MEMS BASED OPTICAL COHERENCE

TOMOGRAPHY IMAGING

Optical coherence tomography (OCT) has the ability to detect cancerous tissues at their early stages due to its micron scale resolution.

Microelectromechanical systems (MEMS) technology enables miniature scanning elements to be embedded inside endoscopic OCT probes.

Traditional method used in diagnosis of cancer is biopsy. Several non invasive imaging methods are US,CT &MRI .

INTRODUCTION

The problems of these methods can be overcome by optical imaging technique.

Current methods using in optical imaging include:-

Optical coherence tomography (OCT) Confocal microscopy (CM) Two-photon(2P) microscopy Photoacoustic microscopy (PAM)

Fig.1 OCT system based on Michelson interferometer

OPTICAL COHERENCE TOMOGRAPHY (OCT)

Depth resolution is determined by inverse Fourier transform

To realize 3D imaging, 2D lateral scan needs to be realized by a moving stage or a scanning mirror (SM).

The resolution of OCT can reach 1 to 15 μm depending on the light source employed.

The penetration depth of OCT is normally 1 to 3mm.

It is an enabling technology that makes devices and systems at the scale of micrometers to millimeters.

ADVANTAGES:-• They are small therefore

miniaturization of probes are possible.• Can operate at high speed.• Easy to integrate.• Cost of the devices is low. • power consumption is low .

MEMS

MEMS BASED OCT PROBES

FIG.2. Packaged Mems probe

MEMS mirrors are placed at the distal ends of the probes.

Their angular rotation directs the light and generates lateral scans on the sample.

Several features are highly desirable of MEMS mirrors for this application.

The footprint of the MEMS device must be small . The mirror aperture must be large and flat. The mirror must be able to scan large .

Actuation Mechanisms :- electrostatic, electromagnetic, electro thermal and piezolelectric.

It is based on electrostatic force which exists between electrically charged particles.

Scanning angles of ±8° have been achieved with voltages in the range of 40 V to 200 V

There are two main types of electrostatic actuators: based on parallel-plate capacitance

and combdrive .Another type of actuator is based on the

vertical combdrive.

ElECTROSTATIC ACTUATION

Fig.3 Electrostatic actuation. (a) a parallel-plate actuator, (b) combdrive actuator

Fig.4.Mems packaged probe

ELECTROMAGNETIC ACTUATION

 It is based on Lorentz force. By controlling current flowing direction, both repulsive attractive driving force can be realized.

The magnetic field is generated by perm alloy or active electric coils. 

Scanning angles of ±30° obtained with ±1.2 V and ±4 V driving voltages .

Fig.5.Packaged mems probe

It realizes on bending motion by applying electric field across a piezoelectric material such as lead zirconate titanate (PZT).

They are composed of metal/PZT/metal sandwich or double layered PZT materials .

Scanning angles of 40° have been reported using voltages up to 13 V.

PIEZOELECTRIC ACTUATION

Uses MEMS mirrors with both straight and curled shaped electrothermal bimorph actuators formed by Al & Si.

The largest mechanical deflection reported was 17◦ at an operation voltage of ∼1.3 V

ELECTROTHERMAL ACTUATION

Fig.6.(a) mems mirror,(b) Packaged mems probe

(a) (b)

 OCT can perform optical imaging deep below the tissue surface with sub cellular resolution.

MEMS technology has been widely used as the scanning engine in endoscopic OCT probes.

Among various MEMS actuation mechanisms, electrostatic actuation is the dominant choice.

Research results have shown great potential for OCT in endoscopic imaging by MEMS scanning mirrors.

CONCLUSION

Zhen Qiu and Wibool Piyawattanametha, “MEMS-Based Medical Endomicroscopes”IEEE JOURNAL VOL. 21, NO. 4(2015)

D. Huang et al., “Optical coherence tomography,” Science, vol. 254, no. 5035, pp. 1178–1181, Nov. 1991.

M. Nakada et al., “Optical coherence tomography by all-optical MEMS fiber endoscope,” IEICE Electron. Exp., vol. 7, no. 6, pp. 428–433, 2010.

Changho Chong, Keiji Isamoto, and Hiroshi Toshiyoshi,IEEE photonic technology letters,vol.18,pp.133-135(2006).

K. H. Kim, B. H. Park, G. N. Maguluri et al., “Two-axis magnetically-driven MEMS scanning catheter for endoscopic high-speed optical coherence tomography,” Optics Express, vol. 15, no. 26, pp. 18130–18140, 2007.

 

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