Biomedical Engineering Reference
In-Depth Information
diagnosis of many retinal diseases is possible because OCT can provide images
of retinal pathology with micron-scale resolution.
OCT has also been applied in a wide range of nontransparent tissues [4,5].
In nontransparent tissues such as skin, muscle, and other soft tissues, the
imaging depth is limited by optical attenuation due to scattering and ab-
sorption. Optical scattering decreases with increasing wavelength. Therefore,
while ophthalmic OCT imaging has primarily been performed at 800 nm
wavelengths, OCT imaging in nontransparent tissues has been typically per-
formed with wavelengths of 1.0-1.3
m. Imaging depths up to 2-3 mm can
be achieved using a system detection sensitivity of 100-110 dB. In early ex-
ploration imaging studies, OCT has been performed in virtually every organ
system to investigate applications in cardiology [6-8], gastroenterology [9,10],
urology [11, 12], neurosurgery [13], and dentistry [14], to name a few. Us-
ing short coherence length, short-pulsed light sources, high-resolution OCT
has been demonstrated with axial resolutions of less than 5
µ
m [15-17]. High-
speed real-time image acquisition rates have also been achieved, which enables
volumetric imaging [18, 19]. New imaging modes in OCT have been demon-
strated, such as Doppler OCT imaging of blood flow [20-22] and birefringence
imaging to investigate laser intervention [23-25]. OCT beam delivery systems
including transverse imaging catheter/endoscopes and forward imaging de-
vices have enabled OCT imaging of internal structures [26-28], and most
recently, catheter-based OCT has been used to perform in vivo imaging in
animal models and human patients [29-33].
µ
8.2 Principles of Operation
OCT performs optical ranging in tissue, using high spatial resolution and
high dynamic range detection of backscattered light as a function of optical
delay. While the speed of ultrasound waves in tissue is relatively slow and
detectable using electronics, the velocity of light is extremely high. There-
fore, the time delays of the reflected light cannot be measured directly and
interferometric detection techniques must be employed. Low-coherence inter-
ferometry or optical coherence domain reflectometry are techniques that are
used in OCT. First developed in the telecommunications field for measuring
optical reflections from faults or splices in optical fibers [34], low-coherence in-
terferometry can also be used for localizing the optical reflections in biological
tissue. Subsequently, the first applications in biological samples included one-
dimensional optical ranging in the eye to determine the location of different
ocular structures [35, 36].
Time delays of reflected light-off of tissue boundaries are typically mea-
sured using a Michelson-type interferometer (Fig. 8.1). Other interferometer
designs, such as a Mach-Zehnder interferometer, have been implemented to
optimize the delivery of the OCT beam and the collection of the reflected
signals [37,38]. Light reflected from the specimen or sample is interfered with
