Parallel Imaging

Question 1

What properties of RF coils are used in parallel imaging?

Answer 1

Each array element is sensitive to a small volume of tissue.

Each array element has a separate reciever.

Question 2

How does SENSitivity Encoding (SENSE) employ parallel imaging?

Answer 2

• Works on data in the image space (post fourier transform).
• Data is undersampled in k-space so that it will alias when Fourier transformed.
• Coil sensitivity data is then used to unwrap that aliasing.
• i.e. Knowing the coil location and the intensity of the signal received from that pixel, its location in space can be calculated.

Question 3

How does GeneRalised Autocalibrating Partially Parallel Acquisition (GRAPPA) employ parallel imaging?

Answer 3

• Aims to reconstruct the missing k-space elements before Fourier transform.
• A small area of k-space is sampled to calculate GRAPPA weights or Autocalibration signal (ACS). This reflects the relationship between neighbouring k-space points. These are usually acquired along with the undersampling so the sampling density is non-uniform throughout the scan.
• By windowing the signal intensity based on the coil sensitivity profile we create a mathematical convolution, effectively smearing the information across neighbouring k-space points.

Question 4

How does SNR change with increased parallel imaging?

Answer 4

SNR_PI=SNR/[g.(R^0.5)]
Where g is the geometry factor based on the coil number and arrangement and R is the acceleration factor of the parallel imaging.

Question 5

Why does SNR definition no apply to GRAPPA images?

Answer 5

Noise is not equally distributed across the image because of the ACS lines.

Question 6

What are the two categories of Parallel imaging artefacts?

Answer 6

Residual aliasing and Noise enhancement

Question 7

What is the most common cause of parallel imaging artefacts?

Answer 7

Too high an acceleration factor for the chosen geometric arrangement of coils
or
Patient/coil movement between the reference scan and anatomical acquisition scan (SENSE)

Question 8

How can artefacts be reduced or eliminated in parallel imaging?

Answer 8

Knowledge of the coil arrangement and phase encoding directions.

Question 9

What is the “hot lips” artefact?

Answer 9

Aliasing of the edge of the image into the centre based on poor coil set up and/or not enough space in the PE direction.