3D Freehand Ultrasound Imaging of Optic Nerve Sheath

The optic nerve sheath expands with elevated intracranial pressure, and its diameter is a sensitive proxy measure. While 2D transorbital ultrasound is well-established for detecting elevated intracranial pressure (>20 mm Hg), it shows limited sensitivity to modest or gradual changes, potentially due to geometric assumptions and imaging misalignment. This study introduces freehand 3D ultrasound imaging of the optic nerve sheath to reduce measurement ambiguity and enhance the fidelity of optic nerve sheath assessment as a noninvasive marker of intracranial pressure.

Twelve healthy participants (28.7 ± 8.2 y; 6 females) underwent freehand 3D optic nerve sheath imaging during 24 hours of normobaric hypoxia in an environmental chamber (FiO2 = 13.1%). Scans were acquired in a head-raised (30 degrees) position at baseline, 6, 12, and 24 hours. Cardiorespiratory parameters, cerebral blood flow, and end-tidal gases were recorded. In 6 participants, postural effects were also assessed at baseline. Nutrition and hydration were standardized to 75% of daily requirements.

The optic nerve sheath's curved, noncircular shape makes the 2D diameter measurement of the sheath a geometric mismatch and an inadequate descriptor. Probe misalignments of 1.2 mm or 15 degrees led to 2D diameter errors (9%/18.7%) exceeding observer variability (2.6%/3.4%). Freehand 3D imaging was reproducible and yielded more informative metrics, including sheath area and thickness. Internal sheath thickness was more responsive to posture and hypoxia than internal 2D diameter (+31.5%, P = 0.01 vs +2.4%, P = 0.9; +8.2%, P = 0.04 vs +1.8%, P = 0.9). A time effect was observed for internal sheath thickness (P = 0.04), and both area and thickness correlated with internal carotid artery velocity (area: r = 0.9, P = 0.05; thickness: r = 0.8, P = 0.07), consistent with physiological expectations.

By eliminating geometric assumptions and misalignment, freehand 3D ultrasound improves the fidelity and sensitivity of optic nerve sheath measurements at the bedside.