espite the fact that inherent contrast in MR imaging can be manipulated to a much greater extent than in other imaging techniques, certain diagnostic questions cannot be answered easily and require the application of contrast agents (Figure 13-01).
Nature likes mimicry; radiologists like to highlight lesions. With plain photography (or MR imaging), the object of the examination might be visible but nor clearly delineated. Changing contrast with an extrinsic agent may help ... for instance, painting the wall in the background or injecting a contrast agent.
In general, contrast manipulation in MR imaging by application of contrast agents (sometimes also called molecular imaging) is most useful when inherent contrast cannot be attained successfully. Important goals and requirements for the development and use of MR contrast agents are listed in Table 13- 01. In many instances, the pattern of enhancement of paramagnetic contrast agents in MR imaging is very similar to that of contrast-enhanced x-ray CT. However, it should be taken into account that in reality MR contrast agents behave differently from CT agents and do not in any case follow the CT enhancement patterns.
Since it is nearly impossible to alter the water content of tissues, contrast agents on the market or in clinical or pre-clinical trials focus on relaxation time and susceptibility changes.
As early as 1946, in one of the first papers describing NMR, paramagnetic catalysts were mentioned to accelerate the T1 relaxation process [⇒ Bloch]. This concept was recognized by Paul C. Lauterbur shortly after his invention of MR imaging and tested and proved in imaging studies in animals [⇒ Lauterbur].
Primary and secondary goals and requirements for the development of contrast agents for magnetic resonance imaging.
Many efforts in contrast agent development were channelled in certain directions more by the relative ease of chemical synthesis than by the goal of specific medical applications. Thus, the first-generation contrast agents available for clinical routine examinations today are relatively safe, good enhancers, but unspecific. This means that they do not highlight specific pathologies but rather unspecific pathological tissue changes. At present, paramagnetic contrast agents are the most frequently used. The most efficient elements are listed in Table 13-02. Particulate agents form a different class.
Table 13-02: Some paramagnetic elements (transition metals) and their properties: gadolinium, manganese, dysprosium, and iron.
13-01-01 Classification of Contrast Agents
Table 13-03 gives an overview of a number of MR contrast agents currently in use, already withdrawn from the market, or being developed. Only perhaps a dozen agents are on the market; however, the number of trade names is far higher and an overview of different brands of the same product difficult.
Classification of some magnetic resonance contrast agents being developed, to be approved, approved, marketed, or withdrawn from development or the market. There are numerous other agents in development.
= agent available for clinical and/or research application; = agent available for clinical and/or research application with severely limited indications ("high risk"); = agent being developed; = development interrupted or discontinued; = agent withdrawn from one or all major markets.
= positively enhancing agent; = negatively enhancing agent.
= globally charged agent; = globally neutral agent.
* = trademark or registered trademark;
** = with high concentrations, negative contrast can be achieved (e.g., first-track bolus);
*** = all ECF agents are also kidney-specific agents;
(et al.) = the compund is sold under different trade names.