mansfield and lauterbur nobel prize 1978 first images

Mansfield and Lauterbur nobel prize1978 first images

MRI – Magnetic Resonance Imaging

1st published MRI images of abdomen

3 Tesla MRI Scanner

“Interesting images, but will never be as useful as CT”neuroradiologist, 1982

First brain MR First brain MR Modern T2 imageModern T2 image

MRI

Advantages Disadvantagessafe expensivegreat soft tissue contrast long timemany contrast options bad for bones

mediocre resolution

CT versus MRI

CT

+Excellent bone imaging

+Excellent new acute hemorrhage detection

+Skull fracture, calcified lesion

+Short scan time, metal devices allowed

-Poor contrast and resolution

-Radiation

MRI

+Excellent grey/white matter contrast & spatial resolution

+Better for old hemorrhage (and new with Diffusion?)

-Long scan time

-Pts cannot have metal devices

-Claustrophobia, obesity problems

+No radiation

- expensive

Magnetic Field of a loop of Wire

SOLENOID

3 Tesla MagneticField (60,000 timesEarths field)

MRI

B0

B0

3

21

3 Tesla magnet field

B0

Protons (hydrogennuclei act like littlemagnets)

Not all the protonsline up – thermal energy

MRI

Collective MagneticMoment of Protons

Classical pictureof Quantum Phenomenon

B0

ModelofHeadCoil

B0

ModelofHeadCoil

collective spins

MRIexcite

B0

Radio Waves

Collective MagneticMoment of Protons

start

end

Why precession?

Just like a top on a table

spin

gravity

B0

magnetic moment

B0

ModelofHeadCoil

excite

3.0 T123 MHz

B0

ModelofHeadCoil

signalwe “hear”

LISTEN

3.0 T123 MHz

B0

Radio Waves 123 MHz

Body Coil - Gradients

MRIexcitewithslice

selection

Only excite One Slice

3.1 T127 MHz

3.0 T123 MHz

2.9 T119 MHz

excite

Like a swing. Got one of the 3 orthogonal spatial dimensions whenwe excite. z

B0

ModelofHeadCoil

LISTEN

signalwe “hear”

3.0 T123 MHz

B0

Image we get of water container

Imageshould be

3.1 T127 MHz

3.0 T123 MHz

2.9 T119 MHz

excite

Like a swing. Got one of the 3 orthogonal spatial dimensions whenwe excite. z

3.1 T127 MHz

3.0 T123 MHz

2.9 T119 MHz

LISTEN

Got second of the 3 orthogonal spatial dimensions whenwe listen.

fast

slow

regular

ModelofHeadCoil

x

signalwe “hear”

B0

Image we get of water container

Imageshould be

3.1 T127 MHz

3.0 T123 MHz

2.9 T119 MHz

excite

Like a swing. Got one of the 3 orthogonal spatial dimensions whenwe excite. z

3.1 T127 MHz

3.0 T123 MHz

2.9 T119 MHz

phase encode(after we excitebefore we listen)

Got second of the 3 orthogonal spatial dimensions whenwe listen.

fast

slow

regular

y

3.1 T127 MHz

3.0 T123 MHz

2.9 T119 MHz

LISTEN

Got second of the 3 orthogonal spatial dimensions whenwe listen.

fast

slow

regular

ModelofHeadCoil

x

signalwe “hear”

Repeat 256 times for a 256x256pixel image

Different phase each time

scan = 4 minutes

Image we get of water container

Imageshould be

180 Degree RF Pulse

correcting gradients

Excite

Z

Y

X

Listen

SPIN ECHO SEQUENCE

TE – echo time

TR – repeat time

Contrast

T1 weighted – (MPRAGE-anatomical)T2 weighted – (fmri)

Spin Relaxation• Spins do not continue to precess forever

• Longitudinal magnetization returns to equilibrium due to spin-lattice interactions – T1 decay

• Transverse magnetization is reduced due to both spin-lattice energy loss and local, random, spin dephasing – T2 decay

• Additional dephasing is introduced by magnetic field inhomogeneities within a voxel – T2' decay. This can be reversible, unlike T2 decay

T1 decay – “spins back down”

Collective MagneticMoment of Protons

end

start

B0

signalwe “hear”

V

Time

T1 Recovery

MRSignal

Time

Typical T1 Graph

T2 decay – separation (dephasing) of “collective magnetic moment”

sometime after RF excitationImmediately after RF excitation

=

collective magnecticmoment

individual spins

separation (dephasing)

a little time later

T2 Decay

MRSignal

Typical T2 Graph

Time

T2 Decay

MRSignal

T1 Recovery

MRSignal

50 ms50 ms 1 s1 s

Proton Density Contrast

TE – echo time TR – repeat time

Proton Density Weighted ImageProton Density Weighted Image

T2 Decay

MRSignal

T1 Recovery

MRSignal

50 ms50 ms 1 s1 s

T1 Contrast

time time

TE – echo time TR – repeat time

T1 Weighted ImageT1 Weighted Image

T2 Decay

MRSignal

T1 Recovery

MRSignal

50 ms50 ms 1 s1 s

T2* and T2 Contrast

TE – echo time TR – repeat time

T2 Weighted IMageT2 Weighted IMage

ProtonProtonDensityDensityWeightedWeightedImageImage

T1 T1 Weighted Weighted ImageImage

T2 T2 Weighted Weighted ImageImage

Properties of Body TissuesTissue T1 (ms) T2 (ms)

Grey Matter (GM) 950 100

White Matter (WM) 600 80

Muscle 900 50

Cerebrospinal Fluid (CSF) 4500 2200

Fat 250 60

Blood 1200 100-200

MRI has high contrast for different tissue types!