Experimental cross-polarization transfer rates, from protons in the local dipolar field to carbons in an applied radio-frequency field, can be used to determine quantitatively the relative contributions of spin-lattice and spin-spin processes to carbon Tlp relaxation. We show that this procedure is to be preferred to determinations based either on various semiquantitative arguments, on the analysis of the Hi dependence of Tlp, or on the use of theoretically calculated rigid-lattice spin-spin transfer rates. We find that average carbon Tip's (near 35 kHz) for a wide variety of glassy polymers at room temperature are predominantly spin-lattice (motional) in character.

A procedure is demonstrated for determining the relative contributions of spin-lattice and spinspin interactions to TIp(C), the relaxation time characterizing the loss of spin-locked carbon polarization. This involves the characterization of relaxation processes associated with several cross-polarization experiments. Three glassy polymer Tlp(C) values are shown to be dominated by spin-lattice effects while a crystalline polymer relaxes through a spin-spin mechanism.The combination of cross-polarization,' high-powered heteronuclear decoupling2 and magic-angle spinning3p4 has made it possible to obtain high-resolution, naturalabundance 13C n.m.r. spectra in solid^.^*^ These spectra are sufficiently detailed to

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The Fletcher Suit Delclos (FSD) ovoids employed in intracavitary brachytherapy (ICB) for cervical cancer contain shields to reduce dose to the bladder and rectum. Many treatment planning systems (TPS) do not include the shields and other ovoid structures in the dose calculation. Instead, TPSs calculate dose by summing the dose contributions from the individual sources and ignoring ovoid structures such as the shields. The goal of this work was to calculate the dose distribution with Monte Carlo around a Selectron FSD ovoid and compare these calculations with radiochromic film (RCF) and normoxic polymer gel dosimetry. Monte Carlo calculations were performed with MCNPX 2.5.c for a single Selectron FSD ovoid with and without shields. RCF measurements were performed in a plane parallel to and displaced laterally 1.25 cm from the long axis of the ovoid. MAGIC gel measurements were performed in a polymethylmethacrylate phantom. RCF and MAGIC gel were irradiated with four 33 microGy m2 h(-1) Cs-137 pellets for a period of 24 h. Results indicated that MCNPX calculated dose to within +/- 2% or 2 mm for 98% of points compared with RCF measurements and to within +/- 3% or 3 mm for 98% of points compared with MAGIC gel measurements. It is concluded that MCNPX 2.5.c can calculate dose accurately in the presence of the ovoid shields, that RCF and MAGIC gel can demonstrate the effect of ovoid shields on the dose distribution and the ovoid shields reduce the dose by as much as 50%.

The regional cerebral blood flow (rCBF) values determined using continuous arterial spin labeling (CASL) are subject to several sources of variability, including natural physiologic variations, sensitivity to the input parameters, and the use of different quantification models. To date, a thorough analysis of the impact of input parameters and the choice of quantification model has not been performed. These sources of variability were investigated through computer simulations using bootstrap techniques on actual CASL data. Coefficients of variation for representative single voxels were 6.7% for gray matter and 29% for white matter, and for eight-voxel regions of interest they were 4.5% for gray matter and 23% for white matter.