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21.
H. P. Hetherington R. I. Kuzniecky J. W. Pan J. T. Vaughan D. B. Tweig G. M. Pohost 《Magnetic resonance imaging》1995,13(8):1175-1180
Previous spectroscopic imaging studies of temporal lobe epilepsy have used comparisons of metabolite content or ratios to lateralize the seizure focus. Although highly successful, these studies have shown significant variations within each of the groups of healthy subjects and patients. This variation may arise from the natural differences seen in metabolite concentration in gray and white matter, the complex anatomy seen about the hippocampus, and the large voxels typically employed at 1.5 T. Using a 4.1 T whole body system, we have acquired spectroscopic images with 0.5 cc nominal voxels (1 cc after filtering) to evaluate the regional variation in metabolite content of the hippocampus, temporal gray and white matter, midbrain, and cerebellar vermis. Using a threshold value of 0.90 for CR/NAA, a value 90% of all normal hippocampal voxels lay below, we have correctly identified the presence of epileptogenic tissue in patients with unilateral as well as bilateral seizures. By using comparisons to healthy values of the CR/NAA ratio, this method enables the visualization of bilateral disease and provides information on the extent of gray matter involvement. 相似文献
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Donald R. Hastie John Gray Vaughan S. Langford Robert G. A. R. Maclagan Daniel B. Milligan Murray J. McEwan 《Rapid communications in mass spectrometry : RCM》2010,24(3):343-348
The on‐line detection of gaseous peroxyacetyl nitrate (PAN) using selected ion flow tube mass spectrometry (SIFT‐MS) has been investigated using a synthetic sample of PAN in air at a humidity of ~30%. Using the H3O+ reagent ion, signals due to PAN at m/z 122, 77 and 95 have been identified. These correspond to protonated PAN, protonated peractetic acid and its water cluster, respectively. These products and their energetics have been probed through quantum mechanical calculations. The rate coefficient of H3O+ has been estimated to be 4.5 × 10?9 cm3 s?1, leading to a PAN sensitivity of 138 cps/ppbv. This gives a limit of detection of 20 pptv in 10 s using the [M+H]+ ion of PAN at m/z 122. Copyright © 2010 John Wiley & Sons, Ltd. 相似文献
24.
Hall and drift mobilities in molecular beam epitaxial grown GaAs 总被引:1,自引:0,他引:1
V. W. L. Chin T. Osotchan M. R. Vaughan T. L. Tansley G. J. Griffiths Z. Kachwalla 《Journal of Electronic Materials》1993,22(11):1317-1321
A series of nominally undoped and Si-doped GaAs samples have been grown by molecular beam epitaxy (MBE) with Hall concentrations
ranging from 1015 to 1019 cm−3 and mobilities measured at 77 and 300K by Hall-van der Pauw methods. Drift mobilities were calculated using the variational
principle method and Hall scattering factors obtained from a relaxation-time approximation to permit cross-correlation of
experimental data with drift or Hall mobilities and actual or Hall electron concentrations. At 77K, both high purity and heavily
doped samples are well represented by either drift or Hall values since piezoelectric acoustic phonon scattering and strongly
screened ionized impurity scattering hold the Hall factor close to unity in the respective regimes. Between n≊1015 and 1017 cm−3, where lightly screened ionized impority scattering predominates, Hall mobility overestimates drift mobility by up to 50
percent and Hall concentration similarly underestimates n. At 300K, polar optical phonons limit mobility and a Hall factor
up to 1.4 is found in the lowest doped material, falling close to unity above about 1016 cm−3. Our calculation also agrees remarkably well with the Hall mobility of the highest purity MBE grown sample reported to date. 相似文献
25.
The spatial wave front characteristics of a laser were examined by two beam interference after amplitude division. Complex interference patterns were observed and are interpreted as due to a helical form of the phase front. This unusual form is implicit in previous analyses of cavity eigenfunctions, but the properties of such a helical co-phasal wavefront have not apparently been previously studied. 相似文献
26.
Brillouin scattering has been used to obtain first order elastic constants in 8CB. In contrast to earlier measurements the interlayer constant is found to be finite as T approaches the smectic A—nematic transition. These results confirm recent theoretical predictions. 相似文献
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Kasyutich V.L. Bale C.S.E. Canosa-Mas C.E. Pfrang C. Vaughan S. Wayne R.P. 《Applied physics. B, Lasers and optics》2003,76(6):691-697
We present an application of cavity-enhanced absorption spectroscopy with an off-axis alignment of the cavity formed by two
spherical mirrors and with time integration of the cavity-output intensity for detection of nitrogen dioxide (NO2) and iodine monoxide (IO) radicals using a violet laser diode at λ=404.278 nm. A noise-equivalent (1σ≡ root-mean-square variation
of the signal) fractional absorption for one optical pass of 4.5×10-8 was demonstrated with a mirror reflectivity of ∼0.99925, a cavity length of 0.22 m and a lock-in-amplifier time constant
of 3 s. Noise-equivalent detection sensitivities towards nitrogen dioxide of 1.8×1010 molecule cm-3 and towards the IO radical of 3.3×109 molecule cm-3 were achieved in flow tubes with an inner diameter of 4 cm for a lock-in-amplifier time constant of 3 s. Alkyl peroxy radicals
were detected using chemical titration with excess nitric oxide (RO2+NO→RO+NO2). Measurement of oxygen-atom concentrations was accomplished by determining the depletion of NO2 in the reaction NO2+O→NO+O2. Noise-equivalent concentrations of alkyl peroxy radicals and oxygen atoms were 3×1010 molecule cm-3 in the discharge-flow-tube experiments.
Received: 4 February 2003 / Revised version: 10 March 2003 / Published online: 12 May 2003
RID="*"
ID="*"Corresponding author. Fax: +44-1865/275-410, E-mail: vlk@physchem.ox.ac.uk 相似文献