image quality and display

Question 1

what can subject contrast be increased by?

Answer 1

-Lowering X-ray beam energy
-reducing kVp & filtration
-this results in higher patient dose compared to a higher kV and lower filtration achieving the same detector dose
-Reducing the effect of scatter

Question 2

Increasing the X-ray beam energy by increasing kV will…

Answer 2

-reduce subject contrast
-reduce the patient dose considerably (if the same dose to the detector is achieved compared to lower kV and reduced filtration)
-increase the effect of scatter
-this may be reduced using anti-scatter measures

Lost contrast could be partly recovered using digital image processing

Question 3

what does scatter depend on

Answer 3

Scatter depends on:
-X-ray beam energy
-image field size
-thickness of irradiated area

It is produced in the patient via Compton scattering
Scatter reduces subject contrast

Question 4

how can scatter be reduced?

Answer 4

Scatter can be reduced using
A grid
An air gap (not covered)
Reducing the thickness of material – for example by compressing the breast during a mammogram
Reducing the area irradiated using collimation

Question 5

what is a grid?

Answer 5

A grid is a device which is placed between the patient and the image receptor
Its purpose is to
transmit as many useful (primary) image forming X-ray photons as possible
reject (absorb) as many scattered (secondary) X-ray photons as possible – leading to better contrast

Question 6

what is a scatter reduction gird usually recommended for?

Answer 6

field sizes > 10 cm2
high kVp (though still used for low kVp techniques e.g. mammography)
soft tissue structures

Question 7

scatter reduction grids

Answer 7

Grids do not eliminate all the scattered X-ray photons
Grids do not transmit all the primary X-ray photons so the exposure must be increased to compensate
Grids increase patient dose
Grids improve subject contrast

Question 8

contrast and XR detectors

Answer 8

-The detection and subsequent display of radiographic image contrast is dependent on
-the contrast resolution of the detector which in turn depends on the bit depth
-bit depth – how many levels of grey that can be represented by the detector

Question 9

what can low bit depth lead to

Answer 9

The detection system must be able to accurately represent the subject contrast available in the latent image
Low bit-depth leads to quantization errors – a discrepancy between the true value of the detector pixel signal and that represented by the digital value resulting from the ADC
These errors can lead to contouring in smooth structures and added image noise
Generally, 10 or 12-bit images are used in radiography

Question 10

window and level

Answer 10

Reducing the window width
Increases the contrast of the structures that have detector pixel values within the window
Reduces the range of detector pixel values displayed on the image
Changing the level moves the window left and right to select a different range of detector pixel values.

Question 11

display contrast

Answer 11

-Displayed contrast depends on many factors including:
-image processing applied prior to display
-the quality of the display monitor
-maximum brightness
-number of grey levels available
-calibration of the monitor
-use of a suitable display curve relating digital values to grey level

Question 12

what are the sources of image unsharpness

Answer 12

Geometric
Detector layer
Pixel size unsharpness
Movement unsharpness

Question 13

geometric unsharpness (penumbra)

Answer 13

refers to the blurring of the edges of an image. This blurring arises due to the geometry of the imaging setup.

this can be due to:
-source size
-Source-to-Object Distance (SOD)
-Object-to-Detector Distance (ODD)

Geometric unsharpness results from having a finite sized X-ray tube focus
Geometric unsharpness imposes a limitation on the imaging system in being able to resolve fine details

Question 14

How can geometric unsharpness be reduced?

Answer 14

Reducing the object to detector distance
Increasing the focus to object distance
Reducing the focal spot size

Question 15

Detector layer unsharpness

Answer 15

-Light scatter in phosphor-based detectors is a source of image unsharpness
-Generally, the thicker the phosphor the more unsharpness produced
-Decreasing the thickness of the detection layer increases the resolution, but decreases the absorption efficiency resulting in more image noise
-The noise can be reduced by increasing the exposure, however this will lead to an increase in patient dose

Question 16

detector pixel unsharpness

Answer 16

-Pixel pitch is the distance between the centre of two adjacent pixels (aka pixel size)
-Reducing the pixel pitch could allow smaller objects to be discriminated in the image
-The pixel pitch sets the upper limit for the spatial resolution of the X-ray imaging system

Question 17

what does reducing the pixel pitch of the XR detector do?

Answer 17

Reducing the pixel pitch of the X-ray detector will generally increase the resolution of the final image only if the fine details in the latent image have not already been lost by detection layer blurring

Reducing the pixel pitch of the X-ray detector requires that the exposure is increased to compensate for the loss of area in order to produce the same signal level from the pixel – this will increase dose to the patient

Question 18

movement unsharpness

Answer 18

-Caused by movement of anatomy during the exposure relative to the source and detector
-Minimized by:
-reducing exposure time (this may impact focal spot selection and increase geometric blurring
-getting the patient to hold their breath
-immobilizing the patient

Question 19

resolution test objects

Answer 19

-Typical resolution test pattern
Groups of lines etched in lead and encased in a plastic support
-Each group is made up of equally spaced equal width lines of lead and air
-The (spatial) frequency of each group is different
-The spatial frequency is quoted in line pairs per mm: LP/mm
-For the pattern shown the spatial frequency extends from 0.1 – 4.88 LP/mm

Question 20

Image noise

Answer 20

-superimposes a random pattern over the image
-if strong enough this pattern can obscure anatomical details such that features may be undetectable

Question 21

sources of image noise

Answer 21

-pattern noise e.g. faulty detector elements, unmatched line amplifiers, etc.
-system noise
-electronic noise
-e.g. amplifiers, ADC, radio interference, etc.
-quantum noise – resulting from using a finite number of X-ray photons to form the image

Question 22

what is quantum noise

Answer 22

-In a properly designed and operated X-ray imaging system the dominant noise source is called quantum noise
-Quantum noise arises from using a finite number of X-ray photons when forming an image
-During an exposure, X-ray photons in the beam emerging from the X-ray tube are randomly distributed in time and space and therefore the beam intensity arriving at the image detector is not uniform across the image field
-The beam intensity across the image field fluctuates about a mean value. The size of these fluctuations is termed the image noise.

Question 23

what is a quantum limited system

Answer 23

-A system where the image noise is the dominated by the quantum noise

-The effects of image noise can be reduced by increasing the beam intensity - using more information carriers (X-ray photons) results in a better representation of the latent image and is therefore less noisy. However this will increase patient radiation dose.

-Image noise in the final image is a sum of the quantum noise and the other system noise sources. However, it should only ever be the quantum noise that is limiting not the system noise