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Biomedical subjects

D Harder

Publications and source records attributed to D Harder.

At least 73 records · Page 4Linked to original sources
PubMed1984

Kinetics of cell survival as predicted by the repair/interaction model.

A functional relationship determining the survival curve for reproductive cell death caused by low-LET radiation is derived on the basis of Lea's repair/interaction model. Radiation-induced primary lesions are assumed to interact pair-wise to form long-lived lethal lesions, simultaneously with the repair of the primary lesions. The formula derived for a short single irradiation gives a typical shouldered survival curve. At doses below the shoulder the interaction entirely occurs at low levels of primary lesion concentration, resulting in the parabolic shape of the logarithmic survival curve. At higher doses the interaction initially proceeds at high levels of primary lesion concentration, a state whose duration is determined by the dose and the finite speed of the first-order reaction of repair; this initial phase of the interaction results in the exponential tail of the survival curve. Survival after fractionated and protracted irradiation are also obtained. The dependence of cell survival on the irradiation conditions can be interpreted in terms of the basic parameters of the model.

Cell Survival↗
PubMed1983

[Location resolving sound impulse transit time measurements].

In this paper a method is demonstrated for recording transverse profiles of the time of flight in samples with a local resolution of a few millimetres and a time resolution of 10 ns. Good agreement between the theoretical and the measured curves is shown for some test objects.

Biophysical Phenomena↗
PubMed1983

Characteristic functions of point monodirectional electron beams.

Narrow electron beams with approximately Gaussian radial fluence distributions were extracted from a clinical 42 MeV betatron by means of a special multi-diaphragm lead collimator with either 3 or 7.5 mm diameter of the effective diaphragm. From each narrow beam the dose distribution in water, was measured with an automated scanning system, and the corresponding dose distribution of the point monodirectional source was obtained by deconvolution. As a reference, the fluence distribution in air at the same distance z was used, corrected for multiple scattering in air where necessary. At 10, 20 and 40 MeV, non-linear pear-shaped curves for the root mean square radial excursion, sigma(z), as functions of the depth, with maxima of 12, 19.7 and 27.8 mm at about 70 per cent of the practical range were obtained. The second characteristic function, I(z), the integral of the dose over the transverse plane at depth z, was obtained from the measured broad beam depth dose curve and the associated transverse dose distributions.

Computers↗
PubMed1982

Characteristics of the pulsed ultrasound field.

Pulsed ultrasound fields show some quantitative differences from continuous fields. In order to support the analysis of bioeffects of pulsed ultrasound fields these differences are characterized. For each observation point, the sound pulse is calculated by wavelet addition. The total pulse length has a maximum near the transducer surface, corresponding to large differences in acoustical pathlength from the surface elements. Accordingly, the spectrum varies strongly in the near field. In the far field, pulse-length and spectrum are almost constant as long as frequency-dependent absorption is negligible. The radiation field at each point is described by the time integral of sound intensity over one pulse (energy transport per unit area) and by the corresponding "dose" (absorbed energy per unit volume in one pulse). Differences between the spatial distributions of these two quantities are mainly detectable in the near field, which is also the region where differences between continuous and pulsed ultrasound (at the same centre frequency) are considerable.

Energy Transfer↗
PubMed1982

[Population kinetics of microcolonies causing "shouldered" survival curves at low LET].

The well-known fact that the dose dependence of cell survival, when measured by the macrocolony-forming test, can be fitted with the multitarget expression is re-investigated by considering the delayed loss of cellular reproductivity in a microcolony. As the reproductive capacity of each single cell of the microcolony must be lost in order to prevent the development of a macrocolony, the reproductive death of a microcolony is in mathematical analogy to the multitarget model and provides the biological explanation for its formal applicability. Published data from colony-size distributions and the model of "latent genetical damage" are utilized to understand the exponential dose dependence of single cell survival in a microcolony. Exponential survival curves obtained at high LET are explained by an undelayed loss of reproductivity in the irradiated cell. The restitution of the "shoulder" in delayed-plating and split-dose experiments is interpreted by repair effects.

Animals↗
PubMed1982

[An Axicon lens for the focusing of ultrasound fields].

In this paper the properties of an ultrasonic axicon lens of 10 cm diameter are investigated. This device generates a narrow ultrasonic beam in a range of 20 cm. The beam width varies from 1.7 to 0.5 mm in the frequency range from 1.7 to 5.5 megahertz. For practical applications a water path is necessary.

Humans↗
PubMed1982

[An axicon lens for focussing ultrasound fields].

In this paper the properties of an ultrasonic axicon lens of 10 cm diameter are Investigated. This device generates a narrow ultrasonic beam in a range of 20 cm. The beam width varies from 1.7 to 0.5 mm in the frequency range from 1.7 to 5.5 megahertz. For practical applications a water path is necessary.

Humans↗
PubMed1981

Analysis of radiation-induced acentric fragments in human G0 lymphocytes.

Acentric fragments induced by low and medium LET radiations and observed in the first mitosis after stimulation were evaluated in the total collective of cells and in the sub-collectives of cells with and without dicentric chromosomes. For 13 MeV electrons and 150 kV X-rays the dose-yield relationships of all acentrics and of excess acentrics (acentrics not associated to dicentrics) were best fitted by linear-quadratic functions. For soft X-rays (10 kV) the dose-yield relationship of excess acentrics was linear. LET-independent proportions of, on the average, 1.03 +/- 0.07 acentric fragments associated with 1 dicentric chromosome and of approximately 0.5 acentrics associated with other aberrations of the binary interaction type have been observed. The linear yield components of the excess acentrics are relatively large, and an LET-dependent proportion, decreasing with increasing LET, exists between the liner components of excess acentrics and dicentrics. This may be taken to indicate the presence of acentrics resulting from single "primary lesions", a result questioning Revell's exchange hypothesis. The evaluated distributions of excess acentrics show overdispersion, which can be explained by the simultaneous formation of two acentrics in certain incomplete exchange aberrations of interstitial deletions.

Chromosome Aberrations↗
PubMed1980

Recovery kinetics of radiation-induced chromosome aberrations in human G0 lymphocytes.

Declining yields of radiation-induced dicentric chromosomes in human G0 lymphocytes were observed in split-dose experiments with time intervals varied up to 8 h. In agreement with microdosimetric intratrack-intertrack interaction models, only the dose-squared yield component was reduced and approached an asymptotic value equal to one half of the corresponding single exposure yield. For 150 kV X-rays and 13 MeV electrons, at total doses up to 6 Gy, the time constant tau of the approximately exponential decline was practically dose- and quality-independent within a range of 100-180 min. For 10 kV X-rays, in the presence of a dominant linear yield component, only a small split-dose effect, but with a consistent tau-value, was observed for a total dose of 5 Gy. Since tau can be interpreted as the mean life time of "primary lesions" in chromatin fibres, its independence from absorbed dose and radiation quality means that radiation damage of the split-dose recovery mechanism can be excluded for doses up to 6 Gy. By correlating the observed split-dose reduction of the acentric fragment yield to the reduction of the dicentric yield, (1.64 +/- 0.03) acentrics/dicentric for 150 kV X-rays and (1.51 +/- 0.11) acentrics/dicentric for 13 MeV electrons were obtained. Acentrics formed in the course of dicentric formation as well as in other binary interactions of "primary lesions" are represented in these ratios. Post-irradiation recovery during time intervals between irradiation and cell stimulation up to 24 h did not occur. The relations to comparable results in cell lethality experiments are discussed, and a hypothesis of "fast" and "slow" binary interactions of "primary lesions" is put forward.

Chromosome Aberrations↗
PubMed1980

Numerical relationship between cells with radiation-induced chromosome aberrations and cells lethally injured by radiation.

The numerical relationship between radiation-induced chromosome aberrations in human G0 lymphocytes and reproductive lethality of human bone-marrow lymphocytes is analysed within a large LET interval. The comparison is based upon the evaluation of the coefficient a of the linear component of the corresponding dose-effect-relation for the frequency of cells without aberrations and for the frequency of surviving cells respectively. The good correlation between these coefficients over a large LET interval supports the hypothesis that structural chromosome aberrations and reproductive cell death both result from the same gross chromatin damage.

Cell Survival↗
PubMed1980

Chromosome aberrations induced in human lymphocytes by ultrasoft Al (K) and C (K) X-rays.

Induction of structural chromosome aberrations was studied in human peripheral blood lymphocytes irradiated in the unstimulated state with ultrasoft X-rays. Aluminium K X-rays (1.49 ke V) and carbon K X-rays (0.28 ke V) were used. The frequencies of dicentric aberrations and of excess acentric fragments were found to increase approximately linearly with absorbed dose of both radiations. Carbon X-rays were more effective than aluminium X-rays. The corresponding linear yield coefficients for dicentrics are (3.7 X 10(-1)) Gy-1 and (2.2 X 10(-1)) Gy-1 respectively. AT low doses both these radiations are more effective than hard X-rays in producing dicentric aberrations, but at high doses aluminium X-rays are less effective than hard X-rays because of the large positive curvature of the hard X-ray dose-response. The most significant observation of these experiments is that electrons of less than 280 eV, produced by carbon X-rays, are efficient in producing dicentric aberrations. This implies that single local energy events of about 14 ionizations with in less than 7 nm are able to produce exchange aberrations.

Cell Division↗