Ординатура / Офтальмология / Английские материалы / Handbook of Optical Coherence Tomography_Bouma, Tearney_2002
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expected with this arrangement when the drive shaft rotates. Using these modifications, a catheter probe suitable for passage through the accessory channel of a standard gastrointenstinal endoscope has been constructed and tested during examinations of human patients at the CWRU hospitals. Figure 4 is an individual OCT frame captured at the 4 fps rate in a normal human esophagus using the CWRU OCT system [1] with the described probe design.
To conclude this section, the advantages and limitations of the circumferential design of OCT scanners can be summarized. First of all, the rotational principle is promising due to its potentiality to accelerate the scanning speed. Regular rotary motion is less sensitive to undesirable inertial and hysteresis effects that occur with high speed oscillating motion. Real-time OCT imaging with a video rate of 20–30 fps is feasible with circumferential scanning when fast enough in-depth scanning and high sources of superluminescent radiation of enough power are at hand. Second, rotation does not prevent the OCT probe from being flexible, which is an important feature for an endoscopic OCT device. The described OCT probes directly employ this property from the standard ultrasound catheter design. Third, the circumferential scanning mode is favorable for the examination of organs with a narrow lumen and is especially suitable for intravascular imaging. On the other hand, there are several problems inherent to this approach. One of them is the difficulty in alignment
Figure 4 Optical coherence tomographic image of normal human esophagus in vivo obtained with circumferential endoscopic OCT probe. Scale markers represent 1 mm. P is the outer surface of the probe sheath. The substructure of the mucusal layer is differentiable, with the squamous epithelium (E), lamina propria (LP), and muscularis mucosae (MM). Light gray-scale (‘‘echo-poor’’) inclusions (arrows) can be seen in the submucosa (SubM) that correspond to blood vessels and glands. (From Ref. 1.)
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of the optical rotary junction, which results in high insertion losses and dependence of the probing power on the rotation angle. The circumferential scanners are also much less appropriate for large lumen channels or hollow organs where it is desirable to probe at different viewing angles to the scanner axis. For these purposes, apparatuses described in the following sections appear to be more attractive.
5.3.2Deflecting Scanners
Design of deflecting OCT scanners employs the principle of beam motion that is similar to that in scanning microscopy. A probing beam is deflected transversely to the direction of its incidence on a stationary lens system that further guides it to the distal end and focuses it on tissue. Deflection is produced either by moving the tip of the optical fiber delivering the probing light from the main interferometer or by an oscillating (rotating) lens or a mirror that directs the beam coming out of the fiber to the stationary lens system. Several mechanisms for scanning the probe beam by moving optical components at the proximal end, including motor-driven galvanometric, and piezoelectric techniques, have already been demonstrated in OCT devices.
Moving the fiber tip in the image plane of the stationary lens system appears to be the most universal and applicable technique for scanners of different sizes and functions. One of the first OCT systems comprising a scanner of this type was demonstrated by the OCT group of the Institute of Applied Physics (Nizhny Novgorod, Russia) [21]. The schematic of this hand-held probe capable of twodimensional lateral scanning of the beam is presented in Fig. 5.
Figure 5 Schematic of deflecting OCT scanner with suspension arrangement in the form of embedded parallelograms. (From Ref. 8.)
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The key mechanical feature in this design is a suspension arrangement in the form of embedded parallelograms formed by thin spring plates that provide independent orthogonal movement of a fiber in two directions relative to the probe case. The lens system and two permanent magnet units are fixed to the case, whereas two coils are attached to the spring frame and impart a deflecting torque whose components are controlled by the current magnitude in the coils. The magnetic field in the magnet bores reaches the value of 4.5–5 kG and produces a deflecting driving force of 0:15 N at a current value of 50–70 mA. With a spring length of 50 mm the swing of transverse scanning in both directions is 2 mm. The image of the fiber tip is relayed by the lens system onto tissue with variable magnification that is controlled by the position of the lenses in the optical assembly and is dependent on the application (skin, eye, or teeth imaging).
As in any other mechanical system, hysteresis takes place when the direction of movement is being reversed. In this design, at the fiber tip with a swinging amplitude of 3 mm and a scanning rate of about 1 scan/s, hysteresis of the fiber position is significantly lower than that obtained with piezoceramic actuators and usually does not exceed 3%. To provide easily controlled periodic motion of the scanner it is important that higher harmonic components in the spectrum of the driving force for each orthogonal displacement do not enter domains of eigenresonances. In the described OCT probe, due to the appropriate choice of the spring material and geometry, the suspension has a lower resonance frequency of 62.5 Hz at a Q factor of 7 along the x coordinate and 30 Hz at a Q factor of 14 along the y coordinate. Eigenoscillations of the suspension are suppressed due to electromagnetic damping by means of a power amplifier with low output resistance. As a result, the mechanical movement of the scanner was practically free from hysteresis and oscillations when scanner was driven by isosceles triangle voltage waveform with 1 s period. Of course, to operate at scanning frequencies of the order of 10 Hz it is necessary to either increase the resonance frequency of the suspension or introduce certain predistortions in the driving current.
One of the scanners of this family, designed in an L-shaped form for better access to the tissue in the oral cavity, is shown in Fig. 6. The magnification of the optical system was 5, thus providing imaging of an area 5 mm wide [14]. For skin imaging where the probe size does not cause a real limitation, lateral beam swinging as large as 15 mm was achieved on the tissue surface using another scanner of this kind. A typical tomogram of a skin nevus recorded with the aid of this scanner is shown in Fig. 7.
To study mucosa of human internal organs, the OCT group from the IAP developed in 1997 a one-dimensional lateral microscanner [17] using the principle of electromagnetic displacement of the fiber described above. It is small enough to be inserted into biopsy channels of standard endoscopic instruments such as fibrogastroscopes, hysteroscopes, laryngoscopes, and laparoscopic trocars. For instance, the diameter of a biopsy (instrumental) channel of many commercial endoscopes does not exceed 2.8 mm. There is also a limitation on the length of the rigid part of the scanner body. It should be short enough to allow penetration of the probe head into slightly curved endoscopic channels. Later in this section we describe the design and performance of such a small electromagnetic microscanner for endoscopic OCT.
In the microscanner design, the internal volume is used with maximal efficiency and the number of instrumental elements is minimal. In construction of a suspen-
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Figure 6 Photograph of L-shaped tip of deflecting scanner (Fig. 5) for examination of oral cavity. Bar ¼ 1 cm.
sion, for instance, the elastic properties of the distal part of a quartz fiber itself are used when it is made an end part of the sample arm of the interferometer. The protective plastic coating is peeled off the optical fiber to reduce hysteresis effects caused by nonelastic deformations in the plastic when it is bent. Whereas at the proximal end of the microscanner the fiber is fixed to a base, the free end of the fiber serves as a console to which a coil of an electromagnetic system is attached (Fig. 8). The console part of the fiber is surrounded by the body of a permanent magnet, with a through-hole being formed by the facing grooves. The magnet is composed of two halves pressed together Their similar poles are aligned to produce fanlike magnetic force lines distributed in the plane of the coil. The coil is made of a thin wire and has thin input leads, which insignificantly increase the hysteresis effects of the mechanical suspension. The driving current applied to the coil causes it to interact
Figure 7 Image of skin nevus obtained with deflecting OCT scanner. Bar ¼ 1 mm.
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Figure 8 (a,b) Schematics of endoscopic microprobe. 1, Output window, 2, focusing lens; 3, fiber tip; 4, coil; 5, fiber; 6, permanent magnet; 7, through-hole in magnet; 8, probe sheath; 9, fiber; 10, fiber basement. (From Ref. 17.) (c) For side-view imaging, output glass window is made in the form of a prism directing the beam at a certain angle to the probe axis.
with the permanent magnet field and a deflecting torque to occur that deviates the fiber from equilibrium. When the current is about 100 A, deviation of the fiber tip is about 0.3–0.5 mm, which is enough to move a focused beam with a swing of 2 mm at a magnification of the lens system of about 4–5.
At its distal end, the microscanner used for en face imaging has an output glass window inclined at an angle of 5–6 . The inclination decreases parasitic reflections of the probing light back into the fiber mode from both window surfaces; the inner surface is also antireflectively coated for the same purpose. The outer surface of the window touches a wet mucosa and becomes almost antireflective due to the refractive index matching. For side-view imaging, an output glass window is made in the form of a prism (see Fig. 8c) directing the beam at a certain angle to the probe axis. A single aspheric lens of a lens system focuses the light beam into the spot positioned near the tissue surface. Thus, operation of the microscanner requires neither an additional alignment of the optical system nor adjustment of the reference arm of the interferometer. As an example, in Fig. 9 an EOCT tomogram of healthy human esophagus is presented that was acquired with a portable IAP OCT device equipped with the microscanner inserted in the operating channel of a standard fibrogastroscope.
An alternative method of moving the fiber is based on the use of piezoelectric actuators. In 1997 the OCT group of MIT [15] demonstrated such scanner designs with piezoelectric cantilevers. The displacement of a cantilever made of a bimorph or polymorph plate several centimeters long can reach 1 mm at an applied voltage of several hundred volts. If a fiber is adhered to the cantilever, a deflecting torque is imparted to the optical system, thus producing scanning of the probe beam according to the applied voltage variation.
A schematic of a hand-held probe of this type is presented in Fig. 10. A 6:4 mm 38 mm lead zirconate titanate piezoelectric cantilever was used with a 38 mm
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Figure 9 Endoscopic OCT image of healthy human esophagus that was obtained with deflecting endoscopic scanner. Bar ¼ 1 mm (From Ref. 17.)
extension tube to double the cantilever arm length for transverse scanning. Total displacement of 2 mm was achieved at 300 V applied voltage. A single-mode optical fiber running along the tube was coupled to a GRIN lens through a narrow air gap. The faces of the GRIN lens and the fiber tip were slightly angled to reduce backreflection to the interferometer and oriented parallel to each other to maximize optical coupling. The focal spot, about 30 m in diameter, was positioned at working distance of 3 mm from the lens. An OCT system with this scanner was tested in in vitro experiments to record images of human ovary and lung (Fig. 11).
Because of the rigid fixing of the GRIN lens to the oscillating tube, the described design possesses some features of translational scanners that are considered in the next section. A similar design without an extension tube and a GRIN lens, employing a stationary telescope system to focus the beam, has some advantages over the one diagramed in Fig. 10. It includes low mass (the fiber only) attached to the cantilever, allowing higher oscillation rates, and variable magnification of the optical system achieved by repositioning or changing lenses.
In general, evaluating the piezoelectric OCT scanner design, its simplicity and low cost are rather attractive. At the same time, it has a number of drawbacks in comparison with electromagnetically driven suspensions. Among them the use of high voltage to attain the same displacement magnitudes and a pronounced hysteresis effect inherent to polymorph piezoactuators are the most important.
Figure 10 Schematic of hand-held probe with a piezoelectric cantilever. (From Ref. 15.)
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Figure 11 Optical coherence tomographic images of human ovary (A) and lung (B) obtained in vitro using hand-held probe with piezoelectric cantilever. pf, primordial follicle; a, lung alveoli.
The probe beam in deflecting OCT scanners can also be swung with galvan- ometer-driven mirrors or lenses. This method permits high acquisition rates and three-dimensional imaging but at the expense of increased size, cost, and complexity. This design was implemented in the first commercial OCT devices for ophthalmology produced by Zeiss Humphrey Systems, Dublin, CA. Another working prototype of a hand-held scanner of this type has recently been demonstrated by the OCT group of MIT [22]. A schematic of this design is given in Fig. 12. It contains a galvanometerdriven mirror changing the angle of incidence of the probe beam on a stationary assembly of relay lenses (Hopkins type).
5.3.3Translational Scanners
The main distinctive feature of the design of this scanner family is the linear translational motion of the optical assembly as a whole with a remotely motor-driven
Figure 12 Schematic of galvanometer-based hand-held probe. (From Ref. 22.)
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carriage. Due to its technical simplicity and an obvious advantage, the absence of geometrical aberrations caused otherwise by variation of the beam path in the focusing system, this approach is quite convenient in bench-top experiments without volume limitation on the layout of an OCT probe. It was the reason why in the very first OCT experiments the translational principle of scanning, applied to a probe or to an object, was implemented. Recently, translational scanners have been demonstrated in hand-held [11,12] and even endoscopic [18] designs.
One of these scanning devices (see Fig. 13) was developed by the OCT group of Lawrence Livermore National Laboratory (LLNL) to image the human oral cavity, primarily the hard dental tissue. A traveling stage with an optical system is displaced by a DC-motor-driven screw along the surface of the tissue. The scan direction (up or down) is set by depressing the appropriate switch and applying the necessary bias to the motor. Terminal switches restrict the range of lateral motion to the desired transverse scan dimension. The optical assembly includes a GRIN lens attached to the tip of a single-mode fiber with an index-matching UV-curable epoxy. The distance from the end of the GRIN lens to the tissue is adjusted by means of a screwtype mechanism, and a removable sterilizable cap is in contact with tissue.
Figure 13 Schematic of translational hand-held OCT scanner [10] for examination of dental tissue.
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The described device permits the recording of two-dimensional OCT images that are free of geometrical aberrations over the entire scanning range with a lateral size as large as several millimeters. The main drawback is the long time needed for image acquisition (several tens of seconds) because of the inertia of the displaced assembly and in part the low power of the light source. Figure 14 shows a typical tomogram of periodontal tissue taken with this scanner at a wavelength of 1:3 m.
Another interesting design of translational scanners has recently been implemented by the OCT group of Harvard Medical School [18]. A narrow diameter, flexible endoscopic OCT probe has been constructed where scanning is conveyed
Figure 14 Optical coherence tomographic images of the human periodontium acquired with translational scanner. Scales are represented in terms of optical distance. E, Enamel; D, dentin; GM, gingival margin; DEJ, dentoepithelial junction; CT, connective tissue; JE, junctional epithelium; MGJ, mucogingival junction; AM, alveolar mucosa; AB, alveolar bone. (From Ref. 10.)
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mechanically to the distal end of the probe from a drive situated outside the human body. In contrast with the circumferential scanners described in Section 3.1, the body of the probe linearly translates within a static outer sheath. This construction eliminates the problem of transferring optical power from a stationary to rotating optical fiber, which is a weak point in the circumferential design. From the viewpoint of application areas, this design is more suitable for imaging internal organs with large lumens and hence to some extent is complementary to an MIT-type catheterendoscope.
A schematic of the translational endoscopic scanner is shown in Fig. 15. As a mechanical actuator of scanning, a linearly displaced carriage driven by a rotating galvanometer at the proximal end of the probe is used. The translational motion is conveyed via a wound multiplayer cable that is attached to the carriage and houses an optical fiber inside. The helicity of the cable is reversed between layers to avoid rotation of the distal end while the cable is stretched or compressed. The sampling arm of the interferometer terminates with a 1.0 mm diameter gradient index lens and a 750 m wide prism fixed to the distal tip of the cable. The optical system directs light transversely to the axis of the scanner and focuses it to a spot of 30 m diameter at a distance of 700 m from the outer wall of the catheter. This distance provides optimal imaging if the catheter is placed in contact with the tissue. To isolate the optical components from the tissue, a transparent disposable sheath is placed over the catheter cable during OCT examination.
A typical image of the normal human esophagus that was acquired in vivo with an OCT system equipped with the translational endoscopic scanner is presented in Fig. 16. This tomogram, recorded at a rate of 4 fps, demonstrates the high and constant quality of image acquisition over the entire lateral scanning range, which exceeded 5 mm. It is an impressive manifestation of the main advantage of the translational technique—the absence of image aberration due to invariable probing beam with respect to the moving optical system.
5.4CONCLUSION
In this chapter, we have surveyed various types of OCT scanners. Their designs meet the main requirements for this instrument: to deliver and focus the probing light on the tissue, collect the scattered radiation, and provide beam motion along the sample surface at a rate adequate to that used for in-depth scanning. According to the
Figure 15 Schematic of translational endoscopic scanner. (From Ref. 18.)