chap 8 image quality, snr, cnr - universitetet i oslochap 8 image quality, snr, cnr – introduction...

CHAP 8 Image quality, SNR, CNR

– Introduction

– SNR

– CNR

– K-space filling strategies

CHAP 9 Off-Resonance effects

– Magnetic Susceptibility

– Implication for Imaging

– Water-Fat shift

CHAP 8 Image quality, SNR, CNR

– Introduction

– SNR

– CNR

– K-space filling strategies

CHAP 9 Off-Resonance effects

– Magnetic Susceptibility

– Implication for Imaging

– Water-Fat shift

Image quality: The art of “trade-off”

SNR definition

SNR in MRI

Practical SNR measurement

CNR (Contrast to Noise Ratio)

Rel

SI,

rel

CN

R

CHAP 8 Image quality, SNR, CNR

– Introduction

– SNR

– CNR

– K-space filling strategies

CHAP 9 Off-Resonance effects

– Magnetic Susceptibility

– Implication for Imaging

– Water-Fat shift

kx

ky

p1

p*1

p2

p*2

Partial k-space sampling

Partial Echo acquisition

TE

Tacq

TE

Tacq

Gx

Partial Echo: pulse sequence

diagram

Acceleration

Acceleration

Scan Percentage

Scan Percentage

Scan Percentage

Scan Percentage

Scan Percentage

Scan Percentage

Scan Percentage

Scan Percentage

Scan Percentage

Scan Percentage

Scan Percentage

Scan Percentage

Scan Percentage

Scan Percentage

Scan Percentage

Scan Percentage

Scan Percentage

Acceleration

Partial Fourier (half scan)

Partial Fourier (half scan)

Partial Fourier (half scan)

Partial Fourier (half scan)

Partial Fourier (half scan)

Partial Fourier (half scan)

Partial Fourier (half scan)

Partial Fourier (half scan)

Partial Fourier (half scan)

Rectangular FOV

Rectangular FOV

Rectangular FOV

Rectangular FOV

Rectangular FOV

Rectangular FOV

Rectangular FOV

Rectangular FOV

Matrix size

Matrix size

Matrix size

FOV Size

FOV Size

FOV Size

FOV Size

FOV Size

FOV Size

FOV Size

CHAP 8 Image quality, SNR, CNR

– Introduction

– SNR

– CNR

– K-space filling strategies

CHAP 9 Off-Resonance effects

– Magnetic Susceptibility

– Implication for Imaging

– Water-Fat shift

Magnetic properties of matter

placed in a magnetic field

Magnetic Susceptibility

CHAP 8 Image quality, SNR, CNR

– Introduction

– SNR

– CNR

– K-space filling strategies

CHAP 9 Off-Resonance effects

– Magnetic Susceptibility

– Implication for Imaging

– Water-Fat shift

In practice…

B pixel

B

pixel

Intravoxel dephasing

Bz

Geometric distortion

Susceptibility artefact

x

B0 homogeneity in the bore

CHAP 8 Image quality, SNR, CNR

– Introduction

– SNR

– CNR

– K-space filling strategies

CHAP 9 Off-Resonance effects

– Magnetic Susceptibility

– Implication for Imaging

– Water-Fat shift

H2O and Fat

Water-Fat-Shift

”in-phase / out-of-phase” evolution

GRE sequences

In-phase / out-of-phase

In-phase / out-of-phase images

Water-fat shift artefact

Field strength and water-fat-shift

Fat and water image: Ankle