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F.A.Q.


 

 

What is the difference between high field and low field scanners ?

High field scanners operate at a stronger magnetic field than low field scanners. The stronger field produces a stronger signal, so high field scanners can produce higher resolution pictures in a shorter time than low field scanners. Both high field and low field scanners have been used in human clinical medicine for many years. Low field scanners have been used for "Open MRI" because they are less claustrophobic. Both have a niche in clincal imaging. High field scanners:

  • are tubular in shape, whereas low field scanners are 'C' or 'U' shaped. Getting the horse into the tube requires general anesthesia, with the associated costs and risks. A published study of equine anaesthetic deaths1 found a 1% risk of death in anaesthetising healthy horses
  • require a larger room and more specialist equipment as the stray magnetic field covers a larger area
  • are much more expensive to purchase, and need regular refills with costly liquid helium, so they tend to be installed only in universities and a very few clinics, and cost more for the scan

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Isn't high field always the better way to image?

Images from high field scanners include a lot of detail and can cover a larger region in a single scan. However this does not necessarily make for a better diagnosis, in much the same way as a veterinarian may choose to use a smaller x-ray or ultrasound machine in circumstances where it is diagnostically appropriate, even if a more powerful machine would give a higher quality image. The resolution of the Hallmarq scanner is typically 0.8mm x 0.8mm in-plane for most scans which allows for the visualisation of fine structures, and many horse owners prefer the lower cost and the ability to scan the horse without general anaesthesia. However if an even higher resolution image should be required one can be obtained by increasing the time taken to collect the signal, under general anaesthesia if the veterinarian should consider it necessary.


Low field scanners, including the Hallmarq system, require continuous air conditioning with close temperature control to maintain a stable environment. High field scanners require no more than normal office air conditioning, though other special room design features are required for safe operation in the presence of liquid helium.

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What is the difference between standing and anesthetised ('down') scanners ?

All high field scanners require anaesthesia with its associated risks of anaesthetic death1. There are two types of low field scanner, one uses a large magnet and also requires anaesthesia, the other (the Hallmarq system) uses a small magnet which fits around the leg and is normally used with the horse standing and sedated but not anesthetised.

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What happens with motion?

In a high field scanner the heartbeat and breathing of the horse can produce motion of the leg in the scanner, which is normally reduced with padding and careful positioning. In the Hallmarq scanner the standing horse is supported by a chest rail, but it may sway during the scan which particularly affects parts of the leg above the foot. Special software corrects the effect of the motion to produce diagnostic scans.

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Why not use other low field scanners?

    The Hallmarq scanner is the only MRI system that can image the standing sedated horse without any requirement for general anaesthesia. It is also convenient for scanning the anaesthetised horse, as the magnet rotates in seconds with no special tools, and moves under electrical motor control making it easy to position the horse into approximately the right location and then move the scanner to position the magnet in exactly the right place.

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Can Hallmarq scanners do all types of sequence ?

Yes - the Hallmarq scanner uses the same MRI electronics as some of the most advanced research MRI scanners in the world and can run the complete range of MR pulse sequences. However the best sequences to run at low field and with the standing horse may not the the same as those used at high field. In particular the proton density (PD) weighted sequence is in common use at high field, where it is obtained in a combined sequence with the T2 weighted fast spin echo for only a small increase in imaging time. In the Hallmarq standing MRI system the same sequence is possible and has been tested, but we choose not to use it because even a modest increase in scan time increases the chance that the horse will move and spoil both scans. It is more effective to collect two separate, quicker images. When a separate scan is to be collected we recommend instead using a T1 weighted sequence:

  • the T1 weighted sequence is faster
  • as T1 is shorter at low field the contrast of a low field T1 weighted image is similar to a high field proton density image
  • because T1 varies more between normal and abnormal tissue at low field than at high field, the T1 weighted image gives more clinically useful information than the proton density image


A proton density image is included with the standard Hallmarq sequences and is available for any user to include in their scanning protocol.

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Can Hallmarq scanners switch phase encode and frequency encode directions ?

    Yes.

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Are all MRI suppliers the same?

The major medical electronic companies such as GE, Siemens, Toshiba, Hitachi and Philips, and smaller specialist MRI companies such as Hallmarq, maintain a strong development and engineering team and strong local support, though for the medical suppliers this is generally focused on systems for human medicine. The Hallmarq team have extensive MRI experience over a very wide range of field strengths and applications. For example the motion correction software developed exclusively by the Hallmarq team of physicists and engineers has been based on techniques used for human cardiac and brain MRI, and uses highly sophisticated algorithms which are not likely to be reproduced by any other MR company.


MR distribution companies and used equipment dealers have no internal engineering team and sell products made by other manufacturers (the Vet-MR and Vet-MR Grande are made in Italy by Esaote). They vary in the degree to which they can provide support and optimisation for veterinary applications. Hallmarq is entirely focused on the veterinary market and understands how vets work.

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How easy is it to use the scanner ?

Hallmarq software is designed to be easy to use by providing the operator with a menu-driven selection of specially optimised scans. Hallmarq organises training which allows the scanner to be operated by any capable veterinarian or veterinary technician, though for safety reasons only trained operators are permitted to run scans.

Hallmarq also provides training in image reading and interpretation, and supports research, user meeting, conferences and seminars where users can continue to develop their skills in the use of MRI for diagnosis.

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How safe is it to operate the scanner ?

There are no known biological hazards of static magnetic fields, and MRI does not use any ionising radiation nor toxic chemicals. There are international safety standards for MRI operators relating to the RF and gradient fields generated during the imaging process, but the low field Hallmarq system falls well within all relevant safety standards. The most significant safety concerns are from magnetic metal objects (eg tools, horse shoes) and the normal hazards of working around horses.

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How safe is MRI for the horse ?

Hallmarq equine MRI systems have been used for over 17,000 standing sedated horse examinations on 44 sites in 11 countries. During this time there have been no fatalities. Had general anesthesia been necessary over 170 of these would have died as a result of the normal mortality rate of approximately 1% following general anesthesia1. Hallmarq firmly believes that the standing MRI product has revolutionised the diagnosis and treatment of lameness by making MRI safe, affordable, and widely accessible to veterinarians and horse owners and has made a significant contribution to equine health and welfare.

1Johnston G.M., Taylor P.M., Holmes M.A. and Wood J.L.N. (1995) Confidential enquiry of peri-operative equine fatalities (CEPEF): preliminary results. Equine vet J 27 193-200

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