Q: Bones of the skull block the transmission of ultrasound, than how transcranial doppler is performed?

Answer: Transcranial doppler uses insonation windows. Insonation windows are regions in skull with thinner walls. 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. Various factors affect bone thickness, and may make some examinations difficult or even impossible.