X-rays - Image quality

Question 1

How does scatter affect image contrast? Hows does the amount of scattered radiation reaching the detector compare to primary radiation?

Answer 1

The amount of scattered radiation (S) reaching a detector is usually several times that of the primary radiation (P)

Scatter reduces image contrast by a factor of 1+ S/P, up to ten times.

Question 2

How do collimation, tissue compression, kV, air gap, and grids affect scatter?

Answer 2

Reduction of scatter
-Collimation: reduce volume of irradiated tissue
-Compress the tissue to reduce height of tissue irradiated (denser tissue = more Compton scatter)
-Reduced kV (x-ray energy): increases PEE vs Compton scatter
-Air gap: move image plate away from patient  reduced scatter hitting image detector plate but produces a magnified image (as distance is increased, increased kV/mA required to maintain photon flux).
-Grid: series of highly attenuating septa parallel to primary x-ray beam. Transmitted x-rays will pass through the grid vs scattered rays, which will mostly be stopped.

Question 3

What does scatter affect the most: spatial resolution or contrast?

Answer 3

Scatter affects both spatial resolution (especially of low contrast structures) and contrast but the reduction in contrast is more significant.

Question 4

What is the effect of kV on scatter?

Answer 4

-Increasing kV may allow for a reduction in patient dose for same detector but beam becomes more penetrating.
-A greater proportion of scatter is formed at the exit side of the patient  more scatter reaches the imaging detector

Question 5

Grids: what is a primary detection factor

Answer 5

-Primary detection factor: primary photons that hit detector – there is a loss of these as some will hit the lead septa.

Question 6

Grids: what is the scatter factor? What is the drig ratio and what is it normally?

Answer 6

-Scatter factor: scattered photons that hit the detector
-Grid ratio: determines number of scattered photons that make it through – depth: width. Usually 8:1

Question 7

Grid: what is the grid factor?

Answer 7

• increase in intensity (mA) required for the detector to have the same exposure/intensity without the grid (values 3-5)

Question 8

Important factors that affect image quality (7)

Answer 8

Noise
Contrast
Resolution

Other important factors:
Unsharpness
Magnification
Distortion
Artefacts

Question 9

Noise -What is quantum noise/mottle?

Answer 9

-Quantum noise/mottle: due to random variation in the number of photons being detected, the detection of individual photons is a random process, when only very small numbers of photons are detected, this statistical difference is a significant proportion of the whole and is represented as variance in the density of film/value given to pixels.

Question 10

Noise - Which type of noise is predominant in Xray/US?

Answer 10

-X-ray: quantum noise due to requirement of ALARP
-US: electronic noise

Question 11

Noise - How is noise affected by dose to patient, thickness of phosphor material and efficiency in photon detection? How is noise expressed in an equation?

Answer 11

-decreases with increasing dose to the patient, thicker phosphor material, increased efficiency
in detecting photons
-Expressed as a proportion of the total number of photons detected in a single area/pixel (M) as 1/M^(1/2)

Question 12

Noise - What is structural noise?

Answer 12

-Caused by variation in the structure of the screen phosphor (this is negligible in precisely made new phosphors).

Question 13

Noise - What is the equation for signal to noise ratio?

Answer 13

-M/M^(1/2)

Question 14

Noise - What will be the effect of increasing the thickness of the intensifying screen on the noise/geometric unsharpness/kVp?

Answer 14

-Increased thickness of screen will allow more photons to be absorbed: reduces dose at the expense of more screen unsharpness and increased noise.
-No effect on kVp.

Question 15

Contrast - What is contrast?

Answer 15

Contrast is the difference in the displayed or image signal intensity between two areas of interest e.g. a lesion and background tissue. A high contrast image has a greater difference between the grey shades displayed but a smaller range of greys. A low contrast image has a smaller difference (i.e. it’s more difficult to make out different areas) but a larger range of greys.

Question 16

What is subject contrast?

Answer 16

-Subject contrast is the ratio of the radiation intensities in different parts of an image due to the quality of the subject being imaged. The contrast is due to the differential attenuation by the tissues.

Question 17

How do structure thickness, kV, attenuation, Compton effect and photoelectric effect affect contrast?

Answer 17

Higher contrast is achieved with:
-Thicker structure being imaged
-Greater difference between the attenuation of the two objects.
-Reduced kV
-Increased density (for Compton-dominant effects) – use of barium or gas as a contrast medium
-Increased atomic number (for photoelectric dominant effects) -eg using iodine or barium as a contrast medium against soft tissue

Question 18

Reduced contrast is caused by (2 things):

Answer 18

-Scatter
-Higher kV

Question 19

What is spatial resolution? What is it best described by?

Answer 19

-Resolution is the measure of how far apart two objects must be before they can be seen as separate details in the image.
-smallest details resolvable is equal to pixel size however you can see smaller high contrast detail (such as microcalcification) due to the partial volume effect.
-Best described by the modulation transfer function (MTF)

Question 20

How does pixel size, phosphor crystal size, phosphor layer thickness, backscatter and the readout laser beams affect spatial resolution?

Answer 20

Spatial resolution: improved by
-Smaller diameter of readout laser beams (thinner line of image plate being read out)
-Smaller pixels
-Smaller size of phosphor crystals
-Thinner phosphor layer
-No light reflection/absorption of the backing layer  produces scatter

Question 21

What is spatial frequency?

Answer 21

-Measure of spatial resolution
-Measured in line pairs per mm (lp/mm). An image with a high llp/mm is a high spatial frequency image as there are many altering light and dark regions in a single mm.

Question 22

Spatial resolution - What is modulation transfer function?

Answer 22

-Measure of how well and image represents the original object (object spatial frequency): ranges from 0-100%. Low spatial frequencies are generally well represented by an imaging system (1). As the spatial frequency goes up, the MTF goes down, until, with the addition of noise, it isn’t possible to visualise details of higher spatial frequencies (called limiting spatial resolution)
-0 = no information is conveyed
-1 = spatial resolution imaged and displayed are the same.

Question 23

Spatial resolution - what is detective quantum efficiency (DQE) of imaging? What does a high DQE mean?

Answer 23

-Indicates the efficiency of photon detection and the effects of added noise to the detected signal.
-The higher the DQE, the more efficiently the detector can record information. DQE 0.25 = detected can only exploit ¼ incident x=ray photons.

Question 24

What are DQE values for CR vs DR systems?

Answer 24

-DQE values for CR system: 0.25 standard image plate and 0.12 high resolution image plate.
-DQE for DR: approx. 65%

Question 25

Unsharpness - name 4 kinds of unsharpness

Answer 25

-Geometric
-Image receptor unsharpness
-Movement unsharpness
-Edge unsharpness

Question 26

What is geometric unsharpness and what is it caused by?

Answer 26

The boundaries between a dark and a light area may be ill-defined, resulting in a blurred edge. This is called “unsharpness”. There are several causes and types of unsharpness as outlined below.
The focal spot is not infinitely small. There will be areas of the image that are:
* High signal: all x-ray photons reach detector
* Low signal: no x-ray photons have passed through the object to reach the detector
* Intermediate: not all photons have passed through the object. The size of this area determines the unsharpness and is called the penumbra.

Question 27

What 3 factors can help reduce geometric unsharpness?

Answer 27

-Decreasing focal spot size
-Decreasing object-detector distance
-Increasing focal spot to detector distance
*Moving an object closer to the focal spot will increase the penumbra and, therefore, the unsharpness.

Question 28

Describe receptor unsharpness

Answer 28

Image receptor unsharpness (for digital images): if a detector element lies across the border between a light and dark area, the pixel displayed will be an average of these two values, creating a blurred border

Question 29

Describe movement unsharpness and edge unsharpness

Answer 29

-Movement unsharpness: object moves during acquisition
-Edge unsharpness: if object has a tapering edge, the attenuation will gradually decrease along the object.

Question 30

Magnification - 2 ways in which this can be reduced

Answer 30

-Using longer focus to detector distance
-Decreasing object to detector distance
*The further the from the detector the object is, the more the image is magnified.

Question 31

What is distortion?

Answer 31

-Refers to a difference between the shape of an object and its appearance as an image.
-Xray beam can be distorted as it passes through and create artefact.

Question 32

Digital image structure: what is a pixel? What is one bit?

Answer 32

-Spatial resolution is determined by pixel size. Each pixel records a value (binary format) related to the intensity of the signal of the corresponding part of the image.
-1 bit is one value of grey
-N bits = 2^n (number of different values of grey).

Question 33

How are pixel and field of view and matrix size related?

Answer 33

-Pixel size = field of view/matrix size

Question 34

What is a byte? What is the relation between bytes and bits?

Answer 34

-Computer memory is measured in bytes
-1 byte = 8 bits
-8 bits = 2^8 = 256 values

Question 35

What is the exposure index?

Answer 35

-Measure of the amount of exposure on the image receptor. (Can’t rely on too bright or too dark like in film screen radiography).
-Manufacturers provide a recommended EI range for optical quality.
Sensitivity number (S)

Question 36

How can exposure index be measured?

Answer 36

-S = 2000/ X (where x is the dose incident on the image plate).
-S number around 200-300
-S < 200: improved signal to noise radio but increased patient dose
-S > 400 used when minial radiation is required (eg paeds).