Introduction - Medical Image Registration - page 2

of the blood-brain barrier. In nuclear medicine, radiopharmaceuticals intro-
duced into the body allow delineation of functioning tissue and measurement
of metabolic and pathophysiological processes. Ultrasound detects subtle
changes in acoustic impedance at tissue boundaries and diffraction patterns
in different tissues, providing discrimination of different tissue types. Doppler
ultrasound provides images of flowing blood. Endoscopy and surgical
microscopy provide images of visible surfaces deep within the body. These
and other imaging technologies now provide rich sources of data on the phys-
ical properties and biological function of tissues at spatial resolutions from 5
mm for nuclear medicine down to 1.0 or 0.5 mm for MR and CT, and 20 to 100

m for optical systems. Each successive generation of image acquisition sys-
tem has acquired images faster, with higher resolution and improved image
quality, and together these have been harnessed for great clinical benefit.
Since the mid 1980s medical image registration has evolved from being
perceived as a rather minor precursor to some medical imaging applications
to a significant subdiscipline in itself. Entire sessions are devoted to the topic
in major medical imaging conferences,

1,2

and workshops have been held on

3

the subject. Image registration has also become one of the more successful

areas of image processing, with fully automated algorithms available in a
number of applications.
Why has registration become so important? Medical imaging is about
establishing shape, structure, size, and spatial relationships of anatomical
structures within the patient, together with spatial information about function
and any pathology or other abnormality. Establishing the correspondence of
spatial information in medical images and equivalent structures in the body
is fundamental to medical image interpretation and analysis.
In many clinical scenarios, images from several modalities may be acquired
and the diagnostician's task is to mentally combine or ''fuse'' this information to
draw useful clinical conclusions. This generally requires mental compensation
for changes in subject position. Image registration aligns the images and so
establishes correspondence between different features seen on different imag-
ing modalities, allows monitoring of subtle changes in size or intensity over
time or across a population, and establishes correspondence between images
and physical space in image guided interventions. Registration of an atlas or
computer model aids in the delineation of anatomical and pathological struc-
tures in medical images and is an important precursor to detailed analysis.
It is now common for patients to be imaged multiple times, either by
repeated imaging with a single modality, or by imaging with different modal-
ities. It is also common for patients to be imaged , that is, to have

sequences of images acquired, often at many frames per second. The ever
increasing amount of image data acquired makes it more and more desirable
to relate one image to another to assist in extracting relevant clinical informa-
tion. Image registration can help in this task: intermodality registration
enables the combination of complementary information from different
modalities, and intramodality registration enables accurate comparisons
between images from the same modality.

dynamically