Page 421 - Adams and Stashak's Lameness in Horses, 7th Edition

Diagnostic Imaging 387

MAGNETIC RESONANCE IMAGING

VetBooks.ir Michael SchraMMe and eMilie Segard‐WeiSSe

INTRODUCTION the nuclei of hydrogen atoms in biological tissues. When
exposed to a short RF pulse specific for hydrogen, which
Magnetic resonance imaging (MRI) is a multiplanar is emitted by the RF coil, hydrogen nuclei absorb this
cross‐sectional imaging modality that uses the inherent pulse and change their alignment within the main magnetic
magnetic properties of tissue and is fast becoming the field. Following discontinuation of the RF pulse, the
gold standard for diagnosis of musculoskeletal injury of hydrogen nuclei resume their previous orientation of
the distal limb in horses. MRI allows soft tissue and parallel alignment with the main magnetic field, thereby
bone structures to be evaluated in ways not possible transitioning from a high‐energy to a low‐energy state.
with other imaging modalities. The information obtained This transition results in energy release that is used to
is based on the structure and biochemical environment generate an electrical signal. The exchange of energy
of hydrogen atoms in the tissues. between different spin states is called the resonance and
Because of its unique mode of image acquisition and thus the name MRI. The time required for the hydrogen
construction, it is important to understand the factors nuclei to resume equilibrium within the main magnetic
that influence the MR signal characteristics that pro­ field is the relaxation time, measured in milliseconds.
duce the diagnostic images and to learn the strengths The relaxation of hydrogen nuclei can be divided into
and weaknesses of the technique. The clinician must two separate components, the longitudinal (T1) and
understand the basic physics and the basic sequences transverse relaxations (T2). T2 is much shorter than T1.
used for acquisition in order to comprehend the rela­ Most tissues can be characterized by their T1 and T2
tionship between signal abnormalities and pathology signal properties. Protons are present in different densi­
and distinguish between signal abnormality and artifact ties and with different T1 and T2 properties between
and between signal change and normal anatomical vari­ different tissues, thereby producing contrast differences
ation. The increasing use of MRI in equine sport medi­ between tissues on MR images. The degree to which the
cine requires every equine practitioner to have a basic hydrogen protons are bound within a tissue determines
knowledge of MRI interpretation. The importance of their relaxation time. Protons in fluid or fat return to
correlating the MRI findings with results of the clinical alignment quickly, whereas those in organized tendi­
examination, diagnostic analgesia, and other imaging nous, ligamentous, or osseous tissue return slowly.
modalities, in order to determine the clinical significance As fat and water both contain a large amount of
of each suspected lesion, cannot be overstated. mobile hydrogen atoms, the strength of the resonance
Accurate knowledge of the advantages and disadvan­ signal also depends on the amount of fat and water in
tages of MRI helps the clinician to make the correct the tissue. High signal areas are white, and low‐signal
imaging choices for the benefit of patients and clients. areas are black.
MRI is not a substitute for in‐depth clinical investiga­
tion and conventional imaging techniques, and many
diagnoses can continue to be made without MRI. EQUIPMENT, HIGH‐ AND LOW‐FIELD MAGNETS,
Nonetheless, the use of MRI has highlighted the limitations AND RF COILS
of both radiography for imaging bone and ultrasonog­
raphy for imaging soft tissue lesions. MRI has proven to The basic components of MR imaging are an RF
be particularly helpful in patients in which the lameness shielded room, a body with mobile protons, a magnet
has been localized to a certain area of the distal limb, generating a strong magnetic field, an RF coil to add
but radiography and ultrasonography do not show an energy to the protons to make them move, a receiver coil
abnormality that explains lameness in a satisfactory to collect the electrical signal generated by the moving
manner. protons, and a computer with acquisition software to
time these events and generate an image from the electri­
cal signals.
GENERAL PRINCIPLES AND PHYSICS OF MRI Magnetic field strength is expressed in tesla units.
One tesla (T) is approximately 20,000 times the strength
The generation of an MR image is based on the move­ of the earth’s magnetic field. High‐strength magnetic
ment of hydrogen atoms generating a measurable elec­ fields measure in excess of 1 T, low‐strength fields
trical current in the body. MRI produces a grayscale less than 0.5 T, and mid‐strength fields between 0.5 and
image of tissue hydrogen protons by placing them in a 1 T. Superconducting, closed, cylindrical bore magnets
strong magnetic field, exposing them to a radiofre­ (Figure 3.206) generate high‐strength magnetic fields,
quency (RF) pulse and measuring the magnetic reso­ while resistive or permanent, open magnets (Figure 3.207)
nance caused in the tissues in response to this pulse. A produce low‐strength fields. Signal strength is propor­
computer interprets the data and creates images that tional to the strength of the magnetic field. Consequently
display the different resonance characteristics of differ­ low‐field systems generate less tissue signal, require
ent tissue types on a gray scale. longer acquisition times, and produce lower‐resolution
The resonance that is measured originates from the images. The uniformity or homogeneity of the main
magnetic properties of the positively charged proton in magnetic field is higher in closed than in open magnets.