MRI estimation of global brain oxygen consumption rate

Abstract

Measuring the global cerebral metabolic rate of oxygen (CMRO(2)) is a valuable tool for assessing brain vitality and function. Measurement of blood oxygen saturation (HbO(2)) and flow in the major cerebral outflow and inflow vessels can provide a global estimate of CMRO(2). We demonstrate a rapid noninvasive method for quantifying CMRO(2) by simultaneously measuring venous oxygen saturation in the superior sagittal sinus with magnetic resonance susceptometry-based oximetry, a technique that exploits the intrinsic susceptibility of deoxygenated hemoglobin, and the average blood inflow rate with phase-contrast magnetic resonance imaging. The average venous HbO(2), cerebral blood flow, and global CMRO(2) values in eight healthy, normal study subjects were 64%+/-4%, 45.2+/-3.2 mL per 100 g per minute, and 127+/-7 micromol per 100 g per minute, respectively. These values are in good agreement with those reported in literature. The technique described is noninvasive, robust, and reproducible for in vivo applications, making it ideal for use in clinical settings for assessing the pathologies associated with dysregulation of cerebral metabolism. In addition, the short acquisition time (approximately 30 seconds) makes the technique suitable for studying the temporal variations in CMRO(2) in response to physiologic challenges.

Figure 1

Interleaved 2D gradient-recalled echo sequence alternating between the two anatomic locations (Gx=frequency-encoding gradient, Gy=phase-encoding gradient, Gz=slice-selection gradient). All but the fourth interleave are flow compensated along the slice-select direction. The fourth interleave is flow encoded with a Velocity ENCoding (VENC)=60 cm/second. The echo time difference (ΔTE) between the first and third interleaves is 2.5 milliseconds (TE=7.025 milliseconds). The dashed boxes represent sequence parameters that changed between the interleaves.

Figure 2

Phase-difference images of the cross-sectional area of the phantoms containing (A) 1 mmol/L, (B) 1.25 mmol/L, (C) 1.5 mmol/L, and (D) 1.75 mmol/L Gd-DTPA in distilled water.

Figure 3

(A) Axial magnitude image of mouth orifice highlighting the major inflow vessels (internal carotid and vertebral arteries; red arrows); (B) corresponding velocity map used for the calculation of total cerebral blood flow rate.

Figure 4

(A) Sagittal, (B) coronal, and (C) axial magnitude images of the brain highlighting the superior sagittal sinus (red arrow). Note that the sinus is fairly straight; the highlighted red line represents the imaging slice location. Zoomed in axial (D) magnitude and (E) phase image of the superior sagittal sinus.

Figure 5

(A) Scatter plot of the cerebral metabolic rate of oxygen (CMRO₂) for each subject over three scanning sessions illustrating reproducibility. The vertical span of each diamond represents the 95% confidence interval (CI) for each group. (B) Correlation between AVO₂D and total cerebral blood flow (CBF) in the eight subjects (R²=0.74, P<0.006). Note that subjects with higher CBF values tend to have lower AVO₂D.