Download How to Understand Differences Between High- and

HOW-TO SESSION:
FIELD IMAGING
How to Understand Differences Between High- and
Low-Field Standing Magnetic Resonance Imaging
Katherine S. Garrett, DVM, DACVS
Author’s address: Rood and Riddle Equine Hospital, PO Box 12070, Lexington, KY 40580-2070;
e-mail: [email protected]. © 2015 AAEP.
1.
Introduction
The availability of magnetic resonance imaging
(MRI) has increased greatly over the past 10 years.
This has led to tremendous advances in our ability
to make more accurate and/or specific diagnoses in a
variety of body regions. For the practitioner, it is
important to understand different types of MRI
scanners to appropriately refer cases.
Multiple types of MRI scanners are currently on
the market—all with advantages and disadvantages. High-field (1.5 T) scanners were the first to
come into clinical use for horses. High-field scanners require general anesthesia and specialized
nonferrous anesthetic equipment. However, they
allow for higher-resolution images, thinner slices,
larger fields of view, and faster scan times. Lowfield (typically 0.3 T) scanners are available in both
recumbent and standing designs. The recumbent
design requires general anesthesia, whereas the
standing design can be used in a standing, sedated
horse or can be rotated for use on an anesthetized
horse. Low-field scanners generally have lowerresolution images, thicker slices, smaller fields of
view, and longer scan times. The standing design
is also prone to artifacts from motion that worsen as
the scan region moves proximally from the foot.
It is important to keep all of these factors in mind
NOTES
416
2015 Ⲑ Vol. 61 Ⲑ AAEP PROCEEDINGS
when choosing where to refer a case, as some cases
are better served with one design over another.
Image quality is generally superior using highfield MRI. Increased magnet strength permits the
use of thinner slices and higher resolution compared
with low-field MRI, which aids in identifying cartilage pathology.1 Oftentimes, short tau inversion
recovery sequences can be extremely useful in diagnosing bony lesions, but these sequences are very
sensitive to motion and may be nondiagnostic in
some standing horses. Other work has shown that
even in cadaver limbs, high-field images are superior at detecting pathology compared with low-field
images.2
Because MRI scans of a unilateral region typically
take approximately 45 to 60 minutes, it is very important to attempt to localize the area of lameness
as specifically as possible. This generally involves
perineural and intra-articular anesthesia. Unfortunately, especially in the case of perineural anesthesia, we have learned that anesthetic agents can
migrate over a great distance, leading to a lack of
specificity. For example, a palmar digital nerve
block can migrate as far proximally as the fetlock
region.3 For this reason, when MRI operators scan
a horse that has blocked to a palmar digital nerve
block, they always include at least one sequence that
HOW-TO SESSION:
includes the pastern and the fetlock to the level of
the proximal sesamoid bones. In some cases, the
foot is relatively normal, but the primary lesion involves the pastern region. The field of view and coil
design of a high-field scanner permit imaging the
pastern region without repositioning the horse or
the coil or performing additional localizer sequences,
making inclusion of this region convenient for the
operator. Unfortunately, the standing MRI design
requires that the coil and magnet be repositioned
with new localizer sequences to obtain images of the
pastern region, and these tasks can be time-consuming. The superior image quality of the high-field
images also allows the interpreter of the images to
be more confident of what the primary source of
lameness may be.
It has been shown that significant diffusion of
contrast material occurs after instillation at the
base of the proximal sesamoid bones.4 In the author’s opinion, if a horse has blocked to an abaxial
sesamoid nerve block without a peritoneal dialysis
performed previously, the lesion could be located in
the foot, pastern, fetlock, or distal metacarpal region. This is a large area to image in a practical
sense, so in this case the author encourages reblocking the horse more specifically. If repeating the
lameness examination with different blocks is not
possible, high-field scanners can accommodate imaging the foot, pastern, and fetlock regions with
much less difficulty than in a standing design, where
multiple repositionings of equipment are necessary.
Additionally, in a standing design, horse compliance
may become an issue with a three-region scan, especially if bilateral images are obtained. It has also
been shown that anesthesia of the proximal metacarpal/tarsal region can anesthetize the distal carpal and tarsal regions and vice versa.5–7 For that
reason, when operators scan a carpus or tarsus, they
include the proximal metacarpal/metatarsal region;
when scanning a proximal metacarpal/tarsal region,
they include the carpometacarpal and middle carpal
joints or the tarsometatarsal and distal intertarsal
joints. Proximal regions of the limb are very prone
to motion in a standing design, and the field of view
often does not permit including both regions in the
same sequence without repositioning.
We have found high-field MRI to be extremely
useful in cases of orthopedic sepsis that are not
FIELD IMAGING
responding to treatment as would be expected.8
Areas of osteomyelitis, physitis, or bony necrosis are
readily identified. In smaller foals, areas as proximal as the pelvis and axial spine can be imaged
using recumbent designs, which is helpful because
radiography and ultrasonography of these areas are
more challenging. Standing MRI is generally not
feasible in these cases, because it may be too painful
for the horse to bear weight and remain motionless
for the time required to obtain the images.
Although high-field MRI is not always an option
or indicated for every case, there are many situations in which it provides valuable or vital information beyond what can be obtained from standing
low-field MRI.
Acknowledgments
Declaration of Ethics
The Author declares that she has adhered to the Principles of Veterinary Medical Ethics of the AVMA.
Conflict of Interest
The Author declares no conflicts of interest.
References
1. Werpy NM, Ho CP, Pease AP, et al. The effect of sequence
selection and field strength on detection of osteochondral defects in the metacarpophalangeal joint. Vet Radiol Ultrasound 2011;52(2):154 –160.
2. Murray RC, Mair TS, Sherlock CE, et al. Comparison of
high-field and low-field magnetic resonance images of cadaver
limbs of horses. Vet Rec 2009;165(10):281–288.
3. Seabaugh KA, Selberg KT, Valdés-Martínez A, et al. Assessment of the tissue diffusion of anesthetic agent following administration of a low palmar nerve block in horses. J Am Vet
Med Assoc 2011;239(10):1334 –1340.
4. Nagy A, Bodo G, Dyson SJ, et al. Diffusion of contrast medium after perineural injection of the palmar nerves: an in
vivo and in vitro study. Equine Vet J 2009;41(4):379 –383.
5. Contino EK, King MR, Valdez-Martinez A, et al. In vivo
diffusion characteristics following perineural injection of the
deep branch of the lateral plantar nerve with mepivacaine or
iohexol in horses. Equine Vet J 2014;47(2):230 –234.
6. Dyson SJ, Romero JM. An investigation of injection techniques for local analgesia of the equine distal tarsus and proximal metatarsus. Equine Vet J 1993;25(1):30 –35.
7. Nagy A, Bodo G, Dyson SJ. Diffusion of contrast medium after
four different techniques for analgesia of the proximal metacarpal region: an in vivo and in vitro study. Equine Vet J
2012;44(6):668 – 673.
8. Easley JT, Brokken MT, Zubrod CJ, et al. Magnetic resonance imaging findings in horses with septic arthritis. Vet
Radiol Ultrasound 2011;52(4):402– 408.
AAEP PROCEEDINGS Ⲑ Vol. 61 Ⲑ 2015
417