Limitations to X-ray quality

Question 1

Limitations to X-ray image quality

Answer 1

Unsharpness
Scatter
Noise

Question 2

Types of Unsharpness

Answer 2

Geometric
Movement
Detector

Question 3

Main causes of unsharpness

Answer 3

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

Question 4

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

Answer 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

Question 5

Calculating Geo unsharp

Answer 5

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

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

Question 6

Reducing Geo unsharpness

Answer 6

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

Question 7

Causes of Receptor Unsharpness

Answer 7

Digital: Detector design / Mat pixel size

- CR: laser spot size

Question 8

Calculating receptor unsharp

Answer 8

Ur = F/M

F = intrinsic unsharp for zero thickness object on receptor.

Question 9

Total Unsharpness

Answer 9

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

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

Question 10

Movement Unsharpness and minimising it

Answer 10

Internal movement: Breathing, heartbeat

Min: Use short as poss exposure time

Reduce mag: Long FFD, short FOD

Use compression in mammo

Question 11

Define scatter

Answer 11

Detected photons with no spatial information

Acts like fog on an image

Question 12

Contrast with and without scatter

Answer 12

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

Question 13

Grid structure calculations

Answer 13

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

d= septa width D = low atten width

Question 14

Grid impact and designs

Answer 14

Inc patient dose

Grid moves to avoid artefacts

Complex design: HTC, complex movement

Question 15

Grid parameters

Answer 15

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

Question 16

Air gap

Answer 16

Gap betw patient exit surface and detector

Photons scatter out of primary area

Cause magnification

1/r effect due to atten of scatter

Question 17

Scatter reduction

Answer 17

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

Question 18

Types of noise

Answer 18

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

Question 19

Impact of Counts on SNR

Answer 19

Noise = sqrt N

SNR = N / sqrt N = sqrt N

4 x N -> 2 x SNR

Question 20

Define quantum noise

Answer 20

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

Dominant source of image noise

Question 21

Define fixed pattern noise

Answer 21

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

Equivalent to a signal prop to dose

Reduce by using flat-fielding in DR

Question 22

Define electronic noise

Answer 22

• Arises from detector and detector electronics / thermal effects

Assumed to be constant

Significant at lower doses

Question 23

Fluoro noise and its reduction

Answer 23

Quantum dominates

Use temporal averaging to reduce effects

All types of noise are present

Question 24

How to combine noise

Answer 24

nt = sqrt(n1 ^2 + n2^2 ….

Question 25

Noise component analysis

Answer 25

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

Question 26

Define Anatomical Noise

Answer 26

Noise due to a normal tissue structure in image task dependent Anatomy masks detail in mammo can reduce with compression and tomo

Question 27

Ways of measuring image quality

Answer 27

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

Question 28

Measures of resolution

Answer 28

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

Question 29

Modulation Transfer function

Answer 29

MTF = Image modulation / Object Modulation

Question 30

Nyquist limit

Answer 30

The reciprocal of 1/(2 x sampling distance)

Question 31

Comparing MTF of DR and CR

Answer 31

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

Question 32

MTF from edge spread function

Answer 32

1) Differentiate to get LSF from ESF | 2) FT ESF to get MTF

Question 33

Limiting spatial resolution & its measurement

Answer 33

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

Question 34

SNR and CNR limits of SNR

Answer 34

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

Question 35

Define noise power Spectrum

Answer 35

A plot of the amount of noise at each spatial frequency Grid can appear as a spike at the grid spacing freq

Question 36

Define ROC testing and limits

Answer 36

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

Question 37

Producing an ROC curve

Answer 37

Plot TPP and FPP based on a range of decision thresholds. TPP = No. TP / TP + FN