Glucose uptake kinetics of Saccharomyces cerevisiae monitored with a newly developed FIA.

The glucose content of the culture liquid during shift experiments and synchronized cultures of Saccharomyces cerevisiae H1022 (ATCC 32167) was monitored using a greatly improved and highly precise FIA. During shift-up experiments on the dilution rate, an overshoot of the glucose-concentration was observed. The amplitude of the overshoot showed a dependency on the duration of undisturbed cultivation before application of the shift. Mutarotational non-equilibrium was excluded as the cause of the observed overshoot. For the first time glucose measurements of oscillating cultures of Saccharomyces cerevisiae are demonstrated with high accuracy and reproducibility. The data strongly support the proposals by Münch et al. (1992a, b) that faint oscillations in glucose concentration are responsible for the persistence of the synchronization. Analytical subsystems prove to be a powerful tool for investigation of the dynamics of metabolic pathways of microbial organisms. Accurate glucose measurements at low concentrations point out the limits and allow refinements of commonly used models.

Glycogen detection by in vivo 13C NMR: a comparison of proton decoupling and polarization transfer.

The performance of gated proton decoupling and polarization transfer with respect to glycogen detection by 13C NMR was investigated. Experiments were performed on a 1.5-T whole-body scanner using a 13C surface coil in combination with a proton head coil. Spectra were acquired from a glycogen phantom and from the lower leg of a healthy volunteer using proton decoupling and the polarization transfer method SINEPT. The signal strength of the C1 resonance of glycogen was determined and compared to a reference spectrum acquired without any form of sensitivity enhancement. In the phantom experiment both decoupling and SINEPT produced a signal gain of 3.5. Under in vivo conditions, the signal gain was approximately 2.5 for both techniques. We conclude that decoupling and polarization transfer are equivalently useful techniques for glycogen detection.

Increased rf power absorption in MR imaging due to rf coupling between body coil and surface coil.

Fears have been voiced that excessive tissue heating could occur in the event that first, a surface coil is placed with its axis parallel to the transmitting rf field leading to a maximal coupling of the two coils and second, the decoupling circuit of the surface coil breaks down. To avoid an rf coupling of the transmitting body coil to the receive-only surface coil, conventionally applied surface coils are equipped with an active electronic rf decoupling circuit. In extensive worst-case experiments on phantoms we have shown that no tissue heating occurs for surface coils which are equipped with semiconductor varicap diodes for tuning and matching. These coils should be safe for patient applications even if the decoupling circuit fails. Surface coils equipped with mechanically variable capacitors are generally passively decoupled. To simulate the worst-case situation phantom experiments were performed in which a surface coil of this type having no passive decoupling circuit was coupled to the transmitter coil by its geometric position. Theoretical calculations, in agreement with the experimental results achieved during a 15-min measurement in a 1.5-T MRI whole-body imager, show that a significant rf power deposition in the tissue underneath the coil wire occurs, leading typically to a local specific absorption rate of 24 W/kg and a local temperature rise of 5.2 degrees C.