mri: contrast mechanisms and pulse sequences
DESCRIPTION
Image ContrastsTRANSCRIPT
MRI: Contrast Mechanisms and Pulse Sequences
Allen W. Song, PhD Brain Imaging and Analysis Center Duke
University Image Contrasts The Concept of Contrast
Contrast = difference in signals emitted by water protons between
different tissues For example, gray-white contrast is possible
because T1 is different between these two types of tissue Two Types
of Contrast Static Contrast:
Image contrast is generated from the static properties of
biological systems (e.g. density). Motion Contrast: Image contrast
is generated from movement (e.g. blood flow, water diffusion).
Static Contrast Imaging Methods
T2 Decay transverse MR Signal T1 Recovery longitudinal MR Signal
time 1 s time 50 ms Most Common Static Contrasts
Weighted by the Proton Density Weighted by the Transverse
Relaxation Times (T2 and T2*) Weighted by the Longitudinal
Relaxation Time (T1) Proton Density Contrast
Contrast solely dependent on proton density, without influence from
relaxation times. The Effect of TR and TE on Proton Density
Contrast
MR Signal MR Signal T1 Recovery T2 Decay t (s) t (ms) Optimal
Proton Density Contrast
Technique: use very long time between RF shots (large TR) and very
short delay between excitation and readout window (short TE) Useful
for anatomical reference scans Several minutes to acquire 256256128
volume ~1 mm resolution Proton Density Weighted Image T2 and T2*
Contrasts Contrast dominated by the difference in
T2 and T2* (transverse relaxation times). Transverse Relaxation
Times
Cars on the same track T2* Cars on different tracks To get pure T2
contrast, we need perfectly
homogeneous magnetic field. This is difficult to achieve, as
sometime even if the actual field is uniform, the presence of
biological tissue will still change the homogeneity. So how do we
then remove the influence of the magnetic field inhomogeneity? Time
Reversal Using 180o RF Pulse
Fast Spin Fast Spin TE/2 t=0 180o turn t = TE/2 Fast Spin Fast Spin
TE/2 t=TE Slow Spin Slow Spin TE/2 t=0 180o turn t = TE/2 Slow Spin
TE/2 Slow Spin t=TE The Effect of TR and TE on
T2* and T2 Contrast TR TE T1 Recovery MR Signal MR Signal T2 Decay
T1 Contrast T2 Contrast Optimal T2* and T2 Contrast
Technique: use large TR and intermediate TE Useful for functional
(T2* contrast) and anatomical (T2 contrast to enhance fluid
contrast) studies Several minutes for 256 256 128 volumes, or
second to acquire 64 64 20 volume 1mm resolution for anatomical
scans or 4 mm resolution [better is possible with better gradient
system, and a little longer time per volume] T2 Weighted Image T2*
Weighted Image T2* Images PD Images T1 Contrast Contrast dominated
by the T1 (longitudinal relaxation time) differences. The Effect of
TR and TE on T1 Contrast TR TE T2 Decay MR Signal
T1 Recovery TR TE Optimal T1 Contrast Technique: use intermediate
timing between RF shots (intermediate TR) and very short TE, also
use large flip angles Useful for creating gray/white matter
contrast for anatomical reference Several minutes to acquire
256256128 volume ~1 mm resolution T1 Weighted Image Inversion
Recovery to Boost T1 Contrast
S = So * (1 2 e t/T1) So S = So * (1 2 e t/T1) -So IR-Prepped T1
Contrast In summary, TR controls T1 weighting and
TE controls T2 weighting. Short T2 tissues are dark on T2 images,
but short T1 tissues are bright on T1 images. Motion Contrast
Imaging Methods
Prepare magnetization to make signal sensitive to different motion
properties Flow weighting (bulk movement of blood) Diffusion
weighting (water molecule random motion) Perfusion weighting (blood
flow into capillaries) Flow Weighting: MR Angiogram
Time-of-Flight Contrast Phase Contrast Time-of-Flight
Contrast
No Flow Medium Flow High Flow No Signal Medium Signal High Vessel
Acquisition Saturation Excitation Pulse Sequence: Time-of-Flight
Contrast
Excitation Image Acquisition RF Gx Gy Gz Saturation Time to allow
fresh flow enterthe slice Phase Contrast (Velocity Encoding)
Externally Applied Spatial Gradient G Spatial Gradient -G Blood
Flow v Time T 2T Pulse Sequence: Phase Contrast
RF Excitation G Gx Phase Image Acquisition -G Gy Gz MR Angiogram
Random Motion: Water Diffusion Diffusion Weighting T 2T Externally
Applied Externally Applied
Spatial Gradient G Externally Applied Spatial Gradient -G T 2T Time
Pulse Sequence: Gradient-Echo Diffusion Weighting
Excitation Image Acquisition RF Gx Gy Gz G -G Large Lobes Pulse
Sequence: Spin-Echo Diffusion Weighting
RF Excitation G G Gx Image Acquisition Gy Gz Diffusion Anisotropy
Determination of fMRI Using the Directionality of Diffusion Tensor
Advantages of DWI The absolute magnitude of the diffusion
coefficient (ADC) can help determine proton pools with different
mobility 2. The diffusion direction can indicate fiber tracks ADC
Anisotropy Fiber Tractography DTI and fMRI A B C D Perfusion The
injection of fluid into a blood vessel in order to reach
an organ or tissue, usually to supply nutrients and oxygen. In
practice, we often mean capillary perfusion as most
delivery/exchanges happen in the capillary beds. Perfusion
Weighting: Arterial Spin Labeling
Imaging Plane Labeling Coil Transmission Arterial Spin Labeling Can
Also Be Achieved Without Additional Coils
Pulsed Labeling Imaging Plane Alternating Inversion Alternating
Inversion FAIR Flow-sensitive Alternating IR EPISTAR EPI Signal
Targeting with Alternating Radiofrequency Pulse Sequence: Perfusion
Imaging
Gx Gy Gz Image 90o 180o Alternating opposite Distal Inversion Odd
Scan Even Alternating Proximal Inversion Odd Scan Even Scan EPISTAR
FAIR Advantages of ASL Perfusion Imaging
It is non-invasive Combined with proper diffusion weighting to
eliminate flow signal first, it can be used to assess capillary
perfusion Perfusion Contrast Perfusion Map Diffusion Perfusion Some
fundamental acquisition methods commonly used to generate
static and motion contrasts, and their k-space views k-Space Recap
Kx = g/2p 0t Gx(t) dt Ky = g/2p 0t Gx(t) dt
Equations that govern k-space trajectory: Kx = g/2p 0t Gx(t) dt Ky
= g/2p 0t Gx(t) dt These equations mean that the k-space
coordinates are determined by the area under the gradient waveform
Gradient Echo Imaging Signal is generated by magnetic field
refocusing mechanism only (the use of negative and positive
gradient) It reflects the uniformity of the magnetic field Signal
intensity is governed by S = So e-TE/T2* where TE is the echo time
(time from excitation to the center of k-space) Can be used to
measure T2* value of the tissue MRI Pulse Sequence for Gradient
Echo Imaging
Excitation Slice Selection Frequency Encoding Phase Encoding
digitizer on Readout K-space view of the gradient echo
imaging
Ky 1 2 3 . n Kx Multi-slice acquisition
Total acquisition time = Number of views * Number of excitations *
TR Is this the best we can do? Interleaved excitation method
readout Excitation Slice Selection Frequency Encoding Phase Readout
TR Spin Echo Imaging Signal is generated by radiofrequency pulse
refocusing mechanism (the use of 180o pulse ) It doesnt reflect the
uniformity of the magnetic field Signal intensity is governed by S
= So e-TE/T2 where TE is the echo time (time from excitation to the
center of k-space) Can be used to measure T2 value of the tissue
MRI Pulse Sequence for Spin Echo Imaging
180 90 Excitation Slice Selection Frequency Encoding Phase Encoding
digitizer on Readout K-space view of the spin echo imaging
Ky 1 2 3 . n Kx Fast Imaging Sequences
How fast is fast imaging? In principle, any technique that can
generate an entire image with sub-second temporal resolution can be
called fast imaging. For fMRI, we need to have temporal resolution
on the order of a few tens of ms to be considered fast. Echo-planar
imaging, spiral imaging can be both achieve such speed. Echo Planar
Imaging (EPI)
Methods shown earlier take multiple RF shots to readout enough data
to reconstruct a single image Each RF shot gets data with one value
of phase encoding If gradient system (power supplies and gradient
coil) are good enough, can read out all data required for one image
after one RF shot Total time signal is available is about 2T2* [80
ms] Must make gradients sweep back and forth, doing all frequency
and phase encoding steps in quick succession Can acquire low
resolution 2D images per second Echo Planar Imaging (EPI)
... Pulse Sequence K-space View Allows highest speed for dynamic
contrast
Why EPI? Allows highest speed for dynamic contrast Highly sensitive
to the susceptibility-induced field changes--- important for fMRI
Efficient and regular k-space coverage and good signal-to-noise
ratio Applicable to most gradient hardware Gradient-Recalled EPI
Images Under Homogeneous Field Distorted EPI Images with Imperfect
Field
x imperfection y imperfection z imperfection Spiral Imaging t = TE
RF Gx Gy Gz t = 0 K-Space Representation of Spiral Image
Acquisition Why Spiral? More efficient k-space trajectory to
improve throughput.
Better immunity to flow artifacts (no gradient at the center of
k-space) Allows more room for magnetization preparation, such as
diffusion weighting. Gradient Recalled Spiral Images Under
Homogeneous Field Distorted Spiral Images with Imperfect
Field
x imperfection y imperfection z imperfection