Journal of Equine Veterinary Science 32 (2012) 655-666

Journal of Equine Veterinary Science

journal homepage: www.j -evs.com

Abstracts

Introduction to Equine MRI

Alexia McKnight DVM, DACVR

Take-home message:MRI is a valuable diagnostic tool, particularly when thesource of a lameness is not apparent on radiography orultrasonography. It is helpful even with known lesions asMRI provides more detailed information, which allowsa more thorough diagnosis, focused treatment, and accu-rate prognosis. Furthermore, MRI is very useful in presur-gical planning and in the prevention of disease progressionwhen early lesions are found. However, it is important tounderstand the various factors that can affect imagequality, so that the study is of most clinical use, and artifactsand lesions are correctly identified and interpreted.Introduction: Magnetic resonance imaging (MRI) is anextremely valuable diagnostic imaging modality thatprovides multi-plane and multi-slice, cross-sectionalimages. It has been considered the imaging gold standardfor neurologic and musculoskeletal problems in humansfor several decades. In the horse, MRI has most commonlybeen used for lameness cases, particularly involving thefoot, where other diagnostic imaging modalities have notadequately revealed the cause of lameness. Evaluation ofthe head is also becoming more common in the horse fordiseases of the brain and other areas, such as the paranasalsinuses.MRI vs. Ultrasound and CT. Compared with the other cross-sectional imaging modalitiesdultrasound and CT(computed tomography)dMRI has advantages and disad-vantages. Each is able to provide multi-plane and multi-slice diagnostic information, such as multiple axial slicesthrough the distal limb.Ultrasound is extensively used in equine practice and offerssuperior spatial resolution; it is comparatively inexpensive,readily available, and easily performed on a standing horsein real time. However, ultrasound is unable to penetratebony surfaces, it has very poor soft tissue contrast, and it isusually limited to the evaluation of relatively superficialsoft tissue structures.CT is often considered superior for bony contrast, whereasMRI provides superior soft tissue contrast. Both CT and MRIare very expensive modalities that are not nearly as avail-able in equine practice, although the clinical benefits are

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rapidly being realized and will be discussed throughoutthese lectures.Factors in Optimal Image Quality. MRI scanners are availablein many different designs, with various quality parameters.Important hardware parameters to understand includemagnetic field strength, field orientation, open- and closed-bore systems, gradient strength and speed, homogeneousfield of view, and available surface coils. Each of these affectscan parameters and image quality such as scan speed,signal-to-noise ratio, spatial and contrast resolution,susceptibility to artifacts, and available pulse sequences, aswell as which areas of the horse can be scanned, and theneed for general anesthesia or standing sedation.With few exceptions, the MRI scanner also needs to beenclosed within a radiofrequency (rf) shielded cage. If notfiltered, extraneous rf noise will be received during dataacquisition and will degrade image quality.Magnet strength and SNR. The strength of the magnet of anMRI scanner is measured in Tesla (T). Most high-fieldscanners used in the horse are usually 1–1.5 T. Commonlyused low-field scanners are usually 0.2–0.3T. Higher signal-to-noise ratio (SNR) is often considered superior withhigher field strength MRI systems. However, there areadditional factors that affect the SNR.Increased SNR is highly preferable, as it can lead to fasterscan times and/or increased spatial resolution. However,most high-field systems are markedly more expensive topurchase and to maintain, and they are usually consid-ered prohibitive for most equine institutions. Low-fieldsystems have been more popular for dedicated equineimaging.In addition to magnetic field strength of the MRI systemitself, numerous other hardware, software, and imagingparameters have a significant effect on the SNR of a givenscan. One of the most notable is the surface coil, placedaround the body part to receive the signal from the tissues.Specially designed surface coils for equine imaging willsignificantly increase the SNR compared with off-the-shelfhuman coils that may not be optimal for the area scanned.For example, the different sizes and shapes of the distallimbs in humans and horses translate to different optimalcoil designs. Not all vendors, especially those representinghigh-field systems, are willing to dedicate resources forequine coil development as readily as those with dedicatedequine systems.

Abstracts / Journal of Equine Veterinary Science 32 (2012) 655-666656

Magnet homogeneity and field of view. The homogeneity ofthe magnet is a very important quality factor, and it alsodictates the imaging field of view. If the homogeneity ofa system is poor, then image quality will be poor. High-fieldscanners designed for whole-body imaging in humansoften have excellent homogeneity, about a 40-cm homo-geneous magnetic field. In comparison, a standing equineMRI system may have only a 12- or 14-cm homogeneousfield of view. It is not possible to scan outside this field, andtoward the edges there will be distortion of the tissue.Artifacts caused by a nonhomogeneous field should berecognized and not misinterpreted as pathology. For largeregions such as the head, a large field of view may be quitehelpful to visualize more necessary anatomical structuresin one scan.Imaging gradients. Another critical component in under-standing the basics of MRI is the imaging gradients. Usedduring certain times of the image acquisition process, thegradients alter the main magnetic field slightly so thata unique magnetic environment can be created for eachvoxel (volumetric pixel) of tissue scanned. It is this uniqueenvironment that ultimatelyallows the signal received tobedecoded and spatiallymapped so that the grayscale value ofthe pixel in the image appropriately corresponds to its voxelof tissue. Scan speed and image sharpness, among otherparameters, are related to the strength and amplitude ofthese gradients.Closed- or open-bore magnets. Magnet designs are eitherclosed bore or open bore. Closed-bore magnets are cylin-drical, whereas open-bore magnets have two opposingpoles surrounding an open space. Typically, closed-boremagnets are superconducting magnets at high field; mostopen-bore magnets are permanent magnets at low field.The bore orientation of the high-field magnets requiresgeneral anesthesia for positioning of the distal limbs andhead. Low-field systems may or may not require generalanesthesia for positioning. One low-field magnet designedfor the horse allows for a standing exam. It is very impor-tant to recognize the difference, however, in an exam ob-tained by the different systems. Compromises exist,primarily with regard to image quality and diagnostic valuewhen shortcuts are made.Pulse sequences. In addition to the hardware describedabove, one of the most important software components ofan MRI examination are the pulse sequences used for dataacquisition. A typical exam includes several different pulsesequences for appropriate tissue characterization, andmultiple scan planes for adequate evaluation of normal andabnormal anatomy. As a simplification, very common pulsesequences include spin echo or gradient echo sequencesusing T1, PD, or T2 weighting (see below), as well as either2D or 3D acquisitions.There are also special methods to improve tissue contrast ina pulse sequence. For instance, a STIR sequence is used toimprove the ability to see fluid accumulation caused bytrauma, inflammation, infection, neoplasia, etc., andparticularly fluid surrounded by fatty tissue, as in theintramedullary cavity of a bone. In the equine foot, forexample, the following sequences may be acquired: spinecho T1 sagittal, spin echo PD axial, spin echo T2 axialoblique, STIR sagittal, STIR axial, and gradient echo 3D T1dorsal. Each of these sequences has advantages and

disadvantages, and their combination increases the yield ofa diagnostic study.Scan time. Scan time is highly variable, and is dependent onmany different factors. For example, it is possible to obtainan image every second or less in MRI “fluoro” mode. Thesevery fast sequences are excellent for necessary positioninginformation, but they are not appropriate for most diag-nostic purposes. At the other extreme, a single pulsesequence may be acquiring data for hours; however, thiswould only be performed in a research environment orwithpost-mortem specimens, as it is not clinically practical. Themajority of pulse sequences used in clinical practice rangefrom approximately 2 mins to 10 mins.The number of pulse sequences used for a study alsodefines the total MRI scan time. In general, signal-to-noiseratio and/or spatial resolution within one pulse sequencecan be improved by scanning longer (with limitations), sothe actual scan time for a given system is defined by theoperator and determined by choosing numerous differentscan parameters. Comparing equal scan parameters withcomparable coils, a high-field MRI system would acquiredata much faster.When general anesthesia is not used in low-field systems,motion is an additional, and often very significant, problem.Otherwise preferable pulse sequences are not practicalbecause the longer scan time would more likely reveal non-diagnosticmotionartifacts. To avoidmotion, pulse sequencesare chosen that are less sensitive to motion or optimized toscan very quickly. This has an effect on the spatial andcontrast resolution; and depending on the type of lesion, thediagnostic information may or may not be compromised.Safety Precautions. Safety precautions around MRI systemsneed to be taken seriously. Severe and even fatal accidentsoccur with carelessness. The danger is predominantly inthe attraction forces of the strong magnetic field withferrous objects. Large ferrous objects, such as an oxygentank, dolly, scissor lifts, floor buffers, etc., pose the greatestdanger and should NEVER enter the rf shielded cage of theMRI suite.Smaller objects, such as hoof testers, wrench, scalpel,stethoscope, etc., will also be pulled into the magnet, and ifa person or animal is in the path of that object, injury mayresult. High-field systems are significantlymore dangerous,as the attraction forces are much greater, but MRI safety isalways important with any system. Erasing a credit cardand affecting the time on a quartz watch are other prob-lems when these objects get too close to the scanner.Cost-Effectiveness of MRI for a Practice. The cost-effective-ness of an MRI system depends strongly on which MRIsystem a practice decides to purchase. A typical break-evenpoint for a dedicated, equine low-field MRI system undergeneral anesthesia can range from as few as 2 or 3 horsesa month, to 6 or 7 horses a month with a more expensivelow-field MRI system. If a practice opts for a low-field,dedicated equine system and is capable of a larger caseload,the practice can profit. Some equine practices havepersonnel and clientele to support a small-animal imagingservice as well, and can more readily profit. The initial andongoing maintenance costs of a high-field MRI system areoften considered prohibitive for a dedicated equine prac-tice. Most practices accept a loss using these systems, evenwith extremely high equine caseloads.

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Basics of MRI Interpretation: A very brief understandingof how MRI works will be presented, as well as explana-tions of common MRI terminology, such as signal-to-noiseratio, imaging field of view, acquisition matrix, voxel size,and spatial resolution. This information provides a goodfoundation for MRI interpretation and artifact recognition.Principles. As a compass needle aligns itself with the earth’smagnetic field, tissue protons align themselves with themagnetic field inside an MRI scanner. While in this energystate, the protons easily absorb radiofrequency energy ata specific frequency. The energy is then immediately releasedand received by an antenna (the surrounding surface coil).Information about the tissue’s anatomy is in this receivedsignal, which is mathematically processed to create a seriesof images.Hydrogen protons (H+) have physical properties such asmass and volume, and in the presence of a magnetic fieldand transmitted rf energy they also have magnetic prop-erties such as T1 and T2 relaxation times. In addition,tissues have proton density (PD) properties. These are allvery important properties that allow the differentiation oftissue types, such as the ability to distinguish joint capsulefrom synovial fluid, white matter from grey matter, carti-lage from subchondral bone, etc.Diagnostic Value of Differently Weighted Sequences. Everytissue has T1, T2, and PD properties, but a particular pulsesequence will more heavily weigh one property overanother. Some tissue, such as synovial fluid, has a very longT1 time, and therefore is darker on a T1 weighted sequencethan fat, which is bright on a T1 weighted scan. Dis-tinguishing among tissue types is the main purpose ofa pulse sequence, and it is primarily done by manipulatingthe repetition time (TR) and echo time (TE) of that pulsesequence. For example:

T1-weighted sequencedshort TE, short TR (water is dark,fat is bright)T2-weighted sequencedlong TE, long TR (water is bright,fat is intermediate)PD-weighted sequencedshort TE, long TR (water isintermediate, fat is bright)

Solid material, such as cortical bone, does not have enoughfree H+ to generate a signal at all, so it is black on allsequences. Most lesions, whether traumatic, inflammatory,infectious, or neoplastic, usually cause an increase in fluid(and thus water protons) and are therefore more conspic-uous on sequences such as T2 or STIR.STIR sequences. A short T1 inversion recovery (STIR)sequence is another very common pulse sequence oftenused inmusculoskeletal MRI.With this sequence, the signalfrom fat is nulled, which significantly improves the contrastand identity of certain lesions. A bone bruise, stress frac-ture, or osteomyelitis for instance, may be best seen witha STIR sequence.Other Common Terms. Other common terms related todifferent pulse sequences and scan parameters include thefollowing:

signal-to-noise ratio (SNR)dA quality parameter that isthe ratio of tissue signal to background electronic noise.Images with poor SNR are noisy or “grainy.”

spin echodA method of acquiring image data thatcompensates for small magnetic field inhomogeneities.It provides more reliable soft tissue evaluation and lesssusceptibility to artifacts compared with gradient echosequences.gradient echodA method of acquiring image data that ishighly susceptible to various artifacts, but therefore isusually better for bony detail and detection of bloodproducts. It can be faster that spin echo sequences, but itmay be unreliable for evaluation of some soft tissuelesions.flip angledAn angle used in gradient echo pulsesequences that affects tissue contrast and imaging time.2DdA two-dimensional sequence that acquiresmultiple slices, each with a particular thickness; likeaccumulating slices of a bread loaf, one slice at a time.3DdA three-dimensional sequence that acquiresa volume of data at once, which is then reconstructed intoslices; like accumulating an entire loaf of bread at once.This technique often allows thinner slices to be obtainedcompared with 2D sequences. With 3D sequences, datacan be reconstructed into other planes; and if obtainedisotropically, the spatial resolution of the reconstructedimage approximates that of the original acquisition.fat suppressiondA technique used to null the signalfrom fat in order to improve tissue contrast and lesionenhancement. A STIR sequence is an example of fatsuppression. At high field, fat saturation can be per-formed which is a different method of separating watersignal from fat signal.saturation bandsdDifferent from fat suppression, satu-ration bands may be applied in some sequences to nullall signal originating from a specific anatomic location.This approach is helpful for decreasing or preventingartifacts, such as from blood flow.slice thicknessdThe total tissue thickness used duringdata acquisition or reconstruction that is represented inan image. Thicker slices significantly increase SNR, butpartial volume artifacts become a problem when slicesare too thick.imaging field of view (FOV)dThe total x and y dimen-sions of the acquired image set by the scanner operator;i.e. the size of the raw image. The field of view is usuallylarger than the size of the body part being scanned.Larger body parts require larger fields of view. With thesame acquisition matrix, a larger field of view hasa greater signal-to-noise ratio.acquisition matrixdThe number of columns and rows inthe acquired raw image. The larger the matrix, thehigher the spatial resolution. However, with a given FOV,larger matrices also have lower signal-to-noise ratios.voxeldThe tiny portion of tissue represented in eachpixel of the image, defined by image FOV/acquisitionmatrix in the x and y dimensions. For example, in a 20- x20-cm FOV with a 256 x 208 acquisition matrix, thetissue voxel size is 0.78 mm x 0.96 mm.pixel – The smallest unit within a digital image. Eachpixel has two dimensions: a width and a length. Thepixel’s signal is from the average of the three-dimen-sional voxel it represents.spatial resolutiondRelated to voxel size; small voxelshave high spatial resolution and large voxels have low

Figure 1. Sagittal STIR image of the foot. The green arrows show (fromproximal to distal): digital sheath effusion, a DDFT tear, mild navicular bonestress injury, and moderate P3 stress injury at the insertion of the distalimpar ligament.

Abstracts / Journal of Equine Veterinary Science 32 (2012) 655-666658

spatial resolution. Higher spatial-resolution scansusually take longer to acquire. They may also have lowerSNR if other scan parameters, such as number ofacquisitions, are not increased to compensate.frequency and phase encodingdThe x and y directions ofthe acquisition matrix. The x (or frequency) direction,when increased, does not affect scan time; its effect onlyrelates to spatial resolution and SNR. The y (or phaseencoding) direction increases scan time when increased,aswell as affecting spatial resolution and SNR. Allmotionartifacts will be seen in the phase encoding (y) direction.number of acquisitionsdThe number of times that data isacquired and averaged together during a pulsesequence. The more acquisitions, the higher the SNR,but at the expense of scan time.

Common MRI Artifacts. It is very important to understandcommonMRI artifacts, as it helps with differentiating a truelesion from an artifact. Described below are several verycommon MRI artifacts:

partial volume artifactdOccurs when the signal withina pixel misrepresents the anatomy (or pathology) of itscorresponding voxel. This occurs more commonly whenthe slice thickness is large relative to a specific tissuetype. It is especially problematic at curved surfaces orwhen the slice orientation is not perpendicular to anedge. The artifact may appear as an abnormal signalwithin otherwise normal tissue or as normal signal inabnormal tissue.flow artifactsdMRI is extremely sensitive to motion;when blood flows through a slice, that motion causesflow artifacts, or “ghosting,” across the image. If notrecognized, these artifacts may be superimposed overnormal tissue and mimic a lesion, such as a core lesionin the deep digital flexor tendon. Flow artifacts willalways course across the phase encoding direction ofthe image.magic angle artifactdThis artifact causes an abnormallyhyperintense signal within a normal tendon or liga-ment. It occurs when the fibers are oriented close to the“magic” 55 degrees relative to themagnetic field. Properpositioning helps eliminate or reduce this artifact wherepossible, but recognizing its presence and under-standing other changes associated with a true tendon-itis or desmitis help prevent misdiagnosis.motion artifactdGross body motion, such as occurs ina standing horse, or excessive respiratory or cardiacmotion that travels through an unsupported limbrenders an exam nondiagnostic.magnetic susceptibility artifactdIn equine musculoskel-etal imaging, this artifact is often due to nail fragmentsin the hoof wall or debris within nail tracts. Theseparamagnetic materials disrupt the local magnetic field,ultimately causing black voids and signal distortions inthe image. Prescreening radiographs must be obtainedprior to the MRI exam to check for any nail fragments,which must then be removed. As radiopaque debriswithin nail tracts may also have paramagnetic proper-ties, cleaning the feet and these tracts is very importantas well. Magnetic susceptibility artifacts are worse withgradient echo sequences and with high-field systems.

nonhomogeneous magnetic field artifactsdWhen themagnetic field is not uniformly homogeneous, tissuedistortion occurs. This is particularly prominent at thelimits of a scanner’s homogeneous field and nearmetallic objects (e.g. nail fragments), and it is moreproblematic in systems with smaller fields of view.Incomplete fat suppressionmay be an additional artifactfrom a nonhomogeneous field and should not beconfused with bone pathology.zipper artifactsdA leak in the rf cage or rf noise withinthe cage (e.g. from anesthesia or monitoring systemsthat are not MRI compatible) may cause one or multi-ples streaks across the phase encoding direction.

The Foot: The foot is one of the most common areas of thehorse for which MRI may be used to reveal the cause of anobscure lameness. Typically, the clinical history reportsa lameness that was localized to the foot, but radiographswere considered unremarkable. And in cases in whichradiographic abnormalities were found, the clinician maybe interested in further assessing the degree of injuryand ruling out additional, complicating pathologies. Forexample, in the case of navicular disease, MRI may be usedto assess the presence and degree of associated soft tissuedamage, such as deep digital flexor tendon tears. Detailedknowledge about the amount of bony involvement,fibrocartilage loss, and associated tendonitis, desmitis,

Figure 2. Axial PD weighted image of the foot, showing a core lesion withinthe lateral lobe of the DDFT (green arrow), breaking out at the dorsal aspectof the tendon, at the level of the proximal navicular bursa.

Figure 3. Dorsal plane isotropic 3D, GE, T1 image, showing decreasedarticular cartilage signal at the proximal-lateral aspect of P3 (green arrows),which suggests cartilage erosion/loss.

Abstracts / Journal of Equine Veterinary Science 32 (2012) 655-666 659

and/or bursitis can have a strong impact on the treatmentplan.Navicular Apparatus. There are multiple structures in thefoot that are often found to be abnormal. Within thenavicular apparatus, common areas where lesions are seeninclude the following:

collateral sesamoidean ligament of the navicular bone(body and insertion)flexor cortex of the navicular boneintramedullary cavityflexor cortical fibrocartilageimpar ligament (body, origin, and insertion)deep digital flexor tendon (DDFT)navicular bursa

Some horses have more significant lesions of the flexormargin of the navicular bone, with minimal bursal or DDFTchanges. Other horses have minimal navicular bonechanges, but significant bursal, fibrocartilage, DDFT, and/orimpar ligament pathology (Figs. 1 and 2).Distal Interphalangeal Joint. It is not uncommon to haveinjuries in the medial and lateral collateral ligaments ofthe coffin (distal interphalangeal) joint. However, knowl-edge of the magic-angle artifact is particularly important atthis site, to be certain that an artifact is not misinterpretedas a lesion. This problem ismore prevalent on low-fieldMRIsystems, where the magnetic field is perpendicular tothe long axis of the limb. Proper limb positioning helpsprevent this artifact. It is also important to know that true oractive lesions are usually present on the T2 sequence, wherethe magic-angle artifact is minimal. In addition, there oftenis heterogeneity within the ligament and an increase insize with periligamentous inflammation. With morepronounced lesions, osteolysis at the ligament’s insertiononto the distal or third phalanx (P3) may also be seen.

Other Lesions. Acute trauma and stress injuries to the firstand second phalanges (P1 and P2) and to P3 can readily beseen on MRI. These lesions have an increased STIR signalwithin the affected bone, often originating at the sub-chondral margin and extending into the intramedullarycavity. Associated articular damage and other soft tissueinjuries may also be evaluated.Other injuries that are readily seen on MRI include acutesoft tissue and bony trauma, such as that caused bya puncture wound. The presence and extent of bone infec-tion can be appreciated; in fact, these changes may be seenon MRI before radiographic abnormalities have developed.Osseous cyst-like lesions and osteoarthritic changes arequite common in the proximal or distal interphalangealjoints, whether as developmental or acquired conditions.Small cyst-like lesions may not be seen on routine radio-graphs, butare readily foundwithMRI. Similarly, adecreasedarticular cartilage signal, consistent with cartilage dehydra-tion and degeneration, may be seen on MRI before peri-articular osteophytosis is seen radiographically (Fig. 3).Examining the Opposite Limb. In the case of a unilaterallameness, examining the opposite limb may reveal similar,and in some casesmore severe, lesions. That is quite helpfulin understanding which lesions are clinically insignificantor that are likely to progress to more severe lesions in thefuture. It may be possible to prevent a future lameness byidentifying MRI lesions that are likely to progress. Routineexamination of both limbs, where appropriate, is also quitehelpful for general comparative purposes and to increasethe MRI knowledge database. Some follow-up exams areequally helpful to monitor healing and return to training.Treatment plans and prognoses can be better tailored toa given injury when the extent of the lesions can be fullydelineated. Further understanding of suspected lesions is

Figure 4. T2 weighted axial image of the fetlock, just distal to the proximalsesamoid bones. The green arrow points to the grossly enlarged andheterogeneously T2 hyperintense lateral oblique distal sesamoidean liga-ment, consistent with active desmitis.

Figure 5. Sagittal STIR image of the fetlock. The green arrows show a focalhyperintense STIR lesion within the proximal subchondral region of P1,surrounded by a significant amount of intramedullary STIR signal emanatingfrom this site. This lesion corresponded with a focal demineralized area onthe 3D, GE, T1 sequence, associated with decreased articular cartilage. Thesefindings are consistent with an active erosive arthropathy involving prox-imal P1.

Figure 6. PD weighted axial image of the proximal metatarsus, showingdesmitis involving the dorsal fibers of the suspensory origin (green arrows).

Abstracts / Journal of Equine Veterinary Science 32 (2012) 655-666660

also quite useful, and helps advance the fields of equinemedicine, surgery, and diagnostic imaging. The choice ofrest, medical therapy, or surgery is often easier to makefollowing thorough MRI evaluation.Correlating Clinical and MRI Findings. Correlating the MRIfindings with the clinical findings is a very important partof the process. MRI is a sensitive modality, so multiplelesions are common. Often, the MRI findings correlate withthe clinical findings, so the diagnosis is straightforward andthe cause of the clinical lameness is identified. However,when the clinical and MRI findings do not completelymatch, and/or multiple pathologies are seen, it is veryimportant to reassess the case to be certain that the MRIfindings are not being overinterpreted or overemphasized.The Distal Limb: The fetlock and proximal suspensoryregion are other commonly imaged areas in the distal limbof the horse.Fetlock and Pastern. The most commonly injured softtissues around the fetlock and pastern include the distalbranches of the suspensory ligaments, the distal ses-amoidean ligaments (Fig. 4), and the flexor tendons andsurrounding digital sheath. Determining the underlyingcause of digital sheath effusion may be a specific reason foran MRI request. And with some known pastern injuries,MRI can help to assess the extent of scar tissue, which mayhelp with surgical planning.With the fetlock, the typical history is of a lameness thatimproves with an intra-articular or low palmar/low four-point nerve block. Within the fetlock joint, subchondrallesions are often seen on the palmar and dorsomedialcondyle regions of the third metacarpal/metatarsal bone.Acute trauma and stress injuries are common here. Mildsubchondral sclerosis is often seen and is considereda physiologic response to training, but discontinuitieswithin the subchondral plate, decreased signal within thearticular cartilage, and/or osteolytic or osteochondralfragments are considered pathologic progressions of sub-chondral bone disease and an osteoarthritic process (Fig. 5).

Figure 8. T1 weighted axial image of the head, at the level of the ethmoidturbinates. The five lower arrows indicate a very large progressive ethmoidhematoma in the caudal maxillary and sphenopalatine sinuses which isdestroying part of the ethmoid turbinates. Extension of the lesion is seen inthe dorsal left portion of the ethmoid turbinates (upper, right area of theimage).

Abstracts / Journal of Equine Veterinary Science 32 (2012) 655-666 661

Proximal Suspensory Region. The proximal suspensoryregion is readily imaged, so acute and chronic suspensorydesmitis are easily identified (Fig. 6). Some of these injuriesare very difficult to visualizewith ultrasound. In some casesof suspected suspensory desmitis, no suspensory ligamentlesion is found and instead other lesions are found that arethe likely cause of the lameness, such as bony injuries of theproximal metacarpus/metatarsus. MRI helps determine thepresence and extent of bone injuries, with or withoutsuspensory desmitis, so that an appropriate treatment plancan be made.Carpus or Tarsus. When a lameness is localized to thecarpus or tarsus, MRI can be used to identify acute orchronic stress injuries of the bone, which are often seen assubchondral and articular lesions, as well as injuries to thesupporting soft tissues. Other lesions include occult non-displaced fractures, osteomyelitis, small osseous cyst-likelesions, and osteochondral fragments. As with any area ofthe body, knowing the extent of bony or soft tissuepathology helps steer the appropriate treatment plan andprognosis.The Head: The brain and paranasal sinuses are the mostcommonly imaged regions of the head. Inflammatory,infectious, neoplastic, and traumatic causes of neurologicsyndromes have been demonstrated. Examples includebrain abscesses, inflammatory and toxic encephalopathies,cerebral hemorrhage, skull fractures, neuritis, congenitalmalformations, and hydrocephalus.With sinus infections (Fig. 7) and tooth root pathologies,MRI has been useful in assessing the extent of sinusinvolvement, which is particularly valuable for presurgicalplanning. Ethmoid hematomas and their associateddamage are readily seen (Fig. 8), often with unexpected

Figure 7. Post-contrast T1 weighted axial image. The rostral maxillary andventral conchal sinuses are filled with a multiloculated, fluid-filled tissuethat has septated and thickened contrast enhancing walls (arrows), consis-tent with an abscess.

results that surprise clinicians and influence the surgicalplan. The guttural pouches, stylohyoid apparatus, andmiddle ear may also be evaluated using MRI.

Abstracts

The biology of stem cells and their role in tissuerepair

Clare Yellowley PhDDepartment of Anatomy, Physiology, and Cell Biology,School of Veterinary Medicine, UCDavis

Take-home message:Stem cells have significant potential for therapeutic use inmusculoskeletal tissue repair and regeneration. An under-standing of the biology of such cells, how theymobilize andmigrate in response to injury, and their exact role in theregenerative process may provide us with new moleculartargets towardwhichmodern therapeutics can be designed.Introduction: There is intense focus on the regenerativeand therapeutic potential of stem cells in both veterinaryand human medicine. By definition, stem cells are unspe-cialized cells that can undergo self renewaldthat is, theycan divide and create identical copies of themselves.Additionally, given the correct cues, these cells are able todifferentiate and create all of the cell types of the body.Embryonic stem cells are derived from the inner cell massof the blastocyst, very early in development. They are