Modality of the Millennium
Dr. Ashish Atre
Color Doppler Imaging has added a new to dimension to sonography. It is an established non–invasive technique, predominantly to study blood flow. Color flow imaging is being used to determine organ expansion and functioning. The technique is based on the Doppler principle, first described by Christian Doppler in 1842. The effect is a change in the perceived frequency of sound emitted by a moving source. A working example of this change is the pitch of a train whistle as it moves past a stationary observer. Though the train is whistling at the same pitch the stationary person perceives an increasing and decreasing pitch as the train moves towards and away from him respectively.

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.

Newer Techniques
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.