PubMed · 10794870
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T R Nelson. 2000. Three-dimensional imaging.. https://doi.org/10.1016/s0301-5629(00)00159-9
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A focusing technique based on the inversion of the propagation operator relating an array of transducers to a set of control points inside a medium was proposed in previous work [Tanter et al., J. Acoust. Soc. Am. 108, 223-234 (2000)] and is extended here to the time domain. As the inversion of the propagation operator is achieved both in space and time, this technique allows calculation of the set of temporal signals to be emitted by each element of the array in order to optimally focus on a chosen control point. This broadband inversion process takes advantage of the singular-value decomposition of the propagation operator in the Fourier domain. The physical meaning of this decomposition is explained in a homogeneous medium. In particular, a definition of the number of degrees of freedom necessary to define the acoustic field generated by an array of limited aperture in a focal plane of limited extent is given. This number corresponds to the number of independent signals that can be created in the focal area both in space and time. In this paper, this broadband inverse-focusing technique is compared in homogeneous media with the classical focusing achieved by simple geometrical considerations but also with time-reversal focusing. It is shown that, even in a simple medium, slight differences appear between these three focusing strategies. In the companion paper [Aubry et al., J. Acoust. Soc. Am. 110, 48-58 (2001)] the three focusing techniques are compared in heterogeneous, absorbing, or complex media where classical focusing is strongly degraded. The strong improvement achieved by the spatio-temporal inverse-filter technique emphasizes the great potential of multiple-channel systems having the ability to apply completely different signal waveforms on each transducer of the array. The application of this focusing technique could be of great interest in various ultrasonic fields such as medical imaging, nondestructive testing, and underwater acoustics.
To focus ultrasonic waves in an unknown heterogeneous medium using a phased array, one has to calculate the optimal set of signals to be applied on the transducers of the array. (In most applications of ultrasound, medical imaging, medical therapy, nondestructive testing, the first step consists of focusing a broadband ultrasound beam deeply inside the medium to be investigated.) Focusing in a homogeneous medium simply requires to compensate for the varying focus-array elements geometrical distances. Nevertheless, heterogeneities in the medium, in terms of speed of sound, density, or absorption, may strongly degrade the focusing. Different techniques have been developed in order to correct such aberrations induced by heterogeneous media (time reversal, speckle brightness, for example). In the companion to this paper, a new broadband focusing technique was investigated: the spatio-temporal inverse filter. Experimental results obtained in various media, such as reverberating and absorbing media, are presented here. In particular, intraplate echoes suppression and high-quality focusing through a human skull, as well as hyper-resolution in a reverberating medium, will be shown. It is important to notice that all these experiments were performed with fully programmable multichannel electronics whose use is required to fully exploit the spatio-temporal technique.
The functional outcome of a group of 75 'high-risk' preterm infants was studied at the corrected age of 12 months. Only infants with high-risk for developmental deviance with gestational ages below 32 weeks and/or birthweights less than 1500 g were included in the study. Additionally the infants were categorised according to their medical history conforming with the 'Neonatal Medical Index' (NMI I to V), with category I describing infants with few medical problems and V characterizing those with the most serious complications. In this study we included only infants with 'high-risk' as categorised in NMI III to V, since infants with 'low-risk' have been described earlier. Infants with cerebral ultrasonographic abnormalities were incorporated into the NMI categories, but also analysed separately to compare outcomes. At 12 months (corrected age) apart from pediatric follow-up, a full neurological assessment was done with emphasis on postural control, spontaneous motility and hand function. Special attention was given to symmetrical development. The infants were then categorised as having optimal, non-optimal or asymmetrical outcomes. An overall optimal outcome on postural control was found in 64% of all infants (67% in NMI III, 60% in NMI IV and 62.5% in NMI V). Too much extension interfering with postural control was found significantly more often in infants in NMI V (15%), compared to infants in NMI IV (8%) and NMI III (4.5%). Poor postural control had a significant influence on other domains of development such as motility (P=0.00), asymmetry (P=0.00) and hand function (P=0.00). Cerebral ultrasonographic abnormalities seemed to have an influence on motility (P=0.03), while no direct relationship was found with postural control, hand function or asymmetry. It is unclear whether this poor coordination of gross motor function will have consequences for appropriate visual-motor and sensorimotor integration therewith hampering later cognitive function, as often described in preterm infants. It is suggested that the poor postural control found in many infants is the result of both myogenic and/or neurogenic deviations and associated with cerebral pathology, but is also caused by the preterm birth and its nursing consequences.