Synopsis

Rapid prOtotyping of 2D non-CartesIan K-space trajEcTories (ROCKET) aims to aid researchers interested in rapid development and testing of new MR methods starting from pulse sequence design to image analysis. This was achieved by utilizing Pulseq for pulse sequence design and graphical programming interface for image reconstruction and analysis. ROCKET was demonstrated on two non-Cartesian k-space trajectories – FID based radial and spiral. Each trajectory was tailored into three different trajectories based on rotating angle – standard, golden angle and tiny golden angle. All studies were performed on Siemens scanner demonstrated on in-vitro phantom and in-vivo healthy brain acquisitions and SNRs were computed.

Introduction

Methods

The FID based radial trajectory was designed using equation $$$k_x=k_{width}cos\theta$$$ and $$$k_y=k_{width}sin\theta$$$, where $$$k_{width}$$$ is the maximum extent of k-space and defined as $$$k_{width}=N\delta k$$$, N is matrix size and $$$\delta k=1/FOV$$$, FOV is the field of view. The general spiral trajectory was defined by equation³ $$$k(\tau)=\lambda \tau^{\alpha}e^{j\omega\tau}$$$ where, τ- function of time, α is the variable density factor, λ=N/(2*FOV), N is matrix size, ω=2πn, n is number of turns. As α increases, the density factory also increases. The designed spiral trajectory is the modified version of the code⁴. Each trajectory was designed for three rotating angles – standard, Golden Angle (GA) and Tiny Golden Angle (TGA). GA is obtained by dividing 180⁰ by the golden ratio of 1.618. The acquired data is spaced by a constant azimuthal increment of 111.26⁰. The limiting factor of GA is the rapid change of eddy currents. TGA provides smaller angular increments by inheriting all the properties of GA. ROCKET was demonstrated on two non-Cartesian k-space trajectories (namely, FID based radial and spiral) with three rotating angles. The standard rotating angle for radial/spiral was obtained by dividing 360⁰ with the total number of spokes/interleaves. The GA and TGA angles were obtained by using equations from the paper^5,6. The radial/spiral GRE pulse sequence were designed with six different gradient waveforms, and all were scanned on in-vivo healthy brain and in-vitro Disease Neuroimaging Initiative (ADNI) phantom (Phantom laboratory, New York). The acquisition parameters were shown in table 1. Figure 1 shows the rapid prototyping using ROCKET. The non-Cartesian GRE pulse sequences were designed in Pulseq and the seq files were generated. Each seq file is typically converted to the vendor specific files by using an interpreter. Here, all the scans were performed on a Siemens 3T Prisma. The generated seq files were exported directly to the MR scanner and raw data was obtained. The trajectory and the k-space data were given as input to the GPI network and images were reconstructed. Finally, line intensity profile was plotted using GPI script (GPI/Recon.net)⁷ to compare the resolutions of the reconstructed images obtained from in-vitro and in-vivo studies.

Results

Discussion and Conclusion

Acknowledgements

References