Spatial-spectral holographic correlator at 1536 nm using 30-symbol quadriphase- and binary-phase-shift keyed codes.

Optical 30-symbol quadriphase-shift keyed (QPSK) and binary-phase-shift keyed (BPSK) codes were processed in a spatial-spectral holographic correlator with the Er(3+): Y(2)SiO(5) spectral hole-burning material operating at 1536 nm in the important 1550-nm communications band. The results demonstrate the ability of spatial-spectral holographic correlators to process QPSK codes and BPSK codes with the same apparatus. The high-fidelity correlations produced by this optical coherent transient device exhibit the low sidelobe characteristics expected for the codes used.

Time-scale detection of microemboli in flowing blood with Doppler ultrasound.

Small formed elements and gas bubbles in flowing blood, called microemboli, can be detected using Doppler ultrasound. In this application, a pulsed constant-frequency ultrasound signal insonates a volume of blood in the middle cerebral artery, and microemboli moving through its sample volume produce a Doppler-shifted transient reflection. Current detection methods include searching for these transients in a short-time Fourier transform (STFT) of the reflected signal. However, since the embolus transit time through the Doppler sample volume is inversely proportional to the embolus velocity (Doppler-shift frequency), a matched-filter detector should in principle use a wavelet transform, rather than a short-time Fourier transform, for optimal results. Closer examination of the Doppler shift signals usually shows a chirping behavior apparently due to acceleration or deceleration of the emboli during their transit through the Doppler sample volume. These variations imply that a linear wavelet detector is not optimal. We apply linear and quadratic time-frequency and time-scale detectors to a set of noise-corrupted embolus data. Our results show improvements of about 1 dB using the time-scale detectors versus an STFT-based detector signifying that embolus detection is best approached as a time-scale problem. A time-scale-chirp detector is also applied and is found to have the overall best performance by about 0.5-0.7 dB while coming fairly close (about 0.75 dB) to a theoretical upper bound.

Sizing emboli in blood using pulse Doppler ultrasound--I: Verification of the EBR model.

An experiment to verify a theory describing ultrasound backscattering from emboli in flowing blood is presented. The theoretical predictions for the backscattered power versus embolus size are based on the measurement of the embolus to blood ratio (EBR) of backscattered acoustic power. This tool is necessary for in vivo clinical application because it removes the need to characterize attenuation and reflection loss in heterogeneous tissue. The experiment presented utilizes a customized ultrasound pulse Doppler that is capable of interrogating a sample volume with two different frequencies concurrently. A flow circuit including a 3.6-mm-diameter conduit in polyacrylamide gel in which emboli are observed with the dual frequency Doppler is described. The flow within the circuit has acoustic backscatter coefficient similar to blood due to a calibrated concentration of 31.1-mu diameter polystyrene microspheres. Polystyrene microsphere "emboli" having nominal diameters of 161 and 239 mu are placed in this flow loop and time series Doppler shift signatures are recorded. These signatures are investigated and a refraction artifact hypothesis is proposed to explain systematic deviation of the signatures away from theoretically expected results. Results show that gross discrimination of embolus size is feasible.

Sizing emboli in blood using pulse Doppler ultrasound--II: Effects of beam refraction.

A theoretical and numerical study of the acoustic field intensity within a curved flow conduit having 1) diameter similar to the wavelength of the interrogating frequency and 2) speed of sound mismatch with the surrounding medium is presented. The field intensity is shown to vary significantly and in a monotonic fashion across the flow conduit. The resulting insonation of emboli transiting through the Doppler sample volume is explored with a Monte Carlo study of the behavior of the embolus to blood power ratio (EBR). The numerical simulation findings are shown to be in good agreement with previously reported experimental results. A method is explored for estimating embolus diameter when this refraction artifact is present, and shown to yield excellent results when applied to experimental data. Further work toward clinical application of these results is discussed.

Composite matched filter output partitioning.

A common pattern recognition problem is finding a library element closest, in some sense, to a given reception. In many scenarios, optimal detection requires N matched filters for N library elements. Since N can often be quite large, there is a need for suboptimal techniques that base their decisions on a reduced number of filters. The use of composite matched filters (CMFs) (also called synthetic discriminant functions or linear combination filters) is one technique to achieve this reduction. For two level CMF outputs, the reduction is from N to log(2)N matched filters. Previously, the coefficients of the CMF output were restricted to positive values-often 0 and 1. We refer to such filters as binary CMFs. An alternative approach is to use -1 and +1 for filter coefficients. This alternative filter will be called a bipolar CMF. This paper demonstrates how the extension from a binary to a bipolar CMF greatly improves the detection performance while still maintaining the reduced computational requirements of the binary CMF. Furthermore, the bipolar CMF is invariant to scale: multiplying the input by a positive constant gives the same processor output. This desirable behavior does not exist for the binary CMF.

Conventional and composite matched filters with error correction: a comparison.

A common pattern recognition problem is finding a library object which most closely matches a received image. For additive white Gaussian input noise, optimal detection performance is obtained using a matched filter for each of the N possible library objects. The use of composite matched filters (CMFs) (also called synthetic discriminant functions or linear combination filters) is one technique of reducing the number of filters required for the recognition problem. For two-level composite matched filter outputs, the reduction is from N to Q = log(2) (N) filters. The CMF's performance, however, can be suboptimum. Using CMFs with bipolar (+1,-1) outputs, this paper examines the detection performance improvement obtained by using error correcting codes. Use of varying levels of error correction is shown to allow trade-off between detection probability and the number of bank filters. Also, we show that in the case of inexact processing, the CMF can perform better than the conventional matched filter.