Lag measurement in an  active matrix flat‐panel imager

Schroeder, C.; Stanescu, T.; Rathee, S; Fallone, B. G.

doi:10.1118/1.1713298

Cited by 17 publications

(13 citation statements)

References 30 publications

(35 reference statements)

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“…Ghosting artifacts have been described by Siewerdsen et al 45,46 for an indirect detection active matrix flat panel detector due to signal lag, and in direct detection active matrix flat panel detectors due probably to space charge effects by several others. 47,48 Ghosting occurs when sensitivity reduction and recovery processes have time constant of the order of reading cycle of the detector. In the imaging experi-ments, the dose delivered to the CdWO 4 array is expected to be less than 10 cGy.…”

Section: Discussionmentioning

confidence: 99%

A bench‐top megavoltage fan‐beam CT using ‐photodiode detectors. I. System description and detector characterization

Rathee

Monajemi

et al. 2006

Medical Physics

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We describe the components of a bench-top megavoltage computed tomography (MVCT) scanner that uses an 80-element detector array consisting of CdWO4 scintillators coupled to photodiodes. Each CdWO4 crystal is 2.75 x 8 x 10 mm3. The detailed design of the detector array, timing control, and multiplexer are presented. The detectors show a linear response to dose (dose rate was varied by changing the source to detector distance) with a correlation coefficient (R2) nearly unity with the standard deviation of signal at each dose being less than 0.25%. The attenuation of a 6 MV beam by solid water measured by this detector array indicates a small, yet significant spectral hardening that needs to be corrected before image reconstruction. The presampled modulation transfer function is strongly affected by the detector's large pitch and a large improvement can be obtained by reducing the detector pitch. The measured detective quantum efficiency at zero spatial frequency is 18.8% for 6 MV photons which will reduce the dose to the patient in MVCT applications. The detector shows a less than a 2% reduction in response for a dose of 24.5 Gy accumulated in 2 h; however, the lost response is recovered on the following day. A complete recovery can be assumed within the experimental uncertainty (standard deviation <0.5%); however, any smaller permanent damage could not be assessed.

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Section: Discussionmentioning

confidence: 99%

A bench‐top megavoltage fan‐beam CT using ‐photodiode detectors. I. System description and detector characterization

Rathee

Monajemi

et al. 2006

Medical Physics

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“…[17][18][19][20]25,31 Image lag is defined as residual signal, i.e., the carryover of trapped charge into subsequent frames acquired with no exposure. With regard to amorphous silicon detectors, the dominant source of image lag has been identified as trapping and release of charge in the sensor elements.…”

Section: Ivb Ghostingmentioning

confidence: 99%

Dose‐response characteristics of an amorphous silicon EPID

2005

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Electronic portal imaging devices (EPIDs) were originally developed for the purpose of patient setup verification. Nowadays, they are increasingly used as dosimeters (e.g., for IMRT verification and linac-specific QA). A prerequisite for any clinical dosimetric application is a detailed understanding of the detector's dose-response behavior. The aim of this study is to investigate the dosimetric properties of an amorphous silicon EPID (Elekta IVIEWGT) with respect to three photon beam qualities: 6, 10, and 25 MV. The EPID showed an excellent temporal stability on short term as well as on long term scales. The stability throughout the day was strongly influenced by warming up, which took several hours and affected EPID response by 2.5%. Ghosting effects increased the sensitivity of the EPID. They became more pronounced with decreasing time intervals between two exposures as well as with increasing dose. Due to ghosting, changes in pixel sensitivity amounted up to 16% (locally) for the 25 MV photon beam. It was observed that the response characteristics of our EPID depended on dose as well as on dose rate. Doubling the dose rate increased the EPID sensitivity by 1.5%. This behavior was successfully attributed to a dose per frame effect, i.e., a nonlinear relationship between the EPID signal and the dose which was delivered to the panel between two successive readouts. The sensitivity was found to vary up to 10% in the range of 1 to 1000 monitor units. This variation was governed by two independent effects. For low doses, the EPID signal was reduced due to the linac's changing dose rate during startup. Furthermore, the detector reading was influenced by intrabeam variations of EPID sensitivity, namely, an increase of detector response during uniform exposure. For the beam qualities which were used, the response characteristics of the EPID did not depend on energy. Differences in relative dose-response curves resulted from energy dependent temporal output characteristics of the accelerator. If ghosting is prevented from affecting the results and all dose-response effects are properly corrected for, the EPID signal becomes independent of dose rate, dose, and exposure time.

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“…29 Much research has been performed to experimentally study the image lag using pulse or step response functions. [28][29][30][31][32][33] Recently, we reported a new static grid-based technique to achieve combined scatter reduction and correction. 34 Specifically, the method uses a 1D preobject grid to directly reduce the scatter and simultaneously measure the remaining scatter for postscan scatter correction.…”

Section: Introductionmentioning

confidence: 99%