Building the Perfect Fiber Optic Probe
GARY BOAS, NEWS EDITOR, GARY.BOAS@PHOTONICS.COMResearchers face old-school challenges in developing state-of-the-art technologies.
Researchers and clinicians use fiber optic-based probes for a wide range of applications – which means the probes must exhibit varying amounts of complexity.
On one end of that spectrum sit the probes used for optical imaging of muscle, which can be as simple as a fiber placed in contact with the skin. On the other end sit probes used for combined optical and x-ray imaging of the breast, with high-density arrays of fiber optic sources and detectors integrated into the paddles of the mammography system.
And somewhere between these you will find brain-imaging probes, according to Arthur “Buzz” DiMartino, president of TechEn Inc. of Milford, Mass. The medical device company manufactures and markets near-IR spectroscopy systems, among other technologies.
Near-IR spectroscopy (NIRS) exploits the properties of near-IR light to produce images of what’s going on inside the body noninvasively. Based on the absorption of light by hemoglobin, for example, we can determine the extent of oxygenation in the blood. This, in turn, can tell us a great deal about brain function, because of the tight neurovascular coupling.
Probe developers have come up with a variety of designs, depending on the particular need.
This probe was developed for use with infants. Images courtesy of Dr. Maria Angela Franceschini,
Massachusetts General Hospital.
Users are exploring the potential of NIRS for applications ranging from testing in children with autism and attention-deficit hyperactivity disorder (ADHD) to stroke rehabilitation, and to monitoring of brain function during surgery. Each has different demands, and so each calls for a different probe design.
“When studying ADHD in children, you have a probe applied to the forehead (to image the prefrontal cortex),” DiMartino said. “At least initially, it’s not a very difficult test. But with stroke, you’re looking at the middle cerebral artery, at areas involved with language and motor skills, and here it gets more challenging.”
At this point, the conversation might turn technical or take a detour into some wildly esoteric territory. But, as it happens, some of the most significant obstacles in developing fiber optic-based probes are decidedly prosaic in nature. DiMartino cites two such hurdles: hair and the contours of the head.
Overcoming obstacles in developing probes
Hair presents a challenge with fiber optic-based probes because the tip of the fiber must maintain good contact with the subject’s scalp. Any hair between one and the other will perturb the signal, so researchers essentially must move the hair aside when positioning the probe on the subject’s head.
An earlier version of the probe. By incorporating the fiber into the horizontal strip of soft material, the developers minimize any discomfort associated with placing a fiber against the head.
Similarly, the contours of the head can make it difficult to maintain contact with the scalp when using a whole-head or even a partial-head probe. If the subject’s head were a perfect sphere, the probe would fit snugly, and all of the fibers would remain against the scalp. But it’s not a perfect sphere, of course. Because of the various ridges and slopes on the head, there will be greater and lesser distances between the fibers and the scalp. Researchers have addressed this mainly through novel probe designs and by incorporating bends into the fibers to help maintain contact.
Another challenge with fiber optic-based probes, said Dr. Maria Angela Franceschini, a researcher with the Martinos Center for Biomedical Imaging at Massachusetts General Hospital, is the weight of the fibers themselves.
“It’s not like EEG electrodes, where you have tiny wires coming out of them,” she said. “We have these heavy glass fibers, and the weight of them can cause the probe to shift on the head. This can be a problem, of course, because if the fibers change position, the measurements will be affected.”
Probe developers have introduced a variety of designs to address the weight of the fibers. These include a scheme in which the fibers are collected at the top of the head before running to the imaging hardware – thus alleviating the weight – and another in which the fibers are fixed in a helmet and individually moved into position once the helmet is placed on the head.
In other cases, developers are incorporating the sources and detectors into the probe itself, avoiding the need for fibers running from the probe to the hardware. To achieve this, they are using LEDs instead of lasers, for example, and pin photodiodes instead of avalanche photodiodes. “You lose sensitivity,” Franceschini said, “but for some applications, it still works well.”
Researchers have come up with many ways to address the various challenges in developing fiber optic-based probes, but translating technology to clinical use comes with its own set of imperatives. Besides all the requirements found in the research setting, probes must have a quick and easy setup, be reproducible without too much adjustment during the measurement session, and – this is important – must adhere to professional standards.
TechEn has come up with an approach to meeting these requirements: combining a headcap with 3-D printed holders. Instead of relying on a series of strips or bands to hold the optodes and the fiber stems in place, the company incorporated the optodes and holders into a cap much like a swimming cap or, more to the point, an EEG cap. This serves several purposes: It helps to hold the fibers in place. It lets users reproducibly position the optodes to the 10-20 system, a widely used method for applying scalp electrodes during EEG measurements. And, significantly, it’s familiar to users in clinical settings.
“People in the medical environment have accepted the form factor of the EEG headcap,” DiMartino said. “So they see this as acceptable as well.”
NIRS and the wearables market
The development of fiber optic-based probes continues in other ways – the probes getting smaller and more portable – and this will open up more opportunities for NIRS devices.
One example: the wearable technology market. This market – which today includes smart watches and eyewear, and connected fitness bands, among other devices – is burgeoning. Shipments of wearable devices will cross the 100 million milestone this year and reach 300 million units by 2019, according to a recent report by BI Intelligence, Business Insider’s subscription research service.
Wearable fitness and medical devices will account for about 60 percent of units shipped this year, say the authors of the BI Intelligence report, and, as time goes on, will claim even bigger pieces of the pie.
This could be good news for NIRS and other optical imaging techniques. “Some wearable technologies, like jogging watches, track vital signs and may include pulse oximeters, which are a kind of optical spectroscopy,” DiMartino said. “So it’s not a stretch to see how those concepts might find their way into brain imaging.”
The devices could have multiple inputs attached through the optodes, he added. These could include a heart rate monitor, measures of cerebral metabolic rate of oxygen, and more.
Researchers have already described a cellphone-sized, wireless NIRS device for diagnosis of bladder dysfunction. In a 2011 study, Dr. Andrew Macnab and colleagues at the University of British Columbia in Vancouver, Canada, reported that the device – which is placed on the patient’s skin and held in place with a strap – was just as reliable as the gold standard invasive tests in determining bladder disease. In another study, published last year, they described use of the device in ambulant patients.