Scanning Configuration Research Articles

Monte Carlo simulation of a GE eXplore VISTA system for quantitative small animal PET imaging

We aim to establish an evaluation platform for the GE eXplore VISTA small animal positron emission tomography (PET) scanner, a dual layer phoswich system, by using Monte Carlo simulation. We developed a detection model based on the Geant4 Application for Tomographic Emission to realistically reproduce the physics of PET, the scanner configuration, and the data collecting system of an eXplore VISTA system. For verification purpose, several different physical phantoms were simulated to perform evaluation tests, including sensitivity, spatial resolution, scatter fraction, and count rate performance, which were compared with an actual scanner. After the experimental validation, our detection model was applied to assess the quantification loss in the reconstructed images associated with photon attenuation, photon scatter, and random coincidences. A simulated sensitivity profile as a function of 18F point source axial position was fitted to the measured results. In terms of spatial resolution, agreement was within 10-18% for the point source at various locations. The simulated and measured scatter fractions differed by less than 4.3 and 5.2% for the physical mouse and rat phantoms, respectively. The count rate performance of our model was matched by the measured results, up to the peak activity concentration of 455 kBq/ml for the mouse-sized phantom and 141 kBq/ml for the rat-sized phantom. Finally, we found that photon attenuation is the dominant physical degrading factor in quantitative analysis (> 13.4%). These results suggested that the proposed detection model is able to produce realistic data from the eXplore VISTA system with knowing the ground truth, thus facilitating its evaluation for small animal PET studies.

A six-line ternary barcode detection system with a dual threshold method

A six-line ternary barcode detection system with a dual threshold detection method has been developed to investigate the possibility of multi-line scanners. This system, characterising the subtraction of an attenuated enveloped line of a detected barcode signal from the original signal, enables detection over a larger range and at higher scanning speeds while including a great amount of information. Through a demonstration of the system with a six-line ternary barcode and a six-mirror-facet raster scanner, the possibility of high-speed detection with a sufficient detection range determined by the raster scanner configuration was confirmed, even with increased scanning lines.

A BPF‐FBP tandem algorithm for image reconstruction in reverse helical cone‐beam CT

Reverse helical cone-beam computed tomography (CBCT) is a scanning configuration for potential applications in image-guided radiation therapy in which an accurate anatomic image of the patient is needed for image-guidance procedures. The authors previously developed an algorithm for image reconstruction from nontruncated data of an object that is completely within the reverse helix. The purpose of this work is to develop an image reconstruction approach for reverse helical CBCT of a long object that extends out of the reverse helix and therefore constitutes data truncation. The proposed approach comprises of two reconstruction steps. In the first step, a chord-based backprojection-filtration (BPF) algorithm reconstructs a volumetric image of an object from the original cone-beam data. Because there exists a chordless region in the middle of the reverse helix, the image obtained in the first step contains an unreconstructed central-gap region. In the second step, the gap region is reconstructed by use of a Pack-Noo-formula-based filteredback-projection (FBP) algorithm from the modified cone-beam data obtained by subtracting from the original cone-beam data the reprojection of the image reconstructed in the first step. The authors have performed numerical studies to validate the proposed approach in image reconstruction from reverse helical cone-beam data. The results confirm that the proposed approach can reconstruct accurate images of a long object without suffering from data-truncation artifacts or cone-angle artifacts. They developed and validated a BPF-FBP tandem algorithm to reconstruct images of a long object from reverse helical cone-beam data. The chord-based BPF algorithm was utilized for converting the long-object problem into a short-object problem. The proposed approach is applicable to other scanning configurations such as reduced circular sinusoidal trajectories.

Split gradient coils for simultaneous PET‐MRI

Combining positron emission tomography (PET) and MRI necessarily involves an engineering tradeoff as the equipment needed for the two modalities vies for the space closest to the region where the signals originate. In one recently described scanner configuration for simultaneous positron emission tomography-MRI, the positron emission tomography detection scintillating crystals reside in an 80-mm gap between the 2 halves of a 1-T split-magnet cryostat. A novel set of gradient and shim coils has been specially designed for this split MRI scanner to include an 110-mm gap from which wires are excluded so as not to interfere with positron detection. An inverse boundary element method was necessarily employed to design the three orthogonal, shielded gradient coils and shielded Z0 shim coil. The coils have been constructed and tested in the hybrid positron emission tomography-MRI system and successfully used in simultaneous positron emission tomography-MRI experiments.

Ganzkörper-MR-Angiografie

Cardiovascular diseases still rank number one in terms of morbidity and mortality statistics in the western industrialized world. The majority of these diseases are due to atherosclerotic vessel wall changes. Atherosclerosis gets symptomatic in an advanced stage where vessel wall changes are apparent. Thus an early diagnosis of this disease is desirable. Especially atherosclerosis but also most of the inflammatory vessel diseases are known as systemic in nature. Magnetic resonance angiography offers an excellent and ethically justifiable method for non invasive imaging of the entire arterial vasculature without ionizing radiation. DSA comparable imaging results can be achieved nowadays. Possibilities to perform a whole body MR angiography and concerning imaging protocols are very dependent on the configuration of the available MR scanner.

SU‐FF‐J‐156: High Throughput Micro‐CT Scanner Using a Distributed Multi‐Beam Field Emission X‐Ray Source

Purpose: To investigate the feasibility of a stationary gantry‐free micro‐CT scanner using spatially distributed multi‐beam x‐ray source. A prototype multi‐beam micro‐CT (MBμCT) scanner has been developed and its imaging performance characterized. Method and Materials: The MBμCT system consists of a 20‐beam carbon nanotube (CNT) field emission x‐ray source; an object table; a flat panel x‐ray detector; a MOSFET based control circuitry and a control console with a customized software user interface written in LabVIEW for data acquisition and processing. The x‐ray beams are individually controlled and can generate a scanning beam covering an angle span of 36°. It takes merely 9 steps to finish a full rotation in less than 10s. The projection images are acquired and stored for imaging processing and CT reconstruction. Results: The performance (focal spot size, field emission I‐V characteristics and current stability) of the multi‐beam x‐ray source has been characterized. The control circuitry and LabVIEW based control software have been implemented and tested. A mouse carcass has been scanned using the prototype MBμCT system and the acquired projection images showed decent imaging quality. We are currently working on refining the system geometry calibration and developing the corresponding CT reconstruction algorithm for this novel MBμCT imaging system, and some preliminary data will be presented. Conclusion: The feasibility of a stationary gantry‐free MBμCT has been evaluated based on the study of the as‐developed prototype system. Our preliminary results showed that the MBμCT scanner has the potential to deliver fast CT scan speed as a result of its novel scanning configuration. As a prototype system, it will be used to perform various feasibility tests for the future development of a fully stationary multi‐beam micro‐CT scanner. Once fully developed, this system would potentially provide fast screening, image registration and guidance for radiotherapy in pre‐clinical studies as well.

Scatter measurements for optical cone-beam computed tomography

Both x-ray and optical cone-beam CT (CBCT) scanners are limited by scattered photons that contaminate the primary images. Transparent radiochromic gels and plastics are anticipated to produce less scattering than polymerization gels and hence result in reconstructed dose distributions with greater contrast and dynamic range. Four methods of scatter measurement were investigated to quantify scatter fractions using an in-house optical CBCT scanner. The methods consisted of generating either "coin" shadows or primary beamlets in transmission images. The four methods generated similar results of 10% scatter fractions in gel transmission images for this particular scanner configuration with both leuco crystal violet and carbon black micelle gels. The scatter contributions were distributed as 5% from micelle gel, 2% from Teflon wall, 2% from refractive index matching liquid and 1% other sources. Applying a uniform background subtraction of 2% of open field intensity resulted in 0.2% difference between laser CT and cone-beam CT reconstructed attenuation coefficients for a black micelle gel finger phantom.

Aerosol optical thickness retrieval over non-Lambertian land surface with synergistic use of AATSR radiance measurements and MODIS derived Albedo Model Parameters

Various algorithms have recently been developed in order to retrieve the aerosol optical thickness (AOT) at continental scales. However, they are, to some extent, subject to large uncertainties resulting from some necessary physical assumptions on land surface anisotropy and level of brightness. In fact, disentangling aerosol and surface signals contained in the top of atmosphere (TOA) radiance received at the satellite level are a matter of difficulty because a single sensor itself cannot gather all required spatial, temporal, spectral and angular information. In particular, each instrument yields limited scanning configuration due to its platform's orbital characteristics. In this regard, a synergetic approach is presented which merges Advanced Along-Track Scanning Radiometer (AATSR) TOA radiances and the MODerate resolution Imaging Spectroradiometer (MODIS) Bi-directional Reflectance Distribution Function (BRDF)/Albedo Model Parameters Product for the retrieval of AOT at 0.55, 0.66, and 0.87 µm wavelengths over non-Lambertian land surface at a 5 km spatial resolution. In this approach, BRDF products serve to assess the surface reflectance in the AATSR geometry as a boundary layer. The peculiarity of the approach is that no specific assumption is required about the spectral characteristics of land surface, thus allowing for a quantitative retrieval of aerosol particles over any arbitrary land unit in virtue of combining forward and nadir AATSR observations. We obtain on average differences within 0.1 τ compared to in situ AErosol RObotic NETwork (AERONET) measurements and 36 retrievals corresponding to 27 January, 12 February, 16 March, 28 May, 26 June, and 21 July 2006, respectively, over the city of Beijing in China. Pearson's correlation coefficient is 0.94 and 0.96 for nadir and forward AATSR, respectively. These suggest that AOT retrieval over land is indeed feasible by taking benefit of the validated MODIS BRDF. Besides, the first results indicate that the AATSR retrievals might be used to evaluate the spectral behaviour of the AOT.

Barriers to Providing Dual Energy X-ray Absorptiometry Services to Individuals with Spinal Cord Injury

To assess barriers to bone mineral density testing in individuals with chronic spinal cord injury, a cross-sectional study of 20 individuals with spinal cord injury was conducted with assessment of physical and logistical barriers to dual energy x-ray absorptiometry scanning. We identified several barriers, including scanner design and configuration in the scanning room that limited accessibility, increased typical scanning time, and made additional staff necessary. For dual energy x-ray absorptiometry to become a routine component of ongoing care in spinal cord injury medicine, we recommend the following changes: (1) install ceiling-mounted hydraulic lifts and grab bars to facilitate transfers in the scanning room; (2) increase staffing during scans; (3) increase time allotment for scans; (4) install the scanner in an adequately-sized room; and (5) partner with administrators and staff to raise awareness of access issues faced by individuals with spinal cord injury.

Measuring information content from observations for data assimilations: connection between different measures and application to radar scan design

The previously derived formulations for using the relative entropy and Shannon entropy difference (SD) to measure information content from observations are revisited in connection with another known information measure—degrees of freedom for signal, which is defined as the statistical average of the signal part of the relative entropy. For a linear assimilation system, the statistical average of the relative entropy reduces to the SD. The formulations are extended for four-dimensional variational data assimilation (4DVar). The extended formulations reveal that the information content increases (or decreases) as the model error increase (or decrease) and/or become strongly (or weakly) correlated in 4-D space. These properties are also highlighted by illustrative examples, and the extended formulations are shown to be potential useful for designing optimum phased-array radar scan configurations to maximize the extractable information contents from radar observations by a 4DVar analysis system.

Measuring information content from observations for data assimilations: connection between different measures and application to radar scan design

The previously derived formulations for using the relative entropy and Shannon entropy difference (SD) to measure information content from observations are revisited in connection with another known information measure—degrees of freedom for signal, which is defined as the statistical average of the signal part of the relative entropy. For a linear assimilation system, the statistical average of the relative entropy reduces to the SD. The formulations are extended for four-dimensional variational data assimilation (4DVar). The extended formulations reveal that the information content increases (or decreases) as the model error increase (or decrease) and/or become strongly (or weakly) correlated in 4-D space. These properties are also highlighted by illustrative examples, and the extended formulations are shown to be potential useful for designing optimum phased-array radar scan configurations to maximize the extractable information contents from radar observations by a 4DVar analysis system.

A novel portable bipolar near-field measurement system for millimeter-wave antennas: construction, development, and verification

As new antenna designs require higher frequencies and smaller sizes, traditional large-scale antenna measurement systems become ill-suited for such measurements. External mixing, room-sized chambers, and expensive test equipment add large costs and burdens to antenna measurement systems. A smaller and more cost-effective system is presented in this paper. Using the bipolar planar scanning technique developed at UCLA, a portable millimeter-wave antenna measurement system has recently been constructed. The system was designed to fit on the end of a standard optical table, and enjoys the spacesaving and accuracy inherent to the bipolar planar configuration. Simple construction of the chamber allows for relatively easy assembly and disassembly, and allows movement of the system from one table to another, if needed. Antennas of diameters up to 24 in can be accommodated, and scan planes of up to a diameter of 60 in can be measured. Millimeter-wave frequencies from around 30 GHz to 67 GHz can be measured, with potential extension to higher frequencies. Planar nearfield-to-far-field techniques are used to construct the antenna's far-field patterns from the measured near field. In particular, the post processing follows the OSI/EFT method for pattern reconstruction and diagnostics. The design of the scanner configuration allows the incorporation of the phase-retrieval techniques developed for the bipolar configuration. These phaseless measurements allow the use of scalar millimeter-wave test equipment, with much lower cost than comparable vector test equipment.

Comparison of SNR and CNR for in vivo mouse brain imaging at 3 and using well matched scanner configurations

Signal-to-noise ratio (SNR) and contrast-to-noise ratio (CNR) for magnetic resonance microimaging were measured using two nearly identical magnetic resonance imaging (MRI) scanners operating at field strengths of 3 and 7 T. Six mice were scanned using two imaging protocols commonly applied for in vivo imaging of small animal brain: RARE and FLASH. An accounting was made of the field dependence of relaxation times as well as a small number of hardware disparities between scanner systems. Standard methods for relaxometry were utilized to measure T1 and T2 for two white matter (WM) and two gray matter (GM) regions in the mouse brain. An average increase in T1 between 3 and 7 T of 28% was observed in the brain. T2 was found to decrease by 27% at 7 T in agreement with theoretical models. The SNR was found to be uniform throughout the mouse brain, increasing at higher field by a factor statistically indistinguishable from the ratio of Larmor frequencies when imaging with either method. The CNR between GM and WM structures was found to adhere to the expected field dependence for the RARE imaging sequence. Improvement in the CNR for the FLASH imaging sequence between 3 and 7 T was observed to be greater than the Larmor ratio, reflecting a greater susceptibility to partial volume effects at the lower SNR values at 3 T. Imaging at 7 T versus 3 T in small animals clearly provides advantages with respect to the CNR, even beyond the Larmor ratio, especially in lower SNR regimes. This careful multifaceted assessment of the benefits of higher static field is instructive for those newly embarking on small animal imaging. Currently the number of 7 T MRI scanners in use for research in human subjects is increasing at a rapid pace with approximately 30 systems deployed worldwide in 2008. The data presented in this article verify that if system performance and radio frequency uniformity is optimized at 7 T, it should be possible to realize the expected improvements in the CNR and SNR compared with MRI at 3 T.

Accurate image reconstruction from few-view and limited-angle data in diffraction tomography.

We present a method for obtaining accurate image reconstruction from highly sparse data in diffraction tomography (DT). A practical need exists for reconstruction from few-view and limited-angle data, as this can greatly reduce required scan times in DT. Our method does this by minimizing the total variation (TV) of the estimated image, subject to the constraint that the Fourier transform of the estimated image matches the measured Fourier data samples. Using simulation studies, we show that the TV-minimization algorithm allows accurate reconstruction in a variety of few-view and limited-angle situations in DT. Accurate image reconstruction is obtained from far fewer data samples than are required by common algorithms such as the filtered-backpropagation algorithm. Overall our results indicate that the TV-minimization algorithm can be successfully applied to DT image reconstruction under a variety of scan configurations and data conditions of practical significance.

A Prototype PET Scanner with DOI-Encoding Detectors

Detectors with depth-encoding allow a PET scanner to simultaneously achieve high sensitivity and high spatial resolution. A prototype PET scanner, consisting of depth-encoding detectors constructed by dual-ended readout of lutetium oxyorthosilicate (LSO) arrays with 2 position-sensitive avalanche photodiodes (PSAPDs), was developed. The scanner comprised 2 detector plates, each with 4 detector modules, and the LSO arrays consisted of 7 x 7 elements, with a crystal size of 0.9225 x 0.9225 x 20 mm and a pitch of 1.0 mm. The active area of the PSAPDs was 8 x 8 mm. The performance of individual detector modules was characterized. A line-source phantom and a hot-rod phantom were imaged on the prototype scanner in 2 different scanner configurations. The images were reconstructed using 20, 10, 5, 2, and 1 depth-of-interaction (DOI) bins to demonstrate the effects of DOI resolution on reconstructed image resolution and visual image quality. The flood histograms measured from the sum of both PSAPD signals were only weakly depth-dependent, and excellent crystal identification was obtained at all depths. The flood histograms improved as the detector temperature decreased. DOI resolution and energy resolution improved significantly as the temperature decreased from 20 degrees C to 10 degrees C but improved only slightly with a subsequent temperature decrease to 0 degrees C. A full width at half maximum (FWHM) DOI resolution of 2 mm and an FWHM energy resolution of 15% were obtained at a temperature of 10 degrees C. Phantom studies showed that DOI measurements significantly improved the reconstructed image resolution. In the first scanner configuration (parallel detector planes), the image resolution at the center of the field of view was 0.9-mm FWHM with 20 DOI bins and 1.6-mm FWHM with 1 DOI bin. In the second scanner configuration (detector planes at a 40 degrees angle), the image resolution at the center of the field of view was 1.0-mm FWHM with 20 DOI bins and was not measurable when using only 1 bin. PET scanners based on this detector design offer the prospect of high and uniform spatial resolution (crystal size, approximately 1 mm; DOI resolution, approximately 2 mm), high sensitivity (20-mm-thick detectors), and compact size (DOI encoding permits detectors to be tightly packed around the subject and minimizes number of detectors needed).

Exact reconstruction of volumetric images in reverse helical cone-beam CT.

Helical scanning configuration has been used widely in diagnostic cone-beam computed tomography (CBCT) for acquiring data sufficient for exact image reconstruction over an extended volume. In image-guided radiation therapy (IGRT) and other applications of CBCT, it can be difficult, if not impossible, to implement mechanically a multiple-turn helical trajectory on the imaging systems due to hardware constraints. However, imaging systems in these applications often allow for the implementation of a reverse helical trajectory in which the rotation direction changes between two consecutive turns. Because the reverse helical trajectory satisfies Tuy's condition, when projections of the imaged object are nontruncated, it yields data sufficient for exact image reconstruction within the reverse helix volume. The recently developed chord-based algorithms such as the backprojection filtration (BPF) algorithm can readily be applied to reconstructing images on chords of a reverse helical trajectory, and they can thus reconstruct an image within a volume covered by the chords. Conversely, the chord-based algorithms cannot reconstruct images within regions that are not intersected by chords. In a reverse helix volume, as shown below, chordless regions exist in which no images can thus be reconstructed by use of the chord-based algorithms. In this work, based upon Pack-Noo's formula, a shift-invariant filtered backprojection (FBP) algorithm is derived for exact image reconstruction within the reverse helix volume, including the chordless region. Numerical studies have also been conducted to demonstrate the chordless region in a reverse helix volume and to validate the FBP algorithm for image reconstruction within the chordless region. Results of the numerical studies confirm that the FBP algorithm can exactly reconstruct an image within the entire reverse helix volume, including the chordless region. It is relatively straightforward to extend the FBP algorithm to reconstruct images for general trajectories, including reverse helical trajectories with variable pitch, tilted axis, and/or additional segments between turns.

VirtualScan: Test Compression Technology Using Combinational Logic and One-Pass ATPG

IC testing based on a full-scan design methodology and ATPG is the most widely used test strategy today. However, rapidly growing test costs are severely challenging the applicability of scan-based testing. Both test data size and number of test cycles increase drastically as circuit size grows and feature size shrinks. For a full-scan circuit, test data volume and test cycle count are both proportional to the number of test patterns N and the longest scan chain length L. To reduce test data volume and test cycle count, we can reduce N, L, or both. Earlier proposals focused on reducing the number of test patterns N through pattern compaction. All these proposals assume a 1-to-1 scan configuration, in which the number of internal scan chains equals the number of external scan I/O ports or test channels (two ports per channel) from ATE. Some have shown that ATPG for a circuit with multiple clocks using the multicapture clocking scheme, as opposed to one-hot clocking, generates a reduced number of test patterns.

Noise Properties of Chord-Image Reconstruction

Recently, there has been much progress in algorithm development for image reconstruction in cone-beam computed tomography (CT). Current algorithms, including the chord-based algorithms, now accept minimal data sets for obtaining images on volume regions-of-interest (ROIs) thereby potentially allowing for reduction of X-ray dose in diagnostic CT. As these developments are relatively new, little effort has been directed at investigating the response of the resulting algorithm implementations to physical factors such as data noise. In this paper, we perform an investigation on the noise properties of ROI images reconstructed by using chord-based algorithms for different scanning configurations. We find that, for the cases under study, the chord-based algorithms yield images with comparable quality. Additionally, it is observed that, in many situations, large data sets contain extraneous data that may not reduce the ROI-image variances.

Cone-beam filtered backprojection image reconstruction using a factorized weighting function

We present several new families of mathematically exact cone-beam image reconstruction algorithms for a general source trajectory that fulfills Tuy's data sufficiency condition. The basic structure of the new algorithms is to reconstruct images via filtered backprojection (FBP) with a 1-D shift-invariant filter. Specifically, the general weighting function w(x,;t) for redundant data was decomposed into three components w1(x,), w2(x,t), and sgn[·y(t)], viz. w(x,;t)=[w1(x,)w2(x,t)sgn(·y(t))]. Based upon the normalization condition of the weighting function, the first component w1(x,) may be calculated using the second component w2(x,t) Thus, the design of the weighting function was reduced to the selection of the second component w2(x,t). Using this scheme, it has been demonstrated that, for a given scanning configuration, one may develop infinitely many different, exact cone-beam FBP image reconstruction algorithms. To demonstrate how this general procedure may be used to develop FBP image reconstruction algorithms, a two-concentric-circle scanning configuration is discussed in detail. Numerical simulations have been conducted to validate several of the derived image reconstruction algorithms. Several possible scan strategies are presented, and the possibility of performing multiple reconstructions with different scan configurations to reduce image noise is described. Noise properties also have been numerically studied for the implemented image reconstruction algorithms, then compared with two other shift-invariant FBP reconstruction algorithms.

Assessment of the structural integrity of a prototypical instrumented IFMIF high flux test module rig by fully 3D X-ray microtomography

An inspection procedure to assess the mechanical integrity of the International Fusion Materials Irradiation Facility (IFMIF) capsules and rigs during the irradiation campaign is necessary. Due to its penetration ability and contrast mechanism, the X-ray microtomography is the only known tool that could meet these requirements. In the high flux test module (HFTM) of IFMIF miniaturized specimens are densely packed in capsules. The capsules, which wear electric heaters and thermocouples, are housed in rigs. To assure a well-defined thermal contact the heater wires have to be attached to the capsules by brazing them into grooves. The examination of the quality of the braze material layer is of crucial interest in order to assure the best heat coupling of the heater wires to the capsule. A high density of the heaters is necessary to maintain the required temperature and, in addition NaK filling of narrow channels is employed for improving the 3D-heat transfer between the irradiation specimens and the capsule wall. Fully 3D tomographic inspections of a prototypical HFTM instrumented capsule, developed and manufactured at FZK, were conducted. In order to identify the optimum irradiation parameters and scanning configuration we carried out a comparative NDT analysis on two microtomography facilities: a compact, high magnification installation at NILPRP and a high-end industrial tomography facility with higher X-ray energy and intensity at HWM. At optimum inspection parameters of a directional microfocus X-ray source ( U = 220 kV and I = 300 μA) the geometry resolution was about 30 microns for characteristic dimension of the sample of 50 mm. Voids of 30 microns diameter and cracks of about 20 microns width can be detected. The absolute error of geometrical measurements is sufficient for the assessment of the structural integrity of the irradiation capsule and for the geometry description within the thermal-hydraulic modeling. The space resolution and the overall reconstruction quality could be further improved by operating the X-ray source at low intensity and, consequently, very small focus size and, in addition, by implementing procedures to compensate geometry and beam hardening artifacts.