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Biomedical subjects

T L Nichols

Publications and source records attributed to T L Nichols.

6 recordsLinked to original sources
PubMed2005

The microdosimetry of boron neutron capture therapy in a randomised ellipsoidal cell geometry.

Two reactions deliver the majority of local dose in boron neutron capture therapy. The ionised particles (protons, alpha particles and lithium nuclei) produced in the two reactions, 10B(n,alpha,gamma)7Li and 14N(n,p)14O, have short ranges that are less than -14 microm (which is on the order of the diameter of a typical human cell). The ionised particles are heavy and are in the 2+ charge state in the case of the boron reactions. These heavy 2+ ions will do significant damage to molecules near their tracks. Thus, the distribution of nitrogen and, in particular, of boron determines the spatial characteristics of the radiation field. Since the distribution of nitrogen is nearly homogeneous in the brain and is not easily altered for the purpose of radiotherapy, the spatial variation in the radiation dose is due mainly to the spatial distribution of boron. This implies that the spatial distribution of boron determines the microscopic energy deposition and therefore the spatial characteristics of the microscopic dose. The microscopic dose from the (n,alpha) and (n,p) reactions has been examined in detail and, as averred, the proton dose is relatively homogeneous except for statistical variability. The statistical variability in essence adds a false spatial variability that would not be seen if a large number of histories were performed. Since the majority of spatial variability occurs in the boron distribution, the (n,p) reaction can be suppressed to better understand the spatial distribution effects on the microscopic dose. Programs have been written in FORTRAN using Monte Carlo techniques to model ellipsoidal cells that are either randomly sized and located in the region of interest or are arranged in a face centred cubic array and are identical except for the location of the nuclei, which may be random. It is shown that closely packed prolate ellipsoidal cells with a large eccentricity in one dimension will receive a larger nuclear dose than cells that are more sparsely packed. This demonstrates that the boron content of a cell and its nucleus can have a significant impact upon the dose to neighbouring cells. The local boron distribution in a region of interest can be shown to affect the macrodosimetric dose, with possible implications for clinical outcomes.

Animals↗
PubMed2000

Transcriptional analysis of a superoxide dismutase gene of Borrelia burgdorferi.

A single superoxide dismutase (Sod) gene was identified in Borrelia burgdorferi strains, Borrelia afzelii Ple and Borrelia garinii Pbi. Recombinant enzymatic activity was detected only when sod expression was controlled by the lacZ promoter in the cloning vector. Northern blot analysis with sod- or secA-specific probes identified a common 3.7-kb transcript. Reverse transcriptase-PCR analysis confirmed that secA and sod constitute a single transcriptional unit in B. burgdorferi. A transcriptional start site of this operon, containing -10 and -35 regions of a sigma(70)-type promoter, was mapped to 100 bp upstream of the ATG start codon of secA.

Adenosine Triphosphatases↗
PubMed1987

Determination of cardiac equivalent generator properties using a numerical expansion method.

The ability of the Karhounen-Loeve (K-L) numerical expansion technique to quantitate cardiac dipolarity was assessed using data recorded from fifteen isolated perfused rabbit hearts. Potentials during the QRS complex, registered from thirty-two surface electrodes, were processed to yield percent dipolarity by a potential equation fit as well as by the K-L expansion. A total of 1174 data points was evaluated. The difference in values computed by the two methods equalled 1.02 +/- 6.99% (mean +/- 1 standard deviation); this difference, although small, was statistically significant (P less than 0.01). The correlation coefficient between the two sets of results equalled 0.53 (P less than 0.001). Thus, experimental support is given to the use of the K-L method to assess field dipolarity. Because ranges of differences were high and predictive accuracy of the regression was limited, the K-L expansion may be used with caution to detect directional shifts in equivalent cardiac generator properties.

Animals↗
PubMed1985

Frequency content of the electrocardiogram. Spatial features and effects of myocardial infarction.

The frequency content of electrocardiographic signals has been used to define amplifier characteristics and, more recently, to detect myocardial pathology. It was the purpose of this study to evaluate the differences in frequency content of ECG signals recorded from different volume conductor surface sites and to determine the effects of coronary ligation on these spatial features. To do so, 15 isolated perfused rabbit hearts, enclosed within a precisely machined sphere, were studied; ten were examined prior to and 15 minutes after ligation of the anterior descending coronary artery. ECG signals, recorded from 32 surface electrodes, were subjected to Fourier analysis. Prior to occlusion, the frequency content of the different electrode signals varied widely, with a greater variance between electrodes than between hearts (p less than 0.01). This variation was related to a reproducible spatial effect, with records over the basal left ventricle having higher frequency components than waves from other sites. After litigation, ECG records from electrodes over electrodes over ischemic areas showed augmented R-wave amplitude and ST-segment elevation. This was associated with a spatial shift of the control high frequency peak to a site overlying the ischemic lesion. Thus the frequency content of ECG is spatially variable and hence lead-dependent, and this spatial variability is directly affected by acute myocardial infarction.

Animals↗
PubMed1975

Taste rejection of nonnutritive sweeteners in cats.

Cats reject saccharin and cyclamate and are indifferent to dulcin, although they, like other mammals, prefer sucrose. The rejection threshold for saccharin found in this experiments, .0001 M, is about 2 log steps lower than a previously reported rejection threshold for sodium saccharin. Water produces a taste in cats adapted to their own saliva. The high sodium saccharin threshold may have resulted because the taste of the sodium saccharin was masked by the taste of the water solvent; however, saccharin may also be somewhat more aversive to the cat than sodium saccharin. Saccharin may produce an aversive taste because it stimulates receptor sites sensitive to substances bitter to man as well as those sensitive to sugars. In addition, saccharin may not be an effective stimulus for all sugar-sensitive sites.

Animals↗