Optic nerve sheath diameter measurement techniques: examination using a novel ex-vivo porcine model.
BACKGROUND
Ultrasound (U/S) and MRI measurements of the optic nerve sheath diameter (ONSD) have been proposed as intracranial pressure measurement surrogates, but these methods have not been fully evaluated or standardized. The purpose of this study was to develop an ex-vivo model for evaluating ONSD measurement techniques by comparing U/S and MRI measurements to physical measurements.


METHODS
The left eye of post mortem juvenile pigs (N = 3) was excised and the subdural space of the optic nerve cannulated. Caliper measurements and U/S imaging measurements of the ONSD were acquired at baseline and following 1 cc saline infusion into the sheath. The samples were then embedded in 0.5% agarose and imaged in a 7 Tesla (7T) MRI. The ONSD was subsequently measured with digital calipers at locations and directions matching the U/S and direct measurements.


RESULTS
Both MRI and sonographic measurements were in agreement with direct measurements. U/S data, especially axial images, exhibited a positive bias and more variance (bias: 1.318, 95% limit of agreement: 8.609) compared to MRI (bias: 0.3156, 95% limit of agreement: 2.773). In addition, U/S images were much more dependent on probe placement, distance between probe and target, and imaging plane.


CONCLUSIONS
This model appears to be a valid test-bed for continued scrutiny of ONSD measurement techniques. In this model, 7T MRI was accurate and potentially useful for in-vivo measurements where direct measurements are not available. Current limitations with ultrasound imaging for ONSD measurement associated with image acquisition technique and equipment necessitate further standardization to improve its clinical utility.
Department of Internal Medicine, Baylor College of Medicine, Houston, TX 7703, USA. nusbaum@bcm.edu
Derek M
DM
Nusbaum
Massachusetts General Hospital, Boston, MA, USA.
Erik
E
Antonsen
University of Texas Health Sciences Center at Houston, TX, USA.
Kurt H
KH
Bockhorst
Department of Internal Medicine, Baylor College of Medicine, Houston, TX 7703, USA.
R Blaine
RB
Easley
Department of Internal Medicine, Baylor College of Medicine, Houston, TX 7703, USA.
Jonathan B
JB
Clark
Department of Internal Medicine, Baylor College of Medicine, Houston, TX 7703, USA.
Kenneth M
KM
Brady
Department of Internal Medicine, Baylor College of Medicine, Houston, TX 7703, USA.
Kathleen K
KK
Kibler
National Space Biomedical Research Institute, Houston, TX, USA.
Jeffrey P
JP
Sutton
University of Texas Health Sciences Center at Houston, TX, USA.
Larry
L
Kramer
Wyle Science, Technology and Engineering Group, Houston, TX, USA.
Ashot E
AE
Sargsyan
D000818:Animals; D066298:In Vitro Techniques; D007427:Intracranial Pressure; D008279:Magnetic Resonance Imaging; D023421:Models, Animal; D009900:Optic Nerve / Q000033:anatomy & histology* / Q000000981:diagnostic imaging; D013525:Surgical Instruments; D013552:Swine; D014463:Ultrasonography