Phantom manufacturing, a threefold process
Designing and manufacturing a diffusion phantoms is challenging task involving three steps :
- designing a specific fibre configuration,
- designing a media to hold the fibres strongly tightened and a container to receive the phantom,
- choosing an adequate recipe to fill the container with a MRI compatible solution.
Of course, not all the details will be provided in this technical description, in order to preserve the equity of the competition. To those who are interested in the conception of such phantoms, a short introduction will be delivered during the workshop introductory session.
Mechanical conception and manufacturing In order to create large bundles, we used hydrophobic acrylic fibres which diameter is of the same order of the diameter of myelinated axons. A polyurethane negative and positive prints of the target bundles were manufactured that are used to strongly tighten the fibres together. Layers of fibres were then interleaved and stack in each other to build several fibre crossing configurations. The density of fibres was close to 1900 fibres/mm². A cylindrical container compatible with MRI head coil antennas was designed to hold the phantom in the magnet. This container was made up of a plexiglass cylinder ended at its extremities by two plastic caps, equipped with taps to fill it.
As standard diffusion-weighted MR imaging uses the ultra-fast echoplanar acquisition scheme that is extremely sensitive to phase inhomogeneities, developing a filling process insuring the absence of any air bubbles in the container is of upmost importance. To do so, a dedicated platform was designed that enables a preliminary degassing of the solution, and a filling under vacuum conditions. A ultrasound beam is finally used to destroy any remaining air bubbles. The container was filled using pure water without any contrast agent.
The mechanical conception of this phantom was supervised by Cyril Poupon, Laurent Laribière, Grégory Tournier, and Denis Fournier. Its manufacturing was taken in charge by Jérémy Bernard, Grégory Tournier and Laurent Laribière from the mechanics lab of the NeuroSpin centre, under the supervision of Laurent Laribière. The MRI acquisitions were realized under the supervision of Cyril Poupon and Irina Kezele. The organizers would like to thank Dr Ghislaine Dehaene (LCOGN Lab, NeuroSpin centre) for funding this project.
Diffusion-weighted MRI acquisitions
Diffusion-weighted data of the phantoms were acquired on the 3T Tim Trio MRI systems of the NeuroSpin centre, equipped with a whole body gradient coil (40 mT/m, 200 T/m/s), and using a 12-channel receive only head coil, in combination with the whole body transmit coil of the MRI system.
A single-shot diffusion-weighted twice refocused spin echo echoplanar pulse sequence was used to perform the acquisitions, while compensating the Eddy current to the first order. As described in the previous article, two datasets were acquired at two different spatial resolutions : 3 mm isotropic and 6 mm isotropic.
The parameters were as follows for the 3 mm isotropic acquisition : field of view FOV=19.2cm, matrix 64x64, slice thickness TH=3mm, read bandwidth RBW=1775 Hz/pixel, partial Fourier factor 6/8, parallel reduction factor GRAPPA=2, repetition time TR=5s, 2 repetitions. Three diffusion sensitizations at b-values b=650/1500/2000 s/mm² corresponding to the echo times TE=77/94/102 ms respectively.
The parameters were as follows for the 6 mm isotropic acquisition : field of view FOV=38.4cm, matrix 64x64, slice thickness TH=6mm, read bandwidth RBW=1775 Hz/pixel, partial Fourier factor 6/8, parallel reduction factor GRAPPA=2, repetition time TR=5s, 1 repetition. Three diffusion sensitizations at b-values b=650/1500/2650 s/mm² corresponding to the echo times TE=77/94/110 ms respectively.
The diffusion sensitization was applied along a set of 64 orientations, uniformly distributed over the sphere.