Large Focal Spot Research Articles

Nonuniform irradiation of laser targets

Smoothing of plasma ablated from a laser target under weakly nonuniform irradiation is discussed. Conduction is assumed restricted to a quasisteady layer enclosing the critical surface (large pellet or focal spot, and long, low-intensity, short-wavelength pulse). Light refraction can make the ablated plasma unstable.

Ablation parameters in KrF laser/plasma interaction: An experimental study

Experimental measurements of ablation parameters in a KrF laser produced aluminum plasma are presented for the laser intensity range of 1011–1013 W/cm2. Plasma created from large focal spots shows single broad ion current pulses as compared to two groups of ions observed from small focal spots. Detailed analysis of the velocity distributions of emitted ions is carried out to understand this behavior and its effect on measurements of the ablation parameters. Scalings with laser intensity of the mass ablation rate (ṁ∼I0.42L) and of the ablation pressure (Pa∼I0.81L) giving ṁ=1.5×105 g cm−2 sec−1 and Pa=5.5 Mbar at a laser intensity of 1013 W/cm2 are obtained. The results are found to be in good agreement with the predictions of a plasma ablation model based on inverse bremsstrahlung dominated absorption.

Effect of lateral energy transport on ablation pressure scaling in laser irradiated planar foil targets

Experimental investigations on ablatively accelerated thin plastic foil targets irradiated by a 6J, 5 nsec Nd: glass laser pulse, were conducted using shadowgraphy technique. A 2 nsec, 0.53 µm probe pulse, derived from the main laser was used for recording the foil motion. It was observed that 6 µm plastic foils could be accelerated to a velocity of about 3 × 106 cm/sec for an incident laser intensity of 5 × 1013 W/cm2 and the corresponding ablation pressure was 0.4 Mbar. Ablation pressure (P) scaling against absorbed laser intensity (I a ) was slower (P ∝I 0.4 ) for a smaller laser focal spot (30 µm) as compared to the scaling (P ∝I 0.7 ) for a larger focal spot (500 µm). This result has been explained considering the loss due to lateral energy transport from the laser plasma interaction region.

Causes of cine image quality deterioration in cardiac catheterization laboratories

Deterioration of cineangiographic image quality can result from malfunctions or technical errors at a number of points along the cine imaging chain: generator and automatic brightness control, x-ray tube, x-ray beam geometry, image intensifier, optics, cine camera, cine film, film processing, and cine projector. Such malfunctions or errors can result in loss of image contrast, loss of spatial resolution, improper control of film optical density (brightness), or some combination thereof. While the electronic and photographic technology involved is complex, physicians who perform cardiac catheterization should be conversant with the problems and what can be done to solve them. Catheterization laboratory personnel have control over a number of factors that directly affect image quality, including radiation dose rate per cine frame, kilovoltage or pulse width (depending on type of automatic brightness control), cine run time, selection of small or large focal spot, proper object-intensifier distance and beam collimation, aperture of the cine camera lens, selection of cine film, processing temperature, processing immersion time, and selection of developer.

X-ray tube focal spot sizes: comprehensive studies of their measurement and effect of measured size in angiography.

Thirty-two focal spot sizes of four x-ray tubes were measured by the pinhole, star pattern, slit, and root-mean-square (RMS) methods under various exposure conditions. The modulation transfer functions (MTFs) and line spread functions (LSFs) were also determined. The star pattern focal spot sizes agreed with the effective sizes calculated from the frequencies at the first minimum of the MTF within 0.04 mm for large focal spots and within 0.01 mm for small focal spots. The focal spot size determined by the slit method was approximately equal to the width of the LSF at the cutoff level of 0.15 +/- 0.06 of the peak value. The RMS method provided the best correlation between the measured focal spot sizes and the corresponding image distributions of blood vessels. The pinhole and slit methods tended to overestimate the focal spot size, but the star pattern method tended to underestimate it. For approximately 90% of the focal spots, the average of the star and slit (or pinhole) focal spot sizes agreed with the RMS focal spot size within +/- 0.1 mm.

An oscillation data collection system for high-resolution protein crystallography

An oscillation data collection system for protein crystallography is described. The system consists of a modified Enraf-Nonius precession camera with cylindrical cassette, and stepping motor driven by a flexible microprocessor control system. The X-ray source is graphite-monochromatized radiation from an Elliott GX-21 rotating-anode generator run at 5.5 kW on a focal spot measuring 0.5 × 5.0 mm. The potential advantages of using a relatively large focal spot in conjunction with a graphite monochromator are discussed. Conditions for optimum collimation and X-ray intensity are considered, and it is shown that appropriately designed collimators with adjustable apertures can have substantial advantages over commercially available pinhole collimators. The oscillation films are processed by a procedure based on that of Rossmann [J. Appl. Cryst. (1979), 12, 225-238]. Determination of the initial alignment of the film is facilitated by a pair of reference pins incorporated in the cylindrical cassette. These pins ensure that the position of the film in the cassette is known, and avoid the need for fiducial marks. The crystal alignment and film measurement technique is fully automatic, requiring no prior input other than the approximate starting orientation of the crystal, the approximate unit-cell dimensions, and the angular oscillation range. An alternative method for the determination of crystal orientation is proposed which has been found to be somewhat superior to that of Rossmann, especially for smaller unit cells.

Visibility and sharpness of lung structure at 90, 140, and 350 kV.

Human lung specimens containing a variety of lesions were radiographed at 90, 140, and 350 kV. Nodules less than 3 mm in diameter and linear shadows less than 0.3 mm in diameter were either not visible or poorly seen at 350 kV. Larger lesions were seen well with all three kilovoltages, although the interfaces were more distinct on the 90- and 140-kV views. The best images were obtained at 90, slightly worse at 140, and least satisfactory at 350 kV; in general, however, most features of the normal and diseased lung were readily seen at all three kilovoltages. The apparent degradation of the lung image at high kV levels may be explained by the large focal spot of the 350-kV system and the attenuation properties of high-energy photons.

Brillouin Scatter in Laser-Produced Plasmas

The absorption of intense laser light is found to be reduced when targets are irradiated by 1.06-\ensuremath{\mu} m light with long pulse widths (150-400 ps) and large focal spots (100-250 \ensuremath{\mu}m). Estimates of Brillouin scatter which account for the finite heat capacity of the underdense plasma predict this reduction. Spectra of the back-reflected light show red shifts indicative of Brillouin scattering.

大形構造物用Ｘ線応力測定装置

The strength of large structures such as vessels, slips or bridges depends mostly not only on the residual stress caused by welding but also on constrained stress, and therefore a non destructive stress measurement on a real object is demanded. Based on the experience that one of the authors previously made X-ray stress measurement of ship construction, authors newly developed a compact type of X-ray diffraction stress analyzer which adopts a modified side inclination method and enables the stress measurement at the corner of structures. This analyzer has also such features as an incident X-ray oscillation mechanism for the stress measurement of coarse grained materials, a direct charge system for making counting circuits small and 5×5mm2 large focal spot X-ray tube which is useful in side inclination method.The relation between the measured stress σx' and the true stressσx is theoretically calculated as follow.σx'=0.96σx+0.006σy-0.08Sin2φ(σ1-σ2)The error of σx' is negligibly small, and is found to be comparable to that of the data by using a conventional type of apparatus, based on the experimental results on S15C and S40C heat-treated specimens under uniaxial stress. Goniometer setting error under ±2mm also does not increase the error in X-ray stress measurement.

A spatially-coded x-ray source.

A new type of x-ray tube which permits three-dimensional imaging on a single film and high spatial resolution in spite of a large focal spot is described. The resolution is approximately the reciprocal of the spatial frequency of a half-tone screen.

A study of the angiographic appearance of the kidney in an aging normotensive population.

That intrarenal vessels undergo morphologic changes in the presence of systemic hypertension has been well documented by microangiographic, histologic, and in vitro as well as in vivo arteriographic studies (3–5, 8, 9, 12). Conflicting evidence exists, however, as to whether or not similar morphologic changes occur in the patient who is both normotensive and free of primary renal disease. Meaney et al. (9) studied injection specimens of 13 normotensive persons, all of whom had normal-appearing arterial trees except for 1 patient with polyarteritis nodosa in whom interlobar arterial aneurysms were seen. Contrariwise, similar studies by Virtama et al. (12) and Greenblatt (4) describe interlobar and arcuate arterial abnormalities similar to those seen in hypertension in some of their specimens from normotensive individuals. A precise understanding of the effect of aging on the development and incidence of arterial abnormalities in the normotensive population was not obtained from previous studies. Such information would be of importance in assessing the clinical significance of arteriographie abnormalities demonstrated in normotensive as well as hypertensive patients undergoing selective renal arteriography. This study was undertaken, therefore, to determine if intrarenal arterial abnormalities usually associated with hypertension also occurred in a normotensive population free of renal disease and, if so, to determine if the abnormalities were age-related in their incidence. Materials and Methods The results of this study are based on information derived from the arterial appearances on radiographs of kidneys removed at autopsy and injected with a barium gel mixture. The study was limited to specimens derived from those subjects whose blood pressure during life was documented clearly to be below 90 mm Hg diastolic and whose clinical history and pathologic (including histologic) examination revealed no evidence of primary renal disease such as pyelonephritis, glomerulonephritis, any form of arteritis, hydronephrosis, or arterial embolization. In order to minimize leakage of the injected material over the surface of the kidney, the organ was removed from the body with the perinephric fat and capsule intact; a cannula was then inserted and secured into the renal artery or arteries. Barium gel5 was injected into the arterial tree through an injection apparatus which permitted control of the injection pressure by a hand pump and manometer. Pressure was maintained at a level approximately 20 mm Hg higher than the systolic pressure of the individual recorded during life. Hardening of the contrast medium gel occurred within twenty minutes after injection, at which time perirenal fat was dissected from the specimen and radiographs were obtained. Kodak mammographic film (SO-378) was used. Exposures were obtained at 65 kV and 300 mAs using a large focal spot.

Serial radiography of virus-induced osteosarcomas in mice.

Serial radiography has provided us previously with information on the latent period, site of origin, and rate of growth of radiation-induced osteogenic sarcomas in mice (1, 2). We are now using the same technic to investigate the growth and development of osteosarcomas in mice inoculated soon after birth with FBJ virus. This virus was first obtained from a spontaneous osteosarcoma of the thoracic spine of an untreated CFl/Anl mouse (3). An extract of that tumor was injected into 5 mice on the day of birth, and in 2 of them osteosarcomas of the spine developed nine and eleven months later. An extract of the tumor appearing at eleven months was injected into 31 newborn mice, and by nine months bone cancer had developed in 12 of the 24 that survived to weaning. Since that time the oncogenic agent has been transferred by inoculating newborn mice with tumor material that has passed through a 0.45 micron, HA type millipore filter. The fact that electron microscopy demonstrates viral particles both in cellfree extracts and in ultrathin sections of the osteosarcomas (Fig. 1) further supports the opinion that these tumors are indeed caused by a virus. Materials and Methods For the present study 87 CFl/Anl mice received 0.1 ml of cell-free filtrate from virus-induced osteosarcomas soon after birth by intraperitoneal or subcutaneous injection. They have been examined radiographically one to three times a week since they were one day old. Sixty-four of the mice survived to weaning. To increase the number of potential tumors, 28 mice inoculated with FBJ virus at birth were added to the serial radiographic series when they were forty days old, and 14 were added at eighty-six days of age. At the present time there are 37 osteosarcomas among the 106 animals. Ventrodorsal radiographs are taken with the unanesthetized mice held in position by clips extending from the feet to the four corners of a rectangular frame. A beryllium-window rotating anode x-ray tube with 0.5 mm and 1.5 mm focal spots is used without filtration. Kilovoltage varies with the size of the animal, 15 kVp being used for newborn mice which weigh about 1.3 g, and 35 kVp being used for mice weighing 30 g or more. Target-to-film distances range from 12.5 to 15 in., and times from three-twentieth to one second. Living mice are radiographed at 200 mA with larger focal spot; after death 50 m A and the smaller focus are used. No-Screen x-ray film gives good results, but Kodak Type AA Industrial film is far superior for these studies. The air dose for typical exposures on AA film is 1 to 5 R, but we felt that in the present experiment exposures of this magnitude would not jeopardize the results. Ten control mice radiographed repeatedly under the same conditions as the virus-treated mice have not shown any ill effects to date. Radiologic Findings and Discussion

CALORIMETRIC MEASUREMENTS OF ABSORBED DOSE FOR LOW- AND MEDIUM-KILOVOLTAGE X-RAYS.

The use of the rad as a unit of dosage in roentgen therapy requires a knowledge of the energy absorbed per gram in the material being irradiated. This figure may be determined relatively easily in the case of cobalt-60 gamma radiation but is obtained with considerable difficulty in the conventional therapy range. There, the absorption coefficients change rapidly with photon energy; small changes in filtration with consequent changes in the beam spectrum may result in large variations in energy absorption. Direct measurements are therefore desirable. Calorimetric technics offer the possibility of directly measuring energy absorption by measuring the heat rise in certain tissue-equivalent materials. We have constructed a calorimeter suitable for making measurements in the low and medium kilovolt range. In general layout this instrument is similar to the calorimeter which we previously constructed for the measurement of x-rays of medium energy (1). Thermal isolation of the absorbers is achieved mainly by the use of an extremely low-density insulating material. Since no vacuum seals are involved, beam entrance windows may be made of material having a negligible attenuation for low-energy x-rays. Previously we reported energy absorption measurements on x-ray beams of h.v.l. 1 to 3 mm. Cu (1). Using the present calorimeter, we have extended this range in both directions and have also made measurements at a depth in tissue-equivalent wax. For absorbers we have used aluminum disks 1 or 2 mm. thick, to which thermistors were attached outside the beam area. The temperature rise in the absorber, which was of the order of 103 C. per minute, caused a resistance change in the thermistor which could be detected and related to the heat rise by methods which have been described previously (2). Ionization measurements were made by removing the lid from the innermost tank of the calorimeter and placing various thimble ionization chambers in a position corresponding to the absorbers. Energy absorption in the aluminum absorbers was then expressed in rads per roentgen, and plotted against the half-value layer of the various beams used, as shown in Figure 1. There are other ways of determining energy absorption in such materials. The chief method is either to calculate or to measure the x-ray spectral distribution and, from this and a knowledge of absorption coefficients, calculate the energy absorption. Most therapy tubes, however, have rather large focal spots, are operated at relatively high currents, and measurements can be made at no more than a few meters distance. These three factors make it difficult to obtain reproducible spectral measurements on conventional therapy equipment which is in clinical use. But several groups have used low-current tubes and long distances to obtain accurate spectral measurements.

Advantages of thre-phase spot-film radiography.

Most radiologists give considerable thought to the selection of a spot-film device. Their choice is usually based on mechanical convenience and mobility. There is evidence, however, that many other factors involved in spot-film radiography receive only slight consideration. This paper will deal with the possibilities of improving spot-film quality by utilizing 3-phase generators and controls. The usual fluoroscopic-radiographic unit in use in the United States is powered by a single-phase generator. Design of the control and a comparative lack of efficiency in the generators used in the single-phase systems present several handicaps in producing spot-films of highest quality and sharpness of detail. No choice of focal-spot size is permitted for making a spot-film. Fluoroscopy is done on the small focal spot, and spot-filming is permitted on the large focal spot only. In most cases the large focal spot is 2 mm. in diameter. Focal Spot To demonstrate the effect of focal-spot size on sharpness of detail, three radiographs of wire mesh laminated in plastic were made, one with a 2-mm. focal spot (Fig. 1, A), one with a 1-mm. focal spot (Fig. 1, B), and a third with a O.6-mm. focal spot (Fig. 1, C). Enlargement technic was used to simulate the increased part to film distances which are usually encountered in making spot-films. All other factors influencing detail were constant. A comparison of the three films demonstrates the marked blurring that results with large focal spots even though the object is stationary while being filmed. Versatility of Controls The controls used with most single-phase apparatus do not permit exploiting the fluoroscopic x-ray tube even to its capacity for single-phase operation. The rigidity of the milliamperage, time, and peak kilovolt relationships imposed by control design frequently preclude loading the tube at or near its maximum. Most 3-phase apparatus, on the other hand, permits the selection of any milliamperage for spot-film radiography within the range of the x-ray tube capacity for a given time and kilovolt setting. In contrast to the single-phase units, this flexibility is available for use with either the large or the small focal spot with the 3-phase unit; in other words, one has a wide choice of milliamperage stations and a choice of two focal spots in spot-filming as compared to a single milliamperage station and one focal spot with the customary single-phase units. “Efficiency” of X-ray Production and Tube Load Another important advantage of 3-phase equipment is the higher load that may be applied on the x-ray tube. In the spot-film timing ranges, the manufacturers' tube rating charts show up to 50 per cent greater loadability with 3-phase equipment as compared to single-phase.

Experiences with the upright fluoroscopic spot-film examination of the gallbladder.

The purpose of this brief paper is to describe further experiences (1) with the upright fluoroscopic spot-film examination of the gallbladder and to compare it with other procedures. Cholecystographic technics fall into four groups: 1. Examination with the patient horizontal and an overhead tube. Today such examination cannot be called acceptable. 2. Examination of the upright patient with an overhead tube, taking advantage of the effect of gravity with stratification. This represents a real advance, but with a number of shortcomings: Aiming is difficult; intestinal gas is frequently superimposed upon the gallbladder shadow; in certain body types the gallbladder is projected upon bony structures of the pelvis. 3. Examination of the patient in the right lateral decubitus with a horizontal ray, as described by Kirklin (2). This offers the gravitational advantages of the upright study, theoretically without the frequent superimposition of intestinal gas and bony structures. Without elaborate accessories, however, such as a table that can be rotated on its long axis, the gallbladder shadow actually is often obscured by gas and bone. 4. Spot-filming with a fine-focus (1.0 mm. square, or less) rotating anode tube beneath the table. This affords definite advantages over the previous technics. Radiography of the gallbladder with a spot-film device that does not have such a tube is, however, not to be advised. The stationary anode with larger focal spot will not allow the definition demanded for detecting lesions in the range of 1 mm. It is strongly urged that the gallbladder and gastrointestinal examinations be combined. We have several cases on file in which one of the two examinations was done repeatedly over the years; only when the other was finally done was a positive diagnosis obtained and the difficulty relieved. We are convinced that a clinical differential diagnosis between gallbladder disease and gastric ulcer is often impossible, especially in women. Our technic is as follows: A high-fat lunch the day before the examination aids in emptying the gallbladder of viscid contents which might interfere with the entrance of the fresh medium-containing bile. A preliminary scout film of the abdomen before taking the six tablets of Telepaque is certainly desirable, but the nature of our practice makes this difficult. After preliminary fluoroscopic examination of the thorax, the abdomen is scanned and the gallbladder shadow is then sought. Palpation with attendant movement of the viscera may help in demonstration of the less dense gallbladders. Spot-films are made with varying degrees of compression and rotation. Occasionally it may be necessary to cleanse the hepatic flexure with a 1.0 per cent tannic acid-water enema if a completely unobstructed view of the gallbladder cannot be obtained with any degree of rotation.

Advantages and limitations of the 0.3-mm. focal spot tube for magnification and other technics.

During the past several years a number of reports have attested to the value of the ultra-fine focal spot in diagnostic radiology. The technics are difficult, however, and still not in general use (1-5). In the course of four years experience with 0.3-mm. focal spot tubes we have had occasion to analyze the factors involved and adapt technics for everyday clinical use. Theoretical and Experimental Evaluation The size of the focal spot affects radiographic sharpness in so far as it directly affects penumbra size. Blurring due to this factor is determined by the relative sizes of the penumbra and the radiographic image. With decrease of the focal spot-object distance and/or increase of the object-film distance, both radiographic image and penumbra are magnified, but not in the same degree. The size of the penumbra is determined only by the relative distances involved and the focal spot size, and is independent of the size of object. With magnification, the penumbra size increases more rapidly than the image size. The practical limit to useful magnification is a little over three times, beyond which finer bony detail is lost. Figure 1 shows the relative sizes of images and penumbrae with 0.3-mm. and 1.0-mm. focal spots, with magnifications up to three times. It is evident in this example that relative penumbra sizes with the ultra-fine focal spot are smaller with magnification than with the larger focal spot without magnification. Magnification is obviously impractical with a 1.0-mm. focal spot or larger. It is also clear that, regardless of focal spot size, the relative penumbra size is minimum with the least possible degree of magnification. In our experience, however, the maximum radiographic detail is obtained with magnifications of about head of the radius picked up with magnification of 1 1/4 but not seen on multiple non-magnified films. The thickness of the upper portion of the fracture line suggests that its size without magnification was less than the defining power of the film. Of far more importance than the overall degree of magnification is the relative magnification that can be obtained. Figure 3 shows films of three strips of wire screening at levels 1, 2 and 3, at distances of 2 1/4, 4 1/2, and 6 3/4 inches, respectively, from the table top. The three exposures were made with focal spot-table top distances of 40, 20, and 20 inches, and with table top-film distances of 0, 20 and 30 inches, respectively. In the first film, where the strips overlap, it is almost impossible to differentiate the three levels. In the second film, the three levels can be distinguished, and in the last film the differential magnification or perspective of the three levels is readily appreciated. With the increased magnification, the screening at level 3 is magnified 34 per cent more than that at level 1.

Certain factors of aberration to be considered in clinical roentgenographic cephalometry

1. 1. The type of x-ray machine is an important factor in roentgenographic cephalometry because of the long Focal-Film distance involved. A more powerful x-ray machine than a dental x-ray machine is desirable. 2. 2. The milliamperage is a factor in radiation for milliamperage determines the density in a roentgenogram. The kilovoltage is another factor in radiation. The higher the kilovoltage, the greater the contrast of an image on a film. An x-ray machine should be calibrated so that the operator can control the milliamperage and kilovoltage. Such control is usually not incorporated in a dental x-ray machine. 3. 3. The focal spot: The smaller the focal spot, the finer the detail sharpness and definition of the image on the film; and, contrariwise, the larger the focal spot the less sharpness of detail. However, this factor can be greatly minimized by a long Focal-Film distance. A tube equipped with a small focal spot has less capacity for the amount of radiation delivered per unit of time exposure than a tube with a larger focal spot. This is an important factor where the Focal-Film distance is long as it is in roentgenographic cephalometry. Radiation varies directly with the square of the distance. 4. 4. The Focal-Film distance is a factor in detail sharpness and magnification distortion in roentgenograms. The longer the Focal-Film distance, the sharper the detail and the less magnification distortion. At least, a Focal-Film distance of 60 inches is desirable in roentgenographic cephalometry. 5. 5. The Object-Film distance is also a factor in detail sharpness and magnification distortion. The shortest possible Object-Film distance will greatly improve definition and minimize, to a marked degree, magnification distortion. 6. 6. Secondary radiation is the causative factor in obscuring sharpness of detail in a roentgenogram. The size and volume of an anatomical part radiographed determines the amount of secondary radiation produced. Limiting devices such as cylinders, cones, and diaphragms attached to the x-ray head concentrate the primary beam of radiation to a particular area of the head to be radiographed. This, in effect, decreases the undesirable aberrant factor of secondary radiation. 7. 7. Cassettes, with intensifying screens, are very necessary in roentgenographic cephalometry. Although they reduce the definition and detail sharpness of a roentgenogram, the gain in the extreme shortness of exposure time more than compensates for loss of definition. Intensifying screens of various speeds determine the factor of time exposure. 8. 8. The kind of equipment set up is another important factor in roentgenographic cephalometry. A setup where the x-ray head and cephalostat are fixed introduces less factors of variability and human error of adjustment than one where the height of the x-ray head and cephalostat are adjusted to the patient. The adjustment of the patient into the cephalostat should be so organized as to produce the least amount of discomfort to the patient. An uncomfortable position of the patient may introduce the undesirable factor of movement.

Further Experiences with Chaoul Therapy

In 1937 we presented our experiences with short-distance low-voltage roentgen therapy before the Fifth International Congress of Radiology (2). At that time we had treated but 80 patients with various neoplastic and inflammatory conditions. We have had the opportunity of treating many more lesions by this method since then. The purpose of the present communication is to record the results of further experiences with short-distance low-voltage irradiation. Physical Data Our experience has been limited to the use of the Chaoul unit for “contact” therapy which was installed in our department in March 1936. Briefly, the x-ray tube is activated by a constant potential generator using a modified Greinacher circuit. The tube itself is a modification of the old Lenard ray tube in which the electron beam passes down a grounded metal cylinder, striking at the end a target of gold-plated nickel. The rays emerge through the target, which, therefore, acts as part of the filtration. In addition the beam passes through the cooling water (2 mm.) and a thin metal foil window, which forms part of the water jacket. The total inherent filtration, as stated by the manufacturer, is approximately equivalent to 0.2 mm. of nickel. While the rated capacity of the tube is 60 kv., 4 rna., we have chosen to operate it at 50 kv., 4 rna. The original tube is still in use, the output being about the same now as it was when first measured. The half-value layer of the emerging beam is 2.4 mm. aluminum. The effective wavelength was found to be 0.38 Å. The focal spot measured approximately 1.6 em. in diameter (pinhole 0.4 mm. in diameter, 61 em. pinhole-film distance). Mayneord called attention to the radial variation of intensity in the emerging x-ray beam (1). We found that the intensity fell off 30 to 40 per cent toward the periphery of the field. While the inverse square law of variation in intensity with distance could not be assumed to hold with such a large focal spot at short distances, our investigations revealed that a close agreement did exist. The back-scatter of the rays emitted by the Chaoul unit varies directly with portal area, approaching a limiting value of about 20 per cent for the largest cone, measuring 4.6 cm. in diameter. The back-scatter for the smallest cone, 2 em. in diameter, is about 7 per cent. The difference in percentage back-scatter between target-skin distances of 3 em. and 5 em. was found to be negligible. The percentage depth dose varies from 65 to 24 per cent at I-em. depth as the target-skin distance decreases from 5 em. to 3 mm. The differences in depth dose with varying target-skin distances are summarized in Table 1. Not enough emphasis has been placed upon the protection of the operator during the treatment of the patient.

Focal Spot Projection and the Position of the X-ray Tube in Radiography

MODERN roentgenography is conspicuous for its inability to stabilize and standardize technical procedures. Reduced to essentials, this is due to failure to measure and control within reasonably close limits the various physical factors involved. The technical characters of an X-ray film may be described as having two essential bases: first, the geometric projection of the shadows onto the film, and secondly, the qualitative reproduction of these shadows in the emulsion. Satisfactory management of both of these factors is indispensable to high grade roentgenography. The qualitative reproduction of projected shadows is in the main a matter of securing contrast between densities. This, in turn, resolves itself into questions, first, of the control of penetration, and secondly, of the management of non-geometric radiations, i.e., the radiations scattered from the tissues, et cetera, and those from intensifying screens. Processing of films is also concerned with the maintaining of contrast. As to the geometric projection of shadows, this should be relatively simple to analyze and control, although to our knowledge such data have not been presented to the roentgenologist. Thus, the shadows projected from a given object bear simple relations to the size of the object, to the object-film and target-film distances, and to the size, shape, and position of the source of the radiations, i.e., the focal spot. It must be the aim of the roentgenologist to have these shadow projections reproduce as closely as possible the size and shape of the causative object, and with the greatest possible sharpness. To attain such ends the geometric characters of the shadows, in terms of the causative object, must be capable of simple determination and control. When these are attained, the satisfactory qualitative reproduction of the shadows is a further problem, but one which must not be confused with the first. The present study constitutes a first step in the mensuration of shadow-projections, and relates to the effects of the position of the focal spot of the X-ray tube with respect to the film. From the data so obtained, suggestions are offered for the placing of the tube in order to attain maximum fidelity of shadow-production. Nature of Focal Spot Projection It is, of course, generally recognized that a large focal spot produces under given conditions a less satisfactory degree of sharpness and radiability than does a smaller focal spot. It is less generally recognized, however, that a given focal spot, in the type of X-ray tube in current use, varies widely in its effective size over different parts of a film. It follows that those radiographic characters which are influenced by focal spot size, namely, sharpness of shadow and the radiability of fine objects, are also variable over different parts of a film. The variable effective size of a focal spot may be expressed as “variable focal spot projection.”

Possibilities of Deep X-Ray Therapy with the Coolidge Water-Cooled Tube and Heavier Transformers

THE purpose of this paper is not to describe any new form of therapy, but to present the apparatus and technic of a more intense application of X-rays than has hitherto been employed in treatment. Treatment of deep-seated lesions with X-radiations has progressed markedly within the past few years. The American type of closed, oil-encased transformer superseded the German open-core type; the gas tube was replaced by the hot cathode tube; machines thus came into use which could function continuously over many hours at a potential of two hundred thousand volts and an intensity of four or five milliamperes. The rays emitted were of a longer wave length and far less penetration than the gamma rays of radium, but possessed a beneficial therapeutic action in the field of malignancy and allied conditions, a form of ray therapy being thus developed by which treatment could be rapidly administered over a wide area, including the growth and its lymphatic connections. The search continued for more powerful transformers and for a type of vacuum tube which could carry higher amperage and higher voltage. A year ago Coolidge reported a water-cooled tube which with adequate transformers generates a far more powerful X-ray beam than has as yet been achieved in X-ray manufacture.2 We have been operating a Wappler machine (Fig. 1) functioning at two hundred thousand volts and four milliamperes for a couple of years, with complete satisfaction and uniformly gratifying results. When Coolidge perfected his invention, however, we hastened to install one of his water-cooled tubes for the sake of the heavier output promised. None of the stock transformers on the market appeared to have sufficient capacity to deliver a stable output at this high amperage and voltage; so we appealed to Dr. Waite of the Waite and Bartlett Manufacturing Company of Long Island City, N. Y. Dr. Waite constructed special heavy transformers which have proven most reliable. Working with this system, we have a complete water circulation under forty to fifty pounds pressure; the X-ray tube is kept at an even low temperature during its excitation by the flow of water through a seamless coil of copper pipe behind the tungsten target. A large automobile radiator of copper mesh with an eighteen-inch fan serves to maintain the low temperature of the water system. The X-ray tube is eight inches in diameter. There is a projecting molybdenum pin in the filament spiral which produces a more uniform heat gradient by producing a large focal spot. We have been able to operate this machine at two hundred and fifty thousand volts and fifty milliamperes, but for practical purposes of treatment we use two hundred and forty thousand volts and forty milliamperes. Advantages of the Water-Cooled Tube Comparative electroscopic measurements of this new device and of our old two hundred thousand volt machine prove the former apparatus to possess twenty times the power generated by the old machine.