Synopsis

Perfusion imaging is a promising application for hyperpolarized tracers, as they provide high signal with no endogenous background. Hyperpolarized 13C labeled tert-butanol is a freely diffusible perfusion agent with long T1 and T2 relaxation times in vivo. Prior work has shown that tert-butanol can be polarized to 5-10% using dynamic nuclear polarization through addition of glycerol as a glassing agent. Here we investigate a formulation based on a water/sucrose/tert-butanol mixture that yields a 1.6-fold improvement in polarization, and illustrate its use in 3D cerebral perfusion imaging in rats.

Purpose:

Introduction:

Methods:

All data were acquired using a 9.4T scanner (Biospec 94/20, Bruker, Billerica MA) equipped with a 28mm transmit/receive ¹³C surface coil (for in vivo imaging) or a 36mm ¹³C saddle coil (for polarization measurements).

Agent preparation: FINLAND and OX063 were obtained from Oxford Instruments (Oxfordshire UK) and DOTA was obtained from Macrocyclics (Plano, TX). All other reagents were obtained from Sigma (St Louis, MO). The FINLAND/glycerol formulation was prepared by combining glycerol and tert-butanol (50/50 v/v) with 16mM FINLAND. The OX063/sucrose formulation was prepared by combining tert-butanol with water (50/50 v/v) and then adding 32mg of sucrose per 100mg of tert-butanol/water mixture. 16mM OX063 was then added. Aliquots of each formulation were prepared containing 0,1,2 or 4mM of either gadoteridol (Bracco Diagnostics) or holmium-DOTA. For in vivo imaging, 1mM gadoteridol was added to the FINLAND/glycerol formulation, and 1.4mM holmium DOTA was added to the sucrose/water formulation.

Polarization and T1 Measurement: 75mg samples were polarized as described previously [1]. Following dissolution, solutions were transferred to a syringe and injected through a thin tube into a vial situated within the scanner. A single spectrum was then acquired with a 45 degree pulse approximately 15s after the end of the dissolution. This was followed by a train of 3 degree pulses (TR=5s) for T1 measurement. The thermal signal was then measured with 45 degree tip angle, TR=240s, and 6 or more signal averages to enable computation of the initial polarization.

In Vivo Imaging: Two female Sprague-Dawley rats were imaged with IACUC approval. Rats were prepared for imaging as described previously [1]. After acquisition of proton anatomic images, a bolus of hyperpolarized solution was injected, and ¹³C image acquisition was initiated at the end of the injection using a 3D balanced steady-state free precession sequence (TR/TE=2.3/1.15ms, 25 degree tip angle). In a first experiment the animal received two 2ml injections, separated by approximately 90 minutes, prepared with FINLAND/glycerol and OX063/sucrose mixtures, respectively. After each injection, sixty-four 3D frames were acquired with 40x26x20 matrix and 1.2mm isotropic resolution. In a second animal, 270mg of the OX063/sucrose formulation were polarized, 3ml of the hyperpolarized solution were injected, and twenty-four 3D frames were acquired with 40x40x32 matrix and 0.7mm isotropic resolution.

Image Reconstruction: k-space data were weighted and summed as described previously [1], zero-filled to 4 times the acquired matrix size, and Fourier transformed. Complex images were multiplied by a constant overall phase to obtain a real-valued signal intensity, and the real part of the images were used for display and analysis.

Results and Discussion:

Figure 1 shows polarization levels and T1 relaxation times as a function of holmium and gadolinium concentration for the two preparations. Holmium and gadolinium provide little or no benefit in the FINLAND/glycerol formulation, with the peak signal obtained with ~1mM gadolinium. The OX063/sucrose preparation shows roughly 1.6-fold improvement in polarization, with gadolinium and holmium yielding comparable improvements in polarization. The holmium preparations have a longer liquid-state T1 than those containing gadolinium, as expected [2].

Figure 2 shows a proton reference image and a representative perfusion-weighted image acquired in the animal that received two bolus injections. The right panels show a comparison of SNR between the FINLAND/glycerol and OX063/sucrose formulations. For the slice shown, the OX063/sucrose preparation yields a peak SNR of 107, a roughly 1.5-fold improvement over FINLAND/glycerol, after correcting for the 3.5% lower t-butanol dose in the FINLAND/glycerol scan.

Figure 3 displays representative proton and perfusion-weighted images acquired in the second rat at 700 micron isotropic resolution. Figure 4 shows reformatted axial, coronal and sagittal images from this data set.

Conclusions:

Acknowledgements

References

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