|ECR 2015 / C-1009|
|Virtual Navigator Real-time Ultrasound-MRI Fusion Imaging of leg muscles in supine and weight bearing|
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MRI image acquisitions:
Dedicated pads were placed over and under the leg (Fig. 4) of the subject being examined to ensure a stable positioning and that the anatomical structures were within FoV magnet's range during weight bearing image acquisitions.
The use of pads did not affect leg muscular shape or produce any major leg shape deformation which would have entailed an MRI-US fusion imaging registration error. Pads were required also for supine image acquisitions in order to assure, as much as possible, that the same leg positioning was achieved for both MRI and Ultrasound images.
There were no issues concerning FoV leg image reconstruction during MRI acquisitions other than with one subject due to an initial pad misplacement which created artefacts over the leg’s skin and prevented the display of images for some pin points (specifically 2 out of 5 in this single case). MRI supine images were first acquired, then followed by weight bearing image acquisitions. Examined subjects were blocked by an appropriate lumbar support when standing (Fig. 5) that would avoid possible falls in the event of impaired postural control (i.e. autonomic failure, pyramidal and/or extrapyramidal syndromes).
Thanks to extremely quiet acquisition operations and to an open bore MRI system, subjects had part of their chest, arms and head outside of the bore and were able to talk with the operator while images were acquired in both positions. Stressful/claustrophobic conditions were therefore greatly diminished which allowed patients to relax and to limit their movements also during weight bearing scans which, in turn, avoided major motion or significant image artefacts.
Ultrasound-MRI Fusion Imaging:
Ultrasound fusion images were acquired using a non-metallic table to avoid any interference with the electromagnetic transmitter. The transmitter was placed on a stable stand with arrow being directed towards the examined leg at the greatest electromagnetic field strength.
An appropriate Motion Control Sensor was positioned over the examined leg’s skin to counteract voluntary or involuntary movements thereby avoiding to re-perform co-registration between MRI and US.
Procedures were followed in order to properly co-register the two imaging modes and to preserve co-registration. During registration procedure the operator avoided to initially press skin with registration pen (not to shift pinpoints positioned on highly movable skin area) and did not press it with US probe during US fusion. These precautions we undertaken to prevent excessive compression of muscles and tissues underneath the skin layer that would allow to match MRI acquisition anatomical conditions as much as possible. By the same token, the operator also used large gel amounts that would avoid applying excessive pressure over the tissues while ensuring proper coupling during Ultrasound examination. Replicating MRI acquisition conditions (leg positions, muscle masses “space distribution”, leg rotation, pillow position over leg during resting position acquisition and over the leg during standing acquisitions) as much as possible was important.
For the examined subject's comfort, pinpoints can also be removed after MRI acquisition once their position is properly marked with a skin marking pen using the pins’ centre hole to mark their position directly over the skin. This procedure would also avoid having pins shift position during co-registration phase (Fig. 6 and Fig. 7).
Once the two image acquisition modes were co-registered together (Fig. 8), image acquisition was performed taking into account both B-Mode, Doppler (Color, Power and Pulsed Wave acquisitions) and Elastosonography modes (Fig. 9). Elastosonography showed the most interesting results as far as highlighting stiffness changes between supine and standing image acquisitions.
The algorithm that automatically selects the highest spatial resolution for the pre-acquired second mode image acquisition, was tested on all examined subjects by changing probe’s scanning orientation to check which resolution was automatically selected by the system for the fused second imaging mode sequence (Fig. 10).
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