Physics of Artefacts

Question 1

What is an artefact?

Answer 1

An artefact is a signal in our image which obscures the reality of the object we are imaging.

Question 2

What types of patient motion artefact are there?

Answer 2

• Gross patient motion
• Respiratory motion
• Cardiac motion
• Peristatic motion

Question 3

How can motion artefacts be avoided?

Answer 3

• Sedation
• Gating scans (ECG, vectorcardiogram, respiratory bellows)
• Fast or non-motion sensitive sequences.

Question 4

How do motion artefacts appear on an image?

Answer 4

Motion artefacts appear as multiple ghosts or blurring in the phase encoding direction. The outline of the object is generally visible.

Question 5

How does the chemical shift artefact arise?

Answer 5

• The motion of electrons produces a weak magnetic field of their own, that is proportional to and opposed the applied magnetic field.
• The nucleus experiences the vector sum of these fields, altering its precessional frequency.
• As frequency encodes position, the scanner reads this as a shift in position.

Question 6

How does chemical shift artefact appear on an image?

Answer 6

Thick black outline on one side of a structure. Often called Indian-ink artefact.

Question 7

How can chemical shift artefact be reduced?

Answer 7

Increasing bandwidth, however this also reduces SNR.

Question 8

What is the water-fat shift in Hz?

Answer 8

435Hz

Question 9

What causes the Phase Wrap Artefact?

Answer 9

• If the anatomy is not fully covered with the FOV, the excited signal from outside the image can be aliased in and over to the other side of the image.
• In parallel imaging, we can get the “hot lips” artefact where the signal wraps to the middle of the image.

Question 10

How can we avoid Phase Wrap artefact?

Answer 10

• Cover the entire FOV in the PE direction

- Pre-Sat pulse to destroy magnetisation outside the FOV.

Question 11

What artefacts arise from a FISP (Fast Imaging with Steady State Free-Precession) imaging sequence?

Answer 11

• Unlike other gradient echo sequences, in FISP the transverse magnetisation is not crushed between each TR.
• Therefore the off-resonance effect of the precession of fat spins can null the water signal.

Question 12

How can off-resonance artefacts be minimised?

Answer 12

• Short TR and TE to give the widest passband signal.
• Ensure that shimming and F0 routines locate the resonance frequency in the centre of the anatomical region of interest (change routine if necessary).
• Manually offset carrier frequency (F0) to move artefact (may not be appropriate near implanted objects).

Question 13

What is a zipper artefact and how can it be minimised?

Answer 13

• A zipper artefact happens because external RF has been detected be the scanner creating a band of speckled interference.
• Since the interference in not coherent with the phase encode gradient it extends across the images in the phase encode direction.
• Causes include external equipment (monitoring equipment etc) that has been brought inside the Faraday cage surrounding the scanner.
• Removal of the equipment will remove the artefact.