Finite beam-width ray model for geometric spectral broadening

The purpose of the study was to compare measured spectral width and maximum frequency with that predicted from ray models of geometric spectral broadening. Zero and finite beam-width models were used. Spectral data were acquired from a string phantom using two commonly-used linear array systems. Beam width and Doppler aperture sizes were measured using a needle hydrophone. The results showed that the experimentally measured data agreed best with the finite beam-width model. The zero beam-width model was in error by up to 50% for calculated spectral width, and up to 10% for maximum frequency. It is concluded that spectral width and maximum frequency are best calculated using the finite beam-width model, and that ultrasound manufacturers could improve the variation in spectral broadening measured at different locations on a single machine by adjusting the aperture size to give a constant subtended angle and beam width.

Female voice communications in high level aircraft cockpit noises--part II: vocoder and automatic speech recognition systems

BACKGROUND

The intelligibility of female and male speech is equivalent under most ordinary living conditions. However, due to small differences between their acoustic speech signals, called speech spectra, one can be more or less intelligible than the other in certain situations such as high levels of noise. Anecdotal information, supported by some empirical observations, suggests that some of the high intensity noise spectra of military aircraft cockpits may degrade the intelligibility of female speech more than that of male speech.

METHODS

In an applied research study, the intelligibility of female and male speech was measured in several high level aircraft cockpit noise conditions experienced in military aviation.

RESULTS

In Part I, (Nixon CW, et al. Aviat Space Environ Med 1998; 69:675-83) female speech intelligibility measured in the spectra and levels of aircraft cockpit noises and with noise-canceling microphones was lower than that of the male speech in all conditions. However, the differences were small and only those at some of the highest noise levels were significant. Although speech intelligibility of both genders was acceptable during normal cruise noises, improvements are required in most of the highest levels of noise created during maximum aircraft operating conditions. These results are discussed in a Part I technical report. This Part II report examines the intelligibility in the same aircraft cockpit noises of vocoded female and male speech and the accuracy with which female and male speech in some of the cockpit noises were understood by automatic speech recognition systems. The intelligibility of vocoded female speech was generally the same as that of vocoded male speech. No significant differences were measured between the recognition accuracy of male and female speech by the automatic speech recognition systems. The intelligibility of female and male speech was equivalent for these conditions.

Quantification of the enhanced backscatter phenomenon from an intravenous and an intra-arterial contrast agent

The phenomenon of enhanced backscatter from myocardial contrast agents was studied using two examples, a robust thicker-walled, intra-arterial agent (AIP 201) and a smaller thinner-walled, intravenous agent (Quantison). Both agents are composed of albumin-encapsulated microbubbles. Samples of the agents were inserted into an in vitro phantom and insonated under different scanning regimes. Upon insonation, Quantison exhibited a pronounced increase in mean backscatter at medium and low concentrations, which decreased dramatically with increasing number of frames of insonation. At high concentrations, no dramatic decrease or increase in mean backscatter was observed over the period of the experiment. AIP 201 exhibited an overall decrease in mean backscatter with increasing number of frames of insonation. These results suggest that the difference in size and wall thickness of the contrast microcapsules can significantly affect the behaviour of the contrast agents in an ultrasound field.

Current trials on angiotensin-converting enzyme inhibitors and endothelial dysfunction

Several mechanisms are thought to play a role in the improvement in endothelial vasodilation produced by angiotensin-converting enzyme inhibitors. Both animal and clinical data are reviewed.

Axonal swellings and degeneration in mice lacking the major proteolipid of myelin

Glial cells produce myelin and contribute to axonal morphology in the nervous system. Two myelin membrane proteolipids, PLP and DM20, were shown to be essential for the integrity of myelinated axons. In the absence of PLP-DM20, mice assembled compact myelin sheaths but subsequently developed widespread axonal swellings and degeneration, associated predominantly with small-caliber nerve fibers. Similar swellings were absent in dysmyelinated shiverer mice, which lack myelin basic protein (MBP), but recurred in MBP*PLP double mutants. Thus, fiber degeneration, which was probably secondary to impaired axonal transport, could indicate that myelinated axons require local oligodendroglial support.

Current concepts of PLP and its role in the nervous system

Proteolipid protein (PLP) and its smaller isoform DM20 constitute the major myelin proteins of the CNS. Mutations of the X-linked Plp gene cause the heterogeneous syndromes of Pelizaeus-Merzbacher disease (PMD) and spastic paraplegia (SPG) in man and similar dysmyelinating disorders in a range of animal species. A variety of mutations including missense mutations, deletions, and duplications are responsible. Missense mutations cause a predicted alteration in primary structure of the encoded protein(s) and are generally associated with early onset of signs and generalised dysmyelination. The severity of the phenotype varies according to the particular codon involved and the influence of uncharacterised modifying genes. There is some evidence that the dysmyelination results from the altered protein acquiring a novel function deleterious to the oligodendrocyte's function. Transgenic mice carrying extra copies of the Plp gene provide a valid model of PMD/SPG due to gene duplication. Depending on the gene dosage, the phenotype can vary from early onset of severe and lethal dysmyelination through to a very late onset of a tract-specific demyelination and axonal degeneration. Mice with a null mutation of the Plp gene assemble and maintain normal amounts of myelin but develop a progressive axonopathy, again demonstrating tract specificity. The results indicate that the functions of PLP are far from clear. There is good evidence that it is involved in the formation of the intraperiod line of myelin, and the results from the knockout and transgenic mice suggest a role in the interaction of oligodendrocyte and axon.

Three-dimensional echocardiographic evaluation of left ventricular volume: comparison of Doppler myocardial imaging and standard gray-scale imaging with cineventriculography--an in vitro and in vivo study

BACKGROUND

Standard gray-scale imaging (GSI), three-dimensional (3D) echocardiography has been shown to be superior to two-dimensional echocardiography in measuring left ventricular volume. However, the often relatively poor quality of transthoracic gray-scale data can limit the potential application of this technique. Doppler myocardial imaging (DMI) is a new ultrasound technique that potentially offers higher-quality 3D images with a transthoracic approach than the 3D GSI technique. This study was designed to compare the accuracy of standard GSI and DMI 3D left ventricular volume measurements in vitro and in vivo.

METHODS AND RESULTS

In vitro, the minimum and maximum volume of the contracting single-chamber, tissue-mimicking phantom was calculated by using both techniques. In vivo, GSI and DMI 3D left ventricular volume measurements were performed in 16 patients. End-diastolic and end-systolic left ventricular volumes were computed for both techniques and compared with those calculated by cineventriculography. In vitro, both methods tended to underestimate the true phantom volume, but the systematic error was smaller for DMI than for GSI (-1.2% +/- 1.5% vs. -4.3% +/- 3%; p < 0.01) and was more constant in the case of DMI over the range of different sizes of true volume. In vivo, for GSI the end-diastolic volume mean difference was -12.6 ml and the limits of agreement were +/-18 ml, and for DMI the corresponding values were -4.2 and +/- 10.6 ml, respectively. The difference for end-systole was -6.5 +/- 10.6 ml and -1.5 +/- 10 ml for GSI and DMI, respectively. The magnitude of the difference in volume measurement between 3D echocardiography and cineventriculography was significantly smaller when using the Doppler technique.

CONCLUSIONS

The results of this in vitro and in vivo study indicate that DMI is superior to GSI as a transthoracic acquisition technique for 3 D volume computation.

Contextual dependencies during perceptual-motor skill performance: influence of task difficulty

The present experiment was designed to address the role of task difficulty for the development of contextual-dependent behaviour during perceptual-motor task acquisition. Task difficulty in the present study was manipulated by restricting the time that the subject; (a) viewed task-relevant information, and (b) was afforded to execute a sequence of key presses. Assuming these manipulations made the acquisition of the key press tasks more difficult, it was hypothesised that reducing the time for either of these processes would result in a greater dependence on reinstating the training context at the time of test. This was based on previous findings reported by Wright and Shea (1991) which indicated that task difficulty can contribute to the development of context-dependent behaviour. The results of the study revealed that viewing time was a potent difficulty manipulation. This is supported by the slower acquisition rates for subjects exposed to short as opposed to long viewing conditions during training. More importantly, the impact of changing the intentional-incidental stimuli relationship that was apparent during training, during the test was greatest for the individuals who experienced shorter viewing times during the training phase. Thus, the proposed influence of task difficulty on contextual-dependent performance was supported. This dependency was manifested as a slowing in both the initiation and execution of the correct key presses. It is proposed that individuals trained in the more difficult viewing condition during training, when faced with an incongruence in the acquisition and test stimuli, must engage additional data-driven verification processes to supplement their extant knowledge in order to execute an appropriate response.