Pulse Sequences

Pulse sequence

Interplay of various parameters leading to a complex cascade of events with RF pulses and gradients to form an MR image

Time to Repeat (TR)

• Time between excitation pulse to the application of the next excitation pulse

• Determines how much longitudinal magnetization recovers between each pulse

• Measured in milliseconds (ms)

Time to Echo (TE)

• Time between the application of RF pulse and peak of the signal

• Measured in milliseconds (ms)

• Time where the signal is formed

Flip Angle

Angle of the RF pulse

1. Spin Echo Sequence (SE)

2. Gradient Echo Sequence (GRE)

3. Inversion Recovery (IR) Sequence

4. Echo Planar Imaging (EPI)

4 TYPES OF PULSE SEQUENCES

Spin Echo Sequence (SE)

• It consists of 90 and 180 degree RF pulses

• Forms the basis for understanding all other sequences

• Gold standard in MRI.

1. Dual Spin Echo Sequence

2. Fast/Turbo Spin Echo Sequence

3. Single-shot Fast Spin Echo Sequence

3 Types of SE Sequences

Dual Spin Echo Sequence

• Two 180 degree pulses are sent after each 90 degree pulse to obtain two echoes per TR

• This sequence is run with long TR

• PD and T2 double echo sequence are examples of this modified sequence

Fast/Turbo Spin Echo Sequence

• Multiple 180 degree rephasing pulses are sent after each 90 degree pulse

• Multiple echoes are obtained per TR, one echo with each 180 degree pulse

• All echoes are used to fill a single K-space

multi spin-echo sequence

Fast/Turbo Spin Echo Sequence is also called

Turbo factor

is the number of 180 degree pulses sent after each 90 degree pulse

echo train length

turbo factor is also called as

TE effective

The TE at which the center of the k-space is filled is called as

Single-shot Fast Spin Echo Sequence

Fast SE sequence in which all the echoes required to form an image are acquired in a single TR, hence the name

Gradient Echo Sequence (GRE)

• Rephasing of TM is done by gradients; particularly by reversal of the frequency encoding gradient 

• The flip angle is smaller, usually less than 90 degrees.

• dephasing effects of magnetic field inhomogeneity are not compensated, as there is no 180 degree pulse.

1. Spoiled/Incoherent GRE sequence

2. Steady State (SS) Sequences

2 types of GRE Sequences

Spoiled/Incoherent GRE sequence

• The residual TM is destroyed by the RF pulse or gradient such that it will not interfere with the next TR

• These sequences usually provide T1-weighted GRE images

• Can be acquired with echo times when water and fat protons are inphase and out of phase with each other

• This in and out of phase imaging is used to detect fat in the lesion or organs

Steady State (SS) Sequences

• Have a very short TR and TE making them fast sequences that can be acquired with breath-hold

• They can be used to study rapid physiologic processes (ex. events during a cardiac cycle) because of their speed.

• Achieved when residual TM is refocused keeping the TR shorter than T2 of the tissues which establishes a steady magnitude of LM and TM after a few TRs.

1. Post-excitation refocused SS sequences

2. Pre-excitation refocused SS sequences

3. Fully refocused SS sequences

3 types of Steady State (SS) Sequences

Post-excitation refocused SS sequences

• Only FID (S+) component is sampled. 

• Since S+ echo is formed after the RF excitation
  

Pre-excitation refocused SS sequences

• Only spin echo (S-) component is used for image formation.

• S-echo is formed just before the next excitation

Fully refocused SS sequences

Both FID (S+) and spin echo (S-) components are used for the image formation.

balanced-SSFP sequences

Fully refocused SS sequences are called ___________________ as gradients in all three axes are balanced making them motion insensitive

Inversion Recovery (IR) Sequence

Consists of an inverting 180 degree pulse before the usual spin echo or gradient echo sequence

time to invert

The time between inversion 180 degree and excitatory 90 degree pulses is called as _________________

STIR

• Example of Short TI sequences

• Fat is suppressed since fat has short T1

MPRAGE

• Example of Medium TI IR sequences

• CSF and water is suppressed

FLAIR

• Example of Long TI ranges

• can be used to get heavily T2-wieghted without problems from CSF partial volume effects and artifacts as it is nulled

80-150ms

Short TI sequences ranges from _________ and an example is STIR

300 to 1200ms

Medium TI IR sequences ranges from ___________ and example is MPRAGE (Siemens)

1500 to 2500ms

Long TI ranges from ____________ and example of this is FLAIR.

Echo Planar Imaging (EPI)

• All the lines of k-space required to form an image are filled in a single TR

• Multiple echoes generated in single TR are phase encoded by different slopes of gradient, where they are either generated by 180 degree rephasing pulses or by gradient

• Can be Spin Echo EPI or Gradient Echo EPI