The stripe artifact in transcranial ultrasound imaging.

OBJECTIVE: Transcranial images are affected by a "stripe artifact" (also known as a "streak artifact"): two dark stripes stem radially from the apex to the base of the scan. The stripes limit the effective field of view even on patients with good temporal windows. This study investigated the angle dependency of ultrasound transmission through the skull to elucidate this artifact. METHODS: In vivo transcranial images were obtained to illustrate the artifact. In vitro hydrophone measurements were performed in water to evaluate transcranial wavefronts at different incidence angles of the ultrasound beam. Both a thin acrylic plate, as a simple bone model, and a human temporal bone sample were used. RESULTS: The imaging wavefront splits into two after crossing the solid layer (acrylic model or skull sample) at an oblique angle. An early-arrival wavefront originates from the direct longitudinal wave transmission through water-bone interfaces, while a late-arrival wavefront results from longitudinal-to-transverse mode conversion at the water-bone interface, propagation of the transverse wave through the skull, and transverse-to-longitudinal conversion at the bone-water interface. At normal incidence, only the direct wavefront (without mode conversion) is observed. As the incidence angle increases, the additional "mode conversion" wavefront appears. The amplitude of the transcranial wavefront decreases and reaches a minimum at an incidence angle of about 27°. Beyond that critical angle, only the mode conversion wavefront is transmitted. CONCLUSIONS: The stripes are a consequence of the angle-dependent ultrasound transmission and mode conversion at fluid-solid interfaces such as between the skull and the surrounding fluidlike soft tissues.

A novel phase assignment protocol and driving system for a high-density focused ultrasound array.

Currently, most phased-array systems intended for therapy are one-dimensional (1-D) and use between 5 and 200 elements, with a few two-dimensional (2-D) systems using several hundred elements. The move toward lambda/2 interelement spacing, which provides complete 3-D beam steering, would require a large number of closely spaced elements (0.15 mm to 3 mm). A solution to the resulting problem of cost and cable assembly size, which this study examines, is to quantize the phases available at the array input. By connecting elements with similar phases to a single wire, a significant reduction in the number of incoming lines can be achieved while maintaining focusing and beam steering capability. This study has explored the feasibility of such an approach using computer simulations and experiments with a test circuit driving a 100-element linear array. Simulation results demonstrated that adequate focusing can be obtained with only four phase signals without large increases in the grating lobes or the dimensions of the focus. Experiments showed that the method can be implemented in practice, and adequate focusing can be achieved with four phase signals with a reduction of 20% in the peak pressure amplitude squared when compared with the infinite-phase resolution case. Results indicate that the use of this technique would make it possible to drive more than 10,000 elements with 33 input lines. The implementation of this method could have a large impact on ultrasound therapy and diagnostic devices.

Noninvasive transesophageal cardiac thermal ablation using a 2-D focused, ultrasound phased array: a simulation study.

This simulation study proposes a noninvasive, transesophageal cardiac-thermal ablation using a planar ultrasound phased array (1 MHz, 60 x 10 mm2, 0.525 mm interelement spacing, 114 x 20 elements). Thirty-nine foci in cardiac muscle were defined at 20, 40, and 60-mm distances and at various angles from the transducer surface to simulate the accessible posterior left atrial wall through the esophageal wall window. The ultrasound pressure distribution and the resulting thermal effect in a volume of 60 x 80 x 80 mm3, including esophagus and cardiac muscle, were simulated for each focus. For 1, 10, and 20-s sonications with 60 degrees C and 70 degrees C peak temperatures in cardiac muscle and without thermal damage in esophageal wall, the transducer acoustic powers were 105-727, 28-117, 21-79 W and 151-1044, 40-167, 30-114 W, respectively. The simulated lesions (thermal dose in equivalent minutes at 43 degrees C > or = 240 minutes) at these foci had lengths of 1-6, 3-11, 3-13 mm and 3-15, 5-19, 6-23 mm, respectively, and widths of 1-4, 2-7, 3-9 mm and 3-9, 4-13, 4-17 mm, respectively. As a first step toward feasibility, controllable tissue coagulation in cardiac tissue without damage to the esophagus was demonstrated numerically.

A numerical study of transcranial focused ultrasound beam propagation at low frequency.

The feasibility of transcranial ultrasound focusing with a non-moving phased array and without skull-specific aberration correction was investigated using computer simulations. Three cadaver skull CT image data sets were incorporated into an acoustic wave transmission model to simulate transskull ultrasound wave propagation. Using a 0.25 MHz hemispherical array (125 mm radius of curvature, 250 mm diameter, 24 255 elements), the simulated beams could be focused and steered with transducer element driving phases and amplitude adjusted for focal beam steering in water (water-path). A total of 82 foci, spanning wide ranges of distance in the three orthogonal dimensions, were simulated to test the focal beam steering capability inside the three skulls. The acoustic pressure distribution in a volume of 20 x 20 x 20 mm(3) centred at each focus was calculated with a 0.5 mm spacing in each axis. Clearly defined foci were retained through the skulls (skull-path) in most cases. The skull-path foci were on average 1.6 +/- 0.8 mm shifted from their intended locations. The -3 dB skull-path beam width and length were on average 4.3 +/- 1.0 mm and 7.7 +/- 1.8 mm, respectively. The skull-path sidelobe levels ranged from 25% to 55% of the peak pressure values. The skull-path peak pressure levels were about 10%-40% of their water-path counterparts. Focusing low-frequency beam through skull without skull-specific aberration correction is possible. This method may be useful for applying ultrasound to disrupt the blood-brain barrier for targeted delivery of therapeutic or diagnostic agents, or to induce microbubbles, or for other uses of ultrasound in brain where the required power levels are low and the sharp focusing is not needed.

[Response of lettuce to different nitrogen forms].

Solution culture experiment was adopted to investigate the influence of different nitrogen forms on lettuce growth and development and their nitrogenous nutritional characteristics. The results showed that the affinity of the roots of lettuce seedlings to ammonium nitrogen (NH4(+)-N) was slightly higher than that to nitrate nitrogen (NO3(-)-N). Among three treatments of NO3(-)-N, NH4(+)-N+ NO3(-)-N and NH4(+)-N, the relative proportion of biological yields of lettuce was 100:56.9:12.4, and that of nitrogen uptake amount was 100:48.9:8.6, respectively. Nitrate nitrogen was the most suitable N source to lettuce growth. When the respective proportion of NH4(+)-N and NO3(-)-N was 50%, the growth of lettuce was inhibited to some extent. When the nitrogen source supplied as solely NH4(+)-N, lettuce was hard to grow normally. When supplying the same amount of NH4(+)-N and NO3(-)-N, lettuce showed a tendency to absorb more NH4(+)-N than NO3-N. At different culture stages, the ratio of NH4(+)-N to NO3(-)-N absorbtion was less than 1. It seemed that lettuce did prefer NH4(+)-N to NO3-N in absorbtion. However, ammonium nitrogen as nitrogen source was not suitable to lettuce for its metabolism. When nitrate nitrogen was not sufficient, it mainly affected the growth of lettuce shoot; when the ammonium nitrogen in nutrient solution was 50%, the root growth of lettuce seedlings was greatly inhibited, and some pathological symptoms appeared. Taking running water as water source (in which, NO3(-)-N concentration was about 0.5 mol.L-1) and lettuce was cultured by supplying sole NO3(-)-N for two weeks and then supplying NH4(+)-N, the growth of lettuce was greatly stimulated, in the meantime, the NO3(-)-N contents and total accumulation amount greatly decreased. Supplying urea as N source, the growth rate of lettuce was apparently inferior to other nitrogen forms, but no pathological symptoms appeared.

Experimental spatial sampling study of the real-time ultrasonic pulse-echo BAI-mode imaging technique.

The ultrasonic pulse-echo backscattered amplitude integral (BAI)-mode imaging technique has been developed to inspect the seal integrity of hermetically sealed, flexible food packages. With a focused 17.3-MHz transducer acquiring radio frequency (RF) echo data in a static rectilinear stop-and-go pattern, this technique was able to reliably detect channel defects as small as 38 microm in diameter and occasionally detect 6-microm-diameter channels. This contribution presents our experimental spatial sampling study of the BAI-mode imaging technique with a continuous zigzag scanning protocol that simulates a real-time production line inspection method in continuous motion. Two transducers (f/2 17.3 MHz and f/3 20.3 MHz) were used to acquire RF echo data in a zigzag raster pattern from plastic film samples bearing rectilinear point reflector arrays of varying grid spacings. The average BAI-value difference (deltaBAI) between defective and intact regions and the contrast-to-noise ratio (CNR) were used to assess image quality as a function of three spatial sampling variables: transducer spatial scanning step size, array sample grid spacing, and transducer -6-dB pulse-echo focal beam spot size. For a given grid size, the deltaBAI and CNR degraded as scanning step size in each spatial dimension increased. There is an engineering trade-off between the BAI-mode image quality and the transducer spatial sampling. The optimal spatial sampling step size has been identified to be between one and two times the -6-dB pulse-echo focal beam lateral diameter.