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Contrast Mechanism and Pulse Sequences. Allen W. Song Brain Imaging and Analysis Center Duke University. III.1 Image Contrasts. The Concept of Contrast. Contrast = difference in signals emitted by water protons between different tissues

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contrast mechanism and pulse sequences

Contrast Mechanism and Pulse Sequences

Allen W. Song

Brain Imaging and Analysis Center

Duke University

the concept of contrast
The Concept of Contrast

Contrast = difference in signals emitted by water protons between different tissues

For example, gray-white contrast is possible because T1 is different between these two types of tissue

slide4

MR

Signal

MR

Signal

T2 Decay

T1 Recovery

Static Contrast Imaging Methods

1 s

50 ms

slide5

Most Common Static Contrasts

  • Weighted by the Proton Density
  • Weighted by the Transverse Relaxation Times (T2 and T2*)
  • Weighted by the Longitudinal Relaxation Time (T1)
slide6

The Effect of TR and TE on

Proton Density Contrast

TR

TE

MR Signal

MR Signal

T1 Recovery

T2 Decay

t (s)

t (ms)

optimal proton density contrast
Optimal Proton Density Contrast
  • Technique: use very long time between RF shots (large TR) and very short delay between excitation and readout window (short TE)
  • Useful for anatomical reference scans
  • Several minutes to acquire 256256128 volume
  • ~1 mm resolution
slide9

T2*

Cars on different tracks

Transverse Relaxation Times

T2

slide10

Since the Magnetic Field Factor is always present,

how can we isolate it to achieve a singular T2 Contrast?

Fast Spin

Fast Spin

TE/2

t=0

180o turn

t = TE/2

Fast Spin

Fast Spin

TE/2

t=TE

Slow Spin

Slow Spin

TE/2

t=0

180o turn

t = TE/2

Slow Spin

TE/2

Slow Spin

t=TE

slide11

The Effect of TR and TE on

T2* and T2 Contrast

TR

TE

T1 Recovery

MR Signal

MR Signal

T2 Decay

T1 Contrast

T2 Contrast

optimal t2 and t2 contrast
Optimal T2* and T2 Contrast
  • Technique: use large TR and intermediate TE
  • Useful for functional (T2* contrast) and anatomical (T2 contrast to enhance fluid contrast) studies
  • Several minutes for 256  256  128 volumes, or second to acquire 64  64  20 volume
  • 1mm resolution for anatomical scans or 4 mm resolution [better is possible with better gradient system, and a little longer time per volume]
slide14

T2* Weighted Image

T2* Images

PD Images

slide15

TR

TE

T1 Recovery

T2 Decay

MR Signal

MR Signal

T1 contrast

T2 contrast

The Effect of TR and TE on

T1 Contrast

slide16

Optimal T1 Contrast

  • Technique: use intermediate timing between RF shots (intermediate TR) and very short TE, also use large flip angles
  • Useful for creating gray/white matter contrast for anatomical reference
  • Several minutes to acquire 256256128 volume
  • ~1 mm resolution
slide18

Inversion Recovery to Boost T1 Contrast

S = So * (1 – 2 e –t/T1)

So

S = So * (1 – 2 e –t/T1’)

-So

slide20

In summary, TR controls T1 weighting and

TE controls T2 weighting. Short T2 tissues

are dark on T2 images, but short T1 tissues

are bright on T1 images.

motion contrast imaging methods
Motion Contrast Imaging Methods

Prepare magnetization to make signal sensitive to different motion properties

  • Flow weighting (bulk movement of blood)
  • Diffusion weighting (scalar or tensor)
  • Perfusion weighting (blood flow into capillaries)
flow weighting mr angiogram
Flow Weighting: MR Angiogram
  • Time-of-Flight Contrast
  • Phase Contrast
time of flight contrast

Acquisition

Excitation

Saturation

No Flow

Medium Flow

High Flow

No

Signal

Medium Signal

High

Signal

Vessel

Vessel

Vessel

Time-of-Flight Contrast
slide24

Time to allow fresh

flow enter the slice

90o

90o

RF

Excitation

Gx

Saturation

Image

Acquisition

Gy

Gz

Pulse Sequence: Time-of-Flight Contrast

slide25

Blood Flow v

Externally Applied

Spatial Gradient -G

Externally Applied

Spatial Gradient G

T

2T

0

Time

Phase Contrast (Velocity Encoding)

slide26

Pulse Sequence: Phase Contrast

90o

RF

Excitation

G

Gx

Phase

Image

Acquisition

-G

Gy

Gz

diffusion weighting
Diffusion Weighting

Externally Applied

Spatial Gradient -G

Externally Applied

Spatial Gradient G

T

2T

0

Time

slide30

Pulse Sequence: Spin-Echo Diffusion Weighting

180o

90o

RF

G

G

Excitation

Gx

Image

Acquisition

Gy

Gz

advantages of dwi
Advantages of DWI
  • The absolute magnitude of the diffusion
  • coefficient can help determine proton pools
  • with different mobility
  • 2. The diffusion direction can indicate fiber tracks

ADC

Anisotropy

perfusion weighting arterial spin labeling
Perfusion Weighting: Arterial Spin Labeling

Imaging Plane

Labeling Coil

Transmission

slide37

Arterial Spin Labeling Can Also

Be Achieved Without Additional Coils

Pulsed Labeling

Imaging Plane

Alternating

Inversion

Alternating

Inversion

EPISTAR

EPI Signal Targeting with Alternating Radiofrequency

FAIR

Flow-sensitive Alternating IR

slide38

Pulse Sequence: Perfusion Imaging

180o

180o

90o

RF

Gx

Image

Gy

Alternating

Proximal Inversion

Odd Scan

Even Scan

Gz

90o

180o

180o

RF

Gx

Image

Gy

Odd

Scan

Alternating opposite

Distal Inversion

Gz

Even

Scan

EPISTAR

FAIR

advantages of asl perfusion imaging
Advantages of ASL Perfusion Imaging
  • It can non-invasively image and quantify
  • blood delivery
  • Combined with proper diffusion weighting,
  • it can assess capillary perfusion
slide42

III.2 Some of the fundamental acquisition

methods and their k-space view

slide43

k-Space Recap

Equations that govern k-space trajectory:

Kx = g/2p 0tGx(t) dt

Ky = g/2p 0tGx(t) dt

These equations mean that the k-space coordinates

are determined by the area under the gradient waveform

gradient echo imaging
Gradient Echo Imaging
  • Signal is generated by magnetic field refocusing mechanism only (the use of negative and positive gradient)
  • It reflects the uniformity of the magnetic field
  • Signal intensity is governed by

S = So e-TE/T2*

where TE is the echo time (time from excitation to

the center of k-space)

  • Can be used to measure T2* value of the tissue
mri pulse sequence for gradient echo imaging
MRI Pulse Sequence for Gradient Echo Imaging

Excitation

Slice

Selection

Frequency

Encoding

Phase

Encoding

digitizer on

Readout

multi slice acquisition
Multi-slice acquisition

Total acquisition time =

Number of views * Number of excitations * TR

Is this the best we can do?

Interleaved excitation method

slide48

readout

readout

readout

TR

Excitation

……

Slice

Selection

……

Frequency

Encoding

……

Phase

Encoding

Readout

spin echo imaging
Spin Echo Imaging
  • Signal is generated by radiofrequency pulse refocusing mechanism (the use of 180o pulse )
  • It doesn’t reflect the uniformity of the magnetic field
  • Signal intensity is governed by

S = So e-TE/T2

where TE is the echo time (time from excitation to

the center of k-space)

  • Can be used to measure T2 value of the tissue
slide50

MRI Pulse Sequence for Spin Echo Imaging

180

90

Excitation

Slice

Selection

Frequency

Encoding

Phase

Encoding

digitizer on

Readout

fast imaging sequences
Fast Imaging Sequences

How fast is “fast imaging”?

In principle, any technique that can generate an entire image

with sub-second temporal resolution can be called fast imaging.

For fMRI, we need to have temporal resolution on the order of

a few tens of ms to be considered “fast”. Echo-planar imaging,

spiral imaging can be both achieve such speed.

echo planar imaging epi
Echo Planar Imaging (EPI)
  • Methods shown earlier take multiple RF shots to readout enough data to reconstruct a single image
    • Each RF shot gets data with one value of phase encoding
  • If gradient system (power supplies and gradient coil) are good enough, can read out all data required for one image after one RF shot
    • Total time signal is available is about 2T2* [80 ms]
  • Must make gradients sweep back and forth, doing all frequency and phase encoding steps in quick succession
  • Can acquire 10-20 low resolution 2D images per second
slide54

...

...

...

Pulse Sequence

K-space View

why epi
Why EPI?
  • Allows highest speed for dynamic contrast
  • Highly sensitive to the susceptibility-induced field

changes --- important for fMRI

  • Efficient and regular k-space coverage and good

signal-to-noise ratio

  • Applicable to most gradient hardware
slide56

RF

t = TE

t = 0

Gx

Gy

Gz

Spiral Imaging

why spiral
Why Spiral?
  • More efficient k-space trajectory to improve
  • throughput.
  • Better immunity to flow artifacts (no gradient at
  • the center of k-space)
  • Allows more room for magnetization preparation,
  • such as diffusion weighting.
slide62

However, if we don’t have a homogeneous field …

(That is why shimming is VERY important in fast imaging)

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