MR image corrections for PET-induced gradient distortions.

The combination of positron emission tomography (PET) and magnetic resonance imaging (MRI) by using PET inserts in existing MRI scanners is an attractive approach. When designing the PET insert, mutual influences of both imaging modalities need to be minimized. The gradient magnetic fields induce eddy currents in all conductive components of the PET insert. Eddy currents produce superimposing magnetic fields distorting the gradient magnetic field. However, the gradient magnetic fields determine how the MRI data is acquired in the k-space. A distorted gradient shape produces a distorted k-space trajectory which then results in a distorted image. The dynamic performance of the gradient system can be characterized by measuring its gradient impulse response function (GIRF). Knowledge of the GIRF enables to correct the k-space trajectory and thereby enables to reduce image distortions. We characterized the influence of a preclinical PET insert, i.e. the Hyperion II, on the gradient performance of a 3 T MRI scanner. The GIRFs up to the second order spherical harmonics were determined with and without the PET insert by measuring frequency-swept gradient pulses with an NMR probe. We calculated a corrected k-space trajectory of a single-shot spiral sequence using the measured GIRFs. The low-pass characteristic of the gradient system only slightly increased in the presence of the PET insert. We showed a minor influence of the PET insert on the GIRFs, demonstrating the high gradient transparency of the PET insert. The single-shot spiral image measured in the presence of the PET insert and reconstructed with the uncorrected k-space trajectory was blurred and distorted. We strongly reduced the blurring and distortion by using the corrected k-space trajectory predicted with the measured GIRFs up to the first order spherical harmonics. Slight blurring remained in the corrected image caused by either second order spherical harmonics or distortions of the static magnetic field.