Source depth estimation based on synthetic aperture beamfoming for a moving source

A continuous wave signal received on a single hydrophone from a moving source is beamformed using the synthetic aperture created by the source, where the signal at each range is steered by a range-dependent phase, relative to the starting point. The range increment (aperture) is determined based on the Doppler shift estimated from the data, knowing the original signal frequency. Given a sufficient signal-to-noise ratio, the source depth can be estimated from the beam output, assuming knowledge of the mode depth functions based on the nominal sound speed and bottom profile in the area. The method is illustrated with simulated data and at-sea data. For real data, the signal phase contains a random, incoherent component caused by the (random) source motion and media fluctuations in addition to the deterministic range-dependent component due to source range change. A phase locked loop is introduced to remove the random component assuming that the random component fluctuates faster with time than the range-dependent phase. When a vertical array of receivers are available covering the depth span of interest, the beam output can be used directly to estimate the source depth. In this case, no knowledge of the acoustic environment is needed.

Data-based matched-mode source localization for a moving source

A data-based matched-mode source localization method is proposed in this paper for a moving source, using mode wavenumbers and depth functions estimated directly from the data, without requiring any environmental acoustic information and assuming any propagation model. The method is in theory free of the environmental mismatch problem because the mode replicas are estimated from the same data used to localize the source. Besides the estimation error due to the approximations made in deriving the data-based algorithms, the method has some inherent drawbacks: (1) It uses a smaller number of modes than theoretically possible because some modes are not resolved in the measurements, and (2) the depth search is limited to the depth covered by the receivers. Using simulated data, it is found that the performance degradation due to the afore-mentioned approximation/limitation is marginal compared with the original matched-mode source localization method. The proposed method has a potential to estimate the source range and depth for real data and be free of the environmental mismatch problem, noting that certain aspects of the (estimation) algorithms have previously been tested against data. The key issues are discussed in this paper.

Acoustic mode coupling induced by nonlinear internal waves: evaluation of the mode coupling matrices and applications

This paper applies the mode coupling equation to calculate the mode-coupling matrix for nonlinear internal waves appearing as a train of solitons. The calculation is applied to an individual soliton up to second order expansion in sound speed perturbation in the Dyson series. The expansion is valid so long as the fractional sound speed change due to a single soliton, integrated over range and depth, times the wavenumber is smaller than unity. Scattering between the solitons are included by coupling the mode coupling matrices between the solitons. Acoustic fields calculated using this mode-coupling matrix formulation are compared with that obtained using a parabolic equation (PE) code. The results agree very well in terms of the depth integrated acoustic energy at the receivers for moving solitary internal waves. The advantages of using the proposed approach are: (1) The effects of mode coupling can be studied as a function of range and time as the solitons travel along the propagation path, and (2) it allows speedy calculations of sound propagation through a packet or packets of solitons saving orders of magnitude computations compared with the PE code. The mode coupling theory is applied to at-sea data to illustrate the underlying physics.

Acoustic mapping of ocean currents using networked distributed sensors

Distributed underwater sensors are expected to provide oceanographic monitoring over large areas. As fabrication technology advances, low cost sensors will be available for many uses. The sensors communicate to each other and are networked using acoustic communications. This paper first studies the performance of such systems for current measurements using tomographic inversion approaches to compare with that of a conventional system which distributes the sensors on the periphery of the area of interest. It then proposes two simple signal processing methods for ocean current mapping (using distributed networked sensors) aimed at real-time in-buoy processing. Tomographic inversion generally requires solving a challenging high dimensional inverse problem, involving substantial computations. Given distributed sensors, currents can be constructed locally based on data from neighboring sensors. It is shown using simulated data that similar results are obtained using distributed processing as using conventional tomographic approaches. The advantage for distributed systems is that by increasing the number of nodes, one gains a much more improved performance. Furthermore, distributed systems use much less energy than a conventional tomographic system for the same area coverage. Experimental data from an acoustic communication and networking experiment are used to demonstrate the feasibility of acoustic current mapping.

Multipath correlations in underwater acoustic communication channels

Uncorrelated scattering (US), which assumes that multipath arrivals undergo uncorrelated scattering and are thus uncorrelated, has been the standard model for digital communications including underwater acoustic communications. This paper examines the cross-correlation of multipath arrivals based on at-sea data with different temporal coherence time, assuming quasi-stationary statistics. It is found that multipath arrivals are highly cross-correlated when the channel is temporally coherent, and are uncorrelated when the channel is temporally incoherent. A theoretical model based on the path phase rates and relative-phase fluctuations is used to explain experimentally observed phenomena, assuming the path amplitudes vary slowly compared with the phases. The implications of correlated scattering for underwater acoustic communication channel tracking are discussed.

Predictors of shunt-dependent chronic hydrocephalus after aneurysmal subarachnoid haemorrhage

INTRODUCTION

Chronic hydrocephalus is a common complication that can occur after aneurysmal subarachnoid haemorrhage (SAH). The purpose of this study was to investigate clinical risk factors that could predict the occurrence of shunt-dependent chronic hydrocephalus after aneurysmal SAH.

METHODS

Eighty-eight consecutive patients who underwent either surgery or transarterial endovascular embolization as a treatment for cerebral aneurysm within 72 h -after experiencing SAH from March 2005 to July 2006 were studied retrospectively to assess the risk factors that might predict shunt-dependent chronic hydrocephalus. Clinical and demographic factors were examined, including age, sex, initial admission mean arterial blood pressure (MABP), blood sugar level at admission, fever frequency, initial external ventricular drainage (EVD), Fisher grade, Hunt and Hess grade, intraventricular haemorrhage (IVH) and treatment methods to define predictors of shunt-dependent hydrocephalus. The length of hospital stay and modified Rankin scale recorded 6 months after SAH were also evaluated; these parameters were compared between the shunt-dependent and non-shunt-dependent groups.

RESULTS

Of the 88 patients, 22 (25%) underwent shunt placement to treat their chronic hydrocephalus. The average length of hospital stay was 33.9 days for the shunt-treated group and 14 days for the non-shunt-treated group. The non-shunt-treated group scored an average of 1.05 on the modified Rankin scale compared with 2.77 for the shunt-treated group. A univariate analysis revealed that several admission variables were associated with long-term shunt-dependent hydrocephalus: (1) increased age (p = 0.023); (2) initial admission MABP (p = 0.027); (3) a high Fisher grade (p = 0.031); (4) a poor admission Hunt and Hess grade (p = 0.030); (5) the presence of IVH (p = 0.029), and (6) initial EVD (p < 0.0001). The factor most commonly associated with shunt-dependent hydrocephalus over the course of hospital days was fever frequency (p < 0.0001).

CONCLUSIONS

Chronic hydrocephalus after aneurysmal SAH has a multifactorial aetiology. Understanding the risk factors that predict the occurrence of chronic hydrocephalus may help neurosurgeons to expedite permanent cerebrospinal fluid diversion, which could decrease both the cost and length of hospital stay and prevent further complications.

Five-year multicenter study of magnetic attachments used for natural overdenture abutments

The purpose of this study was to examine a longitudinal clinical performance of magnetic attachments used for natural overdenture abutments. The study included 131 patients who had used removable prostheses (complete overdentures 31%, partial dentures 69%) more than 5 years (40-90 years old) with 211 magnetic attachments on natural abutments (Magfit 400 or 600; Aichi Steel co., Aichi, Japan) treated in 15 clinics using a standardized protocol. Analyses were performed on the degree of patient satisfaction regarding retention, complications of magnets (corrosion, detachment from denture base), abutments (pain during mastication, periodontal pocket formation, inflammation, mobility), and dentures (fracture etc.). Ninety-seven percent of patients were satisfied with the retention and stability of their dentures. No corrosion of magnet was observed, and 19 magnets were detached. Most frequent complication of abutments was periodontal pocket formation (52%), followed by the inflammation (29%), increase in mobility (27%) and pain (4%). Magnetic attachment on natural tooth abutments provided a viable and long-term treatment option.

Clutter reduction using Doppler sonar in a harbor environment

A high frequency experiment was conducted in the Woods Hole Harbor in Massachusetts to evaluate the effectiveness of Doppler sonar for discriminating targets from reverberation. Using a pulsed linear frequency modulated signal, one finds that the matched filtered outputs are filled with high-level discrete backscattered returns, referred to as clutter, which are often confused with the target echo. The high level non-target returns have an amplitude distribution that is heavy-tailed. Using a Doppler-sensitive binary-phase-shift-keying signal coded with an m-sequence, the target echo and clutter can be separated by Doppler and delay, and tracked using the Doppler spectrogram (Dopplergram). The Doppler filtered time series show a background reverberation with a Rayleigh-like amplitude distribution, with an improved signal-to-(peak) reverberation ratio compared with that without Doppler filtering. The reduced reverberation level with Doppler processing decreases the probability of false alarm (Pfa) for a given threshold level. Conversely, for a given Pfa, the higher signal-to-(peak) reverberation ratio implies a higher probability of detection. Transmission loss measurement was conducted to estimate some of the system parameters, e.g., the source level and target strength relative to the noise level.

Temporal coherence of acoustic rays and modes using the path integral approach

Acoustic propagation can be described by rays and normal modes. Applying the path integral to refractive rays in three dimensional space, Dashen et al. [J. Acoust. Soc. Am. 77, 1716-1722 (1985)] derived the mutual coherence function of the acoustic field. For shallow water where sound interacts with boundaries, the acoustic field can be described by vertical modes and horizontal rays. Applying the path integral to the horizontal rays, one obtains the mutual coherence function of the normal modes. This paper applies this formulation to the derivation of the temporal coherence function of individual modes and also that of the acoustic field in the presence of linear internal waves. The effects of mode coupling due to internal waves on temporal coherence loss are illustrated with numerical calculations.

Properties of underwater acoustic communication channels in shallow water

Underwater acoustic channels are band-limited and reverberant, posing many obstacles to reliable, phase-coherent acoustic communications. While many high frequency communication experiments have been conducted in shallow water, few have carried out systematic studies on the channel properties at a time scale relevant for communications. To aid communication system design, this paper analyzes at-sea data collected in shallow water under various conditions to illustrate how the ocean environments (sea surface waves and random ocean medium) can affect the signal properties. Channel properties studied include amplitude and phase variations, and temporal coherence of individual paths as well as the temporal and spatial coherence of multipaths at different time scales. Reasons for the coherence loss are hypothesized.

Relating the performance of time-reversal-based underwater acoustic communications in different shallow water environments

The performance of underwater acoustic communications, such as the output signal-to-noise ratio (OSNR), is generally dependent on the channel specifics, hence a channel model is normally required as the performance of the channel equalizer depends on the number of tap coefficients used (e.g., a sparse equalizer) which are different for different oceans having different multipath arrivals. This letter presents theoretical arguments, and experimental data from different oceans that suggest that the increase of OSNR with the number of diverse receivers (in terms of the effective number of receivers) and the decrease of OSNR with the channel-estimation error follow a universal relationship using the time-reversal or correlation-based equalizer, despite the fact that the channels have very different properties. The reason is due to the fact that the OSNR is a function of the q function, the auto-correlation of the received impulse responses summed over all receiver channels, and the q function is approximately the same for all shallow waters given a sufficient (≥4-6) number of receivers.

Ruptured aneurysm of the accessory middle cerebral artery associated with moyamoya disease: a case report

The accessory middle cerebral artery can provide collateral blood supply in moyamoya disease. We report a case of unilateral moyamoya disease which demonstrates the anatomy of the right accessory middle cerebral artery and a ruptured peripheral aneurysm on the artery. Our patient was a 56-year-old woman who initially suffered from headache and lethargy. Right caudate nucleus hemorrhage with intraventricular extension and spontaneous subarachnoid hemorrhage were found on brain computed tomography. A ruptured peripheral accessory middle cerebral artery aneurysm associated with unilateral moyamoya disease was diagnosed on cerebral angiography. Surgical intervention to excise the peripheral accessory middle cerebral artery aneurysm assisted by frameless navigation guidance to reduce the risk of damage to collateral vessels was done successfully. Histopathology of excised tissue showed this anomaly was a pseudoaneurysm. The management of an aneurysm in moyamoya disease should be modified based on its location and collateral vessels. Prevention of aneurysm bleeding and preservation of collateral vessels during craniotomy are the critical when managing hemorrhagic moyamoya disease. This case suggests that surgical intervention for ruptured intracranial aneurysms is safe with the use of frameless navigation guidance to minimize collateral vessel injuries.

Interference suppression for code-division multiple-access communications in an underwater acoustic channel

In a code-division multiple-access communications network, the signal from a nearby user often creates a strong interference for the signal from a distant user. This is known as the near-far problem. Power control of source levels is ineffective in an underwater acoustic channel due to the slow sound speed. Interference rejection based on code orthogonality is ineffective using matched-filter processing due to the fact that multipath arrivals effectively destroy the code orthogonality and that the signal arrival times between different users are not synchronized. An algorithm, called hyperspace cancellation by coordinate zeroing, is used in this paper to remove/suppress interference. Using a fast Walsh-Hadamard transform (FWHT) based on the interferer's code sequence, the interference signal is enhanced and removed by coordinate zeroing. The residual signal is transformed back using an inverse FWHT. The filtered data, with the interference signal largely removed, are processed using the desired signal code sequence. Two methods previously developed for direct-sequence spread-spectrum communications in an underwater channel are used to extract the transmitted symbols. Low bit error rate (<10(-2)) is found with the at-sea data for signal-to-interference ratio as low as -8 to -11 dB.

Detection of a novel HLA-B27 allele, B*2740, in Taiwanese volunteer bone marrow donors by sequence-based typing: curiosity rewarded

We report here a novel HLA-B allele, B*2740, discovered in Taiwanese volunteer marrow donors. The new sequence has nucleotide variation at position 527 (T-->A) as compared to B*2708. The nucleotide change caused an amino acid substitution from valine (V) to glutamic acid (E) at codon 152. Since B*2740 carries sequence confers to HLA-Bw6 public epitope we believe that this novel B*27 allele might have been generated from a gene conversion involving a Bw4-specific allele (probably B*2704) and a Bw6-specific allele.

Low probability of detection underwater acoustic communications using direct-sequence spread spectrum

Direct-sequence spread spectrum is used for underwater acoustic communications between nodes, at least one of which is moving. At-sea data show that the phase change due to source motion is significant: The differential phase between two adjacent symbols is often larger than the phase difference between symbols. This poses a challenge to phase-detection based receiver algorithms when the source or receiver is moving. A pair of energy detectors that are insensitive to the phase fluctuations is proposed, whose outputs are used to determine the relationship between adjacent symbols. Good performance is achieved for a signal-to-noise ratio (SNR) as low as -10 dB based on at-sea data. While the method can be applied to signaling using short code sequences, the focus in this paper is on long code sequences for the purpose of achieving a high processing gain (at the expense of a low data rate), so that communications can be carried out at a low input SNR to minimize the probability of detection (P(D)) by an interceptor. P(D) is calculated for a typical shallow water environment as a function of range for several source levels assuming a broadband energy detector with a known signal bandwidth.

Temporal coherence of sound transmissions in deep water revisited

This paper examines the signal coherence loss due to internal waves in deep water in terms of the signal coherence time and compare to data reported in the literature over the past 35 years. The coherence time of the early raylike arrivals was previously modeled by Munk and Zachariasen ["Sound propagation through a fluctuating stratified ocean: Theory and observation," J. Acoust. Soc. Am. 59, 818-838 (1976)] using the supereikonal approximation and by Dashen et al. ["Path-integral treatment of acoustic mutual coherence functions for arrays in a sound channel," J. Acoust. Soc. Am. 77, 1716-1722 (1985)] using the path integral approach; a -1 [corrected] power frequency dependence and a -1/2 [corrected] power range dependence were predicted. Recent data in shallow water in downward refractive environments with internal waves suggested that the signal coherence time of the mode arrivals follows a -3/2 power frequency dependence and a -1/2 power range dependence. Since the temporal coherence of the acoustic signal is related to the temporal coherence of the internal waves, based on the observation that the (linear) internal waves in deep and shallow waters have a similar frequency spectrum, it is argued that the modelike arrivals in deep water should exhibit a similar frequency dependence in deep and shallow waters. This argument is supported by a brute-force application of the path integral to mode arrivals based on the WKB relation between the ray and mode. It is found that the data are consistent with the -3/2 power frequency dependence but more data are needed to further test the hypothesis.

Performance analysis of direct-sequence spread-spectrum underwater acoustic communications with low signal-to-noise-ratio input signals

Direct-sequence spread-spectrum signals collected from the TREX04 experiment are analyzed to determine the bit-error-rate (BER) as a function of the input signal-to-noise ratio (SNR) for a single receiver. A total of 1160 packets of data are generated by adding ambient noise data collected at sea to the signal data (in postprocessing) to create signals with different input-SNR, some as low as -15 dB. Two methods are analyzed in detail, both using a time-updated channel impulse-response estimate as a (matched) filter to mitigate the multipath-induced interferences. The first method requires an independent estimate of the time-varying channel impulse-response function; the second method uses the channel impulse-response estimated from the previous symbol as the matched filter. The first method yields an average BER <10(-2) for input-SNR as low as -12 dB and the second method yields a similar performance for input-SNR as low as -8 dB. The measured BERs are modeled using the measured signal amplitude fluctuation statistics and processing gain obtained by de-spreading the received signal with the transmitted code sequence. Performance losses caused by imprecise symbol synchronization at low input-SNR, uncertainty in channel estimation, and signal fading are quantitatively modeled and compared with data.

Traumatic internal carotid artery pseudoaneurysm mimicking sphenoid sinus tumor

Massive epistaxis following blunt craniofacial trauma should alert clinicians to possible traumatic internal carotid artery (ICA) aneurysm. This article describes a case of a 46-year-old female patient with traumatic ICA pseudoaneurysm presenting similar to a sphenoid sinus mass lesion. Massive bleeding occurred during the endoscopic procedure. Brain angiography revealed two lobulated ICA pseudoaneurysms over a cavernous segment. The patient successfully underwent endovascular stent assisted coil embolization. Timely diagnosis and treatment of the ICA psuedoaneurysm produced a favourable outcome.

Subbottom profiling using a ship towed line array and geoacoustic inversion

Bottom profiling traditionally uses broadband signals received on a line array at long ranges to estimate the bottom layer structure and thickness. In this paper, a subbottom profiling method is developed and applied to a ship-towed line array using the same ship towed source to estimate the subbottom layer structure and thickness. A ship towed line-array system can be used to estimate bottom properties using geoacoustic inversion and can cover a wide area in a short time. It needs some prior information about the subbottom structure and layer thickness, without which the solution can be ambiguous and even erratic when resolving parameters over a wide area. It is shown that the required subbottom information can be obtained from the time-angle relation by beamforming the same acoustic signal data used for geoacoustic inversion. The time-angle analysis is used to expose the prevalent physics intrinsic to geoacoustic inversion. One finds that the tau-p relation of the bottom and the bottom reflection coefficients, sampled at discrete angles associated with bottom and multiple surface-bottom returns, are often adequate, for many practical applications, to uniquely determine the geoacoustic bottom at low (< or =1 kHz) frequencies.

Traumatic anterior cerebral artery aneurysm following blunt craniofacial trauma

BACKGROUND

Traumatic aneurysms of the anterior cerebral artery (ACA) were retrospectively reviewed in an effort to identify patients at high risk of ACA aneurysm.

METHODS

Blunt craniofacial trauma patients featuring vascular injuries over the region of the ACA were studied.

RESULTS

Six patients featuring eight ACA aneurysms were diagnosed between June 1992 and December 2005, inclusively. Seven aneurysms were located at nonbranched sites and one was located over the right ACA-anterior communicating artery junction. One patient died immediately of massive intracranial lobar hemorrhage (ICH). The other 5 patients experienced rebleeding during a period of from 1 to 29 days. Brain computed tomography revealed subarachnoid hemorrhage (SAH) in 1 of these 6 patients, ICH over the medial frontal area or cingulated gyrus in 4 patients, intraventricular hemorrhage (IVH) in 3 patients, and an interhemispheric subdural hematoma (SDH) in 2 patients.

CONCLUSION

Delayed-onset deterioration of neurological deficit is the most common clinical presentation of traumatic ACA aneurysms. Midline hemorrhage such as medial frontal hemorrhage or cingulate gyrus hemorrhage, and the presence of an interhemispheric SDH associated with SAH and IVH subsequent to blunt craniofacial trauma should be further evaluated, as they pre-sent a high risk of traumatic ACA aneurysms to patients.

Single- and multi-channel underwater acoustic communication channel capacity: a computational study

Acoustic communication channel capacity determines the maximum data rate that can be supported by an acoustic channel for a given source power and source/receiver configuration. In this paper, broadband acoustic propagation modeling is applied to estimate the channel capacity for a time-invariant shallow-water waveguide for a single source-receiver pair and for vertical source and receiver arrays. Without bandwidth constraints, estimated single-input, single-output (SISO) capacities approach 10 megabitss at 1 km range, but beyond 2 km range they decay at a rate consistent with previous estimates by Peloquin and Leinhos (unpublished, 1997), which were based on a sonar equation calculation. Channel capacities subject to source bandwidth constraints are approximately 30-90% lower than for the unconstrained case, and exhibit a significant wind speed dependence. Channel capacity is investigated for single-input, multi-output (SIMO) and multi-input, multi-output (MIMO) systems, both for finite arrays and in the limit of a dense array spanning the entire water column. The limiting values of the SIMO and MIMO channel capacities for the modeled environment are found to be about four times higher and up to 200-400 times higher, respectively, than for the SISO case. Implications for underwater acoustic communication systems are discussed.

Diagnosis of Traumatic Internal Carotid Artery Injury

Optimally, internal carotid artery (ICA) injury associated with craniofacial trauma should be treated soon after diagnosis. However, diagnosis is difficult and often delayed. The typical symptoms and signs for diagnosis of traumatic ICA injuries are sometimes easily neglected. Clinically, some patients were initially diagnosed by craniofacial fracture nearby the course of ICA. This investigation retrospectively reviews clinical experience in patients with traumatic ICA injury with a focus on the importance of craniofacial fracture nearby the course of ICA observed on brain or facial bone computed tomography. Eighteen patients with traumatic ICA injury were diagnosed at Chang Gung Memorial Hospital, Taiwan, from June 1998 to April 2004, including 10 patients with pseudoaneurysm formation, seven patients with occlusion, and one patient with laceration. Brain or facial bone computed tomography was reviewed retrospectively. The sample included 14 (78%) patients with skull base fractures involving the intracranial course of ICA and three (17%) patients with mandibular and cervical spine fractures near the course of extracranial ICA. Only one (5%) patient did not have evident fracture. Fractures involving the carotid canal were noted in three (17%) patients. Eight patients received interventional treatments. No further interventional treatments for traumatic ICA occlusion were performed as a result of good collateral flow from contralateral ICA or large infarction noted when diagnosed. Three patients with pseudoaneurysm received expectant management. One patient with arterial laceration with extravasation received no further management. Through meticulously evaluating routine brain and facial bone computed tomography, craniofacial fracture involving intracranial or extracranial course of ICA may be an adjuvant indicator of traumatic ICA injury for early diagnosis.

Beam intensity striations and applications

The single-element spectrogram for a continuous broadband signal, plotted as a function of range, has been shown to exhibit striated bands of intensity maxima and minima. The slope of the striations is an invariant of the modal interference and is described by a waveguide invariant parameter "beta." The striation pattern is analyzed and modeled in this paper for the beam outputs of a horizontal line array obtained by conventional beamforming. Array beamforming makes it possible to measure the waveguide invariant parameter for weak signals due to the enhancement of signal levels by the array gain over that of a single element. It is shown that the signal beam spectrogram as a function of range exhibits the same striation pattern as that (predicted) for a single element. Specifically, for a broadside signal, the beam striation is identical to that of a single-element plus a constant signal gain. For a nonbroadside target, the signal beam intensity will be modified by a frequency-bearing dependent signal gain due to the signal spread over multiple beams, nevertheless the beam spectrogram retains the same striation pattern (slope) as for a single element. The sidelobe beams (outside the canonical cones containing the signal arrivals) exhibit an entirely different striation pattern as a function of frequency and range. For array processing, it is shown that a fast range-rate, close range target and a distant, slow range-rate interference source will have a different striation pattern (slope) in the corresponding beam spectrograms as a function of time, assuming no prior knowledge of the source ranges. The difference in the striations between the beam spectrograms can be used in array processing to suppress the interference contribution. A 5-7 dB interference suppression is demonstrated using simulated data.