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 CTimages 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 (140 keV), (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 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 , a coefficient that relates the x-ray tube current to an incident count rate by , 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 spectrameasured 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.
The authors at DxRay and at Johns Hopkins University acknowledge support in part by NIH/NIBIB Grant No. R44 EB008612. We sincerely thank Jochen Cammin, Ph.D., Somesh Srivastava, Ph.D., and Ronald J. Jaszczak, Ph.D., for their helpful discussions and suggestions. We are grateful to Zhihui Sun, M.Sc., and Hideaki Tashima, Ph.D., for their help with data acquisitions. Finally, we thank an anonymous reviewer who helped us to improve the quality of the paper.
II. ANALYTICAL MODELS
II.A. DxRay’s DXMCT-1 PCXD
II.B. Nonparalyzable and paralyzable detection models
II.C. Energy response
II.D. Deadtime losses
II.E. Distortions of the recorded spectrum
III. EVALUATION METHODS
III.A. Energy response
III.B. Deadtime losses
III.C. Distorted, recorded spectrum with pulse pileup effects
IV. EVALUATION RESULTS
IV.A. Energy response
IV.B. Deadtime losses
IV.C. Distorted, recorded spectrum with pulse pileup effects
V. DISCUSSION AND CONCLUSIONS
Data & Media loading...
Article metrics loading...
Full text loading...