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Biomedical subjects

T Lahtinen

Publications and source records attributed to T Lahtinen.

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

Variational formulation of open-ended coaxial line in contact with layered biological medium.

An open-ended coaxial probe designed to measure layered biological media is analyzed with a new method. The probe is considered as an electrostatic circuit element whose capacitance is solved using a stationary functional. The fundamental transverse electric and magnetic field (TEM)-mode and the series of evanescent wavemodes in the coaxial cable are used as basis functions. The field outside the probe is solved using a Hankel transform. The capacitance is calculated for homogeneous materials and two-layer structures and the results are compared with values measured with a phantom model. The method can be easily extended for structures with an arbitrary number of layers. A practical approximation for two-layer cases, originally developed to take into account the effect of subcutaneous fat in skin measurements, is presented and its validity for different combinations of dielectric constants and the thickness of the first layer is demonstrated. The static approximation limits the frequency range, but it covers biological measurements up to 500 MHz. The developed method is accurate and easy to adopt in practice.

Adipose Tissue↗
PubMed1997

Down-regulation of Ku autoantigen, DNA-dependent protein kinase, and poly(ADP-ribose) polymerase during cellular senescence.

During aging and cellular senescence mutations accumulate in genomic and mitochondrial DNA. Ku autoantigens, DNA-dependent protein kinase, and poly (ADP-ribose) polymerase have an essential role in DNA damage recognition. Our purpose was to find out whether cellular senescence of fibroblasts affects the protein components that recognize DNA damage and induce the repair process. We compared presenescent and replicatively senescent human WI-38 fibroblasts with each other and with SV-40 immortalized and serum-deficient quiescent WI-38 cells. Our results showed that replicative senescence significantly decreased the nuclear level of both p70 and p86 components of Ku autoantigen. SV-40 immortalization and cellular quiescence did not affect the level of the p86 component but slightly increased that of p70. Both replicative senescence and cellular quiescence decreased the activity of DNA-dependent protein kinase in WI-38 fibroblasts. On the other hand, SV-40 immortalization increased the activity of DNA-dependent protein kinase. The protein level of poly(ADP-ribose) polymerase (PARP) was strongly decreased in replicatively senescent fibroblasts. Quiescence of early-passage fibroblasts also slightly reduced the protein level of PARP. Apoptosis was not observed in replicatively senescent fibroblasts. Our results show that replicative senescence and to some extent cellular quiescence down-regulate the recognition system of DNA damage involving Ku autoantigens, DNA-dependent protein kinase, and PARP and hence could enhance the accumulation of DNA damage during aging.

Amino Acid Sequence↗
PubMed1994

Relative output factors of asymmetric megavoltage beams.

A new method for calculating output factors of asymmetric therapy fields is presented. The method uses the output factors of symmetric fields, as well as off-axis ratios measured in air, to calculate the output factor for an arbitrary asymmetric field. Calculations have been checked by measurements in four photon beams (4-18 MV) of three different linear accelerators. The accuracy between the theory and the measurements is generally better than 1%. According to the preliminary results the method may also be suitable for megavoltage electron beams.

Algorithms↗
PubMed1994

Modelling of the dielectric properties of normal and irradiated skin.

We have shown that irradiation changes the dielectric properties of human skin at radiofrequencies. Both the dielectric constant and the conductivity of the irradiated skin decrease, especially at low frequencies. The experimental data were analysed using two bioelectric models. Relevant dielectric parameters were determined by curve fitting. The dielectric relaxation of the radiation-induced acute or late reaction of the skin occurred at higher frequencies than with normal or non-irradiated skin, while the static conductivity and static dielectric constant of the irradiated skin decreased. We conclude that bioelectric modelling provides a useful tool in the evaluation of cellular changes in irradiated skin.

Algorithms↗
PubMed1994

Induction of class II molecules by cytomegalovirus in rat heart endothelial cells is inhibited by ganciclovir.

Cytomegalovirus (CMV) has been demonstrated to induce class II antigen expression in endothelial cells. To study whether ganciclovir (DHPG) has an effect on CMV-induced class II expression, cultured rat heart endothelial cells were infected with rat CMV (RCMV) and treated with different DHPG concentrations. Class II antigens in endothelial cells were detected by a monoclonal antibody and immunoperoxidase technique. Control cells did not express class II antigen, but during RCMV infection 92% of cells were class II-positive. DHPG treatment (1, 10, 100 and 1000 microg/ml) decreased RCMV-induced class II expression from 73% to 59%, 6% and 0%, respectively. As DHPG inhibits CMV DNA polymerase, our present results suggest that DHPG affects RCMV-induced class II expression via the inhibition of RCMV DNA replication.

Animals↗
PubMed1994

Irradiation of the whole neuraxis. A method for field positioning.

When the whole neuraxis is irradiated, as in medulloblastoma, the large target volume makes field arrangement complicated and 3-4 adjacent fields have to be used. A calculation method, based on geometrical matching of field edges, is presented for determining optimal separation between the opposed cranial fields and the posterior spinal field as orthogonal movements of the treatment couch top. The dosimetric verification of the method with a 4 MV photon beam indicates a dose inhomogeneity below +/- 7%.

Central Nervous System↗
PubMed1993

ICRU reference points and maximum doses of rectum and bladder in intracavitary radiotherapy.

The absorbed doses in the reference points of the rectum and bladder defined by ICRU report 38 for reporting intracavitary radiotherapy were compared with the calculated maximum doses of these organs. The calculations were validated by a thermoluminescent probe. A good correlation between the doses at the reference points and the calculated maximum doses was found.

Brachytherapy↗
PubMed1990

Anaplastic astrocytoma and glioblastoma: pion irradiation with the dynamic conformation technique at the Swiss Institute for Nuclear Research (SIN).

Clinical phase I/II studies have been performed at the Swiss Institute for Nuclear Research (SIN) since February 1982. Fifty-two out of 249 patients accepted for pion treatment by the end of 1986 were treated for malignant glioma with high dose pion irradiation. A substantial influence of their radioresistance was expected from increased radiation quality due to the contribution of high LET particles from pion capture, and by the possibility of target volume shaping and dose distribution related to the dynamic spot-scan conformation technique. The patients' treatment followed a dose escalation program with total doses from 2720-3420 cGy, fraction sizes from 170 to 205 cGy (90% isodose, minimum target dose), and treatment times from 4 to 5 weeks. 12/52 patients received an accelerated treatment with 3280 cGy in 14-22 days. 49/52 patients are eligible: 3 with astrocytoma of clinical aggressive behaviour, 14 with anaplastic astrocytoma (median age 42 years), and 32 patients with glioblastoma (median age 52 years). 8/49 patients had total/subtotal tumour resection, 19 patients a stereotactic biopsy. The patients were divided into three groups according to total dose, and a fourth group which received the accelerated treatment. There was no statistically significant difference in the median survival rate between the four groups, which was 13 months for the non-glioblastoma patients and 9 months for the glioblastoma patients. No radiation necrosis and no demyelination was found in 17 patients (6 recraniotomies, 11 autopsies). In 10/17 patients, clearly identifiable tumour cells were not demonstrated. NMR findings showed the tumour-surrounding oedema mostly stimulated by tumour necrosis and tumour progression. From these findings, further dose escalation programs, together with a shaping of the target volume close to the tumour, are not contraindicated.

Adult↗
PubMed1989

Biological demonstration of nerve growth factor in the rat pituitary gland.

Explants of anterior, intermediate and posterior pituitary lobes of rats at various ages evoked an intense nerve fibre outgrowth when co-cultured with neonatal sympathetic superior cervical ganglia in a collagen gel medium. Freezing and thawing of pituitary lobes, prior to culture, did not abolish their growth-promoting effect. Since the addition of antiserum to nerve growth factor in the culture medium inhibited totally the stimulative action of both fresh and frozen pituitary explants it is suggested that the stimulation is mediated by nerve growth factor or an immunologically related molecule. Based on the present results it is concluded that nerve growth factor is synthesized by both the anterior, intermediate and posterior lobes throughout the postnatal period, as well as in adult rats. The function of nerve growth factor in the pituitary gland is discussed.

Animals↗
PubMed1989

Developmental changes in 5-hydroxytryptamine immunoreactivity of sympathetic cells.

5-Hydroxytryptamine was demonstrated immunohistochemically in developing sympathetic chain ganglia, adrenal medullae and retroperitoneal paraganglionic tissue. Initially, all catecholamine-synthesizing cells were immunoreactive to 5-hydroxytryptamine, but during the maturation some sympathetic cell types lose this property. In the ganglia, small intensely fluorescent cells retain their 5-hydroxytryptamine content throughout maturation. This property is expressed transitionally in some neonatal principal neurons, but can be triggered even in adult neurons, e.g. after loading with L-tryptophan. In the adrenal medulla, immunoreactivity to 5-hydroxytryptamine is carried over to the adrenaline-synthesizing cells, while in the main retroperitoneal paraganglion it is observed in both adrenaline and noradrenaline cells. These results suggest that initial expression of 5-hydroxytryptamine immunoreactivity is coupled to the onset of catecholamine synthesis, but is regulated differently in each mature sympathetic organ.

Adrenal Medulla↗
PubMed1989

Age-dependence of the nerve fibre growth-promoting effects of hippocampus and exogenous nerve growth factor on cultured rat septum and superior cervical ganglion.

Explants of hippocampus from rats at various ages evoked an intense nerve fibre growth from cocultured superior cervical ganglion and septum explants taken from newborn rats. The addition of antiserum to nerve growth factor (NGF) into the culture medium inhibited the outgrowth of nerve fibres from superior cervical ganglia, while septum explants still extended nerve fibres in the same medium. Septum explants responded to added NGF, as well as to cocultured hippocampus, during the first postnatal week only, whereas ganglia extended nerve fibres in NGF-containing cultures throughout the postnatal period and even at the age of 6 months if superoptimal concentration of NGF was used. The present results suggest that hippocampus releases NGF and some other growth factor(s) in culture throughout the postnatal period from birth to adulthood. On the other hand, the capacity of septum to extend nerve fibres in response to the growth factors appears to be restricted to the first postnatal week.

Aging↗
PubMed1988

Nerve growth factor-like activity in the rat pituitary intermediate lobe.

Nerve growth factor (NGF)-like immunoreactivity was demonstrated in the rat pituitary intermediate lobe by indirect immunofluorescence method using antisera specific to beta-NGF isolated from adult male mouse submaxillary salivary gland. Co-culture of frozen or fresh intermediate lobes with newborn rat superior cervical ganglion resulted in marked fiber growth from the ganglion, which was totally inhibited by NGF antiserum, suggesting the presence in situ and secretion in vitro of biologically active pituitary NGF. Pituitary stalk transection caused decrease in both the NGF immunoreactivity and biological activity. These findings suggest that pituitary NGF level is under neural regulation.

Animals↗
PubMed1988

Blood flow, histomorphology and elemental composition of the canine femur after physical training or immobilization.

Effects of long-term physical training (treadmill running 4 km/day for 15 weeks) and immobilization (casting of the limb for 11 weeks) on the blood flow, bone marrow uptake of radio-colloid, histomorphology as well as elemental composition of the proximal canine femur were investigated. In the runners both bone marrow and mean bone perfusions measured with a 133Xe washout method were increased by 53.5% (P less than 0.05) and 54.1% (P less than 0.05), respectively, as compared with the age-matched controls. Bone marrow uptake of [99mTc]tincolloid and volumetric fractions of trabecular bone and bone marrow were not significantly altered after training or immobilization. The amount of zinc measured by atomic absorption spectrophotometry was, however, significantly higher (P less than 0.01) after immobilization as compared with the controls. Consequently, long-term physical training is suggested to stimulate bone metabolism by enhancing its blood perfusion even in the resting state, whereas the increase of zinc content was considered to be linked to the obvious bone resorption caused by immobilization.

Animals↗
PubMed1986

Effects of 100-Hz magnetic fields with various waveforms on the development of chick embryos.

Chick embryos were exposed during their 52 first hours of development to 100-Hz magnetic fields. Sinusoidal, square and pulsed waveforms were used at average field strengths from 0.1 A/m to 80 A/m. After exposure, the embryos were examined for abnormalities and classified by the developmental stages. When bipolar oscillations (oscillating at both sides of the zero-level) were used, the percentage of abnormal embryos was significantly increased above 1 A/m. In exposure to unipolar square waves, no significant effect on the percentage of abnormalities could be demonstrated. The developmental stage was possibly affected by unipolar square waves at 0.1 A/m, all other field strengths and waveforms being apparently ineffective.

Animals↗