GU MRI Pulse Sequences PDF
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Galala University
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Summary
This document provides a detailed overview of different MRI pulse sequences, including Inversion Recovery (IR), STIR, FLAIR, and Gradient Echo. It explains the principles behind each technique and their applications in medical imaging. The document includes illustrations of the pulse sequences.
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## GU MRI Pulse Sequences ### Inversion Recovery (IR) * Inversion recovery is a spin echo sequence that begins with a 180° inverting pulse. * The TR is the time between successive 180° inverting pulses. * When the pulse is removed the NMV begins to relax back to B｡. A 90° pulse is then applied a...
## GU MRI Pulse Sequences ### Inversion Recovery (IR) * Inversion recovery is a spin echo sequence that begins with a 180° inverting pulse. * The TR is the time between successive 180° inverting pulses. * When the pulse is removed the NMV begins to relax back to B｡. A 90° pulse is then applied at time interval TI (time from inversion) after the 180° inverting pulse. * A further 180° RF pulse is applied which rephases spins in the transverse plane and produces an echo at time TE after the excitation pulse. ### Inversion Recovery Pulse Sequence A diagram showing the pulse sequence with a time line showing: 180°, 90°, FID, TE, echo at the bottom. TR and 180° markings at the top. ### Inversion Recovery (IR) * The TI is the main factor that controls weighting in IR sequences. Certain TI values result in suppression of signal from tissues. * The TE controls the amount of T2 decay. For T1 weighting it must be short, for T2 weighting, long. * The TR must always be long enough to allow full longitudinal recovery of magnetization before each inverting pulse. ### STIR (Short TI Inversion Recovery) * Uses short TIs such as 100–180 ms. * TIs of this magnitude place the 90° excitation pulse at the time that NMV of fat is passing exactly through the transverse plane. Therefore the 90° excitation pulse produces no transverse component in fat (fat is flipped to 180°) and therefore no signal. * In this way a fat suppressed image results. An image of a knee using this technique. ### FLAIR (Fluid Attenuated Inversion Recovery) * Uses long TIs such as 1700–2200 ms. * Nulls the signal from CSF in exactly the same way fat is suppressed in the STIR sequence. * Because CSF has a long T1 recovery time, the TI must be longer to correspond with its null point (when its NMV is passing through the transverse plane). An image of a brain using this technique. ### Uses of IR sequences * Inversion recovery is a versatile sequence that is used in the CNS (T1 and FLAIR) and musculoskeletal systems (STIR). * The FLAIR sequence increases the conspicuity of periventricular lesions such as MS plaques and lesions in the cervical and thoracic cord. * STIR sequences are often called 'search and destroy' sequences when used in the musculoskeletal system as they null the signal from normal marrow thereby increasing the conspicuity of bone lesions. ### Gradient Echo * Gradient echo pulse sequences are sequences that use a magnetic field gradient to reduce magnetic inhomogeneity effects, as opposed to a 180° RF pulse used in spin echo sequences. * An RF excitation pulse followed by a relaxation period and a gradient reversal to produce rephasing of the spins. * The magnitude and duration of the RF excitation pulse selected determines the flip angle. ### Gradient Echo (cont'd) * A flip angle other than 90° is used, and only part of the longitudinal magnetization is converted to transverse magnetization. Therefore the SNR in gradient echo sequences is less than in spin echo sequences. * After the RF pulse is withdrawn, the magnetic moments within the transverse component of magnetization dephase and are then rephased by a gradient. * A gradient causes a change in the magnetic field strength that changes the precessional frequency and phase of spins. This effect rephases the magnetic moments. ### Gradient Echo (cont’d) * In gradient echo sequence, nuclei are dephased with a negative gradient pulse. The negative gradient slows down the slow nuclei even further and speeds up the fast ones. * The gradient polarity is then reversed to positive. The positive gradient speeds up the slow nuclei and slows down the fast ones. The nuclei rephase and produce a gradient echo. ### Gradient Echo Vs. Spin Echo * Gradient rephasing is less efficient than RF rephasing. * Gradient rephasing does not reverse all of the dephased nuclei. Some nuclei that have been dephased from T2* are not rephased. * However, gradient rephasing is faster than RF rephasing and therefore these sequences have shorter TEs and TRs than spin echo. As a result scan times are short.
