title: The effect of hypercarbia and hyperoxia on the total blood flow to the retina as assessed by magnetic resonance imaging.
abstract: OBJECTIVE
  The feasibility of measuring total blood flow to the retina with Arterial Spin Labeling Magnetic Resonance Imaging (ASL-MRI) has been described previously. In the present study, the hypothesis was that the reactivity that the ASL-MRI detects at the human retina is dominated by the choroidal blood flow, and thus it may serve as a useful tool for quantitative assessment of the choroidal vascular reactivity.
  
  
  METHODS
  Before imaging, the intraocular pressure (IOP) was measured in the study eye of nine clinically healthy subjects (four males) while the subject performed the ventilatory protocol subsequently imaged by the scanner. End-tidal CO₂ partial pressure (P(ET)CO₂) was increased to target 45 mm Hg, (baseline P(ET)CO₂ = 40 mm Hg and baseline P(ET)O₂ = 100 mm Hg). P(ET)O₂ was then increased to target 300 and 500 mm Hg while keeping P(ET)CO₂ constant at 45 mm Hg. A background-suppressed, pulsed-continuous ASL sequence was used for blood flow imaging.
  
  
  RESULTS
  The measured total blood flow increased significantly from 1.55 ± 0.17 μL/mm²/min at the baseline to 1.96 ± 0.18 μL/mm²/min during hypercarbia. With increasing P(ET)O₂, the measured blood flow did not change significantly relative to the hypercarbia condition but remained significantly elevated relative to the baseline. There were no significant changes in systolic, diastolic, or mean blood pressure, heart rate, or IOP during all four breathing conditions.
  
  
  CONCLUSIONS
  The lack of change in the ASL signal under hyperoxic conditions is consistent with the hypothesis that this noninvasive assessment technique is predominantly weighted by choroidal blood flow. The results indicate that a CO₂ provocation challenge, in combination with ASL-MRI, is a promising noninvasive approach for investigating choroidal vascular reactivity under normal and disease states.
authors: Department of Radiology, Children's Hospital Boston and Harvard Medical School, Boston, Massachusetts, USA. nasim.maleki@childrens.harvard.edu
authors: Nasim
authors: N
authors: Maleki
authors: David C
authors: DC
authors: Alsop
authors: Weiying
authors: W
authors: Dai
authors: Christopher
authors: C
authors: Hudson
authors: Jay S
authors: JS
authors: Han
authors: Joe
authors: J
authors: Fisher
authors: David
authors: D
authors: Mikulis
mesh_terms: D000328:Adult; D002245:Carbon Dioxide / Q000097:blood; D002829:Choroid / Q000098:blood supply; D005240:Feasibility Studies; D005260:Female; D005500:Follow-Up Studies; D006801:Humans; D006935:Hypercapnia / Q000097:blood / Q000175:diagnosis / Q000503:physiopathology*; D018496:Hyperoxia / Q000097:blood / Q000175:diagnosis / Q000503:physiopathology*; D008279:Magnetic Resonance Imaging / Q000379:methods*; D008297:Male; D010100:Oxygen / Q000097:blood; D012039:Regional Blood Flow / Q000502:physiology*; D015203:Reproducibility of Results; D012171:Retinal Vessels / Q000473:pathology / Q000503:physiopathology*; D014655:Vascular Resistance / Q000502:physiology*
chemical_list: D002245:Carbon Dioxide; D010100:Oxygen