Transcript RAD 354 Chapt. 13 Intensifying Screens

RAD 354 Chap. 13 Radiographic
Technique
• Four PRIMARY exposure factors:
– kVp
– mA
– Time
– distance
In the next 5 minutes
• Write down “bullets” about what happens
when on RAISES kVp
Memory “jerk” for grids
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Write the following:
5
2
8
4
12
5
16
6
Now What???
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5:1 = 2X mAs
8:1 = 4 X mAs
12:1 = 5 X mAs
16:1 = 6 X mAs
kVp
• Beam Qualtiy
– Primarily responsible for quality, BUT INCREASES
in kVp also make x-ray production SLIGHT more
productive
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Penatration
Beam intensity
HVL
Biggest exposure factor affecting CONTRAST
mA
• DIRECTLY responsible for AMOUNT of
radiation produced (Quantity). As mAs is
doubled, so is the number of photons
produced and so is PATIENT DOSE
• mA stations are responsible for focal spot size
selection
Time
• Exposure times should be practical and short
enough to stop patient motion, but the
shortest times also result in the most radiation
output per unit of time – thus MORE wear and
tear on the x-ray tube
• mAs = time X mA
– mAs is only measured by tube current
– Responsible for Optical Density (OD)
Distance (SID)
• The most “forgotten” exposure factor, but
perhaps the most important
– Inverse Square Law
– Primarily effects Optical Density (OD)
• NO effect on quality
• Other distance related terms:
– FFD, FOD, OFD, FRD, ORD, SSD
• Other geometric factors (F.S. size, pt. size, part
orientation to CR and receptor
Filtration
kVp driven
• Inherent (.5 mm al equiv)
• Added (2.0 which may also include some
filtration from localizer light apparatus, etc.) in
a 70-80 kVp unit
• Total filtration : inherent + added (2.5 mm al
equivalent)
Generators
• Half wave (120 cycles/sec = 60 impulses per
second) – 100% ripple
– “self rectified” is also half wave where the X-RAY TUBE
is the DIODE
• Full wave rectification (120 cycles per second =
120 impulses per second) – 1--% ripple
• 3 phase, 6 pulse = 14% ripple (33% more
radiation per exposure over full wave)
• 3phase, 12 pulse = 4% ripple (40% more per
exposure over full wave
• Hi frequency = <1% ripple