Introduction Why invasive imaging · • ECG-gated respiratory-triggered 3-D SSFP Free breathing &...
Transcript of Introduction Why invasive imaging · • ECG-gated respiratory-triggered 3-D SSFP Free breathing &...
Rodrigo Salgado
Annemiek Snoeckx
Maarten Spinhoven
Bob Corthouts
Bart Op de Beeck
Paul M. Parizel
MR Angiography in 2012
Dept of RadiologyAntwerp University Hospital -Belgium
Basic principles, post-processing & pitfalls
Introduction
Why invasive imaging ?
• Major advantages of catheter angiography• Real time cinematic imaging
• Greater spatial & temporal resolution
• Immediate interpretation of images
• Selective catheterization
• But also...• no direct visualization of plaque: underestimation of atherosclerosis
• Invasive, larger threshold for examination
• Not suitable to confirm normal findings
Puncture site complicationPuncture site complication Reported rates (%)Reported rates (%)
Hematoma (requiring further attention) 0,0 - 0,68
Pseudoaneurysm - AV fistula 0,04 - 0,20
Occlusion 0,0 - 0,76
Distal emboli 0,0 - 0,10
Arterial dissection/subintimal passage 0,43
Subintimal contrast injections 0,0 - 0,44
Major contrast reactions 0,0 - 0,38
Contrast-media related nephrotoxicity 0,2 - 1,4
Why invasive imaging ?
Modern MRA expectations
• Faster Scans• Cover large anatomic volume in minimum amount of time
• Less motion artifacts
• High spatial resolution• Submillimeter isotropic resolution at all times
• Confident diagnosis• High sensitive examination
• Specificity is improving, but depends on indication
Acquiring the data
• 3D TOF is…
a. A technique typically used for evaluation of renal artery stenosis
b. A non-contrast enhanced MR angiography technique
c. Can underestimate a stenosis
d. Excellent for evaluation of large aortic aneurysms
Question
Answer: B
Non-contrast enhanced
Time-of-Flight (TOF)
Phase Contrast Angiography (PCA)
Contrast-enhanced Contrast-enhanced MRAfirst-pass
time-resolved
Intrinsic contrast provided by flowing blood
T1-shortening effect of Gd
MR angiography
Inflow effects
Phase effects
MR angiography: technique
• Time-of-flight (TOF) MRA
• One the earliest MRA techniques
• Gradient echo
• Low signal of stationary tissue
• Weak signal due to partial saturation of spins
• High signal of flowing blood
• Strong magnetization due to inflow of non-saturated spins
Technique
• Time-of-flight
Imaging section
Repeated RF excitationsShort TE
Unsaturated spin
Pre-saturation pulse
Technique
• Time-of-Flight
3D TOF has its typical use in neuro-imaging
MR angiography: technique
• Time of flight MRA
• Long scan times
• Difficult breathhold imaging misregistration artifacts
• More sensitive to through-plane flow than to in-plane flow
• Difficult evaluation of mesenteric vasculature
• Triphasic nature of splanchnic arterial blood flow
• Signal diminishes during retrograde diastolic flow
• Overestimation of stenosis
MR angiography: technique
• Phase-contrast (PC) MRA
• Direct quantitative evaluation of
• Flow direction
• Flow velocity
• Gradient• superimposed magnetic field
• switched on during time T
• field strength depends (linearly) on position
MR angiography: technique
• Changed magnetic field• changes “rotational frequency” of protons (precession)
• therefore introduces phase shift along gradient direction
• Gradient• superimposed magnetic field
• switched on during time T
• field strength depends (linearly) on position
MR angiography: technique
• Bipolar gradient• gradient with strength Gx switched on during time T
• followed by gradient of equal strength but opposed direction during equal time T
• for a stationary spin Gx is equal but opposite and x and T are equal, so there is no net phase change
MR angiography: technique
• Bipolar gradient – moving spins
• spins moving along the gradient direction
• different magnitude or direction in the second gradient compared to the first
• introducing a net phase shift
• phase shift is a measure of speed
MR angiography: technique
• PC MRA - Phase images
• voxel with phase shift = flow = vascular
• sensitive to the direction of flow
• velocity quantification
• along the gradient direction
• Velocity + cross-sect. area
• = flow quantification
MR angiography: technique
• PC MRA - Velocity images
• Repeat
• flow sensitization along all 3 orthogonal directions
• Compose:v = (v2
read + v2phase + v2
slice)
MR angiography: technique
Peak systolic velocity curve profile
normal < 80 cm/sec
Curve profiles Moderate stenosis
High grade stenosis MR angiography: technique
• Phase contrast MRA
• Advantages
• especially useful in quantitative evaluation of arterial stenosis
• renal artery blood flow gradient
MR angiography: technique
• Disadvantages• Signal loss due to magnetic susceptibility artifacts
• deoxyhemoglobin, hemosiderin and ferritin
• partially thrombosed aneurysms
• Full PC MRA measurement is time-consuming• GRE or EPI PC-MRA
• Sensitive to motion artifacts
• Niche technique• quantification of flow (renal arteries)
• Venous occlusive disease
MR angiography: technique
• Contrast-enhanced MRA• Introduced by Martin Prince in 1993
• Exploits differences in longitudinal magnetization
• T1-shortening effect of intravenous Gadolinium
• Contrast is relatively independent of flow dynamics
• Saturation effects are substantially reduced
• First pass effect
• timing is everything!
• Most commonly used MRA technique
MR angiography: technique
• Contrast-enhanced MRA
Dry run Visual bolus triggering
MR angiography: technique
• Contrast-enhanced MRA
Contrast-enhanced MRA Subtraction
Contrast-enhanced MRA
• Advantages
• Excellent image quality
• Large FOV and volume of interest coverage
• Short acquisition times
• Absence of flow-related, saturation and T1-related artifacts
• Good temporal resolution
• Minimally invasive character
Manupulating the data
• Maximum intensity projection is…
a. The best processing technique for visualization of 3D structures
b. Only useful with contrast-enhanced MRA techniques
c. Can underestimate a stenosis
d. Provides the best visualization of the distal segments of the renal arteries
Question
Answer: D
Post-processing
Data acquisition
Other applications
multiplanar reformations
curved MPR
Maximum Intensity Projection
Volume Rendering
MR angiography: technique
• Maximum Intensity Projection• Projection technique !
• Selection of voxel with highest density
• Basically a 2D-technique
Post-processing
Post-processing
• Maximum intensity projection
Full volume MIP Segmental slab MIP Where is the AV shunt ?
?
Post-processing
• Maximum intensity projection
True 3D visualization of anatomy
Post-processing
• Volume Rendering (VR)
Volume rendering
• Transfert-function
• Uses full dataset
• Each voxel within specified range has a contribution
• Interactive and modifiable powerful parameters
• Powerful data-segmentation
• Reliable respresentation of reality
• True 3D view• 3D effect of MIP is false !
• Less detail• Smaller vessels better depicted with MIP
Contrast-enhanced MRA
• Disadvantages
• Hardware requirements
• Powerful & fast gradients
• Suboptimal spatial resolution of small vascular structures
• Variable & unpredictable individual hemodynamic variations
• requiring a test bolus study or automatic triggering
• Possibility of magnetic susceptibility artifacts
Clinical applicationsUseful tricks & common pitfalls
• When evaluating an abdominal aortic aneurysm
a. The best processing technique is MIP
b. 3D TOF will overestimate the size of the aneurysm
c. The best processing technique is volume rendering
d. Only native images can reveal the full extend of the pathology
Question
Answer: D
thrombus
Always look at the native images !
Aortic aneurysm
• Look out for artifacts !
This is not a stenosis…
Vascular Stents
• When screening for renovascular hypertension, the MRA examination
result is normal or non-significant in:
a. 50 %
b. 60 %
c. 80 %
d. 90 %
Question
Answer:D
• Renovascular disease• Causal relationship between renal artery stenosis (RAS) and its clinical
consequences
• Hypertension
• Renal failure
• Complex relation between RAS and hypertension
• Etiology of RAS• Atherosclerosis: 90 %
• Fibromuscular dysplasia: 10 %
• Other: aorta arteritis, thrombosis, dissection,…
Clinical applications
• MR angiography in renovascular disease• Evaluation of
• Adrenal glands
• Kidney morphology & dimensions
• Renal arteries
• Vessel wall
• Protocol• Axial T1- & T2-weighted images (T1 in- and out of phase)
• Coronal TrueFisp images
• Contrast-enhanced MRA
• Phase-contrast MRA
Clinical applications
• Renovascular disease
Clinical applications
• Renovascular disease
Clinical applications
Clinical applications
• Fibromuscular dysplasia
Clinical applications
• Fibromuscular dysplasia
Insufficient evaluation of the distal segments
Improving image qualityUseful tricks & common pitfalls
• Which option is the easiest to improve MRA quality
a. Decrease your flip angle
b. Raise your TR-time
c. Use a high relaxivity contrast agent
d. Use 3T MR imaging system
Question
Answer: C
Spatial resolution
SNR Motion
1.5 vs 3T
Improving image quality
Improving SNR
High-Field Imaging (3T) Parallel Imaging
Same spatial resolution in shorter examination time
Better spatial resolution in same examination time
Better temporal resolution
High relaxivity contrast agents
No IPATBody coil
motion acceptable
Different patients
82-y-old F
Acquisition time: 25 sec
NO PATTR: 4.45 msecTE: 1.245msecFlip angle: 20°Matrix resolution: 512x512Acquisition time: 25 secContrast: 20ml (2 ml/sec) Phase sampling: 50%Pixel bandwidth: 280Coil: Body coilSlice thickness: 1.3 mm
54-y-old M
Parallel Imaging
Different patients, 12-channel phased-array body coil
75-y-old M 76-y-old M
PATx2
Acquisition time: 12 sec
PATx3
Acquisition time: 8 sec
PATx2
Parallel Imaging
PATx3TR: 3.63 msecTE: 1.27 msecFlip angle: 20°Matrix resolution: 384x384Acquisition time: 8.52 secContrast: 20ml (2 ml/sec) for each phasePhase sampling: 70%Pixel bandwidth: 345Coil: 12-channel phased-array body coilSlice thickness: 1.5 mm
PATx312-channel body coil
PATx212-channel body coil
LeRiche Syndrome
Parallel Imaging New developments
• High relaxivity contrast agents• Gadobutrol (Gadovist, Bayer Shering)
• Double Gd-concentration 1.0 M compared to conventional contrast agents (0.5 M)
• Lower dose volume• Typical <10 cc for one body region
• Compact contrast bolus• High arterial contrast
• Excellent image quality
3T
• Excellent for MR-A
3T & parallel imaging & high relaxivity contrast agent
New developments
• Time-resolved CE-MRA
Pelvic varicositas
What about 2012?
MRA of children
• Always technically challenging
Small FOVSmall FOVSmall FOV
MotionMotionMotionResolutionResolutionResolution
SNRSNRSNRIV accessIV accessIV access
Off-label use of CMOffOff--label use of CMlabel use of CM
MRA & children
• Age guidelines
• <3 years
• Sedation in pediatric department
• Chloral hydrate
• Transportation to MR department when sleeping
• >3 years
• Sleep-induced anesthesia
• Presence of anesthesiologist
• Monitoring• ECG & respiratory monitoring
• Visual access
Non-contrast enhanced MRA
• Inflow-enhanced b-SSFP technique
Free-breathing & non-contrast MRA
Serai S et al Non-contrast MRA using an inflow-enhanced, inversion recovery SSFP technique in pediatric abdominal imaging Pediatr Radiol. 2012 Mar;42(3):364-8
Non-contrast MRA
• ECG-gated respiratory-triggered 3-D SSFP
Free breathing & non-contrast MRA
Klee D et al Non-enhanced ECG-gated respiratory-triggered 3-D steady-state free-precession MR angiography with slab-selective inversion: initial experience in visualisation of renal arteries in free-breathing children without renal artery abnormality Pediatr Radiol. 2012 Mar 31 (epub)
Non-contrast MRA
• The endpoint is a diagnosis!
• Beware of 3D images in suboptimal conditions
• Do not underestimate axial images
• HASTE, TrueFISP
• Perfect for anatomy evaluation
• Vascular anomalies
• VCI agenesis/hypoplasia
• Use common sense
Last question
• The favorite Belgian beer of yours truly is:
a. Heineken
b. Duvel
c. Budweiser
d. West-Malle Trappist
Question
Answer: B
Thank you!