Department of Veterinary Clinical Sciences, Purdue University, West Lafayette, IN, USA
New Sonography Technology in Vascular Imaging
Doppler effect is a change in frequency (and wavelength) caused by the motion of a sound source, reflector and receiver. If a reflector is moving toward the source and the receiver, the received echo of the reflector has a higher frequency than would occur without the motion.
On the other hand, if the motion is away from the source and the receiver, the received echo of the reflector has a lower frequency. The amount of the frequency change depends on the speed motion of the reflector, the angle between the wave propagation, direction and the motion direction, and the frequency of the wave emitted by the source. The change cause by the motion is called the Doppler-shift frequency or Doppler shift. The Doppler shift depends on the speed of the reflector and the frequency of the source. It is the difference between the frequency of the source and the returning frequency recorded by the receiver.
In medical ultrasonography, the reflector is the cells in the blood stream, and the source and the receiver is the transducer used in the examination. The Doppler effect is a result of the motion of blood. A maximum positive Doppler shift occurs if the direction of the sound propagation is exactly opposite the flow direction. A maximum negative Doppler shift occurs if the direction of the sound propagation and the flow direction is similar. If the flow is perpendicular to direction of the sound propagation, the Doppler shift, no frequency change and thus no Doppler shift is detected.
There are three forms of displaying Doppler: colour Doppler, power Doppler and spectra Doppler (tissue Doppler). The continuous wave (CW) and pulsed wave (PW) PW are display in a form of spectra Doppler.
Note that the blue colour coded blood vessels represent blood vessels that are flowing away from the transducer (at the top of the image). In the top half of the kidney, they represent renal vein, where the blood flow from the peripheral to the center of the kidney. The red colour coded blood vessels at the top half of the image represent renal arteries that are flowing from the hilus to the peripheral margin of the kidney.
In colour Doppler, we sometimes encounter colour flow aliasing, which will lead to reversal of the colour map showing mixing of blues and reds, which is termed mosaic pattern. Aliasing happens due to high velocity flow with and velocity scale setting error. To optimize the colour Doppler, a lower transduce frequency, and a smaller color sector width should be utilize. Most of the currently ultrasound machine will be able to display a normal black and white image with colour Doppler at the same time.
Power Doppler displays the integrated power of the Doppler signal by a single color map instead of the mean frequency shift. There is no aliasing with power Doppler. Flow direction and velocity information normally is not known. This display is more sensitive then colour Doppler to small vessels and those with slow flow. In some advance ultrasound machines, directional power Doppler, which is sensitive to slow flow but at the same time showing the direction of the flow is available.
In recent ultrasonographic technology advances, multiple companies has produced Doppler that is very sensitive to slow flow. Most of them are available in the premium line of products. All of them has specific name by the company: for example Superb Micro-Vascular Imaging (SMI) of Toshiba, B-Flow of GE, S-Flow™ of Samsung, MicroCPA of Philips and Fine Flow of Hitachi Aloka. One of the advantages of this technology is be able to detect the actual margins of the blood vessels.
The potential application of the advance technology in veterinary medicine is still unknown. The extreme sensitivity of this for detection of blood flow may be useful in the determination of change of blood flow in the treatment of neoplasia, thus evaluating the effective of treatment.