Rolf F Schulte1, Enrico Chiavazza2,3, Axel Haase2, Silvio Aime3, Marion I Menzel1, Markus Schwaiger2, and Markus Durst1,2
1GE Global Research, Munich, Germany, 2Technische Universität München, Munich, Germany, 3Università di Torino, Turin, Italy
Synopsis
An
experimental setup for 15N was established on a clinical 3T whole-body scanner,
in order to measure perfusion with an ultra-long T1 15N compound. The substance
α-trideuteromethyl[15N]glutamine was synthesised, polarised and injected into
healthy rats to study the perfusion of the kidneys. With a T1 time in vivo of
146s, it was possible to acquire perfusion images for over 5 minutes.Target audience
Researchers interested in measuring perfusion and hyperpolarisation of ultra-long T1 15N compounds.
Introduction
Hyperpolarisation
and imaging of isotopically labelled compounds is an active research field, focusing
mainly on the NMR-active, stable 13C isotope. Main limitation of the technology
is still T1 relaxation, which in case of [1-13C]pyruvate is approximately one
minute in vitro and 20s in vivo at B0=3T. While this is relatively long as
compared to normal MR T1 times, it is short as compared to radioactive decay in
case of PET imaging (eg half-life of 18F=110min). The physiological processes
must therefore be rapid to be detectable with hyperpolarised 13C compounds. 15N
labelled compounds can exhibit extremely long T1 relaxation times, such as 4-5
minutes in case of α-trideuteromethyl[15N]glutamine (in vitro) [1]. Goal of this work was to establish an MRI experimental
setup including hard- and software for 15N and measure perfusion in healthy
rats with this 15N glutamine compound.
Methods
The
resonance frequency of 15N is -12.95 MHz at 3T, roughly 40% of 13C and 10% of 1H.
All experiments were performed on a 3T HDx scanner (GE Healthcare), using a
commercially purchased 15N TR switch (Rapid Biomedical) and a custom-built
butter-fly transmit-receive coil. 15N images were encoded using a single-shot
spiral trajectory (nominal matrix size 32×32, FOV=8cm) in a slice through the
rat kidneys (slice thickness = 2cm; flip angle=10°). Image acquisition started
at the beginning of the injection, with 16 images being acquired with TR=1s and
the rest with TR=5s. Gradients were nulled every 16th excitation to
acquire a slice-selective spectrum for control purposes. The data was
reconstructed using NUFFT [2], after a 15-Hz
Gaussian apodisation.
α-Trideuteromethyl[15N]glutamine
(15N-Gln) (Fig. 1) was synthesised according to [1] and polarised using a
mixture of 3M 15N-Gln, 28mM OX063 radical (GE Healthcare) and 2.5mM Dotarem
(Guerbet) dissolved in ethylene glycol and D2O (Sigma Aldrich). A small amount
of [1-13C]pyruvate was added to monitor the build-up of polarisation. After
polarising for 1-3 hours in a HyperSense DNP polariser (OxfordInstruments), the
sample was dissolved rapidly in a D2O solution to a concentration of ~100mM
15N-Gln.The excretion of 15N-Gln was evaluated by mass spectrometry of tissue,
blood and urine samples.
Four
healthy male Lewis rats (Charles River) were anaesthetised with 1-3% Isoflurane
gas inhalation and injected with 1ml of hyperpolarised 15N-Gln or ~150mM
[13C,15N2]urea solution in a separate injection to compare it to 13C perfusion
measurements. The setup for 15N-Gln experiments is shown in Fig. 2. All animal
experiments were approved governmentally by the Tierschutzbehörde (Regierung
von Oberbayern).
Results and
Discussion
α-Trideuteromethyl[15N]glutamine
yields clear, localised images of the rat kidneys (Fig. 3). It is possible to
detect signal much longer with 15N-Gln as compared to 13C urea (Figs. 4 and 5).
T1 of 15N-Gln in vivo is with 146s approximately eight times as long as that of
13C urea. 15N-Gln shows a strong localisation in the kidneys with little
background signal as compared to urea. The rapid excretion of 15N-Gln within
the time scale of the hyperpolarised experiment was confirmed by the mass spectrometry.
Conclusion
The investigated 15N-Gln compound exhibits an
extraordinarily long T1 relaxation both in vitro [1] and in vivo. Cellular
uptake and enzymatic conversion are limited, hence its main application is as a
perfusion agent. Due to its long T1, it is a promising candidate to observe
renal function and filtration, happening in the order of a few minutes. The
advantage of 15N-Gln over gadolinium chelates is its smaller size and direct,
positive contrast.
Acknowledgements
Co-funding
BMBF grant #13EZ1114.References
[1]
Contrast Media Mol Imaging. 2013;8:417-21. 15N-permethylated amino acids as
efficient probes for MRI-DNP applications. Chiavazza E, Viale A, Karlsson M,
Aime S.
[2]
Journal of Magnetic Resonance. 2007;188:191-195. On NUFFT-based gridding for
non-Cartesian MRI. Fessler JA.
[3]
Clin N Am 2008;16(4):597-611, viii. Assessment of renal function with dynamic
contrast-enhanced MR imaging. Magn Reson Imaging Bokacheva L, Rusinek H, Zhang
JL, Lee VS.