For Medical Instruments, Going Small Pays Off Big

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Size does indeed matter: For medical instruments, smaller often is better. The benefits can include greater capabilities, lower cost and better outcomes. In pulling off this trifecta, imaging plays an important role, as illustrated by three cases.

An example of why going small makes sense can be seen in capsule endoscopy, a procedure in which a vitamin-size pill containing a complete video device is swallowed. The minicamera then transits the gastrointestinal tract, taking video and transmitting it wirelessly to a recording device. This allows doctors to spot problems in the esophagus and small bowel. After doing its duty, the device exits the body in the same way as anything else that is ingested.

Capsule endoscopes, such as the PillCam devices shown here, are shrinking in size but expanding in reach. Courtesy of Given Imaging Ltd.

The overall endoscope market is growing at a modest 3 percent a year, said Teri Minor, a senior analyst at the technology market research firm Frost & Sullivan. But there are segments within it, such as the gastrointestinal one, that are growing much faster.

“They have new technologies, like capsule endoscopy, which have growth rates through 2016 that could be as much as 18 or 19 percent a year,” she said.

Fueling that growth are two factors: the increasing number of procedures and technologies such as the pill-size cameras.

Pill-size imaging devices were introduced almost a decade ago by Given Imaging Ltd. of Yokneam, Israel. To do this, the company exploited advances in sensors, lights and wireless technology, a combination that allowed mass production of products that could take and transmit images for hours.

Given is on its second-generation device, which measures 11 x 26 mm and weighs <4 g. The camera snaps as many as 35 pictures a second, and its wide field of view allows doctors to see twice the surface area of that seen with other capsules, said Given CEO Homi Shamir.

Although declining to reveal any details about future products, Shamir did say that there is no technical reason why the devices couldn’t be smaller. But there may be an optical reason not to shrink them. Currently, their dome is optimized in size and shape to enhance imaging performance.

“A smaller capsule may limit the visualization of the small-bowel mucosa,” he said.

In general, the trend is toward smaller and smaller devices, he added, thanks in part to various drivers. One is increased patient satisfaction; for example, studies show that capsule endoscopy is preferred to much more intrusive methods, he said.

Images of an esophagus acquired with a PillCam. Courtesy of Given Imaging Ltd.

Being smaller also allows access to new areas and easier entry to others. Imaging the small intestine without a capsule endoscope, for instance, requires complicated procedures, radiation or surgery, he noted.

Being small also can provide another benefit: low cost. A case in point is a CMOS camera from Medigus Ltd. of Omer, Israel. In September 2009, the company announced a 1.2-mm video camera, claiming to have produced the smallest such device in the world. Medigus currently is working with medical device companies and academic teams around the globe to incorporate it into various devices.

Measuring only 1.2 mm in diameter and 5 mm in length, this CMOS camera for medical applications is as small as the point of a pen. Inside the camera cylinder are a sensor, four lenses and a miniature printed circuit board connected to a four-wire cable. The camera is designed to be used in disposable instruments. Courtesy of Medigus Ltd.

Product development manager Ariel Smoliar noted that the camera uses some of the latest CMOS sensor technology and has 2.2-μm pixels. Another required element comes courtesy of a different photonics technology.

“We needed to miniaturize also our light sources. It’s not based on fibers. It’s based on very low power consumption LEDs,” Smoliar said.

The sensor fits approximately 50,000 pixels into an area that measures 0.7 mm on a side. Future sensors could be smaller because improved pixel sensitivity may make it possible to get good enough results with smaller pixels.

Today, this CMOS approach produces a very small device, one that can be used in minimally invasive surgery. In particular, devices of this size allow doctors to use natural orifices or, perhaps, only one incision. Fewer, or no, scars are not only cosmetically preferred but also could lead to faster healing, less risk of infection and minimal complications.

Because it is an inexpensive imaging system, this camera can be thrown away after a single use. The advantage of this approach is that it eliminates the need for sterilization between procedures, which can be expensive and a source of infection if not done properly.

This array of pixels in a CMOS sensor has microlenses on top, making the device sensitive to RGB colors. Such sensors may be the future for imaging in miniature medical instruments. Courtesy of TowerJazz.

The sensors for the Medigus camera are manufactured by TowerJazz, a global specialty semiconductor foundry with headquarters in Migdal HaEmeq, Israel. The company has a long history in the imager business.

TowerJazz runs 8-in. wafers, which means there can be close to 50,000 half-square-millimeter sensors on each one. That multitude is part of what enables the sensors to be produced cheaply enough to render the camera disposable.

The small size of these sensors – as compared with those in other applications – is not necessarily the result of going with a smaller pixel, said Avi Strum, TowerJazz vice president and general manager. The need to capture clear images in low light makes it difficult – if not impossible – to shrink the pixels much from the size of those used in devices such as cell phones.

This constrains what can be done in terms of shrinking a sensor, Strum said. “If you use the same size pixel and you need an order of magnitude smaller in area sensor, you need to give up two things. One is pixel count.”

Thus, the Medigus sensor has 5 percent of the million pixels found in a typical cell phone camera. Even so, this is five times the number of fibers in a fiber bundle solution, making the small-area sensor effectively high resolution compared with that alternative, Strum said.

Simply shrinking the number of pixels, however, isn’t enough. So the other trick, Strum said, is the removal – as much as possible – of everything but the pixels themselves from the chip. Thus, the output is not converted to a digital signal on the chip, which saves real estate by getting rid of circuitry. Also, the pixel readout is done serially, which enables a reduction in the number of space-consuming, but nonlight-sensitive, input and output pads.

A back-illuminated sensor achieves a higher light-sensitive-to-total-area ratio, or fill factor. The problem is that it is an expensive solution, which works against the desire to use the sensor in a disposable device.

Some miniaturized medical instruments do use fiber bundles. In this approach, optical fibers transmit light, and an image, out of the body to an external camera.

Using a bundle of optical fibers allows doctors to see inside the sinus cavity.

The results can be remarkable, said David W. Sanso, president of BioVision Technologies LLC of Golden, Colo., a company that makes imaging products for doctors and veterinarians.

In making this assessment, Sanso indicated a miniature endoscope his company developed in conjunction with Zibra Corp. of Westport, Mass. The entire system measures only 1.2 mm in diameter, the size of an 18-gauge needle.

It is intended for arthroscopy, or the inspection of joints. Having it be as small as possible is a benefit because areas of the body can be imaged that would be inaccessible otherwise. Such imaging has the advantage of potentially being used to see what a joint looks like from the inside while it is actively being worked by a patient. This can provide doctors with mechanical information that is otherwise unavailable.

The scope has other valuable attributes besides being small, Sanso said. “The image quality is really quite impressive but, more so than that, we were also able to come up with a way to make that scope disposable.”

As for how the required combination of size, image quality and low cost was achieved, Sanso listed a few factors. First, the device was built around a small fiber imaging bundle of only a half millimeter in diameter, with ground lenses on the far end. Second, custom-developed equipment allowed the construction of the optics to be somewhat automated. Third, the design allowed the optics to be less complicated and, therefore, less costly.

An important point to keep in mind for almost all small medical devices is that the working distance is only a few millimeters or, at most, a centimeter. Thus, things must be in focus for distances of only up to about 10 mm.

In the case of optical fiber-bundle-based systems, the camera that is used is tweaked to match the output of the fiber. This ensures the best possible overall image quality.

Zibra President Arthur C. McKinley said that his company routinely builds devices with an outside diameter of 0.4 mm, far smaller than that of any current medical instrument. However, such devices offer limited image quality, a drawback that prevents them from being used in medical applications. He doesn’t foresee this situation changing, largely because of limitations related to fiber size and the wavelength of visible light.

Instead, he predicts that the future for imaging in miniature medical instruments will increasingly belong to CMOS sensors. Fiber bundle techniques are based on a comparatively mature technology. In contrast, CMOS is relatively new and undergoing much more rapid advances. What’s more, it could enable disposable use, a prospect made more likely by the economics of its manufacturing.

As McKinley noted, “It can be made very cheaply if the volume goes way up.”

Published: November 2010
1. A localized fracture at the end of a cleaved optical fiber or on a glass surface. 2. An integrated circuit.
fiber bundle
A rigid or flexible, concentrated assembly of glass or plastic fibers used to transmit optical images or light. See aligned bundle; incoherent bundle.
field of view
The field of view (FOV) refers to the extent of the observable world or the visible area that can be seen at any given moment through a device, such as an optical instrument, camera, or sensor. It is the angular or spatial extent of the observable environment as seen from a specific vantage point or through a particular instrument. Key points about the field of view include: Angular measurement: The field of view is often expressed in angular units, such as degrees, minutes, or radians. It...
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