Artifacts in Cardiac MRI

1
Artifacts in Cardiac MRI

• Andreas Sigfridsson, IMV/IMT/CMIV
  ltsigge_at_imv.liu.segt
• 2007-04-27

2
Dynamic imaging

• Images are acquired line-by-line in k-space
• Motion during acquisition causes artifacts, not
  blurring
• Special care must be taken
• Triggering
• Acquisition ordering

3
Example

• Simulation example
• 256x256, TR20 ms, total scan time 5 s

Image domain
k-space
4
Signal saturation

• Signal recovery - relaxation

5
Signal saturation

• Progress to steady-state

6
Varying signal during inflow

• The MR signal is increased during inflow
• Signal increase related to flow velocity
• More magnetization because of T1 decay/recovery

Flowing spins, fresh signal
Static spins, saturated signal
7
Varying signal intensity

• Simulated six sinusoidal heart beats

Intensity
Image (animation)
8
Varying signal intensity, results

• Reconstructed image

High contrast
Normal contrast
9
Varying signal intensity, k-space

• Image artifacts because of k-space sampling
• Different lines are acquired with different
  signal intensity

10
Varying signal intensity, analysis

• Analyze signal

11
Slow sine variation
12
Varying signal intensity, results

• Reconstructed image

High contrast
Normal contrast
13
Fast sine variation
Intensity
FT(Intensity)
14
Fast sine variation
Intensity
FT(Intensity)
15
Physiological variation

• Aortic flow (handcrafted, not actual data)

Intensity (whole scan)
Intensity (one beat)
16
Physiological variation
Intensity variation (animation)
17
Physiological variation
FT(Intensity)
Intensity
18
Physiological variation

• Reconstructed images

High contrast
Normal contrast
19
Remedies

• Triggering
• Wait for ECG R-tag
• Wait additional trigger delay time
• Acquire one (or several) k-space lines
• Acquisition reordering
• Measure current cardiac (or respiratory phase)
• Choose k-space line to acquire
• Creates a slow varying intensity curve along
  k-space

20
Triggering - random phase

• If triggering is not accurate, random variations
  will still be left

Intensity
FT(Intensity)
21
Random phase, reconstructed
Normal contrast
High contrast
22
Acquisition reordering
Intensity
FT(Intensity)
23
Acquisition reordering
Ordered
Random
24
Acquisition reordering

• Reconstructed images

High contrast
Normal contrast
25
Actual examples

• ECG triggering failed

26
Dynamic imaging

• Images are acquired line-by-line in k-space
• Motion during acquisition causes artifacts, not
  blurring
• Special care must be taken
• Triggering
• Acquisition ordering
• Artifacts might be predictable through use of
  Fourier transform