My notes

• Warm-up procedure


  1. Avoid damage to the X-ray tube

  2. Series of low energy exposures to prevent damage
• 1. ray tube's warm-up procedure

  Purpose is to avoid damage to the X-ray tube
• A general recommendation is proposed by all manufacturers for warming of the X-ray tubes by different series of low energy exposures to prevent such type of damage
• 1. radiation

  Relatively large amounts of electrical energy must be transferred to the x-ray tube to produce it
• Only a small fraction (typically less than 1%) of the energy deposited in the x-ray tube is converted into x-rays; most appears in the form of heat
• If excessive heat is produced in the x-ray tube

  The temperature will rise above critical values, and the tube can be damaged
• Damage to x-ray tube

  Melted anode or a ruptured tube housing
• Preventing x-ray tube damage

  Operator must be aware of the quantity of heat produced and its relationship to the heat capacity of the x-ray tube
• The figure below identifies the factors that affect both heat production and heat capacity
• Heat is produced in the focal spot area by the bombarding electrons from the cathode
• Only a small fraction of the electronic energy is converted in x-radiation, it can be ignored in heat calculations
• Calculating heat produced in a single exposure

  Heat (J) = w x KV p x m A s
• Waveform factor (w)


  Determined by the waveform of the voltage applied to the x-ray tube. Values for most waveforms encountered in diagnostic x-ray machines: constant potential, 1.0; three-phase, 12 pulse, 0.99; three-phase, 6-pulse, 0.96; single-phase, 0.71
• Heat unit (HU)


  Smaller quantity of heat than a joule, 1 joule = 1.4 heat units
• Heat capacity


  Proportional to an object's size/mass and specific heat. As heat is added, temperature increases in proportion to the amount of heat added
• When a given amount of heat is added

  The temperature increase is inversely proportional to the object's heat capacity
• In most x-ray tubes there are three distinct areas with critical heat capacities: focal spot area/track, anode body, and tube housing
• Heat flow in x-ray tube

  Heat moves by conduction throughout the anode body and by radiation to the tube housing, then removed from the tube housing by transfer to the surrounding atmosphere
• Warm-up procedure


  Purpose is to avoid damage to the X-ray tube by performing a series of low energy exposures
• Radiographic technique factors

  kVp, mA, and exposure time
• Time of exposure should be as short as possible to improve image quality by reducing motion blur
• Image-quality factors

  Characteristics of the radiographic image, including OD and contrast
• Optical density (OD)

  Degree of blackening of the finished radiograph, numeric value from 0 (clear) to 3+ (completely black)
• Overexposure and underexposure


  • Unacceptable image quality, may require examination to be repeated
• Optical density control factors

  mAs and SID
• OD increases directly with mAs

  To increase OD, mAs setting must be increased accordingly
• A change in mAs of approximately 30% is required to produce a visible change in OD
• As a general rule, when only the mAs setting is changed, it should be halved or doubled
• Changes in mAs value

  • Decrease OD when decreased by half, increase OD when doubled
• As kVp is increased

  The quality of the beam is increased, and more x-rays penetrate the anatomical part, resulting in a greater number of image-forming x-rays
• Image contrast is affected when kVp is changed to adjust OD, making it much more difficult to optimize OD with kVp
• 15% rule


  If the OD on the radiograph is to be increased with the use of kVp, an increase in kVp of 15% is equivalent to doubling the mAs
• Contrast


  The difference in OD between adjacent anatomical structures, or the variation in OD on a radiograph. Necessary for the outline or border of a structure to be visible.
• Contrast resolution of soft tissues

  Can be enhanced with reduced kVp but at the expense of higher patient radiation dose
• Gray scale of contrast


  The range of ODs from the whitest to the blackest part of the radiograph
• High-contrast radiographs

  Produce short gray scale, exhibit black to white in just a few apparent steps
• Low-contrast radiographs


  Produce long gray scale, have the appearance of many shades of gray
• To reduce contrast


  Increase the kVp to produce a radiograph with longer gray scale contrast and more grays
• Relationships between contrast and kVp

  • High contrast = low kVp
  • Low contrast = high kVp
• Endocrine glands release hormones directly into the bloodstream, while exocrine glands secrete their products onto epithelial surfaces such as skin or mucous membranes.