Introduction
This proof-of-concept study—conducted in collaboration between the Max Planck Institute for Multidisciplinary Sciences and the Centre for Biostructural Imaging of Neurodegeneration (Göttingen, Germany)—investigates safer and more powerful imaging agents to advance MRI clinical diagnostics. The overarching goal is to achieve angiographic performance comparable to other vascular imaging methods while maintaining the key advantages of MRI: non-ionising imaging and broad clinical utility.
Although magnetic resonance imaging (MRI) is essential in modern diagnostics, one of its primary limitations is low sensitivity. Improving sensitivity can unlock clearer visualisation, faster scans, and more comprehensive assessments—particularly in cardiovascular disease settings, where rapid, high-contrast imaging can be clinically valuable.
Why Hyperpolarisation Matters in 13C MRI
Hyperpolarisation can dramatically enhance the MR signal of imaging agents. In particular, 13C hyperpolarisation enables stronger signals with useful traceability, supporting real-time imaging of injected tracers. Compared with alternative enhancement strategies, 13C approaches are attractive because they can be non-toxic and provide biologically relevant contrast without relying on heavy metals or ionising radiation.
This study focuses on parahydrogen-based hyperpolarisation, known as parahydrogen-induced polarisation (PHIP). Historically, PHIP methods were largely limited to molecules containing a double bond. However, the introduction of PHIP-side arm hydrogenation (PHIP-SAH) expands the scope of PHIP by enabling hyperpolarisation of a broader range of metabolites and derivative structures, including those suitable for in vivo imaging.
Clinical Context: Cardiovascular Imaging Needs Better Sensitivity
Cardiovascular diseases (CVDs) remain a leading cause of mortality worldwide, and cardiac magnetic resonance imaging is a major tool for assessing cardiac structure and function. However, sensitivity constraints can limit angiographic applications or require trade-offs in scan time and contrast quality.
Hyperpolarised angiography aims to overcome these limitations. An ideal hyperpolarised MRI contrast agent should be:
-
Biocompatible and low-toxicity
-
Capable of producing strong, background-free signal
-
Stable enough to persist briefly in circulation
-
Preferably an endogenous metabolic end product that is not rapidly metabolised, preserving imaging performance without changing chemical identity in vivo.
The Tracer: Hyperpolarised N-acetyl-alanine Derivative (NAVA)
In this work, the authors use a derivative of N-acetyl-alanine as a novel tracer, abbreviated to NAVA. The tracer is introduced into the bloodstream and ultimately undergoes renal excretion, making it well-suited for vascular imaging with a favourable in vivo profile.
The animal work was conducted under an approved Niedersächsisches Landesamt für Verbraucherschutz und Lebensmittelsicherheit (LAVES) protocol (20/3563). Animals were housed under standard laboratory conditions (5 per cage) with a 12-hour light cycle and food/water available ad libitum.
in vivo. Angiographic Imaging with 13C RARE
Following successful signal enhancement, angiographic imaging was performed using 13C RARE (Rapid Imaging with Refocused Echoes):
-
Anatomical images (coronal slices) were acquired in a 300 MHz spectrometer.
-
Mice received a single dose of 70 mM hyperpolarised N-acetyl-alanine ester.
-
A 13C RARE image was acquired 10 seconds after injection, generating background-free angiographic signal.
-
A superimposed representation showed localised signal in the cardiac region, including the left and right ventricles, with prominent renal signal (notably the right kidney).
-
A schematic summary indicated additional vessels where signal intensity fell below the applied cut-off.
This workflow demonstrates how hyperpolarised 13C techniques can capture vascular information within a short imaging window, supporting rapid angiography-style readouts.
Summary and Key Findings
Overall, the study demonstrates the synthesis and performance of hyperpolarised [1-13C] N-acetyl-alanine ethyl ester as a versatile contrast agent for 13C MRI angiography. With sub-second scan capability using 13C RARE imaging, the approach enabled background-free angiography, clearly visualising major cardiovascular structures such as the heart, vena cava, and aorta within a single scan.
Safety and practical considerations
-
Ester cleavage produces ethanol in equimolar quantities. While the levels used here were below toxic thresholds, ethanol formation remains a consideration for future optimisation and work-up procedures.
-
The T1 of [1-13C] N-acetyl-alanine in H2O was reported to be significantly longer, supporting improved residence time in circulation and extended imaging utility.
-
The post-hyperpolarisation work-up was adapted to use a deuterated buffer (D2O). Related research has reported safe D2O injection in humans with improved signal-to-noise ratio (SNR), suggesting potential translational value for protocol development.
-
N-acetyl-alanine can be stored stably in dry form at low temperature for months without compromising chemical integrity—an advantage for preclinical workflows and repeated experimental use.
How NAVA Compares with Other Angiography Approaches
Compared with other hyperpolarised angiography agents (e.g., urea or water), N-acetyl-alanine derivatives offer flexibility because they may be hyperpolarised via multiple approaches, increasing accessibility for different laboratories and instrument setups.
Alternative angiographic imaging methods include:
-
Gadolinium-based proton MRI, which can involve longer scan times and carries risks related to gadolinium retention and nephrogenic systemic fibrosis in vulnerable patients
-
[15O]H2O PET/CT, which offers strong imaging performance but exposes patients to ionising radiation
While those methods can provide high-detail outputs, the potential safety and workflow benefits of hyperpolarised 13C angiography make NAVA an appealing candidate for ongoing development.
Outlook
Further refinement of hyperpolarisation conditions, work-up design, and imaging protocols will be important to fully realise the potential of hyperpolarised N-acetyl-alanine derivatives as safer, competitive contrast agents for MRI-based angiography. With continued optimisation, PHIP-SAH-enabled tracers may expand the toolkit for high-sensitivity in vivo imaging—particularly for cardiovascular applications where rapid, background-free contrast is clinically valuable.
