Dynamic in vivo oxymetry using overhauser enhanced MR imaging.

A noninvasive method for in vivo measurement of the oxygen concentration has been developed. By introducing a novel contrast medium (CM) based on a single electron substance, it is possible to enhance the proton signal through the Overhauser effect. A low-field magnetic resonance scanner is used to image the proton nuclei of the object. The electron spin transition of the CM is saturated using rf irradiation. As a consequence, the nuclear polarization becomes enhanced through dipole-dipole interaction. The signal enhancement is a function of rf power and of the EPR line width of the substance, which is influenced by the oxygen concentration. The maximum in vivo enhancement has been measured to 60. Image data, generated with different scanning parameters, is used in a postprocessing method to generate images showing pO(2) and the contrast medium concentration, respectively. The mathematical foundation of the postprocessing algorithm is outlined. The results from phantom experiments and animal experiments, in which the oxygen content of the inspired gas was varied, are presented. The potential for human imaging is discussed. J. Magn. Reson. Imaging 2000;12:929-938.

EPR and DNP properties of certain novel single electron contrast agents intended for oximetric imaging.

Parameters of relevance to oximetry with Overhauser magnetic resonance imaging (OMRI) have been measured for three single electron contrast agents of the triphenylmethyl type. The single electron contrast agents are stable and water soluble. Magnetic resonance properties of the agents have been examined with electron paramagnetic resonance (EPR), nuclear magnetic resonance (NMR), and dynamic nuclear polarization (DNP) at 9.5 mT in water, isotonic saline, plasma, and blood at 23 and 37 degreesC. The relaxivities of the agents are about 0.2-0.4 mM-1s-1 and the DNP enhancements extrapolate close to the dipolar limit. The agents have a single, narrow EPR line, which is analyzed as a Voigt function. The linewidth is measured as a function of the agent concentration and the oxygen concentration. The concentration broadenings are about 1-3 microT/mM and the Lorentzian linewidths at infinite dilution are less than 1 microT in water at room temperature. The longitudinal electron spin relaxation rate is calculated from the DNP enhancement curves. The oxygen broadening in water is about 50 microT/mM O2 at 37 degreesC. These agents have good properties for oximetry with OMRI.

Overhauser-enhanced MR imaging (OMRI).

PURPOSE: To evaluate a new single-electron contrast agent for Overhauser-enhanced MR imaging. The contrast agents that are currently available give enhancement factors that are too low to make the technique a valid option for routine clinical use. MATERIAL AND METHODS: MR images were generated directly following the injection of the substance into rats. The MR scanner was operated at a main magnetic field of 0.01 T and equipped with a separate rf-transmitter tuned to the electron paramagnetic resonance frequency of the contrast agent. RESULTS: As expected, the images generated show a high level of enhancement in areas where the contrast agent was present, and a maximum enhancement of 60 times the normal proton signal was obtained in the vascular area. The signal-to-noise ratios in the images were superior to those previously attained. CONCLUSION: The new contrast agent makes it possible to generate MR images with both morphological and functional information at 0.01 T. The signal-to-noise ratios found in the generated images were of the same order as, or better than, those obtained with the standard clinical routine.

On a novel MRI technique (OMRI) for the determination of tissue parameters.

In clinical diagnostic medicine the assessment of key physiological tissue parameters is important. OMRI is a novel ultra-low field MR-technology taking advantage of the Overhauser effect in order to enhance the signal. However, the technology further enables the visualization of tissue oxygen due to its interaction with an administrated CM. Experimental results from rats show morphological images of high resolution and further the alternation of tissue oxygen as a function of various external physiological conditions. It is believed that the novel technique enables and expands bed-side diagnosis.

Magnetic resonance imaging of the masseter muscle of the rabbit.

Gross anatomy of the masseter muscle was studied in eight rabbits using a 2.34 tesla MRI-device. Anatomy was displayed and assessed consistent with previous dissectional findings. It is concluded that MRI, applied as described in the present study, is suitable for assessment of masseter morphology in experimental studies on the rabbit.

MRI contrast media for the liver. Efficacy in conditions of acute biliary obstruction.

The authors investigated in a rat model the efficacy of magnetic resonance imaging (MRI) contrast media for evaluating the liver in conditions of acute biliary obstruction. Two liver-specific MRI contrast media, Cr-DEHIDA and Mn-DPDP, and the nonspecific agent Gd-DTPA were studied in normal rats and in rats whose bile ducts had been ligated before administration of the contrast medium. Images were made using a 2.4 T animal MRI system, and intensity enhancement of liver after contrast medium injection was calculated. Metal analyses of serum and liver tissue and T1 and T2 measurements on liver samples in vitro were performed. The differences in image intensity enhancement of liver between normal rats and rats with ligated bile ducts were not significant for any of the three contrast media. Imaging with Mn-DPDP resulted in the highest intensity enhancement of the liver compared with Cr-DEHIDA and Gd-DTPA. Contrast media concentrations in liver tissue were not significantly different between normal rats and rats with ligated bile ducts; however, Cr-DEHIDA concentrations in serum were higher after bile duct ligation. In vitro measurements of liver tissue indicated unique relaxation properties for Mn-DPDP. This investigation indicates that the contrast media studied may be useful in situations where suspected liver pathology is complicated by acute biliary obstruction.

A magnetic resonance imaging contrast medium for the liver and bile.

A pharmacokinetic investigation of a paramagnetic Cr-HIDA derivative was performed. Blood, bile, and urine were collected during the first 2 hours after injection of Cr-HIDA 0.01, 0.05, and 0.25 mmol/kg in rats or rabbits. The pharmacokinetics of the substance were found to be similar to those of the biliary iodinated contrast media in common use. Magnetic resonance imaging performed at 10 minutes after injection into the animals revealed that it was necessary to use doses higher than 0.01 mmol/kg to obtain a diagnostically significant increase in signal intensity from the liver. The gallbladder, however, was clearly defined at this dose level.

Metabolism and excretion of iopentol in the pig.

The metabolism of iopentol was examined in pigs. The search was focused on the urine and bile excreted during the first day after intravenous injection of a high dose. Chromatographic methods (both TLC and HPLC) indicated that the maximum amount of metabolites was less than 1 per cent of the given dose in urine and less than 0.3 per cent in the bile.