We explored the potential for clinical research of computed tomography (CT) with monochromatic x-rays using the preclinical multiple energy computed tomography (MECT) system at the National Synchrotron Light Source. MECT has a fixed, horizontal fan beam with a subject apparatus rotating about a vertical axis; it will be used for imaging the human head and neck. Two CdWO4-photodiode array detectors with different spatial resolutions were used. A 10.5 cm diameter acrylic phantom was imaged with MECT at 43 keV and with a conventional CT (CCT) at 80 kVp: spatial resolution approximately equal to 6.5 line pairs (lp)/cm for both; slice height, 2.6 mm for MECT against 3.0 mm for CCT; surface dose, 3.1 cGy for MECT against 2.0 cGy for CCT. The resultant image noise was 1.5 HU for MECT against 3 HU for CCT. Computer simulations of the same images with more precisely matched spatial resolution, slice height and dose indicated an image-noise ratio of 1.4:1.0 for CCT against MECT. A 13.5 cm diameter acrylic phantom imaged with MECT at approximately 0.1 keV above the iodine K edge and with CCT showed, for a 240 micrograms I ml-1 solution, an image contrast of 26 HU for MECT and 13 and 9 HU for the 80 and 100 kVp CCT, respectively. The corresponding numbers from computer simulation of the same images were 26, 12, and 9 HU, respectively. MECT's potential for use in clinical research is discussed.

The convergent synthesis of a new class of chiral dendrimers is described. Owing to their structural resemblance to depsipeptides they are called depsipeptide dendrimers. The ex-chiral pool synthesis starts from (R,R)-, (S,S)-, and meso-tartaric acid as branching units and dipeptides or tripeptides consisting of glycine, (L)-alanine, and (L)-leucine as chiral-spacer building blocks. The key intermediates for the convergent assembly of such depsipeptide dendrimers are the peptide-tartaric acid conjugates 13a,b, 19a,b, 25, and 27, which contain either an unprotected C terminus of the peptide chain (13 a,b, 25) or two unprotected hydroxy groups within the tartaric acid termini. Dendra up to the third-generation, by using different combinations of stereoisomeric building blocks, were synthesized and completely characterized. Since this construction principle of chiral depsipeptide dendrimers allows for a wide variation of the length, the primary structure of the peptide spacer, and the configuration of both the amino acid and the tartaric acid moieties, access to new combinatorial libraries is conceptually provided.

Only the region-growing and isocontour algorithms have the potential to be used for tumor volumetry. However, further improvements of these algorithms are necessary before they can be placed into clinical use.

Computed tomography (cr) with monochromatic x-ray beams was used to image phantoms and a live rabbit using the preclinical Multiple Energy Computed Tomography ( M E 0 system at the National Synchrotron Light Source. MECT has a horizontal fan beam with a subject apparatus rotating h u t a vertical axis. Images were obtained at 43 keV for single energy studies, and at energies immediately below and above the 33.17 keV iodine Kedge for dualznergy subtraction CT.Two CdW0,-photodiode array detectors were used. The highresolution detector (0.5