Strain by virtual extensometers and video-imaging optical coherence tomography as a repeatable metric for IOP-Induced optic nerve head deformations

Optical coherence elastography and its applications for the biomechanical characterization of tissues

The biomechanical characterization of the tissues provides significant evidence for determining the pathological status and assessing the disease treatment. Incorporating elastography with optical coherence tomography (OCT), optical coherence elastography (OCE) can map the spatial elasticity distribution of biological tissue with high resolution. After the excitation with the external or inherent force, the tissue response of the deformation or vibration is detected by OCT imaging. The elastogram is assessed by stress-strain analysis, vibration amplitude measurements, and quantification of elastic wave velocities. OCE has been used for elasticity measurements in ophthalmology, endoscopy, and oncology, improving the precision of diagnosis and treatment of disease. In this article, we review the OCE methods for biomechanical characterization and summarize current OCE applications in biomedicine. The limitations and future development of OCE are also discussed during its translation to the clinic.

The Strain Response to Intraocular Pressure Decrease in the Lamina Cribrosa of Patients with Glaucoma

OBJECTIVE

To measure biomechanical strains in the lamina cribrosa (LC) of living human eyes with intraocular pressure (IOP) lowering.

DESIGN

Cohort study.

PARTICIPANTS

Patients with glaucoma underwent imaging before and after laser suturelysis after trabeculectomy surgery (29 image pairs; 26 persons).

INTERVENTION

Noninvasive imaging of the eye.

MAIN OUTCOME MEASURES

Strains in optic nerve head tissue and changes in depths of the anterior border of the LC.

RESULTS

Intraocular pressure decreases caused the LC to expand in thickness in the anterior-posterior strain (Ezz = 0.94 ± 1.2%; P = 0.00020) and contract in radius in the radial strain (Err = - 0.19 ± 0.33%; P = 0.0043). The mean LC depth did not significantly change with IOP lowering (1.33 ± 6.26 μm; P = 0.26). A larger IOP decrease produced a larger, more tensile Ezz (P < 0.0001), greater maximum principal strain (Emax; P < 0.0001), and greater maximum shear strain (Γmax; P < 0.0001). The average LC depth change was associated with the Γmax and radial-circumferential shear strain (Erθ; P < 0.02) but was not significantly related to tensile or compressive strains. An analysis by clock hour showed that in temporal clock hours 3 to 6, a more anterior LC movement was associated with a more positive Emax, and in clock hours 3, 5, and 6, it was associated with a more positive Γmax. At 10 o'clock, a more posterior LC movement was related to a more positive Emax (P < 0.004). Greater compliance (strain/ΔIOP) of Emax (P = 0.044), Γmax (P = 0.052), and Erθ (P = 0.018) was associated with a thinner retinal nerve fiber layer. Greater compliance of Emax (P = 0.041), Γmax (P = 0.021), Erθ (P = 0.024), and in-plane shear strain (Erz; P = 0.0069) was associated with more negative mean deviations. Greater compliance of Γmax (P = 0.055), Erθ (P = 0.040), and Erz (P = 0.015) was associated with lower visual field indices.

CONCLUSIONS

With IOP lowering, the LC moves either into or out of the eye but, on average, expands in thickness and contracts in radius. Shear strains are nearly as substantial as in-plane strains. Biomechanical strains are more compliant in eyes with greater glaucoma damage. This work was registered at ClinicalTrials.gov as NCT03267849.

Mapping Pulsatile Optic Nerve Head Deformation Using OCT

OBJECTIVE

To develop a noninvasive technique to quantitatively assess the pulsatile deformation due to cardiac contractions of the optic nerve head (ONH).

DESIGN

Evaluation of a diagnostic test or technology.

PARTICIPANTS

Healthy subjects with no history of refractive surgery, divided into 2 cohorts on the basis of their axial length (AL).

METHODS

We present a noninvasive technique to quantitatively assess the pulsatile deformation of the ONH tissue by combining high-frequency OCT imaging and widely available image processing algorithms. We performed a thorough validation of the approach, numerically and experimentally, evaluating the sensitivity of the method to artificially induced deformation and its robustness to different noise levels. We performed deformation measurements in cohorts of healthy (n = 9) and myopic (n = 5) subjects in different physiological strain conditions by calculating the amplitude of tissue displacement in both the primary position and abduction. The head rotation was measured using a goniometer. During imaging in abduction, the head was rotated 40° ± 3°, and subjects were instructed to direct their gaze toward the OCT visual target.

MAIN OUTCOME MEASURES

Pulsatile tissue displacement maps.

RESULTS

The robustness of the method was assessed using artificial deformations and increasing noise levels. The results show acceptable absolute errors before the noise simulations grossly exaggerate image degradation. For the group of subjects with AL of < 25 mm (n = 9), the median pulsatile displacement of the ONH was 7.8 ± 1.3 μm in the primary position and 8.9 ± 1.2 μm in abduction. The Wilcoxon test showed a significant difference (P ≤ 0.005) between the 2 paired measures. Reproducibility was tested in 2 different sessions in 5 different subjects with the same intraocular pressure, and an intraclass correlation coefficient of 0.99 was obtained (P < 0.005).

CONCLUSIONS

The computational pipeline demonstrated good reproducibility and had the capacity to accurately map the pulsatile deformation of the optic nerve. In a clinical setting, we detected physiological changes in normal subjects supporting its translation potential as a novel biomarker for the diagnosis and progression of optic nerve diseases.