CR & DR

Question 1

Indirect DR

Scintillation layer?

Absorption efficiency?

Thickness of scintillator layer?

Answer 1

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

Question 2

Indirect DR

Photodiode & Active Matrix?

Answer 2

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)

Question 3

Direct DR

Direct conversion of x-ray photons to electrical charge

?Photoconductor

Answer 3

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

Question 4

Indirect DR Spatial resolution and DQE?

Answer 4

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

Question 5

DQE and Spatial resolution???

DR

vs

Computed Radiography

Answer 5

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

Question 6

Mammography

Answer 6

• 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)

Question 7

Mammography - Spatial Resolution

Answer 7

Digital mammography - 7-10lp/mm

Film screen - 15lp/mm

Question 8

How does sampling frequency affect spatial resolution and sensitivity??

Answer 8

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

Increasing sampling frequency DECREASES sensitivity

Question 9

Digital Image

Answer 9

• 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

Question 10

Exposure Index

Answer 10

A measure of the dose incident on the image plate

• The relationship of dose and exposure index is not linear
• Can vary between manufacturers
  *

Question 11

Absorbed Dose

Answer 11

A measure of energy absorbed per unit mass of medium

• Measured in grays
• Measured in joules/kg

Question 12

Radiographic Image

Answer 12

• Human eye can differentiate 15-28 shades of grey
• Can detect min optical density of 0.04

Question 13

Nyquist frequency

Answer 13

Is the max frequency that can be sampled

• It is equal to half the sampling frequency

Question 14

MTF

Answer 14

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

Question 15

Computed Radiography plate

Answer 15

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

Question 16

Pixel size calculation

Answer 16

Pixel size = FOV/number of pixels

Question 17

File compression techniques

Answer 17

Can be:

• Lossy
• Lossless

Not all result in loss of data

Question 18

PACS

DICOM

?

Answer 18

Picture archiving and communications system

Digital imaging and communications in medicine services

Question 19

Pixel Sampling interval and Fill Factor

Answer 19

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

Sampling interval is number of pixels and space between them

Question 20

DQE

Answer 20

Is ratio of SNR (output:input)²

Dependent on:

• radiation exposure
• MTF
• Detector material

Question 21

DR Detector

Answer 21

• 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
  *

Question 22

Is geometric unsharpness less towards anode side of image?

Answer 22

Yes

Question 23

Is movement unsharpness an issue in Mammography?

Answer 23

Yes due to long exposure times.

Can be greater than 1 second

Question 24

Normal anode angle?

Answer 24

7 - 20 degrees

Question 25

Does focal spot size affect contrast or scatter?

Answer 25

NO it doesnt

Question 26

What increases focal spot size?

Answer 26

Anode angle Length of cathode filament Width of focusing cup

Question 27

What makes heel effect worse?

Answer 27

* **Smaller anode angles (thicker material to get through)** * **Low kVP** * **Worn out anodes** **(Anode rotation speed does NOT directly affect the heel effect)**

Question 28

How does automatic exposure control work?

Answer 28

Uses an ionisation chamber placed close proximity to the detector Terminates exposure when sufficient qualtity or radiation reaches plate

Question 29

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

Answer 29

Always more scatter than primary beam

Question 30

Does collimation reduce scatter and dose?

Answer 30

Yes It reduces area irradiated therefore reducing both **Have NO EFFECT on filtration**

Question 31

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

Answer 31

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

Question 32

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

Answer 32

Yes even despite the resultant increase in mA. Dose is still decreased. An ESD is less as beam more spread out

Question 33

Which LAC is higher - actual LAC or compton LAC?

Answer 33

Actual LAC is always higher

Question 34

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

Answer 34

This happens in parallel grids!

Question 35

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

Answer 35

Focused grids High ratio grids need to be accurately focused **\*\*Grid lines can occur in both moving and stationery grids\*\***

Question 36

What is Grid (Bucky) factor?

Answer 36

Amount of radiation needed with a grid / Without a grid

Question 37

What distance is needed in an air gap?

Answer 37

At least 30cm

Question 38

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

Answer 38

NO

Question 39

How is focal spot size measured?

Answer 39

Star test object ## Footnote Pinhole camera

Question 40

What kvP accuracy is accepted?

Answer 40

Usually +/- 5kV or 5% nominal value