An acoustic double fishnet using Helmholtz resonators.

The acoustic transmission of a closely spaced pair of patterned and perforated rigid plates is explored in air. The structure resembles an acoustic double fishnet design, with each plate modified such that the gap between them acts as an array of Helmholtz resonators. This allows the center frequency of the stop band to be reduced by a factor greater than 2 from the value obtained for the conventional acoustic double fishnet design. Experimental results accord well with the predictions of a finite element model.

Magnetic resonance imaging of rheumatoid arthritis in metacarpophalangeal joints.

OBJECTIVE: To report the development of high-resolution targeted magnetic-resonance imaging (MRI) techniques (not using injections of contrast media) to investigate and monitor rheumatoid arthritis (RA) in the metacarpophalangeal (MCP) joints. DESIGN AND PATIENTS: A total of 25 RA patients (age range 30-68 years) with varying degrees of disease severity ranging from early onset through active disease to the burnt-out stage, were imaged. (One patient subsequently underwent surgery and histological data was obtained.) A series of 10 control subjects were also studied--two for each 10-year age range. All the RA subjects were assessed for disease activity using standard clinical criteria and radiography as part of normal procedures. MRI was carried out using a targeted system and novel radiofrequency coil. Images of the MCP were performed at very high resolution with 1.5 mm slice thickness and in-plane resolution 130 microns. Standard gradient-echo (GE) sequences were used for anatomical imaging, multiple-echo GE sequences used to produce effective spin-spin relaxation time (T2*) maps and optimised binomialpulse presaturation used in conjunction with a GE sequence to generate magnetization-transfer (MT) ratio maps. RESULTS: High-quality high-resolution images of the MCP joints were obtained which highlighted normal anatomy and key features characterising the disease state (e.g. pannus, bone erosions, vascularity). Accurate measurements of T2* and MT with variations of +/- 4% and +/- 2% respectively were achieved. In active disease, variations in T2* and MT could be determined throughout areas of pannus, clearly demonstrating the heterogeneity of this erosive tissue. Pannus in MCP joints with active destruction was found to have high values of T2* varying from 25 ms to 40 ms with pockets up to 100 ms, whereas pannus present in chronic destruction, or burnt-out disease, had T2* values ranging from 21 to 29 ms. MT-active tissue was uniformly distributed in burnt-out disease, which was confirmed histologically in one case, compared with a more heterogeneous distribution in active disease. CONCLUSION: The MRI sequences and targeted system developed allow high-resolution studies of RA disease progression and activity. The data confirm the variable pattern of the disease and, in particular, heterogeneity of pannus.

Optimum setting of binomial pulses for magnetization transfer contrast.

Investigation of the cause of image artifacts generated by a magnetization transfer (MT) sequence using binomial-pulse trains led to findings of imperfections in the pulses. These imperfections caused anomalous direct saturation of the free water, which was localized due to the static magnetic field inhomogeneity. In the case of single binomial pulses a loss of overall MT response across the field of view results. Two methods of correcting the imperfections and removing the artifact have been established using interactive adjustment of sub-pulse lobes and phase swapping of pulse trains. These imperfections may be present in many systems and may have led to erroneous judgements of the value of binomial pulses for MT imaging. A technique for interrogating the frequency spectrum of the binomial-pulse train has been utilized, allowing its optimization. The use of accurate and optimized binomial pulses may yet prove to be preferable to pulsed off-resonance methods for quantitative, clinical MT imaging.

Measurement of acoustic streaming using magnetic resonance.

Magnetic resonance imaging (MRI) has been used to explore acoustic streaming caused in water under ultrasonic exposure conditions similar to those used for diagnostic applications. Streaming was established in an enclosed tube with acoustically transparent end windows, using a pulsed, weakly-focused transducer of acoustic frequency 3.5 MHz. Phase-detection MRI was used to image and quantify streaming profiles in the region of the acoustic focus. Acoustic powers in the range 0.4 mW to 100 mW were used. The sensitivity of the technique enabled streaming velocities down to 0. 1 mm s(-1) to be measured, generated by acoustic power less than 1 mW. In addition, acoustic streaming generated within open meshes with minimum pore dimensions of 3.0 mm and 2.0 mm was measured. The flow velocity in the coarser mesh reached 0.9 mm s(-1) at 95 mW total acoustic power. These observations demonstrate that acoustic streaming is probably a much more general phenomenon in diagnostic ultrasound (ultrasound) than previously recognised. The combination of magnetic resonance and ultrasound shows promise as a diagnostic method for the differentiation of cystic lesions in vivo, and for their characterisation, with sensitivity significantly greater than using ultrasound alone.

A programmable eddy-current compensation system for MRI and localized spectroscopy.

A programmable preemphasis system has been developed for eddy-current compensation in gradient sets with or without active screening. Preemphasis parameters are nonvolatile but can be changed rapidly when working with interchangeable gradient sets. The design allows easy retrofit to existing systems. A method for setting the preemphasis is presented, together with specimen results.

Integration of visual and tactile modalities.

Experiments have been carried out in which subjects were required to discriminate time-varying visual and tactile stimuli presented simultaneously via a 2D array of light-emitting diodes and a vibrator on the fingertip. Contrary to expectations, no problems in parallel visual and tactile perception were apparent. Similar results were obtained in a second experiment in which subjects were require to lipread from a video image and simultaneously identify speech-derived tactile stimuli. The generally poor performance of tactile aids to lipreading has been attributed to problems of parallel perception, but such problems are clearly not present in all circumstances.

Information from time-varying vibrotactile stimuli.

Experiments have been carried out to investigate the information transfer available via a single vibrator on the fingertip. In a first experiment, for stimuli with durations 80 to 320 ms, discrimination of a one-octave step change in frequency at the halfway point was investigated. Results were similar for three stimulus types--sinewave, monophasic pulse and tetraphasic pulse--suggesting temporal cues are more important than spectral cues in this task. In a second experiment, subjects were required to perceive changes in a sequence of stimulus elements. A presentation rate of 6.25 elements s-1 was found to give better results than a rate of 12.5 elements s-1. In the former case, the potential information transfer per element was estimated to be approximately 1.0 bits, corresponding to an information transfer rate of around 6 bits s-1. Implications for the design of a tactile aid to lipreading are discussed.

Coding of acoustic features for a single-channel tactile aid.

Measurements have been made on the discrimination of speech contrasts on the basis of single-channel vibrotactile presentation of a variety of speech-derived signals, coded as amplitude- and frequency-modulated pulse trains. Stimulation was at the index fingertip. The signals chosen for tactile presentation were the speech amplitude envelope, the voice fundamental frequency FO and the zero-crossing frequency in the 1.3-6.6 kHz band. "Two-feature' codings, which present two of these signals simultaneously (one coded as stimulus frequency and one coded as stimulus amplitude), were found to be no more effective than "single feature' codings which present only one signal (coded as both amplitude and frequency). Scores for consonant discrimination were highest for the single-feature coding of zero-crossing frequency, although differences between the codings were not, in general, significant. Scores for emphatic-stress discrimination were highest for the single-feature coding of F0, and this coding produced best results overall. A practical wrist-worn device, whose design is influenced by these experimental results is briefly described.

Modified birdcage coils for targeted imaging.

The design of a radiofrequency coil of U-shaped geometry, specifically intended to image individual knuckle joints of the hand, has been investigated. The coil geometry is that of a "split and opened" birdcage coil. The optimum leg positions for such a U-shaped coil have been theoretically predicted using a novel technique that considers both the signal-to-noise ratio and the homogeneity of the field-of-view. Two particular coils of this type have been constructed, each optimum for a different size of image space and assuming a different source of image noise (sample-dominated or coil-dominated). The experimentally determined radiofrequency fields produced by the coils correspond well to theory. Either coil can be used to obtain good-quality, high-resolution (130 microns x 130 microns in-plane) images in vivo of the first, second, or fifth metacarpophalangeal joints of the hand, sites of particular interest in the study of arthritis.

Vibrotactile and electrotactile perception of time-varying pulse trains.

To establish the best strategy for transmitting speech-derived information via a single tactile channel, measurements were made on the perception of frequency- and/or amplitude-modulated pulse-train stimuli, with a comparison of the electrotactile and vibrotactile modalities. In one experiment, vibrotactile perception of 2-oct step changes in stimulus frequency was found to be significantly better than electrotactile on a time-scale appropriate for the transmission of speech features (e.g., with practiced subjects, information transfer of 69% with 200-ms vibrotactile stimuli, 32% with 200-ms electrotactile stimuli). Perception of step changes in stimulus amplitude was similar in the two modalities when changes in amplitude were tailored to match the different dynamic ranges available. In a second experiment, vibrotactile-perception of voice fundamental frequency with various codings was investigated. Both experiments showed information transfer for vibrotactile stimuli to be greater when frequency and amplitude modulation were used together rather than with one or the other in isolation (sentence-stress identification scores: 66% for FM stimuli, 69% for AM stimuli, 80% for FM/AM stimuli). It is concluded that frequency- and amplitude-modulated vibratory stimulation is a good choice in a practical device for the profoundly hearing impaired.

Water diffusion coefficient measurements in the finger by magnetic resonance imaging.

Diffusion coefficients of water have been measured in the fingers of humans by magnetic resonance imaging. It was found that the measured diffusion coefficients increased with subject age in certain regions of the finger but that these regions differed between males and females. The observation of an increased diffusion coefficient with age appears to be inconsistent with a direct-hydration model and possible explanations are given using other models. It is conjectured that the measured diffusion coefficient of water increases with age as a result of structural changes to proteins.

Coding strategies for a single-channel tactile aid.

Measurements have been made on the ability of normally hearing subjects to identify the stressed word in simple sentences, using only tactile information. Vibrotactile stimuli were presented to the distal pad of the second finger via a single vibrator. A range of coding strategies was investigated, voice pitch or speech amplitude being represented as stimulus frequency and/or intensity. Test results show that, even without specific training, subjects can be quite successful in identifying stress patterns. The most effective coding strategies were (i) voice frequency presented as continuously variable stimulus frequency over the range 40-220 Hz, with a correlated modulation of stimulus amplitude, (ii) speech amplitude presented as two discrete levels of stimulus amplitude.

Electronically simulated loss of frequency resolution.

Circuitry has been developed which degrades speech signals by reducing the spectral resolution available to the listener: each single frequency component in the original speech signal is replaced by a band of noise in the processed output. Measurements were made on the speech perception of normally hearing adult subjects listening to monosyllabic words via such a processing system. The effective resolution bandwidth available to the listener was increased by approximately 100 to 800 Hz, giving a drop of up to approximately 60% in whole-word scores. A three-channel design allowed different degrees of processing in the frequency regions 0 to 2 kHz, 2 to 4 kHz and 4 kHz upwards. Results show that speech frequencies in the range 0 to 2 kHz are more susceptible to this type of degradation than higher frequencies. Future development of this circuitry may allow simulation of the loss of frequency resolution associated with sensorineural hearing loss.

Field trials of a tactile acoustic monitor for the profoundly deaf.

Profoundly deaf subjects were given information about sound level in their environment by means of a body-worn unit coupled to a small vibrator worn on the finger. Results of trials on 19 adults are discussed. The Tactile Acoustic Monitor was found to be useful for identifying domestic sounds by means of their distinctive timing patterns. No significant overall improvement in subject's control of voice level was observed, although some subjects found that having a voice level monitor gave them greater confidence to join conversations. Various design improvements were suggested by the trials. Modifications which have been incorporated into an improved unit are described.

A tactile sound level monitor for the profoundly deaf.

A profoundly deaf person can be made aware of certain features of his acoustic environment by means of a vibrating transducer on the skin. The design of such a system for transmitting sound level information is discussed. Preliminary trials suggest that the device assists profoundly deaf users to control their voice volume and can also give useful information about other sounds in the environment.