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Biomedical subjects

P van Dijk

Publications and source records attributed to P van Dijk.

At least 37 records · Page 2Linked to original sources
PubMed1998

1H MR spectroscopy monitoring of changes in choline peak area and line shape after Gd-contrast administration.

Fifteen percent loss in the peak area of choline containing compounds (Cho) was recently observed in 1H MR spectra of contrast-enhancing tumor at 5-10 min after Gd-contrast administration [Magn. Reson. Med. 37:222-225, 1997]. In this study, chemical shift imaging (CSI, 1500/135 ms PRESS) was used to assess the spectral changes in 47 Gd-enhancing glial brain tumors and metastatic brain tumors measured at 0-5, 5-10, and/or 10-15 min after administration of Gd-contrast. Percent Cho peak area losses measured at these times, 3 +/- 3, 12 +/- 2, and 14 +/- 3 SEM, respectively, coincided with trends of line narrowing and up-field shift of the Cho peak. Significant changes in creatine and N-acetyl acetate signals were not observed. It is concluded that the Gd-induced loss of tumor Cho signal measured after 5 min, typically required for post contrast-MRI and the positioning of the CSI volume on tumor, shows little further change with time, if any.

Adult↗
PubMed1998

Correlated amplitude fluctuations of spontaneous otoacoustic emissions.

The frequency spectrum of a spontaneous otoacoustic emission (SOAE) recording may contain multiple narrow peaks. These peaks correspond to multiple SOAEs emitted by the inner ear. The individual SOAEs exhibit small amplitude fluctuations. The correlation between the amplitude fluctuations of multiple SOAEs in an individual ear was studied in 19 SOAE recordings from humans. For 424 pairs of SOAEs, the normalized amplitude cross-correlation function was computed. Significant correlation was found in 63 pairs (15%): 34 pairs showed positive and 29 pairs showed negative correlation. Some SOAE pairs were part of an equally spaced triplet of emission peaks, with one SOAE apparently generated as a distortion product of the other two "primary" SOAEs. The occurrence of correlation between a distortion product SOAE and a "primary" SOAE was 56%. This is significantly higher than the occurrence of correlation for the entire set of SOAE pairs. In addition, the spectral width of the distortion product SOAE was correlated with the spectral width of the "primary" SOAEs (R = 0.87, p < 10(-6)). For 79% of the correlated SOAE pairs, the cross-correlation function peaked at positive time delay. This indicates that for these pairs the fluctuation of the lower frequency emission lagged the fluctuation of the higher frequency emission. The average peak delay was 11.0 ms (s.d. 22.1 ms). The results demonstrate that interactions between SOAEs in an individual ear frequently occur and are asymmetrical.

Cochlea↗
PubMed1997

1H chemical shift imaging reveals loss of brain tumor choline signal after administration of Gd-contrast.

1H MR spectra obtained by chemical shift imaging (CSI) of contrast-enhancing brain tumors before and after the administration of Gd-contrast agent were quantitated and compared with the results in normal brain tissue included in the volume of interest. Twenty-seven combined magnetic resonance imaging and spectroscopy (MRI, MRS) examinations of brain tumor lesions included T1-weighted MRI and CSI (TR/TE 1500/135 ms double-spin echo) repeated 5-10 min after the administration of Gd-contrast agent (0.1-0.2 mM). In 1H MR spectra of contrast-enhancing tumor Gd-contrast induced a mean loss of 15% of the peak area of choline-containing compounds (Cho, P < 0.001) that was correlated with precontrast Cho linewidth (r = -0.72, P < 0.00001). This phenomenon limits the diagnostic use of brain tumor MRS examinations performed immediately after contrast-enhanced MRI.

Aspartic Acid↗
PubMed1997

Dissecting the frog inner ear with Gaussian noise. I. Application of high-order Wiener-kernel analysis.

Wiener kernel analysis was used to characterize the auditory pathway from tympanic membrane to single primary auditory nerve fibers in the European edible frog, Rana esculenta. Nerve fiber signals were recorded in response to white Gaussian noise. By cross-correlating the noise stimulus and the nerve fiber response, we computed (1) the full second-order Wiener kernel, and (2) the diagonals of the zeroth- to fourth-order Wiener kernels. These diagonals are usually referred to as polynomial correlation functions. The measured Wiener kernels were fitted with a 'sandwich' model. A new fitting procedure was used to compute the response characteristics of (1) the first filter, (2) the static nonlinearity, and (3) the second filter, which form the functional components of the model. The first filter is a bandpass filter. In the majority of low frequency fibers, with best excitatory frequency (BEF) < 800 Hz, this filter was tuned to two frequencies. This dual tuning mechanism gives rise to 'off-diagonal' components in the second-order Wiener kernel. The static nonlinearity resembles a rectifier, and is dominated by second-order (quadratic) nonlinearity. As a function of BEF, the shape of the nonlinearity changes systematically. Finally, the last filter in the model was a low pass filter. Across fibers, its cutoff frequency f-3dB ranged from 106 to 434 Hz.

Acoustic Stimulation↗
PubMed1997

Dissecting the frog inner ear with Gaussian noise. II. Temperature dependence of inner ear function.

The temperature dependence of the response of single primary auditory nerve fibers (n = 31) was investigated in the European edible frog, Rana esculenta (seven ears). Nerve fiber responses were analyzed with Wiener kernel analysis and polynomial correlation. The responses were described with a cascade model, consisting of a linear bandpass filter, a static nonlinearity, and a linear lowpass filter. From the computed Wiener kernels and the polynomial correlation functions, the characteristics of the three model components were obtained. With increasing temperature (1) tuning of the first filter increased in the majority (n = 16) of amphibian papilla fibers (best excitatory frequency, BEF < 1 kHz, n = 21) but remained unchanged in the majority (n = 10) of basilar papilla fibers (BEF > 1 kHz, n = 11), (2) the gain of the first filter remained unchanged, (3) the shape of nonlinear IO function remained unchanged, (4) the combined gain of the static nonlinearity and the second filter usually increased, but displayed considerable scatter across fibers (from -0.7 dB/degrees C to 3 dB/degrees C), and (5) the cutoff frequency of the second lowpass filter increases, with average 0.13 oct/degrees C. The immunity of the shape of the nonlinearity is considered evidence of a temperature independent gating mechanism in the transduction channels. The temperature dependence of the second filter may have resulted from a decrease of the hair cell membrane resistance, but may also reflect changes in subsequent staging of nerve fiber excitation.

Acoustic Stimulation↗
PubMed1997

Wiener kernel analysis of a noise-evoked otoacoustic emission.

In one specimen of the frog species, Rana esculenta, the following were measured: (1) a spontaneous otoacoustic emission; (2) a click-evoked otoacoustic emissions; and (3) a noise evoked otoacoustic emission. From the noise evoked emission response, a first- and a second-order Wiener kernel and the first six polynomial correlation functions were calculated by cross-correlating the stimulus and response signals. A peak near 850 Hz was visible in the spontaneous otoacoustic emission, the click-evoked emission and the noise-evoked emission. The higher order cross-correlation functions did not display any indication of the emission response. Apparently, that the noise stimulus linearizes the evoked emission from the frog's ear.

Acoustic Stimulation↗
PubMed1996

Spontaneous otoacoustic emissions in seven frog species.

Spontaneous otoacoustic emissions were screened for in five advanced frog species (Hyla cinerea, n = 10 ears; Hyla chrysoscelis, n = 10; Hyla versicolor, n = 7; Leptodactylus albilabris, n = 2; Rana pipiens pipiens, n = 8), and two primitive frog species (Xenopus laevis, n = 9; Bombina orientalis, n = 12). Emissions were found in 90% of the advanced species' ears, whereas none of the primitive species' ears had emissions. Emission frequencies ranged from 645 Hz to 1680 Hz. The absence of emissions in the primitive species correlates with (1) the absence of a tympanic membrane, and (2) a reduced length of the caudal extension of the amphibian papilla. In eight frogs, the effect of body temperature on emissions was investigated. As a function of temperature, the frequency changed at a rate between 0.009 and 0.091 oct/degree C, and emission levels displayed a complex but consistent behavior. For 9 out of 15 spectral emission peaks encountered during the temperature experiments, the relation between the peak height and peak width was similar to that of an active oscillator.

Acoustic Stimulation↗
PubMed1996

Frequency response for electromotility of isolated outer hair cells of the guinea pig.

Frequency and impulse responses were determined for isolated guinea pig outer hair cells by electrically stimulating the cells between two wire electrodes with white noise. Cells were attached to the bottom of a small culture dish at one end while the other end was freely moving. Results have the character of a damped second-order system, with a cut-off frequency of approximately 7 kHz. Correspondingly, impulse responses show a single narrow peak roughly 50 microseconds wide and have the shape of the impulse response for a critically damped harmonic oscillator.

Acoustic Stimulation↗
PubMed1996

1H MR spectroscopy detection of lipids and lactate in metastatic brain tumors.

1H MR spectroscopy was used to correlate the metabolite signals in 66 untreated metastatic brain tumors with the results of Gd-DTPA enhanced MRI. Cubic volumes containing brain metastases of lung cancer (n = 17), mammary carcinoma (n = 24), melanoma (n = 12) and those originating from other tumors (n = 13) were examined using the double spin echo technique with CHESS pulses for water suppression and TE = 135 ms. Apart from trends toward reduced signals of choline-containing compounds (Cho) and reduced post-Gd MRI contrast in lung cancer compared with the other pathology groups, the four tumor groups had similar MRI and MRS characteristics. Metastases without lipid or lactate (Lact) signal in the 1H MR spectra were comparatively small in size with homogeneous post-Gd MRI enhancement (33 +/- 5%, means +/- SEM; n = 24) and elevated Cho signals compared with normal contralateral brain tissue (70 +/- 5% of contralateral N-acetyl aspartate signal; p < 0.001). The other metastases showed either unambiguous lipid signals (n = 30) or MRS detectable Lact (n = 12) and were heterogeneous on MRI with divergent signals of Gd-enhancement (49 +/- 5% vs 14 +/- 8%, p < 0.001) and Cho (88 +/- 10 vs 47 +/- 8% of contralateral NAA; p = 0.02). Those with Lact were significantly larger compared with both other groups (p < 0.02, both). It is concluded that brain metastases can be categorized into early stage (Cho), intermediate stage (lipid, higher Cho) and late stage metastases (Lact, lower Cho).

Brain Neoplasms↗
PubMed1995

Hydrogen magnetic resonance spectroscopy follow-up after radiation therapy of human brain cancer. Unexpected inverse correlation between the changes in tumor choline level and post-gadolinium magnetic resonance imaging contrast.

RATIONALE AND OBJECTIVES: The anatomic and metabolic changes in human brain tumors treated by radiation therapy were compared using gadolinium-enhanced magnetic resonance imaging and hydrogen (1H) magnetic resonance spectroscopy. The study was intended to assess the potential of 1H magnetic resonance spectroscopy in monitoring response to therapy. METHODS: Thirteen cases of brain cancer treated by radiation therapy were examined by 1H magnetic resonance spectroscopy and gadolinium-enhanced T1-weighted magnetic resonance imaging and reexamined at 2-month intervals. RESULTS: Follow-up after radiation therapy showed changes in post-gadolinium magnetic resonance imaging contrast that are inversely correlated with the changes in choline level (r = -0.69, P < 0.00001) and in tumor volume (r = -0.35, P < 0.05). CONCLUSIONS: The choline loss in tumors gaining post-gadolinium magnetic resonance imaging contrast after therapy is unexpected in view of previously reported correlation between the two in untreated metastatic brain tumors. Indicated is the use of 1H magnetic resonance spectroscopy to discriminate enhancing brain tumors with a high content of vital tumor cells (high choline) from tumors, combining decreased cell density with increased interstitial space (low choline).

Aspartic Acid↗
PubMed1995

Combining aesthetics and practicality in health care architecture.

Architecture, like health care, is a service profession. It serves not only a functional purpose by providing designs for shelter, but also incorporates social purpose. It allows people to conduct business, congregate for amusement, express their faith, and educate themselves. It is also a collaborative art, creating an environment that influences its users by their interactions with it and how they use it.

Ambulatory Care Facilities↗
PubMed1994

Correlation between choline level and Gd-DTPA enhancement in patients with brain metastases of mammary carcinoma.

Single voxel 1H double spin-echo MR spectroscopy was used to examine 15 cases of brain metastasis of mammary carcinoma (18 lesions) in relation to Gd-DTPA enhanced MR imaging. For lesions larger than 50% of MRS voxel size, there was significant correlation between Gd-DTPA-enhanced MRI signal and MRS-detected signal of choline (Cho) containing compounds (r = 0.86, P < 0.01; n = 8). The observed loss of correlation when including the smaller lesions was overcome by correcting for partial volume effects (r = 0.69, P < 0.002; n = 18). Metastasis spectra showed increased Cho compared with control spectra, except for those lesions showing detectable lactate (Lact) signal. The detection of Lact in four of the larger lesions coincided with comparatively low levels of creatine (Cr) and Cho and heterogeneous Gd-DTPA enhancement (Cr) and Cho and heterogeneous Gd-DTPA enhancement (ring-enhancement). It was concluded that in brain metastases of mammary carcinoma Lact represents a product of ischemia preceding/during tissue decay resulting in central necrosis, rather than tumor specific metabolism resulting in increased glycolysis.

Adult↗
PubMed1994

On the shape of (evoked) otoacoustic emission spectra.

In a preceding paper [Wit et al., (1994) Hear. Res. 73, 141-147] it was shown that realistic time-frequency plots for click evoked otoacoustic emissions can be synthesized by adding a large number of gammatones. It is necessary in such a synthesis to make the inner ear frequency to place map, from which the central frequencies of the gammatones are taken, slightly irregular along the entire cochlear partition. The present paper shows that this approach leads, in a rather straightforward way, to (quasi-) periodicity in synthesized spectra of evoked otoacoustic emissions. These spectra show good resemblance with spectra from real ear emission measurements.

Acoustic Stimulation↗
PubMed1994

Wavelet analysis of real ear and synthesized click evoked otoacoustic emissions.

Wavelet analysis was performed to obtain time-frequency analyses of click evoked otoacoustic emissions from normal ears and one ear with a high-frequency hearing loss; mainly to introduce this relatively new method and to show its potentials for emission analysis. The same analysis was then used to obtain time-frequency decompositions of synthesized emissions. It was found that the introduction of a slightly irregular frequency to place relation for the inner ear yielded synthetic results that were remarkably similar to those obtained from real ears.

Acoustic Stimulation↗
PubMed1994

Wiener kernel analysis of inner ear function in the American bullfrog.

The response of 17 primary auditory nerve fibers in the American bullfrog (Rana catesbeiana) to acoustic noise stimulation of the tympanic membrane was recorded. For each fiber, the first- and second-order Wiener kernels, k1 (tau 1) and k2 (tau 1, tau 2), were computed by cross correlation of the stimulus and the response. The kernels revealed amplitude and phase characteristics of auditory filters of both phase-locking and non-phase-locking fibers. Wiener kernels of high- and midfrequency fibers (best frequency, BF > 500 Hz), implied a simple sandwich model, consisting of a cascade of a linear bandpass filter, a static nonlinearity, a linear low-pass filter, and a spike generator. The bandpass filter was at least of order 7, and had a linear phase response, for both the high- and the midfrequency fibers. Averaged across fibers, filter order 2, and cutoff frequency 451 Hz for the second filter in the model was observed. The responses of low-frequency fibers (BF < 500 Hz) could not be fit with the sandwich model, because the Fourier transform K2 (f1,f2) of the second-order Wiener kernel showed significant components at off-diagonal frequencies f1 not equal to +/- f2. The presence of these off-diagonal components shows that, in addition to the phase and gain characteristics of auditory filters, the Wiener kernel analysis reveals nonlinear two-tone interactions.

Acoustic Stimulation↗
PubMed1994

Correlation between amplitude and frequency fluctuations of spontaneous otoacoustic emissions.

The normalized cross-correlation function between the amplitude and frequency fluctuations of 11 spontaneous otoacoustic emissions was measured. A significant correlation was found in seven subjects. The correlation coefficient ranged from -0.37 to +0.65 across subjects. In four subjects, the amplitude fluctuation lagged the frequency fluctuation. The time lag was between 1.6 and 5.5 ms. The results were interpreted using a noise-perturbed limit-cycle oscillator with nonlinear (Duffing) stiffness as a model for a spontaneous emission. The data show that the relative increase of the nonlinear stiffness in this model was between -0.010 and +0.015. This indicates that an even-order nonlinear stiffness plays a minor role in the emission generator.

Acoustic Stimulation↗
PubMed1994

Intravenous anaesthesia in horses by guaiphenesin-ketamine-detomidine infusion: some effects.

The effects of total intravenous anaesthesia with an intravenous infusion of a combination of guaiphenesin, ketamine and detomidine were studied in 10 patients scheduled for elective surgery. Anaesthesia was maintained by the infusion of guaiphenesin (100 mg/ml), ketamine (2 mg/ml) and detomidine (0.02 mg/ml). The infusion rate was 1 ml/kg/hr. During anaesthesia, pulse rate and mean arterial blood pressure were continuously recorded. Arterial blood gases and pH were determined immediately after induction and at stated times during anaesthesia. Venous blood was sampled to determine plasma glucose, lactate, lactate dehydrogenase (LDH), creatine phosphokinase (CPK) and aspartate aminotransferase (AST) concentrations. Values were compared with those determined in blood sampled before the premedication. All determined parameters with the exception of the plasma glucose concentration, the arterial oxygen tension and the AST concentration did not change significantly and remained within normal ranges. The plasma glucose concentration increased significantly after the induction of anaesthesia compared to the control value but decreased to normal values during anaesthesia. The arterial oxygen tension was on average 30% lower than normally wished. Compared to the control value the plasma concentration of AST was significantly decreased at the end of anaesthesia. Based upon the results of this study an infusion of guaiphenesin, ketamine and detomidine appears to be useful for the maintenance of total anaesthesia in horses.

Anesthesia, Intravenous↗