Pulse Sequences PDF

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Document Details

EasierPeninsula

Uploaded by EasierPeninsula

Galala University

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MRI pulse sequences spin echo medical imaging

Summary

This document provides an overview of pulse sequences used in magnetic resonance imaging (MRI). It covers different types of pulse sequences, their rephasing mechanisms, and their purposes in producing various image contrasts. The document explains how these sequences are used to generate T1, T2, or proton density weighted images and also compares conventional and fast spin echo techniques.

Full Transcript

## 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.

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