Quantum-limited-dose (QLD) and noise-equivalent-dose (NED) are performance metrics often used interchangeably.
Although the metrics are related, they are not equivalent unless the treatment of electronic noise is carefully
considered. These metrics are increasingly important to properly characterize the low-dose performance of flat panel
detectors (FPDs). A system can be said to be quantum-limited when the Signal-to-noise-ratio (SNR) is proportional to
the square-root of x-ray exposure. Recent experiments utilizing three methods to determine the quantum-limited dose
range yielded inconsistent results. To investigate the deviation in results, generalized analytical equations are
developed to model the image processing and analysis of each method. We test the generalized expression for both
radiographic and fluoroscopic detectors. The resulting analysis shows that total noise content of the images processed
by each method are inherently different based on their readout scheme. Finally, it will be shown that the NED is
equivalent to the instrumentation-noise-equivalent-exposure (INEE) and furthermore that the NED is derived from the
quantum-noise-only method of determining QLD. Future investigations will measure quantum-limited performance of
radiographic panels with a modified readout scheme to allow for noise improvements similar to measurements
performed with fluoroscopic detectors.
Our purposes are to develop a conventional computer-aided diagnostic (CAD) scheme and a new fusion CAD
scheme for the detection of lung nodules in multi-projection chest radiography, and to verify that information fused
from the multi-projection chest radiography can greatly improve the performance of the conventional CAD scheme.
The conventional CAD scheme processed each of the three projection images of a subject independently, and
discarded the correlation information between the three images. The fusion CAD scheme registered all candidates
detected by the conventional CAD scheme in the three images of a subject, and integrated the correlation
information between the registered candidates to remove false positives. The CAD schemes were trained and
evaluated on a database with 97 subjects. At the sensitivities of 70%, 65% and 60%, the conventional CAD scheme
reported 20.4, 13.6 and 8.8 false positives per image, respectively, whereas the fusion CAD scheme reported 4.5, 2.8
and 1.2 false positives per image, respectively. The fusion of correlation information can markedly improve the
performance of CAD scheme for lung nodule detection.
The use of flat panel detectors in computed tomography (CT) systems can improve resolution, reduce system cost, and add operational flexibility by combining fluoroscopy and radiography applications within CT systems. However, some prior studies have suggested that flat panel detectors would not perform well in CT applications due to their lack of high dynamic range, lag artifacts, and inadequate frame rate. The purpose of this study was to perform a physical evaluation of a prototype flat panel detector capable of high frame rates and extended dynamic range. The flat panel detector used had a pixel size of 194 microns and a matrix size of 2048x1536. The detector could be configured for several combinations of frame rate and matrix size up to 750 frames per second for a 512x16 matrix size with 4x4 binning. The evaluation was performed in terms of the MTF and DQE as a function of frame rate and exposure at the IEC RQA5 (~75 kVp, 21 mm Al) beam quality. The image lag was evaluated in terms of temporal-frequency dependent transfer function. Offset shift were also evaluated. Preliminary results indicate 0.1 MTF at 0.92 cycles/mm and DQE(0) of approximately 0.8, 0.6, 0.4, and 0.22 at 0.144, 0.065, 0.035, and 0.008 mR per frame exposures. The temporal MTF exhibited a low-frequency drop and a value of 0.5 at the Nyquist frequency. Offset shift was negligible. Considering high frame rate capabilities of the new detector, the results suggest that the detector has potential for use in real-time CT applications including CT angiography.
This paper describes a new flat panel imager designed for use in cardiovascular and mobile C-arm imaging systems. The a-Si sensor array has a 1024 x 1024 matrix with a pixel pitch of 194 μm, resulting in an active area of 198.7 mm x 198.7 mm. The imager allows frame rates of up to 30 fps in full resolution fluoroscopy mode
and up to 60 fps in a 2 x 2 binned low dose fluoroscopy mode. Typically, a 600 μm thick deposited columnar CsI(Tl) layer is used as the scintillator.
Improvements in the pixel architecture, charge amplifier ASICs, and system level electronics resulted in a very low electronic noise floor, such that both the fluoroscopy and low dose fluoroscopy modes of the panel are x-ray quantum limited below 1 μR/frame.
Low power consumption electronics combined with a mechanical design optimized for heat transfer and dissipation makes air-cooling sufficient for most environments. The small size of 24.1 x 24.1 x 6 cm and the weight of only 4.1 kg meet the requirements of C-Arm systems. Special consideration was given to the border around the active area, which has been reduced to 2 cm. Reported performance parameters include linearity, lag, contrast ratio, MTF, and DQE. For the full resolution mode, the MTF is greater than 0.53 and 0.21 at 1
and 2 lp/mm, respectively. DQE measured at 22 nGy/frame was greater than 0.68, 0.50, and 0.23 at 0, 1, and 2 lp/mm, respectively.
Preliminary results are presented from the PaxScan 4030A; a 40x30cm, 2048 x 1536 landscape, flat panel imager, with 194um pixel pitch. This imager builds on our experience with the PaxScan 2520, a 127um real-time flat panel detector capable of both high-resolution radiography and low dose fluoroscopy. While the PS2520 has been applied in C-arms, neuroangiography, cardiac imaging and small area radiographic units, the larger active area of the PaxScan 4030A addresses the broader applications of angiography, general R&F and cone-beam CT. The PaxScan 4030A has the same electrical and software interfaces as the PS2520; however, a number of innovations have been incorporated into the 4030A to increase its versatility. The most obvious change is that the data interface between the receptor and command processor has been reduced to one very flexible and thin fiber-optic cable. A second new feature for the 4030A is the use of split datalines. Split datalines facilitate scanning the two halves of the array in parallel, cutting the readout time in half and increasing the time window for pulsed x-ray delivery to 15ms at 30fps. In addition, split datalines result in lower noise, which, coupled with the larger signal of the 194um pixels, enables high quality imaging at lower fluoroscopy doses rates.
This paper describes a real-time image processing system for correction and enhancement of fluoroscopic (video X-ray) image data obtained from a large area, flat-panel, solid- state medical image sensor. The amorphous silicon sensor is 1536 X 1920 pixels, measuring 20 X 25 cm; for operation at 30 frames per second, the real pixel data rate is approximately 45 MB/sec.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.