Transcript MRI Pulse Sequences

MRI
Pulse Sequences
Jerry Allison Ph.D.
Qui ck Time ™ and a
TIFF (Unco mp res sed) deco mp re ssor
are need ed to se e thi s pi cture .
1017 pages ©2004
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Outline
I. Spin Echo Imaging
Multiplanar
Multislice
Oblique
II. Inversion Recovery (IR)
III. Gradient Recalled Echo
IV. Three Dimensional (Volume)
Techniques
V. Fast Imaging Techniques
VI. Echoplanar Imaging
Image Contrast
Image contrast in radiography and CT is based upon a
few properties of the tissues or contrast agents
involved:
- physical density (g/cc)
- electron density (electrons/cc)
- atomic number
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Image Contrast
Contrast in MRI is more complex and depends
on many properties/parameters, which can be
classified into “intrinsic” properties and
“extrinsic” parameters. Intrinsic properties
relate directly to the tissue. Extrinsic
parameters relate to the characteristics of the
MR imager and the details of the “MRI Pulse
sequence” used for imaging.
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Intrinsic Properties
Proton density
T1 relaxation
T2 relaxation
T2* relaxation
- magnetic susceptibility
Diffusion
Magnetization transfer
-cross relaxation
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Intrinsic Properties
Chemical Shift
Temperature
Perfusion
Changes in tissue composition (e.g. age)
Viscosity
Physiologic motion
Bulk flow
Blood
CSF
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Extrinsic Parameters
Magnetic field strength
-static field
-gradient field
Magnetic field homogeneity
Hardware and software parameters
-coil selection
-number of slices acquired
-slice thickness and gap
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Extrinsic Parameters
Hardware and software parameters
-slice location
-slice orientation
-number of averages or excitations
-RF pulse shape (#sinc lobes)
-RF transmitter bandwidth
-RF receive bandwidth
-pixel size
-matrix size
-field of view
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Extrinsic Parameters
-acquisition mode ( 2D / 3D )
-artifact suppression
-physiologic triggering / gating
-orientation of phase and frequency
encode gradients
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Extrinsic Parameters
RF pulse sequences
-inversion recovery
-spin echo
-gradient recalled echo
-fast scan sequences
-echoplanar (single shot techniques)
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Extrinsic Parameters
Pulse sequence parameters
-repetition time (TR)
-echo time (TE)
-inversion time (TI)
-flip angle ()
-echo train length
Contrast enhancing agents
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MRI Pulse Sequences
An MRI pulse sequence dramatically
impacts the appearance of an MRI image.
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Spin Echo Pulse Sequences
T1 weighted
PD weighted
T2 weighted
TR 510
TR 4500
TR 4500
TE 14
TE 15eff (ETL7)
TE 105eff (ETL7)
2min 7sec for 17 slices
2min 39sec for 24 slices
2min 39sec for 24 slices
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Inversion Recovery Gradient Echo Pulse Sequence
TR 12.1
TE 5.4
3min 11sec for 160 slices
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MRI Pulse Sequences
More specifically, an MRI pulse sequence is
a “sequence” of temporal waveforms:
Radiofrequency (RF) pulses
Gradient (magnetic field) pulses
Data acquisiton intervals
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Here is a pulsesequence diagram.
This shows a
timeline for: 1) RF
pulses; 2) gradient
amplitudes for Gx,
Gy, Gz; 3) the
readout (i.e., A/D),
and 4) the signal of
the excited nuclei.
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Multiplanar Imaging
Axial, sagittal, and coronal images can be
acquired as follows:
Notice that for each plane, the choice of axis for
phase and frequency encoding can vary.
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MRI Image Weighting
Many MRI images are described as:
Proton density weighted
T1 weighted
T2 weighted
(and T2* weighted)
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Spin Echo Images
T1 weighted
PD weighted
T2 weighted
TR 510
TR 4500
TR 4500
TE 14
TE 15eff (ETL7)
TE 105eff (ETL7)
2min 7sec for 17 slices
2min 39sec for 24 slices
2min 39sec for 24 slices
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Proton Density Weighting
Images are (largely) weighted by the mobile
hydrogen content of the tissues (water and
fat).
PD:
PD images:
FAT < WM < GM < CSF
CSF > GM > WM > Fat
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Proton Density Weighting
Proton Density
-The nucleus of most hydrogen atoms is a single particle: the proton
-The number of “mobile” hydrogen nuclei per voxel directly affects the
intensity of the voxel in an MRI image (for all image weightings).
-Proton Density Weighting emphasizes proton density (as opposed to t1, t2 or
T2*)
-Total proton densities
-CSF
-Grey Matter
-White Matter
-Fat
- Protons in lung tissue volume
0.112 g H/cc
0.1058 g H/cc
0.1056 g H/cc
0.1 g H/cc
~ 0.01 g H/cc
So, one of many problems with lung imaging is the low proton density per
volume, leading to very low SNR.
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Proton Density Weighting
-Although white matter and grey matter have
very similar proton density; they are
differentiated in MRI by their lipid and water
content.
Lipid (g H / cc) Water (g H / cc)
Grey Matter
0.0072
0.0910
White Matter
0.0178
0.0796
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T1 Weighting
Images demonstrate good contrast between
soft tissue types (because different tissues
have different “T1” values).
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T2 Weighting
Images demonstrate good contrast between
normal tissue and pathology (because many
pathologies have elevated “T2” values due
to increased free water content).
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Approximate T1 and T2 Values for Human Tissue
(37 oC)
Tissue
Skeletal Muscle
Liver
Kidney
Spleen
Fat
Gray Matter
White Matter
Cerebrospinal Fluid
T1 at 1.5 T
(msec)
T1 at 0.5 T
(msec)
T2
(msec)
870
490
650
780
260
920
790
>4,000
600
323
449
554
215
656
539
>4,000
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58
62
84
101
92
>2,000
T1, T2 Weighting
In images of the head
T1:
T1 images:
FAT < WM < GM < CSF
FAT > WM > GM > CSF
T2:
T2 images:
FAT < WM < GM < CSF
CSF > GM > WM > FAT
Careful: CSF or Fat can be suppressed
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Pulse Sequence Families
Spin Echo: SE
Gradient Echo:
• GE
• Gradient Recalled Echo (GRE)
• Field Echo (FE)
Inversion Recovery: IR
• STIR: short tau inversion recovery
• Fat suppression
• FLAIR: fluid attenuated inversion recovery
• Fluid (CSF) suppression
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Spin Echo Imaging
Easy to control image weighting with SE
• T1 weighted
• T2 weighted
• PD weighted
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Spin Echo Imaging
The Spin Echo imaging technique has the
advantage that it is not as sensitive to static
inhomogeneity of the magnet and
inhomogeneity caused by magnetic
susceptibility of patient tissue.
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Spin Echo Imaging
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Spin Echo Imaging
The pulse sequence must be repeated many times to
produce an MRI image. The time interval between each
execution of the pulse sequence is termed the
Repetition Time (TR).
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Spin Echo Imaging
The value of the repetition time (TR) and the echo
time (TE) can be varied to control contrast in spin echo
imaging. For example:
TR = 2000 msec TE = 20 msec Proton Density Weighting
TR = 2000 msec TE = 80 msec T2 Weighting
TR = 600 msec TE = 20 msec T1 Weighting
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Fast Spin Echo Pulse Sequence (FSE)
Turbo Spin Echo (TSE)
Careful: Fat can be excessively
bright on FSE images (j-coupling)
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Gradient Recalled Echo
Gradient recalled echo techniques have great
versatility. A variety of contrasts can be
produced while imaging rapidly.
GRE techniques include:
GRASS, SPGR, FLASH, FISP, PSIF and many,
many others.
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Gradient Recalled Echo Images
2D-FLASH
TR 25msec
TE 9msec
a = 35o
5.7sec per slice
MIP
(Maximum Intensity Projection)
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Gradient Recalled Echo Image
Multi Planar GRASS
mixed T1/T2 weighting
TR 500msec TE 13msec 2NEX a=60o
3min 14 sec for 15 slices
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Gradient Recalled Echo
Exceptions are:
1. The creation of the echo is accomplished solely by
gradient magnetic fields (no 180o RF pulse).
2. Deposition of RF energy in the patient is lower since
the 180o RF pulses are not used (less heating of patient
tissues).
3. Static inhomogeneity of the magnet and
inhomogeneity caused by magnetic susceptibility of
patient tissue are NOT corrected by gradient recalled
echo techniques.
4. T2 contrast becomes T2* contrast.
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Gradient Recalled Echo
4. The initial flip angle is frequently chosen to
be less than 90o . The flip angle in gradient
recalled echo techniques is called  .
The optimum value of  for a particular TR and
tissue having spin lattice relaxation T1 is called
the Ernst angle.
cos(e) =
e
-TR
T1
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Gradient Recalled Echo
5. 3D or volume imaging can be accomplished
(resulting in thinner slices).
Vancouver, BC
courtesy of Dr. Rawson
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Three Dimensional Volume Techniques
3D voxels are isotropic (or nearly isotropic).
The voxels are the same size in all 3 dimensions.
The dimensions of a typical 3D voxel are
1 mm x 1 mm x 1 mm. The acquisition of
isotropic voxels enables the data set to be
reformatted into any oblique plane without
significant loss of resolution using Post
Processing Techniques.
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Three Dimensional Volume Image
MPRAGE: Magnetization Prepared Rapid Gradient Echo
TR 11.4msec TE 4.2msec a=12o 1.4mm
6min 55sec for 120 slices (168mm slab)
Uses Inversion Recovery
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Inversion Recovery (IR)
Inversion recovery pulse sequences are useful for:
Suppression of selected tissues (e.g. orbital fat, liver
screening, fatty tumors, CSF)
Creation of heavily T1 weighted images without a
dominant contribution from fat (e.g. brain, liver and
musculoskeletal imaging).
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Inversion Recovery (IR)
A basic IR spin echo pulse sequence consists of
a 180o inversion pulse, followed by an inversion
time TI, then a 90o RF pulse.
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Consider two voxels, one of fat and one of H2O
This method of fat suppression is sometimes called “short TI”
inversion recovery or STIR imaging.
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Inversion Recovery (IR)
In spin echo inversion recovery imaging
sequences, the 90o pulse is followed by a 180o
pulse in order to produce a spin echo at time TE
following the 90o pulse
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IR Image
STD T2 weighting
vs
FLAIR: fluid attenuated IR (T2 weighted spin echo)
Inversion time: 2.5sec (CSF is suppressed)
TR 10sec TE 119msec (ETL7)
3min 49sec for 19 slices
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GE MRI Image Annotation
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GE MRI Image Annotation
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GE MRI Image Annotation
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GE MRI Image Annotation
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GE MRI Image Annotation
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GE MRI Image Annotation
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