Effect of Spatial Distribution of Magnetic Dipoles on Lamor Frequency Distribution and MR Signal Decay – a Numerical Approach Under Static Dephasing Conditions

Abstract

Cells loaded with superparamagnetic iron oxide (SPIO) cause relatively strong magnetic field distortions, implying that field position effects of neighboring SPIO loaded cells have to be accounted for. We treated SPIO loaded cells as magnetic dipoles in a homogeneous magnetic field and computed the 3D frequency distribution and the related signal decay using a numerical approach under static dephasing conditions. The volume fraction of dipoles was kept constant for all simulations. For larger randomly distributed magnetic dipoles we found a non-Lorentzian frequency distribution and a non-monoexponential signal decay whereas, for smaller dipoles, the frequency distribution was more Lorentzian and the signal decay was well fitted monoexponentially. Moreover, based on our numerical and experimental findings, we found the gradient echo signal decay due to a single SPIO labeled cell to be non-monoexponential. The numerical approach provides deeper understanding of how the spatial distribution of SPIO loaded cells affects the MR signal decay. This fact has to be considered for the in vivo quantification of SPIO loaded cells, implying that in tissues with different spatial distributions of identical SPIO concentrations, different signal decays might be observed.

Effect of Concentration of SH U 555A Labeled Human Melanoma Cells on MR Spin Echo and Gradient Echo Signal Decay at 0.2, 1.5, and 3T

“…Furthermore, as reported by Simon et al [16], free extracellular ferumoxtran-10 ultrasmall superparamagnetic iron oxide (USPIO) particles have shown a higher R 2 relaxivity as opposed to intracellular compartmentalized USPIO. These findings might be explained with regard to a recent paper [24], which demonstrated faster MR signal decay in case of uniformly distributed magnetic dipoles (e.g., SPIO in liquid solution) as compared to compartmentalized magnetic dipoles (e.g., SPIO ingested in cells).…”

Signal decay due to susceptibility-induced intravoxel dephasing on multiple air-filled cylinders: MRI simulations and experiments

“…Pintaske et al [19] showed that the spatial distribution of magnetic dipoles is an important factor that determines the resonance frequency distribution and hence the MR signal decay. They found the frequency distribution to be nonLorentzian, which led to non-monoexponential TEdependent signal decay.…”

“…In molecular imaging, injection of paramagnetic markers permits passive tracking [4,10], in the in vivo MR imaging of stem cells [11,12] and qualitative and quantitative assessment of holmium-loaded microspheres [13]. Superparamagnetic iron-oxide (SPIO) nanoparticles have recently been exploited as contrast agents for non-invasive cell tracking, quantification of tissue perfusion and target-specific imaging, and for the detection of gene expression [11,[14][15][16][17][18][19][20][21]. Magnetically labeled cells create intense magnetic field distortions within and around the cells, leading to enhanced signal dephasing in GE sequences.…”

Feldverteilung magnetischer Dipole als Modell magnetisch markierter Zellen in der MRT

“…Dieses wurde bereits vorgestellt und zum Studium verschiedener Effekte im Zusammenhang mit paramagnetischen Suszeptibilitätsverteilungen verwandt [14]. Auf die wichtigsten Sachverhalte soll im Folgenden eingegangen werden.…”

“…Dieses Dipolfeld ist identisch mit der Feldstörung außerhalb eines sphäri-schen Objektes mit Radius a (Abb. 3) [14]. (6) Dabei ist M die Magnetisierung des Objektes relativ zum umgebenden Medium.…”