RAD 254 Chapter 16 Image Quality

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RAD 254 Chapter 16Image Quality

• Film factors
• Geometric factors
• Subject factors

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Radiographic Quality

• Resolution ability to image two or more objects
  and distinguish them on the radiograph
• Spatial resolution imaging small objects with
  high subject contrast that are close together
  (breast micro calcifications in breast tissue)
• Contrast resolution ability to distinguish
  items with similar subject contrast (white-gray
  matter)
• Related terms detail recorded detail
  visibility of detail

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NoiseFilm graininess, structure mottle,
quantum mottle, scatter

• Film graininess distribution, size, space of
  silver halide in the emulsion (mammo)
• Structure mottle as above, but involves the
  intensifying screen crystals (mammo)
• Quantum mottle random way x-ray photons
  interact with film/receptor (increased speed
  receptor higher QM)
• Scatter

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Quantum mottle reduction

• High mAs
• Lowest optimum kVps
• Slowest possible film (receptor) screen speeds

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Film factors

• Characteristic curve density, contrast, speed
  latitude
• Relationship of OD and radiation exposure
• Also called H D curve, Hortchkof curve,
  sensitometric curve,
• Terms base fog, toe, average gradient/straight
  line portion, shoulder, solarized portion
• Processing Time, temperature

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Sensitometry/densitometry

• Sensitometry exposing the film to radiation or
  light in step form exposures
• Densitometry measuring the amount of light
  transmitted through each exposed step
• Plotting the optical density (OD) verses log
  relative exposure (LRE) the curve
• Increments in LRE of .3 double the exposure
• Useful range of OD is .2 2.5

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Contrast

• Film/image receptor contrast (inherent in the
  film-screen combination) reflected in the ave.
  gradient portion of the H D curve (more
  vertical curve higher contrast)
• Subject contrast size, shape and attenuating
  characteristics of the body part imaged

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Film Characteristics

• Speed sensitivity and responsiveness of the
  emulsion of the film to x-rays/light
• Latitude range of exposures to create blackness
• wider latitude (responds more slowly to
  light/x-rays) lower contrast.
• Latitude and contrast are INVERSLY PROPORTIONAL

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Geometric factors

• Distortion
• Depends on object thickness, position, shape
• Magnification
• To minimize magnification use long SID and short
  OID
• Blur (focal spot blur) caused by the effective
  focal spot larger on the cathode end of the
  emission spectrum

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Subject Factors

• Contrast thickness, tissue mass density, atomic
  number of tissue being imaged
• Can be somewhat modified by varied kVps
• Motion

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Primary factors

• kVp primary control of radiographic contrast
• mAs primary control of optical density (OD)

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• Four Patient Factors
• Four Image-quality Factors
• Three Types of Technique Charts
• Three Types of AECs

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Patient Factors

• Body habitus
• Sthenic average sized patient
• Hyposthenic thin but healthy
• Hypersthenic big frame usually overweight
• Asthenic small, frail, emaciated, usually
  elderly
• Thickness of part
• Body composition
• Pathology

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Body Composition

• Atomic mass density of body parts to be imaged
• Wide range of atomic mass densities higher
  kVps to be used (CXR)
• Pathology radiolucent (destructive) or
  radiopaque (additive)

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Radiolucent Destructive Pathology

• Active TB
• Atrophy
• Bowel obstruction
• Cancer (for the most part destructive)
• Degenerative arthritis
• Emphysema
• Osteoporosis
• Pnuemothorax

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Radiopaque Additive/constructive

• Aortic aneurysm
• Ascites (abnormal accum. Intraperitoneal fluid)
• Atelectasis (collapse of alveoli)
• Cirrhosis
• Hypertrophy
• Metastases (for the most part)
• Pleural effusion
• Pneumonia
• Sclerosis (hardening of tissue)

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Image quality factors

• Optical Density primarily controlled by mAs
  (20-30 change to see any difference) and SID
• Contrast shades of gray primarily controlled
  by kVp (hi/low long/short scale)
• Image Detail visibility of detail (contrast
  resolution) and sharpness of detail (spatial
  resolution)
• Distortion improper alignment of part

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Technique Charts

• Variable kVp increasing kVps with increased
  body part thickness
• Fixed kVp (Arthur Fuchs) highest OPTIMUM kVp
  atomic mass density driven
• High kVp over 100 kVp barium studies and CXRs

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Automatic Exposure Control

• Usually 3 sensors
• Sensors shut off x-rays when a pre-set amount of
  OD is achieved (pre-set kVps)
• 600 mAs over-ride (if sensors do NOT shut off
  x-rays, the machine automatically terminates the
  exposure at 600 mAs) some units have variable
  back up timers for this

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Anatomically Programmed Radiography

• Microprocessor technology based upon anatomical
  area to be imaged
• Uses anatomic illustrations for the tech to
  select the anatomical part to be imaged and the
  unit does the mA, kV and AEC selection

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