Comparison of four contrast medium delivery protocols in low-iodine and low-radiation dose CT angiography of the aorta.

AIM: To evaluate contrast medium delivery protocols for the optimal enhancement profile of the aorta with both a reduced dose of radiation and contrast medium, called double-low computed tomography (CT) angiography (DLCTA). MATERIALS AND METHODS: DLCTA was performed with 70 kVp and 200 mg iodine/kg in 205 patients following four protocols, namely slow rate (n=52), short duration (n=52), low concentration (n=50), and combined method (n=51), in comparison with a conventional group (120 kVp, 400 mg iodine/kg, n=51). The quantitative measurement of aortic attenuation, homogeneity, and subjective scores were evaluated. RESULTS: Overall, in the four DLCTA groups, the radiation dose was reduced by 62%, and the iodine dose was reduced by 50%. Among the four DLCTA groups, the signal to noise ratio (SNR) and contrast to noise ratio (CNR) of the thoracic aorta were similar, but a significant difference was noted in the abdominal aorta. The short-duration group had the highest peak enhancement, least homogeneity, and worst subjective scores. Good contrast enhancement and good homogeneity were significantly more frequent in the slow-rate (86.6% and 90.4%, respectively) and low-concentration groups (78% and 96.0%, respectively). Subjective scores exhibited a trend of higher scores in the low-concentration group and lower scores in the slow-rate group (p=0.071). CONCLUSION: DLCTA with 70 kVp and 200 mg iodine/kg is feasible for whole-aortic CT angiography. The low-concentration protocol is recommended owing to its most consistent optimal aortic enhancement profile. Alternatively, the slow-rate protocol can be considered for patients with limited venous access.

Ordering of hydrogen bonds in high-pressure low-temperature H2O.

The near K-edge structure of oxygen in liquid water and ices III, II, and IX at 0.25 GPa and several low temperatures down to 4 K has been studied using inelastic x-ray scattering at 9884.7 eV with a total energy resolution of 305 and 175 meV. A marked decrease of the preedge intensity from the liquid phase and ice III to ices II and IX is attributed to ordering of the hydrogen bonds in the proton-ordered lattice of the latter phases. Density functional theory calculations including the influence of the Madelung potential of the ice IX crystal correctly account for the remaining preedge feature. Furthermore, we obtain spectroscopic evidence suggesting a possible new phase of ice at temperatures between 4 and 50 K.

Synaptic plasticity: regulated translation in dendrites.

Synaptic activity can induce neurons to synthesize proteins important for cognition and brain development. Recent results suggest this activity-induced protein synthesis is partially mediated by regulated translation within neuronal dendrites.

Synaptic transmission: spillover at central synapses.

Recent studies suggest that transmitter molecules released at central synapses sometimes diffuse long enough distances to activate receptors located outside the synaptic cleft or even in neighboring synapses. This transmitter 'spillover' may have important physiological consequences.

Synaptic plasticity: going through phases with LTP.

Early and late expressing components of synaptic plasticity may underlie the temporal phases of behavioral memory. New studies argue that a balance between kinase and phosphatase activity regulates the transition between different phases of synaptic plasticity and memory.

The matter of mind: molecular control of memory.

A widely accepted hypothesis suggests that changes in synaptic strength underlie the formation of memories in the brain. LTP is a mechanism of synaptic strengthening. Induction of LTP depends on NMDA receptor activation, and its expression depends in part on protein kinase activity. Studies of knock-out mice suggest that LTP is critical for hippocampus-based memory. Genetic studies in Drosophila implicate cAMP metabolism in classical conditioning, a form of unconscious memory. Consolidating memories for long-term retention depends on the cAMP-inducible transcription factor CREB.

Synaptic plasticity: a role for nitric oxide in LTP.

Nitric oxide is back in the spotlight with a new series of studies showing that it plays an important role in long-term potentiation, the best-studied type of synaptic plasticity in the central nervous system thought likely to play an important role in learning and memory.

Estimating the distribution of synaptic reliabilities.

Using whole cell recording from CA1 hippocampal pyramidal neurons in slices, we examined the progressive decrease of N-methyl-D-aspartate receptor-mediated synaptic responses in the presence of the open-channel blocker MK-801. Previous studies analyzing this decrease have proposed that hippocampal synapses fall into two distinct classes of release probabilities, whereas studies based on other methods indicate a broad distribution of synaptic reliabilities exists. Here we derive the theoretical relationship between the MK-801-mediated decrease in excitatory postsynaptic current amplitudes and the underlying distribution of synaptic reliabilities. We find that the MK-801 data are consistent with a continuous distribution of synaptic reliabilities, in agreement with studies examining individual synapses. In addition, changes in the MK-801-mediated decrease in response size as a consequence of altering release probability are consistent with this continuous distribution of synaptic reliabilities.

Very short-term plasticity in hippocampal synapses.

Hippocampal pyramidal neurons often fire in bursts of action potentials with short interspike intervals (2-10 msec). These high-frequency bursts may play a critical role in the functional behavior of hippocampal neurons, but synaptic plasticity at such short times has not been carefully studied. To study synaptic modulation at very short time intervals, we applied pairs of stimuli with interpulse intervals ranging from 7 to 50 msec to CA1 synapses isolated by the method of minimal stimulation in hippocampal slices. We have identified three components of short-term paired-pulse modulation, including (i) a form of synaptic depression manifested after a prior exocytotic event, (ii) a form of synaptic depression that does not depend on a prior exocytotic event and that we postulate is based on inactivation of presynaptic N-type Ca2+ channels, and (iii) a dependence of paired-pulse facilitation on the exocytotic history of the synapse.

Colour is what the eye sees best.

It has been argued by Watson, Barlow and Robson that the visual stimulus that humans detect best specifies the spatial-temporal structure of the receptive field of the most sensitive visual neurons. To investigate 'what the eye sees best' they used stimuli that varied in luminance alone. Because the most abundant primate retinal ganglion cells, the P cells, are colour-opponent, we might expect that a coloured pattern would also be detected well. We generalized Watson et al.'s study to include variations in colour as well as luminance. We report here that our best detected coloured stimulus was seen 5-9-fold better than our best luminance spot and 3-8-fold better than Watson's best luminance stimulus. The high sensitivity to colour is consistent with the prevalence and high colour contrast-gain of retinal P cells, and may compensate for the low chromatic contrasts typically found in natural scenes.