Reversible Valence Isomerisation

Abstract

RADIOISOTOPE scanning is an exploratory technique performed in an attempt to determine and visualize the distribution of radioactivity in the body. It is becoming rapidly accepted throughout the world because it is sometimes the only method, and often a powerful additional method, of tackling many diagnostic problems in clinical practice. 2. PRINCIPLES OF THE TECHNIQUE Clinical Information RequiredThere are two clinical types of information sought from scanning: (i) the detection and visualization of an organ for the purpose of: ( a ) demonstrating its presence, ( b ) finding its position relative to anatomical features, ( c ) estimating its size and shape, ( d ) estimating the relative concentration of isotope compared uit'h other organs (this may give information on the normality or otherwise of its function). and (ii) the detection and visualization of space occupying lesions within the organ for the purpose of: ( a ) showing their existence, ( b ) finding their position?(c) estimating their size and shape, ( d ) estimating their relative concentra,tion to their surroundings (this may give an indication of the type of lesion). The Scanning ProblemThe object of radioisotope scanning, in its simplest terms, is to show the existence of a target containing a uniform concentration Ct of radioactive material which is embedded in a surrounding medium containing a different uniform concentration CS (see fig. 1).There ma;, in practice, be neighbouring normal structures or organs containing different concentrations which may overlap the target region so that the perception of the target is made much more difficult. It must: in fact, be true that individual blood vessels will act' as neighbouring structures but the present resolution of scanning machines is not good enough to resolve them as discrete regions on the scan.In addition t'o organs containing relatively high concentrations of radioactive material, tissues such as bone? may throw a " shadow " on a scan.As a result, the general configuration of neighbouring organs, blood vessels and muscles give rise t o a normal ' scan pattern for a given W P.M.B. 310 John R. Mallard Fig, 1 (i) Simplified phantom reconstruction of the scanning problem with target (concentration Ct) embedded in surrounding medium (concentration CS).(a) the y-ray flux on surface of phantom ( b ) the count-rate from the detector and collimator (c) the visualizing properties of a linear display ( d ) the visualizing properties of a display with high contrast and ( e ) the visualizing properties of a display controlled bp a, ratemeter (ii) Profile curves of information presented bp saturation with long time-constant.region using a particular radioactive material and it is necessary to know this normal pattern before abnormal patterns can be recognized with certainty.Now the target will only be displayed by the scanning machine as a result of the detector picking up the difference between the gamma-ray flux emerging from the surface of the body over the region of the target and that over the surround...

Studies in valence isomerism of unsaturated paramagnetic species

“…Cyclooctatetraene also exists in equilibrium with its bicyclic isomer (22,23,24), and the concentration of the bicyclic form has been measured as 0.01% at 100° (25). In fact, the rate determining step for Diels-Alder cyclo additions to cyclooctatetraene consists of the bicyclic isomer formation (25).…”