Biomedical Engineering Reference
In-Depth Information
images can be interpreted as single unified data sets and conclusions drawn
with increased confidence. Creating this single unified data set is the process
of “fusion.” In many instances, new information becomes available that
could not have been deduced from inspection of individual images in loose
association with one another.
1.2
Historical Background
Although this is the first topic dedicated to medical image registration, it is
not a new topic. Image registration has been widely used for many years in
x-ray angiography. It is common to acquire x-rays before and after injection
of intravascular contrast and then subtract these images in order to visualize
the blood vessels in isolation. This technique almost invariably uses digital
systems now, but optical subtraction using photographic methods has been
extremely effective. A negative of the radiograph taken before the arrival of
the contrast material, the “mask,” was positioned on a light box over the
radiograph taken after the arrival of contrast and an additional film was
taken. If the patient moved between the acquisition of the precontrast mask
image and the image with contrast, then the subtracted image would contain
edge artifacts. Translating and rotating the films prior to optical subtraction
greatly reduced these artifacts. Photographic subtraction was also used with
MR to correct for patient motion and generate images showing where gado-
linium contrast had been taken up.
4
Image-guided surgery was the first application of medical image registration.
Indeed, the very first radiograph acquired for this purpose was reported to
have been in Birmingham, U.K., only two weeks after the discovery of x-rays
was published in December 1895. A patient had broken a needle in her hand.
A radiograph was taken and the casualty officer aligned the plate with the
hand in order to successfully guide his scalpel to removal of the needle.
5
Other early examples included battlefield surgery for removal of shrapnel by
registering a calibrated pair of x-ray films to the patient so that the x-ray
could guide the surgeon precisely to the target in 3D. The stereotactic frame
was proposed for image-guided neurosurgery as a means of localizing target
structures with respect to anatomical features identified in the patient's
radiographs and
6,7
The frame is rigidly fixed to the skull
and defines a coordinate system for both imaging and treatment. Stereotactic
neurosurgery became more widely used when the technology was comput-
erized and combined with CT,
or a standard atlas.
8,9
and then with multiple preoperative imag-
10
ing modalities.
Stereotactic neurosurgery can only be used for a small
proportion of neurosurgical procedures, because the frame has to be attached
to the patient prior to imaging and left on until surgery, and the presence of
the frame restricts the types of surgery that can be performed, often just to
biopsy and electrode implantation. These problems were overcome with the
