Ultrasound screening for internal carotid disease-I. The temporal artery occlusion test--which periorbital artery?

Measurement of arterial blood flow by Doppler ultrasound

A review is given of quantitative techniques and clinical applications of arterial Doppler ultrasound. The currently available Doppler equipment of stand-alone continuous wave and pulsed wave units, duplex systems and colour flow systems is briefly described. Doppler ultrasound can be divided into procedures concerned with waveform analysis, volume flow measurement and more recently colour flow imaging. Arterial Doppler waveform analysis is considered for a number of areas including carotid, lower limb, renal and renal transplant, obstetrics, adult cerebral, neonatal cerebral, and tumour studies. Using a duplex scanner volume flow in arteries can be measured from estimates of vessel cross sectional area, mean Doppler frequency and beam-vessel angle. The errors associated with each of these measurements is discussed, and reports of experimentally determined in vivo accuracy of volume flow measurements made using this technique are considered. Other volume flow measurement techniques including the promising attenuation compensation method are also explored.

Haemodynamic evaluation of carotid artery disease

Cerebral ischaemia in the region of an internal carotid artery (ICA) stenosis may be caused by embolism or cerebral hypoperfusion. A severe ICA stenosis may be well compensated by collateral blood supply, however, in some patients the capacity of the collateral blood supply is insufficient. Studies evaluating therapeutic modalities or natural history of carotid artery disease should therefore include a test capable of assessing cerebral haemodynamics. However, most studies, invasive as well as non-invasive, have focused on the ability of the test to diagnose the ICA lesions itself, rather than the haemodynamic changes induced by the stenosis. This paper reviews non-invasive methods for haemodynamic evaluation of carotid artery disease. Haemodynamic evaluation of ICA stenoses may be performed accurately by different techniques. Analysis of Doppler waveforms obtained distal to the ICA lesion and CBF reactivity tests may identify patients with severe reduction in ICA perfusion pressure. Periorbital Doppler examination and OPG identifies ICA lesions causing minor pressure gradients, however, they are unable to discriminate between minor and moderate to severe pressure reduction. Transcranial Doppler examination may prove to be useful in haemodynamic assessment of ICA stenoses, but, this remains to be evaluated. Presently, in addition to direct evaluation of the carotid arteries, we use a combination of periorbital Doppler examination and analysis of distal ICA waveforms. In cases of an ICA stenosis and orthograde flow any severe pressure reduction may be ruled out. In cases of inverted flow, analysis of distal ICA waveforms may identify patients with severe reduction in ICA perfusion pressure.

Transcranial measurement of blood velocities in the basal cerebral arteries using pulsed Doppler ultrasound: a method of assessing the Circle of Willis

Transcranial pulsed Doppler ultrasound and spectral analysis were used for detection of blood velocities in the basal cerebral arteries. The Doppler transducer was placed superior to the zygomatic arch and during insonation of the middle cerebral artery care was taken to obtain maximum Doppler-shift frequency signals since this allowed a small angle between the ultrasound beam and this artery. Doppler signals were obtained from the middle, anterior, and posterior cerebral arteries in 20 volunteers with the average depth of the Doppler gate at 4.9 (4.6-5.2 cm), 5.2 (4.9-5.4 cm), and 6.3 cm (6.0-6.9 cm), respectively. These measurements were in agreement with those obtained for 15 cadaver studies, in whom the distance from the proposed site of the Doppler transducer to each basal cerebral artery was measured as 4.7 +/- 0.6, 5.3 +/- 0.5, and 5.9 +/- 0.9 cm, respectively. The reproducibility of middle cerebral artery blood velocity values was tested in seven subjects and showed a variation of not more than 8% in any individual. The method was used in combination with common carotid compression to assess four patients who had occlusive extracranial carotid disease; in three the disease was more severe on one side and reversal of blood flow in the proximal ipsilateral anterior cerebral artery was demonstrated, consistent with cross flow from the contralateral side via the anterior communicating artery of the Circle of Willis. In the fourth patient augmentation of posterior cerebral artery blood velocities during common carotid compression indicated the major collateral source was from the vertebrobasilar system.