====== Red Light ======

[[Intrinsic optical imaging]] is a [[functional imaging]] modality where the reflectance of [[red light]] indicates active portions of [[cortex]], as developed by Grinvald et al. is a powerful technique for monitoring neural function in the in vivo [[central nervous system]]. The advent of this dye-free imaging has also enabled us to monitor human brain function during neurosurgical operations. 
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The objective of a study was to detect [[5 aminolevulinic acid]] ([[ALA]])-induced [[tumor]] [[fluorescence]] from [[glioma]] below the surface of the [[surgical field]] by using red-light illumination. 

To overcome the shallow tissue penetration of blue light, which maximally excites the ALA-induced [[fluorophore]] [[protoporphyrin IX]] (PpIX) but is also strongly absorbed by [[hemoglobin]] and [[oxyhemoglobin]], a system was developed to illuminate the surgical field with red light (620-640 nm) matching a secondary, smaller absorption peak of PpIX and detecting the fluorescence emission through a 650-nm longpass filter. This wide-field spectroscopic imaging system was used in conjunction with conventional blue-light fluorescence for comparison in 29 patients undergoing craniotomy for resection of high-grade glioma, low-grade glioma, meningioma, or metastasis. 

Although, as expected, red-light excitation is less sensitive to PpIX in exposed tumor, it did reveal tumor at a depth up to 5 mm below the resection bed in 22 of 24 patients who also exhibited PpIX fluorescence under blue-light excitation during the course of surgery. 

Red-light excitation of tumor-associated PpIX fluorescence below the surface of the surgical field can be achieved intraoperatively and enables detection of subsurface tumor that is not visualized under conventional blue-light excitation. Clinical trial registration no.: NCT02191488 (clinicaltrials.gov)
((Roberts DW, Olson JD, Evans LT, Kolste KK, Kanick SC, Fan X, Bravo JJ, Wilson 
BC, Leblond F, Marois M, Paulsen KD. Red-light excitation of protoporphyrin IX
fluorescence for subsurface tumor detection. J Neurosurg. 2017 Aug 4:1-8. doi:
10.3171/2017.1.JNS162061. [Epub ahead of print] PubMed PMID: 28777025.
)).