ABSTRACT
The definitions of functional imaging and molecular imaging are relatively vague (1) . Functional imaging encompasses functional magnetic resonance (MR) imaging (e.g., using BOLD technology, macroscopic and microscopic motion imaging and imaging of any molecular biological phenomenon); molecular imaging is best defined as “a new discipline that unites molecular biology and in vivo imaging. It enables visualization of cellular function and following the molecular process in living organisms without perturbing them” (2) . Nuclear medicine (NM) methods are more suitable for this purpose than the other imaging methods. This is because of the exquisite sensitivity of NM detection devices for radioactivity. While x-ray contrast agents change image contrast perceptibly in the millimolar range and MR contrast agents change them in the 10-μ M range, radiopharmaceuticals can be detected in nano-or even picomolar concentration. Further enhancement of the MR signal by using ultrasmall superparamagnetic iron oxide (USPIO) agents at concentrations less than micromolar is possible in principle, but in practice the mobility in the human body of such agents is greatly impeded by their size. This makes targeting difficult and USPIO agents usually are not suitable as clinical molecular imaging agents. An increase in MR field strength improves the signal-to-noise ratio (SNR) approximately in proportion, but going from a 1.5 to a 7 T magnet improves the SNR by no more than a factor of five. The improved sensitivity of radiopharmaceuticals over MR contrast agents is a factor of 10,000 to 1 million. As a result, the radiotracer imaging methods, positron emission tomography (PET) and single photon emission computed tomography (SPECT) are currently the only techniques which permit true molecular imaging. Indeed, NM has been a molecular imaging modality since its conception. Radioactive iodine and its molecular properties were used to image and treat benign and malignant thyroid disease using basic molecular biological mechanisms of uptake and iodination of thyroglobulin (3) (see chap. 30).
