X-ray Detection systems (screen and digital)

Question 1

What is the difference between screen film and digital radiography?

Answer 1

Screen film:
plastic film held up to a light box (cannot be digitally manipulated).

Digital:
digital system with pixel values (can be manipulated) uses cassettes and scintillation
1. Computed radiography: make an image in a cassette that can be processed into a digital copy (analog storage and uses phosphorescence delay in converting energy into light)
2. Digital radiography: refers to way process and store the image
a. Indirect: uses scintillation and fluorescence (immediate conversion of x-rays into light). X-ray photons hit a scintillator layer, which then releases light photons that can hit an active matrix array that digitises the signal
b. Direct: doesn’t require scintillation – photons act directly on a photoconductor layer producing positive and negative charge. The negative charge is attracted to a charge capacitor that stores the latent image. It is then read out by TFT switches pixel by pixel.

Question 2

CR: what is the definition? How does it store energy from the incident x-ray? What is the plate made up of?

Answer 2

Definition: make an image in a cassette that can be processed into a digital copy (analog storage and uses phosphorescence delay in converting energy into light)
-Contains a photostimulable phosphor plate to store energy form the incident x-ray  plate requires light input to release the trapped energy (proportional to x-ray intensity).
-Plate usually made of barium fluorohalide dopped with europium, with surface coat and plate contained in light-tight cassette.

Question 3

Roughly outline process for revealing an image in CR (and erasing the plate)

Answer 3

-X-ray photons absorbed into phosphor crystal: gives rise to a high energy photoelectron. This ionises atoms along it’s track and releases thousands of electrons. Electrons become trapped producing latent image.
-Uses phosphorescence (delayed light production) – commonly barium fluorohalide with divalent europium ions embedded in a polymer binder with the top surface protected by a layer of toughened plastic. Usually 0.3mm thick.
-Scanning takes about 30 seconds.
-CR reader: plate is removed form cassette and scanned with laser beam. Red laser light used (most phosphors release blue light)  rotating mirror is used for scanning.
-Photomultiplier tubes measure the light intensity emitted from each scanned section (line by line)  amplify the recorded signal and convert this to electronic signal using analogue-digital converter.
-Plate is erased by exposing it to a bright light source.

Question 4

What image processing is done for CR? Detail some steps that occur before export to PACS

Answer 4

-Photostimulable phosphors have a very wide dynamic range (10 000:1)
-Initial image processing before export to PACS:
* Ignore signals outside collimation area
* Histogram analysis of the distribution of light intensities in the collimated area
* Application of a gradation curve to optimise readability.

Question 5

How does CR spatial resolution compare to analogue film-screen radiography? Why is this? (resolution and effect of reading)

Answer 5

-CR has poorer spatial resolution than analogue film-screen radiography due to pixel size limitation and effect of laser reading
* Larger plates: 2500 x 3070 (chest and abdo imaging) – yielding in resolution of 3.5lp/mm vs film screen allows for much better resolution eg 8-12lp/mm
* Laser reading stage affects resolution as there is inherent scatter in the phosphor layer. A thinner phosphor layer decreases scatter and improves spatial resolution. Smaller phosphor layer also improves spatial resolution.

Question 6

CR: what is a Detector dose indicator?

Answer 6

-Safety measurements to indicate the level of exposure of a radiograph. A radiograph that may have been overexposed will be automatically adjusted to look as though taken at the perfect exposure.
-ALARP

Question 7

CR: what is type the relationship between contrast and dose? (Compared to screen radiography?)

Answer 7

is dose-dependent and is linear (not characteristic like in film screen radiography).

Question 8

CR: outline some artefacts that can occur (7 in total) at the image acquisition stage

Answer 8

-Moire’ pattern: interference between grid and laser scan lines. Need to use grids with >60lines/cm
-Ghost artefact: imaging plate was not erased after previous use
-Fading of image: delay in acquisition and processing
-Light bulb effect: backscattered radiation
-Over and under exposure
-Cracks or focal radio-opacities: fault with imaging plate or dust.
-Linear radio-opaque or radiolucent lines: malfunctioning plate reader.

Question 9

What do Moire’ and light bulb artefacts look like?

Answer 9

Question 10

Digital radiography: difference between direct and indirect

Answer 10

Indirect systems
-Use a phosphor to absorb x-rays and release light photons which produce the image (eg layer CsI to convert x-rays into light before capturing the light photons via photodiodes in a TFT array or via tiled charge couple device (CCD) detectors.

Direct systems
-Amorphous selenium is used to allow the direct conversion of the x-ray photons to a charge captured by the TFT array.

Question 11

DR Indirect (DRI): describe important features of scintillation layer - function, material. What is the effect on spatial resolution?

Answer 11

Scintilaltion layer:
-Scintillation layer converts photons into light.
-Uses caesium iodide to long tubular crystal structures where single x-ray photon is converted into light and funnelled down a small area.
-Improves spatial resolution but creates a much less intense light signal than gadolinium oxysulfide.

Question 12

DRI: what is the role of the active matrix? What is it made of?

Answer 12

Active matrix
-Made of layer of hydrogenated amorphous silicon and forms the readout electronic.
-Each pixel has: photodiode (amplifies signal from incident light photons), charge capacitor (stores signal of latent image), thin film transistor (TFT switch)

Question 13

DRI: What is a fill factor? Equation? What is the relationship between pixel size and fill factor

Answer 13

-TFT and charge storage capacitor take up a small area of each pixel, prevent formation of image in this area.
-Fill factor = sensitive area/overall area
*Decreasing pixel size improve resolution but circuitry size stays the same t/f efficiency of the array changes.

Question 14

DRI: describe image formation in indirect DR using TFT

Answer 14

1. CsI:TI absorbs x-ray photons and releases light photons
2. These light photons are then absorbed in the photodiodes and the charge stored in the charge storage capacitor at each pixel location
3. The latent image is read out sequentially to a bank of charge sensitive amplifier (TFT switches)
4. The resulting voltage signal is then digitised and transferred to the system computer where the DR image is built up

Question 15

DRI: what other system is there (other than TFT?) Describe the MOA briefly

Answer 15

Charged coupled device (CCD) chip
-Take light and convert to a digital signal.

MOA CCD Indirect DR system:
-Scintillation layer converts x-ray photons into light photons.
-Coupling layer couples light photons to CCD chip.
-Light photons are converted into electrons at CCD chip.
-As electrons are released they stay in dexels (separated by voltage gates) until these are read sequentially by row to digitise the signal into a pixel value.
-The more electrons available: the darker the image appears.

Question 16

DRI: how do TFT and CCD MOAs differ?

Answer 16

-Differs from CCD as in CCD the charged electrons are released through voltage gates  in TFT light photons are converted to electron current via photodiode layer  this is then diverted to the capacitor which can be released when the TFT array is switched on.

Question 17

Digital Radiography Direct (DRD): what are the differences with indirect? What components does it share with indirect? What is the most common photoconductor?

Answer 17

No scintillation required: converts x-ray energy directly into an electronic signal.
Shares the DEL component with the indirect TFT array.
This directly converts x-ray photon energy into free electrical charge carriers (electrons and holes) i.e. the “middle-men” or light photons, are cut out. The most commonly used photoconductor is amorphous selenium (a-Se).

Question 18

DRD: outline how the image is formed

Answer 18

1. X-ray photon absorbed by a-Se photoconductor
2. Electrical charge carriers (negative electrons and positive holes) are created in the a-Se
3. A surface electrode at positive potential attracts and discards all the electrons
4. The positive charges are drawn to the charge storage capacitor forming the latent image
5. The latent image is then read out sequentially by gating each row of TFT switches (each TFT corresponds to one pixel) in turn to read the charge pattern and transfer to a bank of charge sensitive amplifiers
6. The resulting voltage signal is then digitised and transferred to the system computer where the DR image is built up
7. Post-processing

Question 19

DRD: describe 5 artefacts in DRD (detector drops, backscatter, image saturation, ghost image and irregular shading)

Answer 19

-Detector drops: white or black artefacts after individual pixels/electronics are damaged.
-Backscatter: backscattered radiation from detector electronics is visible on image
-Image saturation: loss of information when the processing algorithm dynamic range is exceeded.
-Ghost image: previous image not cleared
-Irregular shading across the field: non uniform variations of the sensitivity or grain of the x-ray absorption layer

Question 20

DRD image processing - correction. What is gain calibration?

Answer 20

Gain calibration: uses previously acquired mask image comprising an image acquired with a uniform x-ray beam and subtracting this gain mask image from the patient’s image

Question 21

DRD image processing - correction. What is pixel calibration?

Answer 21

Pixel-calibration: defects in pixel array can be corrected by interpolating the data values of neighbouring pixels which are functioning correctly using a reference map

Question 22

DRD image processing: what is auto-ranging?

Answer 22

Automated analysis of the image histogram: excludes very high and low values which would otherwise adversely affect the image contrast and brightness. The remainder of the histogram is normalised to maximise image display. The data needs to be matched to the display device.

Question 23

DRD image processing: outline the steps of auto-ranging

Answer 23

1. Identification of relevant image field
2. Generation of a histogram of the data representing the number of pixels at each grey-scale value
3. Analysis of the histogram to exclude ranges of data which contain no clinical information (very high and low values)
4. Selected grey-scale range normalised to match the display image

Question 24

Windowing: what does changing window width do? What does having a large window width do to the range of shades displayed and how does this affect contrast?

Answer 24

-Window width: alters the image contrast. The larger the width, the larger the range of shades displayed and t/f the smaller the difference in contrast between each shade (eg lung and bone window) – results in a smaller difference (ie contrast) in the grey value between the represented Hounsfield units.

Question 25

Windowing: what does changing the window do?

Answer 25

Alters image brightness

Question 26

What does a histogram do? What does it allow in terms of brightness and contrast? What is it used for?

Answer 26

-Histogram demonstrates graphically the number of pixels for each allotted brightness level.
*Alterations to the histogram allow for brightness and contrast manipulation, as well as compressing the range of values displayed.
-Used for greyscale processing

Question 27

Filtering: describe convolution

Answer 27

Convolution: mathematical process involving a predefined Kernel. This process adds weighted values of each surrounding pixel to the original

Question 28

Filtering: describe low pass filtering

Answer 28

Low pass filtering to smooth an image: technique where the greyscale value stored in each pixel is increased by a proportion of the value of the neighbouring pixels and the resultant value is averaged. This causes to smooth the final image but blurs some details.

Question 29

Filtering: describe High pass filtering:

Answer 29

for edge enhancement – high pass filter adds in a proportion of the difference between the greyscale value of the pixel and that of it’s neighbour. Effect is to exaggerate the contrast at the boundary between structures but also generates more noise/false structures.

Question 30

Filtering: describe temporal averaging

Answer 30

uses multiple sets of the same image acquired over time to reduce noise in the image.

Question 31

Quality Assurance (QA): what is the difference between level A tests and level B tests?

Answer 31

Requirement of IRR 1999
-Level A tests: quick/simple, done frequently by equipment user
-Level B tests: take longer, more expensive, done by medical physics department/manufacturer engineers.

Question 32

QA: how do you measure x-ray tube output? How often should it be done?

Answer 32

-1-2 months
-Ionisation chamber placed at known distance from xray tube – measures dose using electrometer to determine: dose per mA, output, repeatability, consistency

Question 33

QA how often should you measure x-ray tube kV? how? What is the allowed deviation from baseline?

Answer 33

-Tested 1-2 years
-Potential measured using electronic kV meter.
-Should be within 5% or +/- 5kV from baseline

Question 34

QA: automatic exposure control - how is this measured?

Answer 34

-AEC terminates exposure once film has received appropriate level of dose.
-Should produce consistent optical density in film for wide range of kV/patient thicknesses.
-Use simulated pt.

Question 35

QA: light beam alignment - how often and how is it tested? What is the allowed variation from baseline?

Answer 35

-Tested 1-2 months
-Edges of light beam marked on film and film exposed. Compare area of exposed field with light field
-Should be within 1cm at 1m from focal spot.

Question 36

Name 2 tests to measure focal spot

Answer 36

-Pinhole test: can estimate size, shape and irregularities this way
-Star test: can calculate focal spot size