No abstract

Dynamic nuclear polarization has enabled hyperpolarization of nuclei such as 13 C and 15 N in endogenous substances. The resulting high nuclear polarization makes it possible to perform subsecond 13 C MRI. By using the dynamic nuclear polarization hyperpolarization technique, 10% polarization was obtained in an aqueous solution of 100 mM 13 C-labeled urea, ready for injection. The in vivo T1 relaxation time of 13 C in the urea solution was determined to 20 ؎ 2 s. Due to the long relaxation time, it is possible to use the hyperpolarized substance for medical imaging. A series of highresolution (Ϸ1-mm) magnetic resonance images were acquired, each with a scan time of 240 ms, 0 -5 s after an i.v. injection of the hyperpolarized aqueous [ 13 C]urea solution in a rat. The results show that it is possible to perform 13 C angiography with a signalto-noise ratio of Ϸ275 in Ϸ0.25 s. Perfusion studies with endogenous substances may allow higher spatial and͞or temporal resolution than is possible with current proton imaging techniques.T he diagnostic quality of magnetic resonance (MR) images depends on the signal-to-noise ratio (SNR) as well as the contrast-to-noise ratio (CNR). The SNR is a measure of what resolution and͞or scan time is needed to separate the NMR signal generated from a given region of interest from the background noise in the image. Every ͌ 2-fold improvement in SNR allows a doubling of the spatial resolution in one direction, and has a tremendous effect on the perceived image quality (1). The CNR is a measurement of how well a region of interest can be separated from surrounding regions, which in turn is the real measure of the clinical usefulness of an MR image. Above a given receiver frequency (when the dominating noise source is the subject to be imaged), the SNR will increase linearly with the main field (2). Consequently, the manufacturers of MRI scanners have directed considerable efforts toward the development of higher field instruments with the aim of improving the diagnostic quality of the images. In recent years, 3.0-T instruments have been introduced for clinical whole-body imaging (3).The use of dynamic nuclear polarization (DNP) techniques such as the Overhauser effect (4) has been suggested as an alternative to increase SNR without the need for high magnetic field strengths (5-7), and a signal enhancement factor of Ϸ60 has been achieved at a main field of 0.01 T (8). This enhancement is smaller than the theoretically expected factor of 329, which is based on dipolar interaction between the electron spins of the administered free radical and the 1 H nuclear spins. The main reason for this discrepancy is that the electron spin system cannot be fully saturated in vivo, because excessive radio frequency irradiation will lead to unacceptable heating of the subject. This problem can be circumvented by performing the polarization process outside the subject and the MR scanner. Nuclear polarizations close to 100% for protons and Ϸ50% for 13 C in various organic molecules have been reported when D...

No abstract

No abstract

No abstract