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

• Write the following:• 5 2• 8 4• 12 5• 16 6

Now What???

• 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

• 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

FiltrationkVp 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