Figure 2 (not included) shows in short the most important parts of an MRI measurement. First the protons in the patient’s body are excited by an RF pulse, which causes the spins to flip from the Z-direction along the external field to the XY-plane. Directly after that, the relaxation process starts and the spins return to their original state. This relaxation creates a signal that can be measured by receiver coils. Due to the controlled gradients in the magnetic field strength, protons at different spatial positions have a slightly different frequency and phase. Frequency dependent measurements of this signal can be transformed into an MRI. Contrast is the possibility to distinguish an object from another object or the background, as for example the letters of this text can be distinguished from the background or the red arrows in figure 2 (not included) from the rest of the figure. The contrast of an object (tissues/organs) to another object or background depends on differences in intrinsic and extrinsic parameters. We can alter the extrinsic parameters easily by changing the measurement parameters (echo time, repetition time, etc), but if this is not enough we also can introduce a contrast agent. This contrast agent will alter the intrinsic relaxivity parameters and this can alter the contrast between tissues significantly. Many kinds of contrast agents are available, like gadolinium, manganese and iron oxide core particles. These contrast agents can be divided in several classifications, like T1 or T2 enhancers and paramagnetic or superparamagnetic particles. A very important point is that contrast agents used in MRI themselves will not generate any signal. It is their influence on neighbouring nuclei what explains the extra contrast that is produced when contrast agents are used. The contrast agents will enhance the local relaxation by shortening T1 and T2 for protons in the proximity. One of the most important contrast agents for MRI is the contrast agent that has an iron oxide core. The iron core particles can be used as a T1 and a T2 contrast enhancer. The usability of the iron oxide particle as a contrast agent depends on many physical factors, like size, shape, applied magnetic field and iron concentration. At the moment iron oxide contrast agents are developed that are more than just an iron oxide core with a simple shell: new smart contrast agents have functional groups in the shell so they are guided to specific tissue. In other current research the contrast agent is added to drugs or human cells, like stem cells, that can be used as a therapy
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