Absence of Selective Deep White Matter Ischemia in Chronic Carotid Disease: A Positron Emission Tomographic Study of Regional Oxygen Extraction
Colin P. Derdeyn
,a,
Nicholas R. Simmonsa,
Tom O. Videena,
Kent D. Yundta,
Susanne M. Fritscha,
David L. Carpentera,
Robert L. Grubb Jr.a and
William J. Powersa
a From the Neuroradiology Section (C.P.D., N.R.S.) and the Division of Radiological Sciences (C.P.D., N.R.S., T.O.V., K.D.Y., S.M.F., D.L.C., R.L.G., W.J.P.), Mallinckrodt Institute of Radiology, and the Department of Neurology and Neurological Surgery (T.O.V., K.D.Y., S.M.F., D.L.C., R.L.G., W.J.P.) and The Lillian Strauss Institute of the Jewish Hospital of St. Louis (W.J.P.), Washington University School of Medicine, St. Louis, MO.
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FIG 1. Positron emission tomographic studies show reduced cerebral blood flow (left image) and increased oxygen extraction (middle image) in the cerebral hemisphere distal to an occluded carotid artery. When autoregulatory vasodilation is insufficient to maintain normal cerebral blood flow (CBF, far-left image), CBF will fall (white arrows). The brain, however, can increase the amount of oxygen extracted from the blood (oxygen extraction fraction) (middle image, white arrows), to maintain normal oxygen metabolism and function (cerebral rate of metabolism for oxygen [CMRO2], far-right image). FIG 2. Regions of interest for superficial (black boxes) and deep white matter (black circles) oxygen extraction fraction measurements. Each region was actually a 15-mm sphere with its center in the center of the box or circle depicted in the figure. All positron emission tomographic data were converted to Talairach and Tournoux atlas space for the reproducible placement of these regions (18). The y coordinates for each coronal slice are shown beneath
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FIG 3. Range of ipsilateral deep white matter:superficial oxygen extraction fraction ratios for all patients (n = 36) compared with values from their contralateral hemispheres (n = 36) and from healthy volunteers (15 volunteers, 30 hemispheres) is very similar. The ratio of deep white matter:superficial oxygen extraction fraction is shown on the y axis and ratios of the three groups of volunteers are shown on the x axis. The boxes indicate the 95% confidence limits for the oxygen extraction fraction ratios. The lines within the boxes indicate the median values, and the bars above and below the boxes indicate the 99% confidence limits. Patients with oxygen extraction fraction ratios beyond the 99% confidence limits are represented with black circles.
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FIG 4. Positron emission tomographic and CT scans.
A, Example of subjects with normal (left) and increased (right, white arrows) oxygen extraction fraction on positron emission tomographic sections at the level of the centrum semiovale. Left and right on positron emission tomographic scans are reversed relative to CT or MR convention. Note the uniformity of oxygen extraction fraction values across the image on the left, compared with the hemispheric increase in oxygen extraction fraction seen on the image on the right. Oxygen extraction fraction values in the centrum semiovale are not quantitatively or qualitatively greater than those in the more superficial regions (white arrows).
B, Corresponding CT scan of patient 30 (Table 1). This patient presented with two episodes of profound left-sided weakness that occurred 10 days apart. These symptoms had nearly completely resolved except for a mild pronator drift at the time of the CT and positron emission tomographic examinations. Angiography showed complete occlusion of the right internal carotid artery at its origin and moderate 25% to 50% stenosis of the contralateral carotid siphon.
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FIG 5. Range of deep white matter:superficial oxygen extraction fraction ratios for the nine patients with high oxygen extraction fraction in superficial regions compared with their contralateral hemispheres and with normal hemispheres is similar. Two patients had deep white matter:superficial oxygen extraction fraction ratios beyond the range seen in the contralateral hemispheres but within the normal range. As with figure 2, the ratio of deep white matter:superficial oxygen extraction fraction is shown on the y axis and the ratios of the three groups of volunteers are shown on the x axis. The boxes indicate the 95% confidence limits for the oxygen extraction fraction ratios. The lines within the boxes indicate the median values, and the bars above and below the boxes indicate the 99% confidence limits. Patients with oxygen extraction fraction values beyond the 99% confidence limits are represented with black circles.
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