Uptake rates of macrominerals and trace elements were characterized in batch and continuous cultures of Spirulina platensis under photoautotropic conditions. The values of yield coefficients were determined using inductively coupled plasma emission spectroscopy (ICP-ES). Further simplifications of culture medium proved possible, mainly in the trace element solutions; concentrations of some elements were lowered and trace elements B, Mo, V, Cr, Ni, Co, W, and Ti were removed.

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liT I (lIs) 6 01 11 5 • 21 ! 4 3 2 4 4 mmolll 3 3 2 2 a O+---------------~---__j o o 01 • 0.51 • 1% o 21 O+-----r-----r-----~-----i o phase correction ofthe images. Possible phase errors can, in fact, result from different sources such as imperfections in the inverting pulse, static and RF field inhomogeneities, or different flip angles over the width of the selected slice.By using a mixed experiment one can assume that the phases of the complex signals (SE and IR) behave similarly, so that the ratio of equivalent pixel values in the SE and IR matrices is less affected. Such a ratio can be made less sensitive to phase errors, b mmol/l Fig. I. Relaxation rates IITI (a) and IIT2 (b) versus copper sulphate concentration. Each line is the least squares fit to the data relative to a different weight percentage of agar.10 I/T2 (lIs) 20~-----------------~___, Material and MethodsAgar gel is obtained when the powder is added to water, while heating and stirring regularly at the same time. Once cooled at room temperature, the gel is optically transparent and solid; its NMR properties are quite well reproducible and stable in time. Doping with transition metals (in our case CuSO. salt) can be done in the liquid phase.The copper sulphate concentration ranged between 0.0 and 3.0xlO· 3 mol/I. The weight percentage of agar varied within the limits 0 and 2 per cent. Sixteen samples were filled with serial dilutions of this solution and placed in a plastic phantom, surrounded by water. Each tube was a 10 cm high cylinder, with a diameter of 1.3 em, The phantom was imaged by a 0.5 T MR Philips scanner, using a coil, 30 em in diameter. The acquisition matrix was 256x256, resulting in a pixel size of I mmx I mm. Images of a slice IS mm thick were reconstructed by the 20FT technique.Tl measurements were performed using the simple two point ratio method with a mixed sequence, where an inversion recovery (lR) and a spin echo (SE) are consecutively combined in one cycle.Commonly the TI calculation needs a technique for performingThe problem of MR simulation of biologic tissues has been approached by many authors (5, 10) with different purposes, such as a better understanding of both relaxation mechanisms (2, 3) and RF power dissipation in tissues (6, 7).The aim of this investigation was to find a phantom material which could be used for easy control of some performances of an MR imaging system. In general, the goal was to check the quality of Tl and T2 computed images and to compare signal intensity of the proposed medium with that of the tissue of interest, in scans routinely performed with standard pulse sequences.As previously proposed 0, 10), an agar gel was chosen, because of its T2 relaxation properties. In fact, owing to the semisolid behaviour when agar is dissolved in water (9), the structure of the gel presents a restricted motion of bound and free water. Moreover, if paramagnetic ions are added to an agar solution, Tl and T2 values are even more strongly reduced than in an aqueous solution, so that, by combining different we...