SPION MRI Contrast & T2* Darkening

SPION Injection

Nanoparticle concentration 60

Core size 14 nm

MRI Readout

Echo time TE 40 ms

Particles in ROI

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Mean ROI signal

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Signal drop

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Dephasing radius

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How it works

Superparamagnetic iron-oxide nanoparticles (SPIONs) don't emit an MRI signal themselves — they distort the local magnetic field around them. Water protons diffusing through that distorted field dephase faster, shortening the effective T2* relaxation time and darkening any voxel near the particles once enough echo time has passed.

S(TE) = S0 · exp(-TE · R2*local)
R2*local = R2_tissue + Σᵢ A(size) / (1 + (d / r_dephase)²)

Concentration — more particles accumulated in the region (e.g. uptake by a lesion) sum their field distortions, darkening a wider patch of the slice.
Core size — a bigger iron-oxide core carries a larger magnetic moment, so each particle darkens a disproportionately large "blooming" halo well beyond its physical size.
Echo time — dephasing accumulates with time; a longer TE lets even a faint field distortion sink the local signal toward black, which is why T2*-weighted sequences are used to hunt for SPION accumulation.

This is the physical basis for using SPIONs to track where nanoparticles have accumulated in brain tissue on an MRI scan — the particles themselves are invisible, but the dark "shadow" they cast on nearby water signal is not.

ROI: Cortical lesion

Signal vs. echo time (ROI mean)

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