X-ray Detection & Aspects of Image Formation

Question 1

Creating a Digital Image

Answer 1

We have seen that when we expose a patient to a uniform beam of X-ray photons, the photons are selectively attenuated depending on the anatomy of a patient.
This creates a beam whose intensity varies as a function of spatial location
We refer to this as the latent image
Latent - (of a quality or state) existing but not yet developed or manifest; hidden or concealed.

Question 2

What is a digital X-ray image receptor ?

Answer 2

A digital X-ray image receptor (or detector) is responsible for detecting the X-ray photons incident upon it and forming a digital image from the latent image
The image is divided into pixels, and each pixel has a value proportional to the X-ray dose incident upon it

Question 3

A Simple Image Receptor

Answer 3

Energy from X-ray photons incident on a pixel are converted to electrical charge in the pixel electronics
A number of intermediate stages may be involved
This charge is passed via an amplifier into an analogue-to-digital convertor

Question 4

Entrance Surface Dose Incident on Detector Pixel Element and Digital Value relationship

Answer 4

Pixel response to dose is usually linear, but can have a logarithmic relationship in some cases.
The gain of the amplifier defines the sensitivity of the imaging system

Question 5

A Simple Image Receptor: The digital (number) value

Answer 5

The digital (number) value produced is proportional to the energy absorbed by the pixel
In practice a square matrix of pixel detector elements is used to produce digital values that are stored in a computer
Typical detector pixel dimensions are 0.1 – 0.2 mm square

Question 6

Creating a Digital Image: pixels

Answer 6

These stored values can be displayed on a monitor
Each pixel element of the image has an associated X and Y position in a matrix
The value of each pixel (i.e. the digital number) defines the shade of grey which is displayed

Question 7

Creating a Digital Image: the size of the displayed image

Answer 7

Note that the size of the displayed image is not generally the same as the size of the detected image
once stored in a computer the image is merely recorded as numbers in a matrix (sometimes called an array)
these numbers can be represented in many different ways before displaying the final image (magnified/reduced)

Question 8

Creating a Digital Image: grey level values

Answer 8

Note that the grey level values (displayed pixel values) of the displayed image are not generally the same as the pixel values initially recorded for the detected image

once stored in a computer the image is merely recorded as numbers in a matrix (sometimes called an array)

these numbers can be manipulated in many different ways before displaying them – for example by adding or subtracting a number to all the values to make the image brighter or darker

Question 9

Energy band concepts of conductors

Answer 9

Looking at electrons right at the outer shell of the atoms of the material.

The valence band- electrons still bound to the atoms.

Have this energy band where electrons can occupy it if they have sufficient energy of the material allowed it.

conduction band- where electrons are still in the material but they’re not bound to the individual atoms, they’re free to move around.

They are then free to conduct the flow of charge is what electrical current is all about.
There’s no gap between the valence band and the conduction band.

Some valence electrons can jump up into the conduction band, but be immediately filled with free electrons from the conduction band.

Question 10

Grey Level Range (Pixel Depth)

Answer 10

The range of values an image pixel can have is quantized and finite
Generally there will be 2N levels possible, where N is the number of binary bits used to store the value in the computer
Typical values of N being 12 or 14
The range will go from 0 to (2N-1)

Question 11

Energy band concepts of insulators

Answer 11

Insulators are different in that their valence band was full, there was no gaps and the conduction band was empty.

There was an energy zone whereby electrons just couldn’t occupy you would need to put in lots of energy into this material before you could get electrons to jump up into this conduction band..

Question 12

Energy band concepts of Luminescence

Answer 12

If electrons drop from there conduction band into the valence band and recombined with a hole, you ocould get light given off depending on what the energy gap was and that’s how light emitting diodes work and that is called luminescence.

Question 13

Energy band concepts of
semiconductors

Answer 13

The in between material is the semiconductor material so it can conduct under certain circumstances.

We do have some electrons in the conduction band.
We can increase the electrons in the conduction band by pumping little bits of energy into the system. And the electrons jump into that conduction band even at room temperature.

A few vacancies (gaps) in the valence band. We call that a semi conductor.

If we apply negative potential here on the left and positive on the right the electrons going to move in the right direction.

Question 14

Energy band concepts of Photostimulated luminescence

Answer 14

Photo stimulated luminescence is what we us in CR image plate readers, the modified material by using small amounts of impurities in the material to create these little charge traps which are between the valence band and the conduction band. In terms of their energy .

Electrons can be stimulated by absorbing x-ray photons into the conduction band.
But when they want to try and drop down back to the valence band we can trap them in the charge traps.
That charge can hold the charge for hours.

Those trap charges correspond to where we absorbed a lot of x-rays.

We want to read where the charge was and we can do that by using some light of a particular wavelength to give that charge a nudge out of that trap, encourage it to recombine into the valence band.

It produces some more lights, but a different colour, a different coloured light for the light which is stimulated

We use a lasor light to get those charges to drop down into the valence band and release some light of a different colour.
2 different types of light: the stimulating lasor light to get this charge to jump out of the trap,
And the stimulated the light that we stimulated are emitted and that is slightly different colour.
We can collect that light at a particular point on a CR plate

Question 15

CR Principles

Answer 15

Uses an imaging plate to store a latent X-ray image
The imaging plate is later scanned by a laser beam which causes the latent image to be released as light
This light is detected and converted to electrical charge using a photomultiplier tube
The charge is readout, amplified and converted to a digital image using an analogue-to-digital (ADC) convertor

Question 16

CR Image Acquisition

Answer 16

Image plate, expose it in this case a neon lasor of a particular wavelength and we scan our lasor across our plate as we are scanning across our plate, we are also moving the plate through the reader as well.
As that’s happening were collecting the stimulated light where the lasor beam actually was at any particular time.
You’ll be stimulating light out of the CR plate and then will use a light guide to guide those stimulated light photons into this this photomultiplier tube which converts the light into charge.
We can then convert that into a digital number by amplifying it and using an analogue to digital converter.
And we got our final image at the end which we cn display on a monitor.

Question 17

Indirect Detectors

Answer 17

Indirect
x-rays

CsI scintillator – converts X-ray photons to light

Light photons converted to electrical charge

charge- Readout electronics

2 types of detector was the direct detector which is a photoconductor (can absorb photons).
We can collect that charge and once we got charge we can convert it to a digital number.
Indirect- we us a a scintillate material which we talk about luminescence. Then we absorb an x-ray, we get light photons. We can convert that light using a photo diode that will produce charge from our light.

Question 18

Direct detectors

Answer 18

Direct
x-rays
a-Se photoconductor converts X-ray photons to electron-hole pairs

charge- Readout electronics

They have conversion layer which is a scintillator. A phosphor converts the x-ray photons to light when you’ve got something like cesium iodide is with a material called palium.
The material has grown as a layer like a crystal.
These columns channel the light down from where the initial x-ray photon was absorbed.

Question 19

Scintillator

Answer 19

The CsI scintillator used on indirect detectors is often around 500 µm thick.
This gives an absorption of the incoming X-ray beam of about 80%.
Each absorbed X-ray photon in the scintillator causes ~3000 light photons to be emitted

Thicker CsI would increase efficiency, thinner CsI would increase resolution.
A 500 mm CsI layer is well matched to a read out matrix with a pixel pitch of around 200 mm.

Question 20

Readout

Answer 20

In turn this causes release of charge carriers in the photodiode (~1500 charge carriers/X-ray photon) which are stored locally on a capacitor
TFT switches control the timing of readout, allowing the charge to be sent to a charge sensitive amplifier
The signal is then digitised by an ADC and readout to the computer for storage and display

Question 21

Readout Electronics

Answer 21

Every pixel has associated with a photo diode, storage capacitor and a switch.
The photo diode converts light to charge, that charge is stored under the capacitor.
That charge is going to be proportional to the light that we got, that light is going to be proportional to the x-ray photons, which produce light.
That store charge then is proportional to absorb x-ray photons.

When we want to read out and amplify it and digitize it we need to switch on a little transiter switch with a voltage.

We switch the waveform switch on and then we can read that charge out and amplify and digitize it.

Question 22

FP imaging chain

Answer 22

Flat panel systems have a rather more complex imaging chain than CR.
Gain and offset corrections correct for differences in amplifier response, CsI variations, etc.
Defect pixel correction is essential for adequate images.
Image enhancement is also performed prior to display.

Question 23

FD Image Corrections

Answer 23

Required for all flat panel systems and comprises:
Offset correction
Gain correction
Defect pixel correction
Requires calibration, which is performed by service engineers

Question 24

Resolution Comparison

Answer 24

Cr has poor resolution, spatial resolution
Somewhere defined- narrow beam of x-ray photons being absorbed
The light gets scattered quite a lot
Direct have the best spatial resolution as there is very little spread of the electron hole pair within the semiconductor.

Question 25

Geometric Magnification

Answer 25

Assumptions
Thin object
Object parallel to image plane
Object at centre of image field
Central ray of X-ray source normal to object and image plane
Point X-ray source

Key Points
Magnification results in size distortion of an object
Magnification is reduced by:
Decreasing distance between object and detector
Increasing the distance between the focus and the detector

Practical Cases
Real objects:
Not thin
Not necessarily at centre of image field
Central ray of X-ray source
Not necessarily normal to object or image plane
Practical X-ray tubes have finite focal spot sizes

Question 26

Geometric Magnification extra

Answer 26

Practical Cases
Real objects:
Not thin
Not necessarily at centre of image field
Central ray of X-ray source
Not necessarily normal to object or image plane
Practical X-ray tubes have finite focal spot sizes

Key Points
Magnification results in size distortion of real objects
Size distortion is reduced by:
Decreasing distance between object and detector
Increasing the distance between the focus and the detector

Magnification results in shape distortion of real objects
Shape distortion is reduced by careful arrangement of the object with respect to the image plane and the X-ray beam

Question 27

1. A radiographic image displayed on a monitor is made up of square _____ elements each having an associated ____ level. Match the correct pair of words.
   a. voxel, colour
   b. pixel, black
   c. pixel, grey
   d. voxel, grey
   e. pixel, white

Answer 27

pixel, grey

Question 28

2. The set of all grey level values available to display a digital image is sometimes referred to as the
   a. brightness
   b. value scale
   c. temperature scale
   d. greyscale
   e. contrast

Answer 28

greyscale

Question 29

3. Reducing the size of the pixels of an X-ray detector will lead to
   a. decreased image noise
   b. increased spatial resolution
   c. increased contrast resolution
   d. increased blurring

Answer 29

increased spatial resolution

Question 30

5. The total physical area imaged by an X-ray detector is given by
   a. the number of pixel elements multiplied by the area for each pixel
   b. the pixel element size squared
   c. the product of the number of pixel elements in the X direction and the number of pixel elements in the Y direction
   d. the area of the displayed image as measured on the display monitor

Answer 30

the number of pixel elements multiplied by the area for each pixel

Question 30

4. X-ray detector pixels sizes for diagnostic radiography (excluding mammography) are generally in the region of
   a. 10-50 microns
   b. 2-4 mm
   c. 0.1-0.2 mm
   d. 0.5-0.7 mm
   e. 10-30 cm

Answer 30

0.1-0.2 mm

Question 31

6. The physical size (area) of a displayed radiographic image on a monitor
   a. is generally the same physical size as the image detected in the image plane
   b. is not generally the same physical size as the image detected in the image plane
   c. is always larger than the image detected in the image plane
   d. is always smaller than the image detected in the image plane

Answer 31

is not generally the same physical size as the image detected in the image plane

Question 31

7. The numeric values that are displayed as levels of grey in a radiographic image on a viewing monitor
   a. are generally the same values of the initial digital image detected
   b. are generally not the same values of the initial digital image detected
   c. are always higher than the initial digital image detected
   d. are always lower than the initial digital image detected

Answer 31

are generally not the same values of the initial digital image detected

Question 32

9. To represent 900 different levels of grey in a digital image would require at most
   a. 6 binary bits
   b. 8 binary bits
   c. 10 binary bits
   d. 12 binary bits
   e. 16 binary bits

Answer 32

10 binary bits

Question 33

8. How many different levels can be represented using 8 (binary) bits?
   a. 16
   b. 64
   c. 256
   d. 1024

Answer 33

c. 256

Question 34

11. A latent image is stored in a computed radiography (CR) image plate as
    a. trapped photons
    b. trapped charge
    c. trapped atoms
    d. trapped protons

Answer 34

b. trapped charge

Question 35

10. Materials that have no forbidden gap between the valence and conduction band are referred to as
    a. insulators
    b. conductors
    c. semiconductors
    d. photoconductors

Answer 35

conductors

Question 36

13. The stimulated light from computed radiography image plate is collected by
    a. an electron guide
    b. a light guide
    c. a photodiode
    d. a transistor

Answer 36

a light guide

Question 36

12. Image plates in a computer radiography (CR) scanner are scanned with
    a. an incandescent light bulb focussed using a lens
    b. a laser
    c. an infrared photodiode
    d. an ultraviolet lamp

Answer 36

a laser

Question 37

14. The stimulated light from a computed radiography image plate is converted to ______ by a ________. Match the correct pair of words.
    a. heat, photo timer
    b. charge, photodiode
    c. frequency, photomultiplier tube
    d. charge, photomultiplier tube

Answer 37

charge, photomultiplier tube

Question 38

15. The wavelength of the stimulating laser used to scan a computed radiography image plate is different from the stimulated light emitted by the image plate.
    a. True
    b. False

Answer 38

. True

Question 39

16. The electrical signal produced by the photomultiplier tube in a computed radiography imaging system is converted to a digital signal using a
    a. digital-to-analogue convertor
    b. a computer
    c. an analogue-to-digital convertor
    d. a photodiode

Answer 39

an analogue-to-digital convertor

Question 40

17. An indirect X-ray detector first converts absorbed X-ray photons to
    a. electric charge
    b. low energy X-ray photons
    c. light photons
    d. protons

Answer 40

light photons

Question 41

18. In an indirect X-ray detector light photons are converted to charge using
    a. a field effect transistor
    b. a photodiode
    c. capacitor
    d. a photomultiplier tube

Answer 41

a photodiode

Question 42

19. In a direct X-ray detector absorbed X-ray photons are first converted in to
    a. low energy X-ray photon
    b. charge
    c. light photons
    d. neutrons
    e. protons

Answer 42

charge

Question 43

20. In either direct or indirect X-ray detectors the electrical charge generated by a pixel element is stored before it is read out and converted to a digital signal. The charge is stored on
    a. a photodiode
    b. a capacitor
    c. a transistor switch
    d. a photomultiplier tube
    e. a memory chip

Answer 43

b. a capacitor

Question 44

21. Size distortion in radiographic images can be reduced by
    a. increasing the distance between the object and the detector & decreasing the distance between the focus and the detector
    b. increasing the distance between the object and the detector & increasing the distance between the focus and the detector
    c. decreasing the distance between the object and the detector & increasing the distance between the focus and the detector
    d. decreasing the distance between the object and the detector & decreasing the distance between the focus and the detector

Answer 44

decreasing the distance between the object and the detector & increasing the distance between the focus and the detector

Question 45

22. The effects of shape distortion in radiographic images can be reduced by
    a. getting the patient to hold their breath
    b. using a large focal spot size
    c. careful arrangement of the object with respect to the image plane and the focus
    d. immobilizing the patient

Answer 45

careful arrangement of the object with respect to the image plane and the focus

Question 46

23. Which one of the following is not a commonly used method of centring the X-ray beam
    a. light beam diaphragm
    b. Varay lamp alignment
    c. laser beam alignment
    d. plumb line alignment

Answer 46

plumb line alignment