  The Blood-Brain Barrier is the gatekeeper of the CNS. It effectively shields
the brain from blood-borne harm but simultaneously represents a significant
challenge for treating neurological diseases. Altering its permeability enables
increasing the local drug concentration and thereby improving the therapeutic
effect. Although permeability increase is achieved by raised tissue
temperature, the determination of the thermal dosage suffers from imprecise
thermometry during hyperthermia application. Knowledge of the thermal dosage is
crucial for improving hyperthermia related interventions of the CNS. Here we
show an approach to determine the thermal threshold for localized Blood-Brain
Barrier disruption estimated by MR thermometry.
  Methods: Using an IR laser ({\lambda} = 1470nm) we showed that highly
localized Blood-Brain Barrier opening can be achieved with mild to moderate
hyperthermia. Non-invasive MR thermometry has been used to determine the
temperature at the heating site. Blood-Brain Barrier opening has been monitored
by DCE-MRI in vivo and post mortem via Evan's Blue extravasation. Results: The
Blood-Brain Barrier permeability can be altered locally with minimal thermal
dosages. Thus mild hyperthermia represents a promising approach to making the
brain accessible for therapeutic interventions.
