Optimization of high-resolution USPIO magnetic resonance imaging at 4.7 T using novel phantom with minimal structural...

To characterize the effect of ultrasmall superparamagnetic iron oxides (USPIOs) on magnetic resonance imaging (MRI) signal at 4.7 T, and to find the highest sensitivity pulse sequence for high-resolution USPIO MRI.

A novel phantom was constructed for optimization of sequence parameters for neuroradiological MR applications, and a wide range of dilutions of the USPIO ferumoxtran-10 was imaged using T(2)/T(1)-, T(1)-, T(2)-, T* (2)-, and PD-weighted sequences. The effect of varying sequence parameters was investigated using phantom measurements and simulations.

The relaxivities r(1), r(2), and r*(2) of ferumoxtran-10 at 4.7 T (21°C) were 5.1, 82.2, and 148.4 mmol(-1) L s(-1), respectively. Gradient echo sequences produced superior susceptibility artifacts at high concentrations; susceptibility artifacts were seen down to a concentration of 137 nmol Fe/mL. A concentration of 17.5 μmol Fe/mL caused a signal void independently of sequence and parameters, and at concentrations ≤273 nmol Fe/mL no signal void was caused. Signal enhancement on T(1)-weighted imaging was seen only at concentrations 137-547 nmol Fe/mL. For the same effective echo time T(2)-weighted rapid acquisition with relaxation enhancement (RARE) yielded significantly higher contrast-to-noise ratio with RARE factor 16 than with RARE factor 8.

At nanomolar concentrations of USPIO, steady-state free precession offers an alternative to T(2)- and T*(2)-weighted sequences. Optimum parameters depend highly on USPIO concentration.