Doppler ultrasound and the neonatal cerebral circulation: methodology and pitfalls

Doppler ultrasound is a popular technique for investigating the haemodynamics of the neonatal cerebral circulation. Different types of instrumentation are available of which a duplex scanner is a suitable choice for spot measurements, whilst a simple continuous wave directional Doppler may be better for long-term monitoring purposes. Doppler signals may be obtained from many arterial and venous sites within the neonatal brain, but most measurements are made from the anterior or middle cerebral arteries. It is usual to process the Doppler signal to obtain an envelope waveform which may represent either mean velocity or maximum velocity, although the latter has some distinct advantages. The zero-crossing detector used for early studies is unsuitable for use with Doppler signals. Changes in the shape of the velocity waveform may be quantified using different methods, of which the so-called resistance index is the most popular. It is also possible to make absolute velocity measurements, although these cannot be converted to flow because it is not possible to measure the diameter of neonatal cerebral vessels. There is considerable controversy as to whether the diameter of the vessels at the sites of ultrasound insonation can be affected by factors such as blood pressure and arterial gases, but it is unlikely that any such changes could invalidate the basic findings of Doppler studies. With care and attention to detail, Doppler ultrasound provides a valuable window on the neonatal cerebral circulation.