Solid State X-ray Image Intensifier Research Articles

TU-A-116-05: Evaluation of Relative X-Ray Imaging Detector Performance Using the Metric of Relative Object Detectability (ROD)

Purpose: To determine the relative detectability performance of two x‐ray imaging detectors for specific clinical objects and devices. Methods: The relative object detectability (ROD) can be defined as the ratio of the integral of the detective quantum efficiency (DQE) of a detector weighted by the square of the Fourier Transform (or frequency spectrum) of the object to the same integral for a second detector. Three different detectors, namely the micro‐angiographic fluoroscope (MAF), the solid state x‐ray image intensifier (SSXII) and a flat panel detector (FPD), were assessed for their relative performance with respect to the detectability of wires and spheres. The RODs were calculated for three sets of detectors: MAF‐FPD, MAF‐SSXII and SSXII‐FPD, for wires of a fixed length of 20 mm and solid spheres made of different materials (aluminum, titanium and nickel) ranging in diameters from 100 micron to 1.0 mm to simulate endovascular devices. Results: For the 100‐micron diameter, the ROD is 1.4 [2.0] for the MAF‐SSXII and SSXII‐FPD sets and 1.9 [3.9] for the MAF‐FPD for the wire [sphere] indicating superior performance of the MAF over the FPD and SSXII and of the SSXII over the FPD for small sized objects. However, the ROD decreased monotonically as the object diameter increased and showed equivalent performance for wires with diameter over 300 microns and spheres with diameters above 400 microns. Conclusion: The ROD depends upon the physical geometry of the object and is useful in quantifying the relative performance of detectors regarding detectability of specific objects. Because of their higher spatial resolution both the MAF and the SSXII perform better than the FPD for viewing wires with diameters under 300 microns and spheres with diameters under 400 microns; however, the MAF performance is shown to be superior to the SSXII as well. NIH Grant 2R01EB002873

SU-D-134-02: Selection of Magnification and Focal Spot Size for Optimizing GMTF for Object Specific Spatial Frequencies

Purpose: To find the optimum geometric magnification (Mopt) for a specific range of spatial frequencies in the object plane by taking the focal spot blur into account for three different detectors. Methods: The generalized modulation transfer functions (GMTFs) (which gives spatial resolution referenced to the object plane) were calculated for a range of magnifications for two focal spots (small [0.3 mm] and medium [0.5 mm]) for three different detectors, namely the micro‐angiographic fluoroscope (MAF), the solid‐state x‐ray image intensifier (SSXII) and a flat‐panel detector (FPD). The magnification factor (Mopt) corresponding to the maximum GMTF value for a particular frequency was determined as a function of frequency in the range of (1–2.5) cycles/mm for the FPD and (1– 6) cycles/mm for the other two detectors. Results: The Mopt decreases from 2 to 1.6 for the FPD and from 2 to 1.2 for both the MAF and the SSXII for the small focal spot in the mid‐frequency range of (1–2.6) cycles/mm while it decreases from 1.2 to 1.1 for both the FPD and the SSXII and from 1.2 to 1.0 for the MAF for the medium focal spot for the same frequency range. For high frequencies (above 2.6 cycles/mm), the Mopt decreases for both the MAF and the SSXII from 1.2 to 1.1 and from 1.1 to 1.0 in the case of small and medium focal spot, respectively. Conclusion: The value of optimum magnification is different in a particular range of frequencies for a given combination of detector and focal spot. For the small focal spot and mid‐frequencies, the Mopt drops sharply for high resolution detectors (MAF and SSXII) compared to the low resolution FPD, while it did not change much for the medium focal spot or for high spatial frequencies for the high resolution detectors. NIH Grant 2R01EB002873

Design, Characterization, and Modeling for a Modular High-Resolution Solid-State X-Ray Image Intensifier (SSXII)

A high resolution x-ray imaging system designated the solid state x-ray image intensifier (SSXII) is being developed as a potential replacement of conventional lower-resolution x-ray detectors for neurovascular interventions. The SSXII consists of a CsI(Tl) phosphor to convert x-rays to light, a fiber optic taper to extend the field-of-view and to couple the phosphor to an Electron-Multiplying CCD (EMCCD) which has a built-in gain to boost the signal above the read-out noise floor. A parallel cascade linear-system model is explored and applied to quantitatively analyze the signal and noise transfer at each stage, and optimize the detector performance. The modulation transfer function (MTF), detective quantum efficiency (DQE) and frequency dependent instrumentation noise equivalent exposure (INEE) can be obtained from the theoretical model and experimental measurement. Agreement of these results validates the effectiveness of the model. Finally, compared with the traditional flat-panel detector (FPD), the SSXII exhibits better performance with higher spatial resolution, higher DQE and lower INEE.

SU-D-218-04: Evaluation of the Performance of the Solid State X-Ray Image Intensifier (SSXII) Using Generalized Metrics.

To evaluate the performance of the Solid State X-ray Image Intensifier (SSXII) using generalized linear-system metrics and to study the effect of different scatter fractions, object magnifications, and focal spots onits performance. The SSXII is a high-resolution and high- sensitivity region-of-interest x-ray imaging detector that provides real-timeimaging with low instrumentation noise. To evaluate the total system performance for a clinical environment, we used generalized metrics that include the effects of scattered radiation, finite focal-spot size, and geometric unsharpness. For comparison, a commercial standard flat-panel detector (FPD) was used. The focal-spot MTF was obtained by taking theFourier Transform of the point-spread function of x-ray pin-hole images. The detector MTF was measured using the standard edge method. The scatter MTF was simulated with a theoretical model. We have calculated the GMTF and GDQE for the SSXII and FPD. Three focal-spots (small, medium, and large), different object magnifications and scatter fractions were used for the GMTF and GDQE comparison. The GMTF andthe GDQE were shown to be degraded significantly from that of the detectoralone at the higher spatial frequencies because of blur due to the finite sizeof the focal-spot, and at the lower frequencies because of scatter. Furthermore, the degradation increases even more as the focal-spot size,object magnification and scatter fraction increases. The GMTF and the GDQE for the FPD were similar to those of the SSXII at lower frequencies, but were limited to frequencies below its 2.5 cycles/mm Nyquist frequency due to the 194-micron pixel size compared to the 18.9 cycles/mm Nyquist frequency of the 26.4-micron pixel-size SSXII. This work demonstrates that the SSXII and the FPD have similar performance at thelower spatial frequencies, whereas, the SSXII demonstrates superior performance over the FPD at higher frequencies. Supported in part by: NIH Grants R01-EB008425, R01-EB002873 and an equipment grant from Toshiba Medical Systems Corp.

TU-A-218-04: Phantom Studies of a Newly Developed Solid State X-Ray Image Intensifier (SSXII) for X-Ray Image Guided Neurovascular Interventions

Purpose: X-ray image-guided interventional treatment of vascular pathology such as aneurysms involves precise delivery of devices such as stents and hence may require very high-resolution imaging capability. We report the first fluoroscopy and angiography phantom studies using a new high-resolution x-ray detector that has been developed to address this need. Methods: The Solid State X-ray Image Intensifier (SSXII) detector is based on Electron-Multiplying Charge Coupled Device (EMCCD) technology (E2V Technologies, Chelmsford, England). An EMCCD with 13- μm pixels and fiber optic window is coupled to a fiber-optic taper with a taper ratio of 3.3 that views a 300-μm thick CsI x-ray scintillator, thus making an effective pixel size of 42.9 μm and a Nyquist frequency of 11.8 lp/mm for this detector. A small aneurysm insert that was part of a flow loop was placed in the base of the brain region of an anthropomorphic head phantom (RS-230, Supertech, Elkhart, IN) to simulate pathology in the Circle of Willis of a patient and interventional procedures were done with similar attenuation and anatomical characteristics that would be experienced in the clinic. A guidewire was threaded into the aneurysm under fluoroscopic image guidance with the SSXII. Digital Subtraction Angiography (DSA) was performed using a standard contrast injection (iodine 350 mg/ml) into the flow loop and angiographic sequences were recorded with the new detector. Results: The operation of the new SSXII detector in both fluoroscopy and angiography exposure ranges was demonstrated for the first time on human phantoms. The resolution of this detector is substantially higher than current-state-of-the-art flat panel detectors with 150 to 200 μm pixels. Conclusions: The newly developed high-resolution EMCCD-based detector can potentially be used for both fluoroscopy and angiography to address the high-resolution imaging needs of x-ray image-guided neuro- vascular interventions. Supported in part by: NIH Grants R01-EB008425, R01-EB002873 and an equipment grant from Toshiba Medical Systems Corp.

TU‐C‐211‐05: A New Solid State X‐Ray Image Intensifier (SSXII) with a 1×2 Modular Array and an Acquisition, Correction, and Display System

Purpose: To provide an extended field‐of‐view (FOV) with higher resolution images, a new SSXII imager with a two‐detector modular array was built. An acquisition, correction, and display system tiles two image modules in real‐time with geometric and brightness matching at the boundary. Methods: Two fiber‐optic tapers (FOTs) were fitted together with minimum dead‐space and embedded in liquid Sylgard 184 silicone elastomer which is allowed to cure and solidify; this 1×2 FOT array was fitted into an optical “head‐box.” The EMCCD sensors were fixed to the small‐ends of the two FOTs while a single CsI x‐ray phosphor plate was fastened onto the large‐ends. The optical head‐box was then combined with custom‐built electronics boards that control the driving clocks and sample the output. The digitized frames were transmitted to a PC through a CameraLink port accommodating a rate of 30 fps for both modules simultaneously. Customized software enabled acquisition, correction (dealing with the rotational and translational misalignments between the sensors), and display of the x‐ray images in real‐time. Other functions including acquisition control, save options, temporal filters, flat‐field correction, and mode selection for fluoroscopy, roadmapping, digital angiography (DA) and digital subtraction angiography (DSA), were also implemented. Results: The processed high‐resolution image was aligned properly along the boundary. The match up error was less than 1 pixel although there was a coupling loss of just under 7% due to chamfers at the edges of the FOTs; remachining the FOTs in future versions will reduce this loss. The two aligned images from each module exhibited balanced brightness between the sensors and flat‐field correction to eliminate fixed patterns introduced by the FOTs. Conclusions: The custom‐built SSXII detector and its acquisition, correction, and display system provides higher spatial resolution and nearly seamless images for the array in real‐time. Larger arrays are planned for future SSXII implementations.Support: NIH Grants R01‐EB008425, R01‐EB002873

WE-E-201C-06: The Next Generation Solid State X-Ray Image Intensifier (SSXII)

Purpose: We report on new advancements with a solid state x‐ray image intensifier (SSXII) to expand the field‐of‐view (FOV) and improve detector performance. Method and Materials: The SSXII is a new high‐resolution, high‐sensitivity radiographic and fluoroscopic imager based on electron‐multiplying CCDs (EMCCDs), which views a CsI:Tl phosphor through a fiber optic taper (FOT). The SSXII has demonstrated the capabilities to significantly improve upon the inherent limitations of current state‐of‐the‐art x‐ray image intensifiers and dynamic flat panel detectors(FPD), with superior performance in terms of MTF, instrumentation noise, and DQE. To expand the FOV, a modular array has been constructed. The EMCCD camera electronics have been designed to enable a 2 × 2 abuttable array configuration. The 27 mm center‐to‐center distance enables use of 3.4:1 magnification ratio FOTs, thereby providing a FOV of 5.44 × 5.44 cm (an increase in area of 290% over the initial single module prototype). Larger arrays can be constructed. Performance of this next generation SSXII was extrapolated based on measured MTFs and transmission efficiencies of the individual components that comprise the detector.Results: Calculations indicate an averaged improvement in the MTF of a factor of 1.2 across the higher spatial‐frequency range (5 to 10 cycles/mm). X‐ray sensitivities are expected to increase by 40% due to a reduction in the FOT magnification ratio (from 4:1 to 3.4:1). The instrumentation noise equivalent exposure (INEE) was calculated to decrease from 0.2 to 0.03 μR, which is two orders of magnitude lower than present FPDs. The DQE was determined to improve by an averaged factor of 2.2 from 5 to 10 cycles/mm. Conclusions: With an expanded FOV, the SSXII is a promising candidate to replace existing state‐of‐the‐art detectors, providing improved resolution, lower instrumentation noise, and better signal‐to‐noise ratio performance. (Support: NIH Grant RO1EB008425)

SU‐FF‐I‐170: Control, Acquisition, Processing, and Image Display System (CAPIDS) for the Solid‐State X‐Ray Image Intensifier (SSXII)

Purpose: A new control, acquisition, processing, and image display system (CAPIDS) for the Solid‐State X‐Ray Image Intensifier (SSXII) was developed and implemented using Laboratory Virtual Instrumentation Engineering Workbench (LabVIEW). Method and Materials: The SSXII is an EMCCD‐based x‐ray detector that was designed to satisfy the high‐resolution and real‐time needs of neurovascular imaging. The SSXII includes a CsI(Tl) scintillator and a fiber‐optic taper (FOT) coupled to an EMCCD camera with fiber‐optic input window. The SSXII can be mounted on and used with a clinical fluoroscopic c‐arm unit (Toshiba Infinix). The new CAPIDS provides controls for several radiographic imaging modes including: fluoroscopy, roadmapping, radiography, and digital‐subtraction‐angiography (DSA). Complete functionality of the SSXII is also provided in research mode, including the ability to modify the detector‐element binning, exposure time, video gain, and electron‐multiplying gain of the camera. A camera control‐panel window can be accessed using a keyboard shortcut; this window shows the status parameters of the camera and allows control for all functions. Status words are saved to a text file for reference by the system control program. Results: The CAPIDS provides a user‐friendly interface that allows control of several clinical radiographic imaging modes using the SSXII including: fluoroscopy, roadmapping, radiographic mode, and DSA. Examples of each mode will be demonstrated. Additional features, including recursive filtering, contrast enhancement, patient registry and image archival have been implemented and should facilitate the clinical use of the new EMCCD‐based detector system. Conclusion: The control, acquisition, and display capabilities along with the electron multiplication gain (1–2000×), high‐resolution, high‐sensitivity and real‐time imaging capabilities of the SSXII should provide angiographers and interventionalists with an improved ability to visualize details of small vessels and endovascular devices (such as stents), making diagnoses and image‐guided interventions more accurate.(Research partially supported by: NIH Grants R01‐EB008425, R01‐EB002873)

TH‐C‐210A‐04: New High Resolution Dynamic Detectors and Flow Modifying Stents for Neuro‐Endovascular Image Guided Interventions

As the field of minimally invasive endovascular image‐guided interventions (EIGI) advances, there has been progress in the development of new endovascular devices such as the asymmetric vascular stent for treatment of aneurysms as well as progress in the image guidance systems needed to improve both the diagnoses and interventions and the methods used to evaluate the improved imaging performance. High resolution imaging systems with MTFs extending past 8 Lp/mm and with ability to visualize not only radioopaque markers but the detailed structure of devices such as stents have motivated the development of new asymmetric devices such as the blood flow modifying asymmetric vascular stent (AVS). The AVS has now come through a number of generations from balloon expandable stainless steel strutted structures with laser micro‐welded mesh flow diverters to new super‐elastic nitinol closedcell self‐expanding stents with organic material flow diverters. The methods of laser machining, surface finishing, and deployment will be described with progress in animal models reported. In parallel with advances in EIGI devices has come new high resolution detector development. The detectors have a unique combination of features such as far superior spatial resolution compared with conventional dynamic flat panel detectors, large dynamic range of sensitivity with negligible lag and ghosting to enable both fluoroscopy and angiography, and low noise to enable quantum limited performance over the full useful range including during lowdose fluoroscopy. The Micro‐Angiographic Fluoroscope (MAF) consists of an x‐ray converter phosphor sensed by a micro‐channel plate light image intensifier which is in turn coupled to a high performance CCD camera using a fiber optic taper. The MAF is a region of interest (ROI) imager with 4 cm field of view centered at the interventional site and may be moved in front of a larger conventional detector when improved resolution is needed. The Solid State X‐ray Image Intensifier (SSXII) while having much of the benefits of the MAF in superior imaging capability achieves its great sensitivity using only electron multiplying CCD sensors. An array design is being developed so that the imaging FOV may be expanded by adding modules each with its own EMCCD‐based detector. To more fully characterize detectors, new evaluation methods are being explored. For example, the accurate determination of MTF from measurements of noise only, without the need for a slit or edge will be reported. Also from a careful analysis of noise, the exposure range for detector quantum limited performance can be well demarcated by the instrumentation noise equivalent exposure (INEE). Finally, more realistic linear system parameters that include focal‐spot size, geometry, and scatter provide generalized MTFs and DQEs or GMTFs and GDQEs. All told, there is much happening in EIGI.Learning Objectives1. Appreciate the progress being made in improved EIGI devices and in particular flow modifiers such as the asymmetric vascular2. stent (AVS) for aneurysm treatment.3. Understand the operation of new high‐resolution micro‐angiographic systems including the MAF and SSXII.4. Understand new objective image detector evaluations including INEE, GMTF, GDQE, and determination of MTF from noise5. measurements alone.(Supported in part by NIH Grants R01EB002873, R01 NS43924, R01EB008425, and the Toshiba Medical Systems Corp.)

SU‐FF‐I‐171: New High‐Resolution Detector Changer for a Clinical Fluoroscopic C‐Arm Unit

Purpose: To investigate a means to alternate image acquisition between a high‐resolution region‐of‐interest (ROI) detector and a standard flat‐panel detector (FPD) when used for neurovascular image‐guided interventions with a clinical fluoroscopic C‐arm so that the larger area detector could be used during initial parts of a procedure and the high‐resolution detector used during critical parts of the intervention when better images are needed. Method and Materials: A new detector changer was designed, fabricated, and mounted on a clinical fluoroscopic unit to allow a ROI detector such as the Micro‐Angiographic Fluoroscope (MAF) or Solid‐State X‐ray Image Intensifier (SSXII) to be introduced in front of the standard FPD to enable high‐resolution fluoroscopy, DSA, and conebeam computed tomography. The x‐ray collimation automatically adjusts the field‐of‐view (FOV) when the changer inserts or retracts the ROI detector so that the correct FOV, smaller for the ROI detector or larger for the FPD, respectively, is implemented. A touch sensor at the front of the ROI detector holder causes the entire imaging unit (FPD and ROI detector) to retract upon physical contact in order to avoid collision with a patient or other objects on or near the table. Results: The new detector changer has been installed onto a Toshiba Infinix clinical fluoroscopic C‐arm unit. The changer provides stable support for the ROI detector, allowing operator selection of high‐resolution ROI or standard FPD. The collision avoidance and automatic collimation features have worked reliably during laboratory evaluation and should provide a patient‐safe imaging capability. Conclusion: The new detector changer mounted on a commercial clinical fluoroscopic C‐arm unit provides a safe and rapid means to exchange imaging detectors to accommodate the resolution requirements of angiographic and interventional vascular imaging during the course of a procedure.(Partial support: NIH Grants R01‐EB008425, R01‐EB002873)

Component analysis of a new Solid State X-ray Image Intensifier (SSXII) using photon transfer and Instrumentation Noise Equivalent Exposure (INEE) measurements.

The SSXII is a novel x-ray imager designed to improve upon the performance limitations of conventional dynamic radiographic/fluoroscopic imagers related to resolution, charge-trapping, frame-rate, and instrumentation-noise. The SSXII consists of a CsI:Tl phosphor coupled via a fiber-optic taper (FOT) to an electron-multiplying CCD (EMCCD). To facilitate investigational studies, initial designs enable interchangeability of such imaging components. Measurements of various component and configuration characteristics enable an optimization analysis with respect to overall detector performance. Photon transfer was used to characterize the EMCCD performance including ADC sensitivity, read-noise, full-well capacity and quantum efficiency. X-ray sensitivity was measured using RQA x-ray spectra. Imaging components were analyzed in terms of their MTF and transmission efficiency. The EMCCD was measured to have a very low effective read-noise of less than 1 electron rms at modest EMCCD gains, which is more than two orders-of-magnitude less than flat panel (FPD) and CMOS-based detectors. The variable signal amplification from 1 to 2000 times enables selectable sensitivities ranging from 8.5 (168) to over 15k (300k) electrons per incident x-ray photon with (without) a 4:1 FOT; these sensitivities could be readily increased with further component optimization. MTF and DQE measurements indicate the SSXII performance is comparable to current state-of-the-art detectors at low spatial frequencies and far exceeds them at higher spatial frequencies. The instrumentation noise equivalent exposure (INEE) was measured to be less than 0.3 μR out to 10 cycles/mm, which is substantially better than FPDs. Component analysis suggests that these improvements can be substantially increased with further detector optimization.

MO-D-332-01: Noise Evaluation of a New Solid-State X-Ray Image Intensifier (SSXII)

Purpose: To measure the impact of thermal and multiplication‐excess noise and the instrumentation‐noise equivalent exposure (INEE) of a new solid‐state x‐ray image intensifier (SSXII) based on electron‐multiplying CCDs (EMCCDs). Method and Materials: The SSXII consists of a 350μm thick CsI(Tl) phosphor grown on a fiber‐optic plate optically coupled to a Peltier cooled EMCCD camera with a fiber‐optic input window via a 4:1 minifying fiber‐optic taper. The EMCCD provides signal multiplication gain prior to the read‐out electronics, thereby eliminating this primary noise source, effectively allowing for instrumentation‐noise free operation even at very low x‐ray exposures. Two additional potential noise sources, thermal and multiplication‐excess noise, were investigated. Thermal noise was determined using a linear fit of dark‐signal variance versus integration time at a constant readout rate. Multiplication‐excess noise was determined using signal variance normalized to gain versus signal plots. The gradient of a linear fit to this data at 1x gain as compared to higher gain values (up to 14x) was used to determine the excess noise factor (ENF). Overall noise‐performance was then determined using the INEE. Results:Thermal noise was measured to be < 2% of the maximal signal (12‐bits) with an EMCCD gain of 275x and an integration time of 30 ms, and is considered an upper bound under typical clinical operating conditions, indicating thermal noise has a negligible impact on overall image quality. The ENF was unity for the range of gains tested, suggesting the inherent excess noise of the electronic‐multiplication process has little effect when imaging secondary quanta from an x‐ray converting phosphor. An INEE of < 0.2μR further demonstrates the SSXII's capability to operate instrumentation‐noise free at very low x‐ray exposures. Conclusion: The SSXII provides quantum‐noise‐limited performance well below typical fluoroscopic exposure levels with negligible thermal and multiplication‐excess‐noise. (Support: NIH Grants R01‐NS43924, R01‐EB002873, Toshiba Medical Systems Corporation).

WE‐C‐L100J‐03: The New Solid State X‐Ray Image Intensifier (SSXII): A Demonstration of Operation Over a Range of Angiographic and Fluoroscopic Exposure Levels

Purpose: To demonstrate the SSXII's capability to image at high angiographic to low fluoroscopic exposure levels using the built‐in adjustable gain. Method and Materials: The prototype SSXII is composed of a 1k×1k, 8μm pixel EMCCD camera system coupled to a 350 μm thick CsI(Tl) structured phosphor via a 1:1 fiber‐optic taper (FOT) and is designed for easy interchange of these front‐end components for task‐based optimization. Variable signal amplification (in solid state) from 1x to 2000x effectively eliminates image degradation due to read‐out noise over all input x‐ray intensities. Images were acquired of a stent crimped on a 1 mm diameter balloon‐tip catheter, for various x‐ray spectra, using both radiographic and fluoroscopic exposures ranging from 6 mR to 145 μR and 80 to 2.7 μR, respectively. The EMCCD gain was adjusted in order to maintain a constant output signal of ∼90% of maximum over these exposures (∼3700 digital units). Results: A series of images, obtained by halving the exposure while doubling the gain, offers a near textbook demonstration of the effect of incident SNR on the resulting image quality. Fine structures of a crimped stent are visible for the entire range of radiographic exposures tested, demonstrating the very high‐resolution capabilities of the SSXII. In pulsed fluoroscopic mode, resolving the crimped stent structures becomes difficult below 40 μR. Using high gain, the object was visible down to 2.7 μR, which corresponds to ∼0.1 incident x‐ray photons per pixel, demonstrating both tremendous sensitivity and extremely low effective instrumentation noise. Conclusion: Images obtained with the prototype SSXII, over a vast range of exposure levels, indicate this new dynamic x‐ray imager has the potential to provide significant improvements over current state‐of‐the‐art detectors (both FPDs and XIIs) for high‐resolution angiographic and low‐noise, real‐time fluoroscopic imaging.(Partial support from the UB Foundation and NIH grants R01‐EB002873, R01‐NS43924.)

Improvements of solid state X-ray image intensifiers

The construction of an electroluminescent-photoconductor sandwich-type solid state X-ray image intensifier is described, and sensitivity and response times presented as a function of X-ray intensity. The application as a practical radiological device is discussed.