For transcranial Doppler imaging requiring deep penetration through the skull, which frequency range is typically used and why?

• A. Higher frequencies around 5–7 MHz to maximize resolution
• B. Lower frequencies around 2–3 MHz to maximize skull penetration
• C. Mid frequencies around 3–4 MHz to balance penetration and resolution
• D. Frequency range is not important

Answer: (B)

Lower-frequency ultrasound penetrates the skull much better than higher-frequency waves. In transcranial Doppler, the goal is to reach intracranial vessels, not to form high-resolution images, so using about 2 MHz provides enough skull penetration while still allowing reliable Doppler velocity measurements. Higher frequencies (like 5–7 MHz) would give better resolution but are mostly blocked by the skull, resulting in weak or unusable signals through the bone. The frequency is important because bone attenuation rises with frequency, so the typical range around 2 MHz is chosen to maximize penetration into the brain.