Physics of MRI

• Course syllabus

– Lecture 1: Intro to NMR Dr. Lowe– Lecture 2: Imaging Sequences I Dr. Buckwalter– Lecture 3: Imaging Sequences II Dr. Buckwalter– Lecture 4: Spatial encoding I Dr. Buckwalter– Lecture 5: Spatial encoding II Dr. Buckwalter– Lecture 6: Spin prepped imaging Dr. Lowe– Lecture 7: Ultrafast imagingDr. Lowe

http://www.indyrad.iupui.edu/public/lectures/mri/iu_lectures/mri_homepage.htm

Loose Ends

Energy Absorption

β0

M0=x M0=z

900 tip

900 RF pulse

ω=ω0

Relaxation

β0

t=t0

RF

t=t1

ML=0t=t2

ML=at=t3

ML=bt=∞ML=1

….

t

ML

t0 t1 t2 t3

Relaxation and Imaging

• FID (free induction decay) is the relaxation behavior following a single RF pulse

• most imaging done with repetitive RF energy deposition

• the interval between the RF energy pulses is called the TR interval (time to repetition)

Relaxation

β0

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

equilibrium

Differential Relaxation

• short TR

• lower absolute ML

• marked difference in relative signal

• long TR

• higher absolute ML

• minimal difference in relative signal

fat protonswater protons

T1 Relaxation

0

0.2

0.4

0.6

0.8

1

1.2

0 1000 2000 3000 4000 5000 6000

msec

ML

long T1

short T1

Image Contrast and T1 Relaxation

• shorter TRs maximize differences in T1 relaxation, generating tissue contrast

• longer TRs minimize differences in T1 relaxation, reducing T1 tissue contrast

Imaging Sequencespart I

• Gradient Echo

• Spin Echo

• Fast Spin Echo

• Inversion Recovery

Goals of Imaging Sequences

• generate an RF signal perpendicular to β0

• generate tissue contrast

• minimize artifacts

Measuring the MR Signalz

y x

RF signal from precessing protons

RF antenna

β0

Gradient Echo• simplest sequence

– alpha flip-gradient recalled echo

• 3 parameters– TR

– TE

– flip angle

• reduced SAR• artifact prone

Gradient Echo

FID gradient recalledecho

αRF pulse

rephase

dephase

signal

gradient

z

y x

z

y x

α0 RF

t=t0 t=t0+

Partial Flip

α0 ML

MXY

M

MXY = M sin(α)

ML = M cos(α)

Dephasing in the xy-planeview from the top

y

xz Mxy

y

xz

Mxy≈0dephase

phase coherency phase dispersion

y

xz Mxy

phase coherencyminus t2* decay

Rephasing in the xy-planeview from the top

rephase

y

xz

Mxy≈0

phase dispersion

MR Signal During Rephasingz

y x

RF signal“echo”

RF antennaβ0

T2* decay

• occurs between the dephasing and the rephasing gradients

• rephasing incompletely recovers the signal

• signal loss is greater with longer TEs

• decay generates image contrast

T2* decay

• T2* decay is always faster than T2 decay

• gradient echo imaging cannot recover signal losses from– magnetic field inhomogeneity

– magnetic susceptibility

– water-fat incoherence

T2 and T2* Relaxation

• T2* relaxation influences contrast in gradient echo imaging

• T2 relaxation influences contrast in spin echo imaging

Gradient Echopulse timing

echo

RF

signal

readout

α0

phase

slice

TE

Gradient Echoadvantages

• faster imaging– can use shorter TR and shorter

TEs than SE

• low flip angle deposits less energy– more slices per TR than SE

– decreases SAR

• compatible with 3D acquisitions

Gradient Echodisadvantages

• difficult to generate good T2 weighting

• magnetic field inhomogeneities cause signal loss– worse with increasing TE times

– susceptibility effects

– dephasing of water and fat protons

Gradient Echochanging TE

TE 9FA 30

TE 30FA 30

susceptibility effect T2* weighting

Gradient Echomagnetic susceptibility

post-surgical change“blooming” artifact

Gradient Echo

• image contrast depends on sequence

• conventional GR scan – aka GRASS, FAST

– decreased FA causes less T1 weighting

– increased TE causes more T2* weighting

Conventional GRTE 20, FA 15

Gradient Echo

• Spoiled GR– aka SPGR, RF-FAST

– spoiling destroys accumulated transverse coherence

– maximizes T1 contrast

Gradient Echo

• Contrast enhanced GR– aka SSFP, CE-FAST

– infrequently used because of poor S/N

– generates heavily T2* weighted images

Gradient Echo

• other varieties– MTC

• T2 - like weighting

– IR prepped• 180 preparatory pulse

– DE (driven equilibrium) prepped• 90-180-90 preparatory pulses• T2 contrast

MTC GRTE 13, FA 50

Spin Echo• widely used sequence

– 90-180-echo

• 2 parameters– TR

– TE

• generates T1, PD, and T2 weighted images

• minimizes artifacts

Spin Echo

FID spinecho

900 RF pulse

readoutfrequency encode

signal

gradient

180 0 RF pulse

Gradient versus Spin Echo

Spin Echo

FID spinecho

900 RF pulse

readoutfrequency encode

signal

gradient

1800 RF pulse

Gradient Echo

FID gradient recalledecho

αRF pulse

rephase

dephase

signal

gradient

900 Flip

z

y x

z

y x

900 RF

t=t0 t=t0+

900

AfterML=0MXY=M

BeforeML=MMXY=0

Dephasing in the xy-planeview from the top

y

xz Mxy

y

xz

Mxy≈0

phase coherency phase dispersion

Dephasing begins immediately after the 900

RF pulse.

t=0 t=TE/2900 RF

y

xz Mxy

phase coherencyminus t2 decay

Rephasing in the xy-planeview from the top

y

xz

Mxy≈0

phase dispersion

t=TE/2 t=TE1800 RF

z

y xz

y x

z

y x

z

y x

t=TE/2 t=TE

1800 RF

t=0

900 RF

dephased

rephased

1800 Flip

Spin Echopulse timing

echo

RF

signal

readout

90 0

phase

slice

TE

1800

WNMR Race

t=0

900 RF

WNMR Race

WNMR Race

t=TE/2

1800 RF

t=TE

WNMR Race

Effects of the 1800 Pulse

• eliminates signal loss due to field inhomogeneities

• eliminates signal loss due to susceptibility effects

• eliminates signal loss due to water/fat dephasing

• all signal decay is caused by T2 relaxation only

Spin Echoadvantages

• high signal to noise

• least artifact prone sequence• contrast mechanisms easier to

understand

Spin Echodisadvantages

• high SAR than gradient echo because of 900 and 1800 RF pulses

• long TR times are incompatible with 3D acquisitions

Spin Echo Contrast

• T1 weighted– short TR (450-850)

– short TE (10-30)

• T2 weighted– long TR (2000 +)

– long TE (> 60)

• PD weighted– long TR, short TE

Spin Echo ContrastT2 Relaxation

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 100 200 300 400 500

msec

Mxy

long T2

short T2

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

T1 weighted - T1 relaxation predominates•Short TE minimizes differences in T2 relaxation•Short TR maximizes differences in T1 relaxation

T2 weighted - T2 relaxation predominates•Long TE maximizes differences in T2 relaxation•Long TR minimizes differences in T1 relaxation

T1 weighted T2 weighted

Spin Echo Contrast

Spin Echo Contrast

PD weighted T2 weighted