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The Impact of Bandwidth on MRI Image Quality

Chen Lin, PhD DABR

Indiana University School of Medicine & Indiana University Health

Disclosure

• Research funding provided by Siemens Healthcare.

Chen Lin, PhD 3/2012

What’s Bandwidth ?

Chen Lin, PhD 3/2012

Radio Signal

Chen Lin, PhD 3/2012

Bandwidth of Radio Signal

Chen Lin, PhD 3/2012

CF

BW

More Waves / Frequencies -> Higher Bandwidth

Chen Lin, PhD 3/2012

BW

Hz

Amplitude

Higher “Bandwidth” = More Lanes

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RF Transmit and Receive in MRI

Chen Lin, PhD 3/2012

Transmit Bandwidth (tBW)

Receive Bandwidth (rBW)

Why care about BWs ?

Chen Lin, PhD 3/2012

tBW

Readout Duration

Excitation & Refocusing Duration

Slice Gradient

Slice Thickness

Spatial Resolution

ESP, TE , TR, etc

rBW Read-out Gradient

PNST & SAR

SNR, Contrast,

Artifacts, Scan time

Tx Frequency

Z

tBW

(

~ 2

KH

z)

Slice/Slab Selection Excitation

Chen Lin, PhD 3/2012

Thick Slice

Thin Slice

Given the same RF pulse, i.e. fixed tBW, thinner slice -> steeper slice selection gradient

Slice selection gradient: Gz Df (z) = g z Gz

What’s appropriate tBW ?

Advantages of high tBW

– Less distortion artifacts

– Shorter excitation pulse -> Shorter TE and Echo Spacing (ESP)

× Disadvantages of high tBW

– Larger minimal slice thickness

– Higher RF Amplitude -> Higher RF power and SAR

– Peripheral Nerve Stimulation PNST

Chen Lin, PhD 3/2012

Slice Profile Distortion

Chen Lin, PhD 3/2012

Z

DB(z) = z Gz

B0

B0 inhomogeneity

Thinner slice -> steeper slice selection gradient -> less slice profile distortion due to B0 inhomogeneity

Air Artifact ?

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0 0.2 0.4 0.6 0.8 1-0.05

0

0.05

0.1

0.15

0.2

rfsincnf = (pi/2)*msinc

nf(128,4)

Time Bandwidth Product

• Higher tBW RF pulse -> Shorter pulse length

– > Higher gradient amplitude

– > Higher RF amplitude

Chen Lin, PhD 3/2012

FT

Time Domain Frequency Domain

-20 -15 -10 -5 0 5 10 15 200

0.2

0.4

0.6

0.8

1

rfsincnf = (pi/2)*msinc

nf(128,4)

BW

Time

Choices of Different RF Pulses

• Low SAR:

– 90/180: 5888ms/7680ms

• Normal:

– 90/180: 4096ms/5120ms

• Fast:

– 90/180: 2048ms/2560ms

Chen Lin, PhD 3/2012

90 180

Min. TE and ESP

RF Pulse (Type)

Min. TE (ms) Min. ESP

(ms) Min. SL (mm)

Low SAR 19 18.6 0.4

Normal 16 16.1 0.6

Fast 14 14.4 1.2

Chen Lin, PhD 3/2012

Normal Gradient Mode & rBW=130 Hz/px

Relative RF Amplitude

• Low SAR

– 90/180: 26V/42V

• Normal

– 90/180: 37V/64V

• Fast

– 90/180: 70V/105V

Chen Lin, PhD 3/2012

RF Power and Absorption

1. High BW RF pulse -> High RF power

2. Short TE and ESP -> High duty cycle

1 & 2 -> High SAR

Chen Lin, PhD 3/2012

1

2( ) ( )P t B t

0

( )

T

SAR P t dt

What’s appropriate tBW ?

Advantages of high tBW

– Larger gradient -> Less distortion artifacts

– Shorter RF pulse -> Shorter TE and Echo Spacing (ESP)

× Disadvantage of high tBW

– Larger min. slice thickness due to max. gradient limit.

– Higher B1 -> High SAR (RF power)

Chen Lin, PhD 3/2012

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Rx Frequency

X

Rx

Ban

d W

idth

(RB

W)

(+/

- 8

– 1

28

KH

z)

Frequency Encode Direction

Frequency Encoding

Df (x) = g x Gx

Chen Lin, PhD 3/2012

Chemical Shift Artifact

x

Gx

Water Signal Fat Signal

220Hz @ 1.5T

Miss-registration of fat and water in the frequency encoding and slice selection direction

Chen Lin, PhD 3/2012

rBW specifications

• GE: kHz

• Philips: WFS in pixels

• Siemens: Hz/pix

(WFS shown as Tool Tip.)

• Hz/pix = rBW / # of pix in freq dir

• WFS = (Hz/pix)/220Hz @ 1.5T or (Hz/pix)/440Hz @ 3.0T

Chen Lin, PhD 3/2012

WFS = 0.4 pix

rBW

What’s appropriate rBW

• Disadvantage of higher rBW

– Lower SNR

– Lower max. in-plane resolution

• Advantages of higher rBW

– Shorter TE and ESP (Faster Scan)

– Less artifacts

Chen Lin, PhD 3/2012

Chemical Shift Artifact

Chen Lin, PhD 3/2012

rBW = 125Hz/px rBW = 490Hz/px

rBW & SNR

• SNR ~ f(Sequence Type, FA, TR, TE, TI …) x B0 PD Dx Dy Dz ( Nphase NEX ) 1/2 / rBW ½

Chen Lin, PhD 3/2012

f f

Low rBW High rBW

Signal

Noise

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rBW and Spatial Resolution (Pixel Size)

• Higher resolution -> Higher readout gradient

• Higher rBW-> Higher readout gradient

Both resolution and rBW are competing for readout gradient

Given max. gradient available, reduce rBW to allow higher max. spatial resolution (smaller pixels in readout direction).

Chen Lin, PhD 3/2012

MRI @ 200nm !

Chen Lin, PhD 3/2012

Min. TE and ESP

RF Pulse (Type)

Min. SL (mm)

Min. TE (ms) Min. ESP

(ms)

Low SAR 0.4 19 -> 18 18.6 -> 17.7

Normal 0.6 16 -> 13 16.1 -> 13.4

FasT 1.2 14 -> 8.7 14 -> 8.74

Chen Lin, PhD 3/2012

Normal Gradient Mode & rBW = 130Hz/px -> 651 Hz/px

rBW and artifacts

• Chemical shift artifacts

• B0 inhomogeneity (susceptibility) artifacts

– Spatial distortion

– Signal lost

• T2 Blurring artifacts in FSE

• Ghosting artifacts in EPI

• Banding artifacts in bSSFP

Chen Lin, PhD 3/2012

TR/TE (ms) 500/66 500/14 500/9

2 kHz 16 kHz 32 kHz

Comparison of rBW

Courtesy of Dr. J. Zhou of UIC

Chen Lin, PhD 3/2012

Susceptibility Artifact

• Increase receiver bandwidth (rBW).

• Use SE instead of GRE sequences.

• Reduce TE

• Imaging at lower field strength.

Chen Lin, PhD 3/2012

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Reducing Susceptibility Artifact 3D SPACE and High Receiver Bandwidth

2D TSE T2 3-D SPACE

T2 Blurring and Edge Ringing

ky

kx

ky Y

I I

Multi-shot FSE/TSE

I-space Point Spread Function

FT

Chen Lin, PhD 3/2012

K-space Intensity Profile

Blurring

Ringing/Ghosting

FSE/TSE Artifact

Chen Lin, PhD 3/2012

CSE TE=30ms TSE ETL=30 EP=10ms TE=30ms

TSE ETL=30 EP=10ms TE=130ms

ky

I

ky

I

ky

I

Geometric Distortion in EPI

• Phase error accumulates in the echo train.

• Minimized with fewer echoes (ETL) and/or shorter echo spacing (ESP)

Chen Lin, PhD 3/2012

Gphase

Gread

B0 inhomogeneity introduces local gradients

How to reduce Echo SPacing (ESP)

• Lower read resolution (Less frequency encoding points)

• High rBW (Faster sampling rate)

• Ramp Sampling

Chen Lin, PhD 3/2012

The Effect of Varying ESP & ETL

Chen Lin, PhD 3/2012

rBW=752Hz/px p=None

rBW=1502Hz/px p=None

rBW=752Hz/px p=2

rBW=1502Hz/px p=2

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Balanced SSFP • Combines signal from both

partial SE and GRE.

• Typically with large a for bright fluid contrast

• Susceptible to off-resonance artifact.

• Short TR to reduce phase error accumulation and fast imaging

• Can be SAR intensive TrueFISP/FIESTA/bFFE

TR a a

Gslice

Gphase

Gread

Chen Lin, PhD 3/2012

2

2 1

1

90 & ,T

S for TR T TT

a

Off-resonance Effect

To Reduce phase accumulation between excitation:

– Improve B0 homogeneity or shift center frequency

– Reduce TR by increasing tBW and rBW Chen Lin, PhD 3/2012

Eur

J R

adio

l. 2

008 J

an;6

5(1

):15

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Cook Book -> Cooking

Chen Lin, PhD 3/2012

Interactive Live Demo and Group Exercises

1. Optimize T1w protocols minimize artifacts due to metal hardware.

1. Optimize T2w protocols for high resolution and low SAR.

Chen Lin, PhD 3/2012

Cooking up great MRI protocols with different BWs, etc.

Chen Lin, PhD 3/2012

Thank you !

clin1@iupui.edu