Traditional methods of measuring magnetization in magnetic fluid
samples, such as vibrating sample magnetometry (VSM), are typically
limited to maximum field strengths of about 1 T. This work demonstrates
the ability of MRI to measure the magnetization associated with two
commercial MRI contrast agents at 3 T by comparing analytical solutions
to experimental imaging results for the field pattern associated with
agents in cylindrical vials. The results of the VSM and fitted MRI data
match closely. The method represents an improvement over VSM
measurements since results are attainable at imaging field strengths.
The agents investigated are Feridex, a superparamagnetic iron oxide
suspension used primarily for liver imaging, and Magnevist, a
paramagnetic, gadolinium-based compound used for tumors, inflammation
and vascular lesions. MR imaging of the agents took place in sealed
cylindrical vials in the presence of a surrounding volume of deionized
water where the effects of the contrast agents had a measurable effect
on the water's magnetization in the vicinity of the compartment of
contrast agent. A pair of phase images were used to reconstruct a B0 fieldmap. The resultant B0
maps in the water region, corrected for shimming and container edge
effects, were used to predict the agent's magnetization at 3 T. The
results were compared with the results from VSM measurements up to 1.2 T
and close correlation was observed. The technique should be of interest
to those seeking quantification of the magnetization associated with
magnetic suspensions beyond the traditional scope of VSM. The
magnetization needs to be sufficiently strong (Ms ≳ 50 Am2/kg Fe for Feridex and Xm ≳ 5 × 10−5 m3/kg
Gd for Magnevist) for a measurable dipole field in the surrounding
water. For this reason, the technique is mostly suitable for undiluted
agents.