Lapis ISSN 2398-2969

Magnetic resonance imaging

Synonym(s): MRI

Contributor(s): Anna Meredith, Livia Benato

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. 
  • 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 include fast spin echo, gradient echo, spoiled gradient echo and short inversion recovery, with or without specialist techniques such as fat saturation. 
  • Images can be described as T1 or T2-weighted, and the relative weighting determines the signal intensity of, and contrast between, different tissues.

Uses

  • Used to evaluate brain, dental diseases   MRI: abscess dorsal from molar root tooth  , spine for disc disease, joints, soft tissue of the extremities and abdominal organs. 
  • Study of lesions of atherosclerosis in rabbit aorta.

Advantages

  • Non-invasive diagnostic test.
  • Painless procedure.
  • Greater sensitivity than radiography, ultrasonography and computed tomography   Computed tomography  .
  • Doesnt involve exposure to radiation. 
  • 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 be achieved. 
  • No use of ionizing radiation.

Disadvantages

  • Rarely readily available.
  • Equipment required for MRI is extremely expensive as this includes not only the magnet itself, but also a specially designed building and MRI-compatible associated equipment. 
  • Rabbit needs to be absolutely still during procedure and for this reason general anesthesia is preferable to sedation.
  • Not suitable for imaging bone.
  • The high respiratory rate of the rabbit may cause artifacts, or image distortions.
  • 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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Further Reading

Publications

Refereed papers

  • Recent references from PubMed and VetMedResource.
  • Yuan C, Skinner M P, Kaneko E et al (1996) Magnetic resonance imaging to study lesions of atherosclerosis in the hyperlipidemic rabbit aorta. Magnetic Resonance Imaging 14 (1), 93-102 PubMed.

Other sources of information

  • Girling S (2002) Mammalian Imaging and Anatomy. In:BSAVA Manual of Exotic Pets. Eds: Meredith A. & Redrobe S. 4th edn. pp 1-12.
  • RadiologyInfo"- a public information website developed and funded by the American College of Radiology (ACR) and the Radiological Society of North America (RSNA) -www.radiologyinfo.org.


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