Optical elastography

Section 1. Imaging the microscale mechanical properties of tissue

The onset and progression of disease is often accompanied by changes in the mechanical properties of tissue. Elastography is an imaging technique that uses contrast in the mechanical properties of tissue to form images and can aid our understanding, detection, and diagnosis of disease. To probe tissue mechanics on the microscale, we are developing elastography techniques based on optical coherence tomography (OCT) [1, 2], including optical coherence micro-elastography for measuring tissue strain with micro-strain sensitivity, and optical palpation for visualizing tactile information.

Section 2. Optical coherence micro-elastography

We have developed optical coherence micro-elastography (OCME), utilizing OCT to map micro-scale sample deformation in response to mechanical loading and forming images, elastograms, of tissue mechanical properties.

Using phase-sensitive OCT, we measure, with nanometer-scale sensitivity, tissue’s axial displacement and from it calculate relative tissue stiffness (local strain) over an axial depth range (50-100 micrometers), defining axial resolution. Transverse resolution matches that of OCT – 11 micrometers in our case.

Illustration of optical coherence micro-elastography on a structured phantom. (a) Sample arm of the OCME system. RP, rigid plate; SP, structured phantom; IW, imaging window; RA, ring actuator; L, Lens; X-Y GM, xy-scanning galvanometer mirrors. Perspective and side-view illustrations of the phantom are also shown. (b) Displacement of the actuator and synchronized x-scanning galvanometer-mirror scan pattern. (c) Illustrations of displacement and local strain at two locations in the phantom.

Illustration of optical coherence micro-elastography on a structured phantom. (a) Sample arm of the OCME system. RP, rigid plate; SP, structured phantom; IW, imaging window; RA, ring actuator; L, Lens; X-Y GM, xy-scanning galvanometer mirrors. Perspective and side-view illustrations of the phantom are also shown. (b) Displacement of the actuator and synchronized x-scanning galvanometer-mirror scan pattern. (c) Illustrations of displacement and local strain at two locations in the phantom.

0.1. Download video: 3D micro-elastogram of a silicone phantom containing a stiff, star-shaped inclusion. The inclusion undergoes less strain than the surrounding soft silicone.

We have used OCME to reveal mechanical heterogeneity of human breast cancer tissues on the micro-scale [3]. The mechanical contrast provided by OCME, combined with the structural information afforded by OCT, could help highlight tumor boundaries during surgery and reduce the number of unnecessary and traumatizing surgeries. The image below shows an OCT image and micro-elastogram, compared to corresponding histology, of surgically excised tissue from a breast cancer patient.

Optical coherence micro-elastography of malignant human breast tissue. (a) En face OCT image at a depth of ~100 μm. (b) Corresponding en face micro-elastogram. (c) Histology, co-registered with OCT and micro-elastogram. A, adipose; D, duct; M, smooth muscle; T, region densely permeated with tumor; and V, blood vessel. Scale bars in the inset, 0.25 mm.

Optical coherence micro-elastography of malignant human breast tissue. (a) En face OCT image at a depth of ~100 μm. (b) Corresponding en face micro-elastogram. (c) Histology, co-registered with OCT and micro-elastogram. A, adipose; D, duct; M, smooth muscle; T, region densely permeated with tumor; and V, blood vessel. Scale bars in the inset, 0.25 mm.

Section 3. Optical palpation: OCT-based tactile imaging

Elastography relies on tracking tissue deformation to extract mechanical properties. But what if we could measure the tactile sensation experienced by a clinician’s fingertips during palpation? We have developed the first OCT-based tactile imaging technique, termed optical palpation. We use a stress sensor consisting of compliant, silicone rubber that is placed on the tissue surface. We use OCT to measure the local strain (percent change in thickness) of the sensor under a compressive load. We have characterized the stress-strain response of the sensor material and can relate the measured strain to the corresponding stress. The resulting optical palpation image is a projection of the stress on the sample, similar to the stress felt by the fingertips during palpation.

Optical palpation has potential for delineating regions of tumor in human breast tissues. The figure below shows an optical palpation image and corresponding histology of a tumor boundary in human breast cancer tissue. The regions of high and low stress in the optical palpation image correspond to regions of invasive tumor on the left and normal fatty tissue on the right, as confirmed by the histology.

Optical palpation image of human breast cancer tissue and corresponding histology. N = necrosis. DS = desmoplastic stroma.

Optical palpation image of human breast cancer tissue and corresponding histology. N = necrosis. DS = desmoplastic stroma.

1) Key applications

2) Key researchers

3) Key publications

  1. B.F. Kennedy, K.M. Kennedy, and D.D. Sampson “A review of optical coherence elastography: fundamentals, techniques and prospects,” IEEE J. Sel. Top. Quantum Electron. 20, 7101217 (2014).
  2. B.F. Kennedy, L. Chin, K.M. Kennedy, P. Wijesinghe, A. Curatolo, S. Es’haghian, P.R.T. Munro, R.A. McLaughlin, and D.D. Sampson, “Optical elastography: a new window into disease,” Optics & Photonics News, in press.
  3. B.F. Kennedy, R.A. McLaughlin, K.M. Kennedy, L. Chin, A. Curatolo, A. Tien, B. Latham, C.M. Saunders, and D.D. Sampson, “Optical coherence micro-elastography: mechanical-contrast imaging of tissue microstructure,” Biomed. Opt. Express 5, 2113 (2014).
3.1. Full list of our publications in elastography