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Pre Exposure > Beam Restriction > Flashcards

Beam Restriction Flashcards

1
Q

Compton scatter is dependent on:

A

the thickness and density of the part
(more matter, more scatter)
(more available electrons)

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2
Q

Increasing kVp increases the amount of…

A

scatter relative to the photoelctric effect

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3
Q

What does the use of grids do?
(2)

A

-absorb scatter radiation within the patient
-improves image contrast at the expense of patient dose

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4
Q

Technique must be increased to compensate for the use of a grid because grids remove part of the primary beam

A

Note

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5
Q

What’s something we can do to reduce patient dose and reduce scatter radiation?

A

restrict the beam or collimate

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6
Q

What is collimation?
(2)

A

-decreases the amount of radiation that interacts with the patient
-reduces the patient dose

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7
Q

By properly collimating we..
(2)

A

-reduce the amount of scatter produced
- reduced the amount of matter that the photons can interact with

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8
Q

Collimation should be limited to

A

-1” beyond the anatomy of interest on all 4 sides on tabletop exams

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9
Q

Collimation should not exceed…

A

the recommended field size for a particular part

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10
Q

When we collmate properly we are…
(4)

A

-reducing scatter radiation
-reducing unnecessary exposure by the primary beam
-increasing contrast
-reducing patient dose

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11
Q

According to Bushong, the atomic number of material has…

A

no effect on likehood of scattering

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12
Q

Atomic number does not directly…

A

affect scatter

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13
Q

What is the device collimator?

A

the foremost method of restricting the primary beam field size

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14
Q

What is the most commonly employed beam restrictor in radiography?

A

Collimator

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15
Q

Most collimators now equipped with an accessory known as a…

A

Positive beam limitation (PBL) device

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16
Q

What does the PBL do?

A

it automatically collimates the beam to the size of the image receptor

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17
Q

What is the collimator function?
(3)

A

-Permits an infinite number of field sizes
-Provides a light source for the radiographers to properly placing the tube and central ray
-Reduces penumbra

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18
Q

What do entrance shutters do?

A

help remove off-focus radiation

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19
Q

Off focus radiation?

A

Xrays created in other parts of the tube not the anode

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20
Q

How many sets do the collimator shutters have?

A

2 sets of shutters
(sometimes 3)

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21
Q

The second set collimator shutter?

A

is the one the technologist adjusts

22
Q

Length and width of field are independently…

A

controlled by technologist

23
Q

The light field?

A

uses a light reflected off mirror to project the area of the x-ray beam

24
Q

Proper adjustment of mirror is necessary because…

A

to accurately display location of exposure field

25
Q

The light field/xray beam coincidence testing should be part of the…

A

QC program
-Needs to be accurate within +/- 2% of the SID

26
Q

The use of a collimator results in some filtration of the xray beam because…

A

the primary beam is passing through the mirror in the collimator
-About 1mm Al/Eq

27
Q

Features of collimator housing?

A

-Central ray must be marked
-Some units project location of photo timer sensors in light field
-An alignment light helps to center beam with the image receptor

28
Q

What is penumbra?

A

a geometric unsharpness around the periphery of the image also known as edge unsharpness

29
Q

X-rays photons are being created in all areas of the focal spot rather than at just a single point

A

note

30
Q

Xrays originating from slightly different spots on the anode take slightly different angles to hit the same point on the detector

A

note

31
Q

Primary photons diverge from the tube at varying angles and intersects with…

A

structures of interest at varying angles when creating image

32
Q

What does the shutter reduce?

A

the penumbra along the periphery of the beam because of their greater distance from the focal spot

33
Q

Closer to the light source it looks blurry and the father away from the light source…

A

you have sharper edges

34
Q

What are other beam restriction devices?

A

Aperture diaphragms and cones/cylinders

35
Q

The cones/cylinders are devices that are historically…

A

used to restrict the beam but are not in common use today

36
Q

What is the aperture diaphragm?
(Simplest of all beam-restricting devices)

A

is a flat sheet of metal, usually lead with an opening cut in the center and attached to the x-ray tube head

37
Q

Disadvantages of aperture diaphragm?

A

Greater amount of penumbra and off focus radiation due to the proximity to the tube port

38
Q

Aperture diaphragm allows…

A

-field to cover area just smaller than image receptor
-can be supplemented with cones and cylinders

39
Q

Cones and cylinders are essentially…

A

circular aperture diaphragms with metal extensions

40
Q

A cone has an…

A

extension that flares or diverges with the upper diameter smaller than the bottom flared end

41
Q

A cylinder does not..

A

flare

42
Q

What are cones and cylinders used for?

A

-for greater detail visibility in small area
-reduction in scatter which make images appear more sharper

43
Q

Some applications of cones and cylinders are?

A

-Head work: sinus or skull
-Spine work: spot films
-Cross: table hip
-Dental

44
Q

Cylinders reduce the amount of irradiated material and therefore…

A

-reduce patient dose
-reduce scatter
-increase contrast

45
Q

Ideally the beam restrictors decrease field size to anatomy of interest and also?

A

-unnecessary tissue exposure decreases
-Scatter decreases
-Contrast increases (Scale of contrast shortens)
-Visibility of detail increases

46
Q

What is ancillary devices?

A

-Devices tailored for a specific use during given procedure
-designed to restrict the beam to a specific shape for particular examination

47
Q

Examples of ancillary of devices

A

Lead blockers and lead masks

48
Q

What is a lead blocker?
Will help absorb scatter that is produce in the soft tissue of the patient’s back

A

its placement of it on a radiography table during radiography of a lower spine in lateral position

49
Q

Lead mask is usually cut to correspond…

A

to the field size desired and is then secured to the end of the collimator.

50
Q

Lead mask were sometimes used during cerebral angiography to help improve…

A

image quality by reducing scatter

Pre Exposure (10 decks)

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