Assessing brain hemodynamics using Arterial Spin Labeling MRI.

In vascular brain diseases vasodilatation of the vessels and increased oxygen
extraction are important to maintain homeostasis and are correlated with the
severity of the disease. In order to obtain full knowledge about brain tissue
hemodynamics it is important to obtain a measurement of these two parameters.

With this study we want to measure the two parameters who determine the
hemodynamic status of the brain tissue. The first parameter we want to
determine is the Cerebrovascular Reactivity (CVR), which is a measure for the
degree in which blood vessels can dilate. The second parameter is the Cerebral
Metabolic Rate of Oxygen (CMRO2), which is a reflection of the oxygen-use in
brain tissue.

To measure the Cerebrovascular Reactivity we will use a respiratory challenge
(RespirAct TM), this because it is well known that an increase in arterial
blood gas CO2 within physiological values causes dilatation of the brain
vessels. This dilatation is reflected in an increase in brain perfusion.
Arterial Spin Labeling (ASL) Magnetic Resonance Imaging (MRI) will be used to
evaluate this increase in brain perfusion. Furthermore we will apply a Velocity
Selective Spin Labeling MRI sequence which selectively labels the blood in the
venous compartment and permits us to assess oxygen-use (CMRO2) in brain tissue.

All volunteers and patients will be exposed to step wise respiratory challenges
with a maximum of 4 minutes of end tidal CO2 pressure (= arterial pressure) of
50 mmHg CO2 and minimum O2 level of 100 mmHg (= normal end-tidal arterial O2
pressure). These CO2 and O2 levels are within physiological ranges.

The controlled gas breathing requires a closed breathing system. Therefore,
subjects have to breath through a mask, which might be a bit uncomfortable. CO2
end-tidal levels of 50 mmHg can induce an increased breathing frequency due to
physiological stress. However, end-tidal levels of 50 mmHg CO2 is within the
physiological range and is experienced repeatedly by most people over the day
(eg. during exercises). If patients experience discomfort they can open a valve
in the mask or we can smitch immediately to 100% oxygen by pushing the red
button on the front of the gas blender. Volunteers and patients can also always
squeeze the panic button in the MRI. Risks associated with controlled gas
breathing of high levels of CO2 are minimized because the minimum O2 level in
the gasses is 10%. Breathing frequency, CO2- and O2-levels are continuously
digitally analysed and monitored by the RespirAct TM device. An independent
blood oxygen saturation monitoring will be performed during the MRI
measurements with fingertip pulse oxymetry.