Billeder på siden
PDF
ePub

escent rings. This latter condition is important, even though at times difficult of accomplishment.

The penetration of the tube is a perfectly measurable quantity, and its actual measurement is an important advance in bringing about improvement in and uniformity of methods.

A simple form of penetrometer may be made by a combination of the Benoist penetrometer and the aluminum window penetrometer. Cylinders of aluminum, of two, four, et cetera, to sixteen millimeters in thickness, and about eight millimeters in diameter, are cut from a rod of aluminum, and the eight cylinders set in apertures in lead, the first four in lead one-half millimeter thick, and the remaining four, ten to sixteen millimeters thick, set in apertures of a strip of lead one millimeter thick. The device is then glued to a thin board, and placed in front of a small fluoroscope. Tubes whose rays penetrate freely the first four or five aluminum cylinders, and the one-half millimeter lead strip not at all, are "soft" tubes, or "low tension" tubes, although the latter term is a misnomer. Tubes whose rays penetrate all the cylinders and the one-half millimeter lead strip to some extent, are "hard" tubes, or "high tension" tubes. The terms hard and soft are purely relative, having to do only with the penetration of the rays.

The Walter penetrometer is the international standard. Soft tubes penetrate four or less platinum windows, while hard tubes penetrate from five to seven windows. Tubes are now designated as Walter 4, 5, 6 or 7, according to their penetration.

Besides being of correct penetrating power the tube must be capable of enduring a heavy current (amperage) for the required time of exposure, that is one-half second to six or eight minutes, without overheating, lowering of vacuum and lessening in penetration. It should be one which emits a large percentage of direct rays, supposedly from the focal point on the anode, and a small percentage of indirect rays, or rays from the walls of the tube, or at least not from the focal point on the anode. If a hard tube be used, it is preferable to use one which has been aged or seasoned to the degree where it will emit rays of at nearly homogeneous character, rather than a mixture of rays varying from those of extreme softness to those of extreme hardness. The tube should be one having a high efficiency as a transformer of electrical energy into Roentgen rays.

At present there is no simple and satisfactory method of measuring directly the volume of rays given off by a Roentgen tube. The nearest approach to it is the measurement of a function which is proportional to it, namely, the current exciting the tube. A milliamperemeter constructed to be used in series with the tube indicates, for a given tube excited by the same apparatus, a current strength proportional to the radiant output of the tube. Or more strictly speaking, the volume of rays is proportional to the square of the current strength, provided the resistance of the tube and the penetration remains constant. is very nearly realized in practice, radiographically, for, in exposing a

structure for radiographic purposes, if two milliamperes through the tube requires an exposure of sixteen seconds, four milliamperes requires not eight seconds, but only about four, and a radiograph may be made of the part exposed in one and one-half to two seconds if we can excite it by six milliamperes. It is possible to secure a good radiograph of a one hundred fifty pound man in one second, by using a Walter 6 tube, excited by six milliamperes, from an induction coil actuated by an electrolytic interrupter.

With reference to the penetration of tubes employed in radiography, Roentgenologists are divided more or less sharply, into two schools, advocates of soft tube technique, with attendant long exposures, employing Walter 3 to 5 tubes, and users of hard tubes, Walter 5 to 7, requiring exposures of one-half second to ten seconds, for radiographing any part of the body, while the Walter 3, 4, or 5 tube requires from twenty seconds to six minutes exposure to do the same work. So far as I know, there are no radiographers who are equally skilled in the use of both hard and soft tubes.

At first thought it might seem that the advantages are all with the use of the Walter 5, 6, and 7 tubes. This, however, is not true, as soft tubes, Walter 5 or less, possess the undeniable advantage of giving greater detail and contrast in soft parts, especially radiography of abdomen and chest, than the average hard tube. In other words, we are able, by using the soft tube, to record upon the plate more marked differentiation between tissues differing but slightly in density. In addition the likelihood of overexposing the plate is very much less than when using the hard tube. On the other hand there is the greatest danger of "burning"-it is the rays of slight penetration which produce changes in the integument.

The difficulty of immobilizing the patient during the long exposure, the absolute impossibility of radiographing lungs and abdomen during suspended respiration, and the numerous failures in attempted radiography of small children who cannot be kept from moving during the exposure, are drawbacks which have from time to time annoyed and discouraged the user of the soft tube. The greatest trouble of all, however, is the rapid heating of the soft tube under heavy current excitation, its quickly lowering vacuum following this, and the resulting lessening of the penetration rendering the tube for the time absolutely worthless for radiographic purposes.

With the use of the hard tube there is extreme improbability of producing a dermatitis, even if a dozen radiographs be taken of the same patient, the same day. It is also possible to secure immobility of all parts of the body. One can radiograph either chest or abdomen during suspended respiration, and avoid all blurring of shadow-detail due to respiratory movements. With a properly seasoned old Roentgen tube we may pass a heavy current through it for a dozen consecutive radiographs without materially interfering with its penetration. Such a tube also possesses a higher efficiency as a transformer of electrical

energy into Roentgen rays. There are some important changes taking place during the life of a tube, which all experienced radiographers have noted. The new tube is naturally soft. It produces really good radiographs only when in this condition. Its vacuum is very unstable. It will heat and run down during exposure, and rise a little higher in vacuum on cooling. After a time it reaches a degree of hardness where its rays freely penetrate the bones. It has reached a penetration of Walter 6 or 7, but it possesses poor definition; that is, the output of direct rays may be as low as twenty per cent of the energy absorbed, while the indirect rays may be as great as forty to sixty per cent, the balance of the energy being transformed into heat. Upon lowering the vacuum successively, it will naturally run longer, without excessive heating, and not rise as rapidly upon cooling. It is approaching a stage where its vacuum is more nearly stable. In the course of a year, if the tube be used for a short time daily, it has undergone remarkable changes. The glass of the anterior hemisphere becomes darkened. The glass annealed, the vacuum more constant, it will stand a heavy current without heating, and instead of fluorescing a greenish-yellow or an olive green, it will fluoresce a sunflower-yellow, a yellow almost as pure as the sodium light. Its penetration is about Walter 6. It gives beautiful definition; the medullary canal of the ulna is clearly defined by the fluoroscope. It has become a seasoned tube. Its output of direct rays may rise as high as seventy per cent of the output of radiant energy, and the rays seem to be much more nearly homogeneous in quality than those from a Walter 6 new tube. Such a tube is worth many times the value of a new tube for radiographic purposes. It is better to save a well seasoned old tube for the more difficult work, such as radiography of skull, chest, renal and biliary calculi.

The use of the diaphragm for cutting off the indirect rays is of the greatest service, particularly when using a comparatively new hard tube. A scientifically constructed diaphragm, with its accompanying compression cylinder or compression ring is a necessary accessory to every fully-equipped radiographic armamentarium.

In the construction of these compression diaphragh cylinders some deplorable mistakes have been made which appear to be due to a lack of knowledge of the fundamental principles of radiant energy. When Roentgen rays or any other form of radiant energy pass from one medium to another of different density, the particles of the second medium act as secondary centres for the production of secondary radiations, according to the principle of Huygens. Hence if we would avoid the secondary rays from the inside of the cylinder, we must make our diaphragm at the top of the cylinder the truncated portion of a cone whose apex is the focal point on the anode, and whose base is coincident with the periphery of the bottom of the cylinder. We are indebted to Albers Shönburg for the compression diaphragm cylinder and to Dessauer for first calling attention to the correct principles of construction of such apparatus.

It is perfectly possible to make a radiograph by means of these secondary r-rays generated from the inside of the cylinder of improper construction. The direct rays may be intercepted by a thick lead disc, and the secondary r-rays, s (Figure 1), allowed to pass through the structure to be radiographed. On the other hand their production may be eliminated by the proper diaphragm, while at the same time minimizing the transmission of indirect radiations from the tube. The accompanying diagram (Figure I) illustrates, in the left hand half of the figure, a cylinder of improper construction, with its indirect and secondary rays, while the right half shows how these may be eliminated by the diaphragm of correct aperture. A diaphragm apparatus of nearly universal application may be constructed by covering a suitable-sized

S

FIGURE I. (D) Apex of cone of direct rays. (1) Indirect rays from walls of tube. (s) Secondary rays from cylinder walls.

board or fiber support with a layer of a mixture of putty and mercurous oxide on the side next the tube. A convenient aperture, of from five to eight centimeters, is left in the center. This diaphragm, being a nonconductor, nonmagnetic, and noninductive, may be placed in direct contact with the tube. Suitable lead diaphragms may be employed for further limiting the field covered by the rays, and two or more rings may be detached to the under side of the diaphragm for purposes of compression or immobilization. The diagram (Figure II) will make clear its construction.

The compression rings of seven and one-half and ten inches in diameter, in addition to a five and one-half inch cylinder, will fulfill about all of the requirements demanded of such apparatus by the radiographer.

Lastly, I wish to mention briefly the photographic technique. The

radiograph is a special negative. It should possess strong and emphatic contrasts. Two of the most important factors in the production of the best grade of radiographs are the correct exposure, and the correct developmental technique. The physician who exposes his plates, and then passes them on to a portrait or landscape photographer for development, will not obtain the highest grade of work, until both learn by experience the important difference between a landscape or portrait negative, and a radiographic negative. A different developing formula and a different duration of development are essential. Especially is this true if the hard tube technique be employed.

In conclusion, I may say that while a great deal depends upon the apparatus and its correct management, there is also the personal factor

D

FIGURE II.-(A) Layer of putty and mercurous oxide. (B) Wood or fiber support. (c) Metal diaphragm. (D) Cone of direct rays.

of the radiographer. If he would succeed he must train his judgment by keeping a careful record of all exposures, and making careful measurements of all the factors which he employs, which are capable of measurement. It is only by persistent systematic training that he can ever hope to be able either to duplicate his best results or to place his work on a scientific basis.

The use of the Roentgen rays in diagnosis has made remarkable strides since the presentation of Roentgen's paper upon its discovery, ten years ago this month. If the next five years are as fruitful in

« ForrigeFortsæt »