The present invention relates to a method and system for graphic representation of dynamic information from a dynamic 2D or 3D object. In particular, the invention relates to a novel method of interpolating dynamic information having values of different signs.
The method of the present invention is particularly useful for the graphic representation of dynamic information, in particular flow data which has been acquired with a colour-Doppler-method. The method of the invention can be used for representing the flow of liquids, such as blood and is therefore useful for human and animal diagnostics.
Although, in principal, any dynamic information can be represented, dynamic information in the sense of the present invention means in particular flow data, such as colour flow data as gained from Doppler sonography measurements. Sonography can be enhanced with Doppler measurements which employ the Doppler effect to assess whether structures, such as blood, are moving towards or away from the probe, and its relative velocity. By calculating the frequency shift of a particular sample volume, for example a jet of blood flow over a heart valve, its speed and direction can be determined and visualised. This is particularly useful in cardiovascular studies and the study of other organs, such as the liver etc.
Doppler ultrasound sonography can be used in continuous wave systems and pulsed wave systems. However, continuous wave Doppler ultrasound is unable to determine the specific location of velocities within the beam and therefore, cannot be used to produce colour flow images. On the other hand, pulsed wave ultrasound methods allow the measurement of the depth or range of the flow site. For colour flow images, pulsed wave ultrasound is used to generate the data for Doppler sonograms.
For display, the dynamic information is conveniently mapped to different colours. The hue of a colour represents the direction and the brightness or shade represents the magnitude of velocity. Usually, two different or contrasting colours, such as red and blue, are used to represent the two opposing flow directions (positive or negative). In general, the colour red is used for flow towards the probe and blue is used for flow away from the probe. When an interpolation operation is performed between two values which have different signs, such an interpolation of colours may produce colours outside the velocity colour map, resulting in misleading colour flow information and their corresponding graphics, such as black or white border lines.
In order to visualize colour flow data represented as sampled volume data, special care has to be taken to reconstruct so called “aliasing effects”, which occur on borders between high negative-high positive flow regions. Normal tri-linear interpolation methods are not suitable for such border regions because they do not emphasize them enough. When pulses are transmitted at a given sampling frequency or pulse repetition frequency, the maximum Doppler frequency that can be measured unambiguously is half the pulse repetition frequency. If the blood velocity and beam/flow angle being measured combine to give a Doppler frequency value greater than half of the pulse repetition frequency, ambiguity in the Doppler signal occurs. This ambiguity is known as aliasing. In the prior art, attempts have therefore been made to increase the pulse repetition frequency in order to avoid aliasing. However, the problem with increasing the pulse repetition frequency is that low velocities may not be identified. Therefore, other ways of avoiding aliasing affects is to use different methods of interpolation.
For this reason, the prior art proposes the use of minimum arc interpolation. Such an interpolation method and system is described in US 2006/0094963 A1 by Sumanaweera et al. This document teaches exact modelling of the minimum arc interpolation which results in a rather complex sign computation carried out for interpolation. Such computations are very hard to implement efficiently on graphics hardware.
Another approach disclosed in US 2006/0094963 A1 splits the velocity into a real/complex number representation which is interpolated separately and is later combined using a pre-computed lookup-table (LUT). This sampled lookup-table can lead to numerical artefacts due to limited precision and interpolation errors when sampling the LUT.
Especially for trigonometric functions (arctan 2), normal byte-values do not provide enough precision and will produce artefacts. Moving to higher precision is only possible on recent GPUs (graphics processing unit) and will result in a severe performance drop.
Another disadvantage of the prior art method is the use of the minimum arc interpolation itself. A small change in input values can lead to a drastic change in velocity. This is the case when the interpolation direction along the arc suddenly changes its direction. Using composition based volume-rendering (where a number of slices through the volume is combined back-to-front or front-to-back), these sudden changes in direction can lead to noticeable artefacts (“grizzling-effect”).