Higher resolution in dynamic radiological imaging such as angiography is increasingly being demanded by clinicians; however, when standard anti-scatter grids are used with such new high resolution detectors, grid-line artifacts become more apparent resulting in increased structured noise that may overcome the contrast signal improvement benefits of the scatter-reducing grid. Although grid-lines may in theory be eliminated by dividing the image of a patient taken with the grid by a flat-field image taken with the grid obtained prior to the clinical image, unless the remaining additive scatter contribution is subtracted in real-time from the dynamic clinical image sequence before the division by the reference image, severe grid-line artifacts may remain. To investigate grid-line elimination, a stationary Smit Rӧntgen X-ray grid (line density: 70 lines/cm, grid ratio 13:1) was used with both a 75 micron-pixel CMOS detector and a standard 194 micron-pixel flat panel detector (FPD) to image an artery block insert placed in a modified uniform frontal head phantom for a 20 x 20cm FOV (approximately). Contrast and contrast-to-noise ratio (CNR) were measured with and without scatter subtraction prior to grid-line correction. The fixed pattern noise caused by the grid was substantially higher for the CMOS detector compared to the FPD and caused a severe reduction of CNR. However, when the scatter subtraction corrective method was used, the removal of the fixed pattern noise (grid artifacts) became evident resulting in images with improved CNR.
A family of imaging task-specific metrics designated Relative Object Detectability (ROD) metrics was developed to enable objective, quantitative comparisons of different x-ray systems. Previously, ROD was defined as the integral over spatial frequencies of the Fourier Transform of the object function, weighted by the detector DQE for one detector, divided by the comparable integral for another detector. When effects of scatter and focal spot unsharpness are included, the generalized metric, GDQE, is substituted for the DQE, resulting in the G-ROD metric. The G-ROD was calculated for two different detectors with two focal spot sizes using various-sized simulated objects to quantify the improved performance of new high-resolution CMOS detector systems. When a measured image is used as the object, a Generalized Measured Relative Object Detectability (GM-ROD) value can be generated. A neuro-vascular stent (Wingspan) was imaged with the high-resolution Micro-Angiographic Fluoroscope (MAF) and a standard flat panel detector (FPD) for comparison using the GM-ROD calculation. As the lower integration bound increased from 0 toward the detector Nyquist frequency, increasingly superior performance of the MAF was evidenced. Another new metric, the R-ROD, enables comparing detectors to a reference detector of given imaging ability. R-RODs for the MAF, a new CMOS detector and an FPD will be presented. The ROD family of metrics can provide quantitative more understandable comparisons for different systems where the detector, focal spot, scatter, object, techniques or dose are varied and can be used to optimize system selection for given imaging tasks.
Anti-scatter grids are used in fluoroscopic systems to improve image quality by absorbing scattered radiation. A
stationary Smit Rontgen X-ray grid (line density: 70 lines/cm, grid ratio: 13:1) was used with a flat panel detector (FPD)
of pixel size 194 micron and a high-resolution CMOS detector, the Dexela 1207 with pixel size of 75 microns. To
investigate the effectiveness of the grid, a simulated artery block was placed in a modified uniform frontal head
phantom and imaged with both the FPD and the Dexela for an approximately 15 x 15 cm field of view (FOV).
The contrast improved for both detectors with the grid. The contrast-to-noise ratio (CNR) does not increase as much in
the case of the Dexela as it improves in the case of the FPD. Since the total noise in a single frame increases
substantially for the Dexela compared to the FPD when the grid is used, the CNR is degraded. The increase in the
quantum noise per frame would be similar for both detectors when the grid is used due to the attenuation of radiation,
but the fixed pattern noise caused by the grid was substantially higher for the Dexela compared to the FPD and hence
caused a severe reduction of CNR.
Without further corrective methods this grid should not be used with high-resolution fluoroscopic detectors because the
CNR does not improve significantly and the visibility of low contrast details may be reduced. Either an anti-scatter grid
of different design or an additional image processing step when using a similar grid would be required to deal with the
problem of scatter for high resolution detectors and the structured noise of the grid pattern.
Relative object detectability (ROD) quantifies the relative performance of two image detectors for a specified object of
interest by taking the following ratio: the integral of detective quantum efficiency of a detector weighted by the
frequency spectrum of the object divided by that for a second detector. Four different detectors, namely the microangiographic
fluoroscope (MAF), the Dexela Model 1207 (Dex) and Hamamatsu Model C10901D-40 (Ham) CMOS xray
detectors, and a flat-panel detector (FPD) were compared. The ROD was calculated for six pairs of detectors: (1)
Dex/FPD, (2) MAF/FPD, (3) Ham/FPD, (4) Dex/Ham, (5) MAF/Ham and (6) MAF/Dex for wires of 5 mm fixed
length, solid spheres ranging in diameter from 50 to 600 microns, and four simulated iodine-filled blood vessels of outer
diameters 0.4 and 0.5 mm, each with wall thicknesses of 0.1 and 0.15 mm.
Marked variation of ROD for the wires and spheres is demonstrated as a function of object size for the various detector
pairs. The ROD of all other detectors relative to the FPD was much greater than one for small features and approached
1.0 as the diameter increased. The relative detectability of simulated small iodine-filled blood vessels for all detector
pairs was seen to be independent of the vessel wall thickness for the same inner diameter. In this study, the ROD is
shown to have the potential to be a useful figure of merit to evaluate the relative performance of two detectors for a
given imaging task.
Neuro-EIGIs require visualization of very small endovascular devices and small vessels. A Microangiographic Fluoroscope (MAF) x-ray detector was developed to improve on the standard flat panel detector’s (FPD’s) ability to visualize small objects during neuro-EIGIs. To compare the performance of FPD and MAF imaging systems, specific imaging tasks related to those encountered during neuro-EIGIs were used to assess contrast to noise ratio (CNR) of different objects. A bar phantom and a stent were placed at a fixed distance from the x-ray focal spot to mimic a clinical imaging geometry and both objects were imaged by each detector system. Imaging was done without anti-scatter grids and using the same conditions for each system including: the same x-ray beam quality, collimator position, source to imager distance (SID), and source to object distance (SOD). For each object, relative contrasts were found for both imaging systems using the peak and trough signals. The relative noise was found using mean background signal and background noise for varying detector exposures. Next, the CNRs were found for these values for each object imaged and for each imaging system used. A relative CNR metric is defined and used to compare detector imaging performance. The MAF utilizes a temporal filter to reduce the overall image noise. The effects of using this filter with the MAF while imaging the clinical object’s CNRs are reported. The relative CNR for the detectors demonstrated that the MAF has superior CNRs for most objects and exposures investigated for this specific imaging task.
KEYWORDS: Sensors, X-rays, CMOS sensors, Electrons, X-ray detectors, Digital filtering, X-ray imaging, Scintillators, Signal to noise ratio, Imaging systems
The detectors that are used for endovascular image-guided interventions (EIGI), particularly for neurovascular
interventions, do not provide clinicians with adequate visualization to ensure the best possible treatment outcomes.
Developing an improved x-ray imaging detector requires the determination of estimated clinical x-ray entrance
exposures to the detector. The range of exposures to the detector in clinical studies was found for the three modes of
operation: fluoroscopic mode, high frame-rate digital angiographic mode (HD fluoroscopic mode), and DSA mode.
Using these estimated detector exposure ranges and available CMOS detector technical specifications, design
requirements were developed to pursue a quantum limited, high resolution, dynamic x-ray detector based on a CMOS
sensor with 50 μm pixel size. For the proposed MAF-CMOS, the estimated charge collected within the full exposure
range was found to be within the estimated full well capacity of the pixels. Expected instrumentation noise for the
proposed detector was estimated to be 50-1,300 electrons. Adding a gain stage such as a light image intensifier would
minimize the effect of the estimated instrumentation noise on total image noise but may not be necessary to ensure
quantum limited detector operation at low exposure levels. A recursive temporal filter may decrease the effective total
noise by 2 to 3 times, allowing for the improved signal to noise ratios at the lowest estimated exposures despite
consequent loss in temporal resolution. This work can serve as a guide for further development of dynamic x-ray
imaging prototypes or improvements for existing dynamic x-ray imaging systems.
The new Solid State X-ray Image Intensifier (SSXII) is a high-resolution, high-sensitivity, real-time region-ofinterest
(ROI) x-ray imaging detector. Evaluations were made of both standard linear systems metrics (MTF, DQE)
and total system performance with generalized linear systems metrics (GMTF, GDQE) including scatter and
geometric un-sharpness for simulated clinical conditions.
The SSXII is based on a 1k x 1k EMCCD sensor coupled to a 300 μm thick CsI(Tl) phosphor through a 2.88:1 fiber
optic taper resulting in a 37 μm effective pixel size and an effective 3.7 cm x 3.7 cm square field-of-view (FOV).
Standard methods were used to calculate MTF, NNPS and DQE. Generalized metrics were calculated and compared
for three different magnifications (1.03, 1.11 and 1.2) and three different focal spots (0.3 mm, 0.5 mm and 0.8 mm)
for a scatter fraction of 0.28.
For an RQA5 spectrum, at 5 cycles/mm the MTF was found to be 0.06 and DQE was 0.04, while the DQE(0) was
0.60. Focal spot un-sharpness and scatter significantly degrades the GMTF and GDQE performance of the detector.
A low frequency drop is caused by scatter and is almost independent of focal spot size and magnification. The
degradation for middle range frequencies is caused by geometric un-sharpness and increases with focal spot size and
magnification. This degradation was least in the case of the small focal spot and almost independent of
magnification.
In spite of this degradation, the high resolution SSXII with a small FOV may have a significant impact on ROI
image-guided neuro-interventions since it demonstrates far better performance than standard current detectors.
Present day treatment for neurovascular pathological conditions involves the use of devices with
very small features such as stents, coils, and balloons; hence, these interventional procedures demand high resolution xray
imaging under fluoroscopic conditions to provide the capability to guide the deployment of these fine endovascular
devices. To address this issue, a high resolution x-ray detector based on EMCCD technology is being developed. The
EMCCD field-of-view is enlarged using a fiber-optic taper so that the detector features an effective pixel size of 37 μm
giving it a Nyquist frequency of 13.5 lp/mm, which is significantly higher than that of the state of the art Flat Panel
Detectors (FPD). Quantitative analysis of the detector, including gain calibration, instrumentation noise equivalent
exposure (INEE) and modulation transfer function (MTF) determination, are presented in this work. The gain of the
detector is a function of the detector temperature; with the detector cooled to 50 C, the highest relative gain that could be
achieved was calculated to be 116 times. At this gain setting, the lowest INEE was measured to be 0.6 μR/frame. The
MTF, measured using the edge method, was over 2% up to 7 cycles/ mm. To evaluate the performance of the detector
under clinical conditions, an aneurysm model was placed over an anthropomorphic head phantom and a coil was guided
into the aneurysm under fluoroscopic guidance using the detector. Image sequences from the procedure are presented
demonstrating the high resolution of this SSXII.
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