Contrast Mechanisms in MRI

Introduction to Cardiovascular Engineering

Michael Jay Schillaci, PhDManaging Director, Physicist

Tuesday, September 16th, 2008

Overview

Image Acquisition Basic Pulse Sequences Unwrapping K-Space Image Optimization

Contrast Mechanisms Static and Motion Contrasts

T1 & T2 Weighting, Field Strength, T2*, Dephasing Endogenous Contrasts

BOLD Imaging Motion Contrasts

Time of Flight, Diffusion, Perfusion

Basic Pulse Sequences

Image Formation Integrate magnetization to get MRI signal

Select a z “slice” and form image of XY plane variations

Contrast from difference in magnetization in different tissues— Image at several times to get average

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XYz

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YX

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Horizontal density

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Basic MRI Scan Terminology Orientation:

Coronal Sagittal Axial

Matrix Size: # of Voxels in dimension

Field of view (FOV): Spatial extent of dimension

Resolution: FOV/Matrix size.

Axial Orientation64x64 Matrix192x192mm FOV3x3mm Resolution

Sagittal Orientation256x256 Matrix256x256mm FOV1x1mm Resolution

CoronalCoronal SagittalSagittal AxialAxial

Image Creation

The scanning process1. Protocol sets Gradients and Encodes K-Space Weights2. Signal is Determined with Fourier Transform

3. Image Created with Inverse Transform

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dtGkt

yy 0

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nyx

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Step 1

Step 2

Step 3

Image Acquisition

sxpeyy TGTGy

FOV

221

TNGTGxFOV

sxsxx

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Gy varies in each cycle

Data Acquisition (DAQ)

Gradient Field Ensures Field Greater on “Top”

Larmor Frequency Depends on z Position

RF pulse Energizes “Matched” Slice

Slice Selection Gradient: Gsl

Fie

ld S

tren

gth

Z Position

Frequency Encoding Gradient: Gro

Apply transverse gradient when we wish to acquire image.

Slice emits signal at Larmor frequency, e.g. lines at higher fields will have higher frequency signals.

X P

ositi

on

Field Strength

Phase Encoding Gradient: Gpe

Apply Orthogonal RF pulse Apply before readout Adjusts the phase along the dimension (usually Y)

Y P

ositi

on

Field Strength

Unwrapping K-Space

TNGTGxFOV

sxsxx

221

sxpeyy TGTGy

FOV

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x

x

N

FOVx

Image Adapted from Prof. Yao Wang’s Medical Imaging course notes at: http://eeweb.poly.edu/~yao/EL5823

y

y

N

FOVy

Pixel Size:

Field of View:

Choose phase encoding

time so that yx

Image Optimization

Adjustment of Flip Angle Parameter Maximum SNR typically between 30 and 60 degrees Long TR sequences (2D)

Increase SNR by increasing flip angle Short TR sequences (TOF & 3D)

Decrease SNR by increasing flip angle

1cos TTR

E e

*2

1

1

cos1

1sin0

TTE

TTR

TTR

Spoil ee

eMS

Maximizing the signal

gives the: Ernst Angle:

1. Assume perfect “spoiling” -transverse magnetization is zero before each excitation:

2. Spin-Lattice (T1) Relaxation occurs between excitations:

1. Assume steady state is reached during repeat time (TR):

2. Spoiled gradient rephases the FID signal at echo time (TE):

Gradient Echo Imaging

coszAzB MM

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TTR

TTR

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TTE

TTR

TTR

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TE

zASpoil eMS

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Spin Echo Imaging

Spin echo sequence applies a 180º “refocusing pulse” Half way between 90º pulse and DAQ Allows measurement of true T2 time

T2

T2*

The “Refocusing Pulse”

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l

0

1

T2

T2*

0.5 TE 0.5 TE

Actual Signal

Spins Rotate at Different Rates Refocusing Pulse Re-Aligns Spins

1mm Gap

2mm Thick

Volume Reconstruction 3D volumes

composed of 2D slices

Slice thickness. Thicker slices have more hydrogen so

more signal (shorter scan time) Thinner slices provide higher resolution

(longer scan time)

Optional: gap between slices. Reduces RF interference (SNR)

Fewer slices cover brain3mm

NVSNR

Static Contrast Mechanisms

T1 and T2 Weighting T1 Contrast

Echo at T2 min Repeat at T1 max

T2 Contrast Echo at T2 max Repeat at T1 min

Net Magnetization is

T1 Contrast Weighting

T2 Contrast Weighting

TE

TR TE

TR

Min T2 Contrast Max T1 Contrast

Max T2 Contrast Min T1 Contrast

decay

T

TE

eryre

T

TR

XY eeMM

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cov

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Static Contrast Images

T2 Weighted Image (T2WI)(Gray Matter – CSF Contrast)

T1 Weighted Image (T1WI)(Gray Matter – White Matter)

Examples from the Siemens 3T

“Anatomical Image” “Diagnostic Image”

Flip Angle Variation

RF Pulse Magnitude Determines Flip Angle Duration and magnitude are important

cosMM Z

Adapted from: http://www.mri.tju.edu/phys-web/1-T1_05_files/frame.htm

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+z

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sinMM XY

Field Strength Effects

Increased field strength Net magnetization in material is greater Increased contrast means signal is increased Image1 resolution is better

1MRI adapted from: http://www.mri.tju.edu/phys-web/1-T1_05_files/frame.htm

Muscle

Tissue

Tissue Contrast and Dephasing

Dephasing of H2O and Fat MRI signal is a composite of Fat and H2O signals

H2O and Fat resonate at different frequencies

T1F = 210 ms, T1W = 2000 ms ( T1F > T1W → fat is brighter) Relative phase gives TE dependence

Anti-Parallel (ΦFW = 180o )

@ TE = 15.66 ms

Parallel ( ΦFW = 0o )

@ TE = 13.42 ms

ΦFW

MF

MW

Endogenous Contrast

BOLD Imaging

Blood Oxyenation Level Dependent Contrast dHb is paramagnetic, Hb is less Susceptibility of blood increases linearly with oxygenation BOLD subject to T2* criteria

Oxygen is extracted from capillaries Arteries are fully oxygenated Venous blood has increased proportion of dHb Difference between Hb and dHb is greater for veins Therefore BOLD is result of venous blood changes

Sources of the BOLD Signal

Neuronal activity Metabolism

Blood flow

Blood volume

[dHb]BOLDsignal

BOLD is a very indirect measure of activity…

Neuronal Origins of BOLD

Adapted from Logothetis et al. (2002)

BOLD response predicted by dendritic activity (LFPs)

Increased neuronal activity results in increased MR (T2*) signal

LFP=Local Field Potential; MUA=Multi-Unit Activity; SDF=Spike-Density Function

BASELINE ACTIVE

The BOLD Signal

BASELINE ACTIVE

BOLD Imaging

Blood Oxyenation Level Dependent Contrast Susceptibility of blood changes with oxygenation Blood flow correlated with task performance Differential activations can be mapped

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2

Static Contrast - T2* Relaxation

T2* accounts for magnetic defects and effects

T2 is relaxation due to spin-spin interaction of nuclei

T2M is relaxation induced by inhomogeneities of main magnet

T2MS is relaxation induced by magnetic susceptibility of material

0BM

MSM TTTT 2

1

2

1

2

1

2

1*

mM

BOLD artifacts

fMRI is a T2* image – we will have all the artifacts that a spin-echo sequence attempts to remove.

Dephasing near air-tissue boundaries (e.g., sinuses) results in signal dropout.

BOLDNon-BOLD

Motion Contrast

Flow Weighting

• Time-of-Flight Contrast

No Flow

Medium Flow

High Flow

No Signal

Medium Signal

High Signal

Vessel

AcquisitionSaturation Excitation

Vessel Vessel

Diffusion Coefficients Magnitude (ADC) Maps “Proton pools” Direction (Anisotropy) Maps “Velocity” Reconstruct Fiber Tracks with “Clustering”

Diffusion Tensor ImagingADCADC AnisotropyAnisotropy

Dtl 2322

3

2TGD

oeSS

FA VectorMD

Indices of Diffusion Anisotropy

2 2 2

1 2 3 3RA

Relative anisotropy:

Fractional anisotropy: 2 2 2 2 2 2

1 2 3 1 2 33 2FA

Healthy

DTI in Stroke Research Examine integrity of fiber tracts

Tractography - trace white matter paths in gray matter Assess neglect as a disconnection syndrome

Stroke

Arterial Spin Labeling

Perfusion Flow of fluid into vessels to supply nutrients/oxygen

The amount and direction of flow matters

AlternatingAlternatingInversionInversion

Pulsed Labeling

AlternatingInversion

Imaging PlaneImaging Plane

FAIRFAIRFlow-sensitive Alternating IRFlow-sensitive Alternating IR

EPISTAREPISTAREPI Signal Targeting with Alternating RadiofrequencyEPI Signal Targeting with Alternating Radiofrequency

ASL Pulse Sequences

RF

Gx

Gy

Gz

Image

90o 180o

Alternating oppositeDistal Inversion

OddScan

EvenScan

180o

RF

Gx

Gy

Gz

Image

90o180o180o

AlternatingProximal Inversion Odd Scan

Even Scan

FA

IRF

AIR

EP

IST

AR

EP

IST

AR

Flow-sensitive Alternating IRFlow-sensitive Alternating IR

EPI Signal Targeting with Alternating RadiofrequencyEPI Signal Targeting with Alternating Radiofrequency