Imaging techniques, including PET, SPECT, MRI, and microCT, are valuable tools in preclinical research just as their counterparts are in clinical diagnostics.
A range of imaging techniques can be employed to obtain structural and functional information about cells and organs, and to acquire insights into molecular activity, such as tissue metabolism. In preclinical research, this can facilitate the determination of biological responses to environmental changes or pharmaceutical compounds under development. In medicine, imaging can identify the presence of cancerous tissue, evaluate healthy tissue, assess bodily functions, diagnose disease, and monitor the effects of a treatment.
The information that can be obtained using such modalities can be increased by using contrast agents, since they can make images easier to interpret.
Research to support the development of new, more effective contrast agents is thus an important enabler of numerous scientific advances.
Contrast agents for magnetic resonance imaging
MRI has become a particularly valuable radiological tool since it provides anatomical information across almost all organs and soft tissues within the body without the need for harmful ionising radiation. It allows distinction between normal and diseased tissue and can be used to perform functional evaluation, such as tissue perfusion.
Contrast-enhanced MRI is commonly used to increase the difference in relaxation characteristics of normal and pathologic tissues. A variety of different categories of contrast agents are currently available for preclinical use. These include paramagnetic agents, such as gadolinium and manganese, superparamagnetic agents, such as iron oxide and iron platinum, and protein-based agents, such as β-galactosidase-activated contrast agents.
The use of contrast agents facilitates the detection of malignant lesions and can enable more accurate characterization of different types of lesions1. The small difference in relaxation properties between a tumour and healthy tissue may be insufficient to produce obvious differences in signal intensity and this can be increased by intravenous administration of a contrast agent. Elusive pathologies thus become more obvious in the presence of a contrast agent, thereby aiding diagnosis.
It is also possible to improve the contrast of different tissues, for example to highlight blood vessels. This can be increased further by using two contrast agents with complementary mechanisms. This is particularly useful when imaging the liver. A superparamagnetic iron oxide is used to darken the background liver and gadolinium serves to brighten the vessels2.
The drive for contrast agents to maximise the information that can be acquired through MRI has resulted in extensive research to develop new, more effective and convenient agents.