rad 354 chapt. 13 intensifying screens physical purpose: to convert x-ray photons into light photons...
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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
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
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