MR coronary angiography in pigs with intraarterial injections of a hyperpolarized ¹³C substance

Abstract

A new diagnostic application of a water-soluble contrast medium (CM) based on the hyperpolarization of a 13 C substance is introduced. The degree of polarization achieved is >30%, which is about a factor of 10 5 higher than the thermal equilibrium polarization level at 1.5 T. Imaging of hyperpolarized (HP) CM during a cardiac interventional MRI procedure was studied. Catheters were positioned in the left and right coronary arteries of pigs. A coil tuned to 13 C was used for nonproton imaging. The HP-13 C CM (ϳ5 ml, 0.5 M, ϳ30% polarization) was injected during projection imaging using a fully balanced steady-state free precession (SSFP) pulse sequence with and without cardiac gating. The contrast agent-filled catheter was clearly visible during the procedure. The coronary arteries were well depicted and the signal-to-noise ratios (SNRs) were in the range of 10 -40. The use of HP- Magnetic resonance imaging (MRI)-guided vascular interventional procedures have several potential advantages compared to X-ray guided procedures (1-5). The patient and the interventionalist are not exposed to ionizing radiation, and the patient can avoid adverse reactions due to an iodinated contrast medium (CM) and its potential nephrotoxic effects.Coronary angiography is one of many interventional procedures that potentially could benefit from the advantages offered by MRI. In proton-based real-time MRI, coronary angiography with intraarterial injections of a conventional gadolinium-type CM is problematic. The Gdbased CM used can to be diluted to obtain optimal relaxation times to enhance signal from the blood in the vessels, but the CM will rapidly redistribute and accumulate in the surrounding tissue during the procedure and thus gradually decrease the contrast effect. A 2D projection technique is efficient for quickly localizing the coronaries without prior knowledge of their exact positions. However, in proton imaging, large signal from background tissue is present in the 2D projection, and the coronary arteries will only be a small fraction of the tissue. Saturation techniques and imaging based on inversion nulling of tissue have been suggested (2,3,5), but these methods reduce the possibility of imaging in real time. Another complicating factor is that the CM loads in the tissue as a function of time after the injection. Consequently, the optimum inversion time for nulling the tissue will change during the procedure.One way to improve the contrast-to-noise ratio (CNR) in projection imaging is to inject a CM that can be imaged at a resonance frequency other than that of protons. Before any contrast is injected, the signal from all tissue is virtually zero on the 13 C resonance frequency. Recently, MRI using hyperpolarized(HP) 13 C was introduced for MR angiography (MRA) in rats (6,7). In these studies it was calculated that the obtainable signal-to-noise ratio (SNR) in the injection solution could be higher by a factor of Ͼ100 than that obtained in a standard 3T proton image. If such high signals can be obtained from the CM solut...

Cardiac metabolism measured noninvasively by hyperpolarized 13C MRI

“…In the case of 13 C-pyruvate the decay time (T 1 ) is Ϸ55 sec as long as the substance is kept in a water solution in the syringe. This T 1 is within the range reported for other hyperpolarized 13 C substances (21,26,27).…”

“…The level of polarization was measured immediately after the dissolution using a low-field NMR spectrometer operating at 2 mT resulting in a 13 C Larmor frequency of 22 kHz. A volume of 40 L was extracted from each polarized dose and inserted into the measuring cell of the spectrometer (21). Throughout the experiments the obtained polarization was in the range of 15-20%.…”

“…The hyperpolarization technique using 13 C, incorporated into small molecules (19,20), has produced polarization levels several orders of magnitude higher than thermal equilibrium values. The use of hyperpolarized 13 C substances for MR angiography (21,22), passive catheter tracking in interventional MRI (23), and perfusion (24,25) has been demonstrated. The dynamic nuclear polarization (DNP) method (20) may be used to enhance the signal of the 13 C-nuclei in endogenous substances such as amino acids, urea, and pyruvate (26,27), making it possible to perform in vivo visualization of the injected substances and their metabolites.…”

Passive catheter tracking during interventional MRI using hyperpolarized 13C

“…Catheterization of aortic branches using a three-lumen 13 C catheter could be used for selective MR angiography (MRA) or perfusion mapping, where a 13 C contrast agent is injected into the endovascular space (14,15). Arterial catheter injections of 13 C contrast agents into arteries that support a specific organ result in very high tissue or vasculature SNR depending on the diffusibility of the 13 C compound and the timing of the image acquisition.…”

Hyperpolarized 13C MR Angiography

“…The bSSFP imaging technique has been demonstrated to be a rapid, high-SNR technique for angiography and spectroscopic applications using hyperpolarized substances ( Figure 2) [21,22,24,25]. The gradient echo (GRE) technique has been used extensively for single resonance HP lung imaging [26][27][28], but less for HP-angiography using 13 C-labeled substances.…”