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Letters to the Editor |
Department of Radiology, Santa Casa de Misericordia de Sao Paulo, Sao Paulo, Brazil
Ricardo B. Fonseca, MD
Department of Radiology, Vanderbilt University Medical Center, 1161 21st Avenue South, MCN CCC-1118, Nashville, TN 37232-2675. e-mail: ricardo.fonseca@vanderbilt.edu
Editor:
We read with great interest the article by Dr Frigon and colleagues (1) in the October 2004 issue of Radiology,"Supplemental Oxygen Causes Increased Signal Intensity in Subarachnoid Cerebrospinal Fluid on Brain FLAIR MR Images Obtained in Children during General Anesthesia." This article deals with a subject that is part of our line of research.
The increase in signal intensity (SI) in the cerebrospinal fluid (CSF) on fluid-attenuated inversion-recovery (FLAIR) magnetic resonance (MR) images that results from inspired oxygen levels has been shown in the literature (2–4). As indicated by the authors, awareness of this phenomenon is important to avoid diagnostic mistakes.
The article fulfills its objective of showing a relationship between a high fraction of inspired oxygen (FIO2) and an elevation in the CSF SI on FLAIR images. However, one methodologic issue deserves further analysis. As the study was performed, it is not possible to categorically determine whether the persistence of high SI in the CSF in some patients after inhaling 30% O2 is due to propofol or an insufficient amount of time between image acquisitions to allow for equilibrium of the blood and CSF O2 tension. We believe, however, that the persistent high SI is due to the order in which the sequences were performed. Had the authors chosen to acquire the images with 30% FIO2 first before acquiring images with 100% FIO2, high SI would not occur with 30% FIO2, and it would be certain that propofol was not responsible for CSF hyperintensity on FLAIR images. After observing high SI in the CSF of anesthetized patients, Fillipi et al (2) proposed propofol as the culprit. However, a meticulous study by Deliganis et al (3) showed that O2 causes the increase in SI in the CSF. Our study did not show any change in the CSF SI with different anesthetic drugs, including propofol (4).
We believe that it is essential to control the FIO2 in patients who are anesthetized during brain MR imaging, especially in patients suspected of having meningeal disease, be it inflammatory or neoplastic. Specifically, if the FIO2 is kept below 50%, there should be no artifactual increase in the SI of the CSF on FLAIR images (4) and, therefore, increased SI in the CSF can be interpreted as pathologic.
On the other hand, we have used increased FIO2 to perform a type of noninvasive MR cisternography, with good results in the identification and characterization of small cystic lesions in the subarachnoid space (5).
Finally, we congratulate Dr Frigon and colleagues for furthering the knowledge on the relationship between oxygen and increased SI in the CSF on FLAIR MR images.
References
,
Susan Heckbert, MD, PhD
and
Ed Weinberger, MD
Department of Anesthesiology, Montreal Children’s Hospital, 2300 rue Tupper, Montreal, QC, Canada H3H 1P3
Departments of Radiology and Epidemiology, University of Washington, Seattle. e-mail: chantal.frigon@muhc.mcgill.ca
We appreciate the interest of Dr Braga and colleagues in our article (1) on the effect of supplemental oxygen on CSF signal at FLAIR MR imaging. We acknowledge their contributions to understanding this phenomenon. To address their comment regarding the methodology of our study, we conducted a prospective trial with a crossover design with all of the children initially receiving 100% oxygen during the first FLAIR sequence and a reduction to 30% during the second FLAIR sequence. To account for the intermediate CSF signal intensity (isointense to brain) on images obtained with the second FLAIR sequence in some children, we hypothesized that the oxygen tension in the CSF did not necessarily have time to equilibrate with the blood by the time the second FLAIR sequence was performed. A more ideal study design would have been to divide the sample in two, with (a) half initially receiving 100% oxygen and afterward receiving a reduction to 30% for the second FLAIR sequence and (b) the other half initially receiving 30% oxygen and afterward receiving 100% for the second FLAIR sequence. This was not attempted, however, because 100% oxygen was always administered at some point during the induction of general anesthesia for the instrumentation of the airway with a laryngeal mask or an endotracheal tube. Since the children started at 100% oxygen were associated with induction, performing the first FLAIR sequence with 100% oxygen and then reducing it to 30% actually provided the longest oxygen washout time.
We do agree, considering this and prior studies (2,3), if clinically prudent in a potentially sick population, that at least marked oxygen-induced CSF hyperintensity on FLAIR images can be largely avoided with FIO2 of 50% or less.
References
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