Equis ISSN 2398-2977

Magnetic resonance imaging

Synonym(s): MRI

Contributor(s): Rachel Murray, Vetstream Ltd

Introduction

  • Magnetic resonance imaging (MRI) is a non-invasive imaging modality that can be used for evaluation of anatomy, function and pathology in many structures.
  • The image is produced by placing the region of interest within a magnetic field, exciting hydrogen nuclei within the tissues at their resonance frequency in the magnetic field, and then detecting the energy released as these nuclei relax. By using different sequences of excitation and relaxation, an image of the tissue can be built up.
  • To excite hydrogen nuclei, radiofrequency pulses from a radiofrequency coil are applied to change the alignment of the hydrogen nuclear within the static magnetic field.
  • Magnetic field strength can be high (1-1.5 Tesla), midfield (0.5-1.0 Tesla) or low field (less than 0.5 Tesla). High field strengths are usually generated by a superconducting magnet, while lower fields may be generated by resistive or permanent magnets.
  • Horses can be imaged staqnding or under general anesthesia, depending which configuration of magnet is used.
  • Hydrogen nuclei that are available for excitation are most abundant in fat and water, so most signal can be derived from these tissues.
  • Three gradient coils change the intensity of the static magnetic field in different directions, which is used to position the signal within space and create a 3-dimensional image.
  • Images are constructed mathematically by computer software using Fourier transformations, with signal intensity shown in gray scale. High signal areas are shown as white and low signal areas shown as black.
  • MR images can be acquired as 2-dimensional slices of varying thickness with the signal averaged across the slice of tissue imaged, or as 3-dimensional data sets. 3D reconstruction allows slices in any plane and thinner slice thickness.
  • The sequence of RF pulses and application of magnetic field gradients used during an MR study is described by the pulse sequence used to create the MR image.
  • Numerous pulse sequences can be used to highlight different tissues and various pathological changes. Pulse sequences used for equine clinical MRI often include fast spin echo and gradient echo, with or without specialist techniques such as fat saturation and short tau inversion recovery (STIR).
  • Images can be described as proton density, T1 or T2-weighted, and the relative weighting determines the signal intensity of, and contrast between, different tissues:
    • Proton density weighted images are based on the density of mobile protons in the tissue.
    • T1 weighted images   Foot: sagittal T1-weighted image - MRI    Carpus: sagittal fat suppressed T1-weighted image - MRI  : tissues with fast T1 relaxation, eg fat, show high signal intensity. Structures with a high fat content are generally bright on T1 weighted images
    • T2 weighted images   Foot: sagittal T2-weighted image 01 - MRI    Foot: sagittal T2-weighted image 02 - MRI    Foot: sagittal fat suppressed T-2 weighted image - MRI    MCP joint: sagittal fat suppressed T2-weighted image - MRI    Carpus: transverse T2-weighted image - MRI  : tissues with slow T2 relaxation, eg water, show high signal intensity so structures with a high water content are generally bright on T2-weighted images.

Print off the Owner Factsheet on Diagnostic imaging to give to your clients.

Uses

Musculoskeletal imaging

Indications
  • Localized, undiagnosed lameness, most frequently at the level of the carpus/tarsus distally. Some MRI systems are also able to acquire imaged up to and including the level of the stifle.
  • Foot pain with no definitive diagnosis.
  • Suspected tendon or ligament injury at a site inaccessible to ultrasound examination or where further information on injury is required.
  • Suspected subchondral, cortical or cancellous bone injury with no radiographic abnormality, with or without scintigraphic abnormality, or where further information on injury is required.
  • Suspected articular cartilage abnormality.
  • Monitoring of pathological processes.
  • Monitoring of response to training.
Detectable pathology
  • Ligament pathology, including damage at the osseous attachments.
  • Tendon pathology, including damage at the osseous attachments.
  • Articular pathology including synovial proliferation, capsular thickening, effusion, subchondral bone defects, irregularity, edema and thickening and articular cartilage defects and pathology.
  • Bone edema, inflammation, necrosis, trabecular damage and fractures.
  • Laminar damage, including disruption of dermal/osseous interface.
  • Damage to other tissues, including nerves, vascularture, connective tissues and their interfaces.
  • Neoplasia.

Brain

Indications
  • Central signs.
  • Seizures.
  • Endocrine abnormality attributable to pituitary abnormality.
  • Trauma.
  • Loss of vision or hearing.
  • For the potential detection of intracranial masses, multifocal inflammatory disease and vascular accidents.
  • For the potential identification of malformations.

Advantages

  • It is possible to show structures that are impossible to assess using other diagnostic imaging techniques including both soft tissues and bone.
  • Many tissues can be assessed at the same time.
  • Anatomical and physiological assessment can be undertaken concurrently.
  • Three dimensional, multiplanar imaging can achieved.
  • No use of ionizing radiation.
  • Highly detailed imaging capability.
  • Can be acquired standing Anesthesia: standing chemical restraint or under general anesthesia Anesthesia: general - overview.

Disadvantages

  • Skilled staff are required for image acquisition and image interpretation.
  • The area of interest must be decided prior to imaging as imaged acquision can be time consuming and limited regions of interest can be assessed during a reasonable sedation or anesthetic time.
  • Only the limbs, head and cranial neck can generally be imaged in the adult horse at the current time.
  • Equipment required for MRI is expensive as this includes not only the magnet itself, but also radiofrequency shielding and MRI-compatible associated equipment.
  • The large number of images acquired per study means interpretation is time consuming.

Requirements

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Preparation

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Procedure

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Aftercare

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Outcomes

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Prognosis

  • Dependent on diagnosis.

Further Reading

Publications

Refereed papers

  • Recent references from PubMed and VetMedResource.
  • Holmes S P (2014) Equine skull magnetic resonance imaging: The where, when and why? Equine Vet Educ 46 (11), 605-609 Wiley.
  • Bolt D Met al (2013) Standing low-field magnetic resonance imaging of a comminuted central tarsal bone fracture in a horse. Equine Vet Educ 25 (12), 618-623 VetMedResource.
  • Judy C E (2013) Cross-sectional imaging: More ways to look at a problem.Equine Vet Educ 25 (12), 624-627 Wiley
  • Black B et al (2013) The effects of perineural and intrasynovial anaesthesia of the equine foot on subsequent magnetic resonance images. Equine Vet J 45 (3), 320-325 PubMed.
  • Daniel A J, Judy C E & Saveraid T (2013) Magnetic resonance imaging of the metacarpo(tars)phalangeal region in clinically lame horses responding to diagnostic analgesia of the palmar nerves at the base of the proximal sesamoid bones: five cases. Equine Vet Educ 25 (5), 222-228 Wiley.
  • Smith M A, Dyson S J & Murray R C (2012) Reliability of high- and low-field magnetic resonance imaging systems for detection of cartilage and bone lesions in the equine cadaver fetlock. Equine Vet J 44 (6), 684-691 PubMed.
  • Dyson S J, Blunden T & Murray R (2012) Comparison between magnetic resonance imaging and histological findings in the navicular bone of horses with foot pain. Equine Vet J 44 (6), 692-698 PubMed.
  • Tranquille C A, Parkin T D H & Murray R C (2012) Magnetic resonance imaging-detected adaptation and pathology in the distal condyles of the third metacarpus, associated with lateral condylar fracture in Thoroughbred racehorses. Equine Vet J 44 (6), 699-706 PubMed.
  • Mair T S & Linnenkohl W (2012) Low-field magnetic resonance imaging of keratomas of the hoof wall.  Equine Vet Educ 24 (9), 459-468 VetMedResource.
  • Biggi M, Zani D D, De Zani D & Di Giancamillo M (2012) Magnetic resonance imaging findings of bone marrow lesions in the equine distal limb. Equine Vet Educ 24 (5), 236-241 Wiley.  
  • Labens R & Redding W R (2012) MR imaging of the equine distal tarsus - too much information? Equine Vet Educ 24 (5), 242-246 Wiley.  
  • Nagy A & Dyson S (2012) Magnetic resonance imaging and histological findings in the proximal aspect of the suspensory ligament of forelimbs in nonlame horses. Equine Vet J 44 (1), 43-50 PubMed.
  • Vallance S A, Bell R J W, Spriet M, Kass P H & Puchalski S M (2012) Comparisons of computed tomography, contrast enhanced computed tomography and standing low-field magnetic resonance imaging in horses with lameness localised to the foot.  Part 1: Anatomic visualisation scores. Equine Vet J 44 (1), 51-56  PubMed.
  • Keller M D, Pollitt C C & Marx U C (2011) Nuclear magnetic resonance-based metabonomic study of early time point laminitis in an oligofructose-overload model. Equine Vet J 43 (6), 737-743  PubMed.
  • Nagy A & Dyson S (2011) Anatomical, magnetic resonance imaging and histological findings in the accessory ligament of the deep digital flexor tendon of forelimbs in nonlame horses. Equine Vet J 43 (3), 309-316 PubMed.
  • Biggi M & Dyson S (2011) High-field magnetic resonance imaging investigation of distal border fragments of the navicular bone in horses with foot pain. Equine Vet J 43 (3), 302-308 PubMed.
  • Biggi M & Dyson S (2010) Comparison between radiological and magnetic resonance imaging lesions in the distal border of the navicular bone with particular reference to distal border fragments and osseous cyst-like lesions. Equine Vet J 42 (8), 707-712 PubMed.
  • Schneider et al (2010) Magnetic resonance imaging features of a benign peripheral nerve sheath tumour with 'ancient' changes in the tongue of a horse. Equine Vet Educ 22 (7), 346-351VetMedResource.
  • Dyson S, Pool R, Blunden T & Murray R (2010) The distal sesamoidean impar ligament: Comparison between its appearance on magnetic resonance imaging and histology of the axial third of the ligament. Equine Vet J 42 (4), 332-339 PubMed.
  • Rodriguez M J et al (2010) Magnetic resonance imaging of the equine temporomandibular joint anatomy. Equine Vet J 42 (3), 200-207 PubMed.
  • Powell S E et al (2010) Standing magnetic resonance imaging detection of bone marrow oedma-type signal pattern associated with subcarpal pain in 8 racehorses: A prospective study. Equine Vet J42 (1), 10-17 PubMed.
  • Garrett K S et al (2009) Diagnosis of laryngeal dysplasia in five horses using magnetic resonance imaging and ultrasonography. Equine Vet J 41 (8), 766-771 PubMed.
  • Murray R C, Mair T S, Sherlock C E & Blunden A S (2009) Comparison of high-field and low-field magnetic resonance images of cadaver limbs of horses. Vet Rec 165 (10), 281-288 PubMed.
  • Spriet M & Zwingenberger A (2009) Influence of the position of the foot on MRI signal in the deep digital flexor tendon and collateral ligaments of the distal interphalangeal joint in the standing horse. Equine Vet J 41 (5), 498-503 PubMed.
  • Olive J, Mair T S & Charled B (2009) Use of standing low-field magnetic resonance imaging to diagnose middle phalanx bone marrow lesions in horses. Equine Vet Educ 21 (3), 116-123 VetMedResource.
  • Werpy N (2009) Diagnosis of middle phalanx bone marrow lesions in horses using magnetic resonance imaging and identification of phase effect cancellation for proper image interpretation. Equine Vet Educ 21 (3), 125-130 Wiley.
  • Blunden A, Murray R & Dyson S (2009) Lesions of the deep digital flexor tendon in the digit: a correlative MRI and post mortem study in control and lame horses. Equine Vet J 41 (1), 25-33 PubMed.
  • Sherlock C, Mair T & Blunden T (2008) Deep erosions of the palmar aspect of the navicular bone diagnosed by standing magnetic resonance imaging. Equine Vet J 40 (7), 684-692 PubMed.
  • Dyson S, Blunden T & Murray R (2008) The collateral ligaments of the idstal interphalangeal joint: Magnetic resonance imaging and Post mortem observations in 25 lame and 12 control horses. Equine Vet J 40 (6), 538-544 PubMed.
  • Coudry V, Denoix J-M, Didierlaurent D, Rossignol F & Audigie F (2008) Use of magnetic resonance imaging to diagnose the cause of proximal metacarpal pain in a Standardbred trotter. Vet Rec 162 (24), 790-792 PubMed.
  • Tucker R L (2008) Magnetic resonance imaging of the equine head and neck region. Equine Vet Educ 20 (6), 294-296 VetMedResource.
  • Jakesova V, Konar M, Gerber V, Brachelente C, Howard J & Tessier C (2008) Magnetic resonance imaging features of an extranodal T cell rich B cell lymphoma in the pharyngeal mucosa in a horse. Equine Vet Educ 20 (6), 289-293 VetMedResource.
  • Maulet B E B, Bestbier M, Jose-Cunilleras E, Scrine J A & Murray R (2008) Magnetic resonance imaging of a cholesterol granuloma and hydrocephalus in a horse. Equine Vet Educ 20 (2), 74-79 VetMedResource.
  • Barrett M F & Zubrod C J (2008) Use of magnetic resonance imaging to detect and direct therapy of an osseous cystic lesion at the solar surface of the third phalanx of a horse. Equine Vet Educ 20 (1), 19-23 VetMedResource.
  • Mair T S & Sherlock C E (2008) Osseous cyst-like lesions in the feet of lame horses: diagnosis by standing low-field magnetic resonance imaging. Equine Vet Educ 20 (1), 47-56 VetMedResource.
  • Nagy A, Dyson S J & Murray R M (2008) Radiographic, scintigraphic and magnetic resonance imaging findings in the palmar processes of the distal phalanx.  Equine Vet J 40 (1), 57-63 PubMed.
  • Sherlock C E, Kinns J & Mair T S (2007) Evaluation of foot pain in the standing horse by magnetic resonance imaging. Vet Rec 161 (22), 739-744 PubMed.
  • Dyson S & Murray R (2007) Magnetic resonance imaging evaluation of 264 horses with foot pain: The podotrochlear apparatus, deep digital flexor tendon and collateral ligaments of the distal interphalangeal joint. Equine Vet J 39 (4), 340-343 PubMed.
  • Dyson S & Murray R (2007) Use of concurrent scintigraphic and magnetic resonance imaging evaluation to improve understanding of the pathogenesis of injury of the podotrochlear apparatus. Equine Vet J 39 (4), 365-369 PubMed.
  • Bischofberger A S et al (2006) Magnetic resonance imaging, ultrasonography and histology of the suspensory ligament origin: a comparative study of normal anatomy of Warmblood horses. Equine Vet J 38 (6), 508-516 PubMed.
  • Franci P, Leece E A & Brearley J C (2006) Post anesthetic myopathy/neuropathy in horses undergoing magnetic resonance imaging compared to horses undergoing surgery. Equine Vet J 38 (6), 497-501 PubMed.
  • Murray R & Mair T (2005) Use of magnetic resonance imaging in lameness diagnosis in the horse. In Practice 27 (3), 138-146 BVA.
  • Boado A, Kristoffersen M, Dyson S & Murray R (2005) Use of nuclear scintigraphy and magnetic resonance imaging to diagnose chronic penetrating wounds in the equine foot. Equine Vet Educ 17 (2), 62-68 VetMedResource.
  • Kinns K & Mair T S (2005) Use of magnetic resonance imaging to assess soft tissue damage in the foot following penetrating injury in 3 horses. Equine Vet Educ 17 (2), 69-73 Wiley.
  • Mair T S, Kinns J, Jones R D & Bolas N M (2005) Magnetic resonance imaging of the distal limb of the standing horse. Equine Vet Educ 17 (2), 74-78 VetMedResource.
  • Boswell J (2004) Magnetic resonance imaging in horses. Vet Review 89, 26-27.
  • Dyson S J, Murray R C, Schramme M & Branch M (2003) Magnetic resonance imaging of the equine foot: 15 horses. Equine Vet J 35 (1), 18-26 PubMed.
  • Widmer A, Buckwater K, Fessler J, Hill M, Van Sickle D & Ivancevich S (2000) Use of radiography, computed tomography and magnetic resonance imaging for evaluation of navicular syndrome in the horse. Vet Radiol Ultrasound 41, 108-116 PubMed.
  • Kleiter M, Kneissl S, Stanek C, Mayrhofer E, Baulain U & Deegan E (1999) Evaluation of magnetic resonance imaging techniques in the equine digit. Vet Radiol Ultrasound 40, 15-22 PubMed.
  • Whitton C, Buckley C, Donovan T, Wales A & Dennis R (1998) The diagnosis of lameness associated with distal limb pathology in a horse - a comparison of radiography, computed tomography and magnetic resonance imaging. The Vet J 155, 223-229 PubMed.
  • Denoix J M, Crevier N, Roger B & Lebas J (1993) Magnetic resonance imaging of the equine foot. Vet Radiol Ultrasound 34, 405-411 Wiley.

Other sources of information

  • Murray R C (2011) Equine MRI. Eds: Murray R. Blackwell Publishing, Oxford.
  • Murray R C (2010) Magnetic Resonance Imaging. In: Diagnosis and Management of Lameness in the Horse. 2nd edn. Eds: Ross M & Dyson S. Elsevier, St Louis.


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