Non-uniform Sensitivity Research Articles

Correcting spatial distortions and nonuniform response in area detectors

Software and hardware methods have been developed to correct images for spatial and intensity distortions produced by optical and electro-optical components in X-ray area detectors. Spatial distortions are divided into two types: gross distortions produced by the inherent properties of the detector components and local distortions formed by irregularities in the components. Intensity distortions are separated into three types: those caused by background nonuniformity; those resulting from pixel-dependent nonuniform intensity response; and those resulting from time-dependent variations in background and incident beam intensity. From background, flat-field, reference and mask images, `forward' and `reverse' interpolation tables are generated to correct for spatial distortions and a lookup table is generated to correct for nonuniform sensitivity. The routines have been used successfully on four different area detectors to correct entire images or to correct intensities of individual Bragg peaks. The spatial-distortion correction is good to within 0.1 pixels and the nonuniformity correction to ≲ 2%.

Signal processing for sensor arrays

Five methods have been developed to process output signals from sensor arrays to improve sensor characteristics. A multielement capacitive force sensor array with nonlinear characteristics and inter-element cross-talk was used to demonstrate and compare the performance of these methods: (1) Variable Threshold, which calculated a threshold value from a histogram of the data to determine whether the output of each element was due to an input or cross-talk; (2) Algebraic Cross-Talk Reduction, which reduced the cross-talk between array elements by solving for the input matrix from the input-output relation of the sensor; (3) 2 x 2 Edge and Shape Detection, which was adapted from image processing techniques and enhanced the resolution of the sensor output; (4) Deconvolution using FFT; and (5) Artificial Neural Network, which was trained to learn the inverse relation of the sensor transfer function. The ability of these methods to reduce the cross-talk seen between sensor elements and more closely estimate the sensor input was studied. Preprocessing of the sensor output, which consisted of calibration of each element using non-linear curve fitting, was used to compensate for the non-linear, non-uniform sensitivity characteristics of the sensor array elements. Postprocessing compensated for the dependency on the number of excited elements. A comparative study of eight force sensor arrays using a 200 element test set showed that the simpler signal processing methods, Variable Threshold and 2 x 2 Edge and Shape Detection, performed well only for a limited number of input conditions especially when forces were applied through small objects. In general, neither reduced the error in determining total force by more than 30 percent. The Artificial Neural Network method, the most complicated and computationally intensive method during initial training but simple to implement thereafter, produced superior results for the majority of the input patterns applied to the sensor array, reducing the overall error by 6 to 83 percent. All five signal processing techniques reduced the overall errors of the sensor outputs, some more consistently than others. However, when the overall error magnitude was less than 10 percent, only the Artificial Neural Network reduced the error consistently, making it the method of choice. The results of this study showed that sensors having outputs with high error values can be converted into sensors with much lower output error using appropriate signal processing methods. Incorporation of such signal processing methods into sensor arrays allows the use of sensors with less than ideal characteristics.

Design of capacitance electrodes for concentration measurement of two-phase flow

Capacitance electrode systems for the measurement of the volumetric concentration of two-phase flows have inherently non-uniform sensitivity distributions over the pipe cross section and therefore have different responses to different flow regimes (e.g. core, annular and stratified flows). In processes with varying flow regimes, this can lead to significant measurement errors. An electrode system which has a uniform sensitivity distribution and consequently a response independent of flow regime is therefore desirable. The system performance parameters of surface electrode systems used with stray-immune measurement transducers have been characterised quantitatively using a two-dimensional finite-element model. The electrode system design parameters have also been identified and their effects on system performance parameters have been investigated. As a result, optimum electrode designs are proposed, with responses to different flow regimes having a minimum deviation from linearity.

Improvement of Piezoelectric Photoacoustic Imaging Using Two Images at Different Modulation Frequencies

A new method is presented to improve piezoelectric photoacoustic imaging. This technique uses two images at different modulation frequencies to cancel the background caused by the coupling condition and the spatially nonuniform sensitivity of the piezoelectric sensor. It is found that the image of an object with sub-surface defects obtained by this technique is much clearer than a conventional image. The obtained photoacoustic image is compared with the ultrasonic image.

Quantitative flow measurement in phase contrast MR angiography.

The quantitative nature of phase contrast magnetic resonance angiography is explored, and a technique of blood flow measurement that is independent of system and patient parameters is presented. Phantom and patient studies demonstrate that quantitative flow measurements by phase contrast angiography can be routinely obtained with good accuracy despite nonuniform sensitivity profiles, blood flow pulsatility, and patient-to-patient changes in system gain. The calibration method requires the acquisition of only two flow images and thus can be performed as part of an angiographic session. Flow calibration of any flow profile can be accomplished with this technique. The solutions for plug flow and parabolic flow are presented.

The development of ASPECT, an annular single crystal brain camera for high efficiency SPECT

A description is given of ASPECT, a totally digital annular SPECT (single-photon-emission computed tomography) camera system designed specifically for 3-D brain imaging. Its detector consists of a single-crystal NaI(Tl) ring (31-cm ID by 8-mm thick by 13-cm wide) coupled through an annular light pipe to a 21*3 array of 51-mm/sup 2/ photomultipliers. A unique multifield collimator system, consisting of set of six parallel-hole collimators oriented in an annular array, is designed to view the patient's head from six angles simultaneously. This ring is rotated with high precision, concentric to the detector, through 2 pi radians to sample gamma-ray emissions for 3-D reconstruction over a 23-cm*9-cm field of view. Preliminary results indicate that central regions of the brain will be detected with an efficiency of about six times that of a conventional rotating parallel-hole camera SPECT system and an effective inherent transverse resolution of within 3.5 mm. The system is designed to achieve a reconstructed resolution FWHM within 7.5 mm at the center of the image field. It also features unique means for digital position analysis, calibration, and user quality control. With ASPECT's precision rotating collimator, artifacts normally associated with detector motion, nonuniform sensitivity, and spatial nonlinearities are expected to be insignificant.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

An alternative discriminator design for the Scanditronix SP-3000 time of flight emission tomograph

The detector modules for the original versions of the Scanditronix SP3000 positron-emission tomograph (PET) exhibit a consistent nonuniform sensitivity in count rate for paired crystals coupled to single photomultiplier tubes. The detector modules and discriminators used in the original SP3000 machines are described, and possible solutions are examined. The response is corrected by adopting a new discriminator design for the machine. >

Phase and sensitivity of receiver coils in magnetic resonance imaging.

Receiver coil response is a major cause of nonuniformities in magnetic resonance images. The spatial dependence of the sensitivity and phase of single-saddle receiver coils has been investigated quantitatively by calculating the H1 field and comparing the results with measurements of a uniform phantom. Agreement between the measurements and calculations is excellent. A method is developed which corrects for both the nonuniform sensitivity and the phase shifts introduced by receiver coils.

The design and use of a dual-frequency surface coil providing proton images for improved localization in 31P spectroscopy of small lesions.

Nuclear magnetic resonance (NMR) spectroscopy of small lesions is restricted by the difficulties of localizing the surface coil with respect to the lesion and the problem of ensuring that signal is only obtained from the lesion and not from surrounding tissue. A double-tuned coil has been developed that permits NMR proton images to be obtained from a region of interest, prior to carrying out 31P spectroscopy of the same region with the same coil, without the need for further adjustment. The coil provides a means of accurately localizing the region from which the 31P signal is obtained, whilst offering a means of accurately applying 31P signal localization methods, and the possibility of making corrections for the nonuniform sensitivity of a given surface coil. The coil makes use of two parallel resonant circuits, with independent rf connections, but sharing a common coil. Simulated shorted and open circuit lambda /4 cables are used, respectively, to open circuit each circuit at the resonant frequency of the other circuit and ensure that the simulated lambda /4 line is short circuited for each circuit at the circuit's resonant frequency. At 63.6 MHz, the Q of the coil was 190 unloaded and 90 loaded, and at 25.7 MHz the Q was 210 unloaded and 140 loaded, for a 4-cm-diam coil. The coil has been used to obtain proton images and 31P spectra. A circuit employing only one input was also developed.

In vitro germination of Nosema apis spores under conditions favorable for the generation and maintenance of sporoplasms

Nosema apis spores were germinated readily with physiological medium following air drying on noninhibitory glass surfaces. Stable sporoplasms with active granular cytoplasm were formed. Cytoplasmic activity was maintained in vitro for periods of up to 7 days. Sporoplasms demonstrated nonuniform osmotic sensitivity with a proportion of the organisms remaining stable and active in media of widely differing osmotic potential. N. apis spores, polar tubes, sporoplasms, and cytoplasmic contents were antigenic, showing all features by means of indirect immunofluorescence staining.

The effect of structure on the mechanical behavior and stretch formability of constitutionally dynamic 3000 series aluminum alloys

For a better understanding of the forming mechanism, microstructural changes occurring during processing and their effects on mechanical behavior and stretch formability were studied. The features studied in stripcast and d.c. cast Al-1.25 Mn alloys (3003) (where the composition is given in approximate weight per cent) were (1) the cast structure, (2) the degree of solute supersaturation of the matrix, (3) the size and distribution of second-phase particles, (4) the grain size and (5) the degree of preferred grain orientation produced in materials by selected processing procedures. The structural variables were obtained by a thermomechanical process which indluced (a) preheat treatment, (b) cold rolling and (c) annealing. Concerning mechanical behavior, the main interests were the tensile yield strength, ultimate tensile strength, uniform and non-uniform elongation, strain-hardening exponent (n value) and strain rate sensitivity parameter (m value), all of which are known to have significant influence of formability. Forming limit diagrams were determined by the use of a punch stretch test. In this study the limitation of n and m values in controlling exclusively the limit strains in stretch formibility tests is pointed out and further the significance of microstructure in obtaining high limit strains is indicated. Any quantitative mathematical model of formability must incorporate certain microstructural features of the material.

Effects of high-contrast peripheral patterns on the detection threshold of sinusoidal targets.

Detection thresholds of sinusoidal gratings in the simultaneous presence of high-contrast peripheral masking stimuli partially overlapping the test gratings were determined as a function of the separation between the center of the test grating and the peripheral stimulus by a two-alternative forced-choice method. The results showed that the threshold-elevating effect of simultaneously present peripheral masking stimuli depends on how much of the test grating is left unexposed. An additional experiment, in which the detection thresholds in the absence of the peripheral stimulus were determined as a function of the number of cycles of the test grating, enabled us to show that the threshold-elevating effect is somewhat higher than the effect of simply cutting the test grating down in size. The threshold-elevating effects caused by high-contrast peripheral masking stimuli can be explained in terms of a lateral inhibition and a probability summation across space, taking into account the nonuniform sensitivity across the visual field.

Investigation of a Technique for Providing a Pseudo-Continuous Detector Ring for Positron Tomography

The use of small bismuth germanate (BGO) detectors (2.5-4.5 mm wide) and a simple coding technique for identification of the crystal of interaction with the impinging gamma-ray was investigated in terms of a technique for providing a positron tomography system of the highest possible intrinsic resolution with the additional capability of providing an artifact free image in a stationary system. The technique employs one photomultiplier per two BGO crystals in a scheme of alternating single PMT/crystal and shared PMT/crystal combinations to form a pseudo-continuous ring. Problems of crosstalk between detectors, nonuniform sensitivities and variable resolution are shown to be minimized once discriminator thresholds are clearly above the energy of the bismuth x-rays induced in the BGO crystals by the 511 keV gammas from the positron annihilations. A stationary circular system employing 4.5 mm wide BGO detectors can provide transverse sampling equal to 1/2 the center to center crystal distance or 2.5 mm sampling. The measured intrinsic resolution of a bench top version of this system is 5 to 6 mm FWHH for a 30cm diameter field of view for a 100 cm system diameter. The 2.5 mm sampling with reconstruction filter of appropriate frequency response should provide a high resolution image on the order of 7 to 8 mm FWHM with good signal to noise characteristics which is free of significant aliasing artifacts.

Blurring in Tomograms Made with X-Ray Beams of Finite Width

Tomographic reconstruction has ordinarily assumed that the measurement data can be regarded as line integrals, but the finite width of the X-ray beam invalidates this assumption. The data can however be expressed in the form of integrals over a strip rather than a line. The strip integral kernel is calculated allowing for extended source and detector, as well as for nonuniform photon emission and detector sensitivity. Strip eccentricity, which occurs in practice, is also taken into account. Even if the measurement data were to cover all scanning angles, there would be imperfect reconstruction expressible as a space-variant point spread function deducible from the strip integral kernel. To deal with this it is convenient to introduce the concepts of generalized projection and generalized Radon transform. Point-spread functions are given for cases involving piecewise-uniform symmetrical source distributions and uniform detectors.

Gel permeation chromatography for determining molecular weight distribution and composition distribution based on the molecular weight of block copolymers

The method proposed for determination of the molecular weight distribution (MWD) of block copolymers involves the use of GPC data along with continuous composition determinations during block copolymer fractionation processes. Apart from the normal procedure of gel chromatogram transformation into MWD the proposed method comprises a further correction of the gel chromatogram to allow for the composition curve, for non-uniform detector sensitivity to block copolymer components and, finally, for the principle of3additivity of intrinsic viscosity rates for block copolymer components. Satisfactory results were obtained with block copolymers of PSPMMAPS, PSPAN, PSPAN, PSPB and PIP-α-MSPi in toluene, DMF and THF. Good agreement was found between average characteristics of the block copolymers calculated in line with GPC data and determined from independent measurements.

Reduction of Circular Artefacts in Fan-Beam Scanners

A non-uniform sensitivity of the detector array used in a fan-beam scanner causes circular artefacts which interfere with the picture of the object slice reconstructed from the measured projections. An additional shift of the detector array is proposed here to reduce the amplitude of the artefacts considerably.

Quantitative comparison of 99mTc-iron hydroxide aggregates and 133Xe pulmonary perfusion, and importance of sensitivity-corrected gamma-camera data

Unilateral and four-quadrant regional pulmonary perfusion fractions were quantitatively compared for 99mTc-iron hydroxide aggregates and superior-vena-cavally administered 133Xe in 23 patients with chronic obstructive lung disease and a “coin” lesion, using a gamma camera/PDP-8/1 system. 99mTc and 133Xe perfusion fractions were equivalent (±0.03), with correlation coefficient ≥0·9. Unilateral 99mTc perfusion fractions, measured only with the camera operated in divided mode, were insignificantly different from computer-processed 133Xe fractions. Correction of camera data for non-uniform sensitivity response was not necessary for determination of unilateral or four-quadrant perfusion fractions, but may be important in determining absolute 133Xe clearance half-times. Simple static 99mTc-iron hydroxide aggregates imaging was as accurate as dynamic 133Xe studies for routine clinical estimation of unilateral or four-quadrant pulmonary perfusion.

Effect of nonuniform sensitivity distribution along a wire ther moane mometer probe on turbulence measurement

The effect is determined of nonuniform distribution of temperatures along a wire probe on the averaged turbulent velocity pulsation along it.

The Effect of Wire Length and Separation on X-Array Hot-Wire Anemometer Measurements

This paper describes a simple method for the calculation of errors due to the finite length and separation of X-array hot-wire anemometers. Results of computations for a typical wire array are presented for total and spectral turbulence measurements. Although the actual errors are slightly underestimated due to the simplifying assumptions used to reduce the computational effort, it is found that the wire length and separation effect is very significant. The analysis does not require complete correlation or spectral functions of the flow and includes the effect of nonuniform wire sensitivity.

Seasonal variations in the semi-diurnal tide in the upper atmosphere as determined from radio meteor observations

Corrections to the wind velocity for finite width of aerial polar diagram, and for non-uniform sensitivity of the radio meteor method to different values of velocity, have been calculated for eight stations in the northern hemisphere. The corrected wind data are used to study seasonal changes in phase and amplitude of the semi-diurnal tide in the upper atmosphere. These seasonal variations are analysed into harmonic components of periods 12, 6, 4 and 3 months; the component of period 12 months is found to have the largest amplitude. Similarities of seasonal behaviour are evident at all eight stations.