In Service MR

Question 1

when does chemical shift artifact occur?

Answer 1

Chemical shift occurs as a result of the Difference in the resonant frequency of protons associated with very hydrogenous (fat) tissues compared to protons associated with water.

Protons are localized by variation of resonance frequencies under the influence of an external magnetic gradient over a specific field of view.

Results in a net range of precessional frequencies across the field of view which is sampled accordin to the MATRIX size.

Question 2

How do you increase chemical shift artifact?

Answer 2

decreasing gradient read out strength.

If the gradient strength is low, that means that a small range of frequencies will be manifested over the FOV, and for a given matrix size, the bandwidth across a single pixel will be similary small.

Since the chemical shift results in frequency shifts of about 3 parts per million, for a 1T magnet, the frequency differences in fat versus water protons is about 1 T x 42.58 MHz/ T x 3 x 10-6 = 125 Hz. If the bandwidth acrosss the pixel is small, then the signals due to fat can potentially be mapped into adjacent pixels, and the anatomical presentation with manifest as “Chemical shift” artifacts.

Question 3

What is the difference in Hz between resonance frequency of fat and water (in a 1 T magnet) ?

Answer 3

125 Hz

Question 4

How do you decrease chemical shift artifact?

Answer 4

• increase receiver bandwidth
• decrease matrix size
• decrease field strength

Question 5

Why does increasing receiver bandwidth decrease chemical shift artifact?

Answer 5

broad bandwidth receiver setting will result in similarly larger bandwidth across each pixel

Question 6

What is the equation for chemical shift

Answer 6

actual chemical shift = relative chem shift (ppm) x B0 x y

Question 7

Why will lower magnetic field strength cause decreased chemical shift artifact?

Answer 7

Lower main magnetic strength will cause the absolute chemical shift to be lower

actual chemical shift = relative chem shift (ppm) x B0 x y

Question 8

Magnetic resonance imaging of the fetus relies MOST heavily on which of the following types of
acquisitions?

Answer 8

Single-shot fast/turbo-spin echo

This type of sequence allows for sub-second image acquisition and the T2 weighting highlights
pathology well.

Question 9

In MR acquisition, decreasing the bandwidth when all other imaging parameters are held constant results in:

Answer 9

Increase SNR

Image SNR is inversely proportional to the square root of the bandwidth. Note: by
reducing the bandwidth, in order to keep the slice thickness the same, the gradient strength must also be reduced.