Helical acquisition

A substantial increase in patient dose results from helical acquisition in cardiac CT. Regardless of the number of X-ray tubes, the table feed must be sufficiently slow such that projection data of 180° gantry rotation are available for image reconstruction at all points in time and space even in patients with variable heart rate. Though radiation exposure can be markedly reduced... 

Very low-risk patients are not good candidates for CT imaging due to the risk of contrast and radiation exposure. Cardiac CT imaging based on the current standard of retrospective helical acquisitions is associated with a radiation dose exposure of 15mSv (three times the average exposure rate of conventional catheterization angiography), precluding use of the method for asymptomatic patients. 

In summary, a prospectively acquired scan with a large detector versus a regular helical acquisition with retrospective gating reduces the dose exposure by a factor... 

Fig. 16.8a,b. Helical versus axial acquisition, a A detector with a total z-axis coverage of less than 16 cm needs to be propagated along the heart in order to cover the volume of interest for coronary angiographic imaging, b This can be achieved either conventionally with a helical acquisition as on the lefior in a stepping... 

The ammonium/methylammonium transport (Amt) proteins of enteric bacteria are required for fast growth at very low concentrations of the uncharged NH3. Homologues exist in all three domains of life. They are essential at low ammonium (NHj + NH3) concentrations under acidic conditions. The Amt protein of S. typhimurium (AmtB) participates in acquisition of NHj /NH3, but cannot concentrate either NH3 or NHJ. In general, Amt proteins appear to be bidirectional channels for NH3. They are examples of protein facilitators for a gas [93], The majority of Amt proteins contain 11 transmembrane helices with the C-terminus facing the cytoplasm [94],... 

Fig. 2. Evolution of the spectrin superfamily. Rounded rectangles represent spectrin repeats. Shaded rectangles denote a-actinin-like repeats involved in dimerization, whereas unshaded rectangles represent repeats that were involved in duplication and/ or elongation events. The incomplete spectrin repeats involved in tetramer formation are proportionally represented depending on the number of repeat helices each protein contributes to the formation of a complete spectrin repeat. (Adapted from Dubreuil, 1991 Pascual et al., 1997.) A dystrophin/utrophin ancestor probably diverged from a-actinin at a relatively early stage and then underwent its own series of duplications and acquisitions of new motifs.

This chapter will discuss the role of CTA in the diagnosis and triage of acute stroke patients. First, the general principles of helical CT scanning will be reviewed, including image acquisition and reconstruction techniques. The stroke CTA protocol will then be described, followed by specific issues regarding the accuracy and clinical utility of stroke CTA. 

The development of helical CT in the early 1990s made possible the rapid acquisition of angiographic-type vascular images, with no greater risk of patient... 

Apolipoprotein A-1 is made up of repeating amphiphilic helices, and a heptad repeat pattern similar to that seen for coUed-coil proteins. However, in aqueous buffer at neutral pH the protein has a random coil stmcture. At pH 3, there is a gain in structure, as the internal fiuorescence shows the three Trp residues are in a hydrophobic pocket (Andreola et al. 2003). CD studies show an acquisition of helical structure at lower pH. The protein is not, however, stable at pH 4, forming insoluble material after around 5 min, the process being one of intermolecular association and then precipitation. Andreola et al. (2003) have hypothesised that the formation of these amyloid aggregates requires the peptide to pass from... 

Undoubtedly, acquisition of such fluctuation motion at the cytoplasmic side of the transmembrane B and C a-helices in the M-like state of D85N mutant (Fig. 24B) is responsible for a transient environmental change from the hydrophobic to hydrophilic conditions both at the Asp 96 and Schiff base (SB) as compared with the ground state (Fig. 24A), resulting in a reduced pAa value of Asp 96 in the M-like state, which makes proton uptake more efficient. Further, it is demonstrated that the presence of the van der Waals contact of Val 49 with Lys 216 at the Schiff base is essential to trigger this sort of dynamic change, as revealed from the NMR... 

It is expected, however, that the backbone dynamics could be substantially modified when such a 2D lattice assembly is distorted or disrupted as in bacterio-opsin (bO) prepared from either an hydroxylamine-treated bR or retinal-deficient ElOOl strain in which retinal-helix interactions is absent owing to lack of retinal. The resultant protein dynamic change caused by the removed retinal through the modified helix-helix interaction resulted in the preferentially suppressed NMR signals of [3-Ala-labeled bO at the loops and transmembrane a-helices near to the membrane surface. At the same time, the NMR spectrum of [l- C]Val-labeled bO was also partly suppressed especially in the region of the loops at lower frequency by acquisition of the fluctuation motions. 

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