Twenty-fold acquisition time improvement in 3D projection reconstruction MPI

Abstract

Summary form only given. Magnetic Particle Imaging (MPI) commonly utilizes a Field Free Point (FFP) magnetic field gradient to localize magnetic nanoparticles [12]. With the benefits of two orders of magnitude reduced acquisition time or one order of magnitude signal-to-noise ratio (SNR) improvement, a gradient called a Field Free Line (FFL), which localizes particles to a line instead of a point, has been theoretically developed [3-6], and experimentally demonstrated [4,6]. In this work, we use a FFL with sample rotation and projection reconstruction to demonstrate experimental images with a 20 fold improvement in acquisition time compared to the first projection reconstruction (PR) MPI results [6]. To gain this 20 fold speed up, we implement a z direction focus field coil configuration instead of the previously utilized translation stage. Our imaging system included a 2.3 T/m permanent magnet FFL, a solenoidal drive coil, two focus field x and z direction electromagnet pairs with Helmholtz configurations, a solenoidal receive coil with a gradiometer configuration, and a motor driven rotary table (see Figure 1). The system drive coil was excited to create a 22.9 kHz drive field with a 1.3 cm z partial field of view (FOV). The x slow shift (focus) field operated with a 3.3 Hz triangle wave, which produced a 5 cm x FOV. A linear ramp z focus field traversed 6 cm in 3 s, once per projection image. The drive and z shift fields summed, producing a 7.3 cm z FOV. There were 20 x axis traversals (10 cycles) per projection image. With this sequence, we acquired 40 images at linearly spaced angles over 180 degrees. The image acquisition time was 2.1 min. We collected all the necessary projection data to produce a MPI tomographic 3D volume using the above parameters. Images were reconstructed using x-space reconstruction with filtered backprojection (FBP) [5-6]. The final imaging volume was limited by bore size and the z slow shift magnets to a 4.8 cm by 4.8 cm by 7.3 cm 3D volume. The 3D volume was exported in DICOM file format and subsequently imported to Osirix (Pixmeo, Switzerland) where maximum intensity projection (MIP) images were rendered. To test our imaging sytem, we have designed a phantom with a 3D distribution of magnetic nanoparticles (see Figure 1). Polyurethane tubing with inner diameter 1.6 mm (outer diameter 3.2) filled with 43 mM Fe Micromod Nanomag-D-spio was wrapped around a cylindrical piece of acrylic with a 3.4 cm outer diameter. Resulting MIP images illustrate the ability of the FFL imager to accurately resolve nanoparticle distributions in 3D. The MIP image can be rotated to any orientation, and two such views are shown in Figure 1. In previous work, an image with a similar FOV would have taken 39 min using a translation stage [6]. The two minute acquisiton time using electromagnetic z shift demonstrates an approximately 20 fold improvement in acquisition time.