X-ray Attenuation Handout PDF

Attenuation of the x-ray beam Cletus Amedu Objectives  Understand the concept of attenuation  Factors that affect attenuation  Importance of attenuation  Attenuation  Decrease in intensity of an X-ray beam as it passes through 1𝑚𝑚 of matter.  Occurs due to interaction between the...

Attenuation of the x-ray beam Cletus Amedu Objectives  Understand the concept of attenuation  Factors that affect attenuation  Importance of attenuation  Attenuation  Decrease in intensity of an X-ray beam as it passes through 1𝑚𝑚 of matter.  Occurs due to interaction between the x-ray photons and the atomic structures that comprise the tissues  *Penetration = Transmission Sprawles, 2016 WHAT? Decrease in intensity of an X-ray beam. WHY? It occurs due to interactions between the X-ray photons and the atomic structures that comprise the tissues. HOW? A decrease is caused by absorption(photoelectric effect) and scattering(Compton scatter).  Attenuation Interactions When X-ray photons pass through an area of a patient, interactions can occur between the photons and the atoms of the medium. A The photon interacts with the atom and is deflected, which may or may be accompanied with a loss of energy. This known as scattering. B The photon interacts with the atom and loses all of its energy to the atom. This is known as absorption. C The photon passes through the material without interacting with any of the atoms. Attenuation = scattering and absorption. 1. Following interaction, there is a lower intensity of radiation after passing through the medium. 2. These types of interaction involve a photon and an orbital electron rather than the photon and the nucleus. 3. Attenuation makes the beam weaker and involves both absorption and/or scattering.  Attenuation Experiment 𝑰𝟎 𝑰𝒕 A = Scattering B = Absorption Original Intensity (𝑰𝟎 ) Vs Transmitted intensity (𝑰𝒕 ) C= Transmission Attenuation  The attenuated x-ray beam that passes through the patient is also called remnant radiation Quizlet, 2020 Factors that affect attenuation  Thickness of the examined region/material  Atomic number (Z) – Tissues with higher Z cause more absorption  Tissue Density – compactness of the atomic particles  X-ray beam Quality/Energy- In diagnostic radiography and radiotherapy, the beam energy can be adjusted. Similar to the impact of speed/K.E. in head on collisions Atomic number, relative density, thickness of the tissue  The radiopacity of various objects and tissues results in radiographs showing different radiopacities, and hence they can be differentiated. Radiopaque tissues/objects result in a whiter image; less radiopaque objects result in a blacker image.  The radiopacity depends on:  1. Atomic number  The higher the atomic number, the more radiopaque the tissue/object:  2. Physical opacity  Air, fluid and soft tissue have approximately the same atomic number, but the specific gravity of air is only 0.001, whereas that of fluid and soft tissue is 1  Therefore, air will appear black on a radiograph, compared with fluid and soft tissue, which appear more grey  3. Thickness  The thicker the tissue/object, the greater the attenuation of X-Rays and the more-white the image will be  When two tissues/objects are superimposed, the composite shadow formed by these will appear more-opaque than either of the two separate tissues/objects (e.g. the area where the two kidneys overlap appears more radiopaque than either kidney itself)  Specific gravity usually means relative density with respect to water Atomic number, relative density, thickness of the tissue Atomic number, relative density, thickness of the tissue Linear attenuation coefficients  Grey scale image 1. X-rays are part of the electromagnetic spectrum, emitted as a result of bombardment of a tungsten anode by free electrons from a cathode. 2. Plain films/Radiographs are produced by a passage through the patient and exposing a radiographic film. 3. Bone absorbs most radiation, causing least film exposure, and the result is white. 4. Air absorbs least radiation; therefore film appears black. Differential tissue absorption results in a grey scale image. 5. Remember it is ionising radiation Factors that affect attenuation  Attenuation of monochromatic radiation  Suppose we have a “narrow beam” of radiation incident on a slab of material of thickness ‘x’. Let us say that the material lets 80% of the radiation through (is transmitted) and 20% of it interacts in some way.  If we place another slab of the same thickness x in the transmitted path a further 20% will be attenuated.  For each additional thickness of x we get the same fractional reduction in intensity. This is known as an exponential change.  So the relationship between incident and transmitted intensity is exponential.  If we plot this function we get a graph showing exponential curve  *Significance of the exponential curve? To explain the relationship between the incident and the transmitted intensity  To expand our knowledge of radiation protection  * Intensity is Kinetic energy of x-ray per unit mass of a tissue Attenuation of monochromatic radiation Relationship between the incident and transmitted beam Total linear attenuation coefficient  Consider the following experiment. We have narrow beam geometry with a source of x-ray of known intensity incident on a slab of material of thickness x. The intensity of the x-rays that pass through the slab (transmitted intensity) without interacting at all can be measured with a suitable detector. The quantities governing this experiment can be expressed as shown below.  This is the fraction of X-rays removed from a parallel beam of monochromatic radiation per unit thickness of a homogeneous attenuator. This value is different at different beam energies. The number of interactions per unit distance through a medium depends on: 1. The area and material presented to the incoming X-ray photons i.e. high atomic number materials have high number of electrons. 2. The spacing between the atoms in the medium. A dense medium provides a higher probability of attenuation then a less dense medium, even if the atomic number is the same.  The linear attenuation coefficient is characteristic for a particular material and also will vary depending on the energy of the incident radiation. Total linear attenuation coefficient Total linear attenuation coefficient Definition  Total linear attenuation coefficient (µ) is the fraction of x-rays removed from the beam per unit thickness of the particular attenuating material  Exponential curve The linear attenuation coefficient is characteristic for a particular material and also will vary depending on the energy of the incident radiation. Lesser thickness of the different atoms transmits higher intensity  Linear attenuation coefficient  Examples  Let the incident intensity I0 be 100 units (grays), the thickness of the material be 2.0 mm or 0.2 cm and the energy of the photons be 60 kV. We can use the attenuation formula to find the transmitted intensity through different materials  Aluminium:   Approx 15% of incident radiation is stopped by 2 mm Aluminium at 60 kV  LEAD The K-absorption edge for lead occurs at 88kev so that the photons whose energies lie between 60 and 88 kev are much less attenuated than those between 88 and 110 kev with a net result that more soft radiation than is desirable is transmitted and the high energy photons are excessively removed Linear attenuation coefficient µ will vary depending on the energy of the incident radiation  Half value thickness (H½) This is that thickness of a substance which will transmit exactly one-half of the intensity of radiation incident upon it In diagnostic radiography we use aluminium to measure the HVTs whereas in therapy we use copper. Summary  Attenuation is the reduction of the intensity of x-ray photons  The combination of attenuation and transmission creates an x-ray image which structurally represents body parts  µ increases with atomic number at very low energies  For water (similar to soft tissue), HVT in the diagnostic range is about 30 mm  Also, at diagnostic energies the HVT for lead is 0.1 mm or less so quite a thin layer of lead provides effective shielding for the door of an x-ray room Attenuation Experiment 𝑰𝟏 𝑰𝟐 A = Scattering B = Absorption C= Transmission School of Health & Psychological Sciences City, University of London Northampton Square London EC1V 0HB United Kingdom T: +44 (0)20 7040 5060 E: [email protected] www.city.ac.uk/department