MRI Pulse Sequences PDF

# MRI Pulse Sequences ## Dr. Mohamed Auf ## Pulse Sequences - A pulse sequence is defined as a series of RF pulses, gradients applications and intervening time periods. - By selecting the intervening time periods (TR & TE), image weighting is controlled. - Pulse sequences are required because w...

# MRI Pulse Sequences ## Dr. Mohamed Auf ## Pulse Sequences - A pulse sequence is defined as a series of RF pulses, gradients 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). ## 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. ## Spins are rephased in two ways: - by using a $180^\circ$ RF pulse - by using magnetic field gradients ## Pulse sequences and their rephasing mechanisms | Use $180^\circ$ 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 | ## The conventional spin echo pulse sequence Conventional spin echo pulse sequences are used to produce T1, T2 or proton density weighted images. - a $90^\circ$ excitation pulse followed by a $180^\circ$ rephasing pulse followed by an echo. - After the removal of the $90^\circ$ RF pulse, spins dephase and NMV decays. - A $180^\circ$ RF pulse flips the dephased nuclei through $180^\circ$. 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. - $180^\circ$ Rephasing pulses may be applied either once or several times to produce either one or several spin echoes. ## RF Rephasing A diagram showing the RF signal being applied to the spin. The spin is first aligned with the magnetic field then is flipped by the 90° RF pulse, dephases and then rephased by the 180° RF pulse. An echo is then detected. ## Fast or Turbo Spin Echo (FSE/ TSE) - Much faster version of conventional spin echo. - Employs a train of $180^\circ$ rephasing pulses, each one producing a spin echo. This train of spin echoes is called an echo train. The number of $180^\circ$ RF pulses and resultant echoes is called the echo train length (ETL) or turbo factor. - Typically 2, 4, 8 or 16, $180^\circ$ 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. ## Inversion Recovery (IR) - Inversion recovery is a spin echo sequence that begins with a $180^\circ$ inverting pulse. The TR is the time between successive $180^\circ$ inverting pulses. - When the pulse is removed the NMV begins to relax back to B₀. A $90^\circ$ pulse is then applied at time interval TI (time from inversion) after the $180^\circ$ inverting pulse. - A further $180^\circ$ RF pulse is applied which rephases spins in the transverse plane and produces an echo at time TE after the excitation pulse. ## Advantages of Spin Echo - Good image quality. - Available on all systems ## Disadvantages of Spin Echo - Long scan times. The document contains 25 pages. Each page has the same background - a blue patterned gradient.