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AJNR - American Journal of Neuroradiology

A, CBF measured with dynamic susceptibility contrast-enhanced MR imaging (MR CBF, left) and CBF measured with [¹⁵O]-H₂O PET (PET CBR, right). Axial view MR image and PET scan are co-registered to the same location, filtered to the same spatial resolution as the PET scan, and displayed with the same color scale. Diffusely decreased CBF in the left hemisphere, ipsilateral to the occluded carotid artery, can be identified on both the MR image and PET scan, with more severely reduced CBF focally within a chronic infarct in the left frontal lobe.

B, Large regions of interest (gray circles) for CBF measurement are shown on MR image obtained in the hemisphere contralateral to the carotid occlusion. Homologous regions of interest were placed in the hemisphere ipsilateral to the occlusion (see Methods).

C, Small subcortical regions of interest (gray circles) in normal appearing white matter of the hemisphere contralateral to the carotid occlusion are shown on MR image. These subcortical white matter regions were used to determine normal white matter CBF for the purpose of scaling relative CBF values measured by MR imaging to physically meaningful units of mL/100 mL/min (see Methods).

A, Graph compares CBF values measured by MR imaging (MR CBF) with those measured by PET (PET CBF). CBF values measured by PET have been corrected for the finite permeability of water through the blood-brain barrier (see Methods). CBF values measured by MR imaging were calculated with AIFs from the proximal MCA, both ipsilateral to the occluded carotid artery (Ipsi AIF, open circles) and contralateral to the occluded carotid artery (Contra AIF, closed squares). The line of identity, with a slope of 1 and a y intercept of 0, is depicted by a solid line. The slopes of the regression line for the data from the ipsilateral AIF (dashed line) and for the data from the contralateral AIF (dotted line) are both close to 1 (Table 2). No statistically significant difference was observed in the slopes or y intercepts of the two regression lines (P > .05). The linear relationship between CBF values measured by MR imaging and those measured by PET is statistically significant for both the ipsilateral AIF (r = 0.88, P < .0001) and the contralateral AIF (r = 0.88, P < .0001). CBF values measured by PET in the subcortical white matter regions used in scaling the CBF values measured by MR imaging was 21.1 mL/100 mL/min, which is close to the assumed value of 22 mL/100 mL/min (see Methods).

B, Plots of the relaxivity R2* versus time for the AIF in the M1 or M2 segment of the MCA ipsilateral to the carotid occlusion (open circles), and that contralateral to the occlusion (closed squares), show that they are almost identical. R2* was determined as the negative log of susceptibility after baseline normalization. Because susceptibility is a relative measurement and not an absolute quantity, R2* is expressed in arbitrary units. The amplitude of the observed change in R2* is not quantitatively comparable between the ipsilateral and contralateral sides because of differences in factors such as vessel orientation and the extent of partial volume averaging of the blood vessel with surrounding tissue and/or CSF within the selected AIF pixels.

A, In a 65-year-old man with symptomatic left carotid occlusion (patient 7), CBF values measured by MR imaging systematically overestimated CBF values measured by PET (PET CBF). The linear regression between CBF values measured by MR imaging and those measured by PET has a slope of 1.65 for the ipsilateral AIF (Ipsi AIF, r = 0.79, P < .0001) and 2.14 for the contralateral AIF (Contra AIF, r = 0.81, P < .0001). This difference in slopes is statistically significant (P < .05). CBF value measured by PET in the subcortical white matter regions used in scaling the CBF values measured by MR imaging was 21.9 mL/100 mL/min, which is very close to the assumed value of 22 mL/100 mL/min.

B, AIFs in patient 7 show a delay in the peak of the ipsilateral AIF (open circles) and a broader peak of the ipsilateral AIF, compared with the contralateral AIF (closed squares).

A, Pooled data from all seven patients shows a statistically significant correlation between CBF values measured by MR imaging (MR CBF) and CBF values measured by PET (PET CBF), when CBF values measured by MR imaging are calculated with an AIF ipsilateral to the carotid occlusion (Ipsi AIF, open circles) (r = 0.60, P < .0001) and with an AIF contralateral to the occlusion (Contra AIF, closed squares) (r = 0.54, P < .0001). No significant differences were observed between the correlation coefficients, slopes, or y intercepts of the regression lines for the ipsilateral AIF (dashed line) and the contralateral AIF (dotted line). Both regression lines have slopes significantly <1 (P < .001) and y intercepts significantly >0 (P < .001). Relative MR CBF values were scaled to absolute units by assuming a normal white matter flow rate of 22 mL/100 mL/min (see Methods).

B, Use of white matter flow rates measured with PET for each patient to scale relative CBF values measured with MR imaging for each patient to absolute units improves the correlation between CBF values measured by MR imaging and those measured by PET, both for the ipsilateral AIF (open circles) (r = 0.85, P < .0001) and the contralateral AIF (closed squares) (r = 0.84, P < .0001). Again, no significant difference was observed between the correlation coefficients, slopes, or y intercepts of the regression lines for the ipsilateral AIF (dashed line) and the contralateral AIF (dotted line). However, both regression lines for PET-scaled CBF values measured by MR imaging versus CBF values measured by PET have slopes significantly >1 (P < .001) and y intercepts significantly <0 (P < .05).