Transcranial Doppler (TCD) and transcranial color Doppler (TCCD) are types of Doppler ultrasonography that measure the velocity of blood flow through the brain's blood vessels by measuring the echoes of ultrasound waves moving transcranially (through the cranium). These modes of medical imaging conduct a spectral analysis of the acoustic signals they receive and can therefore be classified as methods of active acoustocerebrography. They are used as tests to help diagnose emboli, stenosis, vasospasm from a subarachnoid hemorrhage (bleeding from a ruptured aneurysm), and other problems. These relatively quick and inexpensive tests are growing in popularity. The tests are effective for detecting sickle cell disease, ischemic cerebrovascular disease, subarachnoid hemorrhage, arteriovenous malformations, and cerebral circulatory arrest. The tests are possibly useful for perioperative monitoring and meningeal infection. The equipment used for these tests is becoming increasingly portable, making it possible for a clinician to travel to a hospital, to a doctor's office, or to a nursing home for both inpatient and outpatient studies. The tests are often used in conjunction with other tests such as MRI, MRA, carotid duplex ultrasound and CT scans. The tests are also used for research in cognitive neuroscience (see Functional transcranial Doppler, below).
Methods Two methods of recording may be used for this procedure. The first uses "B-mode" imaging, which displays a 2-dimensional image of the skull, brain, and blood vessels as seen by the ultrasound probe. Once the desired blood vessel is found, blood flow velocities may be measured with a pulsed Doppler effect probe, which graphs velocities over time. Together, these make a duplex test. The second method of recording uses only the second probe function, relying instead on the training and experience of the clinician in finding the correct vessels. Current TCD machines always allow both methods.
How it works The ultrasound probe emits a high-frequency sound wave (usually a multiple of 2 MHz) that bounces off various substances in the body. These echoes are detected by a sensor in the probe. In the case of blood in an artery, the echoes have different frequencies depending on the direction and speed of the blood because of the Doppler effect. If the blood is moving away from the probe, then the frequency of the echo is lower than the emitted frequency; if the blood is moving towards the probe, then the frequency of the echo is higher than the emitted frequency. The echoes are analysed and converted into velocities that are displayed on the unit's computer monitor. In fact, because the probe is pulsed at a rate of up to 10 kHz, the frequency information is discarded from each pulse and reconstructed from phase changes from one pulse to the next. Because the bones of the skull block most of the transmission of ultrasound, regions with thinner walls (called insonation windows), which offer the least distortion to the sound waves, must be used for analyzing. For this reason, recording is performed in the temporal region above the cheekbone/zygomatic arch, through the eyes, below the jaw, and from the back of the head. Patient age, sex, race, and other factors affect bone thickness and porosity, making some examinations more difficult or even impossible. Most can still be performed to obtain acceptable responses, sometimes requiring using alternative sites from which to view the vessels.
Implantable transcranial Doppler Sometimes a patient's history and clinical signs suggest a very high risk of stroke. Occlusive stroke causes permanent tissue damage over the following three hours (maybe even 4.5 hours), but not instantly. Various drugs (e.g. aspirin, streptokinase, and tissue plasminogen activator (TPA) in ascending order of effectiveness and cost) can reverse the stroke process. The problem is how to know immediately that a stroke is happening. One possible way is the use of an implantable transcranial Doppler device "operatively connected to a drug delivery system". Battery-powered, it would use an RF link to a portable computer running a spectral analysis routine together with input from an oximeter (monitoring the degree of blood oxygenation, which a stroke might impair) to make the automatic decision to administer the drug.
Functional transcranial Doppler (fTCD) Functional transcranial Doppler sonography (fTCD) is a neuroimaging tool for measuring cerebral blood flow velocity changes due to neural activation during cognitive tasks. Functional TCD uses pulse-wave Doppler technology to record blood flow velocities in the anterior, middle, and posterior cerebral arteries. Similar to other neuroimaging techniques such as functional magnetic resonance imaging (fMRI) and positron emission tomography (PET), fTCD is based on a close coupling between regional cerebral blood flow changes and neural activation. Due to a continuous monitoring of blood flow velocity, TCD offers better temporal resolution than fMRI and PET. The technique is noninvasive and easy to apply. Blood flow velocity measurements are robust against movement artifacts. Since its introduction the technique has contributed substantially to the elucidation of the hemispheric organization of cognitive, motor, and sensory functions in adults and children. fTCD has been used to study cerebral lateralization of major brain functions such as language, face processing, color processing, and intelligence. Moreover, most established neuroanatomical substrates for brain function are perfused by the major cerebral arteries that could be directly insonated. Lastly, fTCD has been used as a brain–computer interface modality.
Functional transcranial Doppler spectroscopy (fTCDS)
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