Flat-field Images Research Articles

Quantitative Image Quality Measurements of a Digital Breast Tomosynthesis System

The aim of this study was to measure the image quality of a digital breast tomosynthesis (DBT) system quantitatively. The signal transfer property (STP), modulation transfer function (MTF), noise power spectrum (NPS), and detective quantum efficiency (DQE) of the Hologic Selenia Dimensions breast tomosynthesis system were measured according to established methods. The NPS was calculated from two-dimensional (2 D) fast Fourier transform (FFT) of flat field images. The presampling MTF of the system was determined for 2 D standard projection mammography and 3 D breast tomosynthesis mode using the edge method. The DQE was derived for different detector air kerma (DAK) values from NPS and MTF measurements. The detector response function was linear for both two-dimensional (2 D) standard projection mammography and three-dimensional (3 D) breast tomosynthesis modes. The gradient of the detector response in the 3 D imaging mode was higher than the gradient in the 2 D imaging mode by a factor of 3.1. The MTF values measured at the Nyquist frequency were 32 % and 39 % in 2 D and 3 D imaging modes, respectively. The DQE was saturated at an air kerma value approximately 3.5 times lower in 3 D mode than in 2 D mode. The measured maximum DQE value was 54 %. The measured DQE values were comparable with breast tomosynthesis systems from other companies (Siemens, GE).

Heel effect adaptive flat field correction of digital x-ray detectors

Anode heel effect renders large-scale background nonuniformities in digital radiographs. Conventional offset/gain calibration is performed at mono source-to-image distance (SID), and disregards the SID-dependent characteristic of heel effect. It results in a residual nonuniform background in the corrected radiographs when the SID settings for calibration and correction differ. In this work, the authors develop a robust and efficient computational method for digital x-ray detector gain correction adapted to SID-variant heel effect, without resorting to physical filters, phantoms, complicated heel effect models, or multiple-SID calibration and interpolation. The authors present the Duo-SID projection correction method. In our approach, conventional offset/gain calibrations are performed only twice, at the minimum and maximum SIDs of the system in typical clinical use. A fast iterative separation algorithm is devised to extract the detector gain and basis heel patterns from the min/max SID calibrations. The resultant detector gain is independent of SID, while the basis heel patterns are parameterized by the min- and max-SID. The heel pattern at any SID is obtained from the min-SID basis heel pattern via projection imaging principles. The system gain desired at a specific acquisition SID is then constructed using the projected heel pattern and detector gain map. The method was evaluated for flat field and anatomical phantom image corrections. It demonstrated promising improvements over interpolation and conventional gain calibration/correction methods, lowering their correction errors by approximately 70% and 80%, respectively. The separation algorithm was able to extract the detector gain and heel patterns with less than 2% error, and the Duo-SID corrected images showed perceptually appealing uniform background across the detector. The Duo-SID correction method has substantially improved on conventional offset/gain corrections for digital x-ray imaging in an SID-variant environment. The technique is relatively simple, and can be easily incorporated into multiple-point gain calibration/correction techniques. It offers a potentially valuable tool for preprocessing digital x-ray images to boost image quality of mammography, chest and cardiac radiography, as well as automated computer aided diagnostic radiology.

TH-A-103-05: Characterization of An Energy-Resolved Photon-Counting Si Strip Detector for Breast CT Imaging

Purpose: To investigate the characteristics of a high resolution energy‐resolved photon‐counting Si strip detector for breast CT imaging. Methods: A prototype Si strip detector was studied for breast CT imaging. A bench‐top CT system was constructed using a tungsten anode x‐ray source with a focal spot of 0.8 mm and a single line Si strip detector with 256 pixels and pixel pitch of 100 μm. A 38 μm diameter tungsten wire phantom was used to determine the point spread function (PSF) of the detector. The tungsten wire phantom was positioned 110 and 7.5cm from the x‐ray tube focal spot and detector, respectively. The system MTF was measured using reconstructed images. For the noise power spectrum (NPS), three different exposures 16, 31, and 46 μGy were used. Flat field images were acquired with and without electric noise at each exposure to investigate the effect of detector's electric noise on the noise property. The simulations with the same phantom and system geometry were also performed for validation purposes. A 16 cm water phantom, which contains various diameters (76, 102, 152, 203, and 254 μm) of aluminum (Al) wire was investigated to confirm the simulation and experimental results. Results: The desired resolution of 100 μm, corresponding to a spatial frequency of 5 lp/mm was demonstrated in both simulation and experimental results. A higher normalized NPS was measured when electronic noise included. The relationship between measured and known diameter for Al wires was quite linear except for the smallest Al wire which is smaller than detector pixel size. Conclusion: The performance of the prototype Si strip detector was evaluated using MTF and NPS. The results show that the investigated detector has excellent potential for high resolution breast imaging at low radiation dose levels.

TU-A-116-07: Comparison and Evaluation of Commercially Available CMOS X-Ray Detectors for Neurological Endovascular Image Guided Interventions (neuro-EIGI' s)

Purpose: To evaluate three commercially available CMOS x-ray detector systems as potential dual fluoroscopic and angiographic, high-resolution, ROI detectors for neuro-endovascular image-guided interventions (neuro-EIGIs) and to compare them to other experimentally evaluated or simulated neuro-EIGI detectors. Methods: Each CMOS detector was exposed to a standard RQA5 x-ray spectrum obtained by adding 21 mm of aluminum to a 70 kVp x-ray beam. Flat-field images were acquired by each detector and exposure per frame was determined using an ion chamber. From these images and exposures, sensitivity in digital numbers (DN/μR) and instrumentation noise-equivalent exposure (INEE) was found for each detector. These values were used to compare the three commercial detectors to other neuro-EIGI detectors previously built or simulated. Results: Detector A has a 12-bit ADC, a pixel size of 100 μm, and a full well capacity (FWC) of 2.2 million electrons (Me-). The sensitivity was found to be 5.8 DN/μR and the INEE was found to be 7.58 μR. Detector B has a 14-bit ADC, a pixel size of 75 μm, and a FWC of 0.36 Me-for the high sensitivity mode and 1.4 Me-for the high saturation mode. The sensitivity was found to be 28.9 DN/μR and the INEE was found to be 2.04 μR in the high sensitivity mode. Detector C has a 14-bit ADC, a pixel size of 100 μm and a FWC of 1.5 Me-. The sensitivity was found to be 15.1 DN/μR and the INEE was found to be 3.04 μR. The previous neuro-EIGI detectors had similar sensitivities of 10s–100s DN/μR, but much lower INEEs of <0.2 μR. Conclusion: While the commercial CMOS x-ray detectors are improving, it remains a desirable goal to further reduce the commercial detector pixel size and instrumentation noise to approach the values seen with the previously built and simulated neuro-EIGI detectors. Supported by NIH Grant 2R01EB002873 and an equipment grant from Toshiba Corporation.

An improved method of spectrum extraction based on 2-D Gauss model for LAMOST

An improved spectrum extraction method based on two-dimensional (2-D) Gauss model is presented for the Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST). As the previous 2-D fiber spectrum extraction methods, the improved method still uses the flat-field images and the calibration lamp images respectively to determine the parameters of the 2-D Gauss model, at the same time the means of evaluating the parameters is modified. The proposed method makes full use of the correlation between the spectrum signals and the non-correlation between the noise. The means of extracting the original spectrum from the observed 2-D spectra is modified as well. We utilize the symmetry of the spectrum contour to weaken the influence of noise. The experiments are performed with the simulated data based on the feature of the real LAMOST data and the real data on LAMOST. The experimental results show that our improved method is more effective and accurate than the previous spectrum extraction methods used on LAMOST.

Streaked radiography of an irradiated foam sample on the National Ignition Facility

Streaked x-ray radiography images of annular patterns in an evolving tantalum oxide foam under the influence of a driven, subsonic radiation wave were obtained on the National Ignition Facility. This is the first successful radiography measurement of the evolution of well-defined foam features under a driven, subsonic wave in the diffusive regime. A continuous record of the evolution was recorded on an x-ray streak camera, using a slot-apertured point-projection backlighter with an 8 ns nickel source (7.9 keV). Radiography images were obtained for four different annular patterns, which were corrected using a source-dependent flat-field image. The evolution of the foam features was well-modeled using the 3D KULL radiation hydrodynamics code. This experimental and modeling platform can be modified for scaled high-energy-density laboratory astrophysics experiments.

Simulation system for understanding the lag effect in fluoroscopic images

Real-time tumor tracking in external radiotherapy can be achieved by diagnostic (kV) X-ray imaging with a dynamic flat-panel detector (FPD). It is crucial to understand the effects of image lag for real-time tumor tracking. Our purpose in this study was to develop a lag simulation system based on the image lag properties of an FPD system. Image lag properties were measured on flat-field images both in direct- and indirect-conversion dynamic FPDs. A moving target with image lag was simulated based on the lag properties in all combinations of FPD types, imaging rates, exposure doses, and target speeds, and then compared with actual moving targets for investigation of the reproducibility of image lag. Image lag was simulated successfully and agreed well with the actual lag as well as with the predicted effect. In the indirect-conversion FPD, a higher dose caused greater image lag on images. In contrast, there were no significant differences among dose levels in a direct-conversion FPD. There were no relationships between target speed and amount of image blurring in either type of FPD. The maximum contour blurring and the rate of increase in pixel value due to image lag were 1.1mm and 10.0%, respectively, in all combinations of imaging parameters examined in this study. Blurred boundaries and changes in pixel value due to image lag were estimated under various imaging conditions with use of the simulation system. Our system would be helpful for a better understanding of the effects of image lag in fluoroscopic images.

Simulation approach for the evaluation of tracking accuracy in radiotherapy: a preliminary study

Real-time tumor tracking in external radiotherapy can be achieved by diagnostic (kV) X-ray imaging with a dynamic flat-panel detector (FPD). It is important to keep the patient dose as low as possible while maintaining tracking accuracy. A simulation approach would be helpful to optimize the imaging conditions. This study was performed to develop a computer simulation platform based on a noise property of the imaging system for the evaluation of tracking accuracy at any noise level. Flat-field images were obtained using a direct-type dynamic FPD, and noise power spectrum (NPS) analysis was performed. The relationship between incident quantum number and pixel value was addressed, and a conversion function was created. The pixel values were converted into a map of quantum number using the conversion function, and the map was then input into the random number generator to simulate image noise. Simulation images were provided at different noise levels by changing the incident quantum numbers. Subsequently, an implanted marker was tracked automatically and the maximum tracking errors were calculated at different noise levels. The results indicated that the maximum tracking error increased with decreasing incident quantum number in flat-field images with an implanted marker. In addition, the range of errors increased with decreasing incident quantum number. The present method could be used to determine the relationship between image noise and tracking accuracy. The results indicated that the simulation approach would aid in determining exposure dose conditions according to the necessary tracking accuracy.

Precision in 2D temperature measurements using the thermographic phosphor BAM

Investigation of optimized spatial precision for surface temperature measurements is performed. The temperature is measured by means of two-color ratio imaging with ICCD cameras, using the thermographic phosphor BAM. The precision in temperature is put in relation to the spatial resolution, two quantities which involve a trade-off in this case: the more spatial smoothing the better precision, but also the worse spatial resolution. Two different setups are used in order to investigate the influence of laser shot-to-shot variations, the flat-field correction and image registration process on the precision. In order to achieve high precision it is crucial to operate the ICCD cameras with a gain setting that does not introduce nonlinearity effects at the present level of irradiance. The results provide guidance on the precision to be expected from surface temperature measurements using the two-color ratio technique in combination with thermographic phosphors and also confirm the importance of highly stable and linear ICCD detectors. At room temperature and low spatial resolution the precision is evaluated to 0.4%.

SU-E-I-103: Detecting Anomalous Pixels and Correlated Artifacts in Digital Detectors from Flat-Field Images.

Anomalous pixels may be defined as those pixels whose exposure response relationship is deviant from the typical, expected or calibrated response. A group of anomalous pixels may Result in visible correlated artifacts. Here we demonstrate an approach to identify anomalous pixels and correlated artifacts using flat-field images. Using manufacturer specific calibration geometry, sets of four flat-field images per detector were obtained with varying input air kerma values (0.5 to 160 μGy) from 9 digital detectors at 6 institutions. Images obtained before and after calibration, with both proper and improper gain maps and structured artifacts were additionally acquired with some detectors. Image analysis methodology under consideration by AAPM Task Group 150 was used.After eliminating 10mm borders, images were divided into square regions (100mm2 ). Anomalous pixels were identified as pixels within each region with valuesabove or below ±3 standard deviations (SD) relative to the mean value of the region. If these pixels were identified in all four images comprising a set, then they were reported as anomalous. Line artifacts were identified as rows and columns with cumulative profile values that were above or below ±3 SD with respect to the mean value of neighboring profiles in the set of four flat-field mages. Results were verified with visual inspection of the images. For four sets of images, the algorithm did not identify any anomalous pixels, and none were spotted on visible inspection as well, while for five sets of images the identified anomalous pixels matched visual inspection results. Anomalous pixel detection failed in regions with an unusually large number of defects and structured noise, since those regions exhibited relatively large SD. Line artifacts consistent with visual analysis were identified correctly when present. A practical approach to identify anomalous pixels and correlated artifacts from flat-field images is demonstrated.

SU-E-I-102: Independent Implementation of AAPM TG-150 Draft Image Receptor Test Recommendations.

To determine whether a proposed suite of generic tests for digital radiography (DR) detectors could be reduced to practice. MATLAB software was developed to analyze images according to descriptions in a document drafted by the TG150 Detector Subgroup. Forprocessing images were acquired directly from the acquisition stations of three DR and one Computed Radiography system. Images included flat-field exposures at the manufacturer's calibration condition, twice the exposure, ½ the exposure, and a low exposure, plus three images of a lead bar pattern in different orientations, also at the calibration condition. The flat field images were analyzed to determine Detector Response; Gain Correction; Signal, Noise, and Signal-to-noise (SNR) Uniformity; SNR Magnitude; and Anomalous Detector Element (del) Identification. The program also allowed visual inspection for evaluation of collimation and non-uniformity. Bar pattern images were analyzed to evaluate spatial resolution by a variance method. Acquisition revealed a number of pitfalls. Some manufacturers have multiple calibration points. For-processing images are not directly available from all systems, and PACS may modify them from their original state. The orientation of the flat field with respect to the anodecathode axis may not be defined by the manufacturer. Care must be taken to ensure collimation outside the edges of detectors, or the software must exclude collimator shadows. The matrix size of images differs among manufacturers, so the size of the region of interest (ROI) for analysis varies from the default size of 100×100 dels, as does the number of ROIs. The approach for dealing with edges and ROIs may affect the numerical results. The detector response function may also affect the interpretation of results. The software successfully implements most of the detector tests recommended by TG150. Comparison of these results with those of the parallel effort will validate the draft test definition.

SU-E-I-101: Initial Implementation and Evaluation of AAPM TG-150 Draft Image Receptor Non-Uniformity Testing Recommendations.

To implement in software the procedures described in AAPM Task Group 150's draft recommendations for image receptor performance testing, and to evaluate the effectiveness and practicality of these procedures. Images of flat fields were acquired using digital x-ray image receptors at 6 cooperating institutions. Four flat field images obtained with each detector spanned a range of input detector air kerma. Software based on AAPM TG150's draft report processed the test images and generated results. Image receptor response and several measures of non-uniformity were evaluated. Images were divided into 10 mm square regions, after eliminating 10 mm borders. For each region, signal (mean), noise (standard deviation) and SNR were calculated. Characteristic signal, noise and SNR were calculated based on average values from all regions. Local non-uniformity for signal (SLN), noise (NLN) and SNR (SNRLN) were expressed as the maximum ratio of the absolute difference between each region's value and its 4 nearest neighbors, to the respective characteristic value. Global non-uniformity (SGN, NGN, SNRGN) were expressed similarly but differences between maximum and minimum values obtained from the regions were used (without comparison to local neighbors). TG150 tests discriminated between good and poorly performing detectors. Improper detector calibration was detectable, with noise non-uniformity proving to be a more sensitive measure than signal or SNR non-uniformity. Detector rotation relative to calibration conditions produced a greater change in signal non-uniformity than the other measures. Image receptor structured noise was characterized by an increase in noise non-uniformity with incident air kerma. AAPM TG150's proposed approach to image receptor testing was implemented and evaluated. The approach appears to be an effective and practical one for routine quality assurance testing of digital radiographic image receptors.

Fiber spectrum extraction for LAMOST based on 2-D exponential polynomial model

A spectrum extraction method based on a two-dimensional (2-D) exponential polynomial model is proposed in this paper. It is presented for extracting the flux of the fiber spectrum from a two-dimensional image observed by Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST). Due to the effects of the environments and the instruments, centers of the fiber spectra deviate from the ideal, and the energy is expanded to the neighbors in the form of a point-spread function (PSF) in two dimensions. Thus we use a 2-D model to fit the PSF. In spatial orientation and wavelength orientation, the flat-field images and the calibration lamp images are respectively used to evaluate the parameters of the 2-D exponential polynomial model. With the 2-D exponential polynomial model, the spectrum flux is extracted based on the least square theory. The experiments are processed through simulated profiles to demonstrate the effectiveness of the proposed algorithm. Especially, its superiority of restraining noise and fitting asymmetric fiber profiles is proved.

Improved micro-pixel detector element for neutron measurement under high pressure

A micro-pixel detector element with robustness for high voltage was developed by an improved fabrication process, and an irradiation experiment was performed using a Cf-252 neutron source. The withstand voltage of the developed element was improved, allowing the element to operate at voltages of up to 750 V at a total pressure of 5 atm (4.1 atm of He and 0.9 atm of CF4). The measured gas gain was approximately 100, and the thermal neutron detection efficiency was estimated to be approximately 70% at a gas pressure of 5 atm. The flat-field image showed good homogeneity and an average gain spread of approximately 8.7%.

Technical Note: Modification of the standard gain correction algorithm to compensate for the number of used reference flat frames in detector performance studies

The x-ray performance evaluation of digital x-ray detectors is based on the calculation of the modulation transfer function (MTF), the noise power spectrum (NPS), and the resultant detective quantum efficiency (DQE). The flat images used for the extraction of the NPS should not contain any fixed pattern noise (FPN) to avoid contamination from nonstochastic processes. The "gold standard" method used for the reduction of the FPN (i.e., the different gain between pixels) in linear x-ray detectors is based on normalization with an average reference flat-field. However, the noise in the corrected image depends on the number of flat frames used for the average flat image. The aim of this study is to modify the standard gain correction algorithm to make it independent on the used reference flat frames. Many publications suggest the use of 10-16 reference flat frames, while other studies use higher numbers (e.g., 48 frames) to reduce the propagated noise from the average flat image. This study quantifies experimentally the effect of the number of used reference flat frames on the NPS and DQE values and appropriately modifies the gain correction algorithm to compensate for this effect. It is shown that using the suggested gain correction algorithm a minimum number of reference flat frames (i.e., down to one frame) can be used to eliminate the FPN from the raw flat image. This saves computer memory and time during the x-ray performance evaluation. The authors show that the method presented in the study (a) leads to the maximum DQE value that one would have by using the conventional method and very large number of frames and (b) has been compared to an independent gain correction method based on the subtraction of flat-field images, leading to identical DQE values. They believe this provides robust validation of the proposed method.

The XDSpres CL-Based Package for Reducing OSIRIS Cross-dispersed Spectra

ABSTRACTWe present a description of the CL-based package XDSpres, which aims at being a complete reducing facility for cross-dispersed spectra taken with the Ohio State Infrared Imager/Spectrometer, as installed at the SOAR telescope. This instrument provides spectra in the range between 1.2 μm and 2.35 μm in a single exposure, with resolving power of R ∼ 1200. XDSpres consists of two tasks: namely, xdflat and doosiris. The former is a completely automated code for preparing normalized flat-field images from raw flat-field exposures. Doosiris was designed to be a complete reduction pipeline, requiring a minimum of user interaction. General steps toward a fully reduced spectrum are explained, as well as the approach adopted by our code. The software is available to the community online.

Calibration and Performance of the Photon-counting Detectors for the Ultraviolet Imaging Telescope (UVIT) of the Astrosat Observatory11Astrosat is an observatory project of the Indian Space Research Organisation (ISRO), with partnership by the Canadian Space Agency (CSA).

ABSTRACTWe describe calibration data and discuss performance of the photon-counting flight detectors for the Ultraviolet Imaging Telescopes on the Astrosat observatory. This article describes dark current, flat-field, and light-spot images for FUV, NUV, and visible band detectors at more than one wavelength setting for each. We also report on nominal-gain and low-gain operations, full- and subwindow read rates, and non-photon-counting modes of operation, all expected to be used in flight. We derive corrections to the event centroids from the CMOS readout arrays, for different centroid algorithms. We derive spatial resolution values for each detector and plots of point-source signal saturation for different flux levels. We also discuss ways to correct for saturation in extended object images.

Dose optimization in pediatric cardiac x‐ray imaging

The aim of this research was to explore x-ray beam parameters with intent to optimize pediatric x-ray settings in the cardiac catheterization laboratory. This study examined the effects of peak x-ray tube voltage (kVp) and of copper (Cu) x-ray beam filtration independently on the image quality to dose balance for pediatric patient sizes. The impact of antiscatter grid removal on the image quality to dose balance was also investigated. Image sequences of polymethyl methacrylate phantoms approximating chest sizes typical of pediatric patients were captured using a modern flat-panel receptor based x-ray imaging system. Tin was used to simulate iodine-based contrast medium used in clinical procedures. Measurements of tin detail contrast and flat field image noise provided the contrast to noise ratio. Entrance surface dose (ESD) and effective dose (E) measurements were obtained to calculate the figure of merit (FOM), CNR2/dose, which evaluated the dose efficiency of the x-ray parameters investigated. The kVp, tube current (mA), and pulse duration were set manually by overriding the system's automatic dose control mechanisms. Images were captured with 0, 0.1, 0.25, 0.4, and 0.9 mm added Cu filtration, for 50, 55, 60, 65, and 70 kVp with the antiscatter grid in place, and then with it removed. For a given phantom thickness, as the Cu filter thickness was increased, lower kVp was favored. Examining kVp alone, lower values were generally favored, more so for thinner phantoms. Considering ESD, the 8.5 cm phantom had the highest FOM at 50 kVp using 0.4 mm of Cu filtration. The 12 cm phantom had the highest FOM at 55 kVp using 0.9 mm Cu, and the 16 cm phantom had highest FOM at 55 kVp using 0.4 mm Cu. With regard to E, the 8.5 and 12 cm phantoms had the highest FOM at 50 kVp using 0.4 mm of Cu filtration, and the 16 cm phantom had the highest FOM at 50 kVp using 0.25 mm Cu. Antiscatter grid removal improved the FOM for a given set of x-ray conditions. Under aforesaid optimal settings, the 8.5 cm phantom FOM improved by 24% and 33% for ESD and E, respectively. Corresponding improvements were 26% and 24% for the 12 cm phantom and 6% and 15% for the 16 cm phantom. For pediatric patients, using 0.25-0.9 mm Cu filtration in the x-ray beam while maintaining 50-55 kVp, depending on patient size, provided optimal x-ray image quality to dose ratios. These settings, adjusted for x-ray tube loading limits and clinically acceptable image quality, should provide a useful strategy for optimizing iodine contrast agent based cardiac x-ray imaging. Removing the antiscatter grid improved the FOM for the 8.5 and 12 cm phantoms, therefore grid removal is recommended for younger children. Improvement for the 16 cm phantom declined into the estimated margin of error for the FOM; the need for grid removal for older children would depend on practical feasibility in the clinical environment.

Lateral diffusion estimation in fully depleted thick CCD using flat field image analysis

Abstract In thick fully depleted CCDs charge carrier transport from the back window to the gates is accompanied by charge diffusion. Lateral diffusion smooths out density variations of the incoming photon flux by redistributing charges spatially. This creates short range positive correlations in recorded amplitudes. Pixel-to-pixel amplitude variations can also be caused by pixel size and quantum efficiency variations. Pixel size variations result in short range negative correlations. Our study shows that the characteristic diffusion width can be extracted from flat field data. The study was performed on fully depleted, thick CCDs produced in a technology study for the Large Synoptic Survey Telescope (LSST). Data were taken in the laboratory at bias voltages between −5 and −40 V. To increase statistical accuracy, images taken in identical conditions were co-added after base line subtraction and master files were produced. A flat field image simulator was developed for statistical comparison of simulated and measured images. Results on diffusion, pixel response variations, data features, analysis and modeling techniques are presented and discussed.

Radiometric Calibration for AgCam

The student-built Agricultural Camera (AgCam) now onboard the International Space Station observes the Earth surface through two linescan cameras with Charge-Coupled Device (CCD) arrays sensitive to visible and near-infrared wavelengths, respectively. The electro-optical components of the AgCam were characterized using precision calibration equipment; a method for modeling and applying these measurements was derived. Correction coefficients to minimize effects of optical vignetting, CCD non-uniform quantum efficiency, and CCD dark current are separately determined using a least squares fit approach. Application of correction coefficients yields significant variability reduction in flat-field images; comparable results are obtained when applied to ground test images.