Velocity measurements are also dependent upon the Doppler angle (theta). Based on this, the computer can determine the direction relative to the probe and the velocity of the moving red blood cell. The ultrasound probe emits a soundwave with a known frequency as this travels into the tissue and hits a moving red blood cell, the frequency changes as it reflects back to the probe. It describes how a sound wave frequency changes when it hits a moving object in this case, a red blood cell is contained within a vessel. To help with this pitfall, the Lindegaard ratio can be used to determine true vasospasm (discussed later).ĭoppler ultrasonography is based on a physics principle called the Doppler effect, first described in the mid-1800s by Christian Doppler. One potential pitfall in TCD examination is elevated velocity secondary to hyperdynamic states, such as in classic triple H therapy. This focal elevation in the velocity is the primary abnormality of interest in the interpretation of transcranial Doppler exams. This fact is irrespective of the cause of the narrowing, be it from vasospasm, atherosclerotic disease, or other causes. This is based on Bernoulli's principle and is because the same volume of blood is trying to cross this narrowed vessel. When there is a decrease in the diameter of a segment of a vessel (as in vasospasm), there is an associated increase in velocity and decrease in pressure. Delayed cerebral ischemia is important to identify and treat, contributing to the high morbidity and mortality of subarachnoid hemorrhage patients. However, this is no longer the mainstay treatment for vasospasm, with multiple newer treatments, including hypertension in a euvolemic patient, among others. Traditionally, vasospasm has been treated with "Triple H" therapy which included hypertension, hypervolemia, and hemodilution. Detection of vasospasm is important in a subarachnoid hemorrhage patient because of its association with delayed cerebral ischemia. After a subarachnoid hemorrhage, the incidence of vasospasm usually occurs between days 4 and 14. While the cause of vasospasm is not definitely identified, one potential cause is decreased production and decreased response of the artery to the vasodilator, nitric oxide. The final carotid segments are C6 (Ophthalmic) and C7 (Communicating). Still, it roughly correlates with the C2 (Petrous), C3 (Lacerum), C4 (Cavernous), and C5 (Clinoid) segments based on the modern classification system as the artery makes multiple bends through the petrous bone. This terminology is based on angiographic studies by Moniz in 1927. Often, the intracranial portion of the ICA is referred to as the carotid siphon. The C1 segment is considered the cervical segment, extending from the ICA origin to the carotid canal. This travels superiorly to the carotid canal of the petrous bone and bifurcates intracranially as the MCA and ACA vessels. The ICA originates at the common carotid artery's bifurcation as it splits into the external carotid and internal carotid arteries. While there is no universally accepted naming system of the carotid segments, the one by Bouthillier is the most commonly used. The internal carotid artery can be broken down into multiple segments, typically C1-C7 based on a classification system by Bouthillier et al. Segmental hypoplasia or aplasia is common, with a complete circle of Willis observed in less than 20% of the population. There are numerous variations to this classic anatomy, many of which cause an incomplete circle. The circle is completed via connections of the distal ICAs to the PCAs via the posterior communicating arteries, and the bilateral ACAs are connected via the anterior communicating artery. The basilar artery then bifurcates into the bilateral posterior cerebral arteries. The basilar artery gives off distinct branches, including the anterior inferior cerebellar artery (AICA), pontine perforating arteries, and the superior cerebellar arteries. The posterior circulation is supplied by the vertebral arteries, which join to form the basilar artery. The anterior circulation is largely provided by the ICAs, which branch intracranially into the middle cerebral and anterior cerebral arteries. The Circle of Willis is an anastomotic ring of vessels that supply the brain. These extracranial vessels supply the Circle of Willis, named after Thomas Willis, an English physician. The intracranial arteries are supplied by the vertebral and the bilateral internal carotid arteries.
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