Range/velocity limitations for time-domain blood velocity estimation

The traditional range/velocity limitation for blood velocity estimation systems using ultrasound is elucidated. It is stated that the equation is a property of the estimator used, not the actual physical measurement situation, as higher velocities can be estimated by the time domain cross-correlation approach. It is demonstrated that the time domain technique under certain measurement conditions will yield unsatisfactory results, when trying to estimate high velocities. Various methods to avoid these artifacts using temporal and spatial clustering techniques are suggested. The improvement in probability of correct detection is derived, and several examples of simulations are shown

Simulating arbitrary-geometry ultrasound transducers using triangles

Calculation of ultrasound fields from medical transducers is often done by applying linear acoustics and using the Tupholme-Stepanishen method of calculation. Here the spatial impulse response is found and, together with the basic one-dimensional pulse, it is used to find both the emitted and pulse-echo field. The spatial impulse response has only been determined analytically for a few geometries and using apodization over the transducer surface generally makes it impossible to find the response analytically. A popular approach to find the general field is thus to split the aperture into small rectangles, and then sum the weighted response from each of these. The problem with rectangles is their poor fit to apertures which do not have straight edges, such as circular and oval shapes. The simulation thus introduces artifacts in the response, that necessitates the use of a large number of rectangles for a precise simulation. A triangle better fits these aperture shapes, and the field fr..

A new approach to calculating spatial impulse responses

Using linear acoustics the emitted and scattered ultrasound field can be found by using spatial impulse responses as developed by Tupholme and Stepanishen. The impulse response is calculated by the Rayleigh integral by summing the spherical waves emitted from all of the aperture surface. The evaluation of the integral is cumbersome and quite involved for different aperture geometries. This paper re-investigates the problem and shows that the field can be found from the crossings between the boundary of the aperture and a spherical wave emitted from the field point onto the plane of the emitting aperture. Summing the angles of the arcs within the aperture readily yields the spatial impulse response for a point in space. The approach makes is possible to make very general calculation routines for arbitrary, flat apertures in which the outline of the aperture is either analytically or numerically defined. The exact field can then be found without evaluating any integrals by merely finding..