Balanced SSFP (bSSFP) sequences can provide superior SNR efficiency for hyperpolarized (HP) ¹³C imaging, by efficiently utilizing the non-recoverable magnetization. It has been applied for HP ¹³C perfusion imaging and angiography using metabolically inactive agents with a single resonance ^1,2. In this work, a spectrally selective bSSFP sequence was developed to enable metabolic imaging of multiple HP resonances. The sequence was optimized for dynamic 3D imaging of individual resonances in experiments with co-polarized [1-¹³C]pyruvate and [¹³C]urea, attaining 2mm isotropic spatial resolution and <1s temporal resolution.

Unique challenges faced in HP ¹³C metabolite mapping include the non-recoverable nature of magnetization and the complex pattern of resonances that must be spectrally resolved. A novel approach for bSSFP spectral selectivity was developed utilizing a combination of optimized multiband RF pulses and a bSSFP pulse train with a carefully chosen TR to avoid banding artifact. This provides the flexibility to separately excite injected substrates and metabolic products with different flip angles (FA), which is key for dynamic studies ³.

bSSFP has an intrinsic periodic frequency response, as shown in Fig.1 (A, B). Spins near the center of each cycle behave as desired, in that a large FA excites magnetization while a small FA has little affect. The opposite behavior exists at the edge of each cycle; where a large FA leaves little longitudinal and transverse magnetization while a small FA produces significant excitation over a small frequency range. When a selective RF pulse is used (Fig.1 (C)), besides the predefined central passband, there are also narrow excitation bands at the edge of each cycle due to accumulated small FA, as shown in Fig.1(D). A new method is proposed to avoid this banding artifact by carefully choosing TR such that the resonant frequency of each compound is located near center of bSSFP frequency response cycles, thus driving the overall spectral selectivity close to the spectral selectivity of RF pulse.

An optimized multiband spectrally selective RF pulse with shortest possible duration ⁴ was designed to keep TR and total scan time short, which is crucial due to the rapid respiratory motion of mice imaged in this study, which necessitates sub-second imaging with respiratory triggering. The minimum pulse duration was achieved by exploiting spectral sparsity and releasing the constraint on “don’t-care” regions. The optimal pyruvate/lactate/urea-only pulse has duration of 1.4ms/2.16ms/1.04ms, much shorter than conventional minimum-phase SLR pulses (~6ms), with one example shown in Fig. 3 (C, D).

A ramp-up preparation pulse was applied before acquisition to reduce transient state signal oscillations. A ramp-down pulse was applied after the acquisition to flip magnetization back to Mz to increase signal, as shown in Fig. 3(A).

Bloch simulation of bSSFP signal evolution around one stopband (urea) is shown in Fig.2, which failed with a sub-optimal TR of 4ms (B), but succeeded with an optimal TR of 3.8ms (D).

Pyruvate acquisitions with single compound phantom at different resonant frequencies are shown in Fig. 4 (A). Ideally, signal should only be seen in the pyruvate image, as shown in the bottom row with TR of 3.8ms. Such selectivity cannot be achieved when TR is slightly off, such as the top row with TR of 4ms. Note that the failed selectivity at urea/alanine/lactate band agrees with the simulation results in Fig.2 (A) where those bands touch the banding artifact region. Additionally, pyruvate/lactate/urea acquisitions of HP pyruvate/lactate/urea solution are shown in Fig.4 (B), with a table of measured spectral selectivity.

In vivo 3D dynamic bSSFP acquisitions of pyruvate/lactate/urea on a normal mouse with injected co-polarized pyruvate/urea are shown in Fig. 5. For injected substrate pyruvate/urea, strong signal was observed in the aorta at early time points. Lactate, converted from pyruvate, was also observed, mostly in kidney.

A spectrally selective 3D dynamic bSSFP sequence was developed for HP ¹³C metabolite mapping. The spectral selectivity (~1%) of the bSSFP sequence was achieved by using optimal RF pulses with shortest duration and carefully choosing TR to avoid banding artifacts.

Compared to other HP ¹³C imaging sequences, such as spoiled gradient-echo sequences with EPI or EPSI readouts ³, bSSFP is more SNR efficient. Compared to spin echo sequences ⁵, bSSFP also has the refocusing performance but can be used in the small FA regime to prolong the observation time window. Compared to multi-echo spectrally selectively bSSFP sequences ⁶, this sequence features easy and robust reconstruction, and the flexibility of exciting different compounds with different FA. Compared to low FA spectrally selectively bSSFP sequences ^7,8, this sequence yields higher SNR with larger FA and also wider passbands.