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11/7/2011
1
MRI Techniques for Cardiovascular Imaging
Chen Lin PhD DABR
Indiana University School of Medicine & IU Health
Declaration of Conflict of Interest or Relationship
Research support from Siemens Healthcare
Cardiac MRI
• Morphology
– Wall movement
– Valve movement
– Blood vessel (aorta, pulmonary vein, coronary artery)
• Function
– Blood volume, flow and cardiac output
• Tissue Property
– Perfusion and delay enhancement
– Tumor / Mass
Chen Lin PhD 09/11
Basic Cardiac MR Techniques
• Acquisition Techniques for cardiac motion
– Breath hold and Navigator (Respiratory motion)
– Prospective and retrospective ECG triggering (Sync with cardiac motion)
– Segmented Acquisition and View sharing (Faster scan)
– Single-shot and radial sampling
• Magnetization preparation and pulse sequences for optimal contrast
– Double IR (Dark Blood) and Triple IR (DB + STIR)
– Tagging (Wall Motion)
– 2D SSFP (TruFi versus FLASH) (CINE image for cardiac function)
Chen Lin PhD 09/11
Unique Cardiac MR Applications
• Myocardium Perfusion
– IR (Delay Enhancement)
– IR/SR-SSFP/EPI (Myocardial Perfusion)
• Cardiac Function
– Phase Contrast (Flow Quantification)
• Cardiac MRA
– 3D SSFP (Coronary Arteries)
– Time Resolved MR Angiography (Large Vessel)
• Cardiac Mass
– DB TSE sequence w. and w/o contrast (Tumor/Mass)
Chen Lin PhD 09/11
ECG TRIGGERING & GATING
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ECG Triggering/Gating
• Prospective Triggering
– Data acquisition take place after predetermined delay from trigger signal.
• Retrospective Gating
– Arbitrary timing of data acquisition.
– Acquired data is sorted into different cardiac phase determined by the delay from trigger signal.
Chen Lin PhD 09/11
ECG
a a a a a a a a a a a a a | | | | | | | | | | | | |
a a a a a a a a a a a a a | | | | | | | | | | | | |
a a a a a a a a a a | | | | | | | | | |
Acquisition
Retrospective vs Prospective Triggering
Prospective 1 2 3 4 5 6 . . . . . . . 1 2 3 4 5 6 . . . . . . .
Retrospective
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 . . . .
1 2 3 4 5 6 7 8 9 10 11 12 13 1 2 3 4 5 6 7 8 9 10 11 12 13
Sorted based on the phase in cardiac cycle
Chen Lin PhD 09/11
Trigger Delay Acquisition window
Prospective Triggering Setup
• Trigger Delay + Acquisition Window = ~ 90% Average Cycle
• TR (Temporal Resolution) X (Phases+1) = Acquisition Window Chen Lin PhD 09/11
Retrospective Triggering Setup
• User defined Calculated phases ( typically 20-30 )
Chen Lin PhD 09/11
Prospective versus Retrospective
• Prospective Triggering
– Cover less than entire cardiac cycle
– Sensitive to arrhythmia
– Acquisition window manually adjusted
– Cine frame-rate determined by data segments
• Retrospective Gating
– Measures through entire cardiac cycle
– Arrhythmia rejection is available
– Acquisition Window automatically adjusted
– Variable user-defined cine frame-rate
Chen Lin PhD 09/11
Retrospectively Gated FLASH Examples
Flash Flash + Grid Tagging
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DATA ACQUISITION SCHEME
Chen Lin PhD 09/11
k-space and Raw Data
Kx
Ky
Phase E
ncodin
g S
teps (
Yre
s =
4)
Frequency Encoded Points (Xres = 8)
Echo/Line/View
Chen Lin PhD 09/11
a a a a a a a a a a a a a | | | | | | | | | | | | |
1 2 3 N
a a a a a a a a a a a a a | | | | | | | | | | | | |
1 2 3 N
TR
Heartbeat 2 Echo 2
Heartbeat 1 Echo 1
Acquisition Window Acquisition Window
ECG
Echoes
Cardiac Phases
Non-segmented
• One echo (k-space line/view) measured for each cardiac phase
• Large number of cardiac phases -> High temporal resolution (< 10ms)
• Long scan time ( # of phase encodes x RR)
Chen Lin PhD 09/11
Heartbeat 2 Echoes 6-10
Heartbeat 1 Echoes 1-5
Acquisition Window Acquisition Window
a a a a a a a a a a . . . a a a a a
1 2 3 4 5 1 2 3 4 5 . . . 1 2 3 4 5
2 N 1
a a a a a a a a a a . . . a a a a a
1 2 3 4 5 1 2 3 4 5 . . . 1 2 3 4 5
2 N 1
TR
ECG
Echoes
Cardiac Phases
Segmented
• Multiple echoes combined for each phase
• Short scan duration but lower temporal resolution
Chen Lin PhD 09/11
ECG
Echoes
Cardiac Phases
Acquisition Acquisition Window
a a a a a a a a a a a a a . . . a a a
1 2 3 4 5 3 1 2 3 4 5 3 1 . . . 3 4 5
N 3
2 4
1
TR
3
2
1
View sharing
• Some of the data are shared for two adjacent cardiac phases.
• Short scan duration and good temporal resolution
Chen Lin PhD 09/11
a a a a a a a a a a a
1 2 3 4 5 3 1 2 3 4 5
View sharing Setup
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• Calculations: – “TR” = segments x time between excitation (TR)
– Scan time = # of phase encodes / segments x RR
• Increasing # of segments: – Longer “TR”, Lower temporal resolution, Blurring
– Shorter scan time, Shorter the breath-hold or more slices.
Segmentation Trade-off
Chen Lin PhD 09/11
“DARK” BLOOD TECHNIQUE
Chen Lin PhD 09/11
Chen Lin PhD 09/11
Null
Blood signal
Myocardium signal
Aquisition Window
Non-selective
IR
“Double IR” or Dark Blood (DB)
Slice Selective IR
Dark Blood Setup
Chen Lin PhD 09/11
With DB W/O DB With DB W/O DB
Double IR (Dark Blood) Example
Chen Lin PhD 09/11 Chen Lin PhD 09/11
Sel IR
Null Null
Blood
Myocardium
Fat Non-sel
IR
Triple IR (TIR)
Sel IR
Aquisition Window
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Triple IR Example
DIR TIR
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TR optimized
TR too short :
systolic motion reduces
myocardial signal
TR too long :
blood signal begins
to recover
Dark Blood Optimization
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Faster HR -> Shorter RR -> Less Recovery Time -> Shorter TI
TI
TI
Adjust TI According to Heart Rate (HR)
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HR (BPM) RR (ms) TR (ms) TI (ms)
60 1000 2000 630
80 750 1500 550
100 600 1200 420
Data TI
TR
Acquisition Timing
• TI = TR – Data Acq; Data Acq = TRmin
• Trigger Delay = 0
• Adjust TR so that Data Acq is in late Diastole. Chen Lin PhD 09/11
100 TRmin
Data Acq = TRmin = 100 ms
TI = TR - TRmin = 600 ms
TR TI
TI, TR Setup
Chen Lin PhD 09/11
For T1 Weighting: • 1 RR • Short ETL • Short TE
For T2 Weighting: • 2-3 RR • Long ETL • Long TE • + FS ?
DB TSE: T1w versus T2w
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“BRIGHT” BLOOD TECHNIQUE
Chen Lin PhD 09/11
Bright Blood Sequences (bSSFP)
Gslice
Gphase
Gread
TR a(f)
TR -a a
Gslice
Gphase
Gread
SSFP (FLASH)
Balanced SSFP (TrueFISP)
Chen Lin PhD 09/11
a(f)
Fin
n, J
. P. e
t al
. Rad
iolo
gy 2
00
6;2
41
:33
8-3
54
SSFP versus bSSFP
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bSSFP Contrast
Tips for TrueFISP Cine
• Image contrast relies on Steady state effects (Ratio of T2/T1)
– Use large flip angle
• TR and TE are set automatically
– Use min. TR.
– TE is half of the TR with the exception of asymmetric echo.
• Position heart near iso-center to improve field homogeneity and reduce off-resonance artifact.
• Common uses of TrueFISP cine are wall motion, valve motion, ventricular function.
Chen Lin PhD 09/11
FLOW QUANTIFICATION
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Motion Dependent Phase Difference
G
f
Bipolar Gradient
Stationary spins
Moving spins
Df = gAt V
t
A
A
Z
Y
X
MXY
t
t
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+180 deg
-180 deg
0 deg
+90 deg
-90 deg
+4096 pix
-4096 pix
0 pix
Velocity ENCoding
Velocity = VENC / 1800 * Df
Df
Chen Lin PhD 09/11
VENC optimal
VENC Optimization
VENC too large
Poor Contrast
VENC too small
Aliasing
-180
+180 +200
-200
+180
-180
+170
-170
+90
+180
-180
-90
Chen Lin PhD 09/11
VENC Optimization
Pulmonary Artery
70-130
Aorta
100 – 175
Carotid Artery
80 – 120
External Iliac Artery
81 – 120
Carotid Syphon
55
Common Femoral Artery
115
Basilar Artery
40
Superficial Femoral Artery
90
Vertebral Artery
40
Popliteal Artery
70
Sagittal Sinus Vein
10
Peripheral Veins
5 – 10
Chen Lin PhD 09/11
ECG
Acq Window Acq Window
Echoes
s1 = flow compensated s2 = flow encoded Velocity ~ fs2 - fs1
s1 s2 s1 s2 s1 s2 s1 s2 s1 s2 s1 s2 s1 s2 s1 s2 s1 s2 s1 s2
Interleaved Acquisition of Flow Compensated and Flow Encoded Echoes
Chen Lin PhD 09/11
Re-phased Magnitude Phase
|M1|
magnitude of flow
compensated signal
|M2 – M1|
magnitude of signal
difference
f2-f1
phase angle of
signal difference
flow bright
background visible
flow bright
background suppressed
forward flow bright
reverse flow black
background mid-gray
Phase Contrast Acquisition
Need to acquire two images: 1) w/o flow encoding and 2) flow encoded
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* Always minimize TE/TR after increasing VENC
VENC, Reconstruction and Direction Setup
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In-Plane
Sagittal
Aorta
Thru-Plane
Axial Aorta
Velocity Encoding Direction
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Aorta
Measuring Pulsatile Flow with Cardiac Trigger
CSF
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Finn, J. P. et al. Radiology 2006;241:338-354
Normal aortic valve
Velocity Flow Chen Lin PhD 09/11
Finn, J. P. et al. Radiology 2006;241:338-354
Aortic Value Stenosis
Velocity Flow Chen Lin PhD 09/11
Human Vascular System • Intra cranial
• Carotid
• Aortic
• Coronary
• Pulmonary
• Abdominal
• Renal
• Peripheral
Chen Lin PhD 09/11
Vascular Abnormities
• Stenosis
• Aneurysm
• Arterial Venous Malformation (AVM)
• Thrombus
• Plaque
• Internal bleeding
• …
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MRA Related Properties of Blood
• Flow – Velocity: 100 – 150 cm/sec in abdominal aorta; 10 – 20
cm/sec in peripheral arteries
– Steady versus Pulsatile: Peak arterial flow @ 150 – 200 ms after ventricular contraction
– Laminar versus Turbulent
• T1
– ~ 1200ms @ 1.5T; ~ 1500ms @ 3T
• T2
– ~ 250ms for arterial blood; ~ 30ms for venous blood
Chen Lin PhD 09/11
MR Angiography Techniques
• Contrast Enhanced MRA (CE-MRA) – High contrast to noise ratio
– No flow induced de-phasing and signal lost
– Fast acquisition -> Time-resolved MRA
– Acquisition timing is important
– Gd related NSF is a concern
• Non-Enhanced MRA (NCE-MRA) – Quantitative
– Prone to artifacts
– Different techniques specific to region
Chen Lin PhD 09/11
Basic CE-MRA Technique
• 0.1-0.2 mM/kg (20-40ml) of Gd contrast injected at 2-3 ml/sec.
• Flush with 20-30ml of saline.
• T1w 3D spoiled gradient echo based sequence.
• Min. TE and Min. TR.
• Partial k-space acquisition.
0.8 x 0.9 x 0.6 mm3
Chen Lin PhD 09/11
CE-MRA Considerations
1. Amount of Gd Contrast (dose) and Injection rate
2. Proper acquisition window and timing
– Accurate bolus timing by test bolus or fluro-trigger
– Centric view ordering
3. Acceleration with partial k-space acquisition
– Partial Echo, Partial Fourier, Parallel imaging, Radial sampling
4. Time resolved MRA with view sharing
– Key-hole, TRICKS/TWIST, 4D-TRAK
5. Multi-station bolus chasing and continuous moving table acquisition for peripheral MRA (pMRA)
Chen Lin PhD 09/11
1. Amount of Contrast
• Pulmonary arteries 0.1 mmol/kg • Aorta 0.1 - 0.2 mmol/kg • Renal arteries 0.1 - 0.2 mmol/kg • Portal vein 0.2 mmol/kg • Peripheral arteries 0.3 mmol/kg
Gd: 20ml Gd: 40ml
Co
urt
esy
of
M. P
rin
ce,
Co
rnel
l, N
Y
Chen Lin PhD 09/11
2. Acquisition Window and Timing
Artery
Vein
Time
Inje
ctio
n
12 sec 18 sec 24 sec 30 sec 0 sec
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Timing Bolus
• 1 – 2 ml of Gd with 20ml Saline and same injection rate.
• Plot signal intensity versus time in the feeding artery.
• Allows individual measurement of arterial and venous enhancement kinetics
Chen Lin PhD 09/11
Fluoro Triggering and Centric View Order
Artery
Vein
Time
Co
ntr
ast
Co
nce
ntr
atio
n Inje
ctio
n
Patient Specific Delay
Recessed Elliptical Centric View Order
• Fluoro Triggering : Realtime 2D scan of ~1 fps) • Test Bolus: 2D fast scan with small dose
Time to k-space Center
Chen Lin PhD 09/11
4. Time-resolved CE-MRA (tMRA)
Artery
Vein
Time
Co
ntr
ast
Co
nce
ntr
atio
n Inje
ctio
n
Chen Lin PhD 09/11
tMRA with High Accelaration Factor
P.Fi
nn
et
al.,
UC
LA, L
os
An
gele
s, U
SA
One volume per sec with iPAT = 4
Chen Lin PhD 09/11
Combined
Acceleration by Sharing of k-space Data
• Divide k-space into central and peripheral regions.
• Sample central k-space region more frequently than peripheral region.
• Share peripheral k-space data in multiple reconstructions.
• Maintain the same SNR.
• Increase frame rate, but temporal base remains same (temporal interpolation).
Chen Lin PhD 09/11
TWIST (Time-resolved imaging WIth Stochastic Trajectories)
Contrast
A B B B A A
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Dynamic Series MIP
Size (%) Density (%)
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EC-TRICKS
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A
B
C
D
Kx
Kz
Ky
ABCD AB AC AD AB AC AD AB AC AD AB …
Time-resolved MRA
Courtesy of Y. Zhou, PhD
4D (Spatial & Temporal) Information
Chen Lin PhD 09/11
Summary
• CE MRA is a fast and robust technique.
• Acquisition timing is critical to ensure optimal SNR and prevent venous contamination.
• Proper screening of high risk patients is needed to avoid NSF caused by Gd contrast agent.
Chen Lin PhD 09/11
Major Non-CE MRA Techniques
1. Time of Flight (TOF) – 3D TOF: Intra-cranial
– 2D TOF: Carotid, Pelvic, Peripheral
2. Phase Contrast (PC) – Intra-cranial, Renal
3. IR Prepared Balanced SSFP – Coronary, Renal
4. ECG Triggered Multi-(cardiac)-phase FSE – Abdominal, Pelvic, Peripheral
Chen Lin PhD 09/11
TOF PC [IR-] bSSFP [ET-] MP-FSE
GE Inhance 2D
Inflow
Inhance 3D Velocity
Inhance / Inflow IR
Philips TOF Balanced FFE
TRANCE (Triggered
Angiography. Non-Contrast
Enhanced)
Siemens TOF PC NATIVE Tru-FISP NATIVE SPACE
Toshiba FBI (Fresh
Blood Imaging)
Time-SLIP: (Spatial Labeling Inversion Pulse)
CIA (Contrast-Free Improved Angiography)
Non-CE MRA Application from Major Vendors
Chen Lin PhD 09/11
Magnetization Preparations in Renal bSSFP MRA
1. Apply IR to suppress background tissue
2. Allow Inflow of arterial blood
3. Apply SAT band to eliminate venous signal
4. Acquire data with balanced SSFP sequence
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Typical Renal IR-bSSFP MRA Protocols
• Basic Sequence: Balanced SSFP
• Contrast: – TE = Min.
– TR = Min.
– FA: 60-70 deg.
– TI = 600 ms
• Orientation: Axial slab for renal
• Coverage: – FOV/PFOV/SLAB: 340 mm / 70-80% / 70-80 mm
• Resolution: – Base/phase/slice thickness: 304 / 80% / 0.8 mm
• Options: – FATSAT, ECG Triggering and Resp Gating
Chen Lin PhD 09/11
IR-bSSFP MRA of Renal Artery
Chen Lin PhD 09/11
Shim
ada,
K. e
t al
. Am
. J. R
oen
tgen
ol.
20
09
;19
3:1
06
-11
2
IR-bSSFP MRA of Portal Vein
Planning MIP
Chen Lin PhD 09/11
ECG
Systolic phase
Diastolic phase
Arterial flow
velocity
ECG delay time 1
Option 1
Use SPACE1 acquisition in a short window
1
Fast arterial flow
Principles of Triggered FSE MRA
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ECG
Systolic phase
Arterial flow
velocity
ECG delay time
Fast arterial flow
Principles of Triggered FSE MRA
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2
Option 2
Use FSE acquisition in a
long window
Diastolic phase
2
Velocity Contrast
Diastolic (V+A) Systolic (V)
- =
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MRA (A)
11/7/2011
13
FSE MRA Technique
• Signal lost due to fast flow during systole.
• Requires ECG triggering and correct setting of acquisition delays.
• Independent of flow direction.
Chen Lin PhD 09/11
Courte
sy o
f LM
U, M
unic
h, G
erm
any
NATIVE versus CE for pMRA
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Native vs CE MRA for Aortic Artery
Source Image
MIP
Chen Lin PhD 09/11
Thank You!
Chen Lin PhD 09/11
