1. Magnetic Resonance Imaging
FRCR Physics Lectures
Anna Beaumont

2. MRI Artefacts

3. MRI Artefacts Aspect of image that is not in original object/patient
May degrade image or be more subtle
Often can be related to a particular encoding direction
May be known as alternative names, some more common than others…
MRI Artefacts

4. Ghosting Due to instabilities in system or motion
Part of anatomy will change its position relative to encoding resulting in spatial blurring
Ghosts can be discrete (periodic motion: cardiac beats, arterial pulsations, respiration) or diffuse (non- periodic), e.g. bowel peristalsis
Principally occurs in PE direction due to disparity in sampling times for FE (ms) and PE (s to min)
Also, motion along any magnetic field gradient results in abnormal phase accumulation, which mismaps the signal along the phase encoding gradient.

5. Example of Ghosting Examples in a test object and patient
The patient image has been windowed to reveal ghosting:
Non-periodic: Eye movement
Periodic: pulsatile motion PE

6. Example of Ghosting
Ghosts may be dark or bright depending on the phase of the pulsating structure w.r.t phase of background. If they are in phase will be bright, out of phase will be dark.

7. Ghosting: Elimination
The separation of ghosts is given by:
SEP = (TR) (Ny) (NEX)
T( motion)
Example
The aorta pulsates according to the heart rate. If HR = 60bpm, T(motion) = 1s
If TR=0.5s, NEX=1, Ny =256,
SEP =(0.5 x 256)/1 = 128/1 = 128 pixels.
→ Get two ghosts in the image.

8. Ghosting: Elimination
Restrict patient motion (compression devices/ sedation)
Periodic motion can be ‘gated’
Cardiac/respiratory triggered scanning
Longer scan times
Alternatively use ‘breath hold’
Modern scanners have very fast acquisition times
Non-periodic motion more difficult
Anti-peristalsis drugs
PE & FE direction swap
Increase separation between ghosts by increasing TR, Ny or NEX (increases scan time)
Saturation bands
Can suppress the signal from moving tissues with signal suppression techniques

9. Wraparound Sometimes called wrapping or aliasing or ‘foldover’
Predominantly ‘phase wrap’
Occurs when anatomy extends beyond FOV
Part of object outside FOV has same phase as a part inside
Outer anatomy is mapped into FOV after reconstruction

10. Wraparound
There is a gradient in a direction, with a max. frequency (fmax) at one end of the FOV and a min. f at the other end, (Nyquist frequency)
Any frequency higher than fmax cannot be detected correctly.
The computer can’t recognise frequencies outside the bandwidth (which determines the FOV).
Any frequency outside of this frequency range is going to get “aliased” to a frequency that exists within the bandwidth.

11. Wraparound
The “perceived” frequency will be the actual frequency -2f Nyq
f (perceived) = f (true) – 2f(Nyquist)
Example:
If frequency bandwidth is 32 kHz (±16kHz).
If we have a frequency in the arm (outside the FOV) of +17kHz
f (perceived) = +17 – 2(16) = -15 kHz
Now the arm will be identified on the opposite side of the image, the low frequency side.

12. FOV
FOV 0°
360°

13. Why do we see phase wrap in the PE direction?
Phase-wrap in a test-object and a patient with the arm placed outside of FOV and wrapped in at bottom of image
Why do we see phase wrap in the PE direction?
No. of PE steps is related to scan time, so time can be shortened by reducing FOV. If it’s shortened by too much can get wraparound.

14. Phase-wrap: Elimination
Increase FOV size
Reduces spatial resolution
Use of surface coils
Saturation of pulses from outside the FOV
Use of ‘No Phase-Wrap’ (NPW)
Doubles FOV in PE direction
Doubles no. of PE steps (to maintain spatial resolution)
Halving no. of excitations (same scan time, but lower SNR)
Displaying image on user-defined FOV

15. Gibbs Artefact Also called ringing or truncation artefact
Appears as parallel lines adjacent to high contrast interfaces (skull/ brain, cord/CSF).
Cause is inability to approximate exactly steplike change in signal intensity
Consequence of using FT to reconstruct signals
Signal represented by infinite sum of sine waves (Fourier theory)
Series is truncated due to finite sampling
FT produced with ripples→ parallel bands
Pronounced at high-low signal interfaces

16. Ringing Example in a test object Artefact is only at edge
This example has 256  64 matrix
Reduced by
increasing the number of PE steps (64 → 256)
Increase sampling time
Common in spine, where can mimic appearance of syringomyelia

17. Chemical Shift (1st kind)
Difference in resonant frequency of water and fat (chemical-shift effect)
At 1.5 T the difference is about 220 Hz
Spatial location in FE direction assumed to be consequence of FE gradient.
Due to shift voxels containing fat will not have expected frequency and will be mis-registered.
Results in signal void and build-up on opposite sides

19. Chemical Shift (1st kind)
Chemical shift is increased by
A stronger magnetic field
A lower bandwidth.
If we decrease BW→ lower BW / pixel, fewer frequencies/ pixel
Example: BW ↓ from 32 to 16 kHz.
BW/ pixel = 160/256= 62.5 Hz per pixel
220 Hz/ 62.5 Hz/ pixel ~ 4 pixels misregistration, as opposed to ~ 2pixels at 32kHz BW

20. Here the effect is shown in an egg
Example in kidney
Here the effect is shown in an egg FE

21. Chemical Shift (1st kind)
Solutions
Fat suppression
Increase pixel size by keeping FOV same & decreasing No. of frequency encoding steps (but resolution ↓)
B0 ↓
BW ↑ (but SNR ↓)
Use long TE (less signal from fat)

22. Chemical Shift (2nd kind)
Only occurs in GE sequences
The absence of 180 refocusing pulse causes phase shift between fat and water when echo is formed.
Difference in resonant frequency of water and fat leads to phase differences when echo is formed.
At 1.5T f shift is 225 Hz = period of 4.4ms
Every 4.4ms they will be IN phase
At 2.2ms they are exactly out of phase
For TE → out of phase voxels containing same amount of fat & water will cancel and produce a black line at tissue/fat borders.
In-phase
Out-of-phase

23. Chemical Shift (2nd kind)
If TE is 2.25, 6.75,11.25ms etc., fat and water will be out of phase and a dark boundary will be seen around organs surrounded by fat, e.g. kidneys and muscles.
Called boundary effect, or India Ink etching
Does not just occur along FE axis (as with chemical shift of the first kind) because of fat and water proton phase cancellation in all directions.
Does not occur in SE because of 180 refocusing pulse.
To reduce:
Choose appropriate TE so in phase
Increase BW (but decreases SNR)
Fat suppression

24. Susceptibility Artefact
natural property of all tissues.
Measure of how magnetised tissue becomes when placed in strong magnetic field
Depends on arrangement of electrons in tissue
Diamagnetic
Weak susceptibility
Produces internal field in opposite direction to applied field.
Most body tissues are diamagnetic; air & dense bone have almost zero susceptibility
Paramagnetic
Stronger susceptibility
Produce a field in direction as main field
E.g. gadolinium, deoxy-haemaglobin and met-haemoglobin
Ferromagnetic
strongly magnetised
Experience large force when placed in an external field.
E.g. metal alloys containing iron, nickel and cobalt.
Q: is blood ferromagnetic?
A: No.
The exchange interaction which produces ferromagnetism does not occur unless iron is found in bulk form. In the body, iron is distributed in various chemical compounds such as haemoglobin (RBC), serum ferritin (iron stores) and haemosiderin (e.g. in clotted blood). These compounds are not ferromagnetic; they are weakly paramagnetic.

25. Susceptibility Artefact
Although the magnetic susceptibility of tissues is small, the differences between the tissues & air are big enough to set up local magnetic field gradients.
Hydrogen atoms on either side of the boundary will be in different magnetic fields & will relax more quickly as they interact with each other
Artefacts occur at interfaces between materials of very different susceptibilities as local distortions are caused in the magnetic field.
Natural boundaries e.g. air in sinuses/ tissue or tissue/ trabecular bone
Also any metal (not just ferromagnetic)
Form part of the static field inhomogeneities seen in T2*
Results in local distortions
Local field distortion can lead to precessional frequency shift → when Gss is performed there is an absence of spin excitation and absence of signal
When signal is acquired → shift of spatial localisation which causes signal loss and/or image distortion.

26. Susceptibility Examples
(Right) a dental device has produced a huge susceptibility artefact
Signal void plus distortion of anatomy
An air bubble in this phantom creates susceptibility effects
-worse at long TE (short TE allows less time for signal loss)
TE = 5 ms
TE = 42 ms
- Can be reduced by using SE (not GRE) or high bandwidths
- Susceptibility artefacts are used to detect haematomas: blood breakdown products (hemosiderin..) cause artefacts that are responsible for a signal loss in T2*-weighted GE images.

27. RF or ‘Zipper’ Artefact
Causes a discrete zip- like line in PE direction
Caused by RF interference
Usually a breakdown in RF integrity of system
Severe artefact on right FE PE

28. Herringbone artefact Also known as “corduroy” artefact.
Series of high and low intensity stripes across the image.
Often appears on just one or two images in a multislice set.
Caused by spike noise in the raw data, whose F.T. is then convolved with all the image information.
A single image with this artefact could be bad luck, but issues with several different scans is a symptom of breakdown of either the RF coils or the decoupling system.

29. Herringbone artefact
Due to interferences while filling‐in the k‐space during read‐out;
 These interferences can be RF spikes which lead to points in k‐space with high intensity;
 In any of these cases, the image will be contaminated by a zebra pattern, usually oblique, which affects the whole image; The spatial frequency of the pattern depends on which points of k‐space were affected;