fMRI_Intro

HOW FMRI WORKS IN ONE SLIDE! Blood Oxygen Level Dependent (BOLD) Contrast • Oxyhemoglobin: diamagnetic; Deoxyhemoglobin: paramagnetic • Paramagnetic  Susceptibility  T2* signal loss (e.g. DSC perfusion) • Brain activity  demand for O2  regional CBF  (“Neurovascular coupling”  “Hemodynamic response”) • CBF  Oxy:Deoxy Hgb (O2 supplied in excess)  T2* signal  • Signal change small (~3%) – not visible – need stats to ID active regions • Acquire T2* image time series while activity repeatedly cycles on/off • Examine T2* signal-vs-time voxel by voxel – look for expected cycling • Cycling present or not? Question of statistical likelihood (with a p-value!)

• Generate topographical map of likelihood and apply threshold (e.g. p<0.05) • Voxels exceeding threshold fill in with color, make the rest invisible • Overlay onto anatomy

QA in fMRI • Things you cannot control (but need to check): • Did the patient attend to the task and perform it adequately?

• Was head motion excessive or highly synchronized with the task? (False +) • Is neurovascular coupling intact? (no BOLD effect without it! False –) • Are there susceptibility artifacts in the region of interest? (False –)

• Things you do control: • Is the statistical threshold set too high or too low? (More art than science) • Is the anatomical underlay appropriate for the lesion? (T1, T2, T2-FLAIR) • Is the anatomical underlay windowed and leveled appropriately?

• Is the DTI opacity optimized for both lesion and tract visibility?

• All 3 planes sent to PACS?