A general comparison of functional imaging in nuclear medicine with other modalities.

New (noninvasive) diagnostic procedures in medicine (ultrasound [US], digital subtraction angiography [DSA], computed tomography [CT], nuclear magnetic resonance [NMR]) create a need for a review of the clinical utility of functional imaging in nuclear medicine. A general approach that is valid for all imaging procedures is not possible. For this reason, an individual assessment for each class of functional imaging is necessary, taking into account the complexity and sophistication of the various imaging procedures. This leads to a hierarchical order: first order functional imaging: imaging of organ motion (heart, lungs, blood); second order functional imaging: imaging of excretory function (kidneys, liver); and third and fourth order functional imaging: imaging of metabolism (except excretory function). First order functional imaging is possible fundamentally, although with limitations in detail, by all modalities. Second order functional imaging is not possible with US. Third and fourth order functional imaging is a privilege of nuclear medicine alone. Up to now, NMR has not proven clinically useful to produce metabolic images in its true sense. This is due to the fact that the signals used are too weak to provide metabolic information with satisfactory local resolution, even after administration of nonradioactive isotopes. First and second order functional imaging of nonradioactive procedures face severe disadvantages, including difficulties in performing stress investigations, which are essential for coronary heart disease, limited capability for true quantitative information (eg, kidney clearance in mL/min), side effects of contrast media and paramagnetic substances, and high costs. Therefore, nuclear medicine functional imaging turns out to be in a favorable clinical position, even in the presence of the competitive diagnostic modalities that have been developed in recent years.