Optical topography

Introduction

Optical topography involves acquiring multiple reflectance measurements of light at small source-detector separations over a large area of tissue simultaneously or in rapid succession. By keeping the separation low, the measured signals are relatively high and therefore may be acquired quickly. Optical topography has been developed by research groups in Europe, Japan, and the USA as a means of real-time monitoring of haemodynamic and oxygenation changes in the brain. In principle, brain activity with characteristic responses as fast as a hundred milliseconds or so can be studied, with images displayed at a rate of a few Hertz or faster. However, small separations also imply an overwhelming sensitivity to surface (cortical) tissues, and little information is revealed about deeper regions of the brain. Optical topography is consequently a cortical mapping technique, analogous to electroencephalography (EEG) which is sensitive to electrical activity occurring near the surface of the brain. Recent advances in optical topography of the human brain are summarised elsewhere.

The UCL Optical Topography system

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Figure 1: The UCL optical topography system.

We have developed our own optical topography system at UCL, shown in figure 1. Several systems have been built so far, including devices for collaborating groups at the Centre for Brain and Cognitive Development (Birbeck College, London) and the Laboratoire de Sciences Cognitives, (L’Ecole des Hautes Etudes en Sciences Sociales, Paris). Our system consists of 32 laser diode sources (16 at 780 nm and 16 at 850 nm) and 16 avalanche photodiode detectors (APDs). Smaller systems have also been produced. All the sources are illuminated simultaneously, but are modulated at different frequencies. By performing a Fast Fourier transform on the signal received by each detector, the signal corresponding to each source can be isolated. This approach to optical topography allows great flexibility in the positioning of sources and detectors, such that a variety of arrangements of sources and detectors can be employed on the tissue surface with only minor changes being needed in the software. The system is described in detail in Everdell et al. The laser diodes, each emitting a power of approximately 2 mW, are driven by frequencies within a single octave (from 2 kHz to 4 kHz) to prevent interference from harmonics. The 16 APDs each have a 3 mm diameter collecting area and a measurement bandwidth of approximately 100 kHz. The system is designed to be able to produce an image at a maximum rate of 10 frames/second using the data from 16 detectors. Sources and detectors are coupled to the tissue via 2 mm diameter optical fibre bundles. These ‘optodes’ can be arranged in various configurations. They are attached to the head with a variety of methods, two examples of which are shown below. Figure 2 shows the design pioneered by Babylab at Birkbeck College, London.

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Figure 2: The Babylab optode array

Figure 3 shows the design produced by the collaboration between our group and the Evelyn perinatal imaging centre in Cambridge (click here for their website).

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Figure 3: The UCL-Cambridge optode array

Click here to see a recent TV appearance.



NOTE: We are very happy to provide a highly competitive quote for supplying optical topography systems to other research groups. For an overview document about the system and its applications click here (downloadable pdf). For more information, please feel free to contact Dr. Nick Everdell by email, or go to the website of our spin-out company, Gowerlabs.


Published work that has employed our system is listed below (references 3 to 10).

References

1. Ferrari M, V Quaresima (2012) “A brief review on the history of human functional near-infrared spectroscopy (fNIRS) development and fields of application” Neuroimage 63(2): 921-935 Download PDF file

2. Everdell, NL, Gibson, AP, Tullis, IDC, Vaithianathan, T, Hebden, JC, and Delpy, DT (2005): A frequency multiplexed near infrared topography system for imaging functional activation in the brain, Review of Scientific Instruments 76, 093705. Download PDF file

3. Blasi, A, Fox, S, Everdell, N, Volein, A, Tucker, L, Csibra, G, Gibson, AP, Hebden, JC, Johnson, MH, and Elwell, CE (2007): Investigation of depth dependent changes in cerebral haemodynamics during face perception in infants, Physics in Medicine and Biology 52, 6849-6864. Download PDF file

4. Delpy, DT, Cope, M, van der Zee, P, Arridge, S, Wray, S, and Wyatt, JS (1988): Estimation of optical pathlength through tissue from direct time of flight measurement. Physics in Medicine and Biology 33(12), 1433-1442. Download PDF file

5. Correia T, Lloyd-Fox S, N L Everdell, A Blasi, C Elwell, J C Hebden, A Gibson (2012) "Three-dimensional optical topography of brain activity in infants watching videos of human movement" Physics in Medicine and Biology 57 1135-1146 Download PDF file

6. Lloyd-Fox S, A Blasi, N L Everdell, C E Elwell, M H Johnson (2011) "Selective cortical mapping of biological motion processing in young infants" Journal of Cognitive Neuroscience 23(9) 2521-2532 Download PDF file

7. Cooper RJ, Hebden, JC, O'Reilly H, Mitra S, Mitchell A, Everdell NL, Gibson AP, Austin T (2011) "Transient haemodynamic events in neurologically compromised infants: A simultaneous EEG and diffuse optical imaging study" Neuroimage 55(4) 1610-1616 Download PDF file

8. Correia T, A Banga, N L Everdell, A P Gibson, J C Hebden (2009) "A quantitative assessment of the depth sensitivity of an optical topography system using a solid dynamic tissue-phantom" Physics in Medicine and Biology 54 6277-6286 Download PDF file

9. Lloyd-Fox S, A Blasi, A Volein, N Everdell, C E Elwell, M H Johnson (2009) "Social Perception in Infancy: a near infrared spectroscopy study" Child Development 80(4), 986-999 Download PDF file

10. Cooper R J, N L Everdell, L C Enfield, ,A P Gibson, A Worley, J C Hebden (2009) "Design and evaluation of a probe for simultaneous EEG and near-infrared imaging of cortical activation" Physics in Medicine and Biology 54, 2093-2102 Download PDF file

11. Cooper R J, D Bhatt, N L Everdell, J C Hebden (2009) "A tissue-like, optically turbid and electrically conducting phantom for simultaneous EEG and near-infrared imaging" Physics in Medicine and Biology 54 N403-N408 Download PDF file