OVERVIEW OF DYNAMIC OCE
Optical elastography includes elastography techniques based on optical imaging modalities. With the inherent high-resolution of optical imaging technologies, optical elastography techniques are unique for measuring biomechanics at the micron-scale tissue level, the cellular level, and even the molecular level. Our lab focuses on the development of optical elastography techniques and their biological and clinical applications, such as breast tumor and human skin biomechanical property measurements.
One important optical elastography technique is optical coherence elastography (OCE), which is a novel elastography technology used to determine tissue biomechanical properties utilizing the in vivo imaging modality OCT. Similar to other elastography technologies, OCE can also be grouped in static or dynamic methods, and external or internal methods, as shown in the figure below with examples in different groups. Speckle tracking methods or cross-correlation algorithms are intrinsic difficulties for static OCE techniques, but can easily be avoided by dynamic OCE techniques, which are based on solving differential wave equations.
|Liang X, Crecea V, Boppart SA. Dynamic optical coherence elastography: A review. J Innovative Optical Health Sciences, 3:221-233 2010.||2010||n/a|
OCE mapping technique
The OCE mapping technique is an external dynamic OCE method, and is derived from the SD-OCT imaging system. In the OCE system a glass window stage was fixed in the sample arm of the OCE system to restrict the upper boundary of the tissue phantoms and tissues, yet enable optical imaging through the window. The sample stage was mounted on a mechanical wave which provided a frequency range of 0.1 Hz to 5 kHz and a maximum amplitude of 7 mm at 1 Hz, decreasing with increasing frequency. The external driving waveform was programmed and synchronized with the image acquisition. Samples were mounted between the upper glass window and the sample stage, with only minimal contact and force prior to data acquisition. Optical backscattering signals were acquired through the upper glass window while the driving mechanical perturbations were exerted simultaneously through the sample stage, compressing the sample vertically. Step and 20 Hz sinusoidal waveforms were used in the OCE experiments. Voigt model was used for mechanical modeling for each A-line in the OCT image, and with transversely scanned laser beam, the quantitative elastic moduli can be mapped as the OCE image showing below.
|Liang X, Oldenburg AL, Crecea V, Chaney EJ, Boppart SA. Optical micro-scale mapping of dynamic biomechanical tissue properties. Optics Express, 16:11052-11065 2008.||2008||PubMed Abstract|
For OCE mapping technique, the applicability for real-time or in vivo diagnostics is limited by their data acquisition and processing speeds because of long acquisition time by taking M-mode OCT images per transverse location and a long curve-fitting processing time. Thus a novel dynamic OCE technique is developed to image biomechanical properties of breast tumor, which combines dynamic mechanical excitation with fast image acquisition and processing. B-mode images were acquired during sinusoidal mechanical compression excitation, and local strain rates were calculated to represent local biomechanical properties. Different excitation frequencies were used to highlight sample regions with distinct mechanical properties, as shown in the figure below. This technique features fast image acquisition and processing speeds, and therefore has the potential for non-destructive volumetric imaging and real-time clinical applications.
|Liang X, Adie SG, John R, Boppart SA. Dynamic spectral-domain optical coherence elastography for tissue characterization. Optics Express, 18:14183-14190 2010.||2010||PubMed Abstract|
External OCE studies as introduced above may suffer from an inability to maintain a sterile in vivo environment. This ability is important when measuring biomechanical properties under circumstances inside the human body or on the surface in trauma. In contrast to this, an internal elastography technique has been studied using radiation forces and ultrasound imaging on breast tumor. Therefore, an internal and dynamic OCE method, namely acoustomotive OCE (AM-OCE), is introduced and studied to show the feasibility of OCE techniques in an internal excitation method. This technique may also have the advantage for exciting and studying local biomechanical properties such as in the cancer microenvironment, instead of bulk excitation. In AM-OCE experiments, acoustic radiation force was applied by a circular, 19-mm-diameter, f/1 lead zirconate titanate (PZT) element transmitting sine-wave bursts at the resonant frequency of 1 MHz. The PZT element was synchronized with the OCT system as step functions of radiation force were excited into the samples. An M-mode OCT image of the inclusion in the sample was recorded. Voigt body was used as the mechanical model, and the OCT phase data were then used to get quantitative shear moduli of the sample, as shown below.
|Liang X, Orescanin M, Toohey KS, Insana MF, Boppart SA. Acoustomotive optical coherence elastography for measuring material mechanical properties. Optics Letters, 34(19):2894-2896 2009.||2009||PubMed Abstract|
Surface wave propagation OCE
Skin is the other main tissue of interest for biomechanical property measurements in this thesis research. A surface wave propagation OCE method and its in vivo application for skin structures and biomechanical property measurements is studied in our lab, in which the mechanical wave driver was used for external mechanical excitation. The mechanical wave driver was synchronized with the OCT system and sinusoidal waves were generated on the skin surface and the surface wave velocity can be calculated by taking M-mode OCT images at different locations on the skin. Surface wave velocity is an important parameter for material mechanical properties and by which Young's modulus can be determined quantitatively. OCE measurements of skin mechanical properties with different driving frequencies and under different hydration conditions in vivo from the volar forearm of a volunteer, as shown below, which indicates that the stratum corneum has been softened by the hydrating process, while this skin layer has become stiffer because of the dehydtrating process. These findings support the physiology that the outer stratum corneum serves to protect the deeper skin layers against dehydrating conditions.
|Liang X, Boppart SA. Biomechanical properties of in vivo human skin by dynamic optical coherence elastography. IEEE Trans Biomed Engr 57:953-959 2010.||2010||PubMed Abstract|