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
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
What is the difference between standing and anesthetised
('down') scanners ?
All high field scanners require anesthesia. There are two types
of low field scanner, one uses a large magnet and also requires
anesthesia, 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.
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
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 anesthesia. It is
also convenient for scanning the anesthetised 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
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
Can Hallmarq scanners switch phase encode and frequency encode
directions ?
Yes, though the user interface to this is being improved
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 MR 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.
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
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.
How safe is MRI for the horse ?
Hallmarq equine MRI systems have been used for almost 10000
standing sedated horse examinations on 34 sites in 10 countries. During this
time there have been no fatalities. Had general anesthesia been
necessary over 100 of these would have died as a result of the
normal approximately 1-3% 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.
Hallmarq Veterinary Imaging Ltd
Unit 5, Bridge Park
Guildford, Surrey, GU4 7BF
UK
Hallmarq Veterinary Imaging
Inc
6 Eastern Road
Acton, MA 01720-5801
USA
