12 MR Contrast Agents 12 MR Contrast Agents Johannes M. Froehlich Image contrast in medical MR imaging results from differences in signal in- tensity (SI) between two tissues and is determined by intrinsic and extrinsic factors. These are respectively properties of the different tissues and proper- ties of the MR scanner, especially of the pulse sequence used. MR contrast media are pharmaceutical preparations that are adminis- tered in MR imaging to further enhance the natural contrast and addition- ally to obtain dynamic (pharmacokinetic) information. To achieve these goals, contrast agents used for MRI must have specific physicochemical properties and also a suitable pharmacokinetic profile. MR contrast media fundamentally alter the intrinsic contrast properties of biological tissues in two ways: – directly by changing the proton density of a tissue or – indirectly by changing the local magnetic field or the resonance proper- ties of a tissue and hence its T1 and/or T2 values. The local magnetic field strength is altered because the unpaired electron spins of the contrast medium (CM) interact with the surrounding hydrogen nuclei of the water, fat, or protein molecules in the tissue. Thus, the mechan- ism of action of an MR contrast agent comprises processes of the electron shell and not just processes at the nuclear level, as does the MR effect. The magnetic moments of electrons are 657 times greater than those of protons. This is one of the reasons why the electron shell has much more powerful paramagnetic properties than a hydrogen nucleus. The interactions occurring between contrast medium electrons and tis- sue protons comprise “inner-sphere relaxation” (through interaction with bound water) and “outer-sphere relaxation” (e.g. arising from the diffusion of water nearby). Both processes contribute substantially to the overall ef- fect of MR contrast media. Before we proceed with our discussion of contrast media, some terminol- ogy must be clarified: – Paramagnetic substances have magnetic moment (resulting from individual spins) because they consist of atoms or molecules that have magnetic moment due to unpaired electron orbits in their outer electron shells or unpaired nucleons in their atomic nuclei. When such a substance is exposed to an external magnetic field, most of the magnetic moments align with the direction of the magnetic field and the magnetic moments add together, resulting in a local increase in the magnetic field (just as with protons). When no external magnetic field is present, the magnetic moments occur in random pattern and there is no net magnetization. Many dissolved metal ions (including iron in blood) but also stable radicals are paramagnetic because they contain unpaired electrons. Examples are Co2+, Co3+, Fe2+, Fe3+, Gd3+, Mn2+, Mn3+, and Ni3+. Because of their powerful magnetic moment (see above), substances with unpaired electrons are preferred as MR contrast media. Most of the clinically available MR contrast media are paramag- netic metal ion compounds (gadolinium chelates, manganese, iron). – Superparamagnetic substances have very strong paramagnetic proper- ties. Their greatly increased magnetic moment (10- to 1000-fold) results from the arrangement of the paramagnetic ions in a rigid crystal lattice, which increases the mobility of their surrounding electrons (e.g. iron oxide in the form of superparamagnetic nanoparticles). Superparamag- netic contrast agents are solid substances that not only have T1 and T2 effects but also markedly distort the local magnetic field (magnetic susceptibility). – Ferromagnetic substances also consist of large groups of atoms whose unpaired electron spins are strongly entwined by exchange coupling (solid state). These substances retain their magnetization even after the external magnetic field has been removed and subsequently become permanent magnets. The best known example is iron (Fe). By far the majority of all substances are diamagnetic (strictly speaking, diamagnetism is present along with the other forms of magnetic properties in these substances). When brought into an external magnetic field, dia- magnetic substances induce very weak overall magnetization in the oppo- site direction (-z) – mostly because the orbital movement of most electrons is counterclockwise. Now we can address the question as to how a contrast medium alters the MR signal and thus enhances contrast on the resultant image. 12 MR Contrast Agents Unlike radiographic contrast media, which are directly seen on an X-ray absorption image, an MR contrast medium, such as a gadolinium complex, acts indirectly by altering the relaxation properties of surrounding hydro- gen protons. In the example shown (▶ Fig. 50), the image obtained after IV administration of a Gd complex (right) depicts two lesions which were not visible on the image acquired before contrast medium administration (left). To produce this effect, the contrast medium must have two properties – first, it must diffuse through the blood-brain barrier and, second, it must be able to interact with the local protons (thereby shortening their T1). MR contrast agents can alter an MR image in one of four ways. Changing Spin or Proton Density The presence of a contrast medium affects the amount of protons present in a voxel. Most agents reduce the number (e.g. freon-like compounds such as perfluoro-octyl bromide [PFOB], barium sulfate, fatty emulsions). The de- crease in local proton density is associated with a signal loss, as for example after oral administration of a barium sulfate suspension. Fig. 50. Schematic representation of the CNS in transverse orientation. T1-weighted MR images before contrast medium administration (left) and after IV administration of 0.1 mmol Gd/kg body weight (right). Two additional lesions are seen on the contrast- enhanced image. The signal increase results from local T1 shortening and the extravas- cular distribution of the contrast medium within the lesions Shortening T1 and T2 Relaxation Times (Most Important) An MR contrast medium can be thought of as a catalyst that accelerates the relaxation of nearby protons by withdrawing the excess energy (in T1 weighting) the protons have previously absorbed from the excitation pulse (spin-lattice interaction). The faster recovery of longitudinal magnetization results in a stronger MR signal. An agent that enhances the signal is called a positive contrast medium. In addition, the magnetic moments of unpaired electrons alter the local magnetic fieldstrength, resulting in faster dephasing due to spin-spin effects and enhancement of T2 relaxation. High contrast medium concentrations, e.g. in the lower urinary tract, produce local field inhomogeneities and T2 shortening, which is seen as a signal loss especially on T2-weighted images. The significance of faster relaxation is nicely illustrated by comparing re- laxation times in different environments: – spontaneous relaxation in a vacuum: 1016 years, – relaxation in a watery solution: about 1 second, – relaxation in a watery contrast medium solution: a few milliseconds. Pathologic tissue that takes up the contrast medium shows a signal change (typically a markedly higher signal on T1-weighted images and a re- duced signal on T2-weighted images when the contrast medium concentra- tion is high) while the surrounding normal tissue not containing any con- trast medium remains unaffected. For optimal visualization of the effects resulting from the selective uptake of the agent into a lesion, it is important to adjust the imaging parameters, especially the weighting (e.g. predomi- nantly T1 weighting and a short TR when a Gd preparation is administered) (▶ Fig. 51). Members of this class that are used in clinical MR imaging are gadolinium-based compounds, manganese compounds, and iron solutions. Faster Dephasing Through Local Field Inhomogeneities (Susceptibility Effects) So-called T2* effects are predominantly seen on T2-weighted images. Local field inhomogeneities caused by the high magnetic moment of the contrast medium accelerate dephasing of the protons beyond normal FID and thus shorten T2 further. This phenomenon is known as magnetic susceptibility and predominantly occurs in the presence of high local field strengths or at interfaces and may also cause artifacts. Susceptibility becomes manifest as a pronounced signal loss that is best appreciated on T2-weighted images. The 12 MR Contrast Agents Fig. 51. Relaxation-time curve of a tissue with and without Gd uptake. The T1-shortening effect and resulting increase in signal intensity (SI) are transient (black area). The signal increase can be appreciated on images acquired T2 effect with T1 weighting and a short reduces SI TR. No SI increase will be seen Tissue without Gd when a longer TR is used (ex- cept as a result of the increasing T2 effect that occurs in the pres- ence of high contrast medium t concentrations) agents producing a signal loss are therefore termed negative contrast media. Examples are superparamagnetic iron oxide nanoparticles (SPIO) that are taken up by the reticuloendothelial system (RES) of normal liver tissue and can thus serve as a liver-specific contrast medium for the selective suppres- sion of the signal from normal liver. Shifting the Resonance Frequency (Dysprosium) Another mechanism of action is shifting of the resonance frequency by sev- eral hundred ppm. This effect is similar to chemical shift and weakens the measured proton signal. Dysprosium-based compounds are known to have this kind of effect but have virtually