Pulse Sequences Types of Pulse Sequences: Spin Echo Gradient Echo Inversion Recovery Echo Planar...
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Transcript of Pulse Sequences Types of Pulse Sequences: Spin Echo Gradient Echo Inversion Recovery Echo Planar...
Pulse Sequences
Types of Pulse Sequences: Spin Echo Gradient Echo Inversion Recovery Echo Planar Imaging
Functional Techniques Diffusion-weighted imaging Perfusion weighted imaging Spectroscopy fMRI
Pulse Sequences
Spin Echo (SE) (Conventional Spin Echo)
RF pulse sequence with a 90o excitation pulse followed by a 180o rephasing or refocusing pulse to eliminate field inhomogeneity and chemical shift effects.
• Main Points: 1) 90o excitation pulse
2)180o rephasing pulse
Pulse Sequences
Spin Echo (SE)
Pulse Sequences
Spin Echo (SE)
Pulse Sequences
Spin Echo – Multiple Echo
Pulse Sequences
Spin Echo – Multi-echo
Pulse Sequences
Spin Echo Pulse Sequence T1 weighted images
Short TR (300 ms – 700 ms) Short TE (10 ms – 30 ms)
T2 weighted Images Long TE ( > 80 ms) Long TR ( > 2000 ms)
Pulse Sequences
T1WI vs. T2WI____________________________________
*Short TR *Long TE
Short TE Long TR
T1WI T2WI
Pulse Sequences
Proton (Spin) Density Weighted
Density of Resonating spins in a given volume (e.g., tissue). The higher the concentration of spin density, the higher
(brighter) the received signal.
• Long TR (1000 ms – 2000 ms)• Short TE (10 ms – 30 ms)
Pulse Sequences
Spin Echo T1 – weighted image (short TR, short TE) Proton Density image (long TR, short TE) T2 – weighted image (long TE, long TR)
Pulse Sequences
Rapid Acquisition with Relaxation Enhancement Also known as RARE Fast Spin Echo (FSE) – GE, Picker, Toshiba &
Hitachi Turbo Spin Echo (TSE) – Siemens & Philips One advantage is speed without loss of S/N.
In CSE, if acquisition time is reduced by 50%, the S/N is reduced by 40%
Pulse Sequences
Fast Spin Echo
Pulse Sequences
RARE
Pulse Sequences
Fast (Turbo) Spin Echo Echo Train Length (ETL) or Turbo Factor (TF) Effective Echo Time (ETE) Echo Train Spacing (ETS)
Pulse Sequences
Rapid Acquisition with Relaxation Enhancement Acquires multiple echoes per excitation at
different phase encoding steps – echo train length (ETL)
The number of echos acquired per excitation pulse is referred to as echo train length
TE is referred to as “Effective TE” (ETE)
Pulse Sequences
Rapid Acquisition with Relaxation Enhancement Time between each echo (phase encoding
step) is referred to as echo train spacing (ETS)
TE is primarily responsible for image contrast. RARE reduces the amount of excitation
pulses. Scan Time – (TR x Gpe x NEX) / ETL
Pulse Sequences
Fast Spin Echo Scan Time = (TR x Gpe X NEX) / ETL
TR = 2000 ms (2000 x 256 x 4) / 8
Gpe = 256 2000 x 256 x 4 = 2048000
NEX = 4 2048000 / 1000 = 2048 secondsETL = 8 2048 / 60 (convert seconds to minutes) = 34.13
minutes
34.13 minutes / 8ETL = 4.26 minutes
Pulse Sequences
Fast Spin Echo Advantage
Reduced Scan Time Disadvantage
Possible flow and motion artifacts Fat may be brighter on FSE than CSE Image blur – Increased TF or ETL
Pulse Sequences
Gradient Echo (GE)
Generally uses an excitation flip angle (FA) of less than 90o degree and a gradient reversal to rephase the protons
• Main Points: 1) Variable Flip Angle (FA)
2) Gradient reversal
Pulse Sequences
Pulse Sequences
Gradient Echo
Pulse Sequences
Advantages: Gradient Echo Much shorter scan times than SE pulse
sequences Low FA allows for faster recovery of
longitudinal magnetization Gradients rephase faster than 180o RF pulses TR and TE values are shorter than spin echo
pulse sequences
Pulse Sequences
Disadvantages: Gradient Echo Susceptible to magnetic field inhomogeneities Contain magnetic susceptibility artifacts T2* weighting
Pulse Sequences
Fast Spin Echo (Advantage over GRE)
FSE uses a 180 degree pulse to eliminate susceptibility artifacts.
Heavy T2 – weighted images cannot be easily acquired with GRE
Pulse Sequences
Gradient Echo: Weighting T1 Weighting - 1o controlled by FA and TR T2* Weight – 1o controlled by TE
Pulse Sequences
Gradient Echo Pulse Sequence T1 – Weighted
Large Flip Angle Short TR Short TE
T2* - weighted Small Flip Angle Long TR Long TE
Proton Density Weighted Small Flip Angle Long TR Short TE
Pulse Sequences
Inversion Recovery
Sequence consisting of an initial 180o RF pulse to invert the magnetization, followed by a spin-echo (90o to 180o) or gradient sequence.
Pulse Sequences
Inversion Recovery
Pulse Sequences
Inversion Recovery FLAIR – Nulls CSF STIR – Nulls FAT
FLAIR STIR
Pulse Sequences
Dixon
Pulse Sequences
Null Point
Pulse Sequences
Echo Planar Imaging (EPI) Allows data to be collected and reconstructed
in less than a second Stronger and faster gradients (slew rate)
required Data collected along GFE (readout) direction
Pulse Sequences
Echo Planar Imaging
Functional Techniques
Diffusion-weighted image (DWI) Technique used to measure the motion of
water molecules Areas with increased diffusion within a tissue
show greater signal loss Terms such as – b-value, Apparent diffusion
coefficient (ADC) Clinical applications – acute cerebral infarcts
(ischemic areas) appear bright
Functional Techniques
Perfusion-weighted image (PWI) Technique used to measure the flow of blood
though the capillary region of an organ or tissue
Dynamic Susceptibility Contrast (DSC) uses gadolinum as a tracer
Arterial Spin Labeling (ASL) noninvasive Terms - Cerebral Blood Flow (CBF), Mean
transit Time (MTT) and Cerebral Blood Volume (CBV)
Functional Techniques
Spectroscopy (MRS) Technique that provides chemical information
about a tissue Nuclei such as 1H, 31P, 13C, 23Na, 39K, 19F Methods used:
Stimulated echo acquisition mode (STEAM) Point-resolved spectroscopy (PRESS) Image-selected in vivo Spectroscopy (ISIS) Chemical shift imaging (CSI) a.k.a. magnetic
resonance spectroscopic imaging (MRSI)
Functional Techniques
Functional MRI (fMRI) Term used to describe any technique that
evaluates brain physiology rather than anatomy
fMRI specifically used to map brain activity Areas of interest – motor cortex, visual cortex Blood oxygenation level-dependent (BOLD)
• BOLD: Blood Oxygen Level Dependence fMRI is the most common for research purposes• Measures the hemodynamic response (amount of
blood flow) related to neural activity in the brain• Increase in magnetic susceptibility when blood is
oxygenated• Direct correlation between brain activity and
cerebral blood flow has been observed, but unsure of exact underlying mechanisms
• Usually uses a T2* weighted contrast • TR: 2-4s• 2-4mm spatial resolution (better with 3-9T)
• Two common types of experimental design• Resting state: Participant has no task, just
lies still in the scanner• Experimental: Participants are presented
with tasks or stimuli at approximately 5s intervals• Or block design due to poor temporal
resolution
5s + 5s … to 2 min Repeat w Next Block of Images
• Although commonly thought of as a direct measure of brain activity, fMRI usually identifies relative differences in brain activity
• Correlation does NOT equal causality• Subtraction Technique• EX: Response to images of smoking
-
Smokers Nonsmokers
=
Difference
• fMRI is not currently used for diagnostic purposes• Used to identify functional maps of neural networks
& research relative deficiencies in brain function• New software allows for the quantification of
network activity via Independent Components Analysis
• Differences in network connectivity has been associated with certain disorders and fMRI should be capable of accurate diagnosis within the next decade• Allows early identification and treatment of
disordered individuals (e.g. Schizophrenia and Alzheimers)
Default Mode NetworkVisual Network
• Diffusion based fMRI• Contrast MR• Arterial Spin Labeling• Magnetic Resonance Spectroscopic
Imaging• Electroencephalography
• Although it has great spatial resolution, fMRI has poor temporal resolution of neuronal activity
• Combined EEG-fMRI may allow quantification of activity at sub-ms and sub-mm resolution