Spatial encoding using the nonlinear field perturbations from magnetic materials

Position and orientation measurement of susceptibility markers using spectrally selective spin-echo projections

PURPOSE

To develop an accurate technique for simultaneously measuring the position and orientation of interventional devices using a projection-based spectrally selective refocusing pulse sequence.

METHODS

Projections along physical axes using spectrally selective excitation were acquired to track a catheter. A 9F passive tracking device capable of generating controllable susceptibility artifacts using susceptibility materials (titanium and graphite) was attached to the catheter. A library of projections for different orientations of the device with respect to the main magnetic field were simulated offline. Cross-correlations with these templates were computed to determine the orientation and position of the device. A phantom study was performed to evaluate the accuracy of the tracking technique. The tracking technique was also evaluated in vivo in the carotid artery of a pig.

RESULTS

Simultaneous and accurate measurement of position and orientation of the tracking device was obtained in the phantom and in vivo studies with reasonable temporal resolution. For the phantom study, the average of absolute errors in the Z-, Y-, and X-axes are 0.37, 0.76, and 0.85 mm, respectively. The mean absolute error and standard deviation of orientation measurement are 1.5 and 1.1 degrees, respectively.

CONCLUSION

This positioning technique, in conjunction with a controllable tracking device, can provide accurate tracking of interventional devices in MR-guided interventions. Magn Reson Med 76:1563-1573, 2016. © 2015 International Society for Magnetic Resonance in Medicine.

Accelerating sequences in the presence of metal by exploiting the spatial distribution of off-resonance

PURPOSE

To demonstrate feasibility of exploiting the spatial distribution of off-resonance surrounding metallic implants for accelerating multispectral imaging techniques.

THEORY AND METHODS

Multispectral imaging (MSI) techniques perform time-consuming independent three-dimensional acquisitions with varying radio frequency offsets to address the extreme off-resonance from metallic implants. Each off-resonance bin provides a unique spatial sensitivity that is analogous to the sensitivity of a receiver coil and, therefore, provides a unique opportunity for acceleration. Fully sampled MSI was performed to demonstrate retrospective acceleration. A uniform sampling pattern across off-resonance bins was compared with several adaptive sampling strategies using a total hip replacement phantom. Monte Carlo simulations were performed to compare noise propagation of two of these strategies. With a total knee replacement phantom, positive and negative off-resonance bins were strategically sampled with respect to the B0 field to minimize aliasing. Reconstructions were performed with a parallel imaging framework to demonstrate retrospective acceleration.

RESULTS

An adaptive sampling scheme dramatically improved reconstruction quality, which was supported by the noise propagation analysis. Independent acceleration of negative and positive off-resonance bins demonstrated reduced overlapping of aliased signal to improve the reconstruction.

CONCLUSION

This work presents the feasibility of acceleration in the presence of metal by exploiting the spatial sensitivities of off-resonance bins.