Brainscape's Knowledge GenomeTM

Browse over 1 million classes created by top students, professors, publishers, and experts.


See full index

Physics > CR & DR > Flashcards

CR & DR Flashcards

1
Q

Indirect DR

Scintillation layer?

Absorption efficiency?

Thickness of scintillator layer?

A

85% absorption efficiency of Caesium iodide crystals

Thickness is 500micrometres (µm)

  • Caesium iodide crystal internally reflect light thus reducing spread of light in the phosphor layer
  • Caesium iodide crystals are grown PERPENDICULAR to the detector surface

Gadolinium oxysulphide is an alternative to caesium iodide

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
2
Q

Indirect DR

Photodiode & Active Matrix?

A

Amorphous Silicon

  • has 100% efficiency
  • Converts light into electrical charge
  • 1 photon produces = 1500 charge storage carriers
  • Cheap

Active matrix:

  1. Photodiode
  2. TFT
  3. Capacitor (stores charge)

Fill factor = sensitive area/whole area

(Increasing fill factor increases sensitivity)

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
3
Q

Direct DR

Direct conversion of x-ray photons to electrical charge

?Photoconductor

A

Amorphous selenium

  • 500µm thick (0.5mm)
  • Absorption of x-ray photon releases positive and negative charge carrier pairs

Positive holes carry the image information

  • negative electrons are attracted away

There is NO potential applied to increase the charge

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
4
Q

Indirect DR Spatial resolution and DQE?

A

Indirect:

  • better DQE than CR and direct DR (caesium iodide has higher K edge than selenium so can absorb more photons)
  • lower dose for same image quality
  • higher SNR
  • worse spatial resolution due to scattering of light in scintillator)

Direct:

  • lower SNR
  • better spatial resolution (no scattering)
  • higher dose needed than indirect
  • better pixel fill factor
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
5
Q

DQE and Spatial resolution???

DR

vs

Computed Radiography

A

DQE:

  • 60% for DR
  • 40% for CR

Spatial Resolution

  • DR is 3-4lp/mm
  • CR is 5lp/mm approx
  • Film screen higher again

Noise can be reduced by low pass filtering

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
6
Q

Mammography

A
  • Angled tube
  • Fixed focus to detector distance
  • Fixed field size - 65cm
  • Compression device (reduces dose and geometrical unsharpness)
  • Moving grid is used
  • Magnification can be achieved by increasing the object to detector distance*
  • Exposures are LONG - up to 2 seconds

Dosage is 1 - 1.3mGy per breast

Beryllium can be used where inherent filtration needs to be minimised (Atomic number 4)

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
7
Q

Mammography - Spatial Resolution

A

Digital mammography - 7-10lp/mm

Film screen - 15lp/mm

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
8
Q

How does sampling frequency affect spatial resolution and sensitivity??

A

Increasing sampling frequency increases spatial resolution (think of it as increasing number of pixels)

Increasing sampling frequency DECREASES sensitivity

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
9
Q

Digital Image

A
  • 12 bits usually required
  • Image displayed as DISCRETE grayscale values
  • Number of bytes depends on the
    • number of pixels
    • matrix size
    • (NOT PIXEL SIZE)

A wide range of data is obtained during exposure which needs to be processed. This is done by Autoranging

Autoranging is a process where only the most important parts of the data range are displayed. This is done automatically when the type of examination is chosen e.g CXR or AXR

  • automatic process
  • does NOT affect spatial resolution

Images of a larger MATRIX will tolerate compression better than a small matrix

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
10
Q

Exposure Index

A

A measure of the dose incident on the image plate

  • The relationship of dose and exposure index is not linear
  • Can vary between manufacturers
    *
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
11
Q

Absorbed Dose

A

A measure of energy absorbed per unit mass of medium

  • Measured in grays
  • Measured in joules/kg
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
12
Q

Radiographic Image

A
  • Human eye can differentiate 15-28 shades of grey
  • Can detect min optical density of 0.04
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
13
Q

Nyquist frequency

A

Is the max frequency that can be sampled

  • It is equal to half the sampling frequency
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
14
Q

MTF

A

MTF goes down as spatial resolution (spatial frequency goes up)

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
15
Q

Computed Radiography plate

A
  1. 0.3mm thick
  2. Barium fluorohalide doped with europium is 85% bromide and 15% iodine
  3. CR reader uses a ROTATING mirror
  4. Plate is wiped before use with bright light
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
16
Q

Pixel size calculation

A

Pixel size = FOV/number of pixels

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
17
Q

File compression techniques

A

Can be:

  • Lossy
  • Lossless

Not all result in loss of data

18
Q

PACS

DICOM

?

A

Picture archiving and communications system

Digital imaging and communications in medicine services

19
Q

Pixel Sampling interval and Fill Factor

A

Fill factor (sensitive area) increases as pixel sampling interval increases

Sampling interval is number of pixels and space between them

20
Q

DQE

A

Is ratio of SNR (output:input)2

Dependent on:

  • radiation exposure
  • MTF
  • Detector material
21
Q

DR Detector

A
  • Amorphous SELEnium when irradiatied produces an electrical charge
  • Upper electrode is POSITIVE (high positive potential) and attracts all negative electrons to waste them
  • Positive charges are then drawn to the TFT array
    *
22
Q

Is geometric unsharpness less towards anode side of image?

23
Q

Is movement unsharpness an issue in Mammography?

A

Yes due to long exposure times.

Can be greater than 1 second

24
Q

Normal anode angle?

A

7 - 20 degrees

25
Q

Does focal spot size affect contrast or scatter?

A

NO it doesnt

26
Q

What increases focal spot size?

A

Anode angle

Length of cathode filament

Width of focusing cup

27
Q

What makes heel effect worse?

A
  • Smaller anode angles (thicker material to get through)
  • Low kVP
  • Worn out anodes

(Anode rotation speed does NOT directly affect the heel effect)

28
Q

How does automatic exposure control work?

A

Uses an ionisation chamber placed close proximity to the detector

Terminates exposure when sufficient qualtity or radiation reaches plate

29
Q

Does more scatter or primary radiation reach detector when taking x rays?

A

Always more scatter than primary beam

30
Q

Does collimation reduce scatter and dose?

A

Yes

It reduces area irradiated therefore reducing both

Have NO EFFECT on filtration

31
Q

Does a tube with a high frequency generator require less filtration than rectified tubes alone?

A

Yes

High frequency generators produce harder beam as kVP is always just a few percent of max (higher AVERAGE kVP)

32
Q

Does increasing focus to film distance reduce the dose to patient and ESD?

A

Yes even despite the resultant increase in mA.

Dose is still decreased. An ESD is less as beam more spread out

33
Q

Which LAC is higher - actual LAC or compton LAC?

A

Actual LAC is always higher

34
Q

In what type of grid does cut off limit max field of view?

A

This happens in parallel grids!

35
Q

In what grids do cut off limit the focus to detector distance?

A

Focused grids

High ratio grids need to be accurately focused

**Grid lines can occur in both moving and stationery grids**

36
Q

What is Grid (Bucky) factor?

A

Amount of radiation needed with a grid / Without a grid

37
Q

What distance is needed in an air gap?

A

At least 30cm

38
Q

Does target material influence the max photon energy in the spectrum graph?

39
Q

How is focal spot size measured?

A

Star test object

Pinhole camera

40
Q

What kvP accuracy is accepted?

A

Usually +/- 5kV or 5% nominal value

Physics (10 decks)

Brainscape helps you reach your goals faster, through stronger study habits.
© 2025 Bold Learning Solutions. Terms and Conditions