The effect of concomitant fields in fast spin echo acquisition on asymmetric MRI gradient systems
The concomitant field (CF) is a source of phase error in FSE acquisitions that originates from the fundamental physical principles (i.e., the Maxwell's equations 8), which state that the divergence and curl of the magnetic field is zero in free space. Consequently, the linear‐varying spatial‐encoding gradient fields must always be accompanied by a series of undesired magnetic fields, termed Maxwell fields or CFs. They are present whenever a gradient is active, and cause accumulation of phase errors. The CF effect on whole‐body MR gradient systems has been described previously. It causes artifacts including ghosting and distortion in echo‐planar imaging (EPI) 9, blurring in spiral 11, and flow‐quantification errors in phase‐contrast acquisition 8.
Conventional, whole‐body MR gradients typically use a symmetric design, with the coil current paths (generating gradient fields) on the service side mirroring that on the patient side (i.e., the gradient isocenter corresponds to the coil geometrical center). The CFs on such systems contain only terms of second‐order spatial dependence or higher, and usually only the second‐order terms are not negligible 8, especially at 1.5 T and higher. The CF effect in FSE on whole‐body MR systems has been investigated, and was shown to cause ghosting in large field‐of‐view (FOV) applications such as spine imaging 12, as the magnitude of the CF scales as the square of the distance from isocenter. Their effects, however, are usually negligible (at 1.5 T and higher) with image FOVs used for typical brain scans (18–24 cm) and the maximum gradient amplitudes (50 and 200 T/m/s) available on most whole‐body gradients 12.
An alternative to the symmetric design is the asymmetric gradient system, in which the gradient isocenter is shifted away from the center of the coil, toward the patient‐entry end 13. The asymmetric design is typically used for head‐only or compact systems 18, in which this design gives patient (head) access to the imaging volume for a smaller‐sized gradient coil. The smaller‐sized gradient coil allows higher maximum gradient slew rate and amplitude as a result of reduced gradient coil inductance and resistance, as well as decreased peripheral nerve stimulation. In FSE and EPI acquisitions, the high gradient slew rate and amplitude can substantially shorten echo spacing, and improve imaging performance by increasing signal level and reducing susceptibility effects 20. However, the asymmetric design results in CFs that include additional terms of zeroth and first‐order dependences, in addition to the usual second‐order terms 16.
In this work, we investigate the effect of the zeroth and first‐order CFs in FSE acquisitions on asymmetric gradients using the extend phase graphs (EPG) simulation 6, as well as phantom and in vivo experiments. As shown here, these additional CFs can cause prominent artifacts even within the FOV typically used for brain exams. We also demonstrate the effectiveness of the real‐time compensation methods of frequency tracking 24 and gradient pre‐emphasis 25, which counteract their effects.