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MRI Pulse Sequences
MRI Pulse Sequences and Parameters
| Question | Answer |
|---|---|
| Spin Echo Pulse Sequence | utilizes 90 degree (alpha) pulse followed by one or more 180 degree rephasing pulses |
| When are the 180 degree rephasing pulses applied? | The first rephasing pulses occurs at Tau (half TE time) |
| What do rephasing pulses do? | They refocus the dephasing spins to generate one or more spin echos |
| Spin Echo classifications | 1. Conventional Spin Echo (CSE) 2. Fast Spin Echo (FSE) 3. Inversion Recovery (IR) |
| Conventional Spin Echo Sequence | utilizes one 90 degree (alpha) pulse followed by one 180 degree rephasing pulse per TR |
| CSE Advantage? | provide excellent image quality with "true" contrast (because they do not involve signal averaging by sampling multiple echos) |
| CSE Disadvantage? | Only samples one line per k-space per TR, which makes longer scan times Also not good for t2 contrast bc TR is not long enough so would be very time consuming to create T2 contrast |
| Fast Spin Echo Sequence (or turbo spin echo) | utilizes 90 degree pulse followed by multiple 180 degree rephasing pulses per TR. |
| FSE Advantages | shorter scan times (fills multiple lines of k-space) decrease magnetic susceptibility artifacts (due to multiple rephasing pulses) |
| FSE Disadvantages | Increase in motion artifacts (one motion recorded multiple times) increase image blurring (ETL later echoes affected by T2 decay) decrease in SNR (loss of signal in later echos from t2 decay) decrease ability to detect small hemorrhages |
| FSE Modifactions | 1. Single-shot Fast Spin Echo 2. Driven Equillibrium Fourier Transform (DRIVE) |
| Single-shot Fast Spin Echo (SS-FSE) | FSE combined with partial fourier that allows for acquisition of all k-space lines in a single TR period. |
| SS-FSE advantages/disadvantages | Drastically decreases scan time decrease SNR and quality due to increase chance of noise and magnetic susceptibility artifacts (ghosting) |
| DRIVE | specialized FSE that uses an additional RF pulse after the echo train to push/"drive" residual transverse magnetization back into the longitudinal plane making the RTM ready for the next TR (increases TR and therefore increasing signal from fluid) |
| Inversion Recovery (IR) | spin echo pulse that begins with 180 inverting RF pulse and subsequent time of inversion (TI) followed by a CSE or FSE pulse sequence |
| How does IR work? | The 180 inverting pulse inverts the NMV of tissues by 180 degrees causing it to align to main mag field and the longitudinal axis in the opposite direction of its natural alignment ultimately nullifying signal from water or fat |
| What is Inversion Time Interval (TI)? | period between the 180 rephasing pulse and 90 degree excitation pulse determines which tissue type will be nullified |
| Advantages | enhances tissue contrast improves lesion detection (with FLAIR of brain and spine) enables fat supression (with STIR) minimizes certain artifacts (bc of nullifying certain tissues) |
| Disadvantages | longer scan times than FSE (bc of addition of TI interval) decreases SNR (signal loss from inversion pulse) sensitive to TI selection |
| Short TI Inversion Recovery (STIR) | an IR pulse that utilizes a short inversion time (120-150 ms on 1.5 t) to suppress signal from fat, enhancing fluid signal. |
| Advantages of STIR | Good at detecting tumors, edema, infection and inflammatory Good for metal implants than T2 bc less prone to mag susceptibility artifacts while enhancing fluid |
| Fluid Attenuation Inversion Recovery (FLAIR) | utilizes long inversion time (1800-2300 ms on 1.5 T) to suppression signal from fluid while fat appears bright. |
| Advantages of FLAIR | visualization of lesions near or surrounding fluid filled spaces (Brain and spinal cord) and MS, ischemic stroke, or abnormalities of periventricular or subarachnoid space. |
| Gradient Recall Echo (GRE or GE) pulse sequences | utilize alpha excitation pulse with variable flip angles (lesser angle) followed by frequency encoding gradient for rephasing purposes |
| Advantages of GE/GRE | shorter scan times t2* contrast (bc of susceptibility to inhomogeneities) enhanced flow/MRA (emphasizes phase shift causes by flowing spins) enhanced visualization of tissues with short t2 contrast (short TE) |
| Disadvantages of GE/GRE | susceptibility to artifacts and motion not as good for T1 weighting bc of sensitivity to t2* lower SNR (rapid loss of trans. magnetization from t2*) |
| Steady State Pulse sequences | maintains longitudinal and transverse magnetization (RTM) by using repeated excitation pulses with variable/consistent flip angles (30-70 degrees) at TR shorter than the T1 and/or T2 relaxation times of tissues which creates stimulated echoes |
| Incoherent (spoiled) Gradient Echo Pulses | utilize an excitation pulse, variable flip angle, and gradients for gradient rephasing where only the gradient echo (formed by FID) is sampled |
| Why is it called Incoherent? | These sequences destroy phase coherence of the residual transverse magnetization by using RF spoiling/dephasing spins or Gradient spoiling from previous cycles to prevent signal build up and eliminates the contribution of stimulated echoes for contrast |
| Advantages of Incoherent GE | faster scans better for t1 weighting sensitive to flow best for 3d volumetric imaging (aquires thin slices in large volumes quickly) |
| Disadvantage of Incoherent GE | magnetic susceptibility artifacts loud gradient noise |
| Coherent (Rewound) Gradient Echo Pulse Sequenence |
Created by:
emjohnson4