Limitations to X-ray quality Flashcards
Limitations to X-ray image quality
Unsharpness
Scatter
Noise
Types of Unsharpness
Geometric
Movement
Detector
Main causes of unsharpness
Focal spot size Focus-object-film distance Detector design (pixel size) Screen-film system Movement
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
Calculating Geo unsharpness
Blur b = yf/x = f(M-1)
Unsharpness = Ug = f(M-1)/M = yf/(x+y)
Reducing Geo unsharpness
- Small focal spot
- Long source-Im dist
- Short obj-im dist
Causes of Receptor Unsharpness
Digital: Detector design / Mat pixel size
- CR: laser spot size
Calculating receptor unsharpness
Ur = F/M
F = intrinsic unsharp for zero thickness object on receptor.
Total Unsharpness
U = sqrt(Ug^2 + Ur^2)
= 1/M sqrt(f^2(M-1)^2 + F^2)
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
Define scatter
Detected photons with no spatial information
Acts like fog on an image
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
Grid structure calculations
Lines per mm N = 1/(D+d)
Grid ratio r = h/D
d= septa width D = low atten width
Grid impact and designs
Inc patient dose
Grid moves to avoid artefacts
Complex design: HTC, complex movement
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
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
Scatter reduction
- Use collimation
- Use a low kV
- Use a grid
- Use an air gap
- Use compression
Types of noise
- Quantum noise
- Fixed pattern (structure) noise
- Electronic noise
- Anatomical noise
Impact of counts on SNR
Noise = sqrt N
SNR = N/ sqrt N = sqrt N
4 x N -> 2 x SNR
Define quantum noise
Caused by statistical fluctuation in the number of photons per unit area absorbed in detector
Dominant source of image noise
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
Define electronic noise
Arises from detector and detector electronic/thermal effects
Assumed to be constant
Significant at lower doses
How to combine noise
nt = sqrt (n1^2 + n2^2)
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 = sqrt (a2(AK) + b2(AK)2 + C2)
st = std lin. pixel value
