Download RAD 354 Chapt. 13 Intensifying Screens

RAD 354 Chapt. 13 Intensifying
Screens
• Physical purpose: to convert x-ray photons
into light photons (done at the phosphor
layer). The RESULT does lower patient dose.
Most in use – if not ALL – are “rare
earth”
• Rare earth crystals include (but are NOT
limited to):
– Gadolinium
– Lanthanum
– Yttrium
Other Intensifying Crystals Used
• Barium lead sulfate (very early phosphor
used)
• Calcium Tungstate
Desired Physical Properties of Crystals
• High atomic number = high absorption
(DETECTIVE QUANTUM EFFICIENCY {DQE})
• Phosphor should emit a LARGE # of light
photons for EACH x-ray photon –
CONVERSION EFFICIENCY (CE)
• Color of light should match the color light the
film is sensitive to – SPECTRAL MATCHING
• ZERO afterglow (“lag”)
Important Screen Terms
• Luminescence – process of giving off light
when stimulated
• Fluorescence – giving off light ONLY when
stimulated
• Phosphorescence – continuing to give off light
after stimualation
• Intensification factor – amount of radiation
reduction WITH screens vs NO screens
Screen Speed
•
•
•
•
Can be judged by intensification factor (IF)
Increasing speed INCREASES noise
Increasing speed REDUCES spatial rresolution
Increasing speed INCREASES quantum mottle
(line-pair test pattern device is used to
measure this)
Tech CONTROLABLE Screen Items
• Screen attributes the tech can control:
– Radiation quality (kVp, grid/no grid, filters, etc.)
– Image processing and temperature
– Care of and cleaning of screens
Cassette Construction
• Rigid, light proof protective housing for the
film and screens
• Felt/rubber/sponge “compression” layer to
assure good film-screen contact
• K-edge of crystals determines light spectrum
Screen Cleaning
• Compare/contrast screen cleaning solutions
(home made vs commercially produced)
• Cotton balls vs 4 X 4’s
Screen – Film Contact Test
• Wire mesh test for screen-film contact and
proper resolution/visibility of detail
RAD 254 Chapt. 14 Control of Scatter
• Break down into: Those that reduce patient
dose and those that are geometrical in nature
and those not
3 (primary) factors affecting scatter
• Increased kVp
• Increased field size
• Increased patient thickness
Spatial Resolution & Contrast
Resolution
• Spatial resolution may be thought of as
geometric in nature (F.S. size, emission
spectrum, OID, SID – dealing with geometric
image formation
• Contrast resolution – driven by scatter and
other sources of “noise”
Scatter
• INCREASED filed sizes = MORE scatter –
collimation is the MOST readily available and
EASIEST thing to lower the amount of scatter
• Patient thickness also INCREASES scatter –
compression may be used to help avoid this
(IVP’s and mammos are examples where
compression may be used)
Beam restricting devices limit the
radiation to the patient
• Aperature diaphram (size and resultant field
size are a DIRECT proportion – draw the damn
picture and figure the problem)
• Cones and cylinders – GREAT for absorbing
scatter, but are circular shaped = great for
improving contrast and removing scatter, BUT
required MUCH MORE mAs as a result
Variable Aperature Diaphram
• Mandated in 1974 by the Food and Drug
Administration (mandate later removed)
– Positive Beam Limitation Device (PBL’s)
• Automatically collimate to the size of the
cassette/receptor in the bucky and CANNOT be a
BIGGER size than the cassette/receptor
Filtration
• Filtration also will DECREASE the low energy
rays and LIMIT patient dose and some scatter
The Grid
• Only “FORWARD” scatter is of any benefit to
the radiographic image – ALL other scatter
degrades the image!
Scatter = LOWER Contrast
• Using a grid (alternating strips of fine leaded
strips with alternating radiolucent interspace
material) can effectively reduce the amount of
ANGLED scatter from reaching the
cassette/receptor
Grid Terms
• Grid ratio = height of the lead lines divided by
the interspace width
• Grid frequency/lines per inch = the MORE
lines per inch, the more clean up
• Grid clean up = scatter w/o a grid vs scatter
reaching the film/receptor with a grid AKA
“Contrast Improvement Factor”
• Grid function = improved image contrast
Bucky Factor
• Refers to the AMOUNT of radiation to the
patient with a grid vs W/O a grid
– The HIGHER the grid ratio, the HIGHER the “bucky
factor”
– The HIGHER the kVp, the HIGHER the “bucky
factor”
• Grid WEIGHT refers to how HEAVY the grid is
– duhhhh- the MORE lead the heavier it is
Grid Types
• Parallel
• Crossed (cross hatch)
• Focused
– Focused crossed
Grid Problems
• Grid cut-off = short SID’s result in the vertical,
parallel strips absorbing the “diverging” beam at
the OUTER margins of the grid/film/receptor;
MOST pronounced at SHORT SID’s
• Most grid problems are positioning related
–
–
–
–
Uneven grid/off level grid
Off centered (lateral decentering)
Off focus grid
Upside down, focused grid
Focused Grid Misalignment
• Off level = grid cutoff across image;
underexposed image (light OD)
• Off Center = ditto
• Off focus = CR centered to one side of the
other of a focused grid
• Upside down grid = SEVER grid cut-off (NO
density/OD) at BOTH sides of the image
Grid Ratio Selection
• 8:1 grid is the MOST widely used
• 5:1 grid is the most PORTABLE use grid ration
• Grid ratio is kVp driven
– Higher kVp’s warrant HIGHER grid ratios
– Higher grid ratios = HIGHER patient dose (more
radiation needed to produce an image)
– As kVp increases pat MAXIUM OPTIMUM kVp,
patient dose INCREASES
mAs – Grid Considerations
• AS grid ratio INCREASES, so must mAs
– 5:1 = 2 X mAs
– 8:1 = 4 X mAs
– 12:1 = 5 X mAs
– 16:1 = 6 X mAs
Air Gap Technique
• By allowing the scatter radiation to “diffuse” in
the atmosphere AFTER the patient but
BEFORE the cassette/receptor, the image has
HIGHER contrast, as the scatter diffuses and
does NOT reach the receptor
– C-spine is a good example of this
RAD 354 Chap. 15 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
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