Split gradient coils for simultaneous PET-MRI

Abstract

Combining positron emission tomography (PET) and MRI necessarily involves an engineering tradeoff as the equipment needed for the two modalities vies for the space closest to the region where the signals originate. In one recently described scanner configuration for simultaneous positron emission tomography-MRI, the positron emission tomography detection scintillating crystals reside in an 80-mm gap between the 2 halves of a 1-T split-magnet cryostat. A novel set of gradient and shim coils has been specially designed for this split MRI scanner to include an 110-mm gap from which wires are excluded so as not to interfere with positron detection. An inverse boundary element method was necessarily employed to design the three orthogonal, shielded gradient coils and shielded Z0 shim coil. The coils have been constructed and tested in the hybrid positron emission tomography-MRI system and successfully used in simultaneous positron emission tomography-MRI experiments.

Figure 1

Cross-sectional diagram of the principal structures of the combined PET-MRI scanner, including the surfaces of the gradient coils and Z0-shim coil shown.

Figure 2

Wires of the (a)X, (c) Z gradient, and (e) Z0 shim coils. Red wires indicate reversed current flow with respect to blue. Contour maps of the axial magnetic field generated by the coils are shown in (b),(d) and (f) with 5μTA⁻¹ contour spacing (thin green) overlaid with the 5% field error contour (thick red), ROU (orange region), ROS (orange line), and wire positions (into, ⊗, and out of, ⊙, the contour plane).

Figure 3

Photographs of the split gradient coil set (a) during construction after the addition of the outer X-gradient layer and (b) the completed gradient set. Approximate scale bars have been added to both images.

Figure 4

One slice of the image data of the micro-Derenzo hot rod phantom with (a) the PET data, (b) the MRI data and combined image using rigid translation registration only.