Optical coherence tomography (OCT) is a noninvasive, noncontact imaging modality that uses coherence gating to obtain high-resolution cross-sectional images of tissue microstructure. In Fourier domain OCT (FD-OCT), the interferometric signal between light from a reference and the back-scattered light from a sample point is recorded in the frequency domain rather than the time domain. After a wavelength calibration, a one-dimensional Fourier transform is taken to obtain an A-line spatial distribution of the object scattering potential. The spectral information discrimination in FD-OCT can be accomplished by using a dispersive spectrometer in the detection arm in the case of spectral-domain OCT (SD-OCT) or rapidly tuning a swept laser source in the case of swept-source OCT (SS-OCT).
The axial or depth resolution of the FD-OCT system is determined by the actual spectral width recorded and used for reconstruction. The axial range over which an OCT image is taken (imaging depth, scan depth or imaging range) is determined by the sampling interval or resolution of the optical frequencies recorded by the OCT system. Specifically, in SD-OCT, the spectrometer disperses different wavelengths to the detector elements. The resolution of the optical frequencies and therefore the imaging depth depends on the width of the portion of the spectrum that is measured by a single detector element or pixel.
In some SS-OCT implementations, the swept-source tunes or sweeps the wavelength of the source over time. In this case, the resolution of the optical frequencies depends on a spectral separation of the measuring light at adjacent points in time. The spectral resolution of the measurements will increase with sampling density unless it is limited by the instantaneous linewidth of the laser. For most of the swept-sources, OCT signals acquired with adjacent points separated by uniform (constant) time intervals result in a non-uniform sample distribution in k (wave vector) space. Normally, these optical frequencies are further numerically re-sampled (or interpolated) to get equally k-spaced samples before the Fourier transform is actually taken. This will digitally affect the actual imaging depth in the OCT reconstruction as the imaging depth is now determined by the resolution in wave-numbers (K). In other implementations, Fabry-Pérot interferometers (FPI or etalon) (see for example Zhang et al “Swept laser source at 1 um for Fourier domain optical coherence tomography,” Applied Physics Letters 89, 073901 2006) and Mach-Zehnder interferometers (see for example Xi et al “Generic real-time uniform k-space sampling method for high-speed swept-source optical coherence tomography,” Optics Express 18(9):9511 2010) can be used to generate external clock signals with uniform k-spacing. In this case, the digitizer (or data acquisition system) of the SS-OCT system is running in an “external clock” mode, whereby it takes the external k-clock signals for point by point sampling.
There are several types of OCT measurements which require particularly dense spatial sampling, but may tolerate relatively low axial resolution. Such measurements include but are not limited to, OCT Angiography methods (e.g. Doppler OCT, phase contrast, phase variance, speckle variance, power of Doppler shift, normalized vector difference, ultrahigh sensitive optical microangiography (UHS-OMAG)), photoreceptor imaging, wide-field anterior segment scans, wide field overview scans. For example, OCT Angiography or OCT photoreceptor images are often displayed as 2D projection images. While a high transverse sampling is required to reduce phase noise in the case of OCT Angiography, or support a high lateral resolution in the case of photoreceptor imaging, the axial resolution is less relevant, because the processed data is only displayed in projected or enface 2D images. OCT Angiography methods typically require an over sampling of about 4-12 samples per transverse position. This increases the acquisition time by the same factor. This is problematic as increased acquisition time directly affects image quality as well as patient comfort. It is desirable to collect data over as large a field of view as possible which also involves acquisition time, patient comfort and motion considerations.
One of the approaches to solve this problem is to use very high speed OCT systems (See for example Klein et al. “The effect of micro-saccades on the image quality of ultrawide-field multimegahertz OCT data,” SPIE Photonices West 2012, Paper #8209-13 (2012) or Blatter et al. “Ultrahigh-speed non-invasive widefield angiography,” J. Biomed. Opt. 17, 070505, 2012 hereby incorporated by reference), however, such systems can be very complex and costly. Also, the high speed systems would require faster detection electronics and would result in large numbers of data sets that could further slowdown the analysis of the data. Additional approaches to overcome the problems include the use of tracking, and montaging of multiple data sets.