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Image Quality
Ho Kyung [email protected]
Pusan National University
Introduction to Medical Engineering
Outline
• Stochastic nature of quantum interactions
• Imaging systems always spread out the impulsive input
• Contrast– contrast vs. resolution
• Spatial resolution– PSF, LSF– FWHM, lp/mm
• Noise– Poisson & Gaussian distributions– SNR, CNR– noise vs. resolution
• Artifact
2
Cut‐view of a digital x‐ray detector
3Image courtesy of GE Global Research Center, Niskayuna, NY
X‐ray detection schemes
4E. Samei | RSNA Categorical Course | 2003
Oblique x‐ray incidence
5D. Walter et al. | 19th World Conference on NDT | 2016
Demonstration of image quality
• Detail resolution (or high spatial‐frequency information) can be lost by added blur and noise
6(Images taken from) E. Samei | RSNA Categorical Course | 2003
Factors influencing on image quality
• Six factors determining image quality:
① Contrast• The difference btwn image characteristics (e.g., shades of gray => i.e., intensity) of an object (or feature
within an object) & surrounding objects or background
② Resolution• The ability of a medical imaging system to depict details
③ Noise• Random fluctuations in image intensity that do not contribute to image quality
– Reducing object visibility by masking image features
④ Artifacts• Image features that do not represent a valid object or characteristics of the patient
– Obscuring important features or being falsely interpreted as abnormal findings
⑤ Distortion• Inaccurate impression of shape, size, position, & other geometric characteristics of features
– Should be corrected to improve the diagnostic quality
⑥ Accuracy• Conformity to truth & clinical utility
7
Image quality
• Resolution– Ability to depict details– Factors affecting image resolution:
• the characteristics of imaging systems (focal spot, detector blur)• the scene characteristics & geometry (subject shape, position & motion)• the viewing conditions
• Contrast– A measure of differences in brightness (or intensity) in adjacent regions
• Noise– Random fluctuations in image intensity– Inevitable due to the statistical nature of imaging (emission, detection, conversion)– Poisson statistics
8
Should not be defined as the linear pixel density (e.g., dots per inch)
• Artifacts– Artificial image features such as dust or scratches in photos– May also be introduced by digital image processing (e.g., edge enhancement)– Should be avoided or at least understood their origins because they hamper the diagnosis or yield
incorrect measurements
9
How to describe the spatial resolution?
• Point‐spread function (PSF)– Smoothed blob obtained from an imaging system when imaging a very small, bright point on a
dark background– Undistinguishable as two separate objects when the distance between their PSFs is less than the
FWHM
• Line‐spread function (LSF)– If the resolution is the same in all directions
lsf 𝑥,
,
10
psf 𝑥, 𝑦𝛿 𝑥, 𝑦
• Full width at half maximum (FWHM)– Indicative measure of resolution– The (full) width of the LSF (or the PSF) at one‐half its max value (usually in units of mm)– The min distance that two lines (or points) must be separated in space in order to appear as
separate in the recorded image
11
Contrast
• The difference in signal intensity of adjacent regions of the image
• Local contrast– Target: an object of interest (e.g., a tumor in the liver)– Background: other objects surrounding the target (e.g., the liver tissue)
Obscuring our ability to see or detect the target
– Local contrast
𝐶𝑓 𝑓
𝑓
12
Contrast is related to spatial resolution
• Contrast degrades as the object size is decreased• The contrast of very small objects is influenced by the resolution• The detail or the size of an object can be described by the term called the “line pairs per
millimeter (lp/mm)”– Spatial frequency: the reciprocal of length [mm‐1]
• The spatial resolution is defined by how small object you can distinguish in terms of lp/mm, and the smallest object is determined by the lowest contrast that you can perceive
13A. Gopal & S.S. Samant | Med. Phys. | (2008)
Example
Assuming that the spatial resolution is only determined by the pixel size of a detector, can you estimate the pixel size of the detector providing the following radiograph?
14
Example
Assuming that the spatial resolution is only determined by the pixel size of a detector, can you estimate the pixel size of the detector providing the following radiograph?
15
Example
Assuming that the spatial resolution is only determined by the pixel size of a detector, can you estimate the pixel size of the detector providing the following radiograph?
16
Noise
• The random component in the image• Always present because of the statistical nature of imaging• Image quality as noise
• In projection radiography:– Quanta or photons: discrete packets of energy arriving at the detector from the x‐ray source– Quantum mottle: random fluctuation due to the discrete nature of their arrival
Textured or grainy appearance in an x‐ray image
• In magnetic resonance imaging:– RF pulses generated by nuclear spin systems are sensed by antennas connected to amplifiers
Competing w/ signals being generated in the antenna from natural unpredictable(i.e., random) thermal vibrations
17
18J. A. Seibert & R. Morin | Pediatric Radiology | (2011)
Underexposed Overexposed
• The source of noise in a medical imaging system depends on the physics & instrumentation of the particular modality
• Consider the noise as the numerical outcome of a random event or experiment• Think of the noise as the deviation from a nominal value predicted from purely
deterministic arguments– e.g., Random nature radioactive emissions in nuclear medicine where gamma ray photons are
emitted at random times in random directions
19
• Random variables
– The numerical quantity associated w/ a random number or experiment– Probability distribution function (PDF)
• 𝑃 𝜂 Pr 𝑁 𝜂
– The probability that random variable 𝑁 will take on a value less than or equal to 𝜂– 0 𝑃 𝜂 1– 𝑃 ∞ 0, 𝑃 ∞ 1– 𝑃 𝜂 𝑃 𝜂 for 𝜂 𝜂
20
• Continuous random variables
– 𝑁 is a continuous random variable if 𝑃 𝜂 is a continuous function of 𝜂– Probability density function (pdf)
• 𝑝 𝜂
– 𝑝 𝜂 0– 𝑝 𝜂 d𝜂 1
– 𝑃 𝜂 𝑝 𝑢 d𝑢
– 𝜇 E 𝑁 𝜂𝑝 𝜂 d𝜂 expected value or mean
» Average value of the RV
– 𝜎 Var 𝑁 E 𝑁 𝜇 𝜂 𝜇 𝑝 𝜂 d𝜂 variance
– 𝜎 𝜎 standard deviation» “Average” variation of the values of the RV about its mean» The larger 𝜎 , the “more random” the RV
21
– Uniform random variable over the interval [a, b]
• pdf 𝑝 𝜂 , for 𝑎 𝜂 𝑏0, otherwise
• distribution function 𝑃 𝜂0, for 𝜂 𝑎
, for 𝑎 𝜂 𝑏1, for 𝜂 𝑏
• expected value 𝜇
• variance 𝜎
22
– Gaussian random variable over the interval [a, b]
• pdf 𝑝 𝜂 𝑒 ⁄
• PDF (distribution function) 𝑃 𝜂 erf
where the error function erf 𝑥 𝑒 ⁄ d𝑢
• expected value 𝜇 𝜇• variance 𝜎 𝜎• Gaussian pdf is characterized completely (& uniquely) by its mean and variance
• Noise in medical imaging systems is the result of a summation of a large number of independent noise sources
• Central limit theorem of probability– A random variable that is the sum of a large number of independent causes tends to be Gaussian– Often natural to model noise in medical imaging system by means of a Gaussian random variable
23
• Discrete random variables– Specified by the probability mass function (PMF)
– Pr 𝑁 𝜂 for 𝑖 1, 2, … , 𝑘• Probability that random variable 𝑁 will take on the particular value 𝜂
– 0 Pr 𝑁 𝜂 1 for 𝑖 1, 2, … , 𝑘– ∑ Pr 𝑁 𝜂 1– 𝑃 𝜂 Pr 𝑁 𝜂 ∑ Pr 𝑁 𝜂
– 𝜇 E 𝑁 ∑ 𝜂 Pr 𝑁 𝜂 expected value or mean
– 𝜎 Var 𝑁 E 𝑁 𝜇 ∑ 𝜂 𝜇 Pr 𝑁 𝜂 variance
– Poisson random variable
• Pr 𝑁 𝑘!
𝑒 for 𝑘 0,1, 2, …
– 𝑎 0 (real‐valued parameter)– 𝜇 𝑎– 𝜎 𝑎
24
Noise is related to spatial resolution
25
The same unity noise varianceCorrelation over only a very short distance Correlation over only a greater distance
• Both contrast and noise are frequency‐dependent
26Image courtesy of Dr. M. J. Yaffe
Relative signal or noise
• No meaningful information if the noise level > the image intensity of an object– Signal‐to‐noise ratio (SNR)
• A high SNR is no guarantee for a good perceptibility of image objects
– Contrast‐to‐noise ratio (CNR)• To distinguish the neighboring objects• (SNR in terminology of images)• Wiener noise‐power spectrum (NPS): the Fourier transform of the autocorrelation of a “flat‐field” image
27
SNR vs. image quality
• Detectability of an object increases as the SNR increases
28R. M. Nishikawa | RSNA Categorical Course | 2004
Different realizations of the same image
𝑁 ; 𝜎 𝑁 ; SNR 𝑁
𝑁 2𝑁 ; 𝜎 2𝑁 ; SNR 2𝑁 2 SNR
𝑁 2𝑁 ; 𝜎 2𝑁 ; SNR 2 SNR
𝑁 2𝑁 ; 𝜎 2𝑁 ; SNR 2 2 SNR
𝑁 2𝑁 ; 𝜎 2𝑁 ; SNR 4 SNR
Measurements
• Contrast– Signal difference = ∆𝑑 𝑑 𝑑
– Contrast = ∆
• Noise– Standard deviation of signal
– CNR ∆
• Spatial resolution– FWHM
• Artifact
• Scatter
29
𝑑𝑑
Image sig
nal
Position
𝜎
𝜎
Image sig
nal
Position
Ideal impulse response
FWHM
Contrast
• Signal difference = ∆𝑑 𝑑 𝑑
• Contrast = ∆
30
Image sig
nal
Position
𝑑𝑑
Noise
• Standard deviation of signal
• CNR ∆
31
Image sig
nal
Position
𝜎
𝜎
Resolution
• Full width at half maximum of the point‐spread function
32
Image sig
nal
Position
Ideal impulse response
FWHM
Wrap‐up
• Stochastic nature of quantum interactions
• Imaging systems always spread out the impulsive input
• Contrast– contrast vs. resolution
• Spatial resolution– PSF, LSF– FWHM, lp/mm
• Noise– Poisson & Gaussian distributions– SNR, CNR– noise vs. resolution
• Artifact
33