Simulation and Experimental Characterization of Lateral Imaging Resolution of Ultrasound Systems and Assessment of System Suitability for Acoustic Optic Nerve Sheath Diameter Measurement

Background and purpose: Optic nerve sheath diameter (ONSD) is used for the estimation of intracranial pressure (ICP). But there are still doubts about the quality of the images and the lateral resolution. Our aim is to investigate the system suitability and best lateral resolution of different ultrasound systems for acoustic ONSD measurement.

Methods: First, we calculated the theoretically lateral imaging resolution at increasing frequencies: 6.6, 10, and 15 MHz using two different ultrasound systems. Second, we created two phantoms consisting of copper wires or polyvinylchloride (PVC) strips and tested the best lateral resolution at different frequencies with the two ultrasound systems. Using the same ultrasound systems, we evaluated the anatomy of optic nerve at increasing transmission frequencies. Finally, the two probes were tested in two patients with different neurological conditions affected by an increase of ICP.

Results: Theoretical resolutions were .63, .43, and .41 mm, respectively, with a frequency of 6.6, 10, and 15 MHz. We found a similar lateral resolution in both phantoms: copper wire; .56 mm at 6.6 MHz, .46 mm at 10 MHz, and .44 mm at 15 MHz; and PVC strips .6 mm at 6.6 MHz, .47 mm at 10 MHz, and .40 mm at 15 MHz in accordance with experimental resolution. The ONSD thickening could be clearly displayed at frequencies higher than 7.5 MHz using the two linear probes and the two patients with an increase of ICP showed thickening of ONSD.

Conclusion: According to our study, both systems are suitable for ultrasound OSND measurement.