There has been recent interest in Simultaneous Multi-Slice (SMS) imaging^1,2. SMS methods allow for significant acceleration of single-shot imaging modalities such as EPI and spiral previously not possible using parallel imaging. SMS has been proven to be useful for numerous applications including increasing fMRI temporal resolution and obtaining higher angular resolutions for DTI. The most common SMS acquisition is blipped-CAIPI EPI that adds alternating z-gradient blips to an EPI sequence for improved slice encoding³. It has also been shown that an SMS 3D k-space picture can be used for arbitrary trajectories including blipped-spiral as well as generalized SENSE reconstructions^4,5. The purpose of this work is to extend the SMS 3D k-space picture to a circular EPI trajectory⁶. Circular EPI is more time efficient than Cartesian EPI and has less image blurring than spiral and may be a good compromise between the two. Phantom and human fMRI data acquired at 3T using a 32-channel coil are presented.

Circular SMS EPI trajectories were designed for N=4, 8 and 12 simultaneous slices over a 20cm FOV with 64x64 in-plane resolution. Different sampling schemes for k_y-k_z were examined including using sine wave modulation along k_z as well as hexagonal and blipped-CAIPI sampling. The x-gradient consisted of triangular waveforms always operating at a maximum slew rate of 150T/m/s. All points along the trajectory were used including ramps such that the shortest possible readout was obtained. The trajectories were all constructed offline in Matlab and inserted into the sequence as external files. SMS excitation was performed using a modulated sinc RF pulse for the desired number of simultaneous slices. The slice thickness was 3mm. Image reconstruction was formulated using a generalized signal model: $$ s_{m}(t)=\int_{}^{} C_{m}({\bf r})\rho({\bf r})\exp(i{\bf k}(t)\cdot{\bf r}+i{\gamma}\tau{\bf G\cdot{\bf r}})d{\bf r}. $$The reconstruction was then performed offline as an inverse problem using iterative SENSE and non-uniform Fast Fourier Transform in a conjugate gradient algorithm. Ghosting artifact was compensated by adding a delay τ to effectively shift the k-space trajectory used in the reconstruction. The delay was varied until a value of τ was found that produced a ghost free image. Scans were performed using a structure phantom as well as human BOLD fMRI using visual/motor activation paradigm. The paradigm consisted of a flashing checkerboard during which the subject was asked to perform bilateral finger tapping. The stimulus was a block design consisting of 15 second blocks over a two minute duration. The fMRI scanning parameters were N=8, 30ms TE, 250ms TR, and 480 time points. Both positively and negatively correlated activation was determined using a general linear model fit. All scanning was performed on a Siemens Trio 3T scanner using a 32-channel head coll. A standard gradient echo FLASH scan was used to acquire the sensitivity maps for image reconstruction.

Figure 1 shows an example of circular SMS EPI gradients and k-space for an N=4 acquisition. A sinusoidal z-gradient was used for slice encoding. Also shown are four simultaneously acquired slices in the structure phantom. Figure 2 shows the same data reconstructed for values of τ=0.0, 2.5, 5.5, and 7.5μs. A value of τ=5.5μs was found by visual inspection to provide the best image quality. Figure 3 shows circular SMS EPI gradients and k-space for an N=12 acquisition. The sampling in the k_y-k_z plane was chosen to be hexagonal for optimal sampling efficiency. Also shown are an example of 12 simultaneous slices acquired in the structure phantom. Figure 4 shows BOLD fMRI results using a blipped-CAIPI circular SMS EPI scan for N=8 simultaneous slices. The CAIPI blip factor was three. Gradients as well as k-space diagrams are shown. Also shown are the positively and negatively correlated activation maps overlaid on slices windowed for t-values from 9-13. Note that the expected motor and visual regions are seen in the positively correlated activation. The negatively correlated activation shows the well known default mode patterns.We presented a circular EPI SMS sequence for rapid imaging using a generalized 3D k-space model and reconstruction. Image quality of circular EPI was comparable to Cartesian EPI (data not shown) and acquired with shorter duration. A standard EPI scan was approximately 35ms where the circular scan was on the order of 25ms for the same parameters. The use of the 3D k-space picture also allowed for arbitrary slice encoding strategies including sinusoidal and hexagonal sampling. EPI ghosting was also easy to incorporate as a simple k-space shift. Further work will investigate using measured trajectories as well as application of the technique to high angular resolution DTI.