Chất lượng hình ảnh trong XQuang (Image quality dx rad imaging physics course)

Stewart, PhD, DABMP 3 Take Away: Five Things You should be able to Explain after the Image Quality Lecture radiographic contrast and what factors affect each an imaging system’s spatial

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Brent K Stewart, PhD, DABMP 1

Image Quality – Chapter 10

Brent K Stewart, PhD, DABMP Professor, Radiology and Medical Education Director, Diagnostic Physics

a copy of this lecture may be found at:

http://courses.washington.edu/radxphys/PhysicsCourse04-05.html

What Makes for a ‘Quality’ Image?

Take Away: Five Things You should be able

to Explain after the Image Quality Lecture

radiographic contrast and what factors affect each

an imaging system’s spatial resolution characteristics

perceived quantum noise and contrast resolution

into lower frequencies and how this affects an image

performance, including Detective Quantum Efficiency (DQE), Contrast-Detail curves and ROC curves

Image Quality - Motivation

its usefulness in determining an accurate diagnosis

image quality important so that radiologists can recognize problems and articulate their cause

used by physicists and engineers to measure image quality, e.g., contrast, spatial resolution and noise

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Contrast

gray scale between closely adjacent regions of the image

result of many steps during acquisition, processing and display

c.f Bushberg, et al The Essential Physics of Medical

Subject Contrast (Cs)

the signal prior to it being recorded

differences in the object, e.g.,

in x-ray intensity based on attenuation

¬ For A=N0e-µxand B=N0e-µ(x+z)

¬ Cs= 1-e-µz

mammography

c.f Bushberg, et al The Essential Physics of Medical Imaging, 2 nd ed., pp 257-258.

Detector Contrast (Cd)

play an important role in producing contrast in the final image

how the detector ‘maps’

detected energy into the output signal

curve): input radiation exposure to output value (analog or digital)

system a digital value for each pixel: gray scale value

Radiographic Contrast (Cr)

analog film OD the output

¬ Cr= ODA-ODB

cannot in general be adjusted

or enhanced on the analog film

reaching the radiologist’s eyes depend on patient thickness (assuming a constant µ)?

¬ X ∝e-µx

¬ OD ∝g ·log10(X)

¬ T = 10-OD

c.f Bushberg, et al The Essential Physics of Medical Imaging, 2 nd ed., pp 259 and 261.

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Digital Radiographic and Displayed Contrast

post-process digital images, the CNR is a more relevant description of the contrast

potentialin the image than simply the contrast itself

through look-up-table (LUT) transformation

image contrast

Huda 2ndEdition – Chapter 5 – Image Quality

Raphex 2003 Diagnostic Question

in a screen-film imaging system?

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contrast) depends on:

¬ A H&D characteristic curve of the film.

affected primarily by:

Spatial Resolution

of an imaging system to accurately depict objects in the two spatial dimensions of the image (x,y)

depict objects as they become smaller and closer together

(x,y) of an image

Spatial Domain: the Point Spread Function

spatial resolution to stimulate system with a point-spread function (PSF)

¬ Isotropic and non-isotropic

entire field-of-view (FOV)

signal to an image acts as a point stimulus, the output image is just the collection of

with the PSF (convolution =

with the PSF (convolution = )

c.f Bushberg, et al The Essential Physics of Medical Imaging, 2 nd ed., p 264.

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Spatial Domain: the Point Spread Function

stationary non-stationary

Input(x,y) PSF(x,y) Output(x,y) Effect: blurring edges and fine detail*

more regarding in the DR lecture

Image Processing Based on Convolution

¬ Convolution: Ch 11 –Digital Radiography and Ch 13 -CT

¬ Defined mathematically as passing a N-dimensional convolution kernel over an N-dimensional numeric array (e.g., 2D image or CT transmission profile)

¬ At each location (x, y, z, t, ) in the number array multiply the convolution kernel values by the associated values in the numeric array and sum

¬ Place the sum into a new numeric array at the same location

Physical Mechanisms of Blurring

screen)

features) in tomography

magnification (Chapter 6)

¬ Line spread function (LSF)

¬ Edge spread function (ESF)

¬ LSF = d(ESF)/dx

¬ PSF = d(LSF)/dy

Spatial Frequency Domain

time-domain audio signal (t)

image (audio signal) can be thought of as the superposition

of spatial frequencies

are separated by shorter distances (mm, x), these objects correspond to high spatial frequencies (cycles/mm, f)

mm (lp/mm)

10K 1K 100 10

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Modulation Transfer Function: MTF(f)

waves of various frequencies (f) into an imaging system:

what is the amplitude of the

output contrast normalized by the input contrast

resolution properties

Modulation Transfer Function: MTF(f)

¬ MTFtotal(f) = ∏MTFi(f)

¬ A: Optics MTF(f)

¬ B: Image intensifier MTF(f)

¬ C: Video camera MTF(f)

¬ System (f) = Optics MTF(f) * Image intensifier MTF (f) * Video camera MTF(f)

chain with worst MTF often determines the overall system,

The LSF, MTF and Fourier Transform

LSF(x) using the Fourier Transform (FT)

operation that converts a spatial domain (x) signal into a spatial frequency (f) function

line pair phantom and star phantom for quick determination of lp/mm

changing to:

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describing the of an imaging system.

¬ A Properties of the characteristic (H&D) curve

Davis Notes - Image Quality

¬ 8 Referring to Figure 1 (right), 8

which demonstrates three different line spread functions (LSF), which LSF will yield the best spatial resolution?

¬ 10 10 Referring to Figure 1 which shows LSFs, and Figure 2 which shows the corresponding modulation transfer functions (MTFs), which MTF corresponds to LSF C?

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¬ 11 Referring to Figure 2 11

illustrating MTFs, the axes should be labeled

for the y-axis and

for the x-axis:

¬ A Relative amplitude, distance (mm)

¬ B Spatial frequency (lp/mm), distance (mm)

¬ C Lateral dimension (mm), Fresnel ratio

¬ D Relative amplitude, spatial frequency (lp/mm)

¬ E Relative amplitude, relative amplitude

Noise

stochastic component into an

of variability, either naturally occurring or random fluctuation:

1

i i

1

N

i i

N

−

=

−

∑

Gaussian Probability Distribution Function

measurements of people and things make for this kind of distribution (Gaussian) hence the term “normal”

grade school children approximates a Gaussian

x

2

1 2

( )

x x

−

− ⎜⎝ ⎟⎠

=

Poisson Probability Distribution Function

one variable

large values of x due to x!

statistics obey P(x)

¬ Radioactive decay

¬ Quantum mottle

( )

!

x m

m

x

−

=

Poisson Distribution

0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4

m=1

m=4

m=8 m=10

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Probability Distribution Functions

observation in a range:

integrate area (for G):

¬ 1 = 68.25%

¬ 1.96 = 95%

¬ 2.58 = 99%

intervals

image by adjusting the mean number of photons used to produce the image

Quantum Noise

Contrast Resolution

low-contrast object

noise there is in the image SNR

reliably identify an object

structure noise both affect the conspicuityof a target

Noise Frequency

random, the noise has a frequency distribution

(still has noise variations)

Spectrum: NPS(f)

a function of spatial freq (f)

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Detective Quantum Efficiency (DQE)

overall system SNR performance and dose efficiency

¬ SNR2

out=

2

out

in

SNR SNR

( ) ( )

MTF f NPS f

( ) ( )

k MTF f

N NPS f ⋅

DQE(f=0) = QDE

distributions?

greater than 10

photons absorbed in the film.

emulsion.

increasing noise by:

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is practicing manual technique factors for a digital photospot system that is not phototimed Circle each of the following techniques that will result in higher

quantum mottle in the image:

by using:

system to distinguish:

angiogram.

Sampling and Aliasing in Digital Images

¬ Sampling (pixel) pitch

¬ Detector aperture width

determines the highest frequency that can be imaged

a lower frequency

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Aliasing due to Reciprocating Grid Failure

Sampling and Aliasing in Digital Images

¬ When input f > FN then the spatial frequency domain signal

at f is aliased down to:

fa= 2FN–f

averaging across the detector aperture

¬

sin(a f) MTF(f)=FT{rect(a)}=sinc(af)=

a f

π π

Contrast-Detail (C-D) Curves

diameter can just be visualized

high-contrast, small detail

low-contrast

Receiver Operating Characteristic (ROC) Curves

abnormal from normal

histograms

normal (L) or abnormal (R)

1-specificity usu @ five threshold levels

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ROC Questionnaire: 5 Point Confidence Scale

Receiver Operating Characteristic Curves

way of analyzing the SNR associated with a specific diagnostic task

concise description of the diagnostic performance of the systems (including observers) being tested

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¬ 26 26 In Figure 5, showing an ROC curve, the X-axis should

be labeled (circle all that are correct):

¬ 27 In Figure 5 showing the 27

ROC curves, the Y-axis should be labeled (circle all that are correct):

represents pure guessing

represents the best diagnostic approach

about 0.3

C

A

D

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End of Lecture, Additional Questions Follow

the following factors?

contrast that is due to the screen-film image receptor?

of the following except:

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output phosphor, a resolution cell measured at the input plane is 0.5 mm At the output phosphor, the resolution cell dimension is now .

¬ 9 9 Referring to Figure 1 showing three line spread functions, the best choices for the axes labels are for the y-axis and _ for the x-axis:

¬ A Frequency, amplitude

¬ B Blur distance (mm), frequency

¬ C Relative amplitude, frequency

¬ D Relative amplitude, distance (mm)

¬ E Distance (mm), relative amplitude

image is variations in the:

exposed film but it lacks good bone contrast They decide to increase the mAs by a factor of 2, allowing them to reduce the kVp

If they was originally at around 100 kVp, what kVp should the technologist choose to get a properly exposed film of the same patient?

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(automatic exposure control) system is used to produce

a clinical radiograph Increased quantum mottle will occur with:

significant loss of:

will increase all of the following except:

effect on image sharpness?

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the same phosphor will have a lower:

¬ 15 15 MTF number

demonstrates the best spatial resolution

¬ 16 MTF number is probably for an image intensifier (9”II in 9”mode)

¬ 17 MTF number is probably for a general screen-film system

¬ 18 MTF number _ has

a maximum resolving power of about 100 mm

¬ 19 MTF number _ has

a cut-off resolution of about 3 lp/mm

3 1 3

1 2

and is practicing manual technique factors for a digital photospot system that is not phototimed Which techniques would result in a higher absorbed dose to the cadaver? :

high contrast objects The fundamental limitation on useful magnification is:

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