INTRODUCTION

Spiral imaging, followed by a metabolite-selective RF excitation, provides a fast-imaging option for hyperpolarization (HP) ¹³C. This imaging approach is particularly suitable for cardiac imaging due to the rapid cardiac motion and coronary blood flow (1, 2). Despite small size of the heart, a large field of view (FOV) is typically prescribed to avoid potential aliasing of neighboring organs such as liver and spleen (3). However, the large FOV requires dense k-space sampling, resulting in unnecessarily long spiral readouts and limited spatial resolution in the reconstructed images. In particular, HP ¹³C cardiac imaging with a large FOV often only showed the heart region in the reconstructed images, due to lack of metabolic activity in its surrounding tissues. A reduced FOV imaging method was previous proposed by reduced k-space sampling density (4, 5). Additionally, we reported T₂* values of HP pyruvate, lactate and bicarbonate in the heart by fitting the signal decay using multi-echo spiral imaging sequence (3). The measured long T2*s were long enough to acquire consecutive multiple echo images using a short spiral readout (e.g., ~5 ms). For instance, T₂* of HP [¹³C]bicarbonate in myocardium was measured as 64.4 ± 2.5 ms. In the current study, we designed a set of short spiral arms, similar to multi-shot spirals, for rotating multi-echo spiral imaging. Variable FOV images can be reconstructed from variable k-space sampling density by combining a selective subset of spirals.

METHODS

Phantom validation
A spectral-spatial RF pulse (3) and the rotating multi-echo spiral imaging trajectories were implemented in a clinical 3T 750w MRI scanner (GE Healthcare, Waukesha, WI). To demonstrate the performance of the proposed sequence, a gadolinium-doped spherical [13C]bicarbonate phantom (1.0 M, diameter = 3 cm) was used with a ¹³C/¹H dual-tuned birdcage transmit/receive RF coil (GE Healthcare). Four spiral readouts were acquired sequentially following the spectral–spatial RF excitation (FOV = 40x40 mm², spatial resolution = 1x1 mm², FA = 90°, ST = 25 mm, TE/TR = 21.4/3000 ms, readout length per interleave = 5.35 ms, receiver spectral bandwidth = 186.9858e Hz, 32 averages). The sequence diagram was summarized in figure 1. The acquired data were reconstructed for three FOVs: a) full-scale FOV image reconstruction using four spiral arms, b) half FOV images by combining two facing spiral arms (1^st/3^rd arms and 2^nd/4^th arms), and c) quarter FOV images from each spiral arm. The image reconstruction matrix size was set to 160x160.

RESULTS

Figure 2 shows the central quarter region of the full-size FOV, reconstructed from each single spiral readout. Each spiral arm was reconstructed separately, and a shirking phantom shape along the number of readouts could be observed. Figure 3 shows reconstructed images with half-size FOV by combining two facing spiral arms. Figure 4 shows the full-size image, reconstructed from the all spiral readouts and also compares the full/half/quarter-sized images. Enlarged phantom images could be reconstructed with increased spatial resolution (voxel size = 2.5x2.5 mm² in figure 4a, 1.25x1.25 mm² in figure 4b and 0.625x0.625 mm² in figure 4c).

DISCUSSION and CONCLUSION

The proposed method collects multi-echo images from the rotating spiral arms. Due to the rotating spiral trajectories, the acquired data can be combined, similar to multi-shot image reconstruction. The flexible FOVs reduce potential risks of having aliasing artifacts in the small FOV images by reconstructing the large FOV image. Simultaneously, by comparing multi-echo images, T₂* information can be verified. More importantly, the flexible FOV allows us to design the spiral arm to cover a larger k-space area (k_max). Ringing artifacts appeared in all reconstructed images, indicating the truncated k-space coverage. Larger k-space coverage will suppress such artifacts. The phantom results demonstrated that the targeted object in the center of FOV can be zoomed in. Limitations of the proposed approach is the short spiral readout lengths as compared with the T₂*. Therefore, heterogenous tissues with short T₂* may not qualify for the approach (3). In conclusion, the proposed acquisition and reconstruction method has been demonstrated variable FOV within single acquisition and would further be applied for ¹³C HP imaging in other small organs such as hearts, prostates, and kidneys.

Acknowledgements

National Institutes of Health of the United States (R01 NS107409); The Welch Foundation (I-2009-20190330).