SPMNPs Used as Magnetic Contrast Agents in MRI

Relax Your Mind

Relaxation Techniques

Get Instant Access

MRI works by the dual application of an external magnetic field Bo (up to 2 T) and a transverse (i.e., transverse to Bo) radio frequency (RF) pulse on protons which are present in large amounts in biological tissues (e.g., in water molecules, membrane lipids, proteins, etc.). Each proton has a tiny magnetic moment that can be aligned by applying an external magnetic field. The external magnetic field first applies and aligns a magnetic moment of each proton. The RF pulse is then directed to the region of interest, and the protons absorb energy from the pulse and spin in a direction different than the direction of the external field. The RF pulse then will be turned off, the protons now begin to return to their original alignment with the external magnetic field and release excess stored energy through a process called relaxation. When this happens, a signal will be given off and the MRI system can detect this signal and process it to transform it into images of the tissue area of interest.

Relaxation can be divided into two independent processes: (1) longitudinal relaxation, in which the longitudinal component of the proton's magnetic moment returns to be aligned with an external field Bo and this is termed 7^-recovery; (2) transverse relaxation, in which the transverse (i.e., transverse to Bo) component of the proton's magnetic moment vanishes and this is called T2-recovery. Different tissues have different T1 and T2 values, therefore they can be differentiated by MRI scanning. Both T1 and T2 can be shortened by the use of magnetic contrast agents. Magnetic contrast agents work by altering local magnetic moments in the region of interest, therefore relaxation signals can be enhanced. For example, T2 can be shortened according to (1):

Where ABo is the variation in the field brought about by variations in the local magnetic field inhomogeneity or local magnetic susceptibility of the system due to the presence of magnetic enhance agents (Leach 1988; Brown and Semelka 1999). Figure 9 shows an example of TEM images of Fe3O4 SPMNPs and schematically illustrates how SPMNPs can affect T2 recovery time.

Fig. 9 TEM bright field images of (a) 6 nm and (b) 12 nm Fe3O4 SPMNPs deposited from their hexane dispersion on an amorphous carbon-coated copper grid and dried at room temperature (Adapted with permission from reference (Sun et al. 2004)), and (c) schematic illustration of SPMNPs bound to a tissue of interest, thereby shortening T2 recovery time

Fig. 9 TEM bright field images of (a) 6 nm and (b) 12 nm Fe3O4 SPMNPs deposited from their hexane dispersion on an amorphous carbon-coated copper grid and dried at room temperature (Adapted with permission from reference (Sun et al. 2004)), and (c) schematic illustration of SPMNPs bound to a tissue of interest, thereby shortening T2 recovery time

Compared to traditional MR contrast agents (such as gadolinium), SPMNPs have several advantages. First, SPMNPs are in the size range in which they can be delivered to the desired tissues more effectively. They are often composed of a superparamagnetic core composed of iron oxides such as magnetite Fe3O4, maghemite (y-Fe2O3), or other insoluble ferrites (generally from 3 to 10 nm in diameter) with or without polymer coatings such as dextran (Mornet et al. 2004). In particular, in the case of MRI for tumor imaging, SPMNPs can be delivered to tumors using passive targeting methods. Second, they have much greater magnetic susceptibility (specifically, two or three orders of magnitude greater than traditional gadolinium superparamagnetic particles (Wang et al. 2001)), meaning they are more responsive to an external magnetic field, than traditional MR contrast agents. For example, superparamagnetic iron oxides are effective as T2 relaxation enhancers due to the induction of local magnetic field inhomogeneity in the surrounding tissues leading to a stronger shortening of the recovery time as in (1).

Was this article helpful?

0 0
Relaxation Audio Sounds Dusk At The Oasis

Relaxation Audio Sounds Dusk At The Oasis

This is an audio all about guiding you to relaxation. This is a Relaxation Audio Sounds with sounds from Dusk At The Oasis.

Get My Free MP3 Audio


Post a comment