MAGNETIC RESONANCE IMAGING

Magnetic resonance imaging (MRI) is one of the newest imaging modalities in medicine and relies on changes in the energy status of atomic nuclei in a strong magnetic field. There is no ionizing radiation involved in this imaging modality, but there is a strong magnetic field present, differentiating it from other conventional imaging methods. In the equine industry MRI is becoming popular as a diagnostic tool for musculoskeletal issues, particularly in the distal limb.


Physics

The basis for MRI is the phenomenon of nuclear magnetic resonance (NMR) that was first discovered independently by Purcell and Bloch in 1946. Each nucleus with an odd number of protons and/or neutrons has a spin, a circular movement along the axis of the nucleus. Because the protons carry an electric charge, the spin creates a magnetic field. For our purposes we only consider the hydrogen nucleus since this is the commonly imaged atom due to its abundance in the body. A hydrogen nucleus consists of one proton. Each hydrogen nucleus (proton) within tissue acts as a small spinning magnet. These “magnets” are randomly aligned until placed within a strong magnetic field.


When a patient is placed within a MR imager, the hydrogen protons try to align themselves with the strong magnetic field (B0). Because the protons have a spin, the magnetic vector of the protons will resist the torque exerted by the B0 and run around the axis of the B0 field in a cone. This movement is called precession and resembles the movement of a spinning top.

When protons in a static magnetic field are exposed to radio waves of the same frequency as that of the precessional movements of the nuclei (precession frequency), the magnetic vector will tilt away from the B0 vector, proportional to the energy absorbed. When the radiofrequency transmitter is then switched off the nuclei will realign with the static magnetic field and give off the extra energy as a radio wave, which is recorded by a radiofrequency receiver. This signal is the basis of MR imaging. The reconstruction involves a complex mathematical process called Fourier transformation which converts amplitude vs. time data to signal strength vs. frequency data. Gradients are used in each of the three planes so that the computer “knows” the location of the recorded signal. The gradients also allow for images to be made in any plane without moving the patient or equipment. A gray scale is applied to the intensity of the recorded signals from each voxel of tissue and is represented by pixels on a television screen at the operator console.

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Clinical Applications

A magnetic resonance imager consists of a magnet, a transmitter coil, computer system, and operator console. The strength of the magnet used in imaging is measured in Tesla units (1 T [tesla]= 10,000 gausss; the earth’s magnetic field is approximately .5 gauss). Examples of magnet strengths used for clinical MRI are .064T, .5T, 1T, 1.5T, 3.0T. Most human hospitals have a 1T or 1.5 T imager. In veterinary medicine, the MRI exam is performed with the animal under general anesthesia and takes approximately 30 minutes to one hour. The main hazard of this magnetic field is its attractive force towards ferrous objects.

MRI is superior to Computed Tomography (CT) for imaging soft tissues, whereas CT is superior to MRI for imaging bone. Tissues are differentiated based on the number of protons they contain, and how those protons are bound by surrounding molecules. This allows for imaging of subtle differences in tissues. Since the body is composed of about 75% water, it is the hydrogen protons of water that emit the signals forming the clinical MR image. Tissues containing little water such s cortical bone, ligaments, and other dense fibrous tissues are black or dark gray. Cancellous bone images very well with MRI, and diseases such as osteomyelitis and ischemic necrosis are readily depicted. Blood vessels within organs can be differentiated from parenchyma, and white matter can be contrasted with grey matter. MRI is also the only modality that can directly image cartilage. Contrast material may be used to highlight abnormally vascularized lesions.

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MRI General References:

1. Horowitz, A.L.; MRI Physics for Radiologists- A Visual Approach, 3rd Edition; Springer-Verlag; New York;1995

2. Shores A: Magnetic resonance imaging. Vet Clin N Am (Small Animal) 23:437-459, 1993.

3. Thomson CE, Konegay JN, Burn RA, et al: Magnetic resonance imaging- general overview of principles and examples in veterinary diagnosis.

4. Vet Radiol & Ultrasound 34:2-17; 1993Wortman JA, Rantanen NW.; Textbook of Veterinary Diagnostic Radiology; ed. D Thrall, W.B. Saunders; 1986. (Chapter 47, pages 533-548)

 
Copyright © 2006 - University of Pennsylvania School of Veterinary Medicine, All rights reserved.
Faculty: Dr. Alexia McKnight
Student:Charles Bradley 2009