RAD 254 Chapter 15Radiographic 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 2

• 8 4

• 12 5

• 16 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 X-

RAY 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