We have seen how the Brems and Characteristic interactions work when electrons are accelerated by kilovolts and the electrons impact on the target

Transcription

1 We have seen how the Brems and Characteristic interactions work when electrons are accelerated by kilovolts and the electrons impact on the target focal spot. This discussion will center over how x-ray photons interact with human tissue or any other kind of matter outside of the x-ray tube. There are numerous interactions that we will discuss. One of them comprises the image formation process which we discussed in an earlier presentation, and another interaction common in the diagnostic range of radiology actually are responsible for the occupational exposure of radiographers and other individuals that work with ionizing radiation. This same interaction that is responsible for the occupational exposure of radiographers, is also responsible for degrading the radiographic image. 1

2 As you are aware, the diagnostic range of kilovolts in radiography can be as low as the mid thirties for mammography and as high as one hundred-forty for chest radiography. Other anatomical areas depending on patient size will require different levels of kilovolts. Sometimes, these procedures are done with what is called automatic exposure control which is an exposure method where the x-ray machine automatically sets the milliampere seconds based on body habitus and anatomical area. In routine radiography, you will find that most of the imaging techniques are within the mid fifties to one hundred and forty. This is what we call the diagnostic range of kilovolts peak. It is important to realize that not all the interactions that are generated during an x-ray procedure occur within the diagnostic range and not all are biologically damaging. The interactions that are part of diagnostic radiology are Classical or Thompson s, and Rayleigh s. These three interactions are called unmodifying interactions because they do not cause any ionization and generally occur at very low levels of kilovolts. The photoelectric and Compton interactions are the primary modifying interactions that occur primarily in the mid to upper diagnostic range of kilovolts and are the ones that contribute most to the image formation process as well as the occupational exposure of the technologist. Pair Production and Photodisintegration occur in the Radiologic Sciences, but mostly in Radiation Therapy and Nuclear Medicine Imaging. We will discuss each in detail. 2

3 The attenuation and absorption of radiation interactions is one of the most critical processes in medical imaging. Attenuation is responsible for helping form the image as well as for contributing to the occupational exposure of the technologist. Attenuation by matter occurs at different rates and this all depends on the physical make up of the matter that is being irradiated. For example, bony tissue attenuates or absorbs matter much more efficiently than does soft tissue or gas filled structures.. Depending on the size and physical state of the bone, it can either attenuate the radiation significantly or if the bone is pathological, it can permit the radiation to penetrate it very easy thereby providing a distinct image that will tell the physician that something is wrong. Soft tissue does attenuate radiation but not very efficiently. Sometimes when there are calcifications within the soft tissue, they do tend to be seen relatively well and can provide the doctor with clues as to the diagnoses of the patient. Normally soft tissue is not seen well unless the kilovolts peak is reduced to a level that is compatible with the soft tissue that the radiation is being transmitted through. Another factor that will help visualize soft tissue is the administration of contrast media that is only done upon the doctor s orders. This substance is atomically very dense and attenuates xrays very well thereby making the internal soft tissue structures such as a stomach visible. Air or gas has the unique ability to actually make it easier for the radiation to penetrate tissue. A good example of this is a chest x-ray where the lungs are full of air. The lungs appear dark because the radiation penetrates very easy. Another area that also has air or gas is the gastro-intestinal system. We can see air or gas in the bowels very easy because it helps the x-rays penetrate easier and you can see the outlines of these structures. 3

4 To determine how exactly radiation is attenuated and how certain interactions occur, it is important to see how matter is structured. We refer to this as atomic density. On the left side of the slide, you see an atom that has relatively few electrons in the energy levels. In reality, the entire atom structure is a lot of open space at the atomic level and interactions in matter only occur when x-ray photons strike electrons and when there is a lot of open space, it will probably mean that the x-ray photon will pass through that matter not striking any electron. On the example on the right, you can see that this atom represents matter that has a very high atomic density. This is because there are many electrons in the energy levels and as an x-ray photon passes through this atom, chances are excellent it will collide with an electron and cause an interaction. When x-ray photons strike electrons, generally the atom will become ionized because when the collision occurs, very often, the electron will be kicked out or ejected. 4

5 Other factors besides the atomic density that contribute to the probability of interactions are three factors we will discuss individually. First is the distance of the source of radiation from the matter that is to be exposed. The greater the distance is, the less probability that the x-rays will strike the matter. The reason for this is that as the distance from the source of radiation increases, the radiation tends to flare out at an angle and the number of photons per square millimeter decreases as the distance from the source increases. The energy of the radiation is also a factor that is very important to understand. As the kilovolts peak is increased, the wavelength of the photon becomes shorter and the photon becomes smaller and more compact. As the photon becomes smaller, the probability of it striking an electron tends to decrease. The source of radiation is also critical. An example would be comparing two x ray exposures. If we compare an exposure of 10 milliampere seconds and another of 20 milliampere seconds and direct the exposure at a structure of some sort, it will be very likely that the larger exposure will have more collisions with electrons than the first exposure would. 5

6 As you can see in this slide, the number of x-rays at twenty inches is significantly greater than the number of x-rays at 40 inches. This is caused by the normal flaring of the radiation as it exits from the tube. If the technologist must change distance with a real patient, compensation will need to be done in order for the image to be diagnostic at the new distance. We will introduce you to the compensation equation in a later unit. 6

7 The energy of the photon also influences the probability of photon interactions. This slide which we have presented before is an excellent example on how the photon wavelength and frequency changes as the energy increases. You will notice that with the low energy photon in the diagram, the wavelength is much longer and the frequency is lower than the high energy x-ray photon. The probability that the low energy photon will interact with the electrons of an absorber is greatly enhanced because the wavelength is so long. The higher energy photon has a narrower profile and this makes it likely that it will not strike electrons in matter as easy as the low energy photon. 7

8 This diagram demonstrates radioactivity. A radioactive source is constantly disintegrating and emitting radiation photons from its nucleus. This represents only one atom, however in a real sample of radioactive substance, the amount of radioactivity emission can be very high and this will certainly cause a high interaction rate with any absorber in the general vicinity. In order to avoid being exposed excessively to radioactive substances, it is critical that the person be well shielded, keep the exposure short, and maintain a safe distance. The larger the distance, the less exposure that will reach the absorber. 8

9 Unmodifying interactions tend to occur at very low energies, usually well under 20 kilovolts. This is critical to understand because while the interactions can cause some general turbulence or stressing of atoms, they do not actually cause any ionization which is a good thing. This occurs at a very low energy range and if it does occur, about the only thing that may occur is that the film may accumulate a small amount of density that can be attributed to this interaction. The name of these interactions is Classical or Thompsons and Rayleigh s. 9

10 The first coherent or unmodifying interaction that we will discuss is Classical. If you study the diagram on this slide, you will see the incident x-ray entering the area of the atom on the left. This photon will interact with one of the electrons in the atom and the incident photon disappears. When this occurs, a secondary photon emerges from the atomic structure and you will notice that is exactly the same size as the incident. It has the same frequency and wavelength as the incident so for all pracitcal purposes, it is a form of scatter radiation. Because no ionization has occurred, this is referred to as an unmodifying interaction. 10

11 The next low energy interaction is Rayleigh s. This interaction is almost identical to classical in that the low energy x-ray photon enters the atom and this time, it causes all of the electrons to be disturbed and shaken up. The incident photon disappears and in its place, emerges a secondary photon that is exactly the same wavelength and frequency as the incident. It is also considered a form of scatter and because no ionization or damage occurs to the atom, it is referred to as an unmodifying interaction. Again, like classical interactions this one only contributes to a very slight density on the film as radiation fog. 11

12 A more critical interaction and one that contributes to the image formation process is the photoelectric interaction. This interaction tends to occur predominantly in the lower kilovolts range, however it does occur at higher voltages also. This is the interaction that is responsible for the latent image formation process. As you may recall when we discussed the Gurney Mott process, an incident photon entered the silver bromide crystal and caused ionization. This is the interaction that is responsible for that. It is more likely to occur with matter that has a higher atomic number such as bone or other hard tissue. The photons tend to pass through without interaction when the matter is of a low atomic density. Additionally, the x-ray photon has a unique characteristic in that it occurs in the inner energy levels of the atom that is struck. 12

13 The photoelectric interaction occurs as follows. In the diagram on the right, you can see the incident photon entering the area of the atom. It moves through the atom and then strikes an inner shell electron. That electron is ejected out and it is then called a photoelectron. When the atom senses that it has lost an electron through the collision with the x-ray photon, electrons from other electron energy levels above will each cascade down until the vacancy is filled. This cascading process is referred to as transition. Immediately after the collision, the incident photon disappears and as the electrons cascade, the atom releases a secondary photon which has the energy equivalent to the difference in binding energies of the energy levels involved. Because the electron binding energies are very low in the human body atoms, the energy of the secondary photon tends to be very weak. You will notice in the diagram that the secondary photon has a significantly larger wavelength than the incident photon. This basically means that the secondary photon does not have much energy and is very often absorbed by a few centimeters of air if it exits the patient at all. If a film happens to be in contact with the patient s anatomy, then the secondary photon may contribute to a small amount of radiation fog. It is important to note that because this is an ionizing interaction, it does contribute to radiation dosage that is accumulated by the patient or whoever is struck by this type of radiation. 13

14 The next major interaction we will discuss is the Compton Interaction. Very often during discussion of this type of radiation, some people think that secondary radiation and compton scatter are the same thing and nothing could be further from the truth. Compton interactions are most definitely modifying interactions and are responsible for radiation workers exposure to radiation as well as being responsible for degrading an image because of the fog or extra density that they can create. In the photo electric interaction, the absorber needed to have a high atomic number in order to occur, however in the case of compton, this interaction does not require a high atomic number at all. As a matter of fact, this interaction will tend to occur with very low atomic number matter as well as with high atomic number matter. 14

15 The basic process for this interaction is that the incident photon tends to strike an outer shell electron of an atom. If you look at the diagram on the right, you can see the high energy incident photon entering from the upper left. It strikes an outer shell electrons and the incident photon is deflected away at almost a ninety degree angle. You can see that as a result of this collision, the electron that was struck is now moving away from the atom at high velocity as well. This electron is now called either the compton or recoil electron. This is referred to as a partial transfer of energy because the incident photon lost some of its energy in the collision and gave it to the electron that was struck. You can see that the incident photon is now moving away from the atom and now has a longer wavelength. At this stage, it still has sufficient energy to undergo additional compton interactions. In this model, you can see that the second interaction has also resulted in the electron being ejected and the incident x-ray now has significantly longer wavelength. At this point, if the incident x ray exits the patient and strikes a technologist, then this is how it contributes to our occupational exposure. 15

16 It is important to note that the scattered radiation can have significant energy and that the incident x-ray can also have an angle of deflection anywhere from 0 degrees to one hundred and eightly degrees. This means that the radiation could be moving back directly towards the x-ray tube if it is deflected exactly one hundred and eighty degrees. When the scattered radiation does this, then it is referred to as backscatter. 16

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18 Compton interactions represent about ninety five percent of the occupational exposure. A well trained radiographer can significantly decrease this percentage by following some simple precautions we will discuss. One simple method of minimizing the production of scatter is to simply limit the beam of radiation to its smallest practical size. When the size of the beam is reduced you want to make sure you don t compromise or cut off any of the required anatomy. With a small beam, you reduce the volume and area of tissue that is irradiated and proportionately, the amount of scatter that is produced is significantly less. 18

19 The patient themselves contribute to the production of scatter. In general, when a patient is larger than average, they tend to generate more scatter for two reasons primarily. One, the technologist will need to use a larger technique or more milliampere seconds and kilovolts. This will of course increase the number of x-rays that will be scattered. And the other reason is the volume of tissue. The more tissue you must go through, the greater the amount of scatter that is going to be generated. Different areas of the body tend to generate more scatter such as going through thick parts of the body. Also, the radiographer must be aware that certain pathological conditions such as the accumulation of fluid will generally require that the radiographer increase the kilovolts and milliampere seconds and thereby increase the rate of scatter radiation. 19

20 In general, there are some simple rules that the radiographer can follow to reduce compton scatter to themselves, the patient, and the image receptor as well. Compton scatter tends to degrade the image because it reduces the definition or sharpness of structures. The image accumulates additional density that simply is not needed. Effective collimation, an adequate distance from the patient, shielding by standing behind a shield or a lead apron will minimize the exposure to the radiographer. Another thing that can also assist is minimize the time that you are exposed to radiation. You can also help minimize the exposure to the patient by simply using good collimation and positioning technique. This certainly helps reduce the repeat rate which can be a significant source of exposure to everyone. 20

21 On occasion the radiation worker may be in the proximity of extremely high energy interactions such as pair production. This interaction occurs when high energies are used in radiation therapy or nuclear medicine. It is important to note that these do not occur in diagnostic radiography because the energy of x-ray machines in a typical department is not capable of this level. 21

22 Pair production occurs when an incident photon with at least 1.02 million electron volts interacts with an atomic nucleus. The incident photon disappears and two particles emerge with.51 million electron volts each. One is called a positron and the other particle is a negatron. Notice that both particles move at ninety degrees away from the trajectory of the original photon. 22