Planar Scanning Research Articles

Far-field uncertainty due to random near-field measurement error

Recent planar near-field scanning tests with ultralow-sidelobe antennas have confirmed that random near-field measurement errors will ultimately limit the accuracy of far-field patterns. A formulation is outlined for estimating the spectral signal-to-noise ratio (SNR) arising from noncorrectable near-field random measurement errors. The formulation applies to arbitrarily directive test antennas and probes-even nulling probes. A far-field parameter, called the scan plane coupling factor, may be computed directly from the near-field data, and then used to form the spectral SNR. The accuracy of the spectral SNR is confirmed by simulation and by actual tests with low-sidelobe AWACS array antenna.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Surface accuracy measurement of a deployable mesh reflector by planar near-field scanning

Using a near-field antenna measurement facility, it is possible to simultaneously evaluate the surface accuracy of a reflector antenna as well as the far-field pattern of the antenna for a short time. The surface errors of a 2-m deployable mesh reflector for satellite use were measured by a planar near-field system. As a result, the influence of periodic structures, due to the antenna ribs, is clearly observed. Also, the surface accuracy obtained with the near field scanning technique coincides well with that obtained by an optical measurement technique.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Interlaboratory comparison of hydrophone calibrations

The results of an interlaboratory comparison of hydrophone calibration techniques in the frequency range 1-10 MHz are reported. Two membrane hydrophones were calculated to six laboratories, and each laboratory determined the end-of-cable loaded sensitivities using their normal calibration methods; these included optical interferometry, planar scanning, reciprocity combined with time-delay spectrometry, and suspended-sphere radiometry. After converting the results to end-of-cable open-circuit sensitivities, in most cases agreement between the various values was within +/-10% at all frequencies.

The influence of nonlinear fields on miniature hydrophone calibration using the planar scanning technique

Miniature polyvinylidene fluoride (PVDF) hydrophones used in determining the power and intensity output of ultrasonic fields, including those radiated from diagnostic ultrasound equipment, were calibrated under a variety of field conditions using the planar scanning technique. A diagnostic B-scan piston-type transducer was used as a source, and the output intensity (spatial-peak pulse-average, or SPPA) was varied from 2 to 30 W/cm(2) while the total power was kept constant. The higher-intensity waveforms were significantly nonlinear in the focal region of the source. When the lateral beam profiles of the source (as measured by the hydrophone to be calibrated) were determined by positive-peak-detecting the ultrasonic pulse, the calibrated pressure sensitivity of the hydrophone systematically decreased as the field became progressively more nonlinear. When the beam profiles were measured using the pulse intensity integral, no systematic dependence of the calibration on field linearity was noted. These results imply that measured values of power and intensity of ultrasound diagnostic equipment may be dependent on the methodology utilized to map the lateral beam profiles of the transducer being measured, and the extent of nonlinear effects in the field under characterization.

Primary biliary cirrhosis: Tc-99m IDA planar and SPECT scanning.

The authors studied ten patients with primary biliary cirrhosis using planar and single photon emission computed tomography (SPECT); results were compared with those from 13 healthy subjects. Patients with primary biliary cirrhosis had six- to tenfold prolongation of mean halflife (t 1/2) hepatic excretion of technetium-99m iminodiacetic acid (IDA) compared with mean t 1/2 excretion in healthy subjects. All patients with primary biliary cirrhosis had diffuse, uniform hepatic isotope retention and normal major bile ducts on planar and SPECT scans. The gallbladder was seen within 60 minutes in nine of nine patients who had intact gallbladders. The mean gallbladder volume was normal, but gallbladder ejection fractions and ejection rates were reduced in patients with primary biliary cirrhosis compared with those of healthy subjects. In contrast with previous studies of patients with sclerosing cholangitis and common bile duct obstruction, patients with primary biliary cirrhosis had different findings on scintiscans. In the early evaluation of patients with cholestasis, Tc-99m IDA hepatobiliary scintigraphy may be useful in the selection of the most appropriate invasive diagnostic test to enable a definitive diagnosis.

Practical aspects of ultrasound dosimetry: Hydrophone performance

Abstract Wideband piezoelectric polymer (PVDF) hydrophones are now available with well documented performance characteristics; these probes are especially well suited for the measurement and comprehensive characterisation of both pulsed and continuous wave acoustic fields generated by ultrasound diagnostic devices. The hydrophones feature flat (to within ± 1.5 dB) frequency response, voltage (pressure) sensitivity uniform well beyond 15 MHz, well-behaved, predictable directivity patterns and excellent linearity in the pressure response up to 10 8 Pa (kidney stone crushers). These characteristics make the polymer probes uniquely applicable to measurements of the acoustic output parameters of ultrasound devices, including peak instantaneous pressure amplitude, as specified in the AIUM/NEMA Standard, IEC Draft document and FDA 510(K) requirements. The hydrophones are absolutely calibrated in terms of the end-of-cable voltage sensitivity (in units V/Pa or their derivatives) with a specified terminating load. Most common designs of ultrasonic polymer probes are the needle and membrane type which when properly designed provide reference hydrophones of good spatial and temporal resolution. Absolute, (end-of-cable) voltage sensitivity calibration has been carried out in the range of intensities from 10 mW/cm 2 to 100 W/cm 2 (SPTP) using both the conventional two transducer reciprocity calibration technique and the Planar Scanning Technique. Calibration techniques indicate that overall uncertainty of typically ±1.5 dB can be obtained. The area where uncertainties mostly occur are in plane wave approximation due to transmitter and receiver size and excitation frequency, and phase variation. Combining one of the absolute calibration techniques (e.g. reciprocity and a swept frequency measurement procedure) allows hydrophones to be calibrated as a continuous function of frequency. Moreover both amplitude and phase response may be determined. The spectral analysis technique, combined with swept frequency system (TDS), constitutes a useful tool for assessment of both the amplitude only and the complex frequency response of acoustic radiators. Transducer impulse response can be determined, based on far field measurements. Transducer artifacts can be removed by inverse filtering (deconvolution) procedures for quantitative ultrasound measurements. The effect of spatial averaging was determined experimentally and it was found that it is not significant at frequencies up to 5 MHz, providing 0.5–0.6 mm diameter probes (the smallest currently available) are used. For frequencies above 5 MHz the experimental data indicate that in the “worst case” the estimated “true” spatial peak value in the measured waveform exceeded the measured value by a factor of 2, depending on frequency and the diameter of the polymer probe used. Work is in progress to estimate more accurately the effect of spatial averaging due to the finite aperture of the hydrophone probes.

An overview of near-field antenna measurements

After a brief history of near-field antenna measurements with and without probe correction, the theory of near-field antenna measurements is outlined beginning with ideal probes scanning on arbitrary surfaces and ending with arbitrary probes scanning on planar, cylindrical, and spherical surfaces. Probe correction is introduced for all three measurement geometries as a slight modification to the ideal probe expressions. Sampling theorems are applied to determine the required data-point spacing, and efficient computational methods along with their computer run times are discussed. The major sources of experimental error defining the accuracy of typical planar near-field measurement facilities are reviewed, and present limitations of planar, cylindrical, and spherical near-field scanning are identified.

Limited Angle Diffraction Tomography and Its Application to Planar Scanning Systems

Abstmct-a method is presented for tomographic reconstruction when diffraction is important and when the range of viewing angles is limited. Employing iterative techniques in both image and transform spaces, the method is particularly applicable to the planar scanning systems which are used extensively in practice. Computer simulations demonstrate an obvious improvement in the reconstructed images employing this algorithm as opposed to using the conventional algorithm of diffraction tomography.

Revised guidance on acceptable limits of exposure during nuclear magnetic resonance clinical imaging.

Revised guidance on acceptable limits of exposure during nuclear magnetic resonance clinical imaging.

Calibration of miniature ultrasonic receivers using a planar scanning technique

This paper discusses a method currently used to calibrate miniature ultrasonic receivers in the 1- to 10-MHz frequency range. The method compares the power radiated by a source transducer to the integrated intensity obtained by scanning the receiver across the ultrasound field in a series of linear parallel lines. Sources of error are presented and theoretical models and experimental data are analyzed to estimate the maximum uncertainty associated with this technique, as used by the Bureau of Radiological Health. Our calculations estimate the accuracy of the intensity calibration constants determined to be within +17%, −23% of the true value. With refinement of this technique an uncertainty within +14%, −13% should be achievable.

Open mast scattering investigation

The Georgia Tech automated planar scanner has been employed to measure the forward scattered plane wave spectrum (PWS) for both a circular cylinder mast and a tripod open mast. The experimentally derived spectra have been used in a plane wave spectrum scattering analysis to compute the main beam gain-loss and pattern effects of the circular cylinder and tripod masts for a paraboloidal antenna. This investigation demonstrates the practical use of a planar scanner to obtain forward scattering patterns for complex scatterers.

Accurate evaluation of a millimeter wave compact range using planar near-field scanning

Significant progress has been made in recent years on planar near-field measurements for antenna calibrations. Such measurements are also useful in the alignment and evalnation of compact ranges because they provide more information than a limited number of analogue plots in one dimension. Contour plots of amplitude and phase data obtained from more complete two-dimensional measurements precisely and accurately locate sources of problems in the range reflector, with phase contour plots being more useful as diagnostic tools.

Systematic measurement errors of band shape in the case of planar scanning of an interference field

Systematic measurement errors of band shape in the case of planar scanning of an interference field

Rotational scanning of the liver.

When intravenous Au198 or rose bengal-I131 is removed from the blood by normal liver tissue, tumors will be demonstrated as defects on the liver scan pattern only if their presence sufficiently affects the counting rate of the scintillation counter. The liver is a solid organ which resembles an irregular hemisphere, having been so molded by surrounding structures that at least half its bulk is curved laterally along the right side of the patient in contact with the ribs. In one-plane scanning of a supine patient, the recorded image emphasizes the left side of the liver. Unfortunately, tumor identification is made difficult in this region by easy confusion with normally occurring liver defects such as the gallbladder fossa, the hilus, and the fissure for the round ligament, all of which can be consistently demonstrated on the photorecording. Nor are defects in the more homogeneous large right lobe demonstrated well, for, as the probe traverses it, the curvature of the liver mass results in an increasing displacement of the surface from the detector and the lobe is actually viewed on edge with maximum thickness. As a consequence, small tumors in the posterior right lobe are masked by the thick intervening layer of liver tissue and may escape detection. To a certain extent, this difficulty might be avoided by making multiple planar scans in anterior, oblique, and lateral views, but this would be time-consuming, would re-quire repeated dosage if rose bengal-I131 were to be used, and each recording would show only a limited region of the liver to advantage. In seeking to refine this study, it became apparent that rotational scanning might be employed to advantage. If, instead of scanning in one plane, the detector were to traverse the upper abdomen in semicircular path, with the probe focused continuously on the anterior border of the vertebral column, the collimator would be held as close as possible to the liver surface at all times and a more uniform thickness of liver tissue would be examined throughout the scan (Fig. 1). The maximum volume of the liver would thus be brought within the range of effective scanning. Experimental studies with a realistic liver phantom have confirmed the validity of this hypothesis. What would be a deeply placed lesion from a frontal planar view now becomes a shallow lesion as the detector rotates laterally. Tumors of the right lobe are scanned with greater effectiveness, increasing the probability of their detection, and yet the left lobe is surveyed just as adequately as with planar scanning (Figs. 2 and 3). A panoramic image of both liver lobes is presented on a single recording. Preliminary investigations suggest that rotational scanning may be a definite improvement of the liver survey technic meriting further development.