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Flashcards in Limitations to X-ray quality Deck (37)
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1

Limitations to X-ray image quality

Unsharpness
Scatter
Noise

2

Types of Unsharpness

Geometric
Movement
Detector

3

Main causes of unsharpness

Focal spot size
Focus-object-film distance
Detector design (pixel size)
Screen-film system
Movement

4

Effect of focal spot size (1), FFD (2),Focus-Obj Dist (3) and Compression (4)

(1) Inc spot size = Inc penumbra
(2) Dec FFD = Inc Mag, Inc unsharpness
(3) Dec FOD = Inc unsharpness
(4) Compression brings structures together, inc sharpness

Mag mammo uses fine focus to decrease penumbra

5

Calculating Geo unsharp

Blur b = yf/x = f(M-1)

Unsharpness = Ug = f(M-1)/M = yf/(x+y)

6

Reducing Geo unsharpness

- Small focal spot
- Long source-Im dist
- Short obj-im dist

7

Causes of Receptor Unsharpness

Digital: Detector design / Mat pixel size
- CR: laser spot size

8

Calculating receptor unsharp

Ur = F/M

F = intrinsic unsharp for zero thickness object on receptor.

9

Total Unsharpness

U = sqrt(Ug^2 + Ur^2)

= 1/M sqrt(f^2(M-1)^2 + F^2)

10

Movement Unsharpness and minimising it

Internal movement: Breathing, heartbeat

Min: Use short as poss exposure time

Reduce mag: Long FFD, short FOD

Use compression in mammo

11

Define scatter

Detected photons with no spatial information

Acts like fog on an image

12

Contrast with and without scatter

Without scatter: C = 1 - e ^ x(μ1 - μ2)

With scatter:

C = 1 - e ^ x(μ1 - μ2) / 1 + R

R = scatter to primary ratio

1/1+R C degradation factor

13

Grid structure calculations

Lines per mm N = 1/(D+d)
Grid ratio r = h/D

d= septa width D = low atten width

14

Grid impact and designs

Inc patient dose

Grid moves to avoid artefacts

Complex design: HTC, complex movement

15

Grid parameters

Primary transmission: ideal 1 real 0.6

Scatter trans fact: ideal 0 real 0.05-0.2

Bucky factor: inc in dose due to grid, kV, patient thickness. Grid ratio is factor 3-8

Contrast imp factor: ratio of contrast deg factor with and without grid

16

Air gap

Gap betw patient exit surface and detector

Photons scatter out of primary area

Cause magnification

1/r effect due to atten of scatter

17

Scatter reduction

- Use collimation
- Use a low kV
- Use a grid
- Use an air gap
- Use compression

18

Types of noise

• Quantum noise (Recap)
• Fixed pattern (structure) noise
• Electronic noise
• Anatomical noise

19

Impact of Counts on SNR

Noise = sqrt N

SNR = N / sqrt N = sqrt N

4 x N -> 2 x SNR

20

Define quantum noise

Caused by statistical fluctuations in the number of photons per unit area
absorbed in detector

Dominant source of image noise

21

Define fixed pattern noise

Variations in pixel sensitivity, filter thickness, table
top attenuation etc

Equivalent to a signal prop to dose

Reduce by using flat-fielding in DR

22

Define electronic noise

• Arises from detector and detector electronics / thermal effects

Assumed to be constant

Significant at lower doses

23

Fluoro noise and its reduction

Quantum dominates

Use temporal averaging to reduce effects

All types of noise are present

24

How to combine noise

nt = sqrt(n1 ^2 + n2^2 ....

25

Noise component analysis

For a given detector air kerma (AK)
• Quantum noise: sq = a(AK)1/2
• Fixed pattern (structure) noise: ss = b(AK)
• Electronic noise se = c
• Total noise st = √ a2(AK) + b2(AK)2 + c2

st = std lin. pixel value

26

Define Anatomical Noise

Noise due to a normal tissue structure in image

task dependent

Anatomy masks detail

in mammo can reduce with compression and tomo

27

Ways of measuring image quality

• Spatial frequency
• Modulation transfer function
• Limiting spatial resolution
• Signal-to-noise ratio and contrast-to-noise ratio
• Noise power spectrum
• Receiver Operating Characteristics Tests

28

Measures of resolution

Point, line and edge spread functions show degree of blurring present
in imaging system

29

Modulation Transfer function

MTF = Image modulation / Object Modulation

30

Nyquist limit

The reciprocal of 1/(2 x sampling distance)