ABSTRACT
A magnetic resonance imaging (MRI) scan is a noninvasive medical diagnostic imaging procedure currently used in clinical practice. This technique provides images that detect tiny changes in the structures within the body. The objective is achieved by measuring parameters related to the relaxation of hydrogen nuclei of water excited by a magnetic ˆeld.1,2 This process is affected by three parameters: proton density, longitudinal relaxation time (T1), and transverse relaxation time (T2). The variation in the proton density between tissues is not very relevant; therefore, the pulse sequence that measures the T1 and T2 provides the necessary contrast for diagnosis. The image resolution could be enhanced using a contrast agent (CA) that improves both the sensitivity and speciˆcity of MRI and provides anatomical and
Introduction ............................................................................................................ 391 Classical Contrast Agents Based on Gadolinium Ion ............................................ 395 Improved Contrast Agents ..................................................................................... 396 Multi-Gadolinium Complexes as Contrast Agents ................................................ 397 Supramolecular Aggregates and Relaxivity Behavior ........................................... 398
Supramolecular Aggregates as MRI Contrast Agents ....................................... 401 Supramolecular Aggregates Derivatized with Peptides or Antibodies ..............406
Conclusions ............................................................................................................ 410 References .............................................................................................................. 411
physiological information beyond the impressive resolution commonly obtained in uncontrasted images.3,4 The CAs can be classiˆed into two categories, T1 agents and the T2 agents. Both CA categories, based on paramagnetic or superparamagnetic agents, shorten both T1 and T2, and the designation of a CA to one category or the other depends on the ratio between the longitudinal relaxivity (r1) and the transverse relaxivity (r2). T1 agents have r2/r1 ratios slightly higher than 1, whereas T2 agents have much higher (>10) r2/r1 ratios. T1 agents in the usual image acquisition mode give a positive contrast.5 This class of agent is usually based on paramagnetic ions; primarily, the gadolinium(III) complexes. On the contrary, superparamagnetic compounds based on iron oxide are T2 agents; they give a negative contrast in the usual image acquisition mode.6
