In transcranial Doppler, the angle of insonification is assumed to be what?

• A. 15 degrees
• B. 0 degrees
• C. 45 degrees
• D. 90 degrees

Answer: (B)

In Doppler ultrasound, the measured shift depends on the angle between the beam and the direction of blood flow. For transcranial Doppler, the beam is aimed so the flow is essentially parallel to the beam, so the angle is assumed to be zero degrees. This makes the velocity calculation straightforward because cos(0) equals 1, meaning the estimated velocity is effectively the true velocity without needing angular correction. Since obtaining an exact skull-window angle is difficult, using 0 degrees provides a practical, repeatable estimate. If the angle were larger, you’d have to divide by cos theta, which adds potential error if theta isn’t known; and at angles near 90 degrees, the Doppler signal would be minimal.