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 unsharpness

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 unsharpness

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
• 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 fluctuation 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 electronic/thermal effects
Assumed to be constant
Significant at lower doses

Question 23

How to combine noise

Answer 23

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

Question 24

Noise component analysis

Answer 24

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 = sqrt (a2(AK) + b2(AK)2 + C2)

st = std lin. pixel value