Purpose Accurate measurement of the non-uniform transmit radiofrequency field is useful for many applications in MRI, such as calibrating the scanner transmit system, evaluating coil performance, and improving image quality and quantitation. The radiofrequency field excitation amplitude (B1) is often obtained by acquiring a B1 map. In this study, a new B1 mapping method is proposed. Theory and Methods The use of two adiabatic full passage pulses with different magnitudes applied as successive refocusing pulses results in a linear relationship between phase and B1 field strength that is insensitive to the repetition time, off-resonance effects, T1, and T2. Using this method, B1 mapping can be localized to a slice or 3D volume, with a spin-echo acquisition that is appropriate for fast projection measurements. Results This new method is shown to agree well with the Bloch-Siegert B1 mapping method for both phantom and in vivo B1 measurements at 1.5T, 3T and 7T. The method’s ability to acquire accurate projection B1 measurements is also demonstrated. Conclusion This method’s high dynamic range, ability to make fast projection measurements, and linear quantitative relationship between phase and B1 make it an ideal candidate for use in robust transmitter gain calibration.

Image quantitation methods including quantitative MRI, multiparametric MRI, and radiomics offer great promise for clinical use. However, many of these methods have limited clinical adoption, in part due to issues of generalizability, that is, the ability to translate methods and models across institutions. Researchers can assess generalizability through measurement of repeatability and reproducibility, thus quantifying different aspects of measurement variance. In this article, we review the challenges to ensuring repeatability and reproducibility of image quantitation methods as well as present strategies to minimize their variance to enable wider clinical implementation. We present possible solutions for achieving clinically acceptable performance of image quantitation methods and briefly discuss the impact of minimizing variance and achieving generalizability towards clinical implementation and adoption.

We have investigated the efficacy of superparamagnetic iron oxide nanoparticles (SPIONs) as positive T1 contrast agents for low-field magnetic resonance imaging (MRI) at 64 millitesla (mT). Iron oxide-based agents, such as the FDA-approved ferumoxytol, were measured using a variety of techniques to evaluate T1 contrast at 64 mT. Additionally, we characterized monodispersed carboxylic acid-coated SPIONs with a range of diameters (4.9–15.7 nm) in order to understand size-dependent properties of T1 contrast at low-field. MRI contrast properties were measured using 64 mT MRI, magnetometry, and nuclear magnetic resonance dispersion (NMRD). We also measured MRI contrast at 3 T to provide comparison to a standard clinical field strength. SPIONs have the capacity to perform well as T1 contrast agents at 64 mT, with measured longitudinal relaxivity (r1) values of up to 67 L mmol−1 s−1, more than an order of magnitude higher than corresponding r1 values at 3 T. The particles exhibit size-dependent longitudinal relaxivities and outperform a commercial Gd-based agent (gadobenate dimeglumine) by more than eight-fold at physiological temperatures. Additionally, we characterize the ratio of transverse to longitudinal relaxivity, r2/r1 and find that it is ~ 1 for the SPION based agents at 64 mT, indicating a favorable balance of relaxivities for T1-weighted contrast imaging. We also correlate the magnetic and structural properties of the particles with models of nanoparticle relaxivity to understand generation of T1 contrast. These experiments show that SPIONs, at low fields being targeted for point-of-care low-field MRI systems, have a unique combination of magnetic and structural properties that produce large T1 relaxivities.