Ординатура / Офтальмология / Английские материалы / Handbook of Optical Coherence Tomography_Bouma, Tearney_2002

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5.2GENERAL REQUIREMENTS AND APPROACHES TO SCANNER DESIGN

A lateral OCT scanner is a complex optomechanical system that satisfies certain engineering requirements. Because it is intended mainly for clinical use, several additional medical requirements are imposed such as safety, sterilizability, and convenience in application. In this section we discuss these issues.

5.2.1Optical System

The main purpose of the scanner optical system is to deliver and focus the probing light onto the tissue surface and collect the scattered radiation back to the interferometer. In the case of a bulk mirror-based interferometer, it provides focusing of the collimated probing beam into a tissue sample. For a single-mode fiber interferometer, the optical system additionally relays the fiber tip image with a certain magnification to the tissue.

The lateral displacement of the probing beam at the sample surface depends on the optical system magnification M. The magnification in all existing OCT systems is typically chosen as a trade-off between the lateral resolution and the scanning depth. To preserve the resolution one usually scans within the range limited by the double Rayleigh length. If the probing beam has a diameter D and a nearly Gaussian intensity profile (which is a quite realistic assumption taking into account that the beam is coupled from a single-mode fiber), the Rayleigh length is D2=4 . At the magnification value M ¼ 4 for a fiber with a 5 m mode diameter and a probing wavelength of 1:3 m, the Rayleigh length is 250 m.

The above numbers mean that a numerical aperture (NA) of a typical scanner optical system should be 0:05 from the output (sample) side and 0.15 from the input (fiber) side. These NA values, although moderate, require aspherical optics to provide acceptable image quality. In most cases, aspherical or GRIN lenses are used; however, microscope objectives are also applicable in optical systems where space and cost are not critical.

5.2.2Mechanical System

Generally, a mechanical system should provide scanning of the focused beam position along the sample surface. The main mechanical requirements for a scanner are size, scanning rate, and safety. Because the allowable size greatly affects a concrete implementation, some features are discussed separately in Section 5.2.3 for benchtop, hand-held, and endoscopic scanners.

The scanning rate is one of the most important issues. The ultimate values for different scanning methods are not clear yet, but there are no principal limitations to acquiring images at a rate of several frames per second (fps) with any of them. Circumferential and galvanometric scanners have already demonstrated high (up to 30 fps) acquisition rate capability [1,2]. Certainly, fast lateral scanning should be matched to the depth scanning rate employed to provide optimal sampling. This means that the lateral displacement during one period of the in-depth scanning should be kept 1.5–2 times less than the lateral resolution (focused beam diameter). Another limitation is the optical power needed to keep an acceptable signal-to-noise ratio as the image acquisition rate is increased. Taking into account that the power

level affordable today in a clinical OCT device is around a few milliwatts, the optical scanning rate is typically 1–8 fps.

From the safety point of view, a low voltage supply is preferable for a scanner, especially for an endoscopic one. This requirement is met for all designs except for piezoceramic bimorph actuators. However, due to the high electrical resistance of piezoceramic elements it might be easy to keep a current through the probe head at the safe level. Another safety requirement (at least for endoscopic probes) is sterilizability. All known designs can be produced hermetically sealed and hence allow cold sterilization.

5.2.3Classification of OCT Scanners in Size: Benchtop, Hand-Held, and Endoscopic Embodiments

The first published OCT experiments with benchtop scanners [3,4] performed linear translation of a probing head or angular rotation of a sample (eye). This approach, being the simplest one, has a number of advantages (e.g., absence of geometrical aberrations) and is still applied in the laboratory studies. An evident alternative is angular scanning of a probing beam in front of an objective lens that is focusing probing radiation on a sample. For instance, Ref. 5 reports lateral scanning performed by mirrors attached to a galvo scanner.

One of the most popular ways to scan laterally with a benchtop-size scanner with a fiber interferometer is to deflect a fiber tip in front of an optical lens system. Working in fact, in all spatial scales from benchtop to tiny endoscopic systems, this proves to be a quite universal solution. Because the same scanning principle is also applicable in various other fields (optical storage devices, lidars, etc.), it has been well known since the 1960s [6], and a number of embodiments were known in the 1970s [7]. They include various systems driving a fiber tip: galvanometric movers, piezoelectric bimorph (or polymorph) plates, magnetostrictive elements, and thermal expansion bimorph plates. In early OCT experiments [8,9] a fiber tip was attached to a needle of a usual microammeter head and the lateral scanning was performed by changing the current through the microammeter.

The most important feature of hand-held scanners in comparison with benchtop scanners is to provide convenient access to examined tissue in a clinical environment. This imposes certain constraints on the size and shape of the instrument while the basic methods of scanning are inherited from the benchtop scanner family. To examine the tissue in the oral cavity, L-shaped hand-held probes were developed that employ either translational [10–12] or galvanometric beam deflection [13] techniques. A straight hand-held scanner with a similar galvanometric actuator was constructed and used in skin research [14]. Another approach based on bimorph or polymorph plate actuators was proposed in Refs. 7 and 15.

Most challenging are the requirements for endoscopic OCT scanners. The diameter of a probe and its flexibility are crucial parameters here to comply with existing medical standards. For example, when examining human internal organs it is necessary for an OCT probe to fit a millimeter-size-diameter operating channel of a corresponding endoscope to provide an approach to the tissue and parallel visual and tomographic observation. As the first attempt to build an endoscopic OCT scanner, replication of an ultrasound intravascular instrument with a rotating distal part was undertaken [16]. This scanner, with a 1.1 mm diameter, was tested in

experiments with animals to image narrow-diameter channels in the respiratory and gastrointestinal tracts. The first microscanner used in clinical experiments with humans [17] to image the gastrointestinal, respiratory, urinary, and genital tracts and the abdominal cavity during laparoscopy was based on an original galvanometric design and had a 2.8 mm diameter. The only moving part in this instrument is a tiny electromagnetic coil with the attached tip of the optical fiber. Another novel design of the endoscopic scanner was implemented [18] in which a remote mechanical actuator produced linear displacement that was conveyed through a cable to the distal end inside the human body. The probe diameter was small enough to fit the operating channel of a fibrogastroscope. Finally, one more type of OCT scanner constructed and tested in clinics [1] is similar to a commercial high frequency endoscopic ultrasound system with rotating probe shaft In general, a variety of endoscopic scanner designs demonstrated recently reflect the deep interest of researchers and clinicians in the potentialities provided by OCT in imaging internal organs.

5.3IMPLEMENTATION OF OCT SCANNERS

5.3.1Circumferential Scanners

Design of circumferential OCT scanners employs the principle of image construction analogous to that in ultrasonography. The probing beam rotates about the OCT probe axis, imaging cross-sectionally through the structure into which the probe is inserted. A recorded tomogram is then presented in polar coordinates; this has also been adopted from ultrasonography and hence is quite common for physicians. Apparently, this approach is adequate if the imaging depth is of the order of or more than the transverse size of the lumen of a hollow organ under observation. Otherwise, A-scans at different polar angles occur under unequal conditions, which leads to narrowing of the sector of quality image acquisition. This requirement is usually met in ultrasound echography with a centimeter-range penetration depth. Having in mind that OCT imaging is limited to 2–3 mm in depth, circumferential scanning is preferable for intravascular examination or investigation of comparatively narrow ducts such as those in the urinary or low respiratory systems.

The first prototype of a circumferential microscanner referred to by the authors as a catheter-endoscope was designed and constructed by the OCT group at MIT in 1996 [16]. It inherits several important features for an intravascular ultrasound probe supplemented with optical delivery and focusing systems. A schematic of the scanner is shown in Fig. 1. It consists of a fixed optical coupling element at the proximal end to introduce light from the main interferometer and a rotating body to direct, focus, and scan the optical beam and collect the backscattered radiation.

Coupling of the incident light from the stationary single-mode fiber to the rotating one is performed through a small air gap without any lens system. It is the most sensitive part of the optical layout, requiring both high precision, face-to- face alignment of the optical fiber apertures and high mechanical stability of the rotary motion. Variation in optical coupling during rotation results in overall losses and a dependence of the probing power on the polar angle (up to 3 dB losses were measured by the authors with the optimal alignment of the scanner). To drive the rotating body, which comprises an optical connector, a hollow cable with the fiber inside and a focusing unit at the distal end (see Fig. 1), a DC motor and a standard

Figure 1 Schematic of catheter-endoscope. (a) Proximal end; and (b) body and distal end. (From Ref. 16.)

gear mechanism are used. The whole rotating assembly is inserted into an exchangeable stationary outer sheath protecting the surrounding tissue against mechanical irritation and biologically hazardous agents. The body of the catheter is flexible to allow bending during the passage of the scanner through channels of the human body to the area to be examined.

The micro-optical system focusing and directing the probing beam is located at the distal end of the catheter-endoscope. It includes a gradient index (GRIN) lens and a right-angle microprism. Attached to each other and to the optical fiber with ultraviolet-curing optical cement, they form a single unit with the rotating body of the catheter. Parameters of the GRIN lens are chosen based on the required working distance in the tissue being imaged (1–3 mm) and spot size comparable with the indepth resolution (15–30 m).

In the first prototype of a device operating at 1:3 m wavelength, a single-mode isotropic fiber with the 9 m core diameter was used in the scanner body to deliver light to a GRIN lens of 0.7 mm diameter and a 0.5 mm microprism. The outer diameter of the catheter was a small as 1.1 mm. An acquisition rate of 4 fps was demonstrated in the first in vivo studies with animals using this circumferential scanner. In Fig. 2 an OCT image of rabbit esophagus is shown as an example demonstrating the performance of the MIT OCT system with the above-described catheter-endoscope probe [19].

Figure 2 Optical coherence tomographic image of rabbit esophagus in vivo [19] obtained with catheter-endoscope [16] allows distinct visualization of different mucosa layer. Bar, 500 m.

The basic design of the circumferential scanner described above has recently undergone several improvements introduced by the OCT group of Case Western Reserve University (CWRU) for development of a gastrointestinal imaging device [1]. At the distal end of the catheter, a Faraday rotator is placed between the GRIN lens and the right-angle prism (see Fig. 3). According to the well-known idea of optical fiber interferometry [20], it provides compensation for birefringence induced in the isotropic fiber due to bending of the sample arm inside the human body. The ultimate result is utilization of the optical power in both orthogonal wave polarizations and the elimination of dynamic polarization distortions caused by fiber bending and temperature drift. At the proximal end of the catheter, a second microsize GRIN lens is added to facilitate the image relay from the fixed optical fiber to the rotating one. Better and less angle-dependent coupling of light to the catheter body is

Figure 3 Schematic illustration of the circumferential endoscopic OCT probe suitable for passage through the biopsy channel of a standard fibrogastroscope. (From Ref. 1.)