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Biomedical subjects

J H Jensen

Publications and source records attributed to J H Jensen.

At least 19 recordsLinked to original sources
PubMed2002

Theory of nonexponential NMR signal decay in liver with iron overload or superparamagnetic iron oxide particles.

A quantitative theory is proposed for the nonexponential NMR proton signal decay observed in liver with iron overload or superparamagnetic iron oxide particles. This effect occurs for Carr-Purcell-Meiboom-Gill (CPMG) sequences and is argued to be a direct consequence of the strong magnetic field inhomogeneities generated by the iron, rather than being due to tissue compartments. An approximate mathematical form is given for the signal decay, which is fit to experimental data for samples of rat liver with iron oxide particles, for samples of marmoset liver with hemosiderosis, and for in vivo human liver with hereditary hemochromatosis. The fitting parameters obtained are consistent with the pattern of iron deposition determined from histology. For the case of hereditary hemochromatosis, a good correlation is found between a parameter characterizing the nonexponential decay and the iron concentration. Implications for practical MR quantification of hepatic iron are discussed.

Animals↗
PubMed2001

Quantitative model for the interecho time dependence of the CPMG relaxation rate in iron-rich gray matter.

A quantitative model is proposed for computing the dependence on the interecho time of the NMR relaxation rate in iron-rich gray matter obtained with a Carr-Purcell-Meiboom-Gill sequence. The model consists of representing oligodendrocytes as identical magnetic spheres arranged in a spatially random pattern, and in approximating water diffusion as isotropic and unrestricted. Predictions of the model are calculated numerically using a Monte Carlo technique and, for the weak field limit, using an analytic formula. The model is shown to provide a good fit to experimental measurements of in vitro samples of monkey brain at field levels of 1.0 T and 1.5 T. These field levels are not sufficient to fully determine the model parameters, but it is argued that this may be possible at 3.0 T. The model is potentially of value for multiple-spin-echo MRI studies of iron-related neurodegenerative disorders, such as Parkinson's disease. In particular, the model can be applied to correlate MRI data with the cellular distribution of iron in gray matter. Magn Reson Med 46:159-165, 2001.

Animals↗
PubMed2000

Strong field behavior of the NMR signal from magnetically heterogeneous tissues.

A theory for the behavior of the nuclear magnetic resonance (NMR) signal obtained from magnetically heterogeneous tissues is developed for the limit of a strong external magnetic field. If BO is the magnitude of the external magnetic field, it is found that a free-induction signal decays in a time scaling as 1/Bo, a single-spin echo signal decays in a time scaling as 1/Bo(2/3), and a multiple-spin echo signal decays in a time scaling as 1/Bo(2). Moreover, it is shown that the form of the signal decay for a multiple-spin echo sequence may deviate significantly from an exponential. Numerical results for a model consisting of randomly distributed magnetic spheres are used to confirm the theory. In addition, good agreement is demonstrated between the theory and experimental measurements obtained with particle suspensions. The validity and application of the theory to biological tissues are discussed.

Animals↗
PubMed2000

MR imaging of microvasculature.

An imaging technique is proposed in which the contrast is correlated to the morphology of capillaries and other small blood vessels. The technique is based on measurements of the relaxation rates 1/T(2) and 1/T*(2) before and after the injection of a contrast agent. An image is then formed by mapping the quantity Q identical with delta R(2)/(delta R*(2))(2/3), where delta R(2) and delta R*(2) are the changes in the relaxation rates due to the contrast agent. If the contrast agent concentration is sufficiently high, it is shown that Q is given approximately by a simple analytic formula that involves only intrinsic properties of the vascular network and the rate of diffusion. In particular, Q is sensitive to the histologic vessel density. Theoretical predictions for Q are shown to be consistent with experimental data obtained with a rat glioma model and normal cerebral cortex. The imaging technique may be useful in characterizing tumor angiogenesis. Magn Reson Med 44:224-230, 2000.

Animals↗
PubMed2000

Cochlear implantation of Danish prelingually deaf children.

The aim of the study was to review the indications, techniques and results of cochlear implant (CI) treatment with the Nucleus CI multi-channel electrode for the first children operated on in Denmark. The study material comprised 16 children (5 girls, 11 boys). Their median age at the time of CI operation was 27 months. Anacusis was caused by pneumococcal meningitis in 31% of the children in this relatively young prelingual material. Obliteration of the cochlea was found in two children, indicating the need for surgery as soon as possible after the verification of post-meningitis, total deafness. There were no technical difficulties with operating on the youngest children in the congenital group and the age of 2 years is optimal for implantation. These preliminary results have contributed definitively to an increase in the number of cochlear implantations being carried out in young prelingually deaf Danish children.

Child↗
PubMed2000

NMR relaxation in tissues with weak magnetic inhomogeneities.

A theory is presented for describing the effect on the transverse NMR relaxation rate of microscopic spatial inhomogeneities in the static magnetic field. The theory applies when the inhomogeneities are weak in magnitude and the nuclear spins diffuse a significant distance in comparison with a length scale characterizing the inhomogeneities. It is shown that the relaxation rate is determined by a temporal correlation function and depends quadratically on the magnitude of the inhomogeneities. For the case of unrestricted diffusion, a simple algebraic approximation for the temporal correlation function is derived. The theory is illustrated by applying it to a model of randomly distributed magnetized spheres. The theory is also used to fit experimental data for the dependence of the relaxation rate on the interecho time for a Carr-Purcell-Meiboom-Gill pulse sequence. The experimental systems considered are in vitro red blood cell suspensions and samples of human gray matter and rat liver. Magn Reson Med 44:144-156, 2000.

Animals↗
PubMed1999

[Cochlear implantation in children. The first Danish results].

The aim of this study is to review indication, technique and results of cochlear implant (CI) treatment with Nucleus CI-multi-electrode of the first ten children operated in Denmark (five children with congenital deafness and five with acquired prelingual deafness due to meningitis). In the literature, the importance of early referral and operation at the age 2-3 (4) years for congenital deafness and as soon as possible on suspicion of acquired deafness (meningitis) is stressed. A short survey of our indications, technique and rehabilitation is presented. The results of treatment after 11-44 months' use of CI are that one child has language almost matching age, four use words and short sentences accompanied by support signing, three have sound reaction and say single words without sentence building, but of these two children have only used CI for a very short time and are improving. Two meningitis sequelae cases had cochlear ossification, which made the implantation difficult. Both patients had to be operated, and one of them is not using the processor.

Age Factors↗
PubMed1997

Experimental pain in the stomach: a model based on electrical stimulation guided by gastroscopy.

BACKGROUND: Abdominal pain is often variable in intensity and difficult to characterise due to its referred pain pattern. Clinical pain is furthermore confounded by various emotional and cognitive factors. AIMS: To develop and apply an experimental model to induce localised gastric pain. SUBJECTS: Twelve healthy male volunteers. METHODS: Stimulating electrodes were mounted on a biopsy forceps and electric stimuli were delivered during gastroscopy. Single, five repeated, and continuous stimuli were given at four locations in the stomach. Pain detection thresholds and pain intensities were assessed together with localisation of the referred pain area. RESULTS: Pain detection thresholds were higher in the prepyloric region compared with those obtained at the lesser and greater curvature. Increasing stimulus intensity resulted in augmented pain perception and repeated stimuli elicited pain at a lower stimulus intensity than single stimuli. Continuous stimuli evoked constant (33%), increasing (33%), or decreasing (33%) pain. The localisation of referred pain varied considerably in the subjects. CONCLUSIONS: The model seems relevant to study basic pain mechanisms elicited by localised stimuli in the stomach. The experimental data support the premise that a gastric focus should always be suspected in patients referred with different kinds of abdominal pain.

Abdominal Pain↗
PubMed1994

Digital feedback suppression (DFS). Characterization of feedback-margin improvements in a DFS hearing instrument.

The introduction of a new power behind-the-ear hearing instrument equipped with an integrated digital feedback suppression (DFS) system, based on adaptive, digital signal processing, creates the need for new methods for evaluating the characteristics of this new technology. A special measuring method based on determination of the complex loop gain of the DFS instrument and the associated feedback path is described. This method yields information about the static feedback-margin improvement due to the DFS system, and the method is usable especially in connection with measurements on real ears. It requires fairly advanced test facilities, including a dual-channel FFT analyzer and, by preference, an anechoic room. Loop gain measurements on the new DFS power behind-the-ear hearing instrument show encouraging results. Groups of profoundly hearing-impaired children and adults were tested, and static feedback-margin improvements in the order of 10 dB for the new DFS power hearing instrument were seen. Variations were largest for groups using own ear moulds and individually fitted instruments.

Acoustic Stimulation↗
PubMed1993

A semiconductor silicon PIN diode matrix detector for measurement of 133Xe washout in small regional skin areas.

Local variations in microvascularization are expected in lesions and lesion-free skin of patients with pressure sores. To investigate these variations, methods for studies of the regional blood flow rate within small skin areas are needed. Regional blood flow can be estimated by measuring the washout of 133Xe from the tissue. This study describes a 2 x 12 Si PIN diode matrix detector capable of 133Xe detection, and equipped with a collimator to improve the spatial resolution. Thus the regional blood flow in subcutaneous skin areas down to about 2 cm2 can be investigated when the atraumatic epicutaneous labelling technique is used. The capability of the matrix detector and its collimator is described by laboratory investigations. In pilot investigations three normal subjects and three paraplegics with pressure sores were studied. In each individual study the detector matrix was divided into six square areas containing four diode detectors each. Thus six 133Xe disappearance rate constants from adjacent subcutaneous tissue could be determined simultaneously.

Female↗
PubMed1993

Experimental pressure induced rupture of the tympanic membrane in man.

The size of the overpressure in the ear canal which causes rupture of the tympanic membrane (TM) in man (rupture pressure, RP) was determined in 90 subjects 7-112 h post mortem in connection with the autopsy. The equipment allowed an overpressure in the ear canal to be applied either gradually or suddenly. In 144 normal TMs it was demonstrated that the tensile strength of the TM increases post mortem. Corrected to the time 0 post mortem, RP of normal TMs ranged 0.5-2.1 kp/cm2, median 1.2 kp/cm2. It was found to be correlated to the age of the patient, i.e. RP decreased with increasing age. No correlation was found between RP and the application speed of the overpressure. Ninety-nine percent of the ruptures were localized to the pars tensa (63% to the anterior part of this structure) and typically had the shape of a minor tear. The RP of 23 TMs with atrophic scars was significantly lower, 0.3-0.8 kp/cm2, and the rupture typically had the shape of a larger defect. The results of this study indicate large intersubject variability of the tensile strength of the human TM. Some individuals are at increased risk of TM rupture at minor overpressures in the ear canal (e.g. during certain watersports, such as diving) which may carry medicolegal implications.

Adolescent↗
PubMed1991

[Brain stem audiometry and unconventional audiometry in small children--a comparison with pure-tone audiometry performed at a later time].

Auditory brain-stem responses (ABR) was performed in the determination of hearing threshold in 71 infants and children born 1980-1985, all belonging to high risk groups or difficult-to-test children (median age 17 months, range 2-80). In a follow up study, 41 of the children were tested with pure tone audiometry approximately 2 1/2 years later. A significant correlation was revealed between the threshold obtained by ABR and the pure tone threshold at 2 kHz (Kendall tau C = 0.59, p less than 0.00005). Compared to pure tone audiometry, the threshold had been estimated correctly by ABR in 81% of the patients (+/- 20 dB HL), in 2% the threshold had been estimated to be more than 20 dB better (false negative), while 17% had the threshold estimated more than 20 dB worse (false positive). The median difference between the thresholds obtained was -10 dB HL, the 25% percentile -20 dB HL and the 75% percentile 0 dB HL. In 28 children, behavioural audiometry and pure tone audiometry could be analyzed. 21% were false negative and 50% false positive. The result of the study indicates that ABR is a reliable testing procedure in the estimation of hearing thresholds in children who cannot cooperate sufficiently for pure tone audiometry. ABR gives a more valid estimation of the hearing than behavioural audiometry and is therefore a most valuable testing procedure in paediatric audiology.

Audiometry, Evoked Response↗
PubMed1991

Treatment of recurrent acute tonsillitis with clindamycin. An alternative to tonsillectomy?

The failure of treatment with penicillin in patients suffering from acute beta-haemolytic streptococcus Group A tonsillitis, may be due to the presence of beta-lactamase-producing bacteria. Several studies indicate that treatment with clindamycin may prevent recurrence and thereby eliminate the need for tonsillectomy. In this prospective study, 54 patients with recurrent acute tonsillitis were investigated. 29 patients were treated with clindamycin, 150 mg 4 times daily for 10 days, while 25 patients received no antibiotic treatment and served as a control group. A significantly reduced number of episodes of acute tonsillitis (P less than 0.01) and of tonsillectomy (P less than 0.001) was found in the group of patients treated with clindamycin.

Acute Disease↗
PubMed1990

Effect of amiodarone on 3H-ouabain binding sites in human skeletal muscle.

Na,K-ATPase, or the Na,K-pump, is essential for the excitability and contractility of muscle tissue. Hypothyroidism in associated with a marked decrease in the Na,K-pump concentration in skeletal muscle and myocardium. In 7 patients on long-term amiodarone treatment there was a 36% reduction in the concentration of 3H-ouabain binding sites in skeletal muscle biopsies compared to 7 healthy subjects. This decrease during long-term amiodarone treatment may represent an equivalent reduction in the concentration of the functional Na,K-pump and it may be important in the adverse effect of amiodarone on muscle.

Adult↗
PubMed1990

Topical application of decongestant in dysfunction of the Eustachian tube: a randomized, double-blind, placebo-controlled trial.

Thirty-six patients, aged 12-75 years, with dry, central tympanic membrane perforations and a negative Valsalva manouevre and/or a negative aspiration/deflation test, were included in a randomized, double-blind, placebo-controlled trial on the effect of a decongestant agent (xylometazoline chloride 0.1%) and placebo (saline 0.9%) applied directly to the pharyngeal opening of the Eustachian tube. Judged by the Valsalva manouevre, tubal patency was significantly improved after application of the active drug (P less than 0.003). In contrast, no effect was demonstrated by the aspiration test (P = 0.80) or the deflation test (P = 0.51). It is concluded that a topical decongestant improves Eustachian tube function but only at unphysiologically high pressures.

Administration, Topical↗
PubMed1990

[Sugar and sweets].

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Dietary Carbohydrates↗
PubMed1989

[Torticollis--an unusual origin].

On account of left-sided chronic otitis with cholesteatoma an otherwise healthy eight year-old girl underwent an otological operation lasting five hours. The course was technically uncomplicated. Postoperatively, persistent torticollis with turning of the head to the right was observed. X-ray and CT-scanning revealed atlanto-axial subluxation. No other cause than the rotation of the head during the operation was demonstrated. It is concluded that subluxation should be considered if torticollis is present after operations during which the head has been rotated.

Child↗