RVT Transcranial Doppler (TCD) Imaging — Questions and Answers
Question 1: Which acoustic window is used most commonly to insonate the middle cerebral artery (MCA) during a transcranial Doppler examination?
- Suboccipital (transforaminal) window
- Orbital window
- Transtemporal window (Correct answer)
- Submandibular window
Correct answer: Transtemporal window
The transtemporal window, located just anterior to the ear above the zygomatic arch, provides the thinnest area of temporal bone and is the standard approach for insonating the MCA, ACA, PCA, and terminal ICA. The other windows access different vessels: suboccipital for vertebrobasilar, orbital for the carotid siphon, and submandibular for the cervical ICA.
Question 2: A patient with subarachnoid hemorrhage has TCD MCA mean flow velocities of 190 cm/s bilaterally on day 7 post-bleed. What does this MOST likely indicate?
- A normal post-hemorrhage hyperemic response
- Cerebral vasospasm requiring clinical attention (Correct answer)
- Bilateral MCA occlusion
- Elevated intracranial pressure with preserved autoregulation
Correct answer: Cerebral vasospasm requiring clinical attention
TCD mean velocities >120 cm/s in the MCA after SAH are consistent with vasospasm, and velocities >200 cm/s indicate severe spasm. Peak vasospasm typically occurs days 5–14 post-SAH. Early TCD detection allows intervention (hemodynamic augmentation, nimodipine, angioplasty) before clinical deficits appear.
Question 3: The Lindegaard ratio is used in TCD to distinguish cerebral vasospasm from hyperemia after subarachnoid hemorrhage. Which vessels are compared to calculate this ratio?
- Anterior cerebral artery to posterior cerebral artery
- Middle cerebral artery to ipsilateral extracranial internal carotid artery (Correct answer)
- Basilar artery to ipsilateral vertebral artery
- MCA to ipsilateral anterior cerebral artery
Correct answer: Middle cerebral artery to ipsilateral extracranial internal carotid artery
The Lindegaard ratio = MCA mean velocity ÷ ipsilateral extracranial ICA mean velocity. In vasospasm the intracranial vessels narrow while the extracranial ICA is unaffected, yielding a ratio >3 (severe spasm >6). In hyperemia, both MCA and ICA velocities rise proportionally, keeping the ratio <3.
Question 4: Which TCD waveform pattern is MOST consistent with severely elevated intracranial pressure?
- High-velocity, low-resistance waveform with increased diastolic flow
- Normal pulsatility index with symmetric velocities
- Reversal of diastolic flow, producing a bidirectional or 'to-and-fro' pattern (Correct answer)
- Mildly increased pulsatility index with preserved systolic peaks
Correct answer: Reversal of diastolic flow, producing a bidirectional or 'to-and-fro' pattern
When ICP approaches or exceeds diastolic blood pressure, diastolic flow ceases and ultimately reverses, creating a bidirectional 'to-and-fro' waveform. This pattern (along with systolic spikes and eventual absence of flow) indicates cerebral circulatory arrest and is one of the TCD criteria used to confirm brain death in some protocols.
Question 5: During TCD microemboli monitoring, which characteristic helps differentiate a gaseous embolus from a particulate embolus?
- Gaseous emboli cause unidirectional signals; particulate emboli cause bidirectional signals
- Gaseous emboli produce high-intensity (typically >25 dB above background), bidirectional signals; particulate emboli are lower intensity and typically unidirectional (Correct answer)
- Particulate emboli generate a chirping audible tone; gaseous emboli are silent on audio
- Gaseous and particulate emboli cannot be reliably differentiated with standard TCD
Correct answer: Gaseous emboli produce high-intensity (typically >25 dB above background), bidirectional signals; particulate emboli are lower intensity and typically unidirectional
High-intensity transient signals (HITS) on TCD mark embolic events. Gaseous emboli (e.g., from cardiac bypass) reflect ultrasound very strongly (>25 dB), are bidirectional, and produce a distinctive 'chirp.' Particulate emboli (e.g., cholesterol, platelet aggregates) are typically lower intensity and unidirectional. This distinction has clinical implications for identifying cardiac vs. atherosclerotic embolic sources.
Question 6: The suboccipital (transforaminal) acoustic window in TCD is used primarily to evaluate which intracranial vessels?
- Middle cerebral artery and anterior cerebral artery
- Carotid siphon (intracranial ICA) and ophthalmic artery
- Basilar artery and intracranial vertebral arteries (Correct answer)
- Posterior communicating artery and anterior choroidal artery
Correct answer: Basilar artery and intracranial vertebral arteries
The probe is placed at the base of the skull with the patient's neck flexed, directing the beam through the foramen magnum. This provides access to the distal vertebral arteries and the basilar artery. These vessels are inaccessible from the transtemporal or orbital windows, making the suboccipital approach essential for posterior circulation assessment.
RVT Transcranial Doppler (TCD) Imaging
Which acoustic window is used most commonly to insonate the middle cerebral artery (MCA) during a transcranial Doppler examination?