Less invasive surgical techniques for performing brain surgery are disclosed in which a dilating obturator and cannula assembly is inserted into brain tissue until the obturator tip and cannula are adjacent the target tissue. The obturator is removed and surgery is performed through the cannula. In preferred embodiments the obturator and cannula are placed using image guidance techniques and systems to coordinate placement with pre-operative surgical planning. A stylet with associated image guidance may he inserted prior to insertion of the obturator and cannula assembly to guide insertion of the obturator and cannula assembly. Surgery preferable is performed using an endoscope partially inserted into the cannula with an image of the target tissue projected onto a monitor.