Transcranial Doppler Ultrasonography
The TCD measures the velocity of blood flow through the major intracranial vessels by emitting a high frequency (>2MHz) wave from an ultrasound probe and detecting a frequency shift between the incident and reflected wave which directly correlates with the speed of the blood (the so called Doppler effect). The measurement is taken over the regions of the skull with thinner walls (temporal region, back of the head, or through the eye) as the bones strongly attenuate the transmission of the ultrasound at these frequencies. TCD is primarily a technique for diagnosing various intracranial vascular disorders such as emboli, stenosis, or vasospasm, and can be used to identify patients who are at risk of developing cerebral ischemia in early phases of traumatic brain injury or stroke.
ICP can be estimated from the TCD measurements because it impedes the blood flow and consequently decreases the velocity of blood flow. Besides the mean velocity, pulsatility index (which is the difference between peak systolic and end diastolic velocity, divided by mean flow velocity), fraction of the cycle in systole and slopes of the TCD waveforms have been correlated with ICP. The estimates are however insufficiently accurate with the margin of error of ±10 - 15 mmHg 1 .
Physiosonics, Inc. used transcranial Doppler ultrasound to measure ICP indirectly by assessing the elasticity of the biological material in a defined part of the brain. However the elasticity in the brain is highly dependent many other factors apart from ICP, including arterial blood pressure, state of cerebral blood flow autoregulation, and the level of edema. Therefore this approach would require calibration and expert positioning.
1 Schmidt B, Czosnyka M, Raabe A, et al. Adaptive noninvasive assessment of intracranial pressure and cerebral autoregulation. Stroke 2003; 34:84-89.