One of the strengths of ultrasound imaging is its ability to measure blood and tissue velocities with high precision and at a high frame rate. Information of blood velocities can in the diagnostic setting be used to identify abnormal blood flow related to pathology, such as the jet flow pattern resulting from a heart valve leakage. Further, information about tissue velocities can be used to quantify the function of the heart, through the identification of areas of the heart muscle with reduced contractibility.
Traditional velocity measurements with ultrasound are based on the Doppler principle, which states that sound emitted from a moving source or sound reflected from a moving target will lead to a shift in the frequency of the sound. This so-called Doppler shift can be measured directly from the received signal through a continuous wave ultrasound emission (CW-Doppler), or sampled through the emission of several ultrasound pulses (PW-Doppler).
Today, Doppler ultrasound measurement is an integral part of commercial scanner systems. Conventional blood flow imaging modalities include spectral Doppler, in which the complete spectrum of velocities within one specific region is displayed. Another modality estimates the mean velocity and direction of blood in many points in a distributed region, which is encoded as a parametric color image, displayed overlaid an image of the anatomy. This latter color flow imaging (CFI) modality has proven very useful for the detection of areas of abnormal blood flow, which can be investigated further using spectral Doppler techniques. In Figure 1, the operation of both CFI and spectral Doppler techniques are shown.
Pulsed Wave Doppler
ISB
Color Flow Imaging
ISB
Figure 1: PW Doppler and Color Flow Imaging are established methods for blod flow imaging.
The measurement of tissue velocities, tissue velocity imaging (TVI) is currently being established as a method for quantifying heart function, by indicating areas of reduced movement. The velocity measurements can further be used to estimate the strain rate in different areas of the heart, a direct measure of the contractibility. This latter method is currently receiving the attention of the clinical community. In Figure 2, an example of TVI is shown, where the tissue velocites are encoded in color and displayed in a transparent manner on top of the image of the anatomy.
Figure 2: TVI image
Although very useful, all Doppler-based methods have limitations which may lessen its value in the diagnostic setting. The methods are inherently limited to only measuring the axial velocity component (along the ultrasound beam direction), and as the Doppler signal is sampled at a given rate (PW-Doppler), the maximum velocity measurable is limited.
At the department of Circulation and Medical Imaging, we investigate new use of Doppler information, and alternative methods that do not suffer from the Doppler limitations, that might increase the diagnostic confidence in the future clinic. This includes methods able to better quantify the velocity of blood and tissue, and also more intuitive methods less prone to misinterpretations. Currently, three-dimensional (3-D) ultrasound Doppler imaging has become available in real-time on commercial scanner systems. The potential of this new technology is currently being investigated, and also further improved and developed.
