Trajectory analysis for field free line magnetic particle imaging Can Barıs ß Top a) , Alper G€ ung€ or, Serhat Ilbey, and H. Emre G€ uven ASELSAN Research Center, 06370 Ankara, Turkey (Received 9 October 2018; revised 13 January 2019; accepted for publication 15 January 2019; published 22 February 2019) Purpose: Magnetic particle imaging (MPI) is a relatively new method to image the spatial distribu- tion of magnetic nanoparticle (MNP) tracers administered to the body with high spatial and temporal resolution using an inhomogeneous magnetic field. The spatial information of the MNP’ s is encoded using a field free point (FFP), or a field free line (FFL), in which the magnetic field vanishes at a point, or on a line, respectively. FFL scanning has the advantage of improved sensitivity compared to FFP scanning as a result of higher signal-to-noise ratio. The trajectory traversed by the FFL or FFP is an important parameter of the MPI system and should be selected to achieve the best imaging quality in minimum scan time, while considering hardware constraints and patient safety. In this study, we analyzed the image quality of different FFL trajectories for a large field of view (FOV) using simula- tions, to provide a baseline information for FFL scanning MPI system design. Methods: We simulated a human-sized FFL scanning MPI configuration to image a circular FOV with 160 mm diameter, and compared Radial, Spiral, Uniform Spiral, Flower, and Lissajous trajecto- ries with different trajectory densities scanned by the FFL for constant scan time. We analyzed the system matrices of the trajectories in terms of mutual coherence and homogeneity of the spatial sensi- tivity. We calculated the maximum electric fields induced on a homogeneous conductive body by the selection field (SF) and the focus field (FF) to compare the trajectories based on the nerve stimulation threshold. The images were obtained using the system matrix reconstruction approach with two dif- ferent image reconstruction methods. In the first one, we used the conventional image reconstruction method, algebraic reconstruction technique (ART), which gives a regularized least-squares solution. In the second one, we used the state-of-the-art alternating direction method of multipliers (ADMM), which minimizes a weighted sum of the l 1 -norm and the total variation (TV) of the images. Results: The Radial and Spiral trajectories resulted in a poor imaging performance at low trajectory densities due to relatively high coherency and poor sensitivity of the measurements, respectively. For ART reconstruction, the highest image quality with the lowest trajectory density was achieved with the Uniform Spiral trajectory. Uniform Spiral, Flower, and Lissajous trajectories yielded comparable performance with ADMM reconstruction. The rotating SF induced higher electric field amplitude compared to the FF. Consequently, maximum allowable gradient at the same trajectory density was greater for the Radial trajectory compared to the other trajectories. Conclusions: For a large FOV coverage, the Uniform Spiral trajectory offers a good compromise between image quality and imaging time, taking safety and hardware limitations into account. The Radial trajectory, especially using l 1 -norm and TV priors in the reconstruction, may be favorable in case the SF induced electric field is higher than that of the FF at the same frequency (e.g., relatively small FOV coverage). In general, ADMM reconstruction resulted in higher contrast and resolution compared to ART, leading to lighter requirements on the density of the trajectory. © 2019 American Association of Physicists in Medicine [https://doi.org/10.1002/mp.13411] Key words: field free line, image reconstruction, magnetic nanoparticles, magnetic particle imaging, open MPI, peripheral nerve stimulation 1. INTRODUCTION Magnetic particle imaging (MPI) is a new method for imag- ing the magnetic nanoparticle (MNP) distribution adminis- tered to the body. 1 MPI can potentially be used for a diverse range of applications in medical imaging and therapy. 2 Vas- cular imaging, 3 imaging of tools for cardiovascular interven- tion, 4 cancer imaging, 5 stem cell tracking, 6 noninvasive thermography, 7 perfusion imaging for acute stroke, 8 func- tional brain imaging, 9 viscosity imaging, 10 and magnetic hyperthermia 11 are some of the proposed applications of MPI. There are several advantages of MPI making it a promising medical imaging method: (a) MPI uses MNP trac- ers which do not possess toxicity problems to the body, (b) there is no ionizing radiation, as in the case for x-ray based angiography, (c) sub-millimetric resolution can potentially be achieved in real time, (d) quantitative imaging is possible since MPI signal is not affected from the tissue, (e) MPI can provide tomographic imaging as human body is fully penetra- ble in the frequency range of MPI. 1592 Med. Phys. 46 (4), April 2019 0094-2405/2019/46(4)/1592/16 © 2019 American Association of Physicists in Medicine 1592