THE PRODUCTION & MEDICAL USE OF X-RAY

 


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Lesson #2

Learning Objective - To develop greater understanding of the technical aspects of producing and using x-rays for medical imaging.

 

Background

The technical method of producing "x" photons has changed relatively little during the century since the discovery and that process remains the basis for the majority of medical images. Physicians who are not radiologists frequently ask, "why do I need to know this technical stuff"? Remember, the majority of medical radiographs are produced in physician's offices. At least one-third of the primary care offices in South Carolina have their own x-ray machine. The South Carolina Department of Health and Environmental Control (Bureau of Radiological Health) will allow any licensed physician to use an x-ray machine. It is not possible to safely and effectively produce x-ray images without understanding the technical aspects of the equipment. Remember it is the physician (not the technologist) who is responsible for the safe and effective use of the x-ray machine.

In this discussion, emphasis will be placed on the methods of controlling the output of x-rays and special techniques for more effective medical imaging.  The diagram below will remind you of where "x-rays" fit in the radiant energy spectrum.

 

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The Production System

The apparatus for "x" photon production is the x-ray tube. This is a fairly typical electronic "vacuum" tube. One electrode is connected to the positive terminal of the power supply and is called the anode. The other electrode is connected to the negative pole of the power supply and is called the cathode. In clinical terminology, the anode is frequently referred to as the target, while the cathode is sometimes called the filament. You can see the arrangement of the tube components in slide #1.

 

SLIDE #1

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There are actually two (2) electrical circuits controlling the performance of the x-ray tube. The high voltage circuit is connected to the anode and cathode while the filament heating (or milliampere) circuit is connected only across the filament at the cathode end of the tube. When this circuit is energized, the filament is heated just like any incandescent filament such as those commonly used in light bulbs. The heating of the filament causes electrons to move away from the surface of the filament into a closely surrounding "space charge". When the high voltage circuit is energized, the anode is made strongly positive relative to the cathode (in the range of 100 keV) and that causes the electrons in the "space charge" to be accelerated to the anode or target.

The interaction of the accelerated electrons with the tungsten of the target is diagramed in slides #2 and #3. Two (2) possibilities for interaction are common. In the first, the accelerated electron interacts directly with an electron in an orbital shell of a tungsten atom. The orbital electron is displaced but the orbital gap is rapidly filled by an electron from a more distant orbit. The difference in the energies of the two (2) electron orbits is radiated as an x-photon the energy of which is characteristic for the specific element and the specific orbital shell (slide #2). The other and more frequent interaction is termed Bremsstrahlung. This German word means "braking" radiation and that describes what happens to the incoming electron. In this interaction, the accelerated electron passes relatively close to the nucleus of a tungsten atom in the anode. The path of the accelerated electron is affected by the nucleus with a resulting change in direction and the kinetic energy of the electron is dissipated. The difference in the kinetic energy before and after interacting with the nucleus is radiated as an "x" photon. This type of interaction (Bremsstrahlung) can result in x-photons of almost any energy, limited only by the tube potential (slide #3). A typical radiation spectrum from a medical x-ray tube is illustrated in slide #4. As you can see from the spectrum, the most frequent photon energy will be approximately one third (1/3) of the tube potential voltage. There will be superimposed "characteristic" photon spikes which correspond in energy to differences of the specific electron orbits.

 

                                                SLIDE #2

                              SLIDE #3

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SLIDE #4

 

The energy (you can consider this the penetrating power) of the x-ray beam is controlled by the voltage adjustment. This control usually is labeled in keV (thousand electron volts) and sometimes the level is referred to as kVp (kilovoltage potential). Do not be confused by the different terminology, just remember there is a control by which the difference in potential between the cathode and anode can be controlled. The higher the voltage setting, the more energetic will be the beam of x-ray. A more penetrating beam will result in a lower contrast radiograph than one made with an x-ray beam having less penetrating power. It is probably obvious that the more energetic the beam, the less effect different levels of tissue density will have in attenuating that beam.

The second control of the output of the x-ray tube is called the mA (milliamperage) control. This control determines how much current is allowed to flow through the filament which is the cathode side of the tube. If more current (and therefore more heating) is allowed to pass through the filament, more electrons will be available in the "space charge" for acceleration to the target and this will result in a greater flux of photons when the high voltage circuit is energized. The effect of the mA circuit is quite linear. If you want to double the number of "x" photons produced by the tube, you can do that by simply doubling the mA. Changing the number of photons produced will affect the blackness of the film but will not affect the film contrast.

The third control of the x-ray tube which is used for medical imaging is the exposure timer. This is usually denoted as an "S" (exposure time in seconds) and is combined with the mA control. The combined function is usually referred to as mAs or milliampere seconds so, if you wanted to give an exposure using 10 milliampere seconds you could use a 10 mA current with a 1.0 second exposure or a 20 mA current for a 0.5 second exposure or any combination of the two which would result in the number 10. Both of these factors and their combination affect the film in a linear way. That is, if you want to double film blackness you could just double the mAs.

 

Automatic Exposure Control

The description of the use of the controls in the paragraphs above is correct for manually controlling the production of the x-ray beam. Frequently in modern x-ray equipment, the exposure is controlled by an arrangement usually described as "phototiming". With this system, the machine automatically measures the exposure in the sense of how many photons  reach the film. With this approach, all the operator has to do is to set the desired degree of film blackness and the machine will monitor the exposure so that the resulting density of the film is very close to what is wanted. That system would work very well if the human body were equally thick in all parts. Unfortunately, the variation in body thickness makes phototiming sometimes give inappropriate exposure results. In routine clinical practice, phototiming might add some convenience but you should always be able to control radiographic exposures using the manual system.

 

Rotating Anode

The modern medical x-ray tube is a little more complex than the diagram you have already studied. In the x-ray tube, the majority of the electrical energy input to the tube is not changed into x-ray but is converted to heat. Actually, approximately 99% of the electrical energy going into the tube is changed into heat while only 1% is radiated as "x" photons. This heat buildup will rapidly damage the target unless some provision is made for its dissipation. The most common solution to the problem in medical x-ray tubes is to mount the anode target on the armature of an electric motor so that the target becomes a spinning disc which has the capacity to absorb the heat over a large area even though the actual focal spot is quite small. Although the details are a little too complex to discuss in this lecture, the size of the focal spot does affect the sharpness of the radiograph. As the focal spot is enlarged, the radiograph becomes less sharp.

 

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Rotating Anode Tube

 

Image Contrast

Here, we need to spend a little more time discussing the issue of radiographic contrast. This is an important concept because image contrast plays a critical part in the interpreter's ability to detect abnormalities which are only slightly different from the density of the surrounding material. It is not possible to say what is the optimal contrast (or the optimal radiographic technique) for all situations. Different body parts have different inherent tissue contrast. This can be illustrated by using the extreme examples of the chest and the breast. In the chest, there is good inherent tissue contrast with densities ranging all the way from bone at the high end to air at the low end. On the other hand, the breast is inherently very low in tissue contrast only containing structures which are water density (glandular material or tumor) or fat density. For the moment, we will disregard small calcifications which are really not normal structures. Because of this difference in inherent tissue contrast, we would be likely to use a very low contrast radiographic technique for the chest because we have good tissue contrast. Conversely we would be likely to use a very high contrast technique for the breast because the breast has minimal, inherent tissue contrast.

Remember, image contrast is controlled by the energy of the "x" photon beam. Therefore, high kV techniques result in low contrast images (the assumption is always made that the image will have approximately the same average film density so if kV is increased, there must be a compensation in mAs to keep film density constant). To increase image contrast in situations where there is low tissue contrast, a low kV, high mAs technique should be used. This is obvious for mammography but you should also remember this possibility for other special situations such as looking for low-density foreign bodies embedded in soft tissue. To improve film contrast for mammograms we would need to use a very low energy x-ray beam. Mammograms are frequently done with beams in the 25 keV range. For the chest x-ray, we would like to use a low contrast technique which requires a relatively high-energy beam. Chest x-rays are frequently done with beam energies above 100 keV. You should understand that for similar film densities, the high keV technique usually results in lower patient radiation exposure. Think about this long enough to clearly understand why less radiation is absorbed in the patient when a high-energy beam is used.

Recording System

The basic system for recording radiographic images is usually described as a film-screen system. Although that has historically been the dominant system, the trend seems to indicate that the future will be electronic recording in a digital format. If you understand the film-screen system, it will be easy to convert to the digital system when that becomes commonly available. In the non-digital (film-screen) system, the image is recorded on radiographic film which is composed of a mylar sheet on which there is a silver halide emulsion coating on each side. This is exposed in a light-tight cassette which has a radiolucent (plastic) surface on the entrance side. Within the cassette, there are two (2) intensifying screens, one adjacent to each emulsion surface of the film. These intensifying screens convert the wavelength of the "x" photon into photons near the blue end of the visible light spectrum. This greatly improves the efficiency of film exposure allowing a large reduction in radiation exposure to the patient.

The radiation exposure benefits of using intensifying screens cannot be achieved without some detrimental effect on the image. The use of any intensifying screen (no matter what speed) always results in a reduction of image sharpness. The speed of the intensifying screen is related to the size of the individual crystals within the screen. The faster the intensifying screen (more light produced) the larger will be the fluorescent crystals. Since any single crystal must either flash or not flash, if the crystal does fluoresce, the entire area of the crystal is exposed on the film. This exposed area will always be larger than the area that would have been exposed by the "x" photon which excited the crystal. Of course, the larger the crystal, the poorer the resulting image detail.

 

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Grids

One of the problems in getting a sharply defined image in clinical radiology is the presence of scattered or secondary radiation. These photons are created in the body of the patient or closely surrounding objects by the interaction of that material and the primary "x" photons coming from the x-ray tube. Several possible interactions occur in the diagnostic energy range. At relatively low energies, the photoelectric effect is probable. The photoelectric effect is actually the desirable, photon/tissue interaction because there is complete absorption of the photon with no production of a secondary photon. The more common tissue interaction at the photon energies used for the majority of clinical procedures is called the Compton effect or coherent scattering. In this interaction, a secondary photon is produced at the site of interaction. The secondary photon will always have lower energy than the primary photon and will be going in an altered direction. These secondary photons, if allowed to reach the film, will actually produce erroneous information by recording gray tone variation (and therefore indicating relative tissue densities) at some distance from the site at which the photon/tissue interaction actually occurred. The net result of allowing a significant number of secondary photons to reach the film is a reduction in image sharpness. There will always be a loss of spatial resolution.

Several methods have been devised to reduce the problem of scattered radiation. The simplest and most direct is to simply limit the field of exposure. If a small image area is adequate to make the clinical diagnosis, the image area should be "coned down" to that small size. For instance, if you want to image the gallbladder, you will get a much sharper picture if you bring the shutters down to include an area only the size of the gallbladder instead of including the entire upper abdomen on the image.  Just remember that the smaller the area of the x-ray beam the fewer scattered photons you will produce.

In the typical clinical imaging situation, the most common method of reducing scatter is to use a radiographic grid. The grid looks like a flat metallic plate the size of the x-ray film if you look at it directly. However, it is more complicated than that. It actually is composed of alternating radiopaque (lead) and radiolucent (aluminum) strips. These are arranged on edge, sort of like looking at the strips of a venetian blind which is arranged to let light come between the strips. The edge of these strips is turned towards the source of x-rays and in the most commonly used grid, the focused grid, the anglulation of the strips is arranged to match the divergence of the x-ray beam.

This arrangement of the radiographic grid will give the highest probability for primary "x" photons passing between the lead grid strips and reaching the film, while the off-focus or secondary photons are likely to interact in the lead strips and never reach the film.

The use of this radiographic grid will greatly improve image sharpness when a relatively thick body part is being imaged. Unfortunately, there is always a trade off. Since the grid does stop some of the photons which would contribute to film blackening, if you just add a radiographic grid without changing the tube settings, the film will be greatly underexposed. If you decide to use a grid, you will have to increase the number of photons produced by the x-ray tube in order to get the correct film exposure. This will result in giving the patient increased radiation exposure. Remember, the position of the grid is between the patient and the film.

 

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The third method of reducing scatter or at least reducing the probability that scattered photons will reach the film is to use an air gap. This is infrequently used in clinical radiography but can still, sometimes be used to an advantage particularly when magnification of the image might be helpful. Ordinarily we would have the film positioned as close to the patient's body as possible for the radiography of any body part. With an air gap technique, the film is moved several inches away from the patient's body. That separation, (because secondary photons are likely to be lower energy and moving at a greater angle than primary photons) will result in a decreased probability of the secondary photon hitting the film. From the diagram below, you will be able to understand that creating the air gap will also result in magnifying the radiographic image. Remember the x-ray beam is produced from almost a point source and it diverges as it goes towards the patient.

 

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Tomography

Tomography is frequently used to simplify clinical radiographic images. It is very common these days to use computed tomography which is a somewhat more complicated and expensive approach than the type of tomography we are talking about here which is frequently described as conventional or geometric tomography. If the body part being imaged is relatively thick and overlapping structures create serious image complexity, you may find it helpful to simplify the image by reducing the recorded structures to a single plane (the thickness of this plane can be defined by the imager). For instance, if you want to determine if there is a minute speck of calcification in a pulmonary nodule (calcification would be highly suggestive of a benign nodule) you can arrange the system so that all of the information on the chest film will be blurred and not visible on the image except for the plane in which the nodule is located.

To create this body section imaging, the x-ray tube and the film cassette are connected to a rigid structure which causes them to move in opposite directions during the period of exposure. The apparent axis of rotation becomes the plane at which there is no blurring of the details of the recorded object. The structures closer to the tube or further away from the tube than the rotational axis will be blurred and the degree of blurring will be proportional to the distance away from the rotational plane. Although only some of the structures remain in focus, the radiation exposure from each tomographic image will be approximately the same as that produced from an image that records all structures throughout the full thickness of the body part.

 

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Fluoroscopy

Fluoroscopic imaging is useful when it is necessary to radiograph a dynamic situation. Fluoroscopy is most commonly used to evaluate the gastrointestinal tract but can also be used to record the motion of ureters, diaphragms, or any other body part in which the component of motion might be helpful in arriving at a diagnostic decision. A fluoroscope is a radiographic machine which has an x-ray tube mounted in a way that the beam can pass through the patient and be recorded on a fluorescent screen. In modern fluoroscopes, the observer does not look directly at the fluorescent screen but looks at a video image produced from a video camera which is focused on the screen. These machines also incorporate a spot film device which will allow the operator to move a film into the beam and take "snap shot" pictures of any abnormality which is observed. This equipment is usually attached to an x-ray table which allows the operator to tilt the patient in various directions and the x-ray tube is most commonly positioned under the table top with the spot film device and the fluorescent screen including an image intensifier being above the patient if the patient is lying supine on the table.

 

Issues

1.  Which commonly used radiographic images are currently available in digital format?

2.  What are the specific advantages of digital images?

3.  How can you be sure that you are producing the highest quality image with the smallest amount of radiation exposure to the patient?

4.  Are there clinical situations in which intensifying screens should not be used?

5.  Are there clinical situations in which grids should not be used?

6.  Relative to conventional radiography, how dangerous is fluoroscopy to the patient? To the observer?

7.  What is the standard method for measuring image quality?

 

Reference Citations

1. Zink FE. X-ray Tubes. RadioGraphics 1997;17:1259-1268. (optional)

 

Related URLs

1.  http://www.mcw.edu/medphys/learning.htm

 


Send us comments: Dr. David Adcock, DAVID@uscmed.SC.EDU.

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