Contrast Enhanced MRA

Contrast Enhanced MRA
How I do it
Scott B. Reeder, MD, PhD
International Society for Magnetic Resonance in Medicine
Sociedad Mexicana de Radiologia e Imagen (SMRI)
Mexico City
June 4, 2014
Department of Radiology
University of Wisconsin
Madison, WI
Disclosure
UW receives support from GE and Bracco
Off-label uses of Gadolinium contrast
Investigational Pulse Sequences
Learning Objectives
• Understand the basic physics of contrast enhanced
MRA
• Be familiar with common applications and diagnoses
• Advantages and considerations of CE-MRA at 3.0T
MRA Methods
Renal Artery MRA
Time-of-Flight
Phase Contrast
Contrast-Enhanced
Contrast Enhanced MRA*:
T1 Shortening of Gadolinium
• Gadolinium agents shorten T1 dramatically
1
  r1 Gd 
T1
• During IV bolus, Gadolinium concentrates in
arteries for about 1 minute
• Gadolinium is a potent T1 relaxation agent
T1 of blood: 1200 ms
<100 ms
*Contrast-enhanced MRA is an off label use
Contrast Enhanced MRA:
T1 Shortening of Gadolinium
30o
Increasing Gd concentration
(shortened T1)
Poor contrast at
low flip angles
5mM
Background suppression
in non-enhancing tissues
k-space
ky
Image
Detail
kx
Contrast
Contrast Curve
signal
Elliptic Centric Acquisition
time
k-Space Data
Contrast-enhanced MRA
Pre
During
Post
Contrast-enhanced MRA
During
Post
Principles of MRA: Contrast Timing
• Critical to understand k-space
behavior
• Center of k-space:
contrast/brightness
• Periphery of k-space: detail
• Goal: time peak of bolus while
acquiring central lines of k-space
Case: 81yo F with HTN resistant to medication
Differential Perfusion
Phase Contrast
Visualization and Grading of Stenoses
• Grading based on diameter stenoses
–Mild <50%
–Moderate 50-75%
–Severe >75%
• Do NOT rely on MIP images to grade stenosis
–Must use thin source images or thin MPR reformats
MRA: Cross-sectional Multi-Planar Reformats
Case Courtesy Stefan Schoenberg, MD
Courtesy of G. Schneider
Case: 36yo F with
h/o HTN
Case:
29yo F, 3 days s/p MVA with new HTN
Case:
29yo F, 3 days s/p MVA with new HTN
Page Kidney from Intracapsular
Hematoma
Why Time Resolved MRA?
• Physiological information
–Collaterals (eg. coarctation, occlusion)
–Shunting (eg. AV fistula, congenital disease)
–Reversed flow (eg. Pelvic congestion syndrome,
endoleaks)
–Filling patterns in aortic dissection
–Enhancement of hypervascular tumors
–Many more applications …
• Timing
–Timing may be difficult or impossible in some
applications
–Avoids the need for timing bolus: point and shoot
Approaches to Time Resolved MRA
• Brute force
•
•
•
•
Commercial
–Repeat acquisition many times
State of the Art
–Parallel imaging can help
–Inefficient, usually inadequate
Exploit k-space behavior of 3D-MRA
–TRICKS/TWIST, keyhole imaging
Exploit sparsity of MRA imaging
–Undersampled projection reconstruction On the near
Exploit constraints of fixed anatomy
Horizon
–HYPR, compressed sensing methods
Combinations of the above …
k-space
ky
Image
Detail
kx
Contrast
5%
10 %
20 %
50 %
5%
10 %
20 %
50 %
3D Time Resolved Imaging of Contrast Kinetics (TRICKS)
(TWIST, TRACKS)
kz
kz
ky
D CB A B C D
ky
Korosec et al.,
Magn. Reson. Med. 1996
3D TRICKS: Technique
Artery
Contrast curve
Vein
Time frame 10 11 12 13 14 15 16 17 18 19 20 21 22
D A C A B A D A C A B A D
...
A
B(I)
C(I)
D(I)
FFT
Image at
time frame 15
...
3D TRICKS
TR = 10.8 (1996)
512 x 128 x16
Frame Time
5.6 s
construction time 1996: 6 hours, one graduate studen
Pelvic Congestion Syndrome
Bolus-chase MRA
0.1mmol/kg Gd shared btw 2 & 3
2. Pelvis: Reverse EC
2
3. Thighs: EC
3
1. Distal Station
Time-resolved MRA
10 cc Gd at 2cc/sec
1
Indications: Pulmonary Embolus
•
•
•
•
•
•
•
Difficult to diagnose clinically
Potentially fatal
CTA commonly used to diagnose PE
PE uncommon (5% of CTA positive)
Young patients
Large radiation dose
Historically, MRA limited by scan time and
spatial coverage
• Parallel imaging for improved coverage
1.5T Thoracic MRA
2D Parallel Imaging (ARC)
Coronal (Acquisition Plane)
Axial Reformats
1.2 x 1.4 x 2.0mm3 in 13-19s
Sagittal Reformats
Normal Pulmonary Angiogram
Pulmonary MRA:
r/o Pulmonary Embolus
LLL PE
Pulmonary MRA:
r/o Pulmonary Embolus
RUL PE
Pulmonary MRA: Value of Perfusion
33 yr old woman with right pleuritic chest pain
Coronal
Axial MPR
Sagittal MPR
Cutoff vessel leading
into perfusion defect
Double Oblique Thin Slab MIP
Case: 61yo M with Pneumonia, r/o PE
Poor opacification of LLL PA’s
Could not exclude PE ….
Pulmonary MRA …no PE on MRA
Case: 61yo M with Pneumonia, r/o PE
Pulmonary abscess, empyema
Advantages of High Field Strength
Why use 3T for MRA??
Physics of 3.0T MRI: Increased
SNR
• Higher field strength
–SNR proportional to field strength: 2x SNR
• Increased spatial resolution
• Shortened Scan Times
–Opportunity to reduce scan time with parallel
imaging with minimal to no SNR penalty
All methods benefit from increased
SNR
Physics of 3.0T MRI: Improved CNR
• Longer T1 at 3T
–Improved Background Suppression
–Results in improved CNR of enhancing tissue
• Overall: improved CNR
– increased SNR
– background suppression
Most important benefit of TOF and CE-MRA at
3T
Relaxivity of Gadolinium Drops at Higher
Field
1
∆ = r1 [Gd ]
Enhancement
Gad
T1
Concentration
Relaxivity: Bang for your $$
Bo (T)
Gd-DPTA Gd-BT-DO3A Gd-BOPTA
(Magnevist)
(Gadavist) (Multihance)
0.2 T
4.7
5.5
10.9
1.5 T
3.9
4.7
8.1
3.0 T
3.3
3.6
6.3
Pintaske et al, Invest. Radiol. 2006
Physics of 3.0T MRI: Decreased Relaxivity
• Decrease in the relaxivity of Gadolinium at 3T
– Reduces the “bang for your buck” of
contrast
• Relatively small effect, outweighed by improved
SNR and increased background T1
Physics of 3.0T MRI:
Improved Background Suppression
Enhancing
Arterial Blood
Unenhancing
Tissue at 1.5T
Unenhancing
Tissue at 3.0T
Longer T1 Suppresses background signal and
improves CNR
Improved CNR at 3.0T: Summary
• Improved SNR alone improves CNR by 2x
• Also have improved contrast from T1 effect
• Small reduction in the relaxivity of Gd for CEMRA
• Overall, Contrast to Noise Ratio improves by
much larger factor, perhaps as high at 3x
(prediction)
–Precise improvement difficult to measure
Physics of 3.0T MRI: Parallel Imaging
• Parallel imaging works better at higher field strength!
• Smaller wavelengths at 3T
• Better SNR performance (lower g-factor)
• More SNR to “burn”
–2x field strength translates to 4x acceleration with same
SNR!!
• Particularly true at higher accelerations, 2D acceleration
• Full capabilities of parallel imaging at 3T still being
explored
Parallel Imaging Gives 3T imaging the
Flexibility to trade the 2x SNR for 4x faster
scanning
Disadvantages: Challenges of MRA at
3T
Field (Bo) Inhomogeneities
RF (B1) Inhomogeneities
SAR
Physics of 3.0T MRI: Bo Field
Homogeneity
• Increasing field strength worsens magnetic field
inhomogeneity due to increased susceptibility
• Susceptibility from stainless steel implants or
ferromagnetic foreign bodies does NOT worsen at
3T:
–induced magnetization is already saturated
• Relatively small impact on most MRA sequences
–Short TR
–high bandwidth
–Not a major issue
• Inject contrast more slowly to prevent T2* effect
Physics of 3.0T MRI:
RF (B1) Inhomogeneities
• Created by dielectric effects and shorter
wavelength of the higher field imaging
• Results in non-uniform B1 amplitude
–Manifests as non-uniform flip angle
Physics of 3.0T MRI:
RF (B1) Inhomogeneities
Enhancing
Arterial Blood
Broad signal response
Unenhancing
Tissue
MRA methods relatively insensitive to
flip angle and RF inhomogeneitites
RF Heating: Specific Absorption Rate (SAR)
• Major challenge for high flip angle sequences
–Fast Spin-Echo (FSE)
–Steady State Free Precession
* (SSFP, FIESTA, trueFISP, BFFE)
–These sequences not typically used for MRA
RF Pulse Duration
RF Amplitude
- doubles at 3T
RF Heating: Specific Absorption Rate (SAR)
• CE-MRA uses body coil for transmit
–Very bad for SAR, but …
–High flip angle (30o), short TR (3ms)
–Scan duration limited (20-30s), which limits SAR
• Parallel imaging reduces SAR if shorter scan times used
Pulmonary MRA: 3T
19s breath-hold
1.2 x 1.3 x 1.6 mm3
(0.6 x 0.6 x 0.8 mm3)
Pre-surgical Localization : Artery of Adamkiewicz
1. The Artery of Adamkiewicz (AOA) is a tiny
artery (0.5-1.0 mm)
– Requires very high spatial resolution
2. The AOA has a variable origin
– A large field of view (FOV) is needed to ensure
visualization
Vein
Artery
3. Great anterior radiculomedullary vein can
mistaken for the AOA
4. Bolus tracking techniques are difficult to time
precisely in TAA patients
Nijenhuis et al, AJNR 2006
Anterior Spinal Artery MRA:
Excellent anatomical detail
Time-resolved MRA at 3T
Yoshioka, K et al. Radiographics 2006
Results: Examples
Bley et al Radiology 2010, 255(3):873-81
Results: Arterial-Venous Separation
with TR-MRA at 3.0T
No se puede mostrar la imagen. Puede que su equipo no tenga suficiente memoria para abrir la imagen o que ésta esté dañada. Reinicie el equipo y , a continuación, abra el archiv o de nuev o. Si sigue apareciendo la x roja, puede que tenga que borrar la imagen e insertarla de nuev o.
Phase 3
No se puede mostrar la imagen. Puede que su equipo no tenga suficiente memoria para abrir la imagen o que ésta esté dañada. Reinicie el equipo y , a continuación, abra el archiv o de nuev o. Si sigue apareciendo la x roja, puede que tenga que borrar la imagen e insertarla de nuev o.
Phase 5
No se puede mostrar la imagen. Puede que su equipo no tenga suficiente memoria para abrir la imagen o que ésta esté dañada. Reinicie el equipo y , a continuación, abra el archiv o de nuev o. Si sigue apareciendo la x roja, puede que tenga que borrar la imagen e insertarla de nuev o.
Phase 7
Learning Objectives
• Understand the basic physics of contrast enhanced
MRA
• Be familiar with common applications and diagnoses
• Advantages and considerations of CE-MRA at 3.0T
Thank you!
• Tom Grist, MD
• Chris Francois, MD
• Scott Nagle, MD, PhD
• Mark Schiebler, MD