Diagnostic Imaging Notes

X-ray Production and Equipment

  • Introduction to Diagnostic Imaging (Chapter 16): Only the first 30 slides will be covered in this lecture.
  • X-ray Safety Lecture: Watch to understand safety protocols in the x-ray room.

What are X-rays?

  • Non-luminous: Invisible electromagnetic radiation.
  • Similar to visible light and radio waves but with shorter wavelengths.
  • Shorter wavelengths = greater energy and penetration.

X-ray Tube

  • Source of x-rays.
  • Anode: Positive (+) electrode (different from chemistry).
  • Cathode: Negative (-) electrode.
  • Enclosed in a vacuum within a lead housing to prevent x-ray leakage.
  • Filaments in Cathode: Tungsten filaments are heated, causing electrons to boil off and form a cloud within the focusing cup.
  • Electrons accelerate across the vacuum and strike the target on the Anode.
  • Collision generates heat and energy; energy is released as x-rays.
  • A small window allows x-rays to exit the tube.

Cathode (Negative Side)

  • Two coiled wire tungsten filaments.
  • Why Tungsten? Its properties allow it to heat up to a certain amount and boils off the electrons without melting.
  • Two Filament Sizes: Smaller for smaller targets/animals, larger for bigger targets/animals.
  • Better quality x-ray for smaller animals with smaller filament size.
  • Focusing Cup: Contains the filaments and directs electrons towards the anode.

Electrical Circuits

  • High Voltage Circuit: Kilovoltage potential (kVp) controls the speed of electrons.
  • Low Voltage Circuit: Milliampere (mA) controls the number of electrons.

Kilovoltage vs. Milliampere

  • kVp (kilovoltage potential): Controls the speed of electrons impacting the target. Think of NASCAR with kVp on the side to remember speed.
  • mA (milliampere): Controls the number of electrons. Think of "man" to remember number of electrons.

Focal Spot (Target)

  • Located on the Anode; tungsten target.
  • Size is determined by the electron beam size from the cathode.
  • Small focal spot = higher quality images.
  • Large focal spot = higher tube current but lower detail.
  • Horse X-rays Example: Horse x-rays are fuzzier due to the need for a larger focal spot to penetrate thicker tissue.
  • Extremities vs. Thorax of Great Dane Example: Extremities have better detail due to the use of smaller focal spot.

Anode Types

  • Stationary Anode.
  • Rotating Anode.

Heel Effect

  • X-ray beam intensity varies; more intense on the cathode side.
  • Thickest part of the patient should be placed toward the cathode side.

Physics of X-ray Production

  • Incoming Electrons: Either miss target atoms (producing heat) or interact with the electron cloud of target atoms (producing x-rays).
  • Energy Conversion: 99% of electron energy converts to heat; <1% converts to x-rays.

Collimation

  • Reduces number of x-rays, limiting the primary beam and scatter.
  • Decreases Direct and Scatter X-ray: Scatter radiation is dangerous.
  • Scatter Radiation: Degrades film quality, increases radiation exposure, and contributes to film blackness without improving the image.
  • Improper Collimation example: Field is too large; increase in kVp; thick animal = scatter radiation creating an image which is unreadable.

X-ray Equipment

  • Table for positioning the animal.
  • Cassette holders.
  • Control panel for kVp and mA.
  • Exposure time.
  • MAS Dial: Milliampere per Second. Number of electrons per millisecond.
  • Portable X-ray Units: Lower power, can be used in the field.
  • Mobile X-ray Units: Medium powered, wheel-mounted.
  • Stationary X-ray Units: More powerful, fixed location.
  • X-ray Tube Movement: Can rotate 90 degrees to take various angle images.

Exposure Factors: kVp, MAS, and Focal Film Distance

  • Focal Film Distance: Typically 3 feet (36 inches); maintain consistency.

Film Exposure

  • Film Exposure: The more exposure to the film, then the blacker the film will get.
  • Density Differences: High density (bones) appear white because they block x-rays.

Tabletop vs. Grid

  • Tabletop: Use for extremities or small animals.
  • Grid: Use for larger animals or thicker body parts to reduce scatter radiation.
  • Vibrating Grid: Found in the table of the x-ray machine, which stops the scatter radiation.

Milliampere (mA) Control

  • Controls the quantity (number) of electrons.
  • Affects the amount of x-rays produced.
  • Controls density; increase mA = increased density (darker image).
  • Under Exposure: X-rays not strong enough therefore not enough electrons to get through the dense tissue/bone.
  • Over Exposure: Too many electrons therefore the image is too black.
  • MAS High: Overexposed.
  • MAS Low: Underexposed.
  • Ideal Exposure: Can see soft tissue, bone and bone marrow.

Exposure Time

  • Fraction of a second during which the anode is positively charged.
  • Longer Exposure Time: More electrons flow, but can result in blurry images.
  • Shorter Exposure Time: The best exposure time to minimize blur.
  • Thorax: Short exposure time to account for breathing.
  • Abdomen: Longer exposure time fine because it remains still at most times.

Kilovoltage (kVp)

  • Controls the energy/speed of the x-ray beam.
  • Affects contrast.
  • High kVp: High energy, low contrast (less difference between soft tissue and bone).
  • Low kVp: Low energy, high contrast (more difference between tissues).

Focal Film Distance (FFD)

  • Distance between the x-ray tube target and the image receptor surface.
  • Typically 70-85 cm for large animals and 90-105 cm for small animals.
  • Keep distance constant to maintain consistent exposure.
  • Inverse Square Law: Increase in distance decreases x-rays non proportionally.

Grids

  • Controls scatter radiation before it reaches the x-ray cassette.
  • Located between the patient and the cassette.
  • Made of lead strips with radiolucent spacers.
  • Requires greater exposure.
  • Use for body parts thicker than 10 cm.

Technique Chart

  • Ensures consistency in x-ray imaging.
  • Machine-specific; based on trial and error exposures.
  • Usually based on kVp adjustments based on size and body part.

Digitized Radiography

  • Digital Fluoroscopy: Moving x-ray (GI tract, kidneys).
  • Computed Tomography (CT): 3D x-ray.
  • Diagnostic Ultrasound: Sound waves (fluid environments).
  • Nuclear Medicine: Injected nuclear material.
  • Magnetic Resonance Imaging (MRI): Magnets to visualize the body.
  • Digital Radiography: All digital exposure.
  • Computed Radiography: Exposure turned into digital.

Digital Image Management

  • Digital File Saving: Images saved digitally and sent to a radiologist.
  • PACS (Picture Archival Computing Systems): Moves images between workstations.
  • DICOM (Digital Imaging and Communications in Medicine): universal standard for storing and transmitting medical images.
  • Radiology Information Systems (RIS) and Teleradiology: Software to coordinate patient data and transmit images to specialists.

Film Systems (Historical)

  • Dip Tanks: Manual film processing in a dark room (developer, fixer, water rinse, dryer).
  • Automatic Processor: Automated film processing; quicker, but still has limitations.

Modern Digital Imaging Systems

  • Ultrasound, CT, Fluoroscopy, Nuclear Medicine, MRI (to be discussed later).