MR contrast agents
|Type of magnetism||Definition||Examples|
|Ferromagnetic||Large magnetic susceptibility i.e. when placed in a magnetic field the field strength is much stronger inside the material than it is outside
Remains magnetised when external magnetic field removed
Consist of some elements of ferromagnetic materials
Magnetic susceptibility between ferromagnetic and paramagnetic
|Paramagnetic||Weakly attracted to magnets and, therefore, only weakly influence a magnetic field
Increases T1 and T2 relaxation times (i.e. decrease the T1 and T2 times)
|Diamagnetic||When placed in a magnetic field will weakly repel the field
Results in loss of signal in bowel on MRI after barium sulphate suspensions administered
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Types of contrast agents
T1 paramagnetic contrast agents
These cause local magnetic field distortions that enhance T1 and T2 relaxation i.e. result in greater T1 and T2 signal. Factors that affect degree of T1 relaxation:
- Concentration of Gd in tissues
- Proximity of surrounding tissues
- Rotational motion of Gd
- Number of water molecules that associate with Gd
- Time that water molecules are around to associated with Gd
The effect of T1 relaxation will increase with the concentration of Gd until an optimum concentration is reached (increasing T1 signal). After this, any further increases in concentration will reduce the T1 signal due to T2 relaxation effects being more prominent.
- E.g. gadolinium diethylenetriamine penta-acetic acid (Gd-DTPA), which on its own is toxic so must be encased in another molecule. The different MR contrast agents use different chelates
- T1 contrast agent
- Allows dynamic phase imaging e.g. arterial, venous and equilibrium
- Usually contain manganese
- T1 contrast agent
- Intracellular (taken up by functioning hepatocytes in health liver tissue)
- Agent slowly taken up by hepatocytes
- Can image up to 24 hours later
- Doesn't allow for dynamic imaging
T2 superparamagnetic contrast agents
The small local magnetic field disruption caused by the contrast agent will slightly alter the precessional frequency of any proton that is in its vicinity. Once the proton moves away it will return to its precessional frequency but with a phase shift (spin dephasing) which speeds up the T2 decay and reduces the T2 signal.
The higher the concentration of the contrast agent the greater the dephasing and the lower the T2 signal.
- Superparamagnetic iron oxide (SPIO) and ultrasmall superparamagnetic iron oxide (USPIO) agents
- Superparamagnetic iron oxide does not leak into the interstitium. It remains in the intravascular space until it is eliminated via the reticuloendothelial system
- Coated with substances to increase uptake by the reticuloendothelial system
- Reduces T2 signal of absorbing tissues which decreases the T2 signal
- Requires injection one hour before images acquired