Calibration standard of body tissue with magnetic nanocomposites for MRI and X-ray imaging

Abstract

We present a first study of a long-term phantom for Magnetic Resonance Imaging (MRI) and X-ray imaging of biological tissues with magnetic nanocomposites (MNC) suitable for 3-dimensional and quantitative imaging of tissues after, e.g. magnetically assisted cancer treatments.We performed a cross-calibration of X-ray microcomputed tomography (XµCT) and MRI with a joint calibration standard for both imaging techniques. For this, we have designed a phantom for MRI and X-ray computed tomography which represents biological tissue enriched with MNC.The developed phantoms consist of an elastomer with different concentrations of multi-core MNC. The matrix material is a synthetic thermoplastic gel, PermaGel (PG).The developed phantoms have been analyzed with Nuclear Magnetic Resonance (NMR) Relaxometry (Bruker minispec mq 60) at 1.4T to obtain R2 transverse relaxation rates, with SQUID (Superconducting QUantum Interference Device) magnetometry and Inductively Coupled Plasma Mass Spectrometry (ICP-MS) to verify the magnetite concentration, and with XµCT and 9.4T MRI to visualize the phantoms 3-dimensionally and also to obtain T2 relaxation times. A specification of a sensitivity range is determined for standard imaging techniques X-ray computed tomography (XCT) and MRI as well as with NMR.These novel phantoms show a long-term stability over several months up to years. It was possible to suspend a particular MNC within the PG reaching a concentration range from 0mg/ml to 6.914mg/ml.The R2 relaxation rates from 1.4T NMR-relaxometry show a clear connection (R2=0.994) with MNC concentrations between 0mg/ml and 4.5mg/ml. The MRI experiments have shown a linear correlation of R2 relaxation and MNC concentrations as well but in a range between MNC concentrations of 0mg/ml and 1.435mg/ml.It could be shown that XµCT displays best moderate and high MNC concentrations. The sensitivity range for this particular XµCT apparatus yields from 0.569mg/ml to 6.914mg/ml.The cross-calibration has defined a shared sensitivity range of XµCT, 1.4T NMR relaxometer minispec, and 9.4T MRI. The shared sensitivity range for the measuring method (NMR relaxometry) and the imaging modalities (XµCT and MRI) is from 0.569mg/ml, limited by XµCT, and 1.435mg/ml, limited by MRI.The presented phantoms have been found to be suitable to act as a body tissue substitute for XCT imaging as well as an acceptable T2 phantom of biological tissue enriched with magnetic nanoparticles for MRI.