Home // Owners & trainers // F.A.Q
 

Owner FAQ

 

 

 

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 approximately 1% mortality rate following general anesthesia. 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. 

return to top

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.
  • 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

return to top


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 anesthesia. 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 anesthesia 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.

return to top

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

    All high field scanners require anaesthesia. 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.

return to top

Doesn’t the horse move?
 

    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.

return to top

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.

 

What are the most common diagnoses of foot pain?

 Lameness is notoriously difficult to diagnose properly.  The usual procedure involves a series of "trial and error" steps using nerve blocks, x-rays and other methods for diagnosis followed by treatment, waiting to see if the treatment is effective and if not, going round the loop again.  While each individual step in this process may cost a few hundred pounds, because each time round the loop only has a low (about 30%) chance of getting a clear diagnosis the total cost soon adds up.  During this process also the owner has other costs (farrier, livery, food, etc) and there is a risk that the problem may get worse, so MRI does on average work out as a economic option for all but the most straightforward cases.

The most common MRI diagnoses in the foot are:


- Navicular bone degeneration
- DDFT tendonitis
- Collateral ligament desmitis
- Traumatic arthritis
- Phalangeal bone bruises
- Navicular collateral desmitis
- Fractures

To explain ...
- Navicular bone degeneration covers a multitude of different diagnoses.  There is still some discussion as to the exact terminology but they fall into three major categories:
-- "Oedema".  This shows up in the MRI as black on T1 images and white on STIR images, and probably relates to an active disorder (like a bruise)
-- "Sclerosis".  This shows up as black in all MR images and probably relates to increased mineralisation where the centre of the bone (which normally contains spongy material that generates blood cells) becomes more mineralised, harder and more brittle
-- "Cyst-like lesions" which are localised regions where the bone structure breaks down

- The DDFT (deep digital flexor tendon) runs down the back of the foot and transfers the lifting force to the foot as the horse moves.  Damage to this again usually falls into one of three categories:
-- "Fibrillation" where the surface of the tendon is damaged as it rubs up against the back of the navicular bone
-- "Tears" where the long fibres of the tendon mostly remain intact but are separated from each other
-- "Core lesions" where the central fibres of the tendon die

In severe cases the damaged tendon may stick to the surrounding structures (adhesions).  This can give rise to the situation where if the horse is rested the lameness appears to get better, but as soon as work is started again the stuck structures tear apart and the foot becomes painful again.

- The collateral ligments of the DIP joint (distal interphalangel joint = coffin joint) join P2 (the short pastern bone) to P3 (the coffin bone) - one ligament either side of the foot just below the level of the coronary band.  They can be strained by an awkward step or twisting of the foot.  Inflammation of the ligaments (desmitis) is being increasingly recognised but cannot be diagnosed without MRI - except for some cases where the strain is right at the top of the ligament and can be detected using ultrasound, but usually damage is further down underneath the hoof wall where it cannot be reached by the ultrasound beam.

- Traumatic arthritis  is simply where the joint, and particularly the cartilage on the bone surfaces, has been damaged by the strain of use

- Bone bruises can occur particularly in P2 (the short pastern bone) and may (though there is no proof of this) be caused by eg the front of the horse's foot hitting a jumping pole.  They show up clearly on MRI but cannot be diagnoses by any other method.  They are very painful but cannot be detected by x-ray in the early stages.  They can go on to recover fully, or they can get worse and turn into other forms of bone change (which can be detected by x-ray)

- The navicular bone is held in place by two ligaments - the upper is the navicular collateral or navicular suspensory ligament, and the lower is the impar ligament.  Damage to either of these can be detected by MRI.  Damage to either ligament is related to pain in the navicular area.

- Fractures of P3 (the coffin bone) or even the navicular bone can be very difficult to detect by x-ray and are sometimes seen on the MRI

All of these diagnoses appear as pain in the foot and thus lameness in the horse, which is blocked by injection of local anaesthetic to the heel area (palmar digital nerve) and/or the coffin joint.  They can all therefore be described by the catch-all term "navicular disease" or "navicular syndrome" which should be heard less and less these days as MRI leads to a more specific diagnosis.

 

How are the causes of lameness narrowed down;  what is the role of 'nerve blocking?'

 

Normally a vet will be able to tell which leg a horse is lame on (or at least which leg is worst) by watching how it moves.  They then narrow down where the problem is by injecting small amounts of local anaesthetic (like the dentist !) to numb specific regions.  The anaesthetic is applied just where certain nerves are known to go.  Since nerve signals go along the leg and once blocked numb everything "downstream", the vet will start at the furthest point and work back toward the body.  The most common starting point is the nerve that goes to the heel area of the foot, the palmar (or plantar) digital nerve (often abbreviated to PD).  The back half of the lower leg is called palmar for the front legs, and plantar for the back legs.  There are branches of this nerve on both the inside (medial) and outside (lateral) sides.  Sometimes vets will try to numb just one side or the other, but as the nerves branch out further down this is not a very reliable way to localise the source of pain to one side or the other.

If the horse moves more freely when this area is blocked the vet knows that the pain is coming from somewhere in the foot, most likely the back half.  Traditionally pain in this area has been believed to come from the navicular bone or the surrounding tissues so the lameness has been labelled first "navicular disease" and then more recently, as it has been recognised that it is not always actually the navicular bone causing the problem, "navicular syndrome".  Today with MRI showing that many different structures can be involved and other research showing that the PD nerve block will numb most of the foot, even the label "navicular syndrome" is becoming less used.

If the vet thinks the pain is in the foot they might try to narrow the location down further with one or two other blocks.  Since both of these are "downstream" of the PD block the horse has to be left some time to recover before either of these are tried.

- Injection of local anaesthetic into the coffin joint will help determine if the problem is in the joint itself or the surrounding tissue.  However as research has again shown that anaesthetic injected into the joint can numb tissues in a large region, so the block is not as specific as the traditional textbooks will have you believe.  Because this injection is right into the joint fliud there is a risk of infection, so it is not normally used as the first test and strict aseptic procedures are needed.

- Injection into the navicular bursa will detect if pain is coming from this area.  The navicular bursa is like a fluid filled plastic bag squished between the back of the navicular bone and the DDFT and extending above the navicular bone, which helps the tendon move smoothly over the bone.  It can become inflamed, or damaged, if the tendon or bone are diseased.  However the navicular bursa is very difficult to inject accurately, with the needle having to go deeply into the heel, so again vets will not use this as their first test and many avoid it altogether.

If the PD block does not make the horse go sound then the vet will work back up the leg blocking nerves closer to the body.  The next block up is the "abaxial sesamoid" which is used to detect pain in the whole foot, pastern, and to some degree the fetlock.  Then even further up you get the "4 point" or, in the hind limbs "6 point" where there are so many nerve branches many different injections are needed to get them all.  Then there are a whole range of different blocks aimed at different structures.

Very often when a horse is blocked in one foot it will then go lame on the other side, as the pain is actually in both feet but one is worse in one than the other.  So then the other foot has to be blocked as well. This makes the whole process very complicated and time consuming.  And given that the only way to assess pain is to watch the horse move, and it can be really difficult even to see which leg is the problem let alone whether it has got better, stayed the same, or switched sides, the whole process of nerve blocks can take a long time and not be very reliable.  However it is a critical first step to narrow down the painful area as MRI can only image a small region (eg a foot, or a fetlock).  It is just not sensible to MRI the whole leg.

 

 


 

return to top