Limitations to X-ray quality Flashcards Preview

Medical Physics 2: Radiology > Limitations to X-ray quality > Flashcards

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

Limitations to X-ray image quality

A

Unsharpness
Scatter
Noise

2
Q

Types of Unsharpness

A

Geometric
Movement
Detector

3
Q

Main causes of unsharpness

A
Focal spot size
Focus-object-film distance
Detector design (pixel size) 
Screen-film system
Movement
4
Q

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

A

(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
Q

Calculating Geo unsharp

A

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

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

6
Q

Reducing Geo unsharpness

A
  • Small focal spot
  • Long source-Im dist
  • Short obj-im dist
7
Q

Causes of Receptor Unsharpness

A

Digital: Detector design / Mat pixel size

- CR: laser spot size

8
Q

Calculating receptor unsharp

A

Ur = F/M

F = intrinsic unsharp for zero thickness object on receptor.

9
Q

Total Unsharpness

A

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

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

10
Q

Movement Unsharpness and minimising it

A

Internal movement: Breathing, heartbeat

Min: Use short as poss exposure time

Reduce mag: Long FFD, short FOD

Use compression in mammo

11
Q

Define scatter

A

Detected photons with no spatial information

Acts like fog on an image

12
Q

Contrast with and without scatter

A

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
Q

Grid structure calculations

A

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

d= septa width D = low atten width

14
Q

Grid impact and designs

A

Inc patient dose

Grid moves to avoid artefacts

Complex design: HTC, complex movement

15
Q

Grid parameters

A

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
Q

Air gap

A

Gap betw patient exit surface and detector

Photons scatter out of primary area

Cause magnification

1/r effect due to atten of scatter

17
Q

Scatter reduction

A
  • Use collimation
  • Use a low kV
  • Use a grid
  • Use an air gap
  • Use compression
18
Q

Types of noise

A
  • Quantum noise (Recap)
  • Fixed pattern (structure) noise
  • Electronic noise
  • Anatomical noise
19
Q

Impact of Counts on SNR

A

Noise = sqrt N

SNR = N / sqrt N = sqrt N

4 x N -> 2 x SNR

20
Q

Define quantum noise

A

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

Dominant source of image noise

21
Q

Define fixed pattern noise

A

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
Q

Define electronic noise

A

• Arises from detector and detector electronics / thermal effects

Assumed to be constant

Significant at lower doses

23
Q

Fluoro noise and its reduction

A

Quantum dominates

Use temporal averaging to reduce effects

All types of noise are present

24
Q

How to combine noise

A

nt = sqrt(n1 ^2 + n2^2 ….

25
Q

Noise component analysis

A

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
Q

Define Anatomical Noise

A

Noise due to a normal tissue structure in image

task dependent

Anatomy masks detail

in mammo can reduce with compression and tomo

27
Q

Ways of measuring image quality

A
  • 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
Q

Measures of resolution

A

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

29
Q

Modulation Transfer function

A

MTF = Image modulation / Object Modulation

30
Q

Nyquist limit

A

The reciprocal of 1/(2 x sampling distance)

31
Q

Comparing MTF of DR and CR

A

MTF for DR is much
higher than for
standard CR

DR MTF also higher at nyquist

Higher theoretical
limiting spatial
resolution for standard
CR but not much signal

32
Q

MTF from edge spread function

A

1) Differentiate to get LSF from ESF

2) FT ESF to get MTF

33
Q

Limiting spatial resolution & its measurement

A

Describes the ability of a system to record fine
detail.

High res = poss to record high detail

Measure with bar pattern test - count groups of lines

less useful than mtf but easier to measure

34
Q

SNR and CNR

limits of SNR

A

SNR relates signal amp to noise

SNR = pv / stdpv

pv = lin pixel value

CNR the difference
in pixel value between
the object and
background, divided by
the standard deviation

CNR = M1 - M2 / sqrt(std1^2 + std2^2 / 2)

std not necessarily a good measure of im quality

35
Q

Define noise power Spectrum

A

A plot of the amount of noise at each spatial frequency

Grid can appear as a spike at the grid spacing freq

36
Q

Define ROC testing and limits

A

1) Statistical approach
2) Review of a large no, clinical images
3) Use a panel of observers and compare their opinion to a ground truth.

Difficult to coord and costly

37
Q

Producing an ROC curve

A

Plot TPP and FPP based on a range of decision thresholds.

TPP = No. TP / TP + FN