Image Quality Parameters and their MeasurementImage Quality in X -ray Detectors ICMP, September 1,...

ICMP, September 1, 2013 / Brighton, England 1 Image Quality in X-ray Detectors

Image Quality Parameters

and their Measurement

John M. Boone, Ph.D., FAAPM, FSBI, FACR

Professor and Vice Chair of Radiology Professor of Biomedical Engineering

University of California, Davis

2 Image Quality

3 Image Quality

4


and their Measurement Linear Systems / Stationary Behavior

Spatial Resolution

field measurements

scientific characterization

Contrast Resolution (noise)

field measurements


Signal to Noise Ratio

field measurements


Summary

Linear Systems / Stationary Behavior

Spatial Resolution



Summary

5




Spatial Resolution



Summary

6

Response of a 400 speed screen-film system (Fuji)

7

Response of a Varian 4030CB Flat Panel Detector

high gain mode

low gain mode

8

Response of a Fuji Computed Radiography (CR) System

9

stationary

non-stationary

Image Quality

point spread functions (PSFs)

10




Spatial Resolution



Summary

11

Field Measurement

12

spatial resolution

line pair phantom

star pattern phantom

wedge

resolution test tools

Image Quality

system input

system output

blurring

Spatial Resolution: Measurements in the Field

modulation

14

15

can resolve 4 bars?

yes

heck no

no

yes

yes

yes

limiting spatial resolution

16

Limiting spatial resolution (~10% of MTF)

17

Scientific Characterization

Image Quality

The Point Spread Function (PSF)

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Image Quality

The Line Spread Function (LSF)

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The Edge Spread Function (ESF)

Image Quality 20

Image Quality

21

22

The Line Spread Function – lsf(x) – “slit image”

measures focal spot resolution measures detector resolution

slit

slit

focal spot

detector

{


resulting image of slit measurement geometry


resulting image of slit actual lsf(x)

position (mm)

gray

sca

le (d

n)

orthogonal trace thru slit

Fourier Representation

25

26

different phase

different amplitude

different frequency

sine waves 101

27

2( )( )

( )

ifxlsf x e dxOTF f

lsf x dx

∞ − π

−∞∞

−∞

= ∫∫

( ) ( )MTF f OTF f=

2 2( ) [ ( )] [ ( )]MTF f OTF f OTF f= ℜ + ℑ

complex

real imaginary real

28

lsf(x)

MTF(f)

2( )

( )

ifxlsf x e dx

lsf x dx

∞ − π

−∞∞

−∞

∫∫

normalizes area to 1.0

modulation transfer function

29

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top view of detector array

side view

Direct Digital Detectors (eg Selenium)

a

aperture – lsf(x)

lsf(x) = RECT{x/a}

∆

pitch – Nyquist

FN = 1 2 ∆

32

other considerations

Pre-sampled LSF / ESF

Type of Image Processing

flat fielding OK

dead pixel correction OK

“for presentation” not OK

Paralax issues

33




Spatial Resolution



Summary

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Field Measurement

35

contrast resolution

1.2 µR/frame 1.8 µR/frame 3.8 µR/frame

36

( )2

2

1

1 ( , )1

N

ii

I x y IN =

σ = −− ∑

2σ = σ

N = number of pixels in ROI hi(x,y): the data in the ROI

“noise”

First Order Determination of Noise

region on interest (ROI)

digital image

37


σ σ

But “noise”, or the standard deviation σ, does not tell the whole story

38

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( )( )2 2 ( )

1

1( , ) ( , ) x yN

i f x f yx y ix y

iNPS f f I x y I e dxdy

N∞ ∞ − π +

=−∞ =−∞=

= −∑ ∫ ∫

The Noise Power Spectrum (NPS) or Wiener Spectrum

10 20 30 40 50 60

10

20

30

40

50

60

N regions on interest (ROIs)

10 20 30 40 50 60

10

20

30

40

50

60

10 20 30 40 50 60

10

20

30

40

50

60

10 20 30 40 50 60

10

20

30

40

50

60

average

analytical equation

40


10 20 30 40 50 60

10

20

30

40

50

60

N regions on interest (ROIs)

10 20 30 40 50 60

10

20

30

40

50

60

10 20 30 40 50 60

10

20

30

40

50

60

10 20 30 40 50 60

10

20

30

40

50

60

average

discrete implementation

41


10 20 30 40 50 60

10

20

30

40

50

60 radial average

spatial frequency (mm-1) N

PS (m

m2 )

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10 20 30 40 50 60

10

20

30

40

50

60 radial average


PS (m

m2 )

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PS (m

m2 )

σ2

2 ( )NPS f dfσ = ∫

uncorrelated noise correlated noise

convolution

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46




Spatial Resolution



Summary

47

Field Measurement

48

Signal difference to noise ratio (SDNR)

Contrast to Noise Ratio (CNR)

signal

background

detail detail detail

cont

rast

cont

rast

cont

rast

A B C

the contrast detail curve

Image Quality 49

the contrast detail curve

interpretation

50

lower dose higher dose

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22 ( )( )

( )outMTF fSNR fNPS f

=

2 2 ( )( )( )

k MTF fNEQ fNPS f

=

The Noise Equivalent Quanta (NEQ)

53

2 2 ( )( )( )

k MTF fDQE fq NPS f

=

( )( ) NEQ fDQE fq

=

2

2

( )( )( )

out

in

SNR fDQE fSNR f

=2 ( )inSNR f q=

54

2 2 ( )( )( )

k MTF fDQE fq NPS f

=

k = mean GS value

q = mean photons/mm2 for this acquisition

Image acquired to compute NPS(f)

55

detective quantum efficiency (DQE)

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2.94 x 107 photons / mm2

120 kV / 1.0 mGy air kerma incident

To determine q: …some spectral modeling required

57 IEC values courtesy Nicole T. Ranger on line presentation

RQA3 50 kV 10 4.0 RQA5 70 kV 21 7.1 RQA7 90 kV 30 9.1 RQA9 120 kV 40 11.5

IEC: International Electrotechnical Commission (IEC 61267-1)

Radiation Quality

Designation Tube Voltage

~added Filtration (mm Al)

HVL (mm Al)

150

cm

DQE measurement geometry

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Spatial Resolution



Summary

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Field measurements of image quality are adequate

but are subjective and imprecise

Scientific measurements are useful in detector evaluation

but their quantitative nature is desirable

Digital radiography QC should convert to quantitative measures in time

Clinical Medical Physicists should understand MTF, NPS, NEQ, DQE

Summary

60




Spatial Resolution



Summary

Image Quality Parameters and their MeasurementImage Quality in X -ray Detectors ICMP, September 1,...

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