RAD 254 Chapter 16 Image Quality Film factors Geometric factors Subject factors.
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Transcript of RAD 254 Chapter 16 Image Quality Film factors Geometric factors Subject factors.
RAD 254 Chapter 16Image Quality
Film factors
Geometric factors
Subject factors
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
“Noise”Film 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
Quantum mottle reduction
• High mAs
• Lowest optimum kVp’s
• Slowest possible film (receptor) screen speeds
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
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
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
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
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
Subject Factors
• Contrast : thickness, tissue mass density, atomic number of tissue being imaged– Can be somewhat modified by varied kVp’s
• Motion
Primary factors
• kVp = primary control of radiographic contrast
• mAs = primary control of optical density (OD)
Four Patient Factors
Four Image-quality Factors
Three Types of Technique Charts
Three Types of AEC’s
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
Body Composition
• Atomic mass density of body parts to be imaged– Wide range of atomic mass densities = higher
kVp’s to be used (CXR)
• Pathology – radiolucent (destructive) or radiopaque (additive)
Radiolucent – Destructive Pathology
• Active TB• Atrophy• Bowel obstruction• Cancer (for the most part destructive)• Degenerative arthritis• Emphysema• Osteoporosis• Pnuemothorax
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)
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
Technique Charts
• Variable kVp – increasing kVp’s with increased body part thickness
• Fixed kVp – (Arthur Fuch’s) highest OPTIMUM kVp – atomic mass density driven
• High kVp – over 100 kVp – barium studies and CXR’s
Automatic Exposure Control
• Usually 3 sensors – Sensors shut off x-rays when a pre-set
amount of OD is achieved (pre-set kVp’s)
• 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
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