Radiographic Imaging, Ch, 7,8,9
5 interactions with Matter (Human tissue)
(Only 2 are in the diagnostic range)
Photoelectric (PE) Absorption (Primary Beam)
Compton (Scatter) (Secondary Beam)
It is helpful for the radiographer to understand the way x ray photons interact with matter:
- 1. It allows the radiographer to reduce the physical effects of x-ray photons on the patient that result in radiation dose and biologic harm.
- 2. allows the radiographer to better manipulate how the particular anatomic (body part) or area of interest appears radio-graphically.
An Interaction called Compton or scattering
Occurs when a photon with low energy collides with an outer shell electron, removing it, then goes off in a different direction from its original path. It will either interact with the IR (image Receptor) or away from it. As it travels it produces moderate to low energy and increased longer wavelengths. Compton is not a useful x-ray due to its scattering effect.
- Less with Low kVp (Undesirable) 20-40 kVp (Gray)
- More patient dose than x-ray quality
- Compton Electron or secondary electron
- Middle shell involved causes 'characteristic cascade'
- May occur in soft tissue and bone in larger, thicker forms
- will negatively impact the image with fogging (Blur) and cause unnecessary radiation to the tech and patient
- Provides no useful information
- Fewer photon interactions occur at a higher KV, but of those a greater % are compton (Scatter)
An interaction called Photoelectric or Absorption (PE)
The photo-electron is an ejected photon that has enough energy to pass through and force an inner shell electron from its orbit thus causing complete and total absorption of the x ray or incoming photon. This high energy interaction is called the Photoelectric effect.
- More with low kVp (desirable) (White)
- Less patient dose and better quality x-ray
- fewer interactions occur at a higher kVp but of those, a smaller percentage are photoelectric (PE) absorption.
- Necessary for an image
- interacts with an inner shell and is completely absorbed.
- probability depends on the energy of the photon and the Atomic # of the tissue. i.e. bone
If Kv goes up, less interaction will occur, Compton (Scatter) goes down but PE will go down faster.
Lower kVp produces whiter images and higher kVp produces gray images
Transmission Interaction (Black)
- High kVp, flies through or passes through matter with no interaction at all.
- At a lower kVp, there are less transmitted photons;
- At a higher Kv there are less interactions and more transmissions. High KV makes transmissions go up and produces a useless interaction exposing the tech and patient to more radiation and produce no usable/visible images.
Differential absorption (Absorption Vs penetration of x ray beam)
- The difference between absorbed (photo-electronically) photons vs photons that penetrate the body.
- Body structures i.e bone or barium that readily absorb x-rays are called radiopaque. (Cannot be penetrated)
- Less dense structures have a much lower probability of absorption and are called radiolucent. i.e. Air (Can be penetrated)
- All body parts absorb different amounts of of x rays, high atomic numbers in tissue show up white in the form e.g. bone or tissue. Air produces a low atomic number.
Beam Attenuation (thinning or weakening)
- As the beam passes through matter it decreases in Quantity and Quality.
3 Processes that occur during attenuation (weakening or thinning)
- Absorption (Photoelectric effect primary beam) White. Must have enough energy to eject inner shell electrons and be absorbed. Depends on tissue characteristics, (thickness or thinness.)
- Scattering (Compton/secondary photon) Gray, depends on energy of incoming photon, not tissue composition and does not exit patient. High Kv causes scattering, fewer interactions occur at a higher kv but of those a smaller % are PE vs compton scatter or vice versa . Photon may be absorbed by the patient, leave the targeted structure and interact with the Image Receptor (IR) and expose personnel in the room.
- Photo Transmission (no interaction at all) Black
Factors affecting beam attenuation (weakness or thinning)
- Increasing thickness (heavier person) increases attenuation and absorption with decreased transmission.
- Decreasing thickness (Thinner person) decreases attenuation and absorption with increased transmission.
- The thicker the body part the more x rays needed ( more quantity, less quality)
- The thinner the part the less x rays needed (less quantity, more quality)
- Beam attenuation reduces quality and/or quantity as it passes through matter.
Factors affecting beam attenuation (weakness or thinning) Part 2
Types of Tissue (Atomic number) (Bone, tissue or Air)
- An Increasing atomic number increases attenuation and absorption and decreases transmission.
- Decreasing atomic number decreases attenuation and absorption and increases transmission.
- Bone attenuates the most due to its high atomic number
- Bone will show a lighter area in the film (White)
- Air attenuates a low atomic number and shows a darker area on the film (Black)
Factors affecting beam attenuation (weakness or thinning) Part 3
Tissue Density (crowded or compact)
- Increasing tissue density increases attenuation and absorption and decreases transmission.
- Decreasing tissue density decreases attenuation and absorption and increases transmission.
- Tissues with a higher atom compaction will absorb more than tissues than with a loose atomic compaction.
- Example: Bone has atomic particles more compact or dense than that of fat. Beam will not go through to other side of bone and it absorbs more attenuation.
- Fat absorbs less attenuation.
Factors affecting beam attenuation (weakness or thinning) Part 4
X ray Beam Quality
- Increasing beam quality decreases attenuation and absorption and increases transmission.
- Decreasing beam quality increase attenuation and absorption and decreases transmission.
- Higher penetrating x-rays (High KV) are more likely to penetrate through tissue without interactions. (transmission)
- Lower penetrating x rays (Low KV) are more likely to interact and either be absorbed or scattered. (Compton scatter or PE photoelectric)
- Exit radiation consists of compton or transmission interactions, black and gray areas.
- Unwanted scatter hitting the Image receptor is called fog.
- Areas that absorb photons (PE) create white or clear areas of increased brightness.
- Areas that are not affected by x-rays are called transmission interactions and they create black images with decreased brightness.
- Less than 5% of x-rays entering the patient or atamic part actually reach the IR and an even lower percentage is used to create the radiograhic image.
- Primary beam is the x rays between the tube and the patient
- Exit (transmission) or remnant (Scatter) radiation is located between the patient and the IR (image receptor) and creates the latent (invisible) image. This latent image is not visible until processed to form the manifest (visible) image.
- 2 types of IR are digital and film screen.
Image Quality Factors ( all need to be good to produce a a good x ray)
- Brightness: (IR Exposure, photographic visibility) Increasing the amount of luminance or light emission of a display monitor. Dark images have excessive density or insufficient brightness.
- Contrast resolution: (photographic visibility) Another term associate with digital imaging and is used to describe the ability of the imaging system to determine between small objects that attenuate the x ray beam with similar densities or shades of grey. Digital imaging is far more advanced than film screen imaging. Subject contrast refers to the absorption characteristics of the anatomic tissue and the quality of the x ray beam. Depends on tissue thickness, density and atomic number. Dynamic range and Bit depth are the same thing, detecting the number of shades of gray that can be displayed. Quality or high resolution, better sharper image. A narrow decreased window width (IR) increases contrast. Low dynamic range show few shades of gray, high dynamic range show many shades of gray. Scatter decreases contrast.
- Spatial resolution: High/Low Def, High/Low resolution, high/low detail. greater amount of pixels the smaller there size. greater amount improves spatial resolution. (Geometric, sharpness) Increasing the pixel density and decreasing the pixel pitch increases spatial resolution.
- Distortion: (Geometric, sharpness) When the image is distorted, recorded detail is also reduced. It results from either the size of magnification or shape of the body part.Both SID and OID (Object to Image distance) determine the amount of magnification of the anatomic parts on the image. Shape distortion can be found in the form of elongation or foreshortening
- Image Noise : Scatter or fogging, grainy pictures. Insufficient exposure or mass (Low quantity) to the IR. More common in digital.
- Quantum Noise: The fewer the photons reaching the image receptor to form the image, the greater the Quantum noise visible on the digital image.
- Exposure Indicator: A numeric value for each image -Indicators of optimum technique to keep the selected exposures within the indicted optimum range for cassette based systems. Images within range can be Windowed, width and length
- Matrix - image displayed by a combo of rows and colums
- Pixel - Smallest component of a matrix
- Pixel density - number of pixels per unit area
- Pixel pitch - distance between the center of adjacent pixels
- An artifact is an unwanted density on a radio-graph, like a necklace.
- double exposure (two films in one cassette), clothing, fog (exposure), scatter CR IP artifacts (image receptors section)
- A plus density artifact is greater in density than the area around it. (black)
- A minus density artifact is less dense than the area around it, (White)