X-ray Detection & Aspects of Image Formation

Question 1

creating a digital image

Answer 1

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

Question 2

what is a latent image

Answer 2

Latent - (of a quality or state) existing but not yet developed or manifest; hidden or concealed.

Question 3

what is a digital XR image receptor responsible for?

Answer 3

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 4

A simple image receptor

Answer 4

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

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 5

creating a digital image- pixels

Answer 5

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 6

creating a digital image- size of the displayed image

Answer 6

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 7

creating a digital image- grey level values

Answer 7

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

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 8

grey level range (pixel depth)

Answer 8

Each pixel must take on a discrete value. Use of 1 bit (binary digit) to “code” for the pixel value means that the pixel can take on one of two discrete values (1 or 0), which would correspond to a pixel that is either black (0) or white (1). If two bits are used to code for a pixel, then four discrete values are possible (i.e., 00, 01, 10, and 11).

In general, if n bits are used to code for one pixel, the number of discrete values is 2n. 8 bits (equal to one Byte) can code for 256 discrete values (shades of gray); adding an extra bit will double the number of discrete values (i.e., 9 bits codes for 512 shades of gray), whereas subtracting one bit halves the number of shades of gray (i.e., 7 bits allows 128 shades of gray).

Question 9

energy band concepts of conductors

Answer 9

in a conductor. there are no band gaps between the valence and conduction bands.
In some metals the conduction and valence bands partially overlap. This means that electrons can move freely between the valence band and the conduction band

Question 10

energy band concepts of insulators

Answer 10

An insulator is a material that does not allow charge or heat to pass through it easily. has a large gap between the valence band and the conduction band.
The valence band is full as no electrons can move up to the conduction band. As a result, the conduction band is empty.

Question 11

energy band concepts of semiconductors

Answer 11

-In a semiconductor, the gap between the valence band and conduction band is smaller. At room temperature there is sufficient energy available to move some electrons from the valence band into the conduction band. This allows some conduction to take place.

An increase in temperature increases the conductivity of a semiconductor because more electrons will have enough energy to move into the conduction band.

Question 12

energy band concepts of lumicescence

Question 13

energy band concepts of photo stimulated luminescence

Question 14

computed tomography principles

Answer 14

-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 15

phosphor plate

Answer 15

-Transparent protective layer
-phosphor crystals with doping (activating element).
-Light reflective layer
-Conduction layer to eliminate static
-Support for strength
-Light shielding layer
-Backing layer

Question 16

what do flat panel (FP) detectors consist of?

Answer 16

-XR photons
-conversion layer
-amplifiers and ADC’s
-a-Si TFT active matric
-Timer and line driver electronics

Question 17

indirect detectors:

Answer 17

-CsI scintillator – converts X-ray photons to light
-CsI scintillator – converts X-ray photons to light
-Readout electronics

Question 18

direct detectors:

Answer 18

-a-Se photoconductor converts X-ray photons to electron-hole pairs
-Readout electronics

Each absorbed X-ray photon in the a-Se layer creates about 1000 charge carriers.
The charge reaching the active matrix is stored on capacitors
Read out then proceeds in much the same was as with indirect detectors

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

Question 20

FP imaging chain

Answer 20

-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 21

FD image corrections

Answer 21

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

Question 22

what is geometric magnification:

Answer 22

the ratio of the actual size of the sample to the size of the X-ray image projected on the X-ray camera.

Question 23

what are the assumptions of geometric magnification

Answer 23

-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

Question 24

What are some key points about geometric magnification

Answer 24

-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

Question 25

geometric magnification practical cases

Answer 25

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 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 26

what are different types of XR beam collimation devices

Answer 26

-cones
-iris diaphragm
-plate diaphragm

Question 27

Xr beam centring devices

Answer 27

Light beam diaphragm
Varay lamps
(e.g. some skull units)
Laser beams
(e.g. Computed Tomography)