Accuracy of magnetic resonance based susceptibility measurements
Hannah E. Erdevig, Stephen E. Russek, Slavka Carnicka, Karl F. Stupic, Kathryn E. Keenan
Magnetic Resonance Imaging (MRI) is increasingly used to map the magnetic susceptibility of tissue to identify cerebral microbleeds associated with traumatic brain injury and pathological iron deposits associated with neurodegenerative diseases such as Parkinson's and Alzheimer's disease. Accurate measurements of susceptibility are important for determining oxygen and iron content in blood vessels and tissue in the brain, which are in turn used for noninvasive clinical diagnosis and treatment assessments. Magnetic field distortions with a resolution of a few parts per billion can be measured using MRI maps of the proton precessional phase. The field distortion maps can then be inverted to obtain a quantitative susceptibility map. The primary focus of this paper is to determine the accuracy of MRI-based susceptibility measurements using simple phantom geometries and to compare with magnetometry measurements where SI-traceable standards are available. The susceptibilities of paramagnetic salt solutions in cylindrical containers were measured at controlled temperatures as a function of orientation relative to the static MRI field. The observed field distortions as a function of orientation of the cylinder were in good agreement with simple models. The MRI susceptibility measurements were compared with SQUID magnetometry using NIST-traceable standards. MRI can accurately measure relative magnetic susceptibilities while magnetometry measures absolute magnetic susceptibility. Given the accuracy of moment measurements of tissue mimicking samples and the need to look at small differences in tissue properties, the use of existing NIST standard reference materials to calibrate MRI reference structures is problematic and better reference materials are required.