = {2}
}