medical imaging overviewwork2007/25 26 27 28 29 30 31 32...overview 1) historical background 2)...

Medical ImagingMedical Imaging

Magnetic Resonance Imaging

Prof. Ed X. Wuhttp://www.cis.rit.edu/htbooks/mri/inside.htm

OverviewOverview

1) Historical Background2) Physical Basis - Spin Physics3) Nuclear Magnetic Resonance4) Magnetic Resonance Imaging

HistoryHistory1946 MR phenomenon - Bloch & Purcell 1952 Nobel Prize - Bloch & Purcell 1950-70 NMR developed as analytical tool in chemistry1971 Patent on medical MRI to R.V. Damadian,

("Tumor Detection by Nuclear Magnetic Resonance." Science (March 19, 1971)).

1972 Computerized Tomography(G.N. Hounsfield, Br. J. Radiol. 46:1016-1022 (1973).)

1973 Backprojection MRI (P.G. Lauterbur Nature 242:190-191 (1973))

1975 Fourier Imaging A. Kumar, D. Welti, R.R. Ernst "NMR Fourier zeugmatography,”J. Magn. Reson. 18:69-83 (1975) and Naturwissenschaften 62: 34 (1975)

1980 MRI demonstrated - Edelstein 1986 Gradient Echo Imaging and NMR Microscope 1987 Echo-Planar Imaging:

B. Chapman, R. Turner, R.J. Ordidge, M. Doyle, M. Cawley, R. Coxon, P. Glover, P. Mansfield, "Real-Time Movie Imaging from a Single Cardiac Cycle by NMR." Magn. Reson. Med. 5:246-254 (1987

1991 Nobel Prize - Ernst 1994 Hyperpolarized 129Xe Imaging,

M.S. Albert, G.D. Cates, B. Driehuys, W. Happer, B. Saam, C.S. Springer Jr., A. Wishnia” Biological magnetic resonance imaging using laser-polarized 129Xe." Nature 370:199-201 (1994).

2003 Nobel Prize – P Lauterbur & P Mansfield

HistoryHistory

OverviewOverview


MRI Imaging Principles: Physics

Nuclear magnetism. Nuclei with net spin (I) have a characteristic magnetic moment (m) and an associated magnetic field, similar to a dipole, such as a bar magnet

Nuclear Spin, Angular MomentumNuclear Spin, Angular Momentum& Nuclear Magnetic Moment& Nuclear Magnetic Moment

Where γ is the gyromagnetic ratio

• Many nuclei have a magnetic dipole moment, µ,and angular momentum, S.

• |µ|/|S| = γ, the gyromagnetic ratio.

• For a proton γ = 43 MHz/T.

r µ = γ

r S = γ

h

2

r I

A spinning object which carries an electric charge constitutes acirculation of electric current and therefore has an equivalent magnetic moment.

Nuclear Magnetic ResonanceNuclear Magnetic Resonance

What happens when a nucleus with magnetic moment is placed in magnetic field? B0

• Magnetic field makes nuclei precess around it,at rate of Lamor frequency ωL

ωL = γ B0

= 43•1.5 = 64.5 MHz for proton in 1.5 T MRI magnet.= 43•0.5•10-4 = 2 kHz for proton in earth magnetic field.

Nuclear Spin Angular MomentumNuclear Spin Angular Momentum& Nuclear Magnetic Moment& Nuclear Magnetic Moment

Magnetic field present:• Magnetic field makes nuclei precess around it.

• The precession rateis the Larmor frequency fL.

• fL = γ B0 = 43•1.5 = 64 MHz for Hydrogen.

B0

z

Is there macroscopic magnetization?

Energy Levels in Magnetic FieldEnergy Levels in Magnetic Field

Ene

rgy

|B0|

mz = +1/2

mz = -1/2

Spin flip transition

∆Ε = hν = γΒ0

B0

zCan transitions be induced? (Hint: two ways!)

(1) magnetic field Β0 constant -> vary radio frequency ν(2) radio frequency ν constant -> vary magnetic field Β0

Macroscopic MagnetizationMacroscopic Magnetization

No external magnetic field:

Is there net macroscopic magnetization?

B0

z

No macroscopic magnetization.

External Magnetic field:

Macroscopic Magnetization Macroscopic Magnetization at Equilibriumat Equilibrium

NN

e e

e BkT kT

MHz TeV K

up

down

EkT

BkT

BkT

= =

− ≈ = =

− −

−

∆ γ

γ γ ω

0

0

1 65 150 04 300

0 0

Boltzman's equation

h (@ . ). (@ )

≈ 0.999999

For every 1,000,000 nuclei in upper state (mz=+1/2) there are 1,000,001 nuclei in lower state (mz=-1/2).

This difference is enough to result in macroscopic net magnetization of material.

k =1.3805x10-23 [J/Kelvin] “Boltzmann constant”

Net Magnetization MNet Magnetization M

Nevertheless, let us assume that such net magnetization as a single spin from now on for easy understanding

MRI Imaging Principles: Physics

Magnetic Resonance Properties of Some Diagnostically Relevant Nuclei

NUCLEUSRELEVANTABUNDANCE (%)

RELATIVESENSITIVITY*

GYROMAGNETICRATIO (MHz/T)

1H99.98

142.58

13C1.11

0.01610.71

23Na100

0.09311.26

31P100

6.6 x 0.06617.23

39K93.1

5.08 x 0.0005081.99

Equipment DiagramEquipment Diagram

Computer

RF generator

RF Probe Magnet

Bo

B1

909000 RF PulseRF Pulse

µ

•You can tip M by applying a circularly polarized RF magnetic field pulse, B1, to the sample.

•B1 is at the Larmor frequency, γB0.

What happens after you switch off RF pulse (rotating transverse magnetic field)?

909000 RF PulseRF Pulse Equipment DiagramEquipment Diagram

Computer

RF generator

Display

Receiver

Rf Probe Magnet

Bo

NMR Excitation by RF PulseNMR Excitation by RF Pulse

Why?

Left hand rule

NMR Excitation & Signal DetectionNMR Excitation & Signal Detection

Rotating frequency (of transverse spin magnetization Mxy; and direction based on left-handed rule) at specific spatial location and time ONLY depends on the magnetic field (i.e., B0+∆B) at that specific location and time;

BUT the accumulated phase of transverse magnetization (spin phase) depends on the prior history (magnetic field as a function of time between excitation and observation)!

RF Excitation RelaxaRF Excitation Relaxation (tion (T1 and T2 relaxation)T1 and T2 relaxation)

Frequency and amplitude of this voltage signal?FT

909000 RF Pulse: TRF Pulse: T11 RecoveryRecovery

T1 - RecoveryMz = Mo ( 1 - e-t/T1 )

The time constant T1, governs the rate the magnetization recovers along the z-axis (taken to be the main field direction). This process is often called longitudinal relaxation and T1 is called the spin-lattice relaxation time.

The major mechanism for T1 recovery is dipole-dipole interactions of neighboring atoms that randomly collide through thermal motion. (T1(liquid)< T1(solid))

t

909000 RF Pulse: TRF Pulse: T22 DecayDecay

Mxy =Mxyo e-t/T2T2 - Decay

The time constant T2 governs the rate that the magnetization disappears in the plane transverse to the main field. This is called the spin-spin relaxation time.

The source of this decay is the dephasing of all single protons with respect to each other due to different field strength in different parts of the sample and/or magnet.

t

Spin RelaxationSpin Relaxation

T1 - RecoveryMxy = Mxyo e-t/T2Mz = Mo ( 1 - e-t/T1 )

T2 - Decay

The spin-lattice relaxation time, T1, is always longer than the spin-spin relaxation time, T2.

TT11 and Tand T22 Time ConstantsTime Constants

90 ± 20390 ± 70White Matter

100 ± 10495 ± 85Gray Matter

50 ± 10400 ± 40Muscle

60 ± 10240 ± 20Fat

T2 [msec]T1 [msec]Tissue

MRI Imaging Principles: Physics & Data Acquisition

Relaxation Times for Different Brain Tissues at 1.4 T From 5-mm Slice at the Level of the Lateral Ventricles in Three Normal Volunteers

ORGANT1 (ms)

T2 (ms)Putamen

747 ± 3371 ± 4

Caudate nucleus822 ± 16

76 ± 4Thalamus

703 ± 3475 ± 4

Cortical gray matter871 ± 73

87 ± 2Corpus callosum

509 ± 3969 ± 8

Superficial white matter515 ± 27

74 ± 5Internal capsule

559 ± 1867 ± 7

Cerebrospinal fluid (lateral ventricle)1900 ± 353

250 ± 3

Equipment DiagramEquipment Diagram

Computer

Rf generator

Display

Receiver

Rf Probe Magnet

Bo

OverviewOverview


How can one measure T1 and T2?

What is Free Induction Decay (FID)?

OverviewOverview

90o-FID Sequence

S = k ρ 1 − e −TR/ T1( )TR :=Time between two sequences

TR

Pulse Sequence

In reality, how does FID decay? Does it really follow T2 decay?

Hint: B0 is microscopically inhomogeneous; T2* decay (T2*<T2)

SS

FID Signal

Spin-Echo Sequence

Echo Signal

S = k ρ 1 − e −TR / T1( ) e−TE / T2

TR :=Time between two repeating sequencesTE := Time between 900 and maximum of echo

TE

S

Does echo signal S decay with respect to TE by T2 rate?

SpinSpin--Echo Sequence IEcho Sequence I

�� .

Rotating frame Illustration (rotating at the base resonance frequency)

SpinSpin--Echo Sequence IIEcho Sequence II

Inversion Recovery Sequence

FID Signal

S = k ρ 1 − 2e −TI / T1 + e −TR / T1( )

TR :=Time between two sequencesTI := Time between 1800 and maximum of FID

S

What is the advantage of this sequence?

Does 180 RF pulse here produce anyFID signal?

Signal AmplitudesSignal Amplitudes

S = k ρ 1 − e −TR/ T1( )S = k ρ 1 − e −TR / T1( ) e−TE / T2

90o-FID Signal:

Spin Echo Signal:

Inversion Recover Signal: S = k ρ 1 − 2e −TI / T1 + e −TR / T1( )

Amplitudes say something about spin density ρ T1, and T2(TR, TE, and TI are set by programmer of pulse sequence)

Where do these formulas come from?By solving Bloch Equation!

These 3 types of sequences are just examplesWe have almost unlimited number of sequences to manipulate NMR signals!

Signal Amplitudes: Example 1Signal Amplitudes: Example 1

S = k ρ 1 − e −TR/ T1( )S = k ρ 1 − e −TR / T1( ) e−TE / T2

90o-FID Signal:

Spin Echo Signal:

Inversion Recover Signal: S = k ρ 1 − 2e −TI / T1 + e −TR / T1( )

You want to determine ρHow do you design NMR experiment?


S = k ρ 1 − e −TR/ T1( )90o-FID Signal:

You want to determine ρHow do you design NMR experiment?

Chose 90-FID set sequence with pulse repetition TR>>T1:

S = k ρ 1 − e −TR/ T1( )= k ρ 1 −1

exp(TR / T1)

⎛

⎝ ⎜ ⎜

⎞

⎠ ⎟ ⎟ ≈ k ρ


S = k ρ 1 − e −TR/ T1( )S = k ρ 1 − e −TR/ T1( ) e−TE / T2

90o-FID Signal:

Spin Echo Signal:

You are using a spin-echo pulse sequence to study adipose tissue with T1 = 0.2 - 0.75 s and T2 = 53 - 94 ms .

If the minimum TE value you can obtain is 20 ms, how much more signal could you obtain with a 90-FID sequence?

Web Book Chapter 4 - 4

α =SFID 90

SSES=

k ρ 1− e−TR /T1( )k ρ 1− e−TR/T1( ) e−TE /T 2

= eTE /T 2

Signal Amplitudes: Example 2Signal Amplitudes: Example 2You are using a spin-echo pulse sequence to study adipose

tissue with T1 = 0.2 - 0.75 s and T2 = 53 - 94 ms . If the minimum TE value you can obtain is 20 ms, how much more

signal could you obtain with a 90-FID sequence?

α =SFID 90

SSES=

k ρ 1− e−TR /T1( )k ρ 1− e−TR/T1( ) e−TE /T 2

Therefore, under these conditions, the 90o FID sequence would provide 1.24 to 1.46 times the signal of a spin-echo sequence.

= eTE /T 2

α1 = eTE /T 2 = e 20ms/ 53ms = 1.46

α 2 = eTE/T 2 = e20ms /94 ms = 1.24

ReferencesReferences

Derivation of Signal Intensity Formulas for FID, Spin Echo, and Inversion Recovery

See Details in

Magn Reson Imaging. 1984;2(1):23-32. Contrast manipulation in NMR imaging. Perman WH, Hilal SK, Simon HE, Maudsley AA.

The past few years have shown rapid growth of NMR imaging in both image quality and diagnostic usefulness. It has become apparent, as the images have been published, that both inter- and intra-group imaging of the same underlying pathology produces images which can have vastly differing appearance. This effect is mainly due to imaging techniques which use different pulse sequence types and timings thus varying the relative contribution of the proton density, T1, and T2 properties of the tissues. In this paper we investigate the contrast manipulation effects and methods for SNR optimization for the saturation recovery, inversion recovery, spin echo, and inversion recovery spin echo pulse sequences when applied to three clinically relevant imaging tasks.

OverviewOverview


So far only homogenous samples!

How can we use NMR signal to image?

Introduction Introduction The principle behind all magnetic resonance imaging is the resonance equation, which shows that the resonance frequency ν of a spin is proportional to the magnetic field, Bo, it is experiencing.

ν ~ γ Bo

For example, assume that a human head contains only three small distinct regions where there is hydrogen spin density.

Introduction Introduction When these regions of spin are experiencing the same general

magnetic field strength, there is only one peak in the NMR spectrum.

How could we distinguish between different positions of three areas?

1D FT of NMR Signal

ν0 = γ Bo

x

z

Magnetic Field GradientMagnetic Field GradientA gradient in the magnetic field will allow us to distinguish different regions in the brain. A one-dimensional magnetic field gradient is a variation with respect to one direction, while a two-dimensional gradient is a variation with respect to two.

x

?ν0

B(x,y,z)=Bo + x Gx

Bo

Apply a magnetic fieldgradient along x (Gx) during

NMR signal detection

A. Frequency EncodingA. Frequency Encoding

The amplitude of the NMR signal is proportional to the number ofspins in a plane or line perpendicular to the gradient. This procedure is called frequency encoding and causes the resonance frequency

to be proportional to the position of the spin.

ν = γ ( Bo + x Gx ) = νo + γ x Gxx = (ν - νo ) / (γ Gx )

What is Gy or Gz?

Gx

1D FT

B(x,y,z)=Bo + x Gx

ν

Backprojection ImagingBackprojection Imaging

Possible to acquire projection data along all directions?How to apply diagonal frequency encoding gradient?

Possible to reconstruct 2D or even 3D water distribution!(2003 Nobel Prize in Medicine)

Backprojection ImagingBackprojection Imaging

Imag

ing

Sequ

ence

(x-gradient)

(y-gradient)

(z-gradient)

time

Different views can be generated by adjusting x and y gradient strength.

Varying the angle θ of the gradient is accomplished by the application of linear combinations of two gradients. Here the Y and X gradients are applied in the following proportions to achieve the required frequency encoding gradient Gf.

Gy = Gf Sin θGx = Gf Cos θ

NMR data sampling during frequency encoding gradient !

B. Slice SelectionB. Slice Selection

Slice selection is achieved by applying a one-dimensional, linear magnetic field gradient during the period that the RF pulse is applied. A 90o pulse

applied in conjunction with a magnetic field gradient will rotate spins that are located in a slice or plane through the object.

Only in that slice is resonance condition fulfilled (∆Ε = γΒ0)

RF Pulse

For the backprojection technique to work for more than one plane, we need to have the ability to image the spins in a thin slice.

Slice SelectionSlice Selection

Only in that slice is resonance condition fulfilled (∆Ε = γΒ0)

RF Pulse

Thicker slice?

Thinner slice?(Gz)

(Gs) (Gs)

Can you change Gs to adjust slice thickness?

The backprojection imaging technique is very educational but never used in state of the art imagers.

Instead, Fourier transform imaging techniques are used.

http://www.cis.rit.edu/htbooks/mri/inside.htmChapter 7.Fourier Transform Imaging Principles

C. Phase Encoding IC. Phase Encoding IAssume we have three regions with spin. The transverse magnetizationvector from each spin has been rotated to a position along the X axis (900 RF pulse). In a uniform magnetic field Bzo they will possess the same Larmorfrequency.

If a gradient in the magnetic field is applied along the X direction the four vectorswill precess about the direction of the applied magnetic field at a frequency giveby the resonance equation ν = γ ( Bo + x Gx) = νo + γ x Gx

While this phase encoding gradient is on, each transverse magnetization vectorhas its own unique Larmor frequency.

phase encoding gradient

?

Phase Encoding IIPhase Encoding IIIf a gradient in the magnetic field is applied along the X direction the four vectors will precess about the direction of the applied magnetic field at a frequency givenby the resonance equation ν = γ ( Bo + x Gx) = νo + γ x Gx

While this phase encoding gradient is on, each transverse magnetization vector has its own unique Larmor frequency.


So far, the description of phase encoding is the same as frequency encoding.

Now for the difference: If the gradient in the X direction is turned off, the external magnetic field experienced by each spin vector is again identical. Therefore the Larmor frequency of each transverse magnetization vector is

identical, however each region has a different phase.

phase encoding gradient(Gx)

Phase Encoding IIPhase Encoding II


?

Signal (from all x locations) that result from different Gx amplitude?

How are these signals related to spin distribution along x direction?

….





∫∞

∞−

= dxxixfF )exp()()( ωω

.1, ... ,1 ,0 ,2exp)(1)(1

0−=⎟

⎠⎞

⎜⎝⎛= ∑

−

=

NkknN

ikfN

nFN

n

π

Phase modulation or encoding is equivalent to Fourier Transform !

Total signal from all x locations is equivalent to FT of spin distribution along x with respect to x!


Putting it all together (for now)Putting it all together (for now)(1) Static External Field (2) Slice Selection,

(gradient field along z-axis)

Rotated into the XY plane these vectors precess at the Larmor frequency.

Putting it all together (for now)Putting it all together (for now)(2) Slice Selection,

(gradient field along z-axis)

Rotated into the XY plane these vectors precess at the Larmor frequency.

x

y

(3) Phase encoding(gradient field along x-axis)

A phase encoding gradient is applied along the X-axis. The spins at different locations along the X axis begin to precess at different Larmorfrequencies. When the phase encoding gradient is turned off the net magnetization vectors precess at the same rate, but possess different phases. The exact phase is determined by the duration and magnitude of the phase encoding gradient pulse.

x = -2, -1, 1, 2

y = 2

y = 1

y = -1

y = -2

Putting it all together (for now)Putting it all together (for now)(3) Phase encoding

(gradient field along x-axis)A phase encoding gradient is applied along the X-axis. The spins at different locations along the X axis begin to precess at different Larmor frequencies. When the phase encoding gradient is turned off the net magnetization vectors precess at the same rate, but possess different phases. The exact phase is determined by the duration and magnitude of the phase encoding gradient pulse.

(4) Frequency encoding(gradient field along y-axis)

A frequency encoding gradient pulse along the y-axis is turned on. Thefrequency encoding gradient causes spin packets to precess at rates dependent on their Y location.

Now each of the nine net magnetization vectors is characterized by a unique phase angle and precessional frequency.

Spins during different time

……

Putting it all together (for now)Putting it all together (for now)

timing diagram for an imaging sequence(3) Phase encoding

(x-axis gradient GΦ)(4) Frequency encoding

(y-axis gradient Gf)(2) Slice Selection,

(z-axis gradient Gs)

and readout

This sequence of pulses is usually repeated 128 or 256 times tocollect all the data needed to produce an image.

Putting it all together (for now)Putting it all together (for now)Timing diagram for a entire imaging sequence

....

Such repetitive data collection is often drawn as

So 2D data are collected here

How to reconstruct 2D spatial distribution of spins?

How to understand spatial encoding from mathematical perspective?

K-space interpretation: Relating “NMR signal at each time point for different gradient

combinations” to “spatial spin distribution”

MRI: K SpaceMRI: K Space

If kx(t) and ky(t) are defined as shown, then they represent the row and column that the value, digitized at time t, should be assigned to in K Space (Fourier Space)

kx ( t) ≡ 2πγ Gx (t)0

t∫ dt

ky ( t) ≡ 2πγ Gy (t)0

t∫ dt

MRI: Driving through K SpaceMRI: Driving through K Space

γ times the integral of the Gx(t) and Gy(t) gives the position in K space

Kx

Ky

A

BCD

EA

B

C

D

E

RF pulse

Gz

Gx

Gy

MRI: Driving through K SpaceMRI: Driving through K Space

Kx

Ky

A

BCD

EA

B

C

D

E

RF pulseGz

Gx

Gy

Gx, Gy, and Gz gradients are for frequency encoding, phase encoding, and slice selection, respectively.

At point A, all spins are rephrased (in phase) after the slice selective excitation along z direction. Thus the corresponding location in k-space is the center.

At point B, all spins are phase-encoded along y direction. Thus, in k-space, the corresponding point traverses along Ky to point B. The distance between B and A in k-space is proportional to the Gy and its duration.

At point C, all spins are dephased along negative x direction by the negative Gx gradient. Thus, in k-space, the corresponding point traverses along the negative Kx direction to point C. The distance between C and B is proportional to Gx and its duration.

At point D, all spins are refocused by the positive Gx gradient along x direction. Thus, in k-space, the corresponding point transverses back to Kx=0.

At point E, all spins are dephased again along x direction by the continued positive Gx gradient.

kx (t) ≡ 2πγ Gx (t)0

t∫ dt

ky (t) ≡ 2πγ Gy (t)0

t∫ dt

MRI GE Pulse SequenceMRI GE Pulse Sequence

RF

Gz = Gslice

Gx = Greadout

Gy = Gphase encode

TR

TE

http://www.fmrib.ox.ac.uk/~stuart/lectures/lecture3

Sampling kSampling k--spacespace A gradient echo (GE) sequence uses a reversal A gradient echo (GE) sequence uses a reversal in gradient direction to sample kin gradient direction to sample k--spacespace

OverviewOverview

1) Historical Background2) Physical Basis - Spin Physics3) Nuclear Magnetic Resonance

- NMR !- RF Excitation & NMR signal detection- NMR Signals from FID, SE, and IR sequences !

4) Magnetic Resonance Imaging- Frequency encoding, slice selection, phase encoding- K space interpretation and understanding of MRI !

5) Instrumentations & Applications

Components of MRI ImagersComponents of MRI ImagersGE 1.5 T Signa Imager

GE 0.2T ProfileImager

MRI Instrumentation Components of MRI ImagersComponents of MRI Imagers

Main MagnetHigh, constant,Uniform Field, B0.

Gradient CoilsProduce pulsed, linear gradients in this field.Gx, Gy, & Gz

RF coilsTransmit: B1 Excites NMR signal ( Free Induction Decay or FID).Receive: Senses FID.

B0

B0

B0

B1

MagnetsMagnetsMRI Instrumentation: Magnet Subsystem

Superconducting Magnet

MRI Instrumentation: Magnet Subsystem

Permanent Magnet (NdFeB)VirgoTM whole-body MRI system (MTI, Vancouver, Canada)

MRI Instrumentation: Magnet Subsystem

Magnet Types

- Permanent

- Resistive

- Superconducting

Requirements: homogeneity, shimming method

Meaning of Meaning of ““Z GradientZ Gradient””

X

Y

Z

•A “Z gradient” introduces a gradient in the magnetic field in the Z direction. The gradient is produced with resistive coils.

•Traditionally the Z gradient is associated with the RF excitation pulse and slice selection.

z•Gz

B0

Meaning of Meaning of ““X&Y GradientsX&Y Gradients””

x•Gx

X

Y

Z

B0

•An “X or Y gradient” introduces a gradient in the B0magnetic field in the X or Y direction.

•These gradients are associated with readout and phase encode, respectively.

Instrumentation: Gradient Subsystem MRI

Simple DesignsStrength: 1-3 Gauss/cm, 150 A current, 0.1-0.5 rise time

MRI Instrumentation: RF Transmitting/Receiving Coil

High Q Factor

Z

A typical superconducting MRI systemA typical superconducting MRI system

• MRI Contrast is created since different tissues have different T1 and T2.

• Gray Matter: T1= 810 ms, T2= 101 ms

• White Matter: T1= 680 ms, T2= 92 ms

Example of MRI Images of the HeadExample of MRI Images of the Head Example of MRI Images of the HeadExample of MRI Images of the Head

• Bone and air are invisible.• Fat and marrow are bright.• CSF and muscle are dark.• Blood vessels are bright.• Grey matter is darker than

white matter.

Example of MRI Images of the HeadExample of MRI Images of the Head Other Advanced MethodsOther Advanced Methods

• Angiography (MRA)• Functional MRI (fMRI)• Spectroscopic Imaging (MRSI)• Diffusion Imaging• Cardiac Imaging• Non-proton Imaging• NMR Microimaging

MRI Clinical Applications: MRA

T2-weighted

MR Angiograph

T1-weighted

MR Angiograph

MRI Clinical Applications: fMRI Based on BOLD (Blood oxygenation level dependent) Signal

Applications: neuroscience & pre-surgical decision making

x

y

Time

MR

Sig

nal

ON OFF ON OFF ON OFF ON OFFOFF

Visual stimulation (8Hz flickering board)

MRI Clinical Applications: fMRI

BOLD (Blood Oxygenation Level Dependent) Signal

Endogenous MRI contrast agent (Deoxyhemoglobin – paramagnetic)

Local neuronal activation Local CBF, CBV increase Local [dHb] decrease Local BOLD signal increase

MRI Clinical Applications: MR Spectroscopic Imaging (MRSI)

Proton Spectrum in Neural Tissue

N-acetyl aspartate (NAA)Choline (Cho)Creatine (Cr)Glutamine, glutamate (Glx)Lactate (Lac)

MRI Clinical Applications: MR Spectroscopic Imaging (MRSI)

Images obtained in a 44-year-old woman with an anaplastic astrocytoma. A,Gadolinium-enhanced T1-weighted image showing enhancing lesion in the left temporal lobe andinsula. B, Multiplanar GRE localization image. C, Axial FDG PET scan. Hypometabolic areasare bluish-green; hypermetabolic areas are yellowish-red. A rim of hypermetabolic tissue(arrows) surrounds the hypometabolic core. D, 1H MRSI metabolite maps for Cho, Cr, NAA,and lactate. Low-intensity areas are grayish-blue; higher-intensity areas are yellowish-red.Deficits are apparent for Cho, Cr, and NAA, and lactate is elevated in the core.

T1 T2 PET FDG

MRSI

MRI Clinical Applications: Diffusion Imaging

Diffusion Imaging: Principles

Intracellular Water: More boundExtracellular Water: More diffusive

T2-w

T1-w

DiffusionImaging

DWI (diffusion weighted imaging): “EKG for Brain Attack”

Diffusion Imaging: Applications

DWI: 4hrs after MCA occlusion in murine at 4T

DTI (diffusion tensor imaging): Fiber trackingS Mori et al, 2002

Total Sodium-23 MRI at 4.2 Tesla (Normal human volunteer, 10 min)

MRI Clinical Applications: Non-proton Spectroscopic Imaging

Cardiac Imaging

Human Heart

Bright Blood ImagingOptimal Contrast & SNR

Mouse Heart - 500 bpm, 8 cardiac points, 3 adjacent slices

Bright Blood ImagingOptimal Contrast & SNR

500 bpm, 8 cardiac points, 3 adjacent slices

High-Resolution Imaging (50umx50umx600um): Mouse Brain

PDW Image vs. Nissel Staining.

T1-weighted PD-weighted T2-weightedALS transgenic mice (mSOD1G93A): T1, T2 and T2 weighted images of lumbar spinal cord (70umx70umx1.5mm, PD up)

Mouse Spinal Cord Obesity

Obesity

3D Body Composition Measurement to study dietary and pharmacolog3D Body Composition Measurement to study dietary and pharmacological manipulationsical manipulations-- An ongoing study using guinea pig modelsAn ongoing study using guinea pig models

Obesity


Obesity


MRI is a multi-parametric imaging

http://www.hku.hk/bisplab/

medical imaging overviewwork2007/25 26 27 28 29 30 31 32...overview 1) historical background 2)...

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