What is the critical difference between frame-based and frameless stereotactic procedures?

B. The use of rendered three-dimensional images and a three-dimensional digitizer

DISCUSSION: Frame-based and frameless procedures both use digitized imaging studies as the basis for converting the scan coordinate system into a treatment coordinate system. Both types of procedures also require rigid fixation of the patient's head to the operating room table and can be performed in the presence or absence of a lesion. The critical difference is the use of a rendered, three-dimensional image and the three-dimensional digitizer, which together allow the alignment to be generated between the patient's imaging studies and the patient; this alignment occurs in frame-based stereotactic procedures because of the imaging study performed after the frame is applied.