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!