The model presented in Chapter 2 did not discuss the
fact that the transformation 
may be rapidly time varying
after the opening of the skull. Leakage of CSF and the surgical
intervention itself are certainly factors causing the shape of the
patient's anatomy to change during the time the surgery is performed.
Several approaches have been proposed to perform a real-time
monitoring of anatomical changes during the surgery. Among others are
interventional MR imaging [50,32], ultrasound
[35], video [26]. These techniques will
themselves present challenging accuracy issues.