Contrast concentration

It should be noted that, in two of these studies, " the perfusion parameter used to define the mismatch was not CBF or MTT, but instead the time it took for contrast concentration to reach peak concentration in each image voxel after contrast injection ( time to peak or TTP). TTP measurements are often used as rough approximations of MTT measurements because calculation of CBF and MTT are somewhat complex, requiring a mathematical process called deconvolution. The details of deconvolution are beyond the scope of this chapter, and the reader is referred to other sources for further explanation. In many clinical settings, maps of parameters like TTP that do not require deconvolution may be available much more quickly than those that do require deconvolution. TTP is less specific than MTT in detecting underperfused tissue because it does not distinguish between delayed contrast arrival time (such as that related to perfusion via collateral vessels) and truly prolonged intravascular transit time. 

Concentrations of V, Mn, Fe, Cr, Co, Cu, Zn, As, Se, Mo, Ag, Cd, Tl, Hg, Pb, and organic mercury (Org-Hg) were determined in liver, kidney, and muscle of healthy Caspian seals (Phoca caspica) collected in 1998. These concentrations were compared with those of seals infected with canine distemper virus (CDV) found stranded along the coastal areas in 2000 (Table 1). Concentrations of toxic elements (As, Ag, Cd, Tl, Hg, Pb, and Org-Hg) in Caspian seals stranded in 2000 were comparable or lower than those of samples collected in 1998 and in other pinnipeds. Thus it may be inferred that these elements were not the causative agents in the deaths of the seals. In contrast, concentrations of Zn and Fe were much higher in diseased Caspian seals than those in other pinnipeds. Zinc concentrations in all tissues of Caspian seals also increased during 1993-2000. Furthermore, negative correlations were found between blubber thickness and hepatic and renal Zn concentrations. These results imply the disturbance... 

Results of the field experiment are shown in Eig. 16.27a, which is based on combined discharge from five extraction weUs. After about 50 days, TCM concentrations decreased. In contrast, concentrations of TCE fluctuated but remain relatively high. PCE concentrations continued to increase over time, exhibiting a higher dissolution rate over the first 100 days of the experiment. These results were used to plot (Eig. 16.27b) the observed relationship between concentration ratio and source transformation by dissolution-induced depletion, together with the equivalent theoretical relationships. Source depletion was calculated from the cumulative mass removed, as determined from monitoring of effluent at specific times, divided by the initial source mass. 

It is particularly reassuring to note that both laboratories observed almost identical trends in data for the mud samples collected from Carlton Marsh (in the vicinity of Sellafield). The sample collected in the last quarter contained concentrations of Cs and that were elevated (by an order of magnitude) compared with those obtained earlier in the year. In contrast, concentrations of Co were noticeably reduced (by 4 fold). A similar, less marked, trend was apparent in the results for sandy mud obtained from the Ravenglass estuary (concentrations of Cs and " Am in the last quarter elevated by 3 fold whilst the Co concentration was halved). At both sites, the temporal variation in activity concentrations was attributed to the sampling, as opposed to any effect from recent Sellafield discharges. It is likely, due to the dynamic environment in which the samples were collected, that the samples from the last quarter consisted primarily of sub-surface... 

In contrast, concentrations of methane over the middle shelf and the coastal region were similar to the concentrations seen in the surface waters of St. George Basin. Over most of the middle shelf, near-bottom concentrations of methane ranged from 400-600 nL/L. 

West of the source region, there are four more sampling locations on the upper reaches of the Lippe river (sites 17 to 14). This area is not densely populated, but there are two major sewage treatment plants (Fig. 2 Paderbom and Lippstadt). The effluent discharge of the STP s leads to significantly higher loads of HHCB and AHTN at sites 16 to 14. In contrast, concentrations are lower than at site 18. This is the result of Lippe water dilution by several small brooks (e.g. the Beke Brook) and by some larger tributaries (e.g. the Alme river). 

Bluemke DA, Eishman EK, Anderson IH (1995) Effect of contrast concentration on abdominal enhancement in the rabbit spiral computed tomography evaluation. Acad Radiol 2(3) 226-231. 

The technical considerations and interpretation of the second portion of the acute stroke protocol, CTA, is discussed in detail in Chap. 4. Importantly, however, the source images from the CTA vascular acquisition (CTA-SI) also supply clinically relevant data concerning tissue level perfusion. It has been theoretically modeled that the CTA-SI are predominantly blood volume, rather than blood flow weighted [20, 27,70], The potential utihty of the CTA-SI series in the assessment of brain perfusion is discussed in detail below. This perfused blood volume technique requires the assumption of an approximately steady state level of contrast during the period of image acquisition [27], It is for this reason - in order to approach a steady state - that protocols call for a biphasic contrast injection to achieve a better approximation of the steady state [71, 72]. More complex methods of achieving uniform contrast concentration with smaller doses have been proposed that may eventually become standard, such as exponentially decelerated injection rates [73] and biphasic boluses constructed after analysis of test bolus kinetics [72, 74]. 

Quantitation and Resolution. While CTP and MR-PWl both attempt to evaluate the intricacies of capillary-level hemodynamics, the differences in technique create several important differences that should be considered (Table 5.5). While dynamic susceptibility contrast (DSC) MR-PWI techniques rely on the indirect T2 effect induced in adjacent tissues by high concentrations of rv gadolinium, CTP relies on direct visualization of the contrast material. The linear relationship between contrast concentration and attenuation in CT more readily lends itself to quantitation, which is not possible with MR-PWI techniques. Comparison with a positron emission tomography reference standard resulted in an accurate CBF calculation with CTP [92], but overestimation of CBF and... 

Concentrations of Imwe (when expressed) were higly variable in different urine samples from individual males, among males within a strain, and among strains. No significant strain or caging differences were observed for Imwe concentrations. In contrast, concentrations of the other esterases were significantly influenced by strain and caging. 

Figure 4.19 shows the contrasting concentration/time curves for a discontinuous fermentation (Moser and Lafferty, 1976). For integral analysis, the typical procedures are represented as if carried out in a microbiological laboratory using shaking flasks. A series of i experiments is carried out, each... 

Trans-7, cis-9 CLA is also present in the body fat of ruminants, and, as with milk fat, it is the second most prevalent CLA isomer (13,14). Endogenous synthesis is the likely source, although this has not been investigated directly. Concentrations of trans-7, cis-9 CLA are neghgible in rumen fluid (61), omasal digesta (68), and duodenal samples (39) from cows. In contrast, concentrations of trans-7, cis-9 CLA in beef fat represent 10-20% of values for cis-9, trans-ll CLA and are several-fold... 

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