Neurosurgery notes/Radiology/Ultrasound scan

Ultrasound scan

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General

  • Intraoperative US offers real-time information about the location, size, vascular relationships and adjacent structures of brain and spinal cord lesions.
  • Allows
    • Adjustment for brain shift,
    • Detection of untoward intraoperative events such as hemorrhage or hydrocephalus,
    • Critical tumor margins
    • Residual tumor
  • Newer technologies
    • Contrast-enhanced US (i.e. microbubble contrast),
    • Integration of US with navigation systems, or with functional navigation systems as a method of assessing and adjusting for brain shift may add additional benefits.
    • Intraoperative doppler-angiography enabled the delineation of vascular anatomy in real-time.

Imaging appearance

  • Bright
    • Blood
    • Choroid
    • Bone

Doppler effect (shift)

  • Ultrasound waves emitted from the Doppler probe are transmitted through the skull and reflected by moving red blood cells within the intracerebral vessels.
  • The difference in the frequency between the emitted and reflected waves (Doppler shift frequency) is directly proportional to their speed.
  • Physiological variables affecting TCD measured mean blood velocity are age, gender, hematocrit, viscosity, carbon dioxide, temperature, blood pressure, and mental or motor activity.
  • Four main acoustic windows have been described
    • Transtemporal
    • Transorbital
    • Submandibular
    • Suboccipital
  • Although each window has unique advantages for different arteries and indications, a complete TCD examination should include measurements from all four windows and the course of blood flow at various depths within each major branch of the circle of Willis should be assessed.
  • Specific arteries of the circle of Willis are identified using the following criteria
    • Relative direction of the probe within a specific acoustic window,
    • Direction of blood flow relative to the probe,
    • Depth of ultrasound (insonation),
    • Response to carotid compression or vibration (when difficult to differentiate anterior from posterior circulation).
  • The LR, defined as the ratio between the time mean average (Vmean) velocity of the MCA to ICA, is the most established of such ratios and helps differentiate hyperemia from vasospasm.
    • Hyperemia would result in flow elevations in both the MCA and ICA and result in an LR < 3
    • Vasospasm would preferentially elevate the MCA flow velocity over the ICA with LR > 6.
    • LR between 3 and 6 is a sign of mild VSP
    • LR > 6 is an indication of severe VSP.
  • TCD flow velocity criteria appear to be most reliable for detecting angiographic MCA and basilar artery vasospasm.
    • Findings in MCA vasospasm include
      • MCA Vmean ≥ 180 cm/s,
      • A sudden rise in MCA Vmean by > 65 cm/s or 20% increase within 24 h during days 3-7 post-subarachnoid hemorrhage,
      • LR ≥ 6,
      • Abrupt increase in Pulsatility index > 1.5 in two or more arteries suggesting increases in ICP and/or vasospasm.
      • TCD is most useful in monitoring the temporal course of angiographic vasospasm following SAH to help guide the timing of diagnostic and therapeutic angiographic interventions.