Diagnostic imaging

• Electromagnetic radiation is part of the electromagnetic spectrum, where all waves travel at a constant speed of light (so wavelength and frequency are inversely proportional). The higher the frequency, the more energy electromagnetic waves have.

   

  X-rays have short wavelength, high frequency and high energy
• X-rays interact with matter either by penetration (direct), absorption or scatter. They are able to penetrate any material which absorbs or reflects visible light, and cause fluorescence in certain materials.

   

  X-rays’ high energy means they can produce ions in the material they penetrate. This can cause biological DNA damage, either directly or through interaction with water, forming free radicals which can themselves damage DNA. Biological responses to ionizing radiation are either repair or mutation. Mutations can be genetic, somatic, teratogenic or cell death.
• Biologic effects of x-rays can be classified into stochastic and deterministic effects:

  Stochastic – the probability of damage increases with dose. There is no known lower threshold so avoidance is our best bet.

  Deterministic – high radiation exposure exceeds a threshold dose, with severity of damage increasing with each dose. These include erythema, haematopoietic damage and cataracts. This is mainly a concern only in high dose events such as radiotherapy and nuclear accidents.
• A photon is a discrete bundle of electromagnetic radiation
• The most radiation sensitive tissues are those which are actively and quickly dividing:

  • Bone marrow
  • Epithelial cells of the GIT
  • Gonadal cells (genetic damage)
  • Embryonic cells (death, congenital malformations, growth defects – particularly at an early stage)
• The most important radiation measurement to us is the effective dose: a measure of radiation and organ system damage in humans, with a quality factor for different radiation. Measured in sievert.

  A gray is the absorbed dose by a patent – the amount of energy transferred by radiation per mass

  Exposure is the amount of ionization energy per mass of air.
• Limited effective doses describe a dose to the whole body received when not a patient. Dose limits define a level below which no harmful effect to a person would be expected. Based on the ionizing radiation regulations of 1999.

  • For medical employees over 18: 20 mSv /year
  • For trainees aged 16-18: 6 mSv /year
  • For the general public: 1mSv /year
  • Pregnant women: 1 mSv /year (so pregnancy must be declared as soon as known)
• Natural annual background exposure to radiation is about 1 mSv annually. A return flight from the UK to Australia accounts for approx. 0.4 mSv.

   

  A chest or head radiograph as a patient is about 0.02 mSv, a lumbar spinal/ pelvic/ abdominal radiograph is about 1 mSv. A CT scan with contrast is about 20 mSv
• The goal of radiation protection is to obtain maximal diagrnostic information with minimal exposure of the patient, radiology personnel and the general public. Protection principles are designed for personnel primarilym not the patient:

  • ALARA: as low as reasonably achievable (dose)
  • Justification – a medical indication must be clear
  • Higher risk groups should not be involved (children, pregnant women)
  • Dose monitoring for personnel
  • Dose limits for personnel
  • Controlled and regulated areas with ionizing radiation
• For radiation protection, manual restraint of a patient is only permissible is there is a good clinical reason and cannot be kept still by other means. Exceptions might include critical illness in which sedation would deteriorate the patient, or a time delay is critical, a specific radiographic technique which requires presence during exposure e.g. swallowing test, or the patient type e.g. large animal.
• Methods of radiation protection:

  • Medical indication required (not for shits n gigs)
  • Filtration of the X-ray tube to remove low energy, harmful radiation which doesn’t even make a picture (unnecessary exposure)
  • Time (good technique and adequate sedation to reduce radiation exposure time)
  • Distance: inverse square law
  • Shielding by radioprotective clothing
  • Collimation to minimise scatter radiation (only expose area of interest)
  • Personnel monitoring (dosimeters)
• The inverse square law states that if the distance from the primary radiation source is doubled the intensity will decrease by a factor of four (so increasing distance from the x-ray tube has a radioprotective effect)

  • Stand as far away from the tube as possible
  • Use cassette holders
• Shielding using lead aprons, gloves and thyroid collars are only protective against scatter radiation. They must be stored non-folded to prevent cracks which could result in radiation leaks. They should also be examined regularly for defects.
• X-rays are produced by conversion of the kinetic energy of accelerated electrons into electromagnetic radiation. This takes place within the x-ray tube.

  Electrons are sourced from a cathode. Acceleration creates a potential energy difference. As they decelerate onto the anode, energy is converted mostly into (99%), and 1% as x-rays. Tube current is measured in mA (the electric current used to heat cathode, describes the number of electrons flowing per second filament -> target.) A longer exposure results in more x-rays produced, so the final measurement is mAs (Milliamperes per second) .
• X-rays are polychromatic (have many different wavelengths), and the one with the shortest wavelength has the highest energy. This is the peak energy (kVp). There is an electric potential between the anode and the cathode, and the higher potential difference, stronger x-rays produced.

  mAs is the number of electrons being produced, kVp is how fast the electrons fly between the cathode (-ve) and anode (+ve)
• X-rays are produced when high energy electrons from the cathode hit the positive anode. The anode has to be a heavy metal such as tungsten with a very high melting point as the majority of energy is given as heat. If the anode rotates, bigger units of x-rays can be produced whilst it stays cooler.
• The line focus principle of anode design allows for a more effective (smaller) focal spot. The xray tube is surrounded by lead with only a small window to allow radiation out, giving a sharper detail in images. A large focal spot produces shadows at the edge of an image.

  By having a small filament, the electron beam is small and focal, for fine detail (but heat concentrates). Large filament sizes have a larger electron beam and focal spot so there is reduced detail but we can use higher exposures.
• If you think of x-rays as an army: increasing mAs will increase the numbers in your army (more x-rays produced). Increasing kVp makes your army stronger (penetrating power of each x-ray increased).
• X-ray image formation is formed by attenuation and transmission:

  • Attenuation: the intensity of the x-ray beam decreases as it passes though matter (either via absorption or scatter)
  • Transmission: x-rays pass through matter without any interaction

  Because x-rays are differently absorbed by different tissues, beam attenuation creates different intensities of shadow.
• The photoelectric effect occurs when an x-ray photon is completely absorbed (when kV is low), and a photoelectron with the same energy as the photon is removed from its shell (ionization). An electron from a higher shell falls into its place, energy is released (characteristic radiation). All energy is absorbed within the body and there is no scatter. This helps differentiate matter, as effect is proportional to cubed atomic number of matter (density .’. differs). Also proportional to thickness and density.
• The compton effect occurs at higher energy. X-ray photon energy does not match photoelectron energy. The x-ray photon ejects an outer shell electron but still has remaining energy, which continues as scatter radiation in a different direction– this can fog the film and is a radiation safety hazard. A higher kV is required for larger patients so in some cases this is unavoidable. Compton effect is independent of atomic number so differentiation is only proportional to thickness and density (density differential unaffected)
• A grid reduces scatter from the compton effect before x-rays reach the cassette. Scatter is when radiation deviates from its course, but is then absorbed by lead lines in the grid. Diagnostic image quality may be improved but you must simultaneously increase kV. This is not a radiation safety device despite being made of lead! It doesn’t prevent scatter reaching the patient or personnel and requires increased exposure to work.
• Conventional radiography involves X-ray film, cassettes with intensifying screens and automatic or manual film processing (with developer and fixer then rinsing and drying). A view box is needed for review of films and an archive space with appropriate environment for film filing (7 years).

   

  Cassettes should only be opened in a darkroom or they will be damaged.
• An x-ray tube emits a beam which passes through the patient and into an x-ray film, generating an image.

  Conventional radiography films have five layers: an external protective coating, a silver halide emulsion and centrally a polyester base. The emulsion is what develops into the image
• In rediography film, silver halide crystals are made up of about 2% silver ions (+ve) and 98% bromine ions (-ve). In each crystal there is a sensitivity speck – a trapped electron which when interacted with by a photon, a silver atom is formed. This generates a latent image which then needs to be developed to become visible.
• Conventional radiograph development occurs in 3 steps:

  1.      Developer – the latent image centre catalyses the reaction and reduces the remaining silver ions into grains of metallic silver

  2.      Fixer – this prevents further development and removes any undeveloped silver bromide from the film

  3.      Wash – removes fixer chemicals that would otherwise discolour the film over time
• Before processing physical radiography films, they must be permanently identified (date, medical record number, animal and owner name, practice name). Development must occur in a darkroom as light deteriorates films.
• Non-screen x-rays are used in direct exposures in cardboard holders. This is most common in dental radiography.

  Screen  x-rays use a single or double emulsion where 95% of exposure is from the light given off by the intensifying screen (fluorescent effect)
• An intensifying screen reduces personnel and patient exposure and increases contrast. They are made up of a protective layer, fluorescent layer and reflective layer on top of a plastic base.

  Within the screen, an x-ray photon interacts with the screen, creating a fluorescent light emission effect and increasing contrast on the film.
• A grid is used to absorb scatter radiation and improve radiographic contrast by reducing fog. A grid is recommended when radiographing a body part thicker than 10cm.

  To use a grid, mAs must be increased by a factor of 2 to 3 to compensate for the proportion of x-rays absorbed by the grid.
• In computed radiography, there is no film, no chemistry and no darkroom. The image is captured on casettes containing a photostimulable phosphor storage layer. The cassette is then placed into a laser film reader, which stimulates phosphor crystals to release their stored light energy. This generates a computer image in DICOM format – this means they cannot be altered. The image is then stored in a server with PACS
• Computed radiography advantages:

  • Time efficient
  • Creates good quality, robust images
  • A lower radiation dose is possible
  • Very cost effective

   

  Computer radiography disadvantages:

  • Laser reader is sensitive to dust
  • Maintenance is required for moving parts
  • There is still manual labour required.
• Direct digital radiography uses an imaging receptor plate which contains many small detector elements which transform x-rays into electrical signals. Images are then viewed on a local worksation and sent to PACS. The detector is often built into the table, but can be wired in or wireless and movable.
• Advantages of direct digital radiography:

  • Image is obtained within several seconds
  • Image quality is good
  • A lower radiation dose is possible

   

  Disadvantages of direct digital radiography:

  • More expensive than other forms
  • Fragile
• Indirect digital radiography: xray photons strike a panel detector which create visible light. The light photons stimulate a photocathode, the electrical energy from which is encoded and transmitted to the computes for reconstruction.

  Direct: x-ray photons strike a selenium plate, the charges from which are encoded and transmitted to the computer for image reconstruction.

   

  Multiple images can also be stitiched together to create one larger image in direct digital radiography
• Dynamic digital radiation provides moving radiographic images, allowing views of motility. 

  Dynamic digital radiation can be used instead of fluoroscopy, where x-rays are directed into a cathode ray vacuum tube (this is large, bulky, expensive and equipment is sensitive). Fluroscopy images are usually poor quality, but frame rate can be high (up to 100 fps). DDR allows for image capture up to 40 fps, but image quality is higher
• The major advantages of computed and digital radiography are the flexibility of acquisition and display: there is no film chemistry development, filing, or risk of loss as all images can be views from a computer with PACS software with remote backup data.
• DICOM is digital Imaging and Communication in Medicine. This is the unified standard of file format and communication protocol which prevents documents from being manipulated.

   

  PACS is Picture Archiving and Communication System which centrally stores images, replacing the need for hard copies of film and their physical storage. It allows for remote access but is restricted to computers which have the software for data protection.
• X-rays interact with matter are absorbed, scattered or transmitted through a patient. Different tissues vary in x-ray absorption, giving us different opacities. Absorption / attenuation depends on density and atomic number3
• There are five different radiopacities, ranging from radiolucent (dark) to radiopaque (light)

  Gas is darkest, then fat, then soft tissue and fluid, then mineral or bone, then metals are lightest.