MRI Imaging Pulse Sequences PDF

Imaging Pulse Sequences 1  Gradient Echo-Based Pulse Sequences The technique of the GRE was introduced in part one of this course In this part, several GRE-Based Pulse Sequences will be introduced It was clear that the GRE technique uses...

Imaging Pulse Sequences 1  Gradient Echo-Based Pulse Sequences The technique of the GRE was introduced in part one of this course In this part, several GRE-Based Pulse Sequences will be introduced It was clear that the GRE technique uses a small flip angle along with a short TR to speed up the scanning time while preserving the image contrast Furthermore, the GRE technique uses magnetic field gradients to dephase and rephase the FID into an echo The dephasing lobe of the frequency encoding gradient destroys the FID and takes it to the left side of the k-space while the positive lobe of the same gradient rephases the FID into an echo and then dephases it The latter allows the acquisition system to sample (digitize) the echo while filling the k-space from left to right 2  Spatial Encoding (Slice Selection, Phase Encoding, Frequency Encoding) RF SS Slice Phase Encoding Selection PE Frequency Encoding FE Signal Time Note that slice selection occurs first, then phase encoding and finally frequency encoding 3 The Longitudinal Magnetization: In GRE, partial longitudinal magnetization component is used due to small flip angle (In Spin Echo the new imaging cycle starts with full longitudinal magnetization as a 90o RF pulse is used) The Transverse Magnetization: In GRE, a residual transverse magnetization is present at the end of each imaging cycle (TR is short and does not allow transverse decay to occur sufficiently). The residual is affected by the next RF pulse. After few cycles, this residual transverse magnetization accumulates and reaches the steady state (Mss) (In SE, TR is long enough to allow a complete dephasing of spins in the x-y plane and thus a negligible x-y magnetization at the end of each imaging cycle) 4  Image Contrast in GRE Image T1w can be maximized using a large flip angle and intermediate TR and short TE T2*w can be maximized using a small flip angle and long TR and long TE PDw can be maximized using a small flip angle, long TR and short TE Note: GRE images are contaminated by T2* weighting and thus no pure T2 weighting in GRE images (not always good for pathology) 5  Advantages and Disadvantages of GRE Advantages – Increases the speed of scanning – Sensitive to magnetic susceptibility (haemorrhage, functional imaging, iron imaging) – Imaging flowing blood (MRA) Disadvantages – Decreased SNR (small FA and short TR, T2* decay) – Increased magnetic susceptibility artifact in regions of paranasal sinuses or the abdomen – T2* decay which results in increased sensitivity to magnetic field inhomogeneity, intra-voxel dephasing and magnetic susceptibility artifacts – Second type of chemical shift 6  Coherent Vs Incoherent GRE The residual transverse magnetization can be either spoiled (destroyed) or Cycled Spoiled GRE is also known incoherent GRE Non-spoiled GRE is also known as coherent GRE 7 Incoherent (spoiled) GRE In Spoiled (incoherent) GRE, the residual transverse magnetization is destroyed and only the longitudinal magnetization is left (no T2 or T2 star is produced). So if TR is long and FA is large, T1 weighted images are produced. However, if long TR with small FA is used, then PD images are produced 8  Ways of spoiling the Mxy A) RF spoiling By adding a phase offset to each successive RF pulse which causes a corresponding phase shift in successive Mss vectors. So successive Mss vectors cancel each other B) Variable magnetic field gradients Addition of gradients with variable strengths from cycle to cycle (crushers) C) Very long TR After a long TR, the Mss is completely dephased 9 Coherent GRE In coherent GRE, the residual transverse magnetization is recycled such that the Mxy increases in length from cycle to cycle (Mss) To preserve this steady state, a rewinder gradient is added. Rewinder is another PE gradient (similar in strength but opposite in polarity to the original PE gradient) If a PE gradient of a strength of +10 mT/m is used at the beginning of the sequence, a -10 mT/m is added at the end of the sequence to rewind the phase shift introduced by the first PE gradient. This makes sure that the new cycle has no dephased spins due to the previous PE gradient 10 Coherent GRE pulse sequence 11  Fast GRE techniques To make a GRE sequence ultrafast, very short TR and TE times must be used This causes some saturation to tissues with relatively long T1 time specially at high Bo (SNR is significantly reduced) So the right way to make a GRE sequence ultrafast is to use one of the following two options: – Inversion Recovery (IR) prepared-GRE (produces T1-W GRE images) – Driven Equilibrium (DE) prepared-GRE (produces T2-W GRE images) 12  Inversion Recovery (IR) prepared-GRE In this technique, a 180o RF is applied before the start of the GRE sequence The 180o inverts the magnetization to –z A delay time is introduced (inversion time) to allow different tissues to recover back towards the +z axis before starting the GRE sequence Depending on the inversion time, different tissues will recover by different degrees and thus by the time of starting the GRE sequence (applying the small FA excitation pulse), each tissue has its own longitudinal magnetization such that tissues with short T1 have larger longitudinal magnetization So, T1 contrast is introduced before starting the actual GRE sequence 13 IR-GRE 14  Driven Equilibrium (DE) prepared-GRE In this technique, a 90o Rf pulse is applied which flips the Mo into xy plane A delay time (TE/2) is introduced before applying the 180 inversion pulse The delay time allows tissues to decay at different rates (T2*) The 180 inverts the dephased spins to –y axis After a time (TE/2), the transverse vectors converge (become overlapped) but each tissue has its own vector length (tissues with short T2 have small vectors while tissues with long T2 have long vectors) At this time, another 90o RF pulse is applied to flip these vectors back to +z axis After the 90o, all tissues are now back along the +z axis but each of them has its own vector A GRE sequence starts with small FA excitation pulse that partially flips all vectors and as a result, each tissue has a different Mxy component (T2 weighted) 15 DE-GRE 16  Spin Echo (SE) Spin Echo (SE) is introduced by Hahn (1950) SE uses  90o RF pulse to flip the magnetization into the xy plane  180o RF pulse, instead of gradients, to refocus (rephase) the dephased transverse magnetization Once a 90o RF pulse is switched off, an FID signal results which is under the effect of D-D and Field inhomogeneity (ΔBo). 17  The basic sequence consists of a 90o pulse followed by a 180o pulse after a time TE/2 The 180o pulse reverses (corrects) the dephasing effects of magnetic field inhomogeneity (ΔBo) leaving only the signal degradation due to irreversible spin-spin (T2) relaxation effects or what is called D-D interaction The 180o pulse used in this way is often referred to as a ‘refocusing pulse’. 18  Conventional (Hann) SE Mo Mxy B1=90o B1=180o 19  90o RF pulse is applied along x′ axis and this causes the magnetization vector Mz to tip away from the +z axis onto the y′ axis During a time TE/2 , magnetic moments of the magnetization vector of Mxy start to dephase with a time constant T2⋆ due to both spin-spin interactions and Bo inhomogeneity In the laboratory frame of reference, some nuclei experience slightly higher magnetic field (they precess at frequency higher than ωL) and others experience slightly lower magnetic field (they precess at frequency lower than ωL In the rotating frame of reference, nuclei that precess at a frequency lower than ωL appear as a magnetization vectors rotating anti clockwise in xy plane while those precess at frequency higher than ωL appear as magnetization vectors rotating clockwise in the same plane 20  The dephasing appears as a ‘fanning out’ of magnetization vectors due to the continuum of magnetic field values in the sample A 180o refocusing pulse is applied along the x′ axis at a time TE/2 after the 90o pulse, causing the ‘fan’ of magnetization to rotate 180o about the x′ axis As a result, the dephased vectors start to converge along the -y′ axis at time TE/2 after the 180o pulse (or TE after the 90o pulse) The amplitude of spin echo corresponds to the amount of transverse magnetization available on the y′ which is smaller than the amplitude of FID due to the irreversible spin-spin relaxation The spin echo signal has an opposite phase to the FID because Mxy converged along -y′. 21  Mo A Carr-Purcell Mxy modification B1=90o B C D E B1=180o F G H I B1=180o J K L M B1=180o N O P Q B1=180o  TE/2 TE/2 TE 180x’ 180x’ 180x’ 180x’ 90x’ 90x’ E I M Q TE1 TE2 TE3 TE4 TR 23  Alternatively, in the Carr-Purcell sequence, multiple 180o refocusing pulses are applied separated by a time TE along the x′ in rotating frame of reference The resulting echoes alternate between -y′ and y′ with a decay representing the true T2 only if the 180o refocusing is perfect However, errors in the 180o pulse will be cumulative and cause the magnetization vector to converge either above or below the transverse plane 24  Carr-Purcell Meiboom-Gill SE A B C D E F G H I J K L M N O P Q 25  Carr-Purcell Meiboom-Gill SE TE/2 TE/2 TE 180y ’ 180y ’ 180y ’ 180y ’ 90x’ 90x’ E I M Q TE1 TE2 TE3 TE4 TR 26  A modification to this sequence is called Carr-Purcell Meiboom-Gill (CPMG) sequence in which the RF carrier wave of the refocusing pulses is phase shifted by π/2 with respect to the 90o pulse Therefore, the B1 field of the refocusing pulses aligns along y while the 90o B1 field is still along x′ axis This modification makes the spins flip about the y′ axis and thus all spin echoes converge along y′ and have the same (positive) phase The above assumes a perfect 180o refocusing pulse. In reality, it is not perfect and thus not all echoes can be used (will be discussed later) 27  Important notes about Spin Echo By applying the 180o refocusing pulse, the effect of T2* is removed leaving the echo with irreversible T2 relaxation (D-D interaction) The application of 180o refocusing pulse has two drawbacks: 1- Increases the scanning time as it needs longer TE to fit the 180o pulse and thus longer TR. 2- Increases the power deposition in the human body The first drawback can be compensated for by using multiple echoes (the sequence is then known as Fast Spin Echo [FSE] or Turbo Spin Echo [TSE]) to reduce the scan time Number of echoes formed is known as echo train length (ETL), and the scanning time is reduced by this factor Scan time (TSE) = Scan time (SE) /ETL 28  Image Contrast in conventional SE Image T1w can be maximized using short TR and short TE T2w can be maximized using long TR and long TE PDw can be maximized using long TR and short TE 29 Typical values of imaging parameters SE Long TR = 2000 ms Short TR = 300-700 ms Long TE = 60-80 ms Short TE = 10-25 ms 30