MRI Estimation of Global Brain Oxygen Consumption Rate

1
MRI Estimation of Global Brain Oxygen Consumption
Rate
V. Jain , M. Langham , J. Magland and F. W.
Wehrli Laboratory for Structural NMR Imaging ,
Department of Radiology, University of
Pennsylvania, Philadelphia, PA

• Introduction
• Measuring the cerebral metabolic rate of oxygen
  (CMRO2) is a valuable tool for monitoring acute,
  severely brain injured patients.
• Measurement of oxygen saturation and flow in the
  major cerebral inflow and outflow vessels can
  provide a global estimate of cerebral metabolic
  rate of oxygen (CMRO2).
• The internal carotid and the vertebral arteries
  are the predominant inflow vessels while the
  internal jugular veins (IJV) are the primary
  route for cerebral venous drainage.
• The current gold standard for quantifying CMRO2
  is by internal jugular bulb oximetry involving
  catheterization 1. However, this method is
• Invasive
• Prone to complications
• Accuracy of the method is highly dependent on the
  catheter position
• Simultaneous measurement of both the left and
  right sides is impractical but important due to
  asymmetric venous drainage
• MRI can be used to non-invasively estimate CMRO2
  by quantifying blood oxygen saturation level
  (HbO2) 2,3 and blood flow 4.
• Previously, T2-based 5 MR oximetry methods have
  been used for estimating blood oxygen saturation
  levels. However, MR susceptometry-based oximetry
  has several advantages
• Individual calibration is not needed
• Relatively straightforward implementation
• Equal sensitivity to all oxygenation levels
• Excellent temporal (5 s) and spatial resolution
  (1 mm2)

• In the flow sequence, the two-step velocity
  encoding is also interleaved. A short TR (20 ms)
  was used to resolve the flow profile and
  acquisition window was set to 120 of average
  RR-interval total scan time 4 mins.
• Two VENCs (80 cm/s and 40 cm/s) were used lower
  VENC improves velocity-to-noise ratio for slower
  flow rate during the diastole in the vertebral
  and carotid arteries.
• CMRO2 was derived as the product of
  arterial-jugular venous difference in oxygen
  content (ADVO2 in mL/100mL blood) and cerebral
  blood flow (CBF in mL/100g/min).
• Written informed consent was obtained prior to
  all human studies following an institutional
  review board-approved protocol.
• Results
• Sample magnitude, phase difference and flow
  contrast images are shown in Figure 2 and the
  corresponding time-resolved blood flow
  quantifications in the left and right internal
  carotid and vertebral arteries are shown in
  Figure 3.
• Preliminary results collected in two healthy
  human subjects, a 36 year old male and a 23 year
  old female, are consistent with the literature
  8.
• The total measured arterial inflow and the
  estimated global CMRO2 was 720 mL/min and 3.9
  mL/100g/min, respectively for the male volunteer
  and 780mL/min and 2.4mL/100g/min for the female
  volunteer.
• The above CMRO2 values lie within the range of
  2.9 4.9 mL/100g/min for males and 1.9 3.9
  mL/100g/min for females as cited in literature.

40 cm/s
1.3 rad
A
Objective To demonstrate the potential of MRI
susceptometry to estimate the global cerebral
metabolic rate of oxygen consumption by
quantifying HbO2 and global CBF.
B
C
0
0
-40 cm/s
-1.3 rad

• Materials and Methods
• MR susceptometry-based oximetry uses a
  phase-mapping pulse sequence to quantify the
  relative susceptibility difference ?? between
  intravascular blood and the surrounding tissue
  that serves as a calibration free reference.
• Relative susceptibility difference, ??? (1
  HbO2) where HbO2 is the volume fraction of
  oxygenated hemoglobin (Hb).
• Local field shift due to ?? can be measured from
  a phase difference image ?? ??B??TE, where ?B
  a ??.
• Velocity (magnitude and direction) of moving
  spins can be quantified by measuring the phase (?
  ?v/VENC) of the MR signal that is encoded
  (user input parameter denoted by VENC) with an
  appropriate gradient pulse.
• The velocity encoding (VENC) parameter
  corresponds to the maximum phase accumulation or
  maximum speed of the moving spins before the
  phase begins to wrap around on itself (phase
  ranges from -? to ?).

(a)
(b)
(c)
Figure 2 (a) Representative axial magnitude image
(A Right internal carotid artery, B Right
internal jugular vein, C Right vertebral
artery) along with the corresponding (b) phase
image used for estimating oxygen saturation
levels and (c) velocity image (obtained from a
different axial slice).
RF
RF
Gz
Interleave 1
I
Gy
Gz
Gx
II
ADC
?TE
Gy

• Conclusion
• In conclusion, MRI-based estimation of CMRO2 is
  feasible.
• The methods potential requires larger sample
  size and further studies analyzing the precision
  and reproducibility of the measurements.
• Techniques to enhance the contrast between
  venous intravascular blood and the surrounding
  tissue will be explored.
• The use of radial acquisition for flow
  quantification will be implemented to reduce the
  pulsatile artifact (due to phase modulation
  caused by variation in heart rate) that can
  interfere with the phase measurement.

Interleave 2
Gx
ADC
(a)
Etc.
(b)
Figure 1(a) Multi-echo spoiled GRE sequence with
fat saturation and flow compensation along the
slice direction. Phase difference images were
constructed between successive interleaved echoes
separated by ?TE 1.16 ms (b) Spoiled GRE
sequence with velocity encoding (VENC) along the
slice direction. Two-step flow encodings I and II
were interleaved.

• References
• De Deyne et al., Jugular Bulb Oximetry Review On
  A Cerebral Monitoring Technique. Acta Anaesth.
  Belg. 1998 4921
• Haacke et al., In Vivo Measurement of Blood
  Oxygen Saturation Using Magnetic Resonance
  Imaging A Direct Validation of the Blood Oxygen
  Level-Dependent Concept in Functional Brain
  Imaging Human Brain Mapping, 1997. 5 p. 341-346.
• Fernández-Seara et al., MR susceptometry for
  measuring global brain oxygen extraction. Magn
  Reson Med, 2006. 55(5) p. 967-73.
• Lotz et al., Cardiovascular Flow Measurement with
  Phase-Contrast MR Imaging Basic Facts and
  Implementation. Radiographics, 2002. 22651 -
  671.
• Wright et al., Estimating Oxygen Saturation of
  Blood in Vivo with MR Imaging at 1.5 T. J Magn
  Reson Imaging, 19911275 283.
• Magland et al., Pulse Programming in a dynamic
  visual environment. 2006 Seattle, WA. P. 578.
• Langham et al., Retrospective Correction for
  Induced Magnetic Field Inhomogeneity in
  Measurements of Large-Vessel Hemoglobin Oxygen
  Saturation by MR Susceptometry. Magn Reson Med,
  In Press.
• Bateman , The Pathophysiology of Idiopathic
  Normal Pressure Hydrocephalus Cerebral Ischemia
  or Altered Venous Hemodynamics?. Am J Radiol
  2008 29(1)98-102.
• Acknowledgment This research was supported by a
  HHMI-NBIB grant and NIH T32 EB000814.