The ultrasound brain helmet: new transducers and volume registration for in vivo simultaneous multi-transducer 3-D transcranial imaging

In this report the authors describe a noninvasive transcranial method of determining the flow velocities in the basal cerebral arteries. Placement of the probe of a range-gated ultrasound Doppler instrument in the temporal area just above the zygomatic arch allowed the velocities in the middle cerebral artery (MCA) to be determined from the Doppler signals. The flow velocities in the proximal anterior (ACA) and posterior (PCA) cerebral arteries were also recorded at steady state and during test compression of the common carotid arteries. An investigation of 50 healthy subjects by this transcranial Doppler method revealed that the velocity in the MCA, ACA, and PCA was 62 +/- 12, 51 +/0 12, and 44 +/- 11 cm/sec, respectively. This method is of particular value for the detection of vasospasm following subarachnoid hemorrhage and for evaluating the cerebral circulation in occlusive disease of the carotid and vertebral arteries.

Time reversal of ultrasonic fields represents a way to focus through an inhomogeneous medium. This may be accomplished by a time-reversal mirror (TRM) made from an array of transmit-receive transducers that respond linearly and allow the incident acoustic pressure to be sampled. The pressure field is then time-reversed and re-emitted. This process can be used to focus through inhomogeneous media on a reflective target that behaves as an acoustic source after being insonified. The time-reversal approach is introduced in a discussion of the classical techniques used for focusing pulsed waves through inhomogeneous media (adaptive time-delay techniques). Pulsed wave time-reversal focusing is shown using reciprocity valid in inhomogeneous medium to be optimal in the sense that it realizes the spatial-temporal matched filter to the inhomogeneous propagation transfer function between the array and the target. The research on time-reversed wave fields has also led to the development of new concepts that are described: time-reversal cavity that extends the concept of the TRM, and iterative time-reversal processing for automatic sorting of targets according to their reflectivity and resonating of extended targets.

Prior estimates of long-term trends in the incidence and severity of stroke have varied; trends in lifetime risk have not been reported. To determine long-term trends in the incidence, lifetime risk, severity, and 30-day mortality of clinical stroke. Prospective evaluation of the community-based Framingham Study original and offspring cohorts. Participants were 9152 men and women free of prevalent stroke and undergoing follow-up for up to 50 years over 3 consecutive periods (1950-1977, 1978-1989, and 1990-2004), with biennial ascertainment of stroke risk factor data and active surveillance for incident clinical stroke and cause-specific mortality. Incidence (age-adjusted, sex-specific), severity, 30-day mortality, and mortality-adjusted 10-year and lifetime risk of stroke in each of the specified periods. There were 1030 incident clinical strokes (450 [44%] in men, 629 atherothrombotic brain infarctions [61%]) in 9152 persons 55 years or older over 174,917 person-years of follow-up. The age-adjusted incidence of first stroke per 1000 person-years in each of the 3 periods was 7.6, 6.2, and 5.3, respectively, in men (P = .02 for trend) and 6.2, 5.8, and 5.1 in women (P = .01 for trend). The lifetime risk at age 65 years decreased from 19.5% to 14.5% in men (P = .11) and from 18.0% to 16.1% in women (P = .61). Age-adjusted stroke severity did not vary across periods; however, 30-day mortality decreased significantly in men (from 23% to 14%; P = .01) but not significantly in women (from 21% to 20%; P = .32). In this cohort of men and women free of prevalent clinical stroke at initial examination, incidence of stroke has decreased over the past 50 years but the lifetime risk has not declined to the same degree, perhaps due to improved life expectancy. The results of this study suggest that improved control of risk factors has lowered stroke incidence but emphasize the need for continued primary prevention efforts.