Transcript RAD 254 Chapter 18 Radiographic Exposure

RAD 254 Chapter 15
Radiographic Technique
4 primary exposure factors are:
kVp, mA, time, distance
In the next 10 minutes:
• Write down “bullets” about what happens
when one raises kVp
Memory jerk for grids
• Write the following:
•
•
•
•
5
8
12
16
2
4
5
6
Now what???
kVp
• Beam quality
– Primarily responsible for quality, BUT
increases in kVp also make x-ray production
slightly more productive
•
•
•
•
Penetrability
Beam intensity
HVL
Biggest exposure factor affecting
CONTRAST
mA
• Directly responsible for amount of
radiation produced (Quantity) As mA is
doubled, so is the number of photons
produced and so is PATIENT DOSE
• mA stations also 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 affects Optical Density (OD)
• No affect on quality
• Other distance related terms:
– FFD, FOD, OFD, FRD, ORD, SSD
• Other geometric factors (focal spot size, patient
size, part orientation to CR and film)
Filtration
• Inherent (.5 mm al equiv)
• Added (2.0 which may also include some
filtration from localizer light apparatus, etc)
• 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 a half wave unit where the XRAY TUBE is the rectifier (no diodes)
• Full wave rectification (120 cycles per second =
120 impulses per second) – 100% ripple
• 3 phase, 6 pulse = 14% ripple (33% more
radiation produced per exposure over full wave)
• 3 phase, 12 pulse = 4% ripple (40% more
radiation produced per exposure over full wave)
• Hi frequency = <1% ripple