Dr. Ashish Atre
The amount of Doppler shift can be calculated by the Doppler equation. The clinical implication of this is that if we know the frequency shift, we can calculate the velocity of blood flow. Continuous wave Doppler uses two Doppler crystals – one acting as a transmitter and the other as a receiver. Because of the continuous mode of transmitting and receiving, information about blood flow is obtained all along the ultrasound beam without any indication of the depth from which this information comes. Pulsed wave Doppler emits multiple small pulses of ultrasound beams which reach the sampling point and return, giving information of blood flow only in the area of interest.
- Detection of flow outside the area where it should be helps to identify aneurysms and tumor vascularity.
- Change in the spectral waveform also determines the exact extent of vessel narrowing.
Power Doppler Imaging
This is a display technique for showing the amplitude or strength of moving targets. Provision of perfusion–like display of blood flow makes it easy to determine the presence or absence of flow in tissue of interest – for example, testicular torsion. 3D colour power angiography offers a rotary three dimensional view of vasculature. Another technique of color velocity imaging measures velocity directly without the Doppler shift principle. Transcranial Doppler is also becoming popular.
With upgrades in technology and increasing operator experience, pitfalls are negligible. Doppler has become almost a definitive test for evaluation of patients with peripheral arterial disease. Detection of venous thrombosis is one field where Doppler has reached near perfection and avoids painful investigation of venography. The basic advantage of Doppler imaging has been the cost–effectiveness. The amount of information that it delivers at low cost makes it likely to continue at the current status that it enjoys – Modality of the Millennium.