Biomedical subjects
Publications and source records attributed to J Meiler.
After some preliminary remarks on the production and spectral composition of X-Rays it is demonstrated, how the total filtration of the X-Rays including the filtration in the anode can be found from the X-Ray Bremsspektrum. The curves for determining this filtration include filters up to 50 mm AI-Equivalent for 40 to 150 kV constant direct voltage.
The article critically examines the available literature on the tangential effect. It is stated in literature that an interface is clearly outlined if the x-ray beam touches it briefly tangentially during tomography; it follows that the more angles of incidence occur, and hence the more complex the course of the focal pathway is, the more details are shown. Since none of these criteria applies, it is concluded that in fact a tangential effect does not exist.
The relevant modulation transver factors and hence the ability of detail identification in the tomogram were determined for transverse tomography. It was found that the conventional angle of incidence of 20 degrees is not to be recommended, and that, likewise, the use of single screens would not be very effective. A considerable improvement in the visibility of fine details can be achieved by using double screens, the exposure being approximately the same, which means the patient is not exposed to stronger radiation, the angle of incidence being e.g. 45 degrees. Pointers are given with regard to an overall improvement of tomogram quality.
The article discusses the factors which govern blurring in transversal tomography. The amount of blurring is a multiple of the degree of blurring which occurs in normal horizontal tomography, assuming the usual conditions, ie small angle of incidence in case of fixed focus, and large diameter of rotation in case of a rotating arrangement. Hence, only large details will show up in transversal tomography, in contrast to the results obtained via comparable horizontal tomograms.
The article goes into the question of the effective values which must be taken into account with the generally poor definition resulting from oblique incidence of the X-rays on the converter. Particular attention is given to the conditions obtaining in horizontal and transverse tomography, including the spiral or pendular movement of the tube.
The article discusses the effects on tomographic results exercised by uniform exposure distribution in linear direction, as well as by larger exposure fractions in the internal half of the pathway and outer pathway sections. Predominance of the interior exposure fraction may yield a tomogram which is equally significant as obtained with constant blurring, although the angle of the pendular movement must be considerably greater. Transferring the medium exposure to the outer pathway sections yields faulty images. Uniform exposure distribution produces the most favorable results. In multidirectional layers, spiral blurring with control-led intensity was found to be most appropriate. This requires the smallest pendular angle compared with other tomograms of equal contrast, and also avoids faulty images.
The minimal contrast was studied which can be recognised by the eye, using similar, neighbouring images under varying conditions of brightness. Subsequently the effect of contrast thresholds on the recognition of detail in radiographs has been examined.
Tomograms with linear movements and 40 degrees swing have shown that the usual T-modulation transmission function is inadequate and that the I-modulation transmission function is as significant as for plain films. Moreover, several selected modulation transmission functions are necessary. Even if resolution is high, detail recognition is reduced in relation to the threshold contrast. This is particularly true for the level of the cut and its close vicinity. On the other hand, pseudoimages are supressed by the threshold contrast.
Investigations on test grids, phantom-kidneys filled with contrast medium and a phantom skull in the case of linear tomography with various angles and constant, triangular or V shaped dose-rate function during tomograms showed that constant exposure over a certain angle of cut leads to minimum thickness. The V shaped distribution remains unregarded since it produces doublepictures. Radiographs exposed mainly in the middle projection give less contrast than those with constant dose rate and smaller planigraphic angles. Concentrating exposure on outer components leads to double pictures.
The deleterious effect of organ movement on radiographic quality is examined. In connection with this, the performance of various focal spots is discussed, particularly of a modern high speed tube with focal spots of 1.2, 1.5 and 1.8, with a resolution in the region of 3 to 6 lp/mm. A detailed discussion of the results is given with examples derived from various techniques used for the stomach and lungs. The results indicate that there is considerable improvement to be derived from high performance tubes with small focal spots. A further significant advance is also due to the high resolution image intensifiers. These have resulted in a marked increase in the use of 70 and 100 mm. film.
Individual modulation transfer functions of the recording system were related to total modulation transfer function; examples derived from radiographs of the lungs, stomach and gall bladder were used to show that proper evaluation of the radiograph requires consideration of the relationship between original contrast and detailed size. The total transfer function shows a rapid decline of the contrast factor with increasing point frequency. It has been shown that the pseudo record which could be expected from the usual transfer functions of the recording system does not, in fact, appear.
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Nine different artificial neural networks were trained with the spherically encoded chemical environments of more than 500000 carbon atoms to predict their 13C NMR chemical shifts. Based on these results the PC-program "C_shift" was developed which allows the calculation of the 13C NMR spectra of any proposed molecular structure consisting of the covalently bonded elements C, H, N, O, P, S and the halogens. Results were obtained with a mean deviation as low as 1.8 ppm; this accuracy is equivalent to a determination on the basis of a large database but, in a time as short as known from increment calculations, was demonstrated exemplary using the natural agent epothilone A. The artificial neural networks allow simultaneously a precise and fast prediction of a large number of 13C NMR spectra, as needed for high throughput NMR and screening of a substance or spectra libraries.
The automated structure elucidation of organic molecules from experimentally obtained properties is extended by an entirely new approach. A genetic algorithm is implemented that uses molecular constitution structures as individuals. With this approach, the structure of organic molecules can be optimized to meet experimental criteria, if in addition a fast and accurate method for the prediction of the used physical or chemical features is available. This is demonstrated using (13)C NMR spectrum as readily obtainable information. (13)C NMR chemical shift, intensity, and multiplicity information is available from (13)C NMR DEPT spectra. By means of artificial neural networks a fast and accurate method for calculating the (13)C NMR spectrum of the generated structures exists. The approach is limited by the size of the constitutional space that has to be searched and by the accuracy of the shift prediction for the unknown substance. The method is implemented and tested successfully for organic molecules with up to 20 non-hydrogen atoms.