Molecular imaging in biomedical research
Molecular imaging is the concept of noninvasive studies of biochemical and physiological processes in living animals and man. It is increasingly important in clinical research and diagnostics, as well as in development of new pharmaceuticals. Two very important technologies are positron emission tomography (PET) and magnetic resonanace imaging (MRI) and recently the fusion between PET and MRI has been realised. This fused technology is known as PET-MRI. In addition to PET there is single photon emission computed tomography (SPECT) which also can be combined with CT.
PET-MRI, a state-of-the art fusion of two technologies
Conventional PET-CT scanners co-register the functional PET images with Computed Tomography (CT) images obtained just prior the PET examination. However, PET images can also be manually co-registered with Magnetic Resonance Imaging (MRI) acquired at a different time point than the PET scan. This is preferable since MRI provides markedly improved contrast between soft tissues and is especially useful for studies of the brain. Additionally, functional MRI (fMRI) enables non-invasive in vivo assessment of physiological parameters.
This has prompted the development of integrated PET-MRI scanners. MRI provides structural imaging of the body by exploiting the behavior of hydrogen nuclei in the presence of strong magnetic fields. As it does not use ionized radiation (as opposed to CT) PET/MRI yields a markedly reduced radiation dose, which enables safer examinations of new patient groups or healthy subjects. In addition, the PET and MRI scanning can be performed simultaneously instead of sequentially which improves image alignment and reduces artifacts due to movement.
Preclinical small animal PET-MRI has improved resolution for both functional (<1 mm) and structural imaging (<100 μm) compared to most current preclinical instruments. In vivo biodistribution, drug pharmacokinetics and pharmacodynamics, therapeutical effects, tissue function and blood-brain barrier penetration can thus be studied at an unparalleled depth and resolution.
The preclinical PET-MRI scanner also offers the potential of several functional in vivo measurements of functional parameters crucial for evaluation of drug or therapy effects. These include tissue perfusion by both PET (H2150) and MRI (contrast enhancement), regional oxygen consumption (oxygen-15 and BOLD, respectively) and molecular imaging (MRI spectroscopy).