Respiration resolved imaging with continuous stable state 2D acquisition using linear frequency SWEEP.

PURPOSE: To investigate the potential of continuous radiofrequency (RF) shifting (SWEEP) as a technique for creating densely sampled data while maintaining a stable signal state for dynamic imaging. METHODS: We present a method where a continuous stable state of magnetization is swept smoothly across the anatomy of interest, creating an efficient approach to dense multiple 2D slice imaging. This is achieved by introducing a linear frequency offset to successive RF pulses shifting the excited slice by a fraction of the slice thickness with each successive repeat times (TR). Simulations and in vivo imaging were performed to assess how this affects the measured signal. Free breathing, respiration resolved 4D volumes in fetal/placental imaging is explored as potential application of this method. RESULTS: The SWEEP method maintained a stable signal state over a full acquisition reducing artifacts from unstable magnetization. Simulations demonstrated that the effects of SWEEP on slice profiles was of the same order as that produced by physiological motion observed with conventional methods. Respiration resolved 4D data acquired with this method shows reduced respiration artifacts and resilience to non-rigid and non-cyclic motion. CONCLUSIONS: The SWEEP method is presented as a technique for improved acquisition efficiency of densely sampled short-TR 2D sequences. Using conventional slice excitation the number of RF pulses required to enter a true steady state is excessively high when using short-TR 2D acquisitions, SWEEP circumvents this limitation by creating a stable signal state that is preserved between slices.

Metabolism of [14C]glucose by postimplantation mouse embryos in vitro.

Carbon dioxide and lactate production from [14C]glucose were measured for post-implantation mouse embryos aged 6 to 9 1/2 days post coitum in static cultures with a defined medium. The rate of metabolism increased rapidly and paralleled the increase in protein content indicating a fairly uniform rate of metabolism throughout the period. At all stages studied more than 90% of the glucose utilized was converted to lactate. Over a quarter of carbon dioxide produced was derived from the C-1 position resulting in high C-1:C-6 ratios, indicating that the Pentose Phosphate Shunt is a major oxidative pathway. The influence of various culture condition on CO2 production showed that high concentrations of glucose did not affect glucose utilization whilst high lactate concentrations had a significant inhibitory effect. Pyruvate had no discernible effect.

Presence of cap structures in the messenger RNA of mouse eggs.

The presence of 5'-terminal 7-methylguanosine cap structures in the messenger RNA molecules in mouse eggs was assessed by (1) translational inhibition in a message-dependent reticulocyte cell-free system with the cap analogue 7-methylguanosine-5'-triphosphate, and (2) enzymatic removal of cap strucures with tobacco acid phosphatase followed by end-labelling with gamma-32P-ATP and polynucleotide kinase. The results suggest that the majority of mRNA molecules contain cap structures. There was no detectable difference in degree of capping in unfertilized versus fertilized egg mRNA.