Topics

• spatial saturation

• TOF imaging

• chemical saturation

• magnetization transfer

Review: Relaxation

t=t0

900 RF

t=t1

ML=0t=t2

ML=at=t3

ML=bt=ML=1

….

t

ML

t0 t1 t2 t3

Relaxation

t=t0

900 RF

t=t3

ML=bt=t4

ML<b

900 RF

t=t3+

ML=0

900 RF

t=t4+

ML=0

t=t5

ML<<b

TR TR

Equilibrium

• after 5 or so repetitions, the system reaches equilibrium

• similar to water flowing into a leaky bucket

relaxation

RF in

equilibriumequilibrium

T1 Relaxation

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 1000 2000 3000 4000 5000

msec

ML

long T1

short T1

longer TR, more recovery of ML

longer TR, more recovery of ML

shorter TR, less recovery of ML

shorter TR, less recovery of ML

TR and ML

• prolonged TRs allow for more recovery of ML

• shorter TRs allow for less recovery of ML

– condition referred to as “partial saturation”

Saturation

• “total” magnetization– application of additional RF pulses

has no effect on proton orientation

• saturation exists only briefly– net magnetization recovers

longitudinal relaxation immediately after protons are “saturated”

Types of Saturation

• spatial

• fat

• water

• magnetization transfer (1st cousin)

Spatial Saturation

• application of an RF pulse immediately prior to the imaging sequence saturates all of the protons under the influence of that pulse

Spatial Saturationpurpose/advantages

• reduce motion artifacts in the phase encoding direction– swallowing

– CSF pulsation

– respiratory motion

• reduce signal from flowing blood

• facilitate angiography/venography

Spatial Saturationdisadvantages

• fewer slices per TR– timing of saturation pulse prolongs

effective TR interval

• higher SAR

Spin Echo

FID spinecho

RF pulse

readoutfrequency encode

signal

gradient

RF pulse

Saturation

no echo

RF pulse

signal

RF pulse

saturation pulse

additional time required for single saturation pulse

Saturation Pulse

z

y x

z

y x

0 sat pulse

t=t0 t=t0+

0

ML=0

SATURATIONSATURATION

z

y x

0 RF

t=t0++

MXY=0no signal

Saturation Pulse and Longitudinal Magnetization

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

time (msec)

ML No Sat Pulse

Sat Pulse

Saturation Pulse and Longitudinal Magnetization

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

time (msec)

ML No Sat Pulse

Sat Pulse

SAT pulses 900 RF pulses

Spatial Saturation

saturation band within

the FOV

arterial

venous

superior saturation pulse(arterial)

superior saturation pulse(arterial)

inferior saturation pulse(venous)

stack of slices2D

acquisition

stack of slices2D

acquisition

Spatial Saturation

outside the FOV

arterialflow

venous flow

fully magnetizedprotons in arteriesfully magnetized

protons in arteries

fully magnetizedprotons in veinsfully magnetizedprotons in veins

partially saturatedprotons in vesselspartially saturatedprotons in vessels

end slices mayhave bright flow in arteries orveins

end slices mayhave bright flow in arteries orveins

middle slices usually have “flow voids” invessels

middle slices usually have “flow voids” invessels

Entry SlicePhenomenon

s1900RF

s1T=TE

bright flow,entry slicephenom

bloodmoves

downstreamflow directionflow direction

vesselvessel

MR Flow Void

s1 s2 s3

unsaturatedspins

unsaturatedspins

s1 s2 s3

s2900RF onsaturated

spins,flow void

saturatedspins

saturatedspins

arterial

venous

superior saturation pulse(arterial)

superior saturation pulse(arterial)

inferior saturation pulse(venous)

stack of slices2D

acquisition

stack of slices2D

acquisition

Summary: Flow Effects

• entry slice phenomenon due to unsaturated spins

• flow void due to saturation of previous slice coupled with downstream migration of spins

• spatial presaturation bands can reduce (eliminate) signal from flowing blood

Magnetic Resonance Angiography

• exploits flow enhancement of GR sequences

• saturation of venous flow allows arterial visualization

• saturation of arterial flow allows venous visualization

• no IV contrast is required

Magnetic Resonance Angiography

AP projection Lateral projection

tumor

right thigh

2D TOF Angiography

• anatomy imaged using a series of gradient echo images– each image is acquired separately– all slices experience entry slice

phenomenon

• saturation pulse placed proximal for venous imaging, distal for arterial imaging

s10RF

s1T=TE

bright flow,entry slicephenom

flow directionflow direction

vesselvessel

2D TOF

s1

unsaturatedspins

unsaturatedspins

s1

presatband

2D TOF Angiography

• saturation band is located the same distance from each slice to maximize its effect– “walking presat”

• vascular images reconstructed using maximum intensity projection technique

lateral projection AP projection

SPIRAL CTANGIOGRAPHY

.

.

.

.

.

.

MIP Reconstruction

2D TOF

• GR images used– short TR (~ 20-40 msec)

– very short TE • shortest TE times minimize intravoxel

dephasing resulting in maximum flow effects

– small to medium flip angles

MIP

2D TOF Carotid Study

Chemical Saturation• similar to spatial saturation

• narrow band RF pulse causes selective saturation of water or fat protons– “chem sat”– “fat sat”

• compatible with many imaging sequences

Fat Sat

frequency

220 Hz1.5 T

220 Hz1.5 T

waterwater

fatfatfat

selectivebandwidth

fat selective

bandwidth

Fat Saturation

echo from water only

RF pulse

signal

RF pulse

fat sat pulse

additional time required for saturation pulse

FAT SAT

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

time (msec)

MLWater

Fat

Fat Satexamples

Fat Satadvantages

• increase conspicuity of fluid on T2 weighted images– widens dynamic range

• addresses FSE fat-fluid isointensity problem

• post-gadolinium T1 weighted fat sat

• reduced respiratory motion artifact

Fat Sat disadvantages

• fewer slices per TR– timing of saturation pulse prolongs

effective TR interval

• higher SAR

• requires homogenous magnet– shimming

Fat Sat disadvantages

• requires uniformly shaped body part– doesn’t work well at base of neck, crook of

ankle, etc.

• not recommended with FOV > 30 cms– unreliable

• works poorly at lower fields• S/N ratio drops

Fat Suppression and SNR

• non fat-suppressed image– each image pixel comprised of signal

from water and fat in the imaging voxel

• fat-suppression– reduces total signal by suppression of

fat from the voxel– reduces SNR

Fat Suppression

• without fat suppresion

• high SNR

• with fat suppression

• lower SNR

frequency

SIwater only

frequency

SI water and fat

TR 550, TE 15.7, 45° TR 450, TE 15.7, 45°

with MTwith MT without MTwithout MT

Magnetization Transfer

Magnetization Transfer

• first cousin of Fat Sat

• off-resonance RF pulse applied similar to Fat Sat pulse

• “bound water” proton pool absorbs the RF energy– energy is transferred to “unbound”

proton pool

Magnetization Transfer

• think of as “tissue SAT”

• tissues high in proteins (brain, muscle) become darker– MT pulse causes a selective saturation effect

• tissues low in proteins relatively unaffected– fat

– free fluid/water/edema

frequency

bound

freeMT pulse

~1000 kHz off-resonance

MT pulse~1000 kHz

off-resonance

Magnetization Transfer

energytransferenergytransfer

saturationeffect

saturationeffect

Magnetization Transfer

echo

RF pulse

signal

RF pulse

MT pulse

additional time required for saturation pulse

Magnetization Transferadvantages

• generates T2-like weighting with GR images– good cartilage sequence

• suppresses background tissues– improved TOF angiography

– increased contrast (gadolinium) visualization

Magnetization Transferadvantages

• magnetic field homogeneity not critical

• generates images with new contrast relationships

• compatible with many sequences; also compatible with fat sat

Magnetization Transfer disadvantages

• fewer slices per TR– timing of saturation pulse prolongs

effective TR interval

• higher SAR

TR 550, TE 15.7, 45° TR 450, TE 15.7, 45°

with MTwith MT without MTwithout MT

Magnetization Transfer