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IR sensor relates blood flow to heat

06 Sep 2005

An infrared sensor being developed in Italy is helping researchers monitor coronary heart-bypass operations.

Scientists in Italy are developing an infrared (IR) instrument to monitor changes in blood temperature during open heart surgery such as bypass operations. By relating these changes in temperature to blood flow through vital arteries, the team says it gains vital information as various procedures are performed. (Review of Scientific Instruments 76 084302)

“Each blood vessel acts like a thermal emitter and the amount of heat is proportional to the blood flow detected by the IR sensor,” explain Valentina Hartwig and her colleagues from the C.N.R Institute of Clinical Physiology in Pisa. “In our experiment, it is possible to find a relationship between the artery temperature and the blood flow.”

While other approaches to monitor this relationship have proved awkward, the Italian team’s idea is to design a portable, affordable and easy-to-use device that can be placed near the region of interest during the bypass surgery.

The instrument is based around a pyroelectric sensor which detects between 7 to 12 microns. “Pyroelectric sensors are low-cost but have a high sensitivity and work at room temperature,” Hartwig told Optics.org. “The alternatives were bolometers but they are noisy or photon detectors but they require cryogenic cooling.”

According to Hartwig, the one disadvantage of the pyroelectric sensor is that it only detects light when a temperature change occurs in the element. To overcome this, the team uses an electronic shutter to chop the IR emission before it is gathered by a spherical lens and focused on to the sensor.

Two laser pointers are used to ensure the instrument is placed in the optimal position – corresponding to the focal point of the spherical lens.

“Our sensor is 2.5 cm in radius and is mounted in an 8 cm long cylinder,” said Hartwig. “The total dimension, including the two laser pointers, is 10 cm in width. The smallest change that the sensor can detect is 0.2°C.”

To test their prototype, Hartwig and colleagues performed trials both on a thin silicon pipe that simulated an artery and in vivo during an operation on a small pig. In each case, the instrument was able to display real-time changes when the blood flow was interrupted and then returned.

“We are now thinking of using alternative sensors to eliminate the chopper and increase the performance,” concluded Hartwig.

Author
Jacqueline Hewett is technology editor on Optics.org and Opto & Laser Europe magazine.

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