Title: Aligning 2D and 3D Multimodality Breast Images

Purpose: The purpose of this work is to describe a framework in which
breast images from MRI, PET, Ultrasound, and Digital Mammography
can be registered (aligned).

Materials and Methods: Prior to image acquisition, an alignment grid
is drawn on the breast skin using a marking pen.  In general, prior to image
acquisition, modality specific markers (vitamin E for MRI, FDG for PET,
and standard lead markers for Digital Mammography) are placed at the
indicated grid points.  Images are then acquired with the external markers
appearing on the images.  In the case of PET, diagnostic images are first
acquired in the absence of markers, then the cameras are turned off,
next the markers are put in place, and finally the cameras are turned back
on.  Ultrasound does not use any markers but uses a position tracking
device which is attached to the probe and is calibrated to the grid.

Results: Given the similarity between pre- and post-contrast breast MRI,
it is not surprising that their alignment is most accurate.  Although 2D, by
tracking the position of the Ultrasound probe with respect to the external
markers during image acquisition, Ultrasound images may be aligned as well.
Using the external marker system, PET images may be aligned as well but the
lack of internal anatomical information inherent in PET images reduces the
accuracy of the subsequent registration.  Differences in patient positioning
during image acquisition (supine for PET and prone for MRI) pose problems
as well.  In the future we will use a dedicated breast PET scanner which
will dramatically increase the accuracy of PET-MRI registration (because 
patient positioning will be similar to that of MRI).  Digital Mammography
to MRI poses the greatest challenge because: 1) Digital Mammography is a 
2D projection image of the 3D breast, and 2) the breast is subjected to
compression during the imaging procedure.

Conclusion: With varying degrees of accuracy, one may register images from
a variety of modalities using an external marker system.