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Imaging: Miniature Drives for Focus and Zoom Control for Endoscopy

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Minimal invasive surgery (MIS) is aiming at causing patients as little discomfort as possible and endoscopes play an important role here. Traditional Chip-on-the-Tip endoscopes use fixed focus optics, which cannot display objects at different distances with optimum resolution. By integrating a miniature drive, it could become possible to achieve variable focusing, so that the object can always be displayed optimally in sharp focus.

Endoscopes that allow minimally invasive surgery (MIS) are used in larynx diagnostics or laparoscopy (a surgery that uses a thin tube fed through an incision in the belly to look at the abdominal organs). Optics and special instruments (scissors, hooks, grasping forceps, ultrasound scissors, etc.) are then introduced into the abdominal cavity through so-called working trocars. Following surgery, the patient experiences less post-operative pain and can be discharged from the hospital earlier, due to the quicker healing process. The risk of wound infections or wound healing disorders is minimized and allows treatment of high-risk patients.

Modern medical technology is aiming at causing patients as little discomfort as possible with their therapies. Endoscopes that allow minimally invasive surgery provide an important contribution, e.g., in laparoscopy.
Modern medical technology is aiming at causing patients as little discomfort as possible with their therapies. Endoscopes that allow minimally invasive surgery provide an important contribution, e.g., in laparoscopy.

Placing the Imaging Sensor on the Endoscope Tip

Traditional endoscope technology is well established and has reached a high degree of sophistication. One of the bottlenecks used to be the image transmission through classical optics. With the rapid evolution of microelectronics and compact imaging sensors, more progress is feasible, such as integrating a small camera into the endoscope tip (Chip-on-the-Tip). The advantage is vastly improved image quality.

With traditional endoscope optics, it was, however, possible to attach a manual zoom lens or focus lens between the eyepiece and camera to set the focus to different object distances and zoom into details. In contrast, the digital zoom function of the Chip-on-the-Tip technology only allows a detail to be enlarged, which is always accompanied by a loss in quality. The absence of a focus function requires a compromise between depth of focus and image brightness.

Optical Zoom Function for Chip-on-the-Tip Cameras in Endoscopes

This type of compromise could soon be a thing of the past, if the actuators required for optical focus and zoom functions could be installed between the optics and the imaging chip. With diameters of typically 10mm, the potential installation space is very small and it seems to be difficult, but not impossible, to find the right drives for the zoom and focus lens.

Piezo stick-slip mini motors are incredibly small as seen on the left. Designed into a drive, they can become an integral part with further size reduction possible. A standard miniature actuator with a high-force piezo inertia motor is shown on the right. (Image: PI)
Piezo stick-slip mini motors are incredibly small as seen on the left. Designed into a drive, they can become an integral part with further size reduction possible. A standard miniature actuator with a high-force piezo inertia motor is shown on the right. (Image: PI)

For example, miniaturized ceramic piezo motors and voice coil drives could open up a new area of application in this field. This is where PI (Physik Instrumente) and its leadership in miniature precision drive mechanisms come into play. These drives offer a solid base for use in modern high-resolution microscopes.

If actuators could be installed between the optics and the imaging chip, optical focus and zoom functions can be added to a Chip-on-the-Tip Camera. With diameters of typically 10mm, the potential installation space is very small and it seems to be difficult, but not impossible, to find the right drives for the zoom and focus lens.
If actuators could be installed between the optics and the imaging chip, optical focus and zoom functions can be added to a Chip-on-the-Tip Camera. With diameters of typically 10mm, the potential installation space is very small and it seems to be difficult, but not impossible, to find the right drives for the zoom and focus lens.

Small Piezo Motors or Voice Coil Drives

Piezo motors make for very compact and precise linear drives. The direct-drive principle eliminates the conversion of rotation into linear motion, along with the need for gears and other mechanical components that can wear and limit reliability. Piezo motors are small, lightweight, and consume very little energy. In fact, they can hold a position without power consumption.

A miniature piezo motor positions an autofocus lens integrated along with the driver on a small PCB (Image: PI)
A miniature piezo motor positions an autofocus lens integrated along with the driver on a small PCB (Image: PI)

There are a number of different piezo motor principles (see videos belows) that are feasible. All provide the necessary precision.  Ultrasonic motors and inertia motors are very compact and cost effective.

Drive Principle of a Mini-Rod Inertia Piezo Motor>
Drive Principle of a Mini-Rod Inertia Piezo Motor>

Drive Principle of an Ultrasonic Piezo Ceramic Motor>
Drive Principle of an Ultrasonic Piezo Ceramic Motor>

Drive Principle of a Flexure-type Inertia Motor>
Drive Principle of a Flexure-type Inertia Motor>

Drive Principle of a Ratchet-type Piezo Motor>
Drive Principle of a Ratchet-type Piezo Motor>

In addition to piezoelectric motors, traditional electromagnetic linear motors (Voice Coils) are also an option.

Miniaturized PIMagTM voice coil linear drive. This type of motor provides high speed and motion up to 25mm, enough for the requirements in endoscopes. (Image: PI)
Miniaturized PIMagTM voice coil linear drive. This type of motor provides high speed and motion up to 25mm, enough for the requirements in endoscopes. (Image: PI)

 

Drive Principle of a Voice Coil Motor>
Drive Principle of a Voice Coil Motor>

Voice coil actuators are related to the transducers used in loudspeakers. They can be manufactured with very small dimensions and provide motion ranges in the millimeter to centimeter range, just enough for endoscopy. Given these drive options, it should be interesting which of them will contribute to better image quality and depth of focus in the next generation of Chip-on-the-Tip endoscopes.

Miniaturized controller for a voice coil drive with integrated position sensor. (Image: PI)
Miniaturized controller for a voice coil drive with integrated position sensor. (Image: PI)

Piezo Scanner Tubes for 2-Photon Endoscopy

Compact piezo XY scanners can be used for high-resolution two-photon endoscopy.  A recent paper describes a spiral fiber scanner based on a miniature piezo scanner tube (1.5mm outer diameter) used for biological imaging of parts of the eye. The compact instrument has a 2mm diameter providing lateral and axial resolution as low as 1.5μm

Miniature piezo scanner tubes compared to a standard piezo-ceramic ring. (Image: PI)
Miniature piezo scanner tubes compared to a standard piezo-ceramic ring. (Image: PI)

 

Design of the two-photon excitation fluorescence endoscope. (a) shows the piezo scanner tube; the complete 2 photon probe is shown in (d). (Source: <a href="http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3018080" target="_blank" rel="nofollow">http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3018080</a>)
Design of the two-photon excitation fluorescence endoscope. (a) shows the piezo scanner tube; the complete 2 photon probe is shown in (d). (Source: http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3018080)

 

Author: Birgit Bauer, Business Development Manager Health Care at PI (Physik Instrumente)

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About PI

PI (Physik Instrumente) is a leading manufacturer of precision motion control equipment, piezo motors, air bearing stages and hexapod parallel-kinematics for semiconductor applications, photonics, bio-nano-technology and medical engineering. PI has been developing and manufacturing standard & custom precision products with piezoceramic and electromagnetic drives for 4 decades. The company has been ISO 9001 certified since 1994 and provides innovative, high-quality solutions for OEM and research. PI is present worldwide with fifteen subsidiaries, R&D / engineering on 3 continents and total staff of more than 1,000.

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