Discrete Design of an Insert Gradient Coil for Head Imaging in High Field MRI

S. Shvartsman, G. DeMeester, M. Morich Philips Medical Systems, Cleveland, Ohio, United States Introduction In this work we present designs for a transverse head insert gradient coil using a discrete coil design method. The targets for the gradient are: 60mT/m gradient strength, 600mT/m/ms slew rate, and 24cm Field-of-View (FoV). The gradient is sized to fit within a whole body gradient coil. In designing such a gradient coil we use the method of z-intercepts. The concept of this approach is that all characteristics of a transverse gradient can be expressed through the set of z-intercepts where the coil current patterns intersect the cardinal axis φ=0, for an X-gradient. Unshielded symmetric design with shoulder cutouts and shielded asymmetric design are both investigated with the new method. Method Small diameter gradients for head imaging take advantage of the fact that the stored magnetic energy scales as radius to the fifth power. This means higher efficiency, less inductance to provide high slew rate, lower Lorentz forces, and a smaller stiffer structure for potentially reduced acoustic noise. In addition there is less volume of a patient in the high dB/dt field to avoid peripheral nerve stimulation. These performance advantages motivate use of small diameter gradients at high fields, e.g., ≥3T, for proton MRI head studies. Corresponding references could be found in the recent paper [1]. There are many challenges in designing a high performance insert gradient. One key challenge is to have an acceptable head-neck access, due to the patient shoulders restrictions, while maintaining good field quality characteristics. The approaches to overcome this problem include using a symmetric unshielded insert gradient with shoulder cutouts [2], or an asymmetric shielded gradient [3,4]. We propose a new method of transverse gradient coil design using a discrete current density function. It is shown that all characteristics of a transverse gradient, such as coil inductance,