Jonathan,

indeed, you find that different tissues have varying amounts of natural iron content. This is reflected in different T2 and thus in different signal intensity in T2-weighted MRI. I.e. this is the natural background situation. This is observed strongly for liver which is rich in iron, or also for substania nigra (nomen est omen!) in the brain which is clearly of lower signal intensity than most other brain regions on T2-weighted MRI.

Within cells, iron is not moved by the magnetic field. So, ripping out of the body is not a problem. However, one can actually direct cells labeled with iron oxide nanoparticles which are in the blood stream. It has been reported that a local strong magnetic field held e.g. against the template of a mouse or rat brain  will hold such labeled cells preferentially in the vicinity of the magnet, thus allowing easier migration from the ascular compartment into the tissue.

Mathias

Thanks Mathias, that does clear things up.  I think I am confused about what magnetite actually is.   I have come across some papers that seem to show that magnetite (not just iron) are present in some tissues.  I thought by definition, magnetite was...well...magnetic.  From your answer, it would suggest that magnetite is just a form of iron in cells and this is not necessarily magnetic, thus this is why the iron (magnetite) is not moved by the magnetic field.  

Could the size of the magnetite crystals be involved here?  Crystals below a certain size are not subjected to a strong enough force to actually cause any movement?

Thanks again for your help,

John

Magnetite (Fe3O4) is ferrimagnetic not ferromagnetic, so the magnetic moment is somewhat smaller than in iron, but tyically these particles are smaller (

It is very good explanation of Peter Blumler. In fact, the magnetite is superparamagnetic, and does not remain magnetized such as iron. Magnetite under controlled conditions of crystallization (particle size (

So if the magnetite is only magnetized in a magnetic field, then would it not be magnetized when exposed to the large magnetic field inside of an MRI?  So I am still confused, why would the magnetic field of the MRI not pull on the magnetite in the tissue (if only when the tissue is moving inside of the MRI)?

I have a feeling that I am out of my depth here. I am just trying to conceptualize why big magnets (or metal objects) are dangerous in an MRI, but small endogenous magnetite (which would be magnetized in the MRI magnetic field) is not affected.  

Correct: the superparamagnetic particles are too small to be permanently magnetized. Meaning: they behave only like magnets inside a strong magnetic field, they "focus" or bundle the flux lines from such a superimposed field. Hence, they can also experience a magnetic force, BUT a force is only present in an inhomogeneous magnetic field and the MRI fields are rather homogeneous. Even in strong gradients (10 T/m or so, much stronger than the spatially encoding gradients in an MRI experiment) these forces on 100 nm particles are in the femto-Newton regime and easily compensated by the viscosity of the body liquids. For more detail see: S. S. Shevkoplyas et al. Lab on a Chip 7 (2007) 1294-1302.

Thank you very much Peter!  That makes it very clear.  

It is necessary to functionalize the mnp in order to put in others compartments than liver. Also the mnp don´t pass through the BBB. I think that mnp are more promising for the liver diagnostic and therapeutic. Today have appeared nano molecular formulations with a very insensitive magnetism easier to be handle for theranostics proposes in particular for MRI studies.

For invivo imaging in mice fe3o4 is better or Gd؟