Optical imaging techniques can be classified into two groups: purely optical imaging modalities and hybrid optical imaging modalities. A purely optical imaging modality probes and detects with light, whereas a hybrid optical imaging modality combines optical imaging with another method for probing or detecting.
1.4.1 Optical Coherence Tomography
Optical Coherence Tomography (OCT) is currently one of the most advanced optical imaging modalities. It is commercially available, and it has already been applied extensively for ophthalmic imaging to obtain images of the retina [7]. OCT relies on optical
interferometry to obtain high resolution superficial images of a target. A low coherence beam of light is split into a reference beam and a sample beam. The reference beam passes through an optical path with an adjustable path length, while the sample beam is used to illuminate the sample. The light reflecting form the sample is then recombined with the reference beam
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and the resulting interference pattern is analyzed. Constructive interference occurs when the path length of the sample arm is similar to the path length of the reference arm (within the coherence length of the illumination source). The sample arm is scanned in two axes and the reference path length is scanned in one axis to generate a three-dimensional image of the target. Alternatively, a full field illumination scheme can be employed to collect three- dimensional images by scanning only the reference arm [8]. The short coherence length serves as a scatter rejection mechanism as only photons that have travelled a similar path length as the reference arm are imaged. Multiply scattered photons typically travel longer distances before reaching the detector compared to a non-scattered photon. Due to the efficiency of the scatter rejection, OCT produces high resolution images (~10 µm typical or 1-2 µm for ultrahigh-resolution OCT) [7,9]. However, it is limited to superficial depths (2-3 mm) due to the lack of ballistic photons at larger depths.
1.4.2 Time domain imaging
Like OCT, time domain imaging discriminates between scattered photons and unscattered photons by the path length of the photon [10-12]. In time domain imaging, the path length is measured by the time of flight of a particular photon. Given that the speed of light is relatively constant through an imaging sample, the time of flight can be used to deduce the path length of the photon. In time domain imaging, an ultrafast light source (fs) is combined with an ultrafast detector (fs or ps resolution) to capture images of photons which have travelled within a given range of path lengths. The detector is controlled by a delay generator which can vary the time delay of the detector, and correspondingly vary the path length of the detected photons. When a time gate corresponding to the theoretical time of flight of an unscattered photon is employed, a near ballistic image will be generated. When the time gate is relaxed to an extent, a quasi-ballistic image can be obtained. Finally, with a long time gate, a scatter image can be obtained. A typical time domain ballistic imaging system is capable of imaging through a thin sample (< 1 mm tissue) with a diffraction-limited resolution, or through a slightly thicker sample (3.5 mm tissue) in the quasi-ballistic or snake regime with a resolution of ~200 µm [13]. Using this method, the bone within a human finger can be visualized [12]. One of the main disadvantages of time domain imaging is the expense
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of the equipment required for a typical system. Femtosecond light sources and picosecond cameras are 10-100 times the cost of a comparable continuous wave (CW) light source and detector. An optical Kerr cell detection scheme significantly reduces the cost of the detector, but still requires an ultrafast illumination source.
1.4.3 Diffuse optical tomography
Diffuse optical tomography probes a sample with a series of sources and detectors [2]. Based on the geometry and scattering properties of the sample, the potential paths of a large population of photons can be modeled using various photon propagation theories or simulated via Monte Carlo simulations. A typical DOT system operates in the multiply scattered regime, where photons undergo several large angle scattering events before
reaching the detector. In this regime, thick tissue samples can be imaged (3 cm – 10 cm). Due to the use of multiply scattered photons, the resolution of a typical DOT system (~1 cm) is much larger than an optical ballistic or quasi-ballistic system.
1.4.4 Hybrid optical ultrasound imaging
In addition to purely optical imaging modalities, several hybrid optical imaging technologies have been developed. Photoacoustic and acousto-optic imaging both combine ultrasonic imaging with optical imaging [14]. In photoacoustic imaging, a high powered short pulse of light is used to illuminate a sample. An optically absorbing object embedded within the sample will then absorb the pulse of light and convert the optical energy to heat. The rapid heating form the short light pulse will cause a thermoelastic expansion of the target resulting in the emission of a pressure wave. The pressure wave can then be detected with an ultrasonic detector, and the position of the target can be triangulated via various
reconstruction algorithms.
Acousto-optic imaging utilizes a continuous wave laser as an illumination source. The illumination passing through the scattering sample will form a speckle pattern. The sample is then probed with a focused ultrasound transducer which ultrasonically tags a small region in the imaging volume. The ultrasound waves modulate the local refractive index, and displace
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the scatterers within the sample, resulting in a modulation of the speckle pattern at the ultrasound frequency. By analyzing the modulation, the system is able to measure the optical properties of a small region within the sample [15]. A purely optical imaging system is unable accomplish this precise sampling of an imaging volume. In both photoacoustic and acousto-optic imaging systems, the high resolution of ultrasound is combined with the high contrast of optical imaging.