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Piloting for Success

Achieve the best possible scan images with an effective pilot

28/3/13


The uninitiated could be forgiven for thinking that the hardest part of performing an MRI is interpreting the images. However Hallmarq’s MRI operators know that taking images of 3 different planes at once takes a lot of skill and accuracy to do well. A vital part of any scan is setting up the subject and equipment to give the best possible view. This is not easy with a lethargic sedated horse and a magnet that weighs half a tonne.

After placing the horse, the fine positioning of the magnet is done by taking a quick ‘pilot’ scan, adjusting the magnet as necessary, then performing another pilot (the ‘pilot of a pilot’, or a ‘pilot2’). Each pilot scan takes about 7 seconds and produces an image of each of the 3 planes with crosshairs showing the centre of the field of view. The magnet is adjusted until the crosshairs fall over the centre of the region. It is not unusual to perform 4 or 5 pilots to get a perfect position, but this generally decreases with practice.

81% of scans run on Hallmarq systems are of the foot, so this is the most important pilot to master. As a general rule, the crosshairs should be over the junction of the distal phalanx, the middle phalanx and the distal sesamoid bones, as shown above. The crosshairs should also be central in the medial/lateral directions. There is sometimes a natural degree of asymmetry in the foot and the operator must decide whether repositioning will improve the image.

When examining a specific part of the foot it can be tempting to move the crosshairs closer to that region, however this is unnecessary. Hallmarq’s standing equine MRI can image a volume of approximately 13 cm diameter, or about the size of a grapefruit. Placing the crosshairs as described above is the best way to get homogenous resolution of the whole foot. Moving the crosshairs a few centimetres one way or another so that they fall over a specific region will not give a better picture of that area, diagnosis may be even more difficult.

There are of course exceptions. The textbook placement described here will often cut off the very tip of the toe. While usually not a problem, if the pathology is known to be in P3 then it might be prudent to move the crosshairs slightly down and to the left. However, when part of the image is cut off there is often an area of hyperintense signal at the edge of the field of view. This is an artefact due to a high concentration of field lines. The brightness of the artefact signal can be such that it overwhelms the signal from the target region. In this case, to achieve a good image run either a presaturation sequence (PSAT) or adjust the greyscale.

Rotation crosshairs

The first pilot is when lateral corrections are made. The pilot of a pilot is solely to adjust for rotation and should never be used to correct for poor positioning. Lateral adjustment at this stage does not change the relationship between the foot and the magnet, adjusting the pilot2 laterally will result in poor images and make life harder for the image reader. The pilot2 is designed to correct for rotation. Conformation sometimes means that a horse is slightly toe-in or –out in the magnet and even a few degrees of adverse rotation will reduce the usefulness of scan slices. Rotate by shift-clicking and dragging the crosshairs until your image is as symmetrical as possible.

It’s disheartening when you get the perfect pilot and then the horse or coil moves and you have to start it all again. Good piloting is nothing without skilled horse handling and careful sedation monitoring. It’s necessary for the whole team to work together to get the outstanding images that are needed to diagnose the horse’s lameness. 
Stay tuned for a future feature on effective horse handling.
 

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