Pulse Sequences PDF

## Pulse Sequences - A pulse sequence is defined as a series of RF pulses, gradient applications and intervening time periods. - By selecting the intervening time periods (TR & TE), image weighting is controlled. - Pulse sequences are required because without a mechanism of refocusing spins, there...

## Pulse Sequences - A pulse sequence is defined as a series of RF pulses, gradient applications and intervening time periods. - By selecting the intervening time periods (TR & TE), image weighting is controlled. - Pulse sequences are required because without a mechanism of refocusing spins, there is insufficient signal to produce an image (because dephasing occurs almost immediately after the RF excitation pulse has been removed). ## Pulse Sequences and Their Rephasing Mechanisms | Use 180° pulses to rephase spins | Use gradients to rephase spins | |---|---| | Spin echo | Gradient echo | | Fast spin echo | Coherent gradient echo | | Inversion recovery | Incoherent gradient echo | | STIR | Steady state free precession | | FLAIR | Ultrafast sequences | ## Spins are Rephased in Two Ways: - By using a 180° RF pulse. - By using magnetic field gradients. ## The Main Purposes of Pulse Sequences Are: - To rephase spins and remove inhomogeneity effects. - To enable manipulation of the TE and TR to produce different types of contrast. ## The Conventional Spin Echo Pulse Sequence Conventional spin echo pulse sequences are used to produce T1, T2 or proton density weighted images. A 90° excitation pulse followed by a 180° rephasing pulse followed by an echo. After the removal of the 90° RF pulse, spins dephase and NMV decays. A 180° RF pulse flips the dephased nuclei through 180°. The faster-precessing nuclei are now behind the slow ones. The fast eventually catch up with the slow reforming the NMV. This is called rephasing. ## The Conventional Spin Echo Pulse Sequence (cont'd) The signal in the receiver coil is regenerated and called a spin echo. 180° Rephasing pulses may be applied either once or several times to produce either one or several spin echoes. ## A Single Spin Echo Pulse is Used to Produce a T1 Weighted Set of Images. The TR is the time from one 90° RF pulse to the next 90° RF pulse. The TE is the time from the 90° RF pulse that begins the pattern to the peak of the signal generated after the 180° RF pulse. ## A Dual Echo Sequence Consists of Two 180° Pulses Applied to Produce Two Spin Echoes. This is a sequence that provides two images per slice location: one that is proton density weighted and one that is T2 weighted. The first echo has a short TE and a long TR and results in proton density weighted images. The second echo has a long TE and a long TR and results in T2 weighted images. ## Advantages of Spin Echo - Good image quality. - Available on all systems. ## Disadvantages of Spin Echo - Long scan times. ## Fast or Turbo Spin Echo (FSE/TSE) - Much faster version of conventional spin echo. - Employs a train of 180° rephasing pulses, each one producing a spin echo. This train of spin echoes is called an echo train. The number of 180° RF pulses and resultant echoes is called the echo train length (ETL) or turbo factor. - Typically 2, 4, 8 or 16, 180° RF pulses are applied during every TR. The scan time is therefore reduced to 1/2, 1/4, 1/8 or 1/16 of the original scan time.