W Wang

Noise analysis of MAP - EM algorithms for emission tomography

The ability to theoretically model the propagation of photon noise through PET and SPECT tomographic reconstruction algorithms is crucial in evaluating the reconstructed image quality as a function of parameters of the algorithm. In a previous approach for the important case of the iterative ML-EM (maximum-likelihood-expectation-maximization) algorithm, judicious linearizations were used to model theoretically the propagation of a mean image and a covariance matrix from one iteration to the next. Our analysis extends this approach to the case of MAP (maximum a posteriori)-EM algorithms, where the EM approach incorporates prior terms. We analyse in detail two cases: a MAP-EM algorithm incorporating an independent gamma prior, and a one-step-late (OSL) version of a MAP-EM algorithm incorporating a multivariate Gaussian prior, for which familiar smoothing priors are special cases. To validate our theoretical analyses, we use a Monte Carlo methodology to compare, at each iteration, theoretical estimates of mean and covariance with sample estimates, and show that the theory works well in practical situations where the noise and bias in the reconstructed images do not assume extreme values.

Novel multiplexer to enable multiple-module imaging with adjustable high spatial resolution and predetermined display bandwidth for array medical imaging systems

We describe a custom multiple-module multiplexer integrated circuit (MMMIC) that enables the combination of discrete Electron multiplying charge coupled devices (EMCCD) based imaging modules to improve medical imaging systems. It is highly desirable to have flexible imaging systems that provide high spatial resolution over a specific region of interest (ROI) and a field of view (FOV) large enough to encompass areas of clinical interest. Also, such systems should be dynamic, i.e. should be able to maintain a specified acquisition bandwidth irrespective of the size of the imaged FOV. The MMMIC achieves these goals by 1) multiplexing the outputs of an array of imaging modules to enable a larger FOV, 2) enabling a number of binning modes for adjustable high spatial resolution, and 3) enabling selection of a subset of modules in the array to achieve ROI imaging at a predetermined display bandwidth. The MMMIC design also allows multiple MMMICs to be connected to control larger arrays. The prototype MMMIC was designed and fabricated in the ON-SEMI 0.5μm CMOS process through MOSIS (www.mosis.org). It has three 12-bit inputs, a single 12-bit output, three input enable bits, and one output enable, so that one MMMIC can control the output from three discrete imager arrays. The modular design of the MMMIC enables four identical chips, connected in a two-stage sequential arrangement, to readout a 3×3 collection of individual imaging modules. The first stage comprises three MMMICs (each connected to three of the individual imaging module), and the second stage is a single MMMIC whose 12-bit output is then sent via a CameraLink interface to the system computer. The prototype MMMIC was successfully tested using digital outputs from two EMCCD-based detectors to be used in an x-ray imaging array detector system. Finally, we show how the MMMIC can be used to extend an imaging system to include any arbitrary (MxN) array of imaging modules enabling a large FOV along with ROI imaging and adjustable high spatial resolution.

Computerized biological brain phantom for evaluation of PET and SPECT reconstruction

A digital brain phantom was created from available primate autoradiographic (AR) data for use in emission computed tomography studies. The tissue was radio-labelled with a functional analogue of the PET agent [/sup 18/F]-fluoro-deoxyglucose (FDG). Following sacrifice of the animal, film records from serial 20 /spl mu/m thickness sections were digitized and calibrated to obtain ground truth 2D spatial distributions of relative radionuclide density. A 3D version was constructed by using a video subtraction method to align consecutive slices. In order to assess the effects of accurate modelling of activity, the AR data, containing cortical and basal ganglia structures, was used as a phantom in the context of a partial-volume correction method for obtaining accurate regional quantitation. A second phantom, less realistic in terms of activity assignment, was constructed and also tested. The results indicate that quantitation errors due to effects of nonuniform activity in the AR phantom are significant and comparable in magnitude to errors due to non-phantom effects.

Region-of-Interest Micro-Angiographic Fluoroscope Detector Used in Aneurysm and Artery Stenosis Diagnoses and Treatment.

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.

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

TU‐C‐211‐02: Characterizing the Dynamic Range and Noise Performance of a High‐Resolution EMCCD‐Based X‐Ray Detector Having Large Variable Electronic Gain Enabling Use in Both Fluoroscopy and Angiography

Purpose: To quantitatively characterize the dynamic range and noise performance of a new custom‐built, 1k by 1k, 26.4 micron square pixel, high‐resolution, solid‐state EMCCD‐based x‐ray detector. Methods: We performed two sets of experiments: higher‐exposure angiographic imaging with lower gain ( 200). For each experiment, 150 flat‐field images are acquired over the range of exposures for each multiplication gain. Average signal digital number (DN) versus exposure, and average variance (DN) versus exposure or signal are plotted and linearly fitted to quantitatively characterize the electronic gain, background or dark noise equivalent rms electrons, and instrumentation noise equivalent exposure (INEE), respectively. Results: For the low gain measurements, the noise electron rms value and INEE decreases with increasing gain virtually eliminating the effect of read‐noise, which is the main source of noise for conventional detectors. When the gain is increased for the low exposure measurements, dark noise from dark current and clock induced charge (CIC), which are constant with gain, become dominant and set the detection limit. With the same procedure, an additional experiment is performed to evaluate the dark noise and INEE at high gain with cooling. At room temperature, the dark noise is 1.03 e‐ rms and the INEE is 1.46 microR. Cooling the sensor effectively reduces the noise electrons (0.66 e‐ rms at 200mA pelticooler current and 0.50 e‐ rms at 400mA pelticooler current) and INEE (0.52 microR at 200mA pelticooler current and 0.33 microR at 400mA pelticooler current). Conclusions: The EMCCD‐based x‐ray detector with large variable gain has linear performance over a wide range of exposures with a low noise floor that can be further decreased by moderate cooling. The INEE for this unique detector is substantially less than the 2 to 3 microR of conventional flat panels. Support: NIH Grants R01‐EB008425, R01‐EB002873

SU‐E‐I‐193: Dynamic Gain Adjustment of a High Resolution Microangiographic Fluoroscope (MAF) for Improved Imaging of Intracranial Aneurysm Coiling

Purpose: To provide improved MAF image guidance during intracranial aneurysm treatment (IAT) with detachable coils. Method and Materials:The MAF is an ultra‐high resolution (35 μm pixels), high speed detector mounted on a clinical Flat Panel (FP) C‐arm used whenever high resolution is needed in a small field of view. During IATs the interventionalists fill the aneurysm dome with platinum or highly x‐ray attenuating detachable micro‐coils. For medium and large aneurysms, the coil mass becomes increasingly dense making any additional coil nearly impossible to visualize with the standard FP. Thus estimation of dome filling relies heavily only on experience rather than actual imagery. The MAF gain is capable of adjusting dynamically during the procedure based on the average pixel value indicated in a custom or preset ROI. In this study we selected an ROI fitted over the aneurysm, so that as the aneurysm fills with coils, the MAF gain increases to keep the ROI average pixel value the same even with increased x‐ray attenuation. The aneurysm was coiled using MAF fluoroscopic guidance for an elastomer aneurysm phantom superimposed on an anthropomorphic head phantom. The technique was repeated for the FP for comparison purposes. Results: The MAF used with dynamically adjustable gain offered excellent coil visualization during the entire process while the microcatheter tip position and deployment in the aneurysm within the coil mass was visible at all times even though background values increased during deployment to saturation. For the FP, imaging of the catheter tip was not distinguishable after 1/3 coil filling. Conclusions: Improved image guidance for coil deposition in aneurysms is presented. Catheter tip visualization was possible during the entire process which could result in better aneurysm dome coil filling due to improved image guidance with the MAF. NIH Grants R01‐EB008425, R01‐EB002873.

MO‐E‐204C‐01: Radionuclide Imaging with a CCD‐Based, High‐Resolution X‐Ray Detector in Single Photon Counting Mode

Purpose: To demonstrate use of the high‐resolution Micro‐Angiographic Fluoroscopic (MAF) detector in single‐photon counting (SPC) mode for nuclear medicine imaging.Method and Materials: The MAF uses a CCDcamera and fiber‐optic taper, has 1024 × 1024 pixels with effective pixel size of 35 microns and is capable of real‐time imaging. A 300 micron cesium iodide is coupled to a light image intensifier through a fiber optic taper. Large variable gain of the LII provides quantum‐limited operation with essentially no additive instrumentation noise and enables the MAF to operate in both energy‐integrating (EI) and the very‐sensitive low‐exposure SPC modes. To evaluate the MAF in SPC mode, a custom‐made phantom, with hot rods ranging from diameters 0.9mm to 2.3mm, filled with 1mCi of 125I was used as a test object. A medium‐energy, gamma‐camera collimator with 1‐mm diameter parallel holes was placed between the phantom and the MAF. Data was acquired at 20 fps. The collimator was used in two ways: (i) Stationary and (ii) Moving multiple times in random directions during data acquisition to blur the septal pattern with approximately the same number of counts detected per position. Each frame of the two sets was processed using two algorithms to localize events: (i) simple‐threshold and (ii) weighted‐centroid methods. The processed frames were added to give the final image.Results: (i) Stationary collimator: The image showed a grid‐like pattern corresponding to the septa walls of the collimator. All of the hot rods in the phantom can be identified, (ii) Moving collimator: The septal pattern is blurred out, and all of the rods can be clearly identified. Conclusion: The same MAF can be used in both nuclear‐medicine imaging and x‐ray imaging in SPC mode for dual‐mode imaging. Currently we are limited by collimator resolution for radionuclide imaging. Support from: NIH Grants R01EB002873 and R01EB008425.

SU-GG-I-182: Adjustment of Angiographic Time-Density-Curves Temporal Parameters for More Accurate Healing Prediction of Stent-Treated Aneurysms

Purpose: Immediate treatment assessment of intracranial aneurysms using flow modifying stents, can be done using comparative analysis of pre‐ and post‐stented normalized time density curves (NTDC). Previous study showed poor correlation of the time‐related NTDC parameters with the treatment outcome. A parameter normalization method to improve the NTDC parameters‐treatment outcome correlation, is proposed and investigated. Method and Materials: Elastase animal aneurysms model were treated using Asymmetric Vascular Stent prototypes. Angiograms were acquired pre‐ and post‐treatment and after four‐weeks. NTDCs parameters: time‐to‐peak TTP, mean‐transit‐time MTT and wash‐out‐time WOT, were measured and normalized to the corresponding quantities derived from main artery bolus TDC. Aneurysms displaying small area with contrast filling localized at the aneurysm neck were dropped from the analysis. The results are further presented in terms of pre‐stented/post‐stented ratios to describe the flow changes due to stent treatment. Based on a four week follow‐up angiogram, a five grade scale was used to generate a “healing” grade. The correlation between the parameter ratios and healing grade was calculated using Spearman correlation factor (SCF). Results: The pre‐/post‐stented ratio parameters after normalization were: TTP=0.30±0.28, MTT=0.42±0.42 and WOT=0.29±0.28. The SCFs between the time‐related ratios and the healing grade before the correction were under 0.52. After corrections implementation, all SCFs were above 0.82. Conclusion: After implementation of the proposed adjustments the measured parameters agree better with the treatment outcome, hence a better treatment assessment scale can be built for accurate aneurysm occlusion prediction (Support: R01NS43924 and R01EB002873 and Toshiba Medical Systems Corp)