Transcranial Doppler is a delightfully simple procedure. The capital cost of the equipment is relatively low and the running costs are limited to a small amount of gel and a sheet of paper to print the results on. There is, however, a major drawback to successfully utilizing transcranial Doppler and that is that is it the most operator dependent technique in diagnostic ultrasound. It is certainly easy to set up the TCD machine and. It is also easy to apply the probe to the patient's head. However, the success rate of insonating a specific vessel is quite variable. It may be difficult, especially in elderly African American females who have greater calcification, to obtain a good velocity waveform. In approximately 30% of this population, even the experienced sonographer will not be able to get an acceptable signal.

Here are the three golden rules to be considered even before you start your examination
1. Make yourself comfortable
2. You cannot use too much gel
3. Remember, small adjustments

A good technique is to sit at the head of the patient so that you can rest your wrist on the pillow. This will prevent your arm from becoming tired; it will also tend to stop the probe from slipping downwards in the gel. With the remote control either in the other hand or on the pillow you can control the system and the examination without strain on your back or arm.
It is also easy to apply insufficient gel. When you are searching for that elusive temporal window the gel will inevitably be transferred from the probe to the skin. A good tip is to reapply gel each time you move the position of the probe.
Once you have found the window you will follow the MCA to the bifurcation by adjusting the angle of the probe. At a scanning depth of say 70 mm a very small change in angle will be a large change in target area. Look at your thumbnail. That is about the size of the circle of Willis, and you literally have to find that in the dark.
The first step in TCD examination is to find a temporal window where the ultrasonic beam can penetrate. The next step is to identify exactly where you are the different segments of the arterial network. Most operators will start at a depth of 50 cms and angle the probe at 90 degrees to the skull attempting to find the Mid Cerebral Artery.
Using the temporal approach one clearly has to insonate through bone. In most adults it is usually possible to obtain good signals from a relatively large area, but in some elderly patients it may be difficult to obtain signals through a very small window. The temporal windows are found above the zygomatic arch. There are four sites of interest (a) the frontal, (b) the anterior, (c) the middle and (d) the posterior window. The middle temporal window facilitates an approximately direct medial insonation, whereas the probe has to be aimed obliquely in a slightly posterior direction from the anterior temporal window and it has to be aimed interiorly to reach the arteries in the circle of Willis.
There are three main sources of information for artery identification: (a) the spatial resolution of the signal to other intracranial signals (including information of depth and angle of the probe), (b) the direction of flow (towards or away from the transducer) and the spatial distribution and, (c) the response of the signal to compression or vibration maneuvers . By using proper examination techniques, it is generally possible to achieve a relatively high degree of accuracy in artery identification even without compression maneuvers. The main 'landmark' for orientation is the branching of the supraclinoid ICA into the ACA and the MCA. The MCA runs laterally and slightly anteriorly as a continuation of the intracranial ICA. It has the highest volume flow of the branches from the circle of Willis, carrying about 80% of the flow to the hemisphere. An anterior-temporal window allows almost zero degree insonation, whereas from a posterior window a somewhat blunter angle may be expected. The MCA (together with its branches) is normally the only artery seen between depth of 50 and 25 mm from the temporal window. The criteria for MCA identification are the Doppler signal can be followed laterally with only slight probe movements from the termination of the ICA up to about 30 mm and the flow is toward the probe.Having successfully found the MCA, ACA and PCA the procedure now has to be repeated on the contra-lateral side.

Transcranial Doppler can rapidly and non-invasively measure blood flow in the major basal intracranial arteries. Its accuracy makes it acceptable for use in screening for haemodynamically significant intracranial stenoses or vessel occlusions. Although it has a relatively limited field of view and is not technically feasible in some cases, the information obtained is becoming increasingly relevant to therapeutic decision-making in the prevention and management of stroke. Transcranial Doppler ultrasound has the advantages of relatively low cost, ease of repeatability, and excellent safety and tolerability

A typical approach is lie the patient on their side and lower the chin onto the chest. With the probe set to a depth of 85 mm it is placed on the midline about 1 inch below the occipital bone. The probe is then aimed at the bridge of the patients nose. Vessel identification is by flow below the line i.e. away from the probe and a mean flow velocity of approximately 30 cms/s.

Recording these seven arteries concludes the basic TCD examination.