Description
Image registration is the process of aligning images so that corresponding features can easily be related. The term is also used to mean aligning images with a computer model or aligning features in an image with locations in physical space. The images might be acquired with different sensors (e.g., sensitive to different parts of the electromagnetic spectrum) or the same sensor at different times. Image registration has applications in many fields; the one that is addressed in this book is medical imaging. This encompasses a wide range of image usage, but the main emphasis is on radiological imaging.
The past 25 years have seen remarkable developments in medical imaging technology. Universities and industry have made huge investments in inventing and developing the technology needed to acquire images from multiple imaging modalities. Medical images are increasingly widely used in healthcare and biomedical research; a very wide range of imaging modalities is now available. X-ray computed tomography (CT) images are sensitive to tissue density and atomic composition, and the x-ray attenuation coefficient and magnetic resonance imaging (MR) images are related to proton density, relaxation times, flow, and other parameters. The introduction of contrast agents provides information on the patency and function of tubular structures such as blood vessels, the bile duct, and the bowel, as well as the state of the blood-brain barrier. In nuclear medicine, radiopharmaceuticals introduced
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 physical 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 system 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 the subject. 3 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 imaging 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 structures in medical images and is an important precursor to detailed analysis.
Contents:
Section I Methodology
Section II Applications of Rigid Body Registration
Section III Techniques and Applications of Nonrigid Registration