spine surgery
Thorasic Spine: As a general rule the starting point for insertion of Pedicle screws is the point " where the superior border of the transverse process converges with the lamina along the lateral border of the articular facet. this point is typically in line with or just medial to the pars.
Lumbar Spine: The starting point is at the junction of the lateral facet and the transverse process - taken from Spine Surgery
The other way is "Bisection of a vertical line through the facet joints and a horizontal line through the transverse process can also serve as a useful landmark for lumbar pedicle entry site".