Biomedical subjects

Andrew Primak

Publications and source records attributed to Andrew Primak.

6 recordsLinked to original sources

PubMed2007

Scalar localization of the electrode array after cochlear implantation: a cadaveric validation study comparing 64-slice multidetector computed tomography with microcomputed tomography.

HYPOTHESIS: Improved resolution available with 64-slice multidetector computed tomography (MDCT) could potentially be used clinically to localize the cochlear implant (CI) electrode array within the basal turn. BACKGROUND: In CI surgery, the electrode array should be inserted into and remain within the scala tympani to avoid injury to Reissner's membrane and the scala media. Correlating the position of the electrode in the basal turn with surgical technique and implant design could be helpful in improving outcomes. METHODS: After a standard left mastoid exposure of the round window niche through the facial recess performed on a cadaver head, an electrode array from a Nucleus Softip Contour CI was fully inserted through a cochleostomy. The head was then scanned axially on a 64-slice MDCT with 0.4-mm slice thickness and reconstructed into the oblique axial, oblique coronal, and oblique sagittal planes of the cochlea. The temporal bone was then harvested and imaged on a microcomputed tomographic scanner using 20-microm slice thickness. Identical reconstructions were made and compared with the 64-slice images to confirm exact location of the electrode array. RESULTS: The 64-slice MDCT accurately localized the electrode array to the scala tympani. This was best demonstrated in the oblique sagittal plane, identifying the electrode array in the posterior inferior portion of the basal turn, posterior to the spiral lamina. CONCLUSION: This ex vivo validation study suggests that 64-slice MDCT has the potential to allow accurate localization of the CI electrode array within the basal turn of the cochlea.

Cadaver↗

PubMed2004

Characterization of the high-spin heme x in the cytochrome b6f complex of oxygenic photosynthesis.

X-ray structures at 3.0-3.1 A resolution of the cytochrome b(6) f complex from the cyanobacterium Mastigocladus laminosus [Kurisu, G., Zhang, H., Smith, J. L., and Cramer, W. A. (2003) Science 302, 1009-1014] and the green alga Chlamydomonas reinhardtii [Stroebel, D., Choquet, Y., Popot, J.-L., and Picot, D. (2003) Nature 426, 413-418] showed the presence of a unique heme, hemex, that is covalently linked by a single thioether bond to a Cys residue (Cys35) on the electrochemically negative (n) side of the cytochrome b(6) polypeptide. Heme x faces the intermonomer quinone exchange cavity. The only axial ligand associated with this heme is a H(2)O or OH(-) that is H-bonded to the propionate of the stromal side heme b(n), showing that it is pentacoordinate. The spectral properties of this heme were hardly defined at the time of the structure determination. The pyridine hemochromagen redox difference spectrum for heme x covalently bound to the cytochrome b polypeptide isolated from SDS-PAGE displays a low-amplitude broad spectrum with a peak at 553 nm, similar to that of other hemes with a single thioether linkage. The binding of CO and a hydrophobic cyanide analogue, butyl isocyanide, to dithionite-reduced b(6) f complex perturbs and significantly shifts the redox difference visible spectrum. Together with EPR spectra displaying g values of the oxidized complex of 6.7 and 7.4, heme x is defined as a ferric high-spin heme in a rhombic environment. In addition to a possible function in photosystem I-linked cyclic electron transport, the five-coordinate state implies that there is at least one more function of heme x that is related to axial binding of a physiological ligand.

Binding Sites↗

PubMed2004

Biaxial nematic phase in bent-core thermotropic mesogens.

A biaxial nematic phase had been predicted with D(2h) symmetry, wherein the mesogen's long and short transverse axes are simultaneously aligned along the two orthogonal, primary and secondary directors, n and m, respectively. The unique low-angle x-ray diffraction patterns in the nematic phases exhibited by three rigid bent-core mesogens clearly reveal their biaxiality. The results of x-ray diffraction can be readily reproduced by ab initio calculations that explicitly include the bent-core shape in the form factor and assume short-range positional correlations.

Journal Article↗

PubMed2002

Effect of mosaicity in x-ray studies of critical behavior at the nematic to smectic-A transition.

Previous studies of critical behavior at the nematic to smectic-A transition by high-resolution x-ray scattering were performed using low magnetic fields of 0.1-0.8 T. In those studies, the transverse resolution was limited by the sample mosaicity which complicated data analysis. In order to understand the effect of sample mosaicity on the measured values of critical exponents, the divergence of the smectic order correlation lengths xi(||, perpendicular ) and susceptibility sigma(o) was studied in a magnetic field ranging from 0.25 to 5 T. The use of high (5 T) field reduced the sample mosaicity and improved the effective transverse resolution by almost two orders of magnitude. Three liquid crystals, two mixtures of 6th and 7th homologs of 4,4(')-dialkylazoxybenzene (DnAOB) and 4-n-octylcyanobiphenyl (8CB) were studied. 15 wt% (D6.15AOB) and 40 wt% (D6.4AOB) mixtures of D7AOB in D6AOB have a wide nematic range, while 8CB has a narrow nematic range. Analysis of the data at different fields revealed a different and proper way to apply the mosaicity correction. The Gaussian mosaicity correction was found to be temperature independent but significantly ( approximately 3.5 times) smaller than the width of the sharpest q( perpendicular )-scan, which has traditionally been used for mosaicity correction in all previous studies. The values of the critical exponents measured over almost four decades of reduced temperature were: nu(||)=0.79+/-0.02, nu( perpendicular )=0.69+/-0.02, gamma=1.46+/-0.04 for D6.15AOB; nu(||)=0.79+/-0.02, nu( perpendicular )=0.67+/-0.02, gamma=1.44+/-0.04 for D6.4AOB; and nu(||)=0.70+/-0.02, nu( perpendicular )=0.52+/-0.02, gamma=1.24+/-0.04 for 8CB. The results for the two mixtures suggest that in wide temperature range nematics, far from the tricritical point, the exponents may be material independent. No significant effects of mosaicity on the values of the coefficient c of the fourth-order term in the structure factor were observed.

Journal Article↗

PubMed2002

X-ray diffraction study of Anodisc filters.

X-ray diffraction from Anodisc membrane filters is measured and analyzed. The patterns are consistent with pores of constant diameter and a Gaussian distribution of pore-pore spacing. The mean distance between pores, measured using x rays is 0.37 microm compared to 0.32 microm calculated from the nominal density of pores/cm(2). The results may be modeled both in terms of the convolution of a structure factor of the pores with the resolution function, and by modeling the source as a collection of incoherent sources with an ideal monochromator crystal. The incoherent source analysis provides an explanation for the common observation that the resolution function in many x-ray spectrometers is better fit to a sum of Lorentzians, rather than a single Lorentzian.

Journal Article↗

PubMed2002

Critical behavior at the nematic-to-smectic-A transition in a strong magnetic field.

The critical divergence of the smectic-order correlation lengths parallel and perpendicular to the director, xi( parallel, perpendicular), and the susceptibility, sigma(0), near the nematic-to-smectic-A phase transition has been measured in a 5 T magnetic field. The high magnetic field reduced the mosaicity and improved the effective transverse resolution by almost 2 orders of magnitude and permitted measurements nearly over four decades of reduced temperature. The results provide important new insights into mosaicity correction. The values of the critical exponents of xi( parallel, perpendicular) and sigma(0) were nu( parallel) = 0.79+/-0.02, nu( perpendicular) = 0.68+/-0.02, gamma = 1.45+/-0.04, respectively. They become smaller for a narrower nematic range.

Journal Article↗