Evaluation of the imaging properties of an amorphous selenium‐based flat panel detector for digital fluoroscopy

Abstract

The imaging performance of an amorphous selenium (a-Se) flat-panel detector for digital fluoroscopy was experimentally evaluated using the spatial frequency dependent modulation transfer function (MTF), noise power spectrum (NPS), and detective quantum efficiency (DQE). These parameters were investigated at beam qualities and exposures within the range typical of gastrointestinal fluoroscopic imaging (approximately 0.1 - 10 microR, 75 kV). The investigation does not take into consideration the detector cover, which in clinical use will lower the DQE measured here by its percent attenuation. The MTF was found to be less than the expected aperture response and the NPS was not white which together indicate presampling blurring. The cause of this blurring was attributed to charge trapping at the interface between two different layers of the a-Se. The effect on the DQE was also consistent with presampling blur, which reduces the aliasing in the NPS and thereby reduces the spatial frequency dependence of the DQE. (The DQE was independent of spatial frequency from 0.12 to 0.73 mm(-1) due to antialiasing of the NPS.) Moreover, the first zero of the measured MTF and the aperture response appeared at the same spatial frequency (6.66 mm(-1) for a pixel of 150 microm). Hence, the geometric fill factor (77%) was increased to an effective fill factor of 99 +/- 1%. A large scale ( approximately 32 pixels) correlation in the noise due to the configuration of the readout electronics caused increased noise power in the gate line NPS at low spatial frequency (< 0.1 mm(-1)). The DQE (f = 0) was exposure independent over a large range of exposures but became exposure dependent at low exposures due to the electronic noise.

Photon-counting hexagonal pixel array CdTe detector: Spatial resolution characteristics for image-guided interventional applications

“…5,6 Evaluations of direct-conversion a-Se based detector with 150 µm pitch have shown a limiting resolution of approximately 6 cycles/mm. 6,9 Characterization of a MAF employing a 300 µm thick CsI:Tl scintillator coupled to a CCD with 35 µm pitch via a light image intensifier and fiberoptic taper showed a limiting resolution of 4 cycles/mm. 11 The observed 10% MTF of ∼12 cycles/mm with the investigated detector in the SPC mode compares favorably with the added advantage of effectively eliminating electronic noise through appropriate selection of the energy threshold.…”

“…Image intensifierbased systems were the mainstay for several decades, but suffered from veiling glare, distortions, and degradation of image quality with time. Hence, indirect conversion CsI:Tl scintillator coupled amorphous silicon (a-Si) flat-panel detector, [4][5][6] tiled large-area charge-coupled devices (CCDs) coupled to CsI:Tl scintillator, 7,8 and direct conversion amorphous selenium (a-Se) photoconductor-based flat-panel detector 9 were developed. Current generation of interventional C-arm systems utilize flat-panel detectors with pixel pitch ranging from 143 to 200 µm, depending on the manufacturer and the intended application.…”

Scintillator avalanche photoconductor with high resolution emitter readout for low dose x‐ray imaging: Lag

“…With N s = 128 and further increase in t p to 3.4 S, the first frame lag is 4.3%, which is within the range of lag values measured in AMFPIs. [13][14][15] Further reduction in lag could be realized by incorporating the charge clearance procedure described in Sec. IV D 2.…”

“…In a fluoroscopic system, lag is usually required to be less than 10% after the first frame, 12 and the values measured from most indirect active matrix FPIs ͑AM-FPIs͒ are between 2% and 10% depending on the x-ray exposure and detector operation. [13][14][15] The temporal performance of CsI ͑Tl͒ has been studied extensively for indirect AMFPI. It was found that the lag due to the afterglow of the CsI ͑Tl͒ is ϳ0.1% after 5 ms, and the typical values of ghosting are 1% and 3% at the exposure levels of 1 and 10 mGy, respectively.…”

The x‐ray light valve: A low‐cost, digital radiographic imaging system—Spatial resolution

“…1 The current leading technology in digital x-ray systems in terms of image quality and dose to the patient is based on large area active matrix flat panel imagers ͑AMFPIs͒. 2 While AMFPIs provide excellent image quality and increased functionality over traditional film-screen systems, they are very expensive and are therefore used primarily for specialized procedures such as fluoroscopy, 3 cone-beam computed tomography ͑CT͒, 4 and digital tomosynthesis. 5 The most commercially successful digital radiographic x-ray systems are cassette-based computed radiography ͑CR͒ systems.…”