Saturation

Question 1

What is saturation?

Answer 1

when the spins are completely in transverse plane and have no longitudinal component left.

Question 2

How can saturation be achieved?

Answer 2

by exposing the spins to fast repeating RF pulses (and de-phasing gradient)

Question 3

True or false. A saturation sequence starts with a saturation pulse, then 90 RF, then 180 RF, and ends with no echo

Answer 3

True

Question 4

True or false. Saturation exists only briefly.

Answer 4

True. Net magnetization recovers (longitudinal relaxation) immediately after protons are “saturated”

Question 5

What are the types of saturation?

Answer 5

spatial and chemical (water & fat)

Question 6

What is spatial saturation?

Answer 6

The application of an RF pulse immediately prior to the imaging sequence saturates all the protons under the influence of that pulse (so that slice has no signal)

The pulse is slice selective

Question 7

What are the advantages of spatial saturation?

Answer 7

• Reduce motion artifacts in the phase encoding direction (swallowing, CSF pulsation, respiratory motion)
• Reduce signal from flowing blood
• Facilitate angiography/venography

Question 8

What are some disadvantages of spatial saturation?

Answer 8

• Higher SAR (strong RF pulse)
• Drop in overall SNR (because you’re killing the signal)
• Fewer slices per TR (timing of saturation pulse prolongs effective TR interval

Question 9

What is chemical saturation?

Answer 9

similar to spatial saturation; narrow band RF pulse causes selective saturation of water or fat protons

Question 10

What is the order of a fat saturation (fat sat) sequence?

Answer 10

Fat sat pulse, 90 RF, 180 RF, and echo from water only

Question 11

Why is the echo smaller in a fat sat?

Answer 11

because of the overall SNR drop

Question 12

A Fat Sat gives better visualization of ___.

Answer 12

fluid

Question 13

What are the advantages of a fat sat?

Answer 13

• Increases conspicuity of fluid on T2 weighted images (widens dynamic range)
• Post-gadolinium T1 weighted is best with fat sat
• Reduced respiratory motion artifact

Question 14

What are some disadvantages/limitations of a fat sat?

Answer 14

• Fewer slices per TR (timing of saturation pulse prolongs effective TR interval)
• Higher SAR
• Lower SNR
• Requires homogenous magnet (good shimming; not good at FOV > 30 cm^2)
• Requres uniformly shaped body part (doesn’t work well at base of neck, crook of ankle, etc.)
• Works poorly at lower fields

Question 14

The failure of a fat sat could be due to:

Answer 14

• body shape at the imaging area
• location at the bore (center or side)
• metallic objects close to area of imaging
• type of fat sat used

Question 15

What can be done if there is a fat sat failure?

Answer 15

• reposition the body part
• rerun with better shimming
• use sat band (on metals or are with hgih susceptibility issue)
• use different saturation system

Question 16

What happens during fat suppression?

Answer 16

Fat supression reduces total signal by suppression of fat from the voxel; therefore reduces the SNR

Question 17

What is water excitation?

Answer 17

This mean that you tune the RF very narrowly to only flip the water signal; resulting in an image where the fat signal is weak and hence looks like a fat sat image

used on 3D gradient echoes for post contrast imaging