[Show abstract][Hide abstract] ABSTRACT: Over recent years hyperpolarization by dissolution dynamic nuclear polarization has become an established technique for studying metabolism in vivo in animal models. Temporal signal plots obtained from the injected metabolite and daughter products, e.g. pyruvate and lactate, can be fitted to compartmental models to estimate kinetic rate constants. Modelling and physiological parameter estimation can be made more robust by consistent and reproducible injections through automation. An injection system previously developed by us was limited in the injectable volume to between 0.6 and 2.4 ml and injection was delayed due to a required syringe filling step. An improved MR-compatible injector system has been developed that measures the pH of injected substrate, uses flow control to reduce dead volume within the injection cannula and can be operated over a larger volume range. The delay time to injection has been minimised by removing the syringe filling step by use of a peristaltic pump. For 100 ul to 10.000 ml, the volume range typically used for mice to rabbits, the average delivered volume was 97.8% of the demand volume. The standard deviation of delivered volumes was & ul for 100 ul and 200 ul for 10.000 ml demand volumes (mean S.D. was 9 ul in this range). In three repeat injections through a fixed 0.96 mm O.D tube the coefficient of variation for the area under the curve was 2%. For in vivo injections of hyperpolarized pyruvate in tumor-bearing rats, signal was first detected in the input femoral vein cannula at 3-4 s post injection trigger signal and at 9-12 s in tumor tissue. The pH of the injected pyruvate was 7.1± 0.3 (mean ± S.D., n=10). For small injection volumes, e.g. less than 100 ul, the internal diameter of the tubing contained within the peristaltic pump could be reduced to improve accuracy. Larger injection volumes are limited only by the size of the receiving vessel connected to the pump.
Journal of Magnetic Resonance 01/2013; · 2.30 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: It has recently been demonstrated experimentally that cardiac pulsations seem significantly to affect the arterial spin labelling (ASL) signal. In this paper, we introduce a new theoretical model to examine this effect. Existing models of ASL do not take such effects into account since they model the transit of the ASL signal assuming uniform plug flow with a single transit delay. In this study, we model cardiac pulsations through the coupling of the Navier-Stokes equations with the three-dimensional mass transport equation. Our results complement the experimental findings and suggest that the ASL signal does depend on the timing of the onset of the cardiac cycle relative to the tagging and imaging locations. However, cardiac pulsatility only appears to have a small effect on the quantification of perfusion estimates.
Physics in Medicine and Biology 02/2010; 55(3):799-816. · 2.70 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Recent experimental results have shown that effects such as dispersion and cardiac pulsation have a significant effect on the arterial spin labeling (ASL) signal. These have not been incorporated into the existing ASL models potentially leading to inaccuracies in flow calculation. In this study, we develop a new model, based on physical principles, to model the transit of the ASL signal from the tagging band to the imaging band using the mass transport equation. We relax the assumption of a uniform plug flow, and account for the dispersion caused by the viscous nature of blood. The model also provides a framework within which other physiological aspects can easily be examined. Here, we examine the effects of flow dispersion on the ASL signal, and hence the quantification of cerebral perfusion. Our results suggest that not accounting for flow dispersion may result in inaccurate values of cerebral perfusion.
[Show abstract][Hide abstract] ABSTRACT: A major application of dynamic nuclear polarization has been in the study of the conversion of hyperpolarized 13C1-pyruvate to lactate in various disease models. In a typical experimental protocol, hyperpolarized pyruvate is converted from solid to liquid state with superheated fluid and collected in a receiving vessel. The hyperpolarized solution is then rapidly transferred by hand from the polarizer to the imaging magnet, where it is manually injected through an intravenous cannula into the test subject by an experienced operator. Such a procedure leads to inconsistencies in timing, injection rate and volume, all of which can influence the time resolved signal as the pyruvate is metabolized. We have developed a fully magnetic-resonance-compatible withdraw/infuse syringe pump made entirely of plastic so that it can be operated within the bore of an unshielded 7 T (310 mm) magnet. The injector can be programed for variable injection volumes and rates to permit the rapid and reproducible injection of hyperpolarized material without human intervention. The injector was designed for use with 1 ml or 3 ml syringes with a maximum delivery volume of 2.4 ml. The standard deviation of delivered volume from the desired volume was found to be 0.7 % across a volume range of 0.6–2.4 ml.
Applied Magnetic Resonance 43(1-2). · 0.83 Impact Factor