Publication: Measurement of cerebral blood flow in the adult pig by depth-resolved broadband near-infrared spectroscopy

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Title Measurement of cerebral blood flow in the adult pig by depth-resolved broadband near-infrared spectroscopy
Authors/Editors* JT Elliott, M Diop, KM Tichauer, T-Y Lee, K St. Lawrence
Where published* Journal of Cerebral Blood Flow & Metabolism
How published* Proceedings
Year* 2009
Volume 29
Pages S394-S406
Publisher Nature
Objectives: Near-infrared spectroscopy (NIRS) has the potential to improve management of life-threatening neurological emergencies by providing bedside measurements of cerebral blood flow (CBF). We have developed a method for measuring CBF using the NIRS contrast agent, indocyanine green (ICG).1 The challenge to adapting our technique to adult studies is to remove signal contamination caused by the extra-cerebral layer (ECL). To achieve this, we developed a multi-channel, broadband NIRS system. Multi-distance measurements provide some depth sensitivity to assist in separating cerebral and extra-cerebral signals and broadband spectroscopy is used to quantify ICG concentration. The objective of this study was to compare depth-resolved NIRS measurements of CBF to those obtained by CT perfusion.2 Methods: Experiments were conducted on four pigs (3 to 4 weeks old) anesthetized with isoflurane; a cannula was inserted into an ear vein of each pig for administrating ICG and a CT contrast agent (Omnipaque). Animals were placed in the CT scanner with three NIRS optodes positioned on the head: one emission optode and two detection optodes placed 1 and 4-cm from the emitter. Brain and ECL ICG concentration curves were extracted from multi-distance NIRS data using a two-layer modified Beer-Lambert Law. Partial pathlengths (PPL) for each layer were selected from a library generated by Monte Carlo simulations3 based on the geometry of the ECL and with the constraint that water concentration in the brain is 80%.4 CBF was determined from brain ICG concentration curves as described previously.1 For comparison, CBF was also calculated using 4-cm emitter-detector distance raw ICG curves. All NIRS results were evaluated against concomitant CT perfusion measurements of CBF. Results: Figure 1 shows ICG curves obtained at source-detector distances of 1 and 4-cm and the corresponding ICG curves for brain and ECL from the PPL approach. CBF from CT perfusion ranged from 37 to 83 mL/min per100 g and the mean thickness of the ECL was 9.2±1.7 mm. Analysis of the 4-cm ICG data resulted in underestimations in CBF (by 33%±23%), and was more erroneous for pigs with thicker ECLs. In contrast, there was a significant correlation between CBF measurements from CT perfusion and the PPL approach (mean difference of −2%±20% between the techniques). Conclusion: These preliminary results suggest that CBF in adults could be measured by depth-resolved broadband NIRS using our PPL approach. Further work is required to assess the precision of the CBF measurements and the sensitivity to the thickness of the ECL.
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