AbstractID: 7916 Title: Transformation of shot coordinates for repeated application of
Leksell head frame for volume staged treatment of large arteriovenous malformations
This paper describes a technique for transforming shot coordinates between two
consecutive treatments for volume staged radiosurgical treatment of large arteriovenous
malformations (AVM). There are usually two fractions given from 12 to 18 months apart.
The second fraction is given based on a new angiographic examination and it targets a
still active AVM nidus. This creates a need to superimpose the first fraction dose
distribution on the second fraction dose distribution in order to minimize the region of the
dose overlap and ensure that the possible overlap is confined to the treated nidus.
Because the placement of the Leksell head frame changes from fraction to fraction there
is a need to determine a transformation between two different coordinate systems. We
have developed a simple and reasonably accurate transformation technique using MRI
studies, which are acquired for AVM procedures to complement the traditional bi-planar
X-ray angiograms. The set of five or more corresponding anatomical landmarks is
defined using the old and new MRI studies and input to the least-square fitting algorithm
to determine the transformation matrix from the old to the new coordinate system. Once
the transformation matrix is determined the treatment shots coordinates from the first
fraction can be transformed to the coordinate system of the second head frame
application. For the 0.5 x 0.5 x 1.0 mm MRI slices the registration error has been
estimated to be around 2.5 mm. The registration accuracy can be in one-millimeter range
for careful selection and placement of the landmarks