MR perfusion imaging using encapsulated laser‐polarized ³He

Abstract

In this work, the use of a new carrier agent for intravascular laser-polarized 3 He imaging is reported. Lipid-based helium microbubbles were investigated. Their average diameter of 3 m, which is smaller than that of the capillaries, makes it possible to conduct in vivo studies. The NMR relaxation parameters T 1 , T 2 , and T* 2 of a microbubble suspension were measured as 90 s, 300 ms, and 4.5 ms, respectively, and in vivo images of encapsulated 3 He with signal-to-noise ratios (SNRs) larger than 30 were acquired. Dynamic cardiac images and vascular images of encapsulated 3 He were obtained in rats using intravenous injections of microbubble suspensions. Excellent preservation of Several optical pumping techniques (1-3) have been developed for polarization of the nuclear magnetic moments of two noble gas isotopes, 3 He and 129 Xe. These techniques offer NMR signal enhancements of five orders of magnitude compared to thermally polarized nuclei at room temperature.One important biomedical application of these hyperpolarized (HP) gases is the visualization by MRI of cavities such as the lungs (4 -7). Another potential application is their use as intravascular contrast agents. Initial in vivo NMR studies of blood-dissolved HP gas have already been performed on animals using polarized 129 Xe. The polarized nuclei can be delivered to blood and tissues directly from inhaled gas (7-11) or can be alternatively dissolved (12,13) in an appropriate emulsion (typically a lipid-enriched solution) and injected in a venous or arterial vessel.3 He possesses a larger nuclear magnetic moment value than 129 Xe and, at present, higher levels of polarization can be achieved with 3 He. Hence, for an equivalent number of nuclear spins, a typical gain of 10 in NMR signal amplitude can be achieved using 3 He instead of 129 Xe. Furthermore, due to the threefold-larger magnetic moment of helium, there is less demand on gradient magnetic field strength in 3 He MRI. Finally, xenon is well known for its lipophilic and anesthetic properties that may limit the amount of dissolved xenon acceptable in the blood.However, compared to xenon, helium is characterized by a much lower solubility (ϳ0.01 ml of dissolved gas per ml of fluid in blood and water (14) vs. ϳ0.1 for xenon). This precludes an efficient delivery of helium to the intravascular system from inhaled polarized helium. Similarly, major difficulties are encountered in the preparation of an injectable solution of dissolved helium. The first in vivo intravascular helium images were obtained (15) using helium microbubble suspensions. However, the large mean bubble diameter of these suspensions reported by the authors (on the order of 30 m) represented a major limitation for the safe in vivo use of these solutions.Fortunately, the problem of low dissolution coefficient can be overcome using an appropriate carrier agent. Recently, in vivo vascular images have been reported using encapsulated 3 He in microspheres of 5.3 m diameter (16) formed from human serum albumin.In this work, we propose...

Cerebral perfusion assessment by bolus tracking using hyperpolarized 13C

“…However, a potential problem is the influence of tracer depolarization, a result of the nonequilibrium nuclear spin population, on the derived parameters. Perfusion studies using hyperpolarized tracers have been performed earlier using 129 Xe for cerebral applications (16,17), and 3 He encapsulated in microbubbles has been used for studies of various tissues (18,19). For both of these isotopes the potential signal enhancement due to high polarization was of limited benefit due to the low concentration of the tracer.…”

Contrast-enhanced magnetic resonance angiography

“…The Ostwald coefficient for xenon in pure blood is much lower, approximately 0.17 (93). For 3 He-gas encapsulated in microbubbles, the concentration depends on the concentration and size of the bubbles in the carrier suspension (12,17). The HP 13 C-nucleus is part of a water-soluble molecule, and the deliverable concentration is in this case only limited by the concentrations that can be reached during the polarization procedure (paper IV).…”

Indirect detection of lung perfusion using susceptibility‐based hyperpolarized gas imaging

“…Since these studies rely on reporting changes in the T2 (or phase) of the protons of the surrounding tissues, their application is severely restricted in the lungs, where the proton concentration is low. Finally, there are two published reports (27,28) that have investigated the possibility for using HP gas and injection of superparamagnetic iron oxide nanoparticles to measure lung perfusion. One of the significant differences between their studies and our report is that they rely on magnitude images to gain insight into perfusion.…”