In radiography, one of the best methods to eliminate image-degrading scatter radiation is the use of anti-scatter grids. However, with high-resolution dynamic imaging detectors, stationary anti-scatter grids can leave grid-line shadows and moiré patterns on the image, depending upon the line density of the grid and the sampling frequency of the x-ray detector. Such artifacts degrade the image quality and may mask small but important details such as small vessels and interventional device features. Appearance of these artifacts becomes increasingly severe as the detector spatial resolution is improved. We have previously demonstrated that, to remove these artifacts by dividing out a reference grid image, one must first subtract the residual scatter that penetrates the grid; however, for objects with anatomic structure, scatter varies throughout the FOV and a spatially differing amount of scatter must be subtracted. In this study, a standard stationary Smit-Rontgen X-ray grid (line density - 70 lines/cm, grid ratio - 13:1) was used with a high-resolution CMOS detector, the Dexela 1207 (pixel size - 75 micron) to image anthropomorphic head phantoms. For a 15 x 15cm FOV, scatter profiles of the anthropomorphic head phantoms were estimated then iteratively modified to minimize the structured noise due to the varying grid-line artifacts across the FOV. Images of the anthropomorphic head phantoms taken with the grid, before and after the corrections, were compared demonstrating almost total elimination of the artifact over the full FOV. Hence, with proper computational tools, antiscatter grid artifacts can be corrected, even during dynamic sequences.
A small animal micro-CT system was built using an EMCCD detectors having complex pre-digitization amplification technology, high-resolution, high-sensitivity and low-noise. Noise in CBCT reconstructed images when using predigitization amplification behaves differently than commonly used detectors and warrants a detailed investigation. In this study, noise power and contrast sensitivity were estimated for the newly built system. Noise analysis was performed by scanning a water phantom. Tube voltage was lowered to minimum delivered by the tube (20 kVp and 0.5 mA) and detector gain was varied. Contrast sensitivity was analyzed by using a phantom containing different iodine contrast solutions (20% to 70%) filled in six different tubes. First, we scanned the phantom using various x-ray exposures at 40 kVp while changing the gain to maintain the background air value of the projection images constant. Next, the exposure was varied while the detector gain was maintained constant. Radial NPS plots show that noise power level increases as gain increases. Contrast sensitivity was analyzed by calculating ratio of signal-to-noise ratios (SNR) for increased gain with those of low constant gain at each exposure. The SNR value at low constant gain was always lower than SNR of high detector gain at all x-ray settings and iodine contrast. The largest increase of SNR approached 1.3 for low contrast feature for an iodine concentration of 20%. Despite an increase in noise level as gain increases, the SNR improvement shows that signal level also increases because of the unique on-chip gain of the detector.
The aim of this work is to develop a method to calculate lens dose for fluoroscopically-guided neuro-interventional
procedures and for CBCT scans of the head. EGSnrc Monte Carlo software is used to determine the dose to the lens of
the eye for the projection geometry and exposure parameters used in these procedures. This information is provided by a
digital CAN bus on the Toshiba Infinix C-Arm system which is saved in a log file by the real-time skin-dose tracking
system (DTS) we previously developed. The x-ray beam spectra on this machine were simulated using BEAMnrc. These
spectra were compared to those determined by SpekCalc and validated through measured percent-depth-dose (PDD)
curves and half-value-layer (HVL) measurements. We simulated CBCT procedures in DOSXYZnrc for a CTDI head
phantom and compared the surface dose distribution with that measured with Gafchromic film, and also for an SK150
head phantom and compared the lens dose with that measured with an ionization chamber. Both methods demonstrated
good agreement. Organ dose calculated for a simulated neuro-interventional-procedure using DOSXYZnrc with the
Zubal CT voxel phantom agreed within 10% with that calculated by PCXMC code for most organs. To calculate the lens
dose in a neuro-interventional procedure, we developed a library of normalized lens dose values for different projection
angles and kVp’s. The total lens dose is then calculated by summing the values over all beam projections and can be
included on the DTS report at the end of the procedure.
A novel amorphous selenium (a-Se) direct detector with CMOS readout has been designed, and relative detector performance investigated. The detector features include a 25μm pixel pitch, and 1000μm thick a-Se layer operating at 10V/μm bias field. A simulated detector DQE was determined, and used in comparative calculations of the Relative Object Detectability (ROD) family of prewhitening matched-filter (PWMF) observer and non-pre-whitening matched filter (NPWMF) observer model metrics to gauge a-Se detector performance against existing high resolution micro-angiographic fluoroscopic (MAF) detectors and a standard flat panel detector (FPD). The PWMF-ROD or ROD metric compares two x-ray imaging detectors in their relative abilities in imaging a given object by taking the integral over spatial frequencies of the Fourier transform of the detector DQE weighted by an object function, divided by the comparable integral for a different detector. The generalized-ROD (G-ROD) metric incorporates clinically relevant parameters (focal- spot size, magnification, and scatter) to show the degradation in imaging performance for detectors that are part of an imaging chain. Preliminary ROD calculations using simulated spheres as the object predicted superior imaging performance by the a-Se detector as compared to existing detectors. New PWMF-G-ROD and NPWMF-G-ROD results still indicate better performance by the a-Se detector in an imaging chain over all sphere sizes for various focal spot sizes and magnifications, although a-Se performance advantages were degraded by focal spot blurring. Nevertheless, the a-Se technology has great potential to provide break- through abilities such as visualization of fine details including of neuro-vascular perforator vessels and of small vascular devices.
Cone beam computed tomography (CBCT) systems with rotational gantries that have standard flat panel detectors (FPD) are widely used for the 3D rendering of vascular structures using Feldkamp cone beam reconstruction algorithms. One of the inherent limitations of these systems is limited resolution (<;3 lp/mm). There are systems available with higher resolution but their small FOV limits them to small animal imaging only. In this work, we report on region-of-interest (ROI) CBCT with a high resolution CMOS detector (75 μm pixels, 600 μm HR-CsI) mounted with motorized detector changer on a commercial FPD-based C-arm angiography gantry (194 μm pixels, 600 μm HL-CsI). A cylindrical CT phantom and neuro stents were imaged with both detectors. For each detector a total of 209 images were acquired in a rotational protocol. The technique parameters chosen for the FPD by the imaging system were used for the CMOS detector. The anti-scatter grid was removed and the incident scatter was kept the same for both detectors with identical collimator settings. The FPD images were reconstructed for the 10 cm x10 cm FOV and the CMOS images were reconstructed for a 3.84 cm x 3.84 cm FOV. Although the reconstructed images from the CMOS detector demonstrated comparable contrast to the FPD images, the reconstructed 3D images of the neuro stent clearly showed that the CMOS detector improved delineation of smaller objects such as the stent struts (~70 μm) compared to the FPD. Further development and the potential for substantial clinical impact are suggested.
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.
High-resolution 3D bone-tissue structure measurements may provide information critical to the understanding of the bone regeneration processes and to the bone strength assessment. Tissue engineering studies rely on such nondestructive measurements to monitor bone graft regeneration area. In this study, we measured bone yield, fractal dimension and trabecular thickness through micro-CT slices for different grafts and controls. Eight canines underwent surgery to remove a bone volume (defect) in the canine’s jaw at a total of 44 different locations. We kept 11 defects empty for control and filled the remaining ones with three regenerative materials; NanoGen (NG), a FDA-approved material (n=11), a novel NanoCalcium Sulfate (NCS) material (n=11) and NCS alginate (NCS+alg) material (n=11). After a minimum of four and eight weeks, the canines were sacrificed and the jaw samples were extracted. We used a custombuilt micro-CT system to acquire the data volume and developed software to measure the bone yield, fractal dimension and trabecular thickness. The software used a segmentation algorithm based on histograms derived from volumes of interest indicated by the operator. Using bone yield and fractal dimension as indices we are able to differentiate between the control and regenerative material (p<0.005). Regenerative material NCS showed an average 63.15% bone yield improvement over the control sample, NCS+alg showed 55.55% and NanoGen showed 37.5%. The bone regeneration process and quality of bone were dependent upon the position of defect and time period of healing. This study presents one of the first quantitative comparisons using non-destructive Micro-CT analysis for bone regenerative material in a large animal with a critical defect model. Our results indicate that Micro-CT measurement could be used to monitor invivo bone regeneration studies for greater regenerative process understanding.
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.
Compton scatter is the main interaction of x-rays with objects undergoing radiographic and fluoroscopic imaging
procedures. Such scatter is responsible for reducing image signal to noise ratio which can negatively impact object
detection especially for low contrast objects. To reduce scatter, possible methods are smaller fields-of-view, larger air
gaps and the use of an anti-scatter grid. Smaller fields of view may not be acceptable and scanned-beam radiography is
not practical for real-time imaging. Air gaps can increase geometric unsharpness and thus degrade image resolution.
Deployment of an anti-scatter grid is not well suited for high resolution imagers due to the unavailability of high line
density grids needed to prevent grid-line artifacts. However, region of interest (ROI) imaging can be used not only for
dose reduction but also for scatter reduction in the ROI. The ROI region receives unattenuated x-rays while the
peripheral region receives x-rays reduced in intensity by an ROI attenuator. The scatter within the ROI part of the image
originates from both the unattenuated ROI and the attenuated peripheral region. The scatter contribution from the
periphery is reduced in intensity because of the reduced primary x-rays in that region and the scatter fraction in the ROI
is thus reduced. In this study, the scatter fraction for various kVp’s, air-gaps and field sizes was measured for a uniform
head equivalent phantom. The scatter fraction in the ROI was calculated using a derived scatter fraction formula, which
was validated with experimental measurements. It is shown that use of a ROI attenuator can be an effective way to
reduce both scatter and patient dose while maintaining the superior image quality of high resolution detectors.
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.
Focal spot size is one of the crucial factors that affect the image quality of any x-ray imaging system. It is, therefore,
important to measure the focal spot size accurately. In the past, pinhole and slit measurements of x-ray focal spots were
obtained using direct exposure film. At present, digital detectors are replacing film in medical imaging so that, although
focal spot measurements can be made quickly with such detectors, one must be careful to account for the generally
poorer spatial resolution of the detector and the limited usable magnification. For this study, the focal spots of a
diagnostic x-ray tube were measured with a 10-μm pinhole using a 194-μm pixel flat panel detector (FPD). The twodimensional
MTF, measured with the Noise Response (NR) Method was used for the correction for the detector blurring.
The resulting focal spot sizes based on the FWTM (Full Width at Tenth Maxima) were compared with those obtained
with a very high resolution detector with 8-μm pixels. This study demonstrates the possible effect of detector blurring on
the focal spot size measurements with digital detectors with poor resolution and the improvement obtained by
deconvolution. Additionally, using the NR method for measuring the two-dimensional MTF, any non-isotropies in
detector resolution can be accurately corrected for, enabling routine measurement of non-isotropic x-ray focal spots. This
work presents a simple, accurate and quick quality assurance procedure for measurements of both digital detector
properties and x-ray focal spot size and distribution in modern x-ray imaging systems.
Increasing complexity of endovascular interventional procedures requires superior x-ray imaging quality. Present stateof-
the-art x-ray imaging detectors may not be adequate due to their inherent noise and resolution limitations. With recent
developments, CMOS based detectors are presenting an option to fulfill the need for better image quality. For this work,
a new CMOS detector has been analyzed experimentally and theoretically in terms of sensitivity, MTF and DQE.
The detector (Dexela Model 1207, Perkin-Elmer Co., London, UK) features 14-bit image acquisition, a CsI phosphor, 75
μm pixels and an active area of 12 cm x 7 cm with over 30 fps frame rate. This detector has two modes of operations
with two different full-well capacities: high and low sensitivity. The sensitivity and instrumentation noise equivalent
exposure (INEE) were calculated for both modes. The detector modulation-transfer function (MTF), noise-power spectra
(NPS) and detective quantum efficiency (DQE) were measured using an RQA5 spectrum. For the theoretical
performance evaluation, a linear cascade model with an added aliasing stage was used.
The detector showed excellent linearity in both modes. The sensitivity and the INEE of the detector were found to be
31.55 DN/μR and 0.55 μR in high sensitivity mode, while they were 9.87 DN/μR and 2.77 μR in low sensitivity mode.
The theoretical and experimental values for the MTF and DQE showed close agreement with good DQE even at
fluoroscopic exposure levels.
In summary, the Dexela detector’s imaging performance in terms of sensitivity, linear system metrics, and INEE
demonstrates that it can overcome the noise and resolution limitations of present state-of-the-art x-ray detectors.
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.
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.
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.
High resolution imaging capabilities are essential for accurately guiding successful endovascular
interventional procedures. Present x-ray imaging detectors are not always adequate due to their
inherent limitations. The newly-developed high-resolution micro-angiographic fluoroscope
(MAF-CCD) detector has demonstrated excellent clinical image quality; however, further
improvement in performance and physical design may be possible using CMOS sensors. We
have thus calculated the theoretical performance of two proposed CMOS detectors which may be
used as a successor to the MAF.
The proposed detectors have a 300 μm thick HL-type CsI phosphor, a 50 μm-pixel CMOS
sensor with and without a variable gain light image intensifier (LII), and are designated MAFCMOS-
LII and MAF-CMOS, respectively. For the performance evaluation, linear cascade
modeling was used. The detector imaging chains were divided into individual stages
characterized by one of the basic processes (quantum gain, binomial selection, stochastic and
deterministic blurring, additive noise). Ranges of readout noise and exposure were used to
calculate the detectors' MTF and DQE.
The MAF-CMOS showed slightly better MTF than the MAF-CMOS-LII, but the MAF-CMOSLII
showed far better DQE, especially for lower exposures.
The proposed detectors can have improved MTF and DQE compared with the present high
resolution MAF detector. The performance of the MAF-CMOS is excellent for the angiography
exposure range; however it is limited at fluoroscopic levels due to additive instrumentation noise.
The MAF-CMOS-LII, having the advantage of the variable LII gain, can overcome the noise
limitation and hence may perform exceptionally for the full range of required exposures;
however, it is more complex and hence more expensive.
We investigate methods to increase x-ray tube output to enable improved quantum image quality with a higher
generalized-NEQ (GNEQ) while maintaining a small focal-spot size for the new high-resolution Micro-angiographic
Fluoroscope (MAF) Region of Interest (ROI) imaging system. Rather than using a larger focal spot to increase tubeloading
capacity with degraded resolution, we evaluated separately or in combination three methods to increase tube
output: 1) reducing the anode angle and lengthening the filament to maintain a constant effective small focal-spot size, 2)
using the standard medium focal spot viewed from a direction on the anode side of the field and 3) increasing the frame
rate (frames/second) in combination with temporal filter. The GNEQ was compared for the MAF for the small focal-spot
at the central axis, and for the medium focal-spot with a higher output on the anode side as well as for the small focal
spot with different temporal recursive filtering weights. A net output increase of about 4.0 times could be achieved with
a 2-degree anode angle (without the added filtration) and a 4 times longer filament compared to that of the standard
8-degree target. The GNEQ was also increased for the medium
focal-spot due to its higher output capacity and for the
temporally filtered higher frame rate. Thus higher tube output, while maintaining a small effective focal-spot, should be
achievable using one or more of the three methods described with only small modifications of standard x-ray tube
geometry.
We demonstrate a novel approach for achieving patient dose savings during image-guided neurovascular
interventions, involving a combination of a material x-ray region of interest (ROI) attenuator and a spatially different
ROI temporal filtering technique. The part of the image under the attenuator is reduced in dose but noisy and less bright
due to fewer x-ray quanta reaching the detector, as compared to the non-attenuating (or less attenuating) region. First the
brightness is equalized throughout the image by post processing and then a temporal filter with higher weights is applied
to the high attenuating region to reduce the noise, at the cost of increased lag; however, in the regions where less
attenuation is present, a lower temporal weight is needed and is applied to preserve temporal resolution.
A simulation of the technique is first presented on an actual image sequence obtained from an endovascular
image guided interventional (EIGI) procedure. Then the actual implementation of the technique with a physical ROI
attenuator is presented. Quantitative analysis including noise analysis and integral dose calculations are presented to
validate the proposed technique.
Due to the need for high-resolution angiographic and interventional vascular imaging, a Micro-Angiographic
Fluoroscope (MAF) detector with a Control, Acquisition, Processing, and Image Display System (CAPIDS) was
installed on a detector changer, which was attached to the C-arm of a clinical angiographic unit at a local hospital. The
MAF detector provides high-resolution, high-sensitivity, and
real-time imaging capabilities and consists of a 300 μm thick
CsI phosphor, a dual stage micro-channel plate light image intensifier (LII) coupled to a fiber optic taper (FOT),
and a scientific grade frame-transfer CCD camera, providing an image matrix of 1024×1024 35 μm effective square
pixels with 12 bit depth. The changer allows the MAF
region-of-interest (ROI) detector to be inserted in front of the
Image Intensifier (II) when higher resolution is needed during angiographic or interventional vascular imaging
procedures, e.g. endovascular stent deployment. The CAPIDS was developed and implemented using Laboratory
Virtual Instrumentation Engineering Workbench (LabVIEW) software and provides a user-friendly interface that enables
control of several clinical radiographic imaging modes of the MAF including: fluoroscopy, roadmapping, radiography,
and digital-subtraction-angiography (DSA). The total system has been used for image guidance during endovascular
image-guided interventions (EIGI) for diagnosing and treating artery stenoses and aneurysms using self-expanding
endovascular stents and coils in fifteen patient cases, which have demonstrated benefits of using the ROI detector. The
visualization of the fine detail of the endovascular devices and the vessels generally gave the clinicians confidence on
performing neurovascular interventions and in some instances contributed to improved interventions.
Intracranial aneurysm (IA) embolization using Gugliemi Detachable Coils (GDC) under x-ray fluoroscopic guidance is
one of the most important neuro-vascular interventions. Coil deposition accuracy is key and could benefit substantially
from higher resolution imagers such as the micro-angiographic fluoroscope (MAF). The effect of MAF guidance
improvement over the use of standard Flat Panels (FP) is challenging to assess for such a complex procedure. We
propose and investigate a new metric, inter-frame cross-correlation sensitivity (CCS), to compare detector performance
for such procedures. Pixel (P) and histogram (H) CCS's were calculated as one minus the cross-correlation coefficients
between pixel values and histograms for the region of interest at successive procedure steps. IA treatment using GDC's
was simulated using an anthropomorphic head phantom which includes an aneurysm. GDC's were deposited in steps of
3 cm and the procedure was imaged with a FP and the MAF. To measure sensitivity to detect progress of the procedure
by change in images of successive steps, an ROI was selected over the aneurysm location and pixel-value and histogram
changes were calculated after each step. For the FP, after 4 steps, the H and P CCSs between successive steps were
practically zero, indicating that there were no significant changes in the observed images. For the MAF, H and P CCSs
were greater than zero even after 10 steps (30 cm GDC), indicating observable changes. Further, the proposed
quantification method was applied for evaluation of seven patients imaged using the MAF, yielding similar results (H
and P CCSs greater than zero after the last GDC deposition). The proposed metric indicates that the MAF can offer
better guidance during such procedures.
Phantom equivalents of different human anatomical parts are routinely used for imaging system evaluation or dose
calculations. The various recommendations on the generic phantom structure given by organizations such as the AAPM,
are not always accurate when evaluating a very specific task. When we compared the AAPM head phantom containing 3
mm of aluminum to actual neuro-endovascular image guided interventions (neuro-EIGI) occurring in the Circle of Willis,
we found that the system automatic exposure rate control (AERC) significantly underestimated the x-ray parameter
selection. To build a more accurate phantom for neuro-EIGI, we reevaluated the amount of aluminum which must be
included in the phantom. Human skulls were imaged at different angles, using various angiographic exposures, at kV's
relevant to neuro-angiography. An aluminum step wedge was also imaged under identical conditions, and a correlation
between the gray values of the imaged skulls and those of the aluminum step thicknesses was established. The average
equivalent aluminum thickness for the skull samples for frontal projections in the Circle of Willis region was found to be
about 13 mm. The results showed no significant changes in the average equivalent aluminum thickness with kV or mAs
variation. When a uniform phantom using 13 mm aluminum and 15 cm acrylic was compared with an anthropomorphic
head phantom the x-ray parameters selected by the AERC system were practically identical. These new findings indicate
that for this specific task, the amount of aluminum included in the head equivalent must be increased substantially from
3 mm to a value of 13 mm.
Region-of-interest (ROI) fluoroscopy takes advantage of the fact that most neurovascular interventional activity is
performed in only a small portion of an x-ray imaging field of view (FOV). The ROI beam filter is an attenuating
material that reduces patient dose in the area peripheral to the object of interest. This project explores a method of
moving the beam-attenuator aperture with the object of interest such that it always remains in the ROI. In this study, the
ROI attenuator, which reduces the dose by 80% in the peripheral region, is mounted on a linear stage placed near the xray
tube. Fluoroscopy is performed using the Microangiographic Fluoroscope (MAF) which is a high-resolution, CCD-based
x-ray detector. A stainless-steel stent is selected as the object of interest, and is moved across the FOV and
localized using an object-detection algorithm available in the IMAQ Vision package of LabVIEW. The ROI is moved to
follow the stent motion. The pixel intensities are equalized in both FOV regions and an adaptive temporal filter
dependent on the motion of the object of interest is implemented inside the ROI. With a temporal filter weight of 5% for
the current image in the peripheral region, the SNR measured is 47.8. The weights inside the ROI vary between 10% and
33% with a measured SNR of 57.9 and 35.3 when the object is stationary and moving, respectively. This method allows
patient dose reduction as well as maintenance of superior image quality in the ROI while tracking the object.
In this study, we evaluated the imaging characteristics of the high-resolution, high-sensitivity micro-angiographic
fluoroscope (MAF) with 35-micron pixel-pitch when used with different commercially-available 300 micron thick
phosphors: the high resolution (HR) and high light (HL) from Hamamatsu. The purpose of this evaluation was to see if
the HL phosphor with its higher screen efficiency could be replaced with the HR phosphor to achieve improved
resolution without an increase in noise resulting from the HR's decreased light-photon yield. We designated the detectors
MAF-HR and MAF-HL and compared them with a standard flat panel detector (FPD) (194 micron pixel pitch and 600
micron thick CsI(Tl)). For this comparison, we used the generalized linear-system metrics of GMTF, GNNPS and
GDQE which are more realistic measures of total system performance since they include the effect of scattered radiation,
focal spot distribution, and geometric un-sharpness. Magnifications (1.05-1.15) and scatter fractions (0.28 and 0.33)
characteristic of a standard head phantom were used. The MAF-HR performed significantly better than the MAF-HL at
high spatial frequencies. The ratio of GMTF and GDQE of the MAF-HR compared to the MAF-HL at 3(6) cycles/mm
was 1.45(2.42) and 1.23(2.89), respectively. Despite significant degradation by inclusion of scatter and object
magnification, both MAF-HR and MAF-HL provide superior performance over the FPD at higher spatial frequencies
with similar performance up to the FPD's Nyquist frequency of 2.5 cycles/mm. Both substantially higher resolution and
improved GDQE can be achieved with the MAF using the HR phosphor instead of the HL phosphor.
We evaluate a new method for measuring the presampled modulation transfer function (MTF) using the noise power
spectrum (NPS) obtained from a few flat-field images acquired at one exposure level. The NPS is the sum of structure,
quantum, and additive instrumentation noise, which are proportional to exposure squared, exposure, and a constant,
respectively, with the spatial-frequency dependence of the quantum noise depending partly on the detector MTF.
Cascaded linear-systems theory was used to derive an exact and generic relationship that was used to isolate noise terms
and enable determination of the MTF directly from the noise response, thereby circumventing the need for precision test
objects (slit, edge, etc.) as required by standard techniques. Isolation of the quantum NPS by fitting the total NPS versus
exposure obtained using 30 flat-field images each at six or more different exposure levels with a linear regression
provides highly accurate MTFs. A subset of these images from indirect digital detectors was used to investigate the
accuracy of measuring the MTF from 30 or fewer flat-field images obtained at a single exposure level. Analyzing as few
as two images acquired at a single exposure resulted in no observable systematic error. Increasing the number of images
analyzed resulted in an increase in accuracy. Fifteen images provided comparable accuracy with the most rigorous slope
approach, with less than 5% variability, suggesting additional image acquisitions may be unnecessary. Reducing the
number of images acquired for the noise response method further simplifies and facilitates routine MTF measurements.
Due to the high-resolution needs of angiographic and interventional vascular imaging, a Micro-Angiographic Fluoroscope (MAF) detector with a Control, Acquisition, Processing, and Image Display System (CAPIDS) was installed on a detector changer which was attached to the C-arm of a clinical angiographic unit. The MAF detector
provides high-resolution, high-sensitivity, and real-time imaging capabilities and consists of a 300 μm-thick CsI phosphor, a dual stage micro-channel plate light image intensifier (LII) coupled to a fiber optic taper (FOT), and a
scientific grade frame-transfer CCD camera, providing an image matrix of 1024×1024 35 μm square pixels with 12 bit
depth. The Solid-State X-Ray Image Intensifier (SSXII) is an EMCCD (Electron Multiplying charge-coupled device)
based detector which provides an image matrix of 1k×1k 32 μm square pixels with 12 bit depth. The changer allows the
MAF or a SSXII region-of-interest (ROI) detector to be inserted in front of the standard flat-panel detector (FPD) when higher resolution is needed during angiographic or interventional vascular imaging procedures. The CAPIDS was developed and implemented using LabVIEW software and provides a user-friendly interface that enables control of several clinical radiographic imaging modes of the MAF or SSXII including: fluoroscopy, roadmapping, radiography,
and digital-subtraction-angiography (DSA). The total system has been used for image guidance during endovascular
image-guided interventions (EIGI) using prototype self-expanding asymmetric vascular stents (SAVS) in over 10 rabbit aneurysm creation and treatment experiments which have demonstrated the system's potential benefits for future clinical use.
The new Solid State X-Ray Image Intensifier (SSXII) has the unique ability to operate in single photon counting (SPC)
mode, with improved resolution, as well as in traditional energy integrating (EI) mode. The SSXII utilizes an electron-multiplying
CCD (EMCCD), with an effective pixel size of 32μm, which enables variable signal amplification (up to a
factor of 2000) prior to digital readout, providing very
high-sensitivity capabilities. The presampled MTF was measured
in both imaging modes using the standard angulated-slit method. A measured detector entrance exposure of 24μR per
frame was used to provide approximately 0.8 interaction events per pixel in the 10μm-wide slit area. For demonstration
purposes, a simple thresholding technique was used to localize events in SPC mode and a number of such frames were
summed to provide an image with the same total exposure used for acquiring the EI image. The MTF for SPC mode,
using a threshold level of 15% of the maximum 12-bit signal and 95% of the expected events, and for EI mode (in
parentheses) was 0.67 (0.20), 0.37 (0.07), 0.20 (0.03), and 0.11 (0.01) at 2.5, 5, 7.5, and 10 cycles/mm, respectively.
Increasing the threshold level resulted in a corresponding increase in the measured SPC MTF and a lower number of
detected events, indicating a tradeoff between resolution and count efficiency is required. The SSXII in SPC mode was
shown to provide substantial improvements in resolution relative to traditional EI mode, which should benefit
applications that have demanding spatial resolution requirements, such as mammography.
The MTF, NNPS, and DQE are standard linear system metrics used to characterize intrinsic detector performance.
To evaluate total system performance for actual clinical conditions, generalized linear system metrics (GMTF,
GNNPS and GDQE) that include the effect of the focal spot distribution, scattered radiation, and geometric
unsharpness are more meaningful and appropriate. In this study, a two-dimensional (2D) generalized linear system
analysis was carried out for a standard flat panel detector (FPD) (194-micron pixel pitch and 600-micron thick CsI)
and a newly-developed, high-resolution, micro-angiographic fluoroscope (MAF) (35-micron pixel pitch and 300-
micron thick CsI). Realistic clinical parameters and x-ray spectra were used. The 2D detector MTFs were calculated
using the new Noise Response method and slanted edge method and 2D focal spot distribution measurements were
done using a pin-hole assembly. The scatter fraction, generated for a uniform head equivalent phantom, was
measured and the scatter MTF was simulated with a theoretical model. Different magnifications and scatter fractions
were used to estimate the 2D GMTF, GNNPS and GDQE for both detectors. Results show spatial non-isotropy for
the 2D generalized metrics which provide a quantitative description of the performance of the complete imaging
system for both detectors. This generalized analysis demonstrated that the MAF and FPD have similar capabilities at
lower spatial frequencies, but that the MAF has superior performance over the FPD at higher frequencies even when
considering focal spot blurring and scatter. This 2D generalized performance analysis is a valuable tool to evaluate
total system capabilities and to enable optimized design for specific imaging tasks.
X-ray equipment testing using phantoms that mimic the specific human anatomy, morphology, and structure is a very
important step in the research, development, and routine quality assurance for such equipment. Although the NEMA
XR21 phantom exists for cardiac applications, there is no such standard phantom for neuro-, peripheral and cardiovascular
angiographic applications. We have extended the application of the NEMA XR21-2000 phantom to evaluate
neurovascular x-ray imaging systems by structuring it to be
head-equivalent; two aluminum plates shaped to fit into the
NEMA phantom geometry were added to a 15 cm thick section. Also, to enable digital subtraction angiography (DSA)
testing, two replaceable central plates with a hollow slot were made so that various angiographic sections could be
inserted into the phantom. We tested the new modified phantom using a flat panel C-arm unit dedicated for endovascular
image-guided interventions. All NEMA XR21-2000 standard test sections were used in evaluations with the new "headequivalent"
phantom. DSA and DA are able to be tested using two standard removable blocks having simulated arteries
of various thickness and iodine concentrations (AAPM Report 15). The new phantom modifications have the benefits of
enabling use of the standard NEMA phantom for angiography in both neuro- and cardio-vascular applications, with the
convenience of needing only one versatile phantom for multiple applications. Additional benefits compared to using
multiple phantoms are increased portability and lower cost.
The solid-state x-ray image intensifier (SSXII) is an EMCCD-based x-ray detector designed to satisfy an increasing need
for high-resolution real-time images, while offering significant improvements over current flat panel detectors (FPDs)
and x-ray image intensifiers (XIIs). FPDs are replacing XIIs because they reduce/eliminate veiling glare, pincushion or s-shaped
distortions and are physically flat. However, FPDs suffer from excessive lag and ghosting and their performance
has been disappointing for low-exposure-per-frame procedures due to excessive instrumentation-noise. XIIs and FPDs
both have limited resolution capabilities of ~3 cycles/mm. To overcome these limitations a prototype SSXII module has
been developed, consisting of a 1k x 1k, 8 μm pixel EMCCD with a fiber-optic input window, which views a 350 μm
thick CsI(Tl) phosphor via a 4:1 magnifying fiber-optic-taper (FOT). Arrays of such modules will provide a larger field-of-
view. Detector MTF, DQE, and instrumentation-noise equivalent exposure (INEE) were measured to evaluate the
SSXIIs performance using a standard x-ray spectrum (IEC RQA5), allowing for comparison with current state-of-the-art
detectors. The MTF was 0.20 at 3 cycles/mm, comparable to standard detectors, and better than 0.05 up to 7 cycles/mm,
well beyond current capabilities. DQE curves indicate no degradation from high-angiographic to low-fluoroscopic
exposures (< 2% deviation in overall DQE from 1.3 mR to 2.7 μR), demonstrating negligible instrumentation-noise,
even with low input signal intensities. An INEE of < 0.2 μR was measured for the highest-resolution mode (32 μm
effective pixel size). Comparison images between detector technologies qualitatively demonstrate these improved
imaging capabilities provided by the SSXII.
New advances in catheter technology and remote actuation for minimally invasive procedures are continuously
increasing the demand for better x-ray imaging technology. The new x-ray high-sensitivity Micro-Angiographic
Fluoroscope (HS-MAF) detector offers high resolution and real-time image-guided capabilities which are unique when
compared with commercially available detectors. This detector consists of a 300 μm CsI input phosphor coupled to a
dual stage GEN2 micro-channel plate light image intensifier (LII), followed by minifying fiber-optic taper coupled to a
CCD chip. The HS-MAF detector image array is 1024X1024 pixels, with a 12 bit depth capable of imaging at 30 frames
per second. The detector has a round field of view with 4 cm diameter and 35 microns pixels. The LII has a large
variable gain which allows usage of the detector at very low exposures characteristic of fluoroscopic ranges while
maintaining very good image quality. The custom acquisition program allows real-time image display and data storage.
We designed a set of in-vivo experimental interventions in which placement of specially designed endovascular stents
were evaluated with the new detector and with a standard x-ray image intensifier (XII). Capabilities such fluoroscopy,
angiography and ROI-CT reconstruction using rotational angiography data were implemented and verified. The images
obtained during interventions under radiographic control with the HS-MAF detector were superior to those with the XII.
In general, the device feature markers, the device structures, and the vessel geometry were better identified with the new
detector. High-resolution detectors such as HS-MAF can vastly improve the accuracy of localization and tracking of
devices such stents or catheters.
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