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