Perfusion MRI using endogenous deoxyhemoglobin as a contrast agent: preliminary data

Background: The paramagnetic properties of deoxyhemoglobin shorten T2* as do gadolinium based contrast agents. Induction of abrupt changes in arterial deoxyhemoglobin concentration ([dOHb]) can simulate intra-vascular injections of gadolinium for perfusion imaging. Aim: To demonstrate the feasibility of making rapid changes in pulmonary venous hemoglobin saturation and employing the resulting changes in T2* to calculate flow metrics in the brain. Methods: A gas blender with a sequential gas delivery breathing circuit and software enabling prospective arterial blood gas targeting was used to implement rapid isocapnic lung changes in the partial pressure of blood oxygen (PaO2). Lung PO2 was initially lowered to induce a low baseline [dOHb]. PaO2 was then rapidly raised to PaO2 ~ 100 mmHg for 10 seconds and then rapidly returned to baseline. Blood oxygenation level dependent (BOLD) MRI signal changes were measured over time. Results: Arrival delay, signal amplitude and change in BOLD discriminated between large arteries, tissue and veins. The median half-time of BOLD signal in the middle cerebral artery was 1.7 s, indicating minimal dispersion confirming effective rapid modulation of pulmonary venous PO2. The contrast-to-noise ratio in the cortex was 3. Calculations of arrival delay, cerebral blood volume, mean transit time and cerebral blood flow were within normal ranges from published literature values. Conclusion: Non-invasive induction of abrupt changes in [OHb] may function as a novel non-invasive vascular contrast agent for use in perfusion imaging.