Researchers interested in hetero-nuclei imaging and non-Cartesian acquisition scheme. ³¹P MR spectroscopy has been used to quantify non-localized high-energy phosphate in vivo. Spectroscopic imaging offers both spatial and spectral encoding. However, due to the low metabolite concentration, ³¹P spectroscopic imaging with high spatial resolution requires long acquisition time that renders the technique impractical. Alternatively, imaging using spectrally selective excitation allows only one metabolite to be imaged using conventional imaging sequence. Further acceleration can be achieved by using a balanced steady-state free precession (bSSFP) approach. In addition, the relatively long T₂ relaxation time of phosphocreatine (PCr) provides more favorable T₂/T₁ contrast in bSSFP¹. In this study, a fast PCr imaging method was developed by combining spectrally selective bSSFP with single-shot spiral-in/out encoding for measuring PCr distribution in vivo.

A user-defined Gaussian pulse with 4 ms duration was designed to spectrally select PCr. For image acquisition, a single-shot, zero moment compensated spiral trajectory was designed using the minimum-time gradient method². The spiral waveform was executed twice within one repetition time (TR) in a spiral-in/out fashion and the center k-space was acquired at the middle of each TR. The spiral-in/out trajectory was measured manually³ and images were reconstructed with NUFFT⁴ using the measured trajectory. 2D imaging was performed by combining the spectrally selective pulse and outer volume saturation without slice selection gradient.

All experiments were performed on a Bruker 9.4T horizontal scanner (Bruker Biospin Co., Billerica, MA) using an in-house built ³¹P transmit/receive saddle coil. The selectivity of the Gaussian pulse was evaluated on a 500 mM inorganic phosphate (Pi) phantom by sweeping the carrier frequency from -5 to 5 ppm with a step size of 0.5 ppm. The Gaussian pulse combined with the spiral-in/out bSSFP sequence was validated on two phantoms with 40 mM phosphocreatine (PCr) and 20 mM adenosine triphosphate (ATP), respectively.

In vivo PCr imaging of the calf muscle was performed on a 10 month-old Fischer rat. An M2M volume coil was used for proton imaging. The B₁ fields of the ³¹P coil and the ¹H coil were placed orthogonal to minimize the coupling between the two coils. Localized shimming using PRESS was performed before the imaging experiments. Reference ¹H image was acquired using FLASH sequence. Acquisition parameters were: TR, 500 ms; TE, 7 ms; flip angle, 40°; FOV 4×4 cm²; matrix size 256×256; slice thickness, 5 mm. PCr image was acquired using the proposed spiral-in/out bSSFP sequence with outer volume saturation. Acquisition parameters were: TR, 20 ms; TE, 10 ms; flip angle, 25°; FOV 4×4 cm²; matrix size 32×32; slice thickness, 5 mm. The saturation module was implemented at every 32 TRs, followed by 10 dummy scans. A delay of 350 ms was also inserted between every 32 TRs to prevent gradient from overheating. A total of 48000 repetitions were acquired in 30 min. Image reconstruction used an average of 1600, 3200, 8000, 16000, 24000, 32000, 40000, and 48000 repetitions, corresponding to an acquisition time of 1, 2, 5, 10, 15, 20, 25, and 30 min, respectively.

Designed and measured trajectory is shown in Fig. 1. Fig. 2 shows the selectivity of the 4-ms Gaussian pulse. Residual signal at 2.5 ppm, where γATP resonance occurs, was 5.8%. This profile allowed the excitation of PCr only. Fig. 3 shows the reference anatomy image (Fig. 3a) and PCr selective image (Fig. 3b) using the proposed method on PCr and ATP phantoms. In vivo PCr images were reconstructed using different number of signal averages, corresponding to an acquisition time of 1, 2, 5, 10, 15, 20, 25, and 30 min, respectively, and the corresponding signal to noise ratio (SNR) was calculated (Fig. 4). 2 min acquisition yielded an SNR of ~9. The SNR was increased to 25.5 with 30 min acquisition. Representative images with 5, 10, 20, and 30 min acquisition time are shown in Fig. 5. PCr images on phantom and in vivo were zero padded to matrix size 64×64, and FOV of 2cm×2cm is shown.This study shows a fast PCr imaging method with spiral encoding. PCr map of rat calf muscles with high spatial resolution (1.25 mm×1.25 mm×5 mm) was achieved in acceptable acquisition time. With a different carrier frequency, this method can also be used for imaging other metabolites such as ATP. Quantification of the absolute metabolite concentration can be achieved with appropriate T₁, T₂, and flip angle calibration/correction. This method also has the potential for dynamic PCr imaging in exercise-recovery or ischemia-reperfusion studies.