Capillary flow or microscopic random directional coherent flow as a model of perfusion is investigated both theoretically and experimentally. In the model, we assumed that molecular motion within a finite resolvable volume element (voxel) is a superposition of flow of randomly oriented small capillaries. In such a case, the observed signal from the capillary flow within a voxel will be attenuated in signal amplitude without any change in phase. Although this attenuation effect is similar to the diffusion phenomenon, it differs basically in the following aspects: since the motion in each capillary segment is coherent, phase cancellation occurs at even echoes due to spin rephasing, while the diffusion phenomenon is a purely random Brownian motion of the thermally agitated molecules, changing both in direction and speed during the measurement period. Because of the random character of diffusion, even-echo rephasing cannot be observed. Thus capillary flow or perfusionlike microscopic flow can be measured based on the above distinct flow characteristics, i.e., signal restoration at even echoes versus signal amplitude attenuation at odd echoes. By applying a suitable mathematical algorithm, information on the capillary flow alone can be extracted from the two separate distinct measurements, i.e., one with a single echo and the other with a double echo. Both a theoretical calculation of the capillary flow, as well as the experimental results with a human volunteer by a 0.6-T nuclear magnetic resonance imager, are presented.

General strategies for the synthesis of macrocycles are quite limited. Aside from several very specialized reactions that include fragmentation of bicyclic rings, the basic approach relies on ring closure of an R,ω-difunctional linear precursor under conditions of high dilution (eq 1). 1 We report a new strategy for the synthesis of macrocycles. The approach draws upon our discovery of the polyhomologation of trialkylboranes. 2 Polyhomologation results in repetitive methylene insertions into the carbon-boron bond. When applied to boracyclanes, the reaction generates macrocyclic rings (eq 2). These macrocyclic organoboranes can be elaborated to carbocyclic rings and other derivatives.The polyhomologation reaction employs the ylide dimethylsulfoxonium methylide (1) 3 as monomer, and the polymerization is initiated by trialkylboranes. For simple trialkylboranes, all three alkyl groups on boron participate, giving rise to a star polymethylene organoborane 2 (Scheme 1). These oligomeric and polymeric organoboranes (2) are amenable to synthetic transformations that can be used to terminate the polymethylene chain with functionality. 4 For example, peroxide cleavage of the carbon-boron bonds in star 2 results in hydroxyl terminated linear polymethylene 3 in yields over 90% (Scheme 1). 2 The synthesis of macrocycles by the polyhomologation reaction requires boracycles as starting materials. These compounds are readily prepared by hydroboration of dienes. Since all three boron-carbon bonds undergo polyhomologation, we chose thexylborane 5 as the hydroborating agent. The thexyl group has been shown to exhibit a low migratory aptitude in rearrangements of organoboranes. 6 The suppressed tendency toward migration should permit exclusive

Two-dimensional diffusion coefficient maps (images) of a carefully controlled diffusion phantom have been measured by a new diffusion imaging sequence using a 0.6-T whole-body nuclear magnetic resonance (NMR) scanner having a gradient field strength of 2.5 mT/m. The free induction decay (FID) data for the diffusion coefficient images were collected by varying the duration of the readout gradient in the conventional two-dimensional Fourier imaging sequence. The experimental results obtained by the proposed NMR diffusion measurement technique indicate a close agreement with other previous measurements. The selection of optimum spin-echo time for maximum signal-to-noise ratio (SNR) in diffusion imaging is studied and also experimentally confirmed. Finally, a preclinical study with human volunteers has been performed and results are presented.