2-D motion estimation using two parallel receive beams.

We describe a method for estimating 2-D target motion using ultrasound. The method is based on previous ensemble tracking techniques, which required at least four parallel receive beams and 2-D pattern matching. In contrast, the method described requires only two parallel receive beams and 1-D pattern matching. Two 1-D searches are performed, one in each lateral direction. The direction yielding the best match indicates the lateral direction of motion. Interpolation provides sub-pixel magnitude resolution. We compared the two beam method with the four beam method using a translating speckle target at three different parallel beam steering angles and transducer angles of 0, 45, and 90 degrees. The largest differences were found at 90 degrees, where the two beam method was generally more accurate and precise than the four beam method and also less prone to directional errors at small translations. We also examined the performance of both methods in a laminar flow phantom. Results indicated that the two beam method was more accurate in measuring the flow angle when the flow velocity was small. Computer simulations supported the experimental findings. The poorer performance of the four beam method was attributed to differences in correlation among the parallel beams. Specifically, center beams 2 and 3 correlated better with each other than with the outer beams. Because the four beam method used a comparison of a kernel region in beam pair 2-3 with two different beam pairs 1-2 and 3-4, the 2-to-1 and 3-to-4 components of this comparison increased the incidence of directional errors, especially at small translations. The two beam method used a comparison between only two beams and so was not subject to this source of error. Finally, the two beam method did not require amplitude normalization, as was necessary for the four beam method, when the two beams were chosen symmetric to the transmit axis. We conclude that two beam ensemble tracking can accurately estimate motion using only two parallel receive beams.

Speckle tracking for multi-dimensional flow estimation.

Speckle tracking methods overcome the major limitations of current Doppler methods for flow imaging and quantification: angle dependence and aliasing. In this paper, we review the development of speckle tracking, with particular attention to the advantages and limitations of two-dimensional algorithms that use a single transducer aperture. Ensemble tracking, a recent speckle tracking method based upon parallel receive processing, is described. Experimental results with ensemble tracking indicate the ability to measure laminar flow in a phantom at a beam-vessel angle of 60 degrees, which had not been possible with previous 2D speckle tracking methods. Finally, important areas for future research in speckle tracking are briefly summarized.

A novel interpolation strategy for estimating subsample speckle motion.

Multidimensional, high-resolution ultrasonic imaging of rapidly moving tissue is primarily limited by sparse sampling in the lateral dimension. In order to achieve acceptable spatial resolution and velocity quantization, interpolation of laterally sampled data is necessary. We present a novel method for estimating lateral subsample speckle motion and compare it with traditional interpolation methods. This method, called grid slopes, requires no a priori knowledge and can be applied to data with as few as two samples in the lateral dimension. Computer simulations were performed to compare grid slopes with two conventional interpolation schemes, parabolic fit and cubic spline. Results of computer simulations show that parabolic fit and cubic spline performed poorly at translations greater than 0.5 samples, and translations less than 0.5 samples were subject to an estimation bias. Grid slopes accurately estimated translations between 0 and 1 samples without estimation bias at high signal-to-noise ratios. Given that the grid slopes interpolation technique performs well at high signal-to-noise ratios, one pertinent clinical application might be tissue motion tracking.

Ensemble tracking for 2D vector velocity measurement: Experimental and initial clinical results.

We describe a new method, called ensemble tracking, for estimating two-dimensional velocities with ultrasound. Compared to previous speckle tracking techniques, ensemble tracking measures motion over smaller times and distances, increasing maximum velocities and reducing errors due to echo decorrelation. Ensemble tracking uses parallel receive processing, 2D pattern matching, and interpolation of the resulting tracking grid to estimate sub-pixel speckle translations between successive ultrasonic acquisitions. In this study, small translations of a tissue mimicking phantom were quantified at transducer angles of 0 degrees , 45 degrees , and 90 degrees . Measurements over three parallel beam spacings and all transducer angles had mean errors from -4% to +11%, when parallel beam amplitudes were normalized. Such amplitude normalization substantially improved results at 45 degrees and 90 degrees . The amplitude, spacing, and correlation between the parallel beams were quantified, and their effects on the accuracy and precision of estimates are discussed. Finally, initial clinical results demonstrate the ability to track and display blood flow in the carotid artery.

Dexamethasone and bacterial meningitis. A meta-analysis of randomized controlled trials.

A meta-analysis of 5 randomized, controlled trials using dexamethasone as adjunctive therapy in the treatment of bacterial meningitis in children was done to assess the efficacy in reducing sequelae. A 6th study including both children and adults was analyzed separately. Results of the 5 pediatric studies indicated no significant difference in case-fatality rate between the placebo and dexamethasone groups. Significantly more neurologic sequelae were found in the placebo group during the period from discharge from hospital to 6 weeks after discharge (relative risk [RR] = 1.99, 95% confidence interval [CI] 1.13 to 3.53) and during the period beginning 6 months after discharge (RR = 3.90, 95% CI 1.72 to 8.85). The incidence of neurologic sequelae from 6 weeks to 6 months after discharge, though less with dexamethasone administration, did not reach statistical significance. The frequency of bilateral hearing loss was significantly greater in the placebo group (RR = 4.12, 95% CI 1.74 to 9.79), but unilateral loss was not statistically different in the two groups. Dexamethasone administration in addition to antimicrobial therapy appears to be effective in reducing neurologic sequelae and bilateral hearing loss associated with bacterial meningitis in children.