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Effect of Vascular Radioactivity on Regional Values of Cerebral Blood Flow: Evaluation of Methods for H₂¹⁵O PET to Distinguish Cerebral Perfusion from Blood Volume

McConnell Brain Imaging Center, Montreal Neurological Institute, McGill University, Montreal, Canada; and PET Unit, Research Institute, Shiga Medical Center, Moriyama, Japan

View larger version (12K): [in a new window]   FIGURE 1. Two models of compartment analysis used in this study. One-compartment model (A) in conventional H215O PET studies includes radioactivity from both brain tissue and vascular volume, which are calculated separately in 2-compartment analysis. (B) One-compartment model: Ca, radioactivity level in intravascular space; f, blood flow (mL/min/100 g); Ct, tissue radioactivity; , partition coefficient; , decay constant for H215O. Two-compartment model: K1 (mL/g/min) and k2 (min-1), rate constants; Me, radioactivity level in extravascular space.

View larger version (53K): [in a new window]   FIGURE 2. Two slice-level images of rCBF in baseline condition obtained by 2 calculation methods. Images for 12 subjects were averaged after transformation into Talairach space. Left column shows rCBF images calculated by 2-WI method, and right column shows those calculated by 3-WI method. Images show large differences between (A) 1- and (B) 2-compartment models in rCBF values in large vessels and bilateral insula, which is considered affected by middle cerebral arteries in Sylvian fissure. L and R indicate left and right sides of brain.

View larger version (23K): [in a new window]   FIGURE 3. Pixel-by-pixel comparison of values from averaged rCBF images between 2 kinetic models using same slice levels as in Figure 2 (levels in internal carotid artery (A) and basal ganglia (B)). rCBF values in brain tissue corresponded well between 2 methods in most regions. Discrepancy between methods was greatest in regions located close to large vessels, resulting in much lower correlation between methods seen in left graph (A). Areas of bilateral insula and temporal lobes affected by vascular radioactivity in Sylvian fissure also showed overestimation in right graph (B) when using 1-compartment model. Line is a line of identity.

View larger version (69K): [in a new window]   FIGURE 4. SPM results comparing rCBF obtained by 1- (2-WI) and 2- (3-WI) compartment methods in baseline condition. Major arteries supplied from bilateral ICA and surrounding areas were expressed, suggesting that arterial phase of tracer was seen as difference. R indicates right side of brain.

View larger version (38K): [in a new window]   FIGURE 5. Differences in rCBF calculated from data after eliminating initial frames and from entire PET frames using autoradiographic method. SPM was used to determine location affected by elimination. Areas of significant difference in comparison were similar to those shown in Figure 4 when initial 30–40 s of frames were eliminated from whole set of data. R indicates right side of brain.

View larger version (52K): [in a new window]   FIGURE 6. Regional differences in rCBF between 2 conditions in activation study were analyzed using SPM. Results from 2-WI (A) and 3-WI (B) were similar in visual activation study. Location of peak Z values are (x,y,z) = (8,-88,10; Z = 7.97), and (12,-88,12; Z = 7.32) for 2- and 3-WI methods, respectively. V0 comparison also showed a significant increase in blood volume in visual cortex and surrounding areas in visual stimulation (C) ((x,y,z) = (6,-78,12); Z = 4.79). Note that areas expressed as significant difference in V0 comparison (C) expanded out of brain because V0 image includes superficial vascular volume of brain. R indicates right side of brain.