European Nuclear Medicine Guide
If we think of all the medical imaging modalities that are available today, they all take advantage of particular features of the tissue and mode of generating the imaging signal. The pictograms in the figure below give a visual overview of the various basic technologies being used to generate medical images together with an example.
Figure 1. Major medical imaging modalities as used in the clinical routine: standard planar X-ray, planar scintigraphy, positron emission tomography (PET), Ultrasound (US), (x-ray) computed tomography (CT), single photon emission computed tomography (SPECT), magnetic resonance (tomographic) imaging (MRI) and optical imaging (OI).
Moreover, all of those modalities are particularly superior to others in specific features of the generated signal. The table below compares the most interesting features and an overview of the preferred modality for a given target.
As previously described, each imaging method has its advantages and disadvantages, so when selecting a medical imaging modality, one should choose the method best suited to answer the question being asked. Obviously, one of the main goals is to image the structure of the body and its physiological circumstances as well as its alterations in case of benign or malign diseases. Structural imaging modalities, however, often are not specific and/or sensitive enough on a functional level, that is, in providing information about the physiology or pathophysiology of the body on a molecular level. For a comprehensive and as complete as possible visualization and quantification of variations in structure and in the functional and metabolic behaviour of diseases on a cellular and molecular level, the meaningful combination of some of the available medical imaging modalities seems to be desirable. Beginning in the early nineties, the first prototypes of combined SPECT and CT systems were investigated and proposed[127]. In the late nineties and at the beginning of the new millennium, combined PET and CT systems pioneered by Townsend and Beyer entered the clinical arena, and now, with over 6000 installations worldwide, these are one of the major successes in multimodality imaging equipment[70,128,129]. About ten years later, equipment combining PET and MRI became feasible, and today it has evolved to the state where it is available in about 200 sites worldwide. Some companies have combined more than 2 medical imaging modalities to create very versatile systems. However, despite the packaging of three modalities in one system with a unique patient handling system/bed at hand, the imaging still has to be done sequentially. This is one reason why tri-modality systems have not advanced beyond the proof of principle stage at some singular sites. Combinations of two modalities focusing mainly on combining structural and functional imaging have gained the most acceptance. This is why we speak of “Hybrid Imaging” today if cross- and multimodality imaging is meant. More details about the principles and utility of the most common hybrid imaging modalities can be found in the chapters below.