01 INTRODUCTION

MRI Learning Objectives:

. You should be able to make an MRA / MRV examination

. You should be able to

• use the correct MRA / MRV methods as required
• also depending on pathology with a good IQ
• if needed with the correct use of a contrast agent

Pre-Requirements

It should be nice if you have pre-knowledge regarding:

• Pathologies and indications for an MRI exam
• RF sequences
• Basic supporting techniques
• How to optimise the IQ:
  • Resolution
  • Contrast behavior
  • Artefact handling
  • S/N and C/N ratio

With this e-module an impression will be of the state of the art in MR Angio and MR Venography, examining the vessels. The e-module is vendor-independent using the terminology and cases mainly from Siemens, GE, Philips, Canon and Hitachi and could be used as a subject for MRI education on colleges etc. Basic knowledge of MRI is recommended.

As alternatives for MRA we have nowadays DSA, Duplex Ultrasound and CTA.

When we can use MRA?

We can use MRA for occlusions, dissections, stenosis, aneurysms and for selecting candidates for an interventional examination or for surgery.

We have currently a growing number of methods to do an MRA / MRV exam: Vessels can be shown with bright blood or black blood, combinations are also often indicated regarding e.g. the vessel wall.

MRA / MRV Techniques:

• No contrast agent or NCE (Non-Contrast-Enhanced) MRA methods:
  • TOF (Time-Of-Flight) / Inflow
  • PCA: Phase Contrast + Flow quantification
  • QISS (Quiescent Interval Single‐Shot)
  • bSSFP (Fiesta-True FISP- Bal. F(T)FE)
  • 3D bSSFP based and NCE gated/triggerd 3D bSSFP with IR prep pulses: NATIVE TrueFISP (“Non-contrast MRA of ArTerIes and VEins using Sampling Perfection with Application optimized Contrasts using different flip angle Evolution and True Fast Imaging with Steady-state Precession.”), Inhance Inflow IR (IFIR), B-TRANCE (“Balanced TRiggered Angiography Non-Contrast Enhanced”), VASC- (“Veins and Arteries Sans Contrast”) or Time-SLIP (“Time Spatial Labeling Inversion Pulse”)
  • Cardiac-Gated 3D T(F)SE, NCE-MRA (FBI, NATIVE SPACE, 3D Delta Flow, TRANCE) VISTA, SPACE, CUBE RF sequences can be used.
  • ASL: Spin Labelling
  • Black (Dark) Blood: T(F)SE RF sequences with long TE’s or T(F)SE -HASTE with Black blood pre-pulses
  • SWI (Swan – BSI – SWIp)
  • DWI (Black Blood)
  • (i)MSDE T(F)SE RF sequences

• With contrast agent or CE (contrast enhanced) methods:
  • PCA (full dose increase venc. / velocity encoding
  • Black (Dark) Blood Gadolinium > Ti BB RF pulse)
  • CE-Angio: 3D HR or Time Resolved: 4D
  • Steady state with Ablavar (earlier Vasovist)

The way the MRA methods are grouped is very different in the literature: some are using conventional and advanced MRA. The conventional MRA methods Time of Flight – Inflow and Phase Contrast are based on flow of blood: flow gives the signal in the images. Other authors divide them in White and Black Blood MRA/MRV methods. And last but not least MRA/MRV methods can also be divided in CE (Contrast Enhanced) and non- CE MRA.

The ones non- CE MRA’s are the (balanced) SSFP, the Black-Dark Blood MRA, the non CE subtraction MRA, the Susceptibility (Vein Bold) method, DWI and Arterial Spin Labeling. But do not forget the conventional ones: the TOF and PCA methods.

The CE MRA’s can be done in 2D, 3D and 4D (Time resolved) and divided in MRA’s  with extra cellular contrast agents or with a  blood pool agent which we can combine with Steady State Imaging.

White blood Imaging RF sequences are:

• GRADIENT ECHO
• TURBO (FAST) GRADIENT ECHO
• bSSFP (balanced Steady State Free Precession): Balanced FFE /TFE – True Fisp – Fiesta
• T(F)SE non CE subtraction with ECG synchronization /  gating

Black Blood Imaging

• DUAL TIR – BB T(F)SE / DB HASTE
• T(F)SE with long TE’s
• SE-EPI
• GRASE / TGSE
• (i)MSDE T(F)SE RF sequences

02 Disclaimer

This document should not be construed to represent a definitive interpretation of the regulatory statutes regarding MRI and MRS, and the reader should be aware that regulations might change and render possible out-of-date information specified herein.

Although every attempt has been made to verify the information contained in this e-module, the author cannot guarantee its 100% accuracy. Each effort is made by the editorial board to see that no inaccurate or misleading data, opinion or statements occur. EMRIC sarl cannot accept responsibility for the completeness. This document may not be distributed or re-posted without the express written permission of EMRIC sarl.

I know not everything can be covered, but at least I would like to give an overview of MR knowledge, trying to stay updated, and also putting together what I picked up in the last 36 years. I still like to encourage everybody to do the same!

Several images are taken from my own database.

If you find an aspect of MRI and MRS that I have not covered, do let me know and I will resolve or add it.

03 Post Processing MRA – MRV

Image data

The use of AI (Artificial Intelligence) and DL (Deep Learning) made it possible to boost a lot the signal during the reconstruction process.

The latest AI signal boost methods (like GE AIR RECON DL from GE) is changing the MR world and makes it easier to fine-tune protocols while the S/N Ratio is playing a minor role. The acquisition times can be shortened and the spatial resolution can be increased. It brings also consistency for the radiologists where they can rely on.

Furthermore, substantial improvement in IQ, and especially in image contrast, can be attained through post processing techniques.

• Zero padding: the spatial resolution can be increased with interpolation. Zero padding involves filling out peripheral lines of k-space with zeroes prior to perform the Fourier Transformation and more images will be reconstructed with a smaller spacing. E.g.: with two-fold zero padding in the slice direction , if the slice thickness is 3mm, the Fourier Transform will reconstruct additional images that also have a 3mm slice thickness but at 1.5mm spacing with 50% overlap (same technique as OS = over contiguous slices) This reduces volume averaging and makes small vessels a smoother visualization on the reformatted MIP images.
• MR DSA (Digital Subtraction Angiography). The image contrast can be increased by digital subtraction of pre-contrast image data from dynamic, arterial, or venous phase image data. Subtraction can be done both slice-by-slice or before the Fourier transform by using a complex subtraction method, so the gadolinium dose needed can be decreased. However, the patient should not move between the pre-contrast and dynamic CE series and is difficult to reach in the thorax and abdomen, where respiratory, cardiac and peristaltic motions appear. Note complex subtraction is generally automatically performed during the reconstruction before creating any of the images.

Regarding Post Processing one uses mostly Projected Views: on most MR systems we can get automatically orthogonal MIP’s (Maximum Intensity Projection): an axial, sagital and coronal orientation and can be looked to the brightest pixels where the background is rather suppressed.

Using Black Blood Imaging we need MinMIP’s (Minimum Intensity Projection): where we get the darkest, most black pixels being viewed at and calculated.

MIP: on most workstations the MIP’s can be handled differently. Thin slab MIP’s can be made as part can be cut out of the 3D volume which are overlapping the vessels which need to be viewed.

MPR (Multi Planar Reformatting): From the original images also different orientations can be reconstructed. In this case it is an advantage if the original dataset is required with isotropic voxels.

With the data set we also can do more and more advanced Post Processing like e.g.  Volume Rendering, packages where we can navigate through the vessels etc. Many IT companies specialize in it. But source images still stay most important.

• Maximum Intensity Projection
  • Multiple angulated thin MIP!!
• Minimum Intensity Projection for black Blood
• Curved MPR
• Shaded surface display
• Endoview
• Volume Rendering
• Segmentation and registration
• Pressure and flow measurements

MRI Histology

Influence of the resolution on vessel size

The Resolution is defined by the voxel size, which has a certain thickness (slice thickness) and pixel size, defined by the phase encoding and frequency encoding gradient. On most systems the pixel size is related to the FOV : the larger the FOV the larger the pixel. On later generations of MR systems the pixel size can be separated from the FOV.

The resolution of an acquisition can affect the apparent size of the vessel being processed. Clear voxels are outside of, or fully contained within, the ROI. Grey voxels are partial-volumed. The edge of the ROI is the outer edge of the partial-volumed voxels.

The voxel size also influences the Intra-voxel  De-phasing: the more protons in a voxel which move (flow) the more de-phasing in a voxel (the larger the voxel the more de-phasing can appear).

So the more turbulence, retrograde flow and different velocities we have in a voxel, the more de-phasing which gives signal-loss.

The Intra-voxel  de-phasing can be reduced (if it is FOV related) by reducing the FOV (smaller pixels), higher scan matrix and increasing the pixel in phase or frequency encoding direction (getting more square pixels)

Flow Enhancement- Signal Loss

Enhancement of flow in the images is influenced by many scan parameters like TR and TE but also; in several MRA / MRV methods the following parameters can also have impact on the flow:

. Slice order: e.g. ascending – descending, perpendicular or parallel to the blood flow. If the slice order is chosen opposite to the flow direction (depending on the velocity), blood will not give any signal.

. Slice thickness:  slice perpendicular to the flow, increases the signal

Spatial mis-registration can happen by pulsatile flow if the moment of the blood excitation is different than the measured moment so the measured blood will be positioned on another place in k-space. It is shown as a repeating artefact in the phase encoding direction having always the same distance.

. Angulation : the overestimation of the area of a vessel increases if the slice is angulated through the vesse