Biomedical subjects

P Van Dijk

Publications and source records attributed to P Van Dijk.

7 recordsLinked to original sources

PubMed2006

Functional imaging of the central auditory system using PET.

In the last few decades functional neuroimaging tools have emerged to study the function of the human brain in vivo. These techniques have increased the knowledge of how the brain processes stimuli of different sensory modalities, including auditory processing. Positron emission tomography (PET) has been used for nearly 20 years to study changes in cerebral blood flow associated with auditory stimulation in normal and hearing impaired subjects. PET studies gave insight into the neural base of processing basic sound features such as frequency and intensity, but complex stimuli such as speech and music have also been investigated extensively. Knowledge of the normal auditory function of the brain helps us to understand the neural base of hearing deficits and provides ideas for possible treatments. Although functional magnetic resonance imaging (fMRI) is replacing PET in many neuroimaging studies nowadays, PET still holds unique advantages and can give us valuable knowledge about the auditory cortex and auditory perception.

Acoustic Stimulation↗

PubMed1997

Gd-enhanced MR imaging of brain metastases: contrast as a function of dose and lesion size.

MR imaging contrast of brain metastases after cumulative doses of gadolinium chelate is quantitated and compared in order to assess the clinical utility of high dosage. T1-weighted spin-echo MR images of 39 patients with metastatic brain tumors were made before and after each of three equal doses cumulating to 0.1, 0.2 and 0.3 mmol Gd-complex per kg body weight. Quantitation of MRI contrast was limited to homogeneous brain metastases larger than 3 mm (n = 246). Post-Gd MRI contrast doubled with dose escalation from 0.1 to 0.3 mmol/kg and also increased with lesion size, by a factor of 2.5 between metastases of 3 and 16 mm diameter, that is after correcting for partial volume effect. At 0.2 and 0.3 mmol/kg the respective numbers of visible metastases increased by 15% and 43% compared with 0.1 mmol/kg (p < 0.0001, both). Image contrast figures differed significantly between doses (p = 0.018). Both the number of metastases and the image contrast is significantly higher when dose escalation is performed. It is indicated that the number of detected metastases will increase further at Gd doses beyond 0.3 mmol/kg. Post-Gd MRI contrast increases with lesion size, to an extent that can not be attributed to partial volume attenuation.

Analysis of Variance↗

PubMed1996

Temperature dependence of spontaneous otoacoustic emissions in the edible frog (Rana esculenta).

The change in frequency of individual emissions in the European edible frog (Rana esculenta) when the temperature of the frog is modified, is part of a complex pattern of interaction between spontaneous otoacoustic emissions. At high temperatures (above 24 degrees C) two emissions are always detected (e.g., one near 800 Hz and one near 1200 Hz). The higher-frequency emission is lower in level and has a wider bandwidth than the lower-frequency emission. It is also often asymmetric and sometimes breaks into two emissions when an external suppressor tone is applied. When the temperature is decreased, these emissions are reduced in frequency at a rate of 0.04 octave/degree C. The higher-frequency emission becomes narrower and taller, and the lower-frequency emissions becomes broader and less intense. At approximately 18 degrees C the lowest of these emissions (now between 600 and 700 Hz) disappears and is replaced by a new emission approximately 100 Hz lower in frequency. When the temperature is carefully controlled the two emissions can exist simultaneously. The lowest-frequency emission changes 0.015 degree C/octave suggesting that the mechanisms controlling the frequency of this emission may be different than those determining the frequencies of the other emissions. All but the lowest-frequency emissions are maximal in level and have minimal bandwidth when the frequency is close to 700 Hz, which is interpreted as evidence that these emissions are filtered by a temperature-independent process.

Acoustic Stimulation↗

PubMed1995

Non-T1-weighted 31P chemical shift imaging of the human liver.

A 31P chemical shift imaging (CSI) protocol was developed for human liver studies. It is shown that at the commonly used repetition time (TR) of 1 s T1-weighting reduces the integrated intensities of liver phosphate metabolite signals by 18 +/- 15% (inorganic phosphate, Pi) to 46 +/- 10% (phosphodiester, PDE), that is for an RF pulse angle of 60 degrees (weighted average) in liver. The loss in signal-to-noise ratio (S/N) at TR = 20 s, sufficient to eliminate spectral distortions caused by saturation, compared with TR = 1 s (47-65%) can be overcome by using one-dimensional (1D)-phase encoding with a small number of phase-encode steps. The liver spectra obtained by 1D-CSI with 4-step phase-encoding (spatial resolution 10 cm) have the highest S/N and, after multiplication of the PDE signal by a factor of 1.4, closely reflect the liver metabolite levels. It is concluded that clinical 31P MR studies of liver function can be performed without T1-weighting and that the current practice to compromise the MRS quantification of lever metabolites with uncertainties caused by (differential changes in) T1-weighting is not warranted.

Humans↗

PubMed1988

Hypnoanalgesia with R 8110/fentanyl in the dog: pharmacodynamic and pharmacokinetic interactions.

The pharmacokinetics and clinical effects of the short-acting hypnotic R 8110 and of the narcotic analgesic fentanyl were studied in the dog. The effects of separate intravenous (i.v.) injections of R 8110 (4 mg/kg) and fentanyl (0.015 mg/kg) and of concurrent i.v. injection of the two were studied. After administration of R 8110, induction of hypnosis occurred within 1 min, maximal depth of anaesthesia and satisfactory analgesia and muscle relaxation were obtained after 5 min. The effects had decreased within 15 min and full recovery occurred within 30 min. Fentanyl alone produced neither hypnosis nor muscle relaxation. When fentanyl and R 8110 were given simultaneously, the duration of hypnosis was doubled in comparison with R 8110 alone. Moreover, markedly improved and longer lasting analgesia and muscle relaxation were observed with the combination. When the drugs were injected together, the plasma concentrations of R 8110 were initially much higher than after separate injection of R 8110, but they became similar after 30 min. Although statistically non-significant, fentanyl reduced the total plasma clearance of R 8110 (31.1 +/- 6.9 vs. 21.9 +/- 2.3 ml/kg/min) and decreased the volume of distribution (3.78 +/- 1.83 vs. 2.23 +/- 0.90 l/kg, P less than 0.05). Fentanyl did not alter the elimination half-life of R 8110. R 8110 had no apparent influence on the pharmacokinetics of fentanyl.

Animals↗

PubMed1987

Comparison of the effects of etomidate and its fluoro analogue, R 8110, on plasma cortisol, 11 beta-deoxycortisol, 17 alpha-hydroxyprogesterone and testosterone concentrations in dogs.

R 8110, an imidazole derivative, was shown to be clinically superior to etomidate for induction and maintenance of anaesthesia in dogs. The present study compared the effects of intravenous (i.v.) R 8110, etomidate and Ringer solution on cortisol biosynthesis by the adrenal gland in seven male labradors. A tetracosactide challenge was carried out 30 min after the i.v. injection of 3 mg/kg of both drugs and after i.v. Ringer solution (1 ml/kg). Etomidate and R 8110 both suppressed the cortisol response to tetracosactide almost completely and increased the plasma 11 beta-deoxycortisol levels more than 20 fold. Maximal 11 beta-deoxycortisol values were reached 120 min after R 8110, and not less than 300 min after etomidate. Plasma 17 alpha-hydroxyprogesterone and testosterone concentrations did not differ between placebo and R 8110 treatment, but they decreased after etomidate. These results indicate that the effects of R 8110 on steroid biosynthesis in dogs are less pronounced than those of etomidate and are largely limited to a temporary inhibition of the 11 beta-hydroxylase in the adrenal gland.

17-Hydroxycorticosteroids↗

PubMed1984

Proton spin relaxation studies of fatty tissue and cerebral white matter.

Proton spin longitudinal (T1) and transverse (T2) relaxation and proton density studies were carried out on human fatty tissue and bovine white matter, both in the native state and after immersion in D2O. It is concluded that nuclear magnetic resonance signals from fatty tissue result mainly from methyl and methylene protons of hydrocarbons. No contribution from lipid protons could be detected for white matter, although it contains a high percentage of lipids. Imaging experiments, resulting in T1, T2, and proton density maps, support the results obtained with spectroscopic relaxation studies.

Adipose Tissue↗