Ouhsc Exposure & Instrumentation Test 3

Front 1

Scatter radiation is produced during a _________ interaction

Back 1

Compton (no use in demonstrating structures of interest)

Front 2

Do we worry about coherent scatter in diagnostic radiography?

Back 2

No because diagnostic energy range is 50-100 keV and the energy required for coherent is 10 keV

Front 3

Scatter radiation will reduce contrast by adding...

Back 3

More shades of gray

Front 4

Why does increasing scatter increase the density?

Back 4

Each photon that hits the IR creates density; a single high energy photon can cause multiple interactions, causing more density

Front 5

As scatter increases, radiographic density _____________ (increases/decreases) and there is a __________ (increase /decrease) in contrast

Back 5

Increases; decrease (more grays)

Front 6

As scatter decreases, radiographic density _____________ (increases/decreases) and there is a __________ (increase /decrease) in contrast

Back 6

Decreases; increase (fewer grays)

Front 7

If we increase kV and leave all other technical factors constant, what will happen?

Back 7

Overall increase in density b/c more photons are exiting the patient & striking the film

Front 8

What kind of interactions increase with an increase in kVp?

Back 8

Compton interactions (outer shell electron)

Front 9

As tissue volume increases, the amount of scatter radiation __________ (increases/decreases)

Back 9

Increases

Front 10

What are the 2 reasons to reduce your field size to only the area of interest?

Back 10

1) Controls amount of scatter produced
2) Reduce patient dose

Front 11

Which interaction is preferred for diagnostic imaging, Compton or Photoelectric?

Back 11

Photoelectric

Front 12

As the volume of the irradiated tissue increases, which type of interaction increases?

Back 12

Compton Scatter

Front 13

As the atomic number of the irradiated material increases, which type of interaction increases?

Back 13

Compton

Front 14

An increase in _______ (mAs/kVp) would need to occur when the field size is collimated small.

Back 14

mAs

Front 15

Aperture Diaphragm

Back 15

- simplest type of beam-restricting device
- flat piece of lead w/ an opening (aperture) to allow radiation to pass through
- opening can be any shape; size of opening isn't adjustable
- inexpensive

Front 16

In an aperture diaphragm, the field size to be radiated is controlled by the _____

Back 16

SID (increasing SID will increase field size to be radiated)

Front 17

Penumbra

Back 17

- "fuzzy area" out toward the edges of the primary beam
- the less we have, the better our image detail will be ("use the smallest focal spot")

Front 18

Aperture Diaphragm effect upon Penumbra

Back 18

Little impact

Front 19

Cylinder effect upon Penumbra

Back 19

Great effect

Front 20

Cones/Cylinders

Back 20

- circular aperture diaphragm w/ metal extensions (cones are flared @ ends)
- don't have much of an effect on DR as they do on film
- cylinders are constant in diameter, adjustable in length

Front 21

Which, a cone or a cylinder, is better at reducing penumbra?

Back 21

Cylinder

Front 22

Adjustable Cylinders

Back 22

- attach beneath & on the outside of tube housing
- as cylinder length increases the amount of radiation reaching patient/film decreases
- increasing cylinder length will decrease the field size
- off-focus radiation will be reduced

Front 23

Equation for Determining Cone Field Size

Back 23

Projected Image Size = (SID x lower diameter of cone)/distance from focal spot to bottom of cone

Front 24

What's the most frequently used method for beam restriction?

Back 24

Collimation

Front 25

What do the bottom shutters of the collimator control?

Back 25

Reduce penumbra along periphery of beam

Front 26

What do the upper shutters of the collimator control?

Back 26

Attenuate most of the off-focus radiation from reaching film

Front 27

Collimator Test

Back 27

- a test done to check accurate alignment of the light source & the x-ray beam
- US Federal Standard = 2% SID error (ex. 40" SID x 2% = .8" allowable error)

Front 28

Automatic Collimators

Back 28

- positive beam limitation (PBL) device
- adjusts dimension of x-ray beam to IR placed in bucky
- will allow you to collimate smaller, but not bigger than the field size of the IR

Front 29

What does the mirror in a collimator act as?

Back 29

A filter that absorbs low energy x-ray photons to 1 mm/Al eq

(Added Filtration)

Front 30

What is the total filtration goal standard?

Back 30

2.5 mm of aluminum equivalent (mm/Al eq)

Front 31

Ancillary Devices

Back 31

- lead blockers are pliable lead sheets that prevent radiation from striking the film cassette
- lead masks cut to specific shape & size

Front 32

What's the purpose of a grid?

Back 32

To improve the contrast of the image by absorbing the patient-generated scatter radiation (Compton) before it strikes the film

Front 33

When are grids used?

Back 33

1) Body parts larger than 10 cm
2) Procedures using 60 kVp or higher

Front 34

What are the vertical strips in a grid designed to do?

Back 34

Absorb scatter radiation (lead) RADIOPAQUE

Front 35

What are the interspace materials in a grid designed to do?

Back 35

Allow remnant photons to strike the film (aluminum or plastic fiber) RADIOLUCENT

Front 36

How will the grid strips be running in a rectangular grid?

Back 36

With the long axis of the rectangle (used for portables)

Front 37

3 Expectations about Outcome of Image when Using Grids

Back 37

1) Higher contrast
2) Reduction in density (fewer photons hit IR)
3) Grid artifacts

Front 38

What is the grid ratio?

Back 38

The relationship of the HEIGHT of the lead strips to the DISTANCE BETWEEN the strips

GR = h / D

Front 39

What does the thickness of the lead strips in a grid represent in the grid ratio?

Back 39

Nothing (irrelevant in terms of calculating grid ratio)

Front 40

A(n) __________ (proportional/inverse) relationship exists between the distance between the lead strips & grid ratio WHEN the height of the grid strips is constant

Back 40

Inverse

Front 41

High Grid Ratio

Back 41

- stops scatter radiation better
- higher contrast images
- requires higher adjustment in technique
- attenuate more of the remnant beam (have to increase mAs)

Front 42

What is the grid frequency?

Back 42

The number of vertical strip per unit of length/width

Vertical strips will be thinner as the frequency per unit of distance increases

Front 43

Grid Patterns

Back 43

- linear (parallel strips of lead)
- focused grids [diagnostic]
- cross-hatched or criss-cross (2 linear patterns 90 degrees to each pattern) [mammo]

Front 44

What causes grid cutoff with a cross-hatched grid?

Back 44

Tube angulation

Front 45

Do linear grids allow tube angulation?

Back 45

Yes; must be parallel w/ lead strips

Front 46

Focused Grids

Back 46

- linear-positioned lead strips
- angled similar to angulation of primary beam divergence
- REQUIRES PROPER SID (to reduce scatter & allow remnant photons to pass)

Front 47

Latitude

Back 47

- freedom from usual restraints, limitations, or regulations
- high latitude = larger allowable changes
- grid latitude: amount you can be off & still get acceptable image

Front 48

In focused grids, as grid ratio increases, the latitude for SID __________ (increases/decreases)

Back 48

Decreases (small error in SID can cause large grid cutoff)

Front 49

Where are permanent grids located?

Back 49

- within Potter-Bucky diaphragm, above film location
- grid lines parallel w/ table length, or vertical direction of wall bucky

Front 50

As grid ratio or frequency increases, the radiographic density ___________ (increases/decreases)

Back 50

Decreases (more scatter removed along w/ more remnant; less photons striking film)

Front 51

As grid ratio or frequency increases, the radiographic contrast ___________ (increases/decreases)

Back 51

Increases (beam less polyenergetic, more homogenous)

Front 52

Which grid type is used most frequently?

Back 52

Focused grids

Front 53

Grid Conversion Factors (GCF)

Back 53

- "constants" used to determine change in technical factors needed for specific grid ratios
- mAs adjustments must be made when changing grid ratio

Front 54

GCF Formula

Back 54

GCF = mAs with grid / mAs without grid

mAs with grid = GCF x mAs without grid

mAs without grid = mAs with grid/GCF

Front 55

GCF - No grid

Back 55

mAs increase 1X, GCF = 1

Front 56

GCF - 5:1 Grid

Back 56

mAs increase 2X, GCF = 2

Front 57

GCF - 6:1 Grid

Back 57

mAs inceases 3X, GCF = 3

Front 58

GCF - 8:1 Grid

Back 58

mAs increase 4X, GCF = 4

Front 59

GCF - 12:1 Grid

Back 59

mAs increase 5X, GCF = 5

Front 60

GCF - 16:1 Grid

Back 60

mAs increase 6X, GCF = 6

Front 61

Off-Level Grid Error

Back 61

- occurs when tube is angled across the long axis of the grid strips or grid is unleveled in comparison to tube
- typically happens in portable grids
- appearance: decrease in density
- To Fix: level grid under patient, angle tube parallel w/ length of grid

Front 62

Off-Center Grid Errors

Back 62

- Tube isn't centered (CR) w/ the center of the focused grid (tube not centered to center of grid cassette)
- occurs in fixed or portable grids
- decrease in density

Front 63

Off-Focus Grid Errors

Back 63

- SID outside acceptable range of focused grid
- occurs w/ fixed or portable grids
- decrease in density at lateral margins of image

Front 64

Upside-Down Grid Error

Back 64

- center of grid will allow the x-ray beam to pass unrestricted flipped upside down or not
- on edges, grid strips are angled opposite to angle of divergence
- severe decrease in density @ lateral margins

Front 65

Does collimation change the diverging angle of the primary beam?

Back 65

No

Front 66

Air Gap Technique

Back 66

- alternate method to use a grid if one isn't available
- use large OID (will reduce amount of scatter reaching the film with the air gap)
- need to increase SID to reduce magnification
- kVp = 70 at least

Front 67

Reverse-Cassette Technique

Back 67

- flip cassette so remnant radiation strikes back side of cassette first
- foil acts as filter, stopping Compton radiation
- increase in mAs needed to maintain density

Front 68

Grids do not CREATE scatter, they __________ scatter

Back 68

ABSORB (patient creates scatter)

Front 69

What kind of latitude does a high grid ratio grid have?

Back 69

Narrow concerning SID (precise) & alignment (centered & leveled)

Front 70

What size GCF do high grid ratio grids have?

Back 70

Larger (takes more mAs to get good density)

Front 71

Who are the forefathers of technique?

Back 71

Ed C. Jerman (1920s), Arthur Fuchs (1940s)

Front 72

What's the goal of any exposure system?

Back 72

Consistency

Front 73

Exposure System Selection

Back 73

- often incorporated into pre-programmed control consoles
- suggested starting points
- variables change according to anatomical part thickness

Front 74

Calipers

Back 74

- device that measures part thickness
- important QC measure when not using AEC

Front 75

What are the 2 main methods of part measurement?

Back 75

- Central ray entrance & exit
- Thickest part

Front 76

Fixed kVp Systems

Back 76

- developed by Fuchs during WW2
- reduction in patient dose, provides more info within image, increased consistency in image receptor exposure, shorter exposure times
- lengthens exposure latitude (gives variability in quantity of mAs to try to hit "sweet spot")
- Disadvantage: higher amount of scatter results in reduced image contrast

Front 77

Variable kVp Systems

Back 77

- developed by Jerman (1925)
- first exposure "method"
- used when kVp range was limited by equipment generators
- allow for small changes in exposure for small changes in part thickness, higher contrast
- Disadvantage: more patient dose, higher repeat rate, less latitude

Front 78

Anatomically Programmed Radiography (APR) Systems

Back 78

- commonly used computerized exposure control
- provides choices of anatomical parts w/ suggested exposure factors

Front 79

APR Systems & AEC

Back 79

- selection of exposure time (mAs) eliminated
- must still choose mA, kVp, distance

Front 80

Establishing Fixed kVp Exposure Systems

Back 80

- kVp held constant (mAs varies, eliminates effects of multiple variables)
- kVp has effect on contrast, type of interaction, scatter, average energy of beam
- provides consistency, user friendly

Front 81

Optimal kVp

Back 81

- maximum kVp that result in images w/ appropriate contrast
- produces lower contrast & minimum patient dose

Front 82

Establishing a Variable kVp Exposure System

Back 82

- kVp is varied while mAs is constant
- thicker body part requires higher kVp value; mAs is specified for each body part
- Disadvantage: varying kVp from image to image lessens control control over the variables it controls

Front 83

Historical Development of Radiographic Practice

Back 83

- +2 for each additional cm of tissue
- (2 kVp x part cm) + 30 = new kVp
- not accurate (often too low kVp)
- mAs values determined through trial & error

Front 84

Variable kVp System Flaws

Back 84

- 15% rule not thoroughly understood (used +/- 10 kVp as compensation for doubling or halving mAs)
- at low kVp, dramatic effect on image; not as profound at high kVp

Front 85

Establishing Stepped Variable kVp System

Back 85

- straight variable kVp charts often resulted in too much or too little contrast
- remedied w/ multiple kVp scales

Front 86

3 Criteria of kVp Scale

Back 86

1) All film contrast is acceptable to radiologist
2) Small part kVp recommendations provide adequate penetration
3) Large part kVp recommendations limit scatter & fog

Front 87

mAs Scale

Back 87

Step 1: acceptable kVp range = 70-84 kVp (for all part thickness)
Step 2: must compensate w/ mAs via 15% rule
Step 3: fine tuning (measure w/ calipers)

Front 88

Automatic Exposure Controls (AEC)

Back 88

- devices designed to reproduce images w/ consistent density & contrast regardless of differences in anatomical sizes or textures of same anatomical part
- Goal: reproducibility & consistency btw. patients & technologists

Front 89

AED (Automatic Exposure Devices)

Back 89

- phototiming: older method of AEC
- ionization chambers: most current method used

Front 90

Phototimers

Back 90

- use a photomultiplier tube to detect the amount of remnant radiation exiting from the patient
- phosphor screen emits light when struck by radiation, light gathered by tube & converted to electric current, charges the capacitor, reaches a preset level, terminates exposure (quick process)
- cannot use a lead-backed cassette w/ these

Front 91

Ionization Chambers

Back 91

- placed between the patient & the IR (measures remnant radiation leaving patient)
- filled w/ air; measure amount of ionization that occurs in air within chamber
- terminates exposure when predetermined amount of air has been ionized
- typical 3 chamber configuration seen in clinic

Front 92

What is a critical skill to have when using ionization chambers?

Back 92

Critical positioning skills are needed to place the area of interest over the correct ionization chamber

Front 93

The use of AEC requires the art of ________________

Back 93

Positioning

Front 94

What does the AEC do when all 3 chambers are selected?

Back 94

Averages the signals from the cells; when appropriate density is reached the exposure is terminated

Cell receiving most radiation will contribute the greatest electrical signal

Front 95

Density Controls

Back 95

- regulate image density (-2, -1, 0/N, +1, +2)
- DENSITY CONTROLS SHOULD NOT BE USED TO COMPENSATE FOR PATIENT PART THICKNESS OR KVP CHANGES

Front 96

What will poor patient positioning increase while using AEC?

Back 96

Repeat image rate

Front 97

Collimation w/ AEC

Back 97

- do NOT collimate close to areas of ionization chamber
- too wide of collimation will produce scatter radiation that may undercut the patient

Front 98

Minimum Response Time

Back 98

Time necessary for AEC to respond to the ionization & send a signal to terminate exposure (0.001 second)

Front 99

What's the purpose of a back-up timer?

Back 99

Prevents extreme over exposure of the patient to radiation

Should be set to 150% of anticipated manual exposure time

Front 100

Anatomically Programmed Radiography - Cookbook Radiography

Back 100

Specific "recipes" are used for different sized patients

Front 101

Using Film/Screen Combos w/ AEC

Back 101

- most AEC's are calibrated to a particular film/screen speed
- changing the speed will result in undesired high/low density unless setting are changed to accommodate the speed change

Front 102

Extremity cassettes used in conjunction w/ AEC calibrated for par screens will result in __________________ (overexposed/underexposed) films

Back 102

Underexposed

Front 103

Is 100 speed considered a higher or lower speed?q

Back 103

Lower (less than 400)

Means that sensitivity to radiation is lower, need more radiation to get proper image

Front 104

AEC Don'ts

Back 104

DON'T use AEC on extremities such as hands/feet/ankles

DON'T increase mA to increase the density of image

DON'T increase the kV to increase the density of the image