What is radiation?
Radiation: is the transfer of energy in the form of particles or waves.
Energy: the ability to do work (Force·Distance)
X-rays are electromagnetic radiation. Electromagnetic radiation is a form of pure energy which is carried by waves of photons. Electromagnetic radiation is also known as light. Visible light, Radio and X-rays are all forms of electromagnetic radiation which vary in energy and thereby wavelength and frequency as well (Figure 1-1).
Figure 1-1: Forms of Electromagnetic Radiation
Courtesy of Phil Rauch and Laura Smith, 2000
Ionizing Radiation
X-ray radiation contains more energy than ultraviolet, infrared, radio waves, microwaves or visible light. X-ray radiation has sufficient energy (>30 eV) to cause ionizations. An ionization is a process whereby the radiation removes an outer shell electron from an atom (Figure 1-2).
Figure 1-2: The Ionization Process
Alan Jackson, 2001
Non-ionizing radiation does not contain sufficient energy (30 eV) to cause ionizations (Figure 1-3). While some non-ionizing radiation can be harmful, the ionization process is clearly able to cause chemical changes in important changes to biologically important molecules (e.g. DNA).
Figure 1-3: Non-Ionizing Radiations
Courtesy of Alan Jackson, 2001
X-ray Production
X-rays are produced when high velocity electrons are decelerated (slowed or stopped) or by a nucleus of an atom especially by high atomic number material, such as the tungsten target (anode) in a X-ray tube. An electrically heated filament (cathode) within the X-ray tube generates electrons that are accelerated from the filament to the tungsten target by the application of a high voltage to the tube. The energy gained by the electron is equal to the potential difference (voltage) between the anode and cathode. This electron energy is typically expressed in kilovolts (kV). The accelerated electron interacts with the target (anode) nucleus. As the electric field of the electron interacts with nucleus, the electron releases energy in the form of X-rays. This method of of x-ray production is called bremsstrahlung or braking radiation (Figure 1-4).
FIgure 1-4: X-ray Production (Bremstrahlung)
Courtesy of the University of Michigan Student Chapter of the Health Physics Society
Since the degree of interaction of the accelerated electron with the target nucleus can vary, the energy spectrum, or distribution of energy, of the X-rays produced by the bremsstrahlung process is continuous.
As smaller number of characteristic x-rays are also produced as excited electrons interact with the electrons of the target atoms. The X-rays produced from this interaction, with a given orbital electron, have a single specific energy (discrete) instead of a continuous spectrum. Mammographic x-ray tubes are designed to maximize characteristic production to optimize breast tissue imaging. The amount of characteristic X-rays in a fluoroscopy beam is relatively low.
The lower energy X-rays are absorbed within the X-ray tube. This reduces the number of lower energy X-rays in the resultant spectrum since the lower energy X-ray are less penetrating. The beam is considered "harder" when there is more filtration. Most X-ray manufacturers add filtration, commonly consisting of aluminum, since lower energy X-rays do not contribute to images and add to patient dose.
The resulting x-ray spectrum energy (Figure 1-5) is a mixture of the characteristic and bremsstrahlung radiation, less the primarily low energy X-rays absorbed by the X-ray tube (and added filtration). The maximum energy of the X-ray produced is equal to the maximum potential applied across the x-ray tube. This peak X-ray energy is usually described with the unit kVp (kilovolt peak or kilovolt potential). The type of target anode, potential (kVp) and added filtration produce a beam of a given "quality" which implies specific shape of an X-ray spectrum.
Figure 1-5: Simplified X-ray Spectrum
Alan Jackson, 2001
X-ray Machine Parameters
The quantity of electron flow (current) in the X-ray tube is described in units of milliamperes (mA). The rate of X-ray production is directly proportional to the X-ray tube current. Higher mA values indicate more electrons are striking the tungsten target, thereby producing more X-rays. The voltage (kVp) primarily determines the maximum X-ray energy produced but also influences the number of X-rays produced. Increasing the kVp attracts more electrons from the filament increasing the rate of X-ray production. However, this relationship is not directly proportional but higher kVp setting will result in a substantial increase in the number of X-rays produced. The total number of X-rays produced at a set kVp depends directly on the product of the mA and exposure time and is typically described in terms of mA-s or mAs. Fluoroscopy is usually performed using 2 to 5 mA current at a peak electrical potential of 75 to 125 kVp.
X-ray Production Efficiency and Heat Loading
The production of x-rays is a relatively inefficient process so that only a small fraction of the energy imparted by the decelerating electrons is converted into X-rays. The remaining energy is converted to heat. Thus, the production and dissipation of heat in the X-ray tube is a serious consideration. Thus, most x-ray machines have rotating anodes to spread out the heat to prevent anode melting. This is the reason why you can hear an X-ray machine make noise. Most fluoroscopic x-ray machine anodes are primarily based on tungsten due to tungsten's high melting point, excellent heat transmission, and high atomic number. In spite of tungsten's favorable qualities, with sufficiently high usage, X-ray production is prevented by the system to protect the tube. Substantial improvements have been recently made in the ability of fluoroscopic X-ray equipment to remove waste heat and thereby maintain high beam outputs.
Figure 1-6: Heat Production
Courtesy of: Phil Rauch and Laura Smith, 2000
Divergent Nature of X-ray Radiation
Once generated, the X-rays are emitted in all directions in a uniform manner (isotropically). The lead housing surrounding the X-ray tube limits X-ray emission through a small opening or port in the X-ray tube. The resulting primary beam of useful radiation is shaped by additional lead shutters, or collimators, that can be adjusted to provide different beam shapes or sizes.
Inverse-Square-Law (Radiation intensity with distance)
Since the initial beam travels in straight but divergent directions, geometry in a three dimensional world dictates that the radiation intensity will decrease with the inverse square of the distance. Consequently, the number of X-rays traveling through a unit area decreases with increasing distance. Likewise, radiation level decreases with increasing distance since exposure is directly proportional to the number of X-rays interacting in a unit area. The intensity of the radiation is described by the inverse square law equation:
Where XA is the radiation exposure rate at distance DA compared with the exposure rate (XB) at some other distance (DB).
This effect is shown graphically in Figure 1-7:
Figure 1-7: Inverse Square Law
Courtesy of Scott Sorenson, 2000
1-Meter Distance: 1,000 X-rays pass through a
unit area. The amount of X-rays per unit area is
1,000.
2-Meter Distance: With increasing distance, the
beam diverges to an area 4 times the original area.
The same 1,000 X-rays are evenly distributed
over the new area (4 times the original). Thus the
amount of X-rays per unit area is 250 or 1/4 the
original. The resulting radiation exposure is 1/4
less.
This relationship indicates that doubling the distance from a radiation source decreases the radiation level by a factor of four. Conversely, halving the distance, increases the radiation level by a factor of four. Intelligent application of inverse square law principles can yield significant reductions in both patient and operator radiation exposures.
Example 1:
An operator normally stands 1 meter away from the patient during cineangiography. The exposure rate at this point is 15 mrem/min (this unit will be explained later) and total cineangiography time is 2 min. What is the reduction should the operator stand 1.2 meters away?
Solution 1:
The original exposure was 30 mrem (15 mrem/min for 2 min). The new exposure would be:
A 31% percent reduction in radiation exposure is achieved in this example.
X-rays Interactions with matter
X-rays have several fates as they traverse tissue. These fates fall into 3 main categories (Figure 1-8):
Figure 1-8: X-ray Interaction-Imaging Considerations
Courtesy of Scott Sorenson, 2000
No interaction: X-ray passes completely throughtissue and into the image recording device. Producing an image
Complete absorption: X-ray energy is completely absorbed by the tissue. This produces radiation dose to the patient.
Partial absorption with scatter: Scattering involves a partial transfer of energy to tissue, with the resulting scattered X-ray having less energy and a different trajectory. This interaction does not provide any useful information (degrades image quality) and is the primary source of radiation exposure to staff.
X-ray Interaction with Matter
The probability of X-ray interaction is a function of tissue electron density, tissue thickness, and X-ray energy (kVp). Electron dense material like bone and contrast dye attenuates more X-rays from the X-ray beam than less dense material (muscle, fat, air). The differential rate of interaction provides the contrast that forms the image.
Tissue Electron Density Interaction Effects:
As electron density increases, the interaction with X-rays substantially increases. Higher atomic number materials have increased electron density. Thus, bone, which is substantially comprised of calcium, produces more attenuation, than tissue, which is comprised of carbon, hydrogen and oxygen (all of which have a lower electron density or atomic number than calcium). Thus, the image of bone and soft tissue has contrast, or difference, between bone and soft tissue.
The concept of contrast and electron density X-ray interaction can be shown in Figure 1-9.
Assume 1,000 X-rays strike the following body portions. The number of X-rays reaching the recording media (film, TV monitor) directly effect the image's brightness.
Figure 1-9: Electron Density and Image Contrast
Courtesy of Scott Sorenson, 2000
In this example, 900 X-rays are capable of penetrating the soft tissue, while only 400 penetrate the bone (Higher electron density compared with soft tissue). The contrast between the bone and soft tissue is (900-400)/900 = 0.56.
Tissue Thickness
As tissue thickness increases, the probability of X-ray interaction increases. Thicker body portions remove more X-rays from the useful beam compared to thinner portions (Figure 1-10). In fluoroscopy, this effect must be compensated for while panning across variable tissue thickness to provide consistent information to the image-recording device.
Figure 1-10: X-ray Penetration as a Function of Thickness
Courtesy of Scott Sorenson, 2000
Note that on average, only 1 percent of the X-rays reach the image-recording device (e.g., image intensifier, film), yielding useful information. Thus, 99 percent of the X-rays generated are either absorbed within the patient (patient radiation exposure) or are scattered throughout the examination room (staff radiation exposure).
Energy
Higher kVp X-rays are less likely to interact with tissue and are described as more "penetrating." Increasing kVp, thereby generating more penetrating radiation, reduces the relative image contrast (or visible difference) between dense and less dense tissue. Conversely, less radiation dose results to the patient since less X-rays are absorbed. Figure 1-11 illustrates this effect. The X-rays that do not reach the image recording device are either absorbed in the patient (patient radiation dose) or are scattered throughout the exam room (staff radiation dose)
Figure 1-11: X-ray Penetration as a Function of Energy
Courtesy of Scott Sorenson, 2000
Application of Radiation Physics
How does these principles of physics relate to radiation safety? (Figure 1-12)
Figure 1-12: Square Hole-Round Peg
Courtesy of Alan Jackson, 2001
The fundamental tie between Physics and fluoroscopy Radiation Safety occurs during the sequence of steps which lead to radiation biological effects. These steps are deposition of energy (Figure 1-13) and biochemical changes caused by X-rays (Figure 1-14).
Figure1-13: Deposition of Energy
Alan Jackson, 2001
Figure1-14: Biochemical changes
Alan Jackson, 2001
The biochemical changes produced by ionizing radiation radiations are the fundamental event leading to radiation damage. The amount of energy absorbed in a system is the best way to quantify the radiation damage. The amount of energy absorbed per mass is known as radiation dose.
Description of Radiation Exposure/Units
There is a myriad of terms describing amount of absorbed by a system. This is often confusing even to those quite familiar with radiation physics. Terms which the operator should be aware of include those are:
The X-ray machine output is described in terms of Entrance Skin Exposure (ESE) and is the amount of radiation delivered to the patient's skin at the point of entry of the X-ray beam into the patient. The unit for ESE are Roentgens (R) (or C/kg air in SI units). The unit Roentgen is defined in terms of charge (Coulombs) created by ionizing radiation per unit mass (kg) of air (1 R =2.58 * 10-4 C/kg air). The radiation exposure can also be measured at other locations and is the quantity indicated by many radiation detectors such as Geiger-Muller meters. This unit recognizes Wilhelm Conrad Roentgen (Röntgen), who invented (discovered) X-rays, and gave this technology to the world without personal profit (Figure 1-15). For this achievement, he received the Nobel prize in Physics.
Figure 1-15: Wilhelm Conrad Roentgen
Courtesy of Scott Sorenson, 2000
The unit Roentgen is only defined for air and can not be used to describe dose to tissue. Radiation dose is the energy (joules) imparted per unit mass of tissue and has the US units of rad (radiation absorbed dose). Patient exposures, particularly in Radiation Oncology are described in units of radiation dose. There is an international (SI) unit for dose termed the Gray (Gy). The conversion between the units is: 100 rad = 1 Gy.
The biological effectiveness of radiations vary. The unit rem (radiation equivalent man, now person) is used to compare dose received by different types of radiations (e.g. alpha particles) which have a different capacity for causing harm than X-ray radiation. This unit is properly termed dose equivalent. The dose equivalent is the product of the dose times a quality factor. Occupational radiation exposure is described in terms of dose equivalent. There is an international (SI) unit for dose equivalent termed the Sievert (Sv). The conversion between the units is: 100 rem = 1 Sv.
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