Scanner Characteristics Research Articles

Test Performance of CT Colonography: A Systematic Review of the Literature and Meta-Analysis

Background: Computed Tomography Colonography (CTC) is being proposed as a colorectal cancer screening modality that may improve screening adherence rates. The purpose of this study was to systematically review the test performance of CTC. Methods: We searched PubMed, Medline, EMBASE and Cochrane Controlled Trials Register for articles published in English language between January 1975 and October 2004 with the following MeSH terms or text words: virtual colonoscopy, CT colonography, CT colography or CT pneumocolon. We included only prospective blinded studies of adults undergoing CT colonography after full bowel preparation, using colonoscopy or surgery as the gold standard. Further, these studies had to use state-of-the-art technology with at least a single-detector CT scanner, both supine and prone positioning, air or CO2 colon insufflation of the colon, CT collimation < 5 mm, and both 2D & 3D views during interpretation. The data extracted included the overall sensitivity and specificity, as well as sensitivity and specificity for polyps <6mm, 6-9 mm and > 9mm in size. When data was sufficiently homogenous, we calculated weighted (by sample size) sensitivities and specificities. Heterogeneity was examined using stratified analyses and meta-regression. Results: A total of 31 studies were included that provided data from 5731 patients. There was a wide range of CTC sensitivities on a per patient analysis (21%-96%) that prevented pooling, even for polyps > 9 mm (range: 48%-100%). This heterogeneity was not explained by year published, demographics or cancer risk, imaging software used or CT reconstruction interval. Of note, collimation intervals, type of scanner and mode of imaging used did have an impact on heterogeneity. Data for specificity was homogenous with CTC being 92% specific (95% CI: 0.91-0.94) for polyps 6-9mm and 96% (95% CI: 0.96-0.97) for polyps > 9 mm. Discussion: While CTC is highly specific, there is a broad range of sensitivities that is not clearly related to patient or scanner characteristics--though collimation, type of scanner and mode of imaging do explain some of the disparity. The substantial heterogeneity underlines the concerns that CTC is not yet ready for widespread application. Experts may have valuable insights into CTC technique, but this study defines some of the specific technical issues that have impacted optimal test performance and supports the assertion that further scrutiny of CT Colonography is required. Background: Computed Tomography Colonography (CTC) is being proposed as a colorectal cancer screening modality that may improve screening adherence rates. The purpose of this study was to systematically review the test performance of CTC. Methods: We searched PubMed, Medline, EMBASE and Cochrane Controlled Trials Register for articles published in English language between January 1975 and October 2004 with the following MeSH terms or text words: virtual colonoscopy, CT colonography, CT colography or CT pneumocolon. We included only prospective blinded studies of adults undergoing CT colonography after full bowel preparation, using colonoscopy or surgery as the gold standard. Further, these studies had to use state-of-the-art technology with at least a single-detector CT scanner, both supine and prone positioning, air or CO2 colon insufflation of the colon, CT collimation < 5 mm, and both 2D & 3D views during interpretation. The data extracted included the overall sensitivity and specificity, as well as sensitivity and specificity for polyps <6mm, 6-9 mm and > 9mm in size. When data was sufficiently homogenous, we calculated weighted (by sample size) sensitivities and specificities. Heterogeneity was examined using stratified analyses and meta-regression. Results: A total of 31 studies were included that provided data from 5731 patients. There was a wide range of CTC sensitivities on a per patient analysis (21%-96%) that prevented pooling, even for polyps > 9 mm (range: 48%-100%). This heterogeneity was not explained by year published, demographics or cancer risk, imaging software used or CT reconstruction interval. Of note, collimation intervals, type of scanner and mode of imaging used did have an impact on heterogeneity. Data for specificity was homogenous with CTC being 92% specific (95% CI: 0.91-0.94) for polyps 6-9mm and 96% (95% CI: 0.96-0.97) for polyps > 9 mm. Discussion: While CTC is highly specific, there is a broad range of sensitivities that is not clearly related to patient or scanner characteristics--though collimation, type of scanner and mode of imaging do explain some of the disparity. The substantial heterogeneity underlines the concerns that CTC is not yet ready for widespread application. Experts may have valuable insights into CTC technique, but this study defines some of the specific technical issues that have impacted optimal test performance and supports the assertion that further scrutiny of CT Colonography is required.

Profound influence of microarray scanner characteristics on gene expression ratios: analysis and procedure for correction.

BackgroundHigh throughput gene expression data from spotted cDNA microarrays are collected by scanning the signal intensities of the corresponding spots by dedicated fluorescence scanners. The major scanner settings for increasing the spot intensities are the laser power and the voltage of the photomultiplier tube (PMT). It is required that the expression ratios are independent of these settings. We have investigated the relationships between PMT voltage, spot intensities, and expression ratios for different scanners, in order to define an optimal scanning procedure.ResultsAll scanners showed a limited intensity range from 200 to 50 000 (mean spot intensity), for which the expression ratios were independent of PMT voltage. This usable intensity range was considerably less than the maximum detection range of the PMTs. The use of spot and background intensities outside this range led to errors in the ratios. The errors at high intensities were caused by saturation of pixel intensities within the spots. An algorithm was developed to correct the intensities of these spots, and, hence, extend the upper limit of the usable intensity range.ConclusionsIt is suggested that the PMT voltage should be increased to avoid intensities of the weakest spots below the usable range, allowing the brightest spots to reach the level of saturation. Subsequently, a second set of images should be acquired with a lower PMT setting such that no pixels are in saturation. Reliable data for spots with saturation in the first set of images can easily be extracted from the second set of images by the use of our algorithm. This procedure would lead to an increase in the accuracy of the data and in the number of data points achieved in each experiment compared to traditional procedures.

High resolution reconstruction of PET images using the iterative OSEM algorithm

Summary Aim: Improvement of the spatial resolution in positron emission tomography (PET) by incorporation of the image-forming characteristics of the scanner into the process of iterative image reconstruction. Methods: All measurements were performed at the whole-body PET system ECAT EXACT HR+ in 3D mode. The acquired 3D sinograms were sorted into 2D sinograms by means of the Fourier rebinning (FORE) algorithm, which allows the usage of 2D algorithms for image reconstruction. The scanner characteristics were described by a spatially variant line-spread function (LSF), which was determined from activated copper-64 line sources. This information was used to model the physical degradation processes in PET measurements during the course of 2D image reconstruction with the iterative OSEM algorithm. To assess the performance of the high-resolution OSEM algorithm, phantom measurements performed at a cylinder phantom, the hotspot Jaszczack phantom, and the 3D Hoffmann brain phantom as well as different patient examinations were analyzed. Results: Scanner characteristics could be described by a Gaussian-shaped LSF with a full-width at half-maximum increasing from 4.8 mm at the center to 5.5 mm at a radial distance of 10.5 cm. Incorporation of the LSF into the iteration formula resulted in a markedly improved resolution of 3.0 and 3.5 mm, respectively. The evaluation of phantom and patient studies showed that the high-resolution OSEM algorithm not only lead to a better contrast resolution in the reconstructed activity distributions but also to an improved accuracy in the quantification of activity concentrations in small structures without leading to an amplification of image noise or even the occurrence of image artifacts. Conclusion: The spatial and contrast resolution of PET scans can markedly be improved by the presented image restauration algorithm, which is of special interest for the examination of both patients with brain disorders and small animals.

Atomic force microscope with improved scan accuracy, scan speed, and optical vision

We have developed a new atomic force microscope (AFM), with a z scanner independent of the xy scanner. Unlike conventional AFM systems, our xy scanner moves only the sample in the xy plane, while the z scanner controls the AFM probe along the z axis. The xy scanner is a single module parallel-kinematics flexure stage that guarantees high orthogonality and minimum out-of-plane motion. The z scanner is a one-dimensional flexure stage with negligible out-of-axis motion. Separating the z scanner from the xy scanner practically eliminates the x–z cross coupling problem inherent in conventional AFMs. Furthermore, the z servo response is no longer limited by the xy scanner characteristics, allowing us to make full use of our high performance z scanner. Our system uses the laser beam bounce detection method, and only the cantilever and the photodetector are mounted on the z scanner to realize a lightweight probing unit. We have devised a unique design such that the photodetector signal measures only the cantilever deflection and not the z-scanner motion. Our new AFM provides fast z servo response and high scan accuracy.

Characterization of a single LSO crystal layer High Resolution Research Tomograph

The purpose of this study was to determine the performance of a single lutetium oxy-orthosilicate (LSO) crystal layer High Resolution Research Tomograph (HRRT) positron emission tomography (PET) scanner. The HRRT is a high resolution PET scanner designed for human brain and small animal imaging. The scanner consists of eight panel detectors, which have one layer of 2.1 × 2.1 × 7.5 mm thick LSO crystals.Several phantom studies were performed to determine scanner characteristics, such as resolution, scatter fraction, count rate and noise equivalent count rates (NECR). NECR curves were measured according to both NEMA NU2-1994 and NU2-2001 for three different energy windows, i.e. lower level discriminators (lld) of 350, 400 and 450 keV and an upper level discriminator (uld) of 650 keV. Accuracy of scatter and single photon attenuation corrections was evaluated according to NU2-1994. Data were acquired using a ring difference of 67 and a span of 9. Reconstructions were performed using FORE + 2D FBP or OSEM.Transaxial resolution varied from 2.7 to 2.9 mm FWHM between 1 and 10 cm off centre locations, and axial resolution varied from 3.2 to 4.4 mm FWHM. Scatter fractions (NU2-1994) equalled 0.31, 0.42 and 0.54 for lld of 450, 400 and 350 keV, respectively. NECR data were highest for an lld of 400 keV and showed a maximum of 46 kcps at 38 kBq cm−3. Lower NECR values were observed according to NU2-2001, but were still optimal for an lld of 400 keV. After scatter and attenuation corrections, pixel values within water, air and teflon inserts of the NU2-1994 phantom were 14, 4 and 35% of the background activity, respectively.The single layer LSO HRRT scanner shows excellent spatial resolution, making it suitable for small animal studies. The low count rate performance, due to the small amount of LSO, prohibits studies of the human brain, but is sufficient for studies in small laboratory animals.

Cross-sectional structural parameters from densitometry

Bone densitometry has previously been used to obtain cross-sectional properties of bone from a single X-ray projection across the bone width. Using three unique projections, we have extended the method to obtain the principal area moments of inertia and orientations of the principal axes at each scan cross-section along the length of the scan. Various aluminum phantoms were used to examine scanner characteristics to develop the highest accuracy possible for in vitro non-invasive analysis of cross-sectional properties. Factors considered included X-ray photon energy, initial scan orientation, the angle spanned by the three scans (included angle), and I min/ I max ratios. Principal moments of inertia were accurate to within ±3.1% and principal angles were within ±1° of the expected value for phantoms scanned with included angles of 60° and 90° at the higher X-ray photon energy (140 kVp). Low standard deviations in the error (0.68–1.84%) also indicate high precision of calculated measurements with these included angles. Accuracy and precision decreased slightly when the included angle was reduced to 30°. The method was then successfully applied to a pair of excised cadaveric tibiae. The accuracy and insensitivity of the algorithms to cross-sectional shape and changing isotropy ( I min/ I max) values when various included angles are used make this technique viable for future in vivo studies.

Positional calibration of galvanometric scanners used in laser Doppler vibrometers

The knowledge of where one has measured the response of a structure is sometimes as important as the response measured. This paper describes a method for the calibration of the horizontal and vertical scanners in a laser Doppler vibrometer. The residual of the scanner position with respect to a first-order and second-order regression model will be discussed. Comparisons will be made to vendor-measured scanner characteristics. Detailed analysis of the scanner calibration data will result in a very accurate knowledge of where the measured data from a structure is located and directed. Moreover, this knowledge will enhance the accuracy of three-dimensional laser vibrometry.

Responses to Users: The Continuing Evolution of Commercial Digital Photogrammetry

Although well established, digital photogrammetry continues to change. More sources offer imagery which can be treated photogrammetrically. Users need improved scanner performance as they attempt to offset costs by feeding clusters of workstations or establishing bureau services; increased speed, variable pixel size, use of roll film and “on the fly” dodging are desirable scanner characteristics. On the workstation side, the demand for the Windows NT platform has been irresistible. New applications and the quest for higher productivity have resulted in demands including output of orientation data to analytical plotters, processing of data from airborne GPS and inertial measurement unit (IMU), triangulated irregular network (TIN) DTMs, automated radiometric balancing in mosaicking, smoother operation within feature collection and batch processes. Moreover, the market place itself changes, with suppliers, their customers and the end users all competing for position.

Use of scanner characteristics in iterative image reconstruction for high-resolution positron emission tomography studies of small animals

Use of scanner characteristics in iterative image reconstruction for high-resolution positron emission tomography studies of small animals

Use of scanner characteristics in iterative image reconstruction for high-resolution positron emission tomography studies of small animals.

The purpose of this work was to improve of the spatial resolution of a whole-body positron emission tomography (PET) system for experimental studies of small animals by incorporation of scanner characteristics into the process of iterative image reconstruction. The image-forming characteristics of the PET camera were characterized by a spatially variant line-spread function (LSF), which was determined from 49 activated copper-64 line sources positioned over a field of view (FOV) of 21.0 cm. This information was used to model the image degradation process. During the course of iterative image reconstruction, the forward projection of the estimated image was blurred with the LSF at each iteration step before the estimated projections were compared with the measured projections. The imaging characteristics of the high-resolution algorithm were investigated in phantom experiments. Moreover, imaging studies of a rat and two nude mice were performed to evaluate the imaging properties of our approach in vivo. The spatial resolution of the scanner perpendicular to the direction of projection could be approximated by a one-dimensional Gaussian-shaped LSF with a full-width at half-maximum increasing from 6.5 mm at the centre to 6.7 mm at a radial distance of 10.5 cm. The incorporation of this blurring kernel into the iteration formula resulted in a significantly improved spatial resolution of about 3.9 mm over the examined FOV. As demonstrated by the phantom and the animal experiments, the high-resolution algorithm not only led to a better contrast resolution in the reconstructed emission scans but also improved the accuracy for quantitating activity concentrations in small tissue structures without leading to an amplification of image noise or image mottle. The presented data-handling strategy incorporates the image restoration step directly into the process of algebraic image reconstruction and obviates the need for ill-conditioned "deconvolution" procedures to be performed on the projections or on the reconstructed image. In our experience, the proposed algorithm is of special interest in experimental studies of small animals.

Carotid duplex criteria for a 60% or greater angiographic stenosis: Variation according to equipment

The purpose of this study was to evaluate the carotid duplex criteria for a > or = 60% angiographic internal carotid artery (ICA) stenosis and the degree of variation among duplex scanners. Carotid duplex criteria for a > or = 60% angiographic stenosis were evaluated in two ICAVL-accredited vascular laboratories with different brands of duplex scanners (Siemens-Quantum and Diasonics in Laboratory A, ATL and Diasonics in Laboratory B). Analysis was performed for 360 carotid bifurcations in 180 consecutive patients who had concurrent angiographic and duplex evaluation. Blinded angiogram evaluation was performed with precision electronic calipers on magnified views, with stenosis calculated by criteria of the Asymptomatic Carotid Atherosclerosis Study and the North American Symptomatic Carotid Endarterectomy Trial. Duplex data included internal carotid artery peak systolic velocity (ICA PSV), ICA end-diastolic velocity, and the ratio of ICA PSV to common carotid artery (CCA) PSV (ICA/CCA ratio). The most accurate determination of a > or = 60% ICA stenosis was obtained with ICA/CCA ratio and ICA PSV, but the optimal threshold differed for all four scanners. The optimal ICA/CCA ratio varied from 2.6 to 3.3, and the optimal ICA PSV varied from 190 to 240 cm/sec. All four scanners produced criteria that give a positive predictive value > 90% while maintaining accuracy at > or = 90%. Logarithmic transformation of duplex variables created a linear relationship between duplex values and angiographic stenosis, allowing statistical evaluation of scanner operating characteristics by linear regression analysis and analysis of covariance. This analysis revealed that the mathematic equation relating duplex values with angiographic percent stenosis was statistically different for one of the four scanners (p < 0.05). Scanner differences did not appear to be due to technologists, because the regression lines were nearly identical for the two Diasonics scanners despite use by different technologists. Ignoring the significant difference in operating characteristics for one of the four scanners would result in a mean error for predicting a 60% stenosis of 14% to 18% (equating a 46% or 78% stenosis with a 60% stenosis). We conclude that the correlation of duplex data with angiographic percent stenosis and the duplex criteria for a > or = 60% stenosis are machine-specific. Regression analysis can determine whether apparent differences are due to chance or significant differences in scanner characteristics. Future studies should include regression analysis according to equipment type.

A phantom study to assess the accuracy of stereotactic localization, using T1-weighted magnetic resonance imaging with the Leksell stereotactic system.

This phantom study assesses the accuracy of stereotactic localization using the Leksell G frame (Elekta Instruments AB, Stockholm, Sweden) with T1-weighted magnetic resonance imaging (Siemens 1.5 T Magnetom; Erlangen, Germany). The coordinates of an array of solid perspex rods were determined and compared with measured values in a series of transverse, coronal, and sagittal images. The maximum absolute errors observed (X = 2.7 mm, Y = 7.0 mm, Z = 8.0 mm) were discouraging. However, the more reasonable mean errors (X = 0.4 mm, Y = 0.7 mm, Z = 1.3 mm) reflect considerable variation in accuracy throughout the volume assessed and limitation of maximum errors to specific areas. We present details of the spatial variation and discuss possible mechanisms for improving accuracy. The overall results are of direct relevance only to the scanner used. These results are, however, an indication of the need to approach with caution stereotactic localization using magnetic resonance imaging and to emphasize the requirement for quality assurance and for a comprehensive study of the scanner's characteristics.

Measurement of Doppler gate length using signal re-injection

Signal re-injection provides a versatile way of assessing the performance of ultrasonic scanners. The technique was used to measure the Doppler gate length of Duplex ultrasonic scanners operating in the pulsed Doppler mode. Five scanners were assessed, each with two different gate lengths, and the nominal gate length was compared with that measured. For most scanners the nominal gate length was less than that measured, the difference (measured minus nominal) varying between -0.9 mm and +8.0 mm. However, incomplete knowledge of the scanner characteristics, in particular receiver focusing and automatic gain control of the audio signal, made the results difficult to interpret. The test equipment used a perspex block to provide a fixed delay between the emitted and re-injected ultrasonic pulse. A more accurate assessment would have required a re-injection system with a variable delay between the pulses.

An improved stochastic approach to RF amplitude analysis in ultrasonic cardiac tissue characterization

We carried out a series of studies to improve the reproducibility of methods for ultrasonic myocardial tissue characterization using a stochastic approach to amplitude analysis of radiofrequency signals previously reported from our laboratory. Analysis of transducer scanner characteristics, data acquisition and processing, and data display from studies in tissue phantoms permit us to define some features of a parameter for expression of tissue character. The ratio of mean to standard deviation of the amplitude histogram from our system is explored as now implemented in our laboratory for reproducible measurements. The theoretical basis for understanding the utility of this method in defining tissue architecture and pathologic conditions requires further work.

An improved stochastic approach to rf amplitude analysis in ultrasonic cardiac tissue characterization.

We carried out a series of studies to improve the reproducibility of methods for ultrasonic myocardial tissue characterization using a stochastic approach to amplitude analysis of radiofrequency signals previously reported from our laboratory. Analysis of transducer scanner characteristics, data acquisition and processing, and data display from studies in tissue phantoms permit us to define some features of a parameter for expression of tissue character. The ratio of mean to standard deviation of the amplitude histogram from our system is explored as now implemented in our laboratory for reproducible measurements. The theoretical basis for understanding the utility of this method in defining tissue architecture and pathologic conditions requires further work.