An optical-tomography instrument designed for noninvasive monitoring of blood volume and oxygenation could save the lives and brain functions of premature infants. Developed by Adam Gibson and colleagues at the Department of Medical Physics and Bioengineering at the University College London (UCL; London, England), MONSTIR (multichannel optoelectronic near-IR system for time-resolved image reconstruction) uses picosecond pulses of laser energy to acquire and measure the flight times of photons transmitted between pairs of points on the surface (32 parallel time-resolved detectors) and generate 3-D images of biological tissues. The absorption of visible and near-IR light by hemoglobin and other natural chromophores enables the optical tomography instrument to reveal information about tissue oxygenation, hemodynamics, and metabolism.
“This is not optical-coherence tomography, which measures back-reflected light that has not been scattered and so can only penetrate a few millimeters of tissue,” Gibson said. “We record light that has diffused across the entire head. We get much better penetration (about 10 cm) but much worse spatial resolution (about 5 to 10 mm).”
While MONSTIR has been in development for about 10 years, it is currently undergoing clinical study at UCL hospitals, recently demonstrating the ability to produce 3-D whole-head optical-tomography images of passive motor-evoked responses in neonates. Gibson and his team use a plastic, foam-lined helmet that is custom-built for each infant. The outer shell of the helmet is lined with soft near-IR-absorbing foam. Combined source-detector fiber bundles are attached to the helmet by small sockets mounted on the thermoplastic shell, and their positions are measured with a 3-D digitizer.
In a process that takes about eight minutes, the researchers acquire a reference measurement using the same helmet and an object with precisely known optical properties: a balloon filled with a solution of intralipid and near-IR dye, with optical properties similar to those of brain tissue. The researchers acquire three views across the 3-D absorption image of the infant at 780 nm, generate a 3-D image of the scatter coefficient simultaneously, and acquire similar images of both coefficients at 815 nm.
“These are the first clinical tests of optical tomography across the entire baby head,” Gibson said. “Other researchers have imaged part of the head, and some others have imaged the entire breast (for breast cancer). We are the first (and only) group to reconstruct 3-D images of the entire neonatal head.”
Simon Arridge, a member of UCL’s image reconstruction, theory, and modeling group, has developed a software package known as TOAST (time-resolved optical absorption and scattering tomography) that works in conjunction with MONSTIR. Using an iterative finite-element-modeling-based model-fitting approach, TOAST models light transport in highly scattering, diffuse media and reconstructs the optical parameters inside a medium from boundary measurements of light transmission.
Gibson hopes that every neonatal intensive care unit will one day have a MONSTIR instrument, which is also being tested on breast and muscle tissue. While the current prototype is about as big as a standing refrigerator-freezer, Gibson and his team are working to reduce the size, improve the speed, and have a commercially available product in five years. They are currently replacing the fast-photon-counting electronics with time-correlated single-photon-counting (TCSPC) cards in an industrial personal computer, which will reduce the total volume of the system by about half. They also want to replace the detectors and other components to improve performance and substitute diode lasers for the fiber laser. According to Gibson, these changes should yield a system the size of a small refrigerator.