Paul Edholm

Ectomography-A New Radiographic Reconstruction Method-I. Theory and Error Estimates

Radiographic technology has advanced considerably during the last years with the advent of reconstruction techniques allowing visualization of slices through the body. In spite of the advantage of computed tomography compared to conventional radiographic methods, there are still some shortcomings with the method If a different section of the body is desired, another recording has to be made, the width of the dice reconstructed is fixed, and a full 1800 view angle is required.

Ectomography A New Radiographic Method for Reproducing a Selected Slice of Varying Thickness

The mathematical basis is described of a new radiographic method by which an arbitrarily thick layer of the patient may be reconstructed. The reconstruction is performed from at least 60 images of the volume under examination. Each of these images, which have to be in digital form, is subjected to a special filtration process of its spatial frequencies. The combination of all the images will form the resulting image of the layer--the ectomogram. The method has been analysed and tested in experiments simulated with a computer.

Ectomography-A New Radiographic Reconstruction Method-II. Computer Simulated Experiments

In a special radiographic process, ectomography, an image of a slice is produced by simple summation of a set of specially filtered component images, of which each represents one of at least 60 different projections of the object. After being digitized, they are stored, filtered, and summed in a computer. Images representing any slice of any thickness in the object may be produced from the same set of component images. All details within the slice are pictured correctly while details outside are almost completely eliminated.

PRIMARY X-RAY DODGING.

By using compensating filters which can be optically aligned in the beam, radiographic image quality is increased, the dose to the patient is reduced to approximately one-half, and film costs are reduced accordingly. The improvement in image quality is the result of compensation for the varying thickness of the object and reduction in background fogging due to reduced scattering.

Elimination of blur in linear tomography.

The interpretation of the diagnostic content of a linear tomogram is complicated by the presence of blur. This is the main reason for other types of tomography being preferred, where practicable, as for instance those based on circular and hypotrochoidal motion in which the blur is less marked. Linear tomography possesses however the advantages that the apparatus is inexpensive and simple in design, that a high level of precision can be obtained and that the exposure times are short; the ordinary roentgen unit can moreover often be adapted for linear tomography. As linear tomography is the only practicable technique in neuroradiologic examinations requiring certain projections, a method of eliminating blur from linear tomograms seems to be urgently needed.

Nomogram for measuring the anteversion angle and angulation of fracture from roentgenograms.

Most of the numerous methods for computing the anteversion angle rely on the use of complicated nomograms and tables (BILLING 1954, DUNLAP et colI. 1953, JOHNSON 1941, MAGILLIGAN 1956, LEGER 1952, RYDER & CRANE 1953, SCHERTLEIN 1929). Nomograms and trigonometric formulae have also been devised for computing the angulation of a fracture (BOGDANOV 1950) while graphical methods have been envolved for the construction of this and the anteversion angle (LEFEBVRE ett coll. 1961, NORMAN 1965). Practically all the methods necessitate the two projections being perpendicular to each other as well as to the axis of one of the fragments, and the method now presented is based on a nomogram having the same requirements. More general means of measuring angles exist (EDHOLM 1966) but the simplicity and mode of application of the nomogram would appear to warrant its description.

Instrument for measuring angles from roentgenograms.

An angle in the body as depicted in roentgenograms is almost invariably distorted. The angle can be measured directly on the film only when the latter has been obtained parallel to the plane of the angle. In theory, an angle represented by two projections may be determined if the angle between the projections is known; this can be measured trigonometrically or graphically in a number of ways, although either procedure is too complicated for routine use. An instrument by means of which the projected angle can be obtained graphically will now be described. The method is a general one insofar as the directions of projection need not be perpendicular to each other nor need there be right angles between the beam and the film or between the beam and either side of the angle.

Correction of the pelvis in bladder exstrophy. A radiographic analysis.

The theoretical possibility of closing the diastasis of the pubic symphysis in bladder exstrophy by a modified form of osteotomy of the ilium has been investigated. It was believed that this operation might enable a better correction of the pelvis than is possible with the more conventional techniques. Roentgenograms from patients with this anomaly and from a group used as controls, obtained in a radiographic analysis of the orientation of the pelvic bones, were compared. The comparison was based on measurements of the angles between lines joining various skeletal reference points. This method of measurement has the advantage over the measurement of distances in that the determinations are independent of difference in size at different ages as well as in magnification between various radiographic projections.