Measurement of transcranial Doppler insonation angles from three-dimensional reconstructions of CT angiography scans

Blood velocities measured by Transcranial Doppler (TCD) are dependent on the angle between the incident ultrasound beam and the direction of blood flow (known as the Doppler angle). However, when TCD examinations are performed without imaging the Doppler angle for each vessel segment is not known. We have measured Doppler angles in the basal cerebral arteries examined with TCD using three-dimensional (3D) vessel models generated from computed tomography angiography (CTA) scans. This approach produces angle statistics that are not accessible during non-imaging TCD studies. We created 3D models of the basal cerebral arteries for 24 vasospasm patients. Standard acoustic windows were mapped to the specific anatomy of each patient. Virtual ultrasound transmit beams were generated that originated from the acoustic window and intersected the centerline of each arterial segment. Doppler angle measurements were calculated and compiled for each vessel segment. Doppler angles were smallest for the middle cerebral artery M1 segment (median 24.6 degrees) and ophthalmic artery (median 25.0 degrees), and largest for the anterior cerebral artery A2 segment (median 76.4 degrees) and posterior cerebral artery P2 segment (median 75.8 degrees). The ophthalmic artery had the highest proportion of Doppler angles that were less than 60 degrees (99%) while the anterior cerebral artery A2 segment had the lowest proportion of Doppler angles that were less than 60 degrees (10%). These angle measurements indicate the expected deviation between measured and true velocities in the cerebral arteries, highlighting specific segments that may be prone to underestimation of velocity.