Page 424 - Adams and Stashak's Lameness in Horses, 7th Edition

390 Chapter 3

of GREs is to decrease acquisition times further and to Moreover, excessive scanning times and prolonged
allow three‐dimensional (3D) volumetric acquisitions recumbence within a confined magnet space with lim­
VetBooks.ir where motion artifacts and relatively low signal‐to‐noise A routine high­field MRI examination takes approxi­
ited padding can result in post‐anesthetic complications.
with thin slices. This is especially useful in standing MRI
mately 60–90 minutes to complete.
ratio are of concern. However, the advantage of fast 3D
acquisition of gradient echoes comes at the expense of For optimal assessment of an anatomical region,
disadvantages like decreased soft tissue contrast, images are generally obtained in three orthogonal
increased susceptibility to magnetic field inhomogenei­ planes, sagittal, dorsal, and transverse. Sagittal images
ties, and susceptibility artifacts. Most current clinical serve as anatomical reference images for transverse
protocols in horses use a mixture of FSE and GRE sections. Transverse images are most useful to the iden­
sequences. tification of pathology, and the presence of asymmetry
As many lesions tend to have increased signal inten­ between the medial and lateral halves of the images may
sity on MR images while at the same time fat retains help identify lesions. Dorsal images are useful for defini­
high signal intensity in all sequences, eliminating the tion of joint space width and cartilage thickness and
high signal intensity from fat (fat suppression) markedly also serve as reference images for transverse section
increases the ability of recognizing high signal intensity planes.
from a lesion on an MR image. Fat suppression can be
achieved by inversion recovery (STIR) or by a process of
fat saturation.
As a result of fat suppression, adipose tissue appears
black instead of white, making fluid the only remaining ARTIFACTS OF MRI
source of high signal on a fat‐suppressed image, amidst MRI produces a wide range of artifacts and varia­
dark bone, soft tissues, and fat. Fat suppression is com­ tions that can confuse the interpreter. In addition, MRI
monly used in orthopedic imaging for the detection of is susceptible to artifacts that are created by acquisition
abnormal fluid in bone (e.g. the navicular bone). of images at oblique angles. Even slightly asymmetric
Depending on whether T1 or T2 relaxation is positioning of image slices can create significant prob­
measured during acquisition, sequences are called T1 lems of image interpretation. Consequently, MRI is an
weighted, T2 weighted, or proton density (PD) weighted imaging technique that can easily lead the examiner to
(between both T1 and T2). T1 and T2 relaxation times overinterpret images as well as miss lesions.
have characteristic values for each type of tissue and can Artifacts can be classified as motion artifacts, mag­
be used in combination to describe the magnetic proper­ netic field heterogeneity artifacts, and digital imaging
ties of all tissues (Table 3.2) (Figure 3.209). PD images artifacts.
display any change in the density of protons as a change Motion artifacts generally result in ghosting, which
in signal intensity. results from displaced reduplications of the image in the
An imaging protocol or series of different sequences phase‐encoding direction, or in marked blurring of the
is typically grouped together to image a region of inter­ image. Swaying of a standing sedated horse results in
est. Both high‐ and low‐field musculoskeletal protocols blurring of images, which may render the entire sequence
typically include PD spin echo, T2‐weighted FSE, short nondiagnostic. STIR sequences are worst affected by
tau inversion recovery (STIR) FSE, and T1‐ and T2*‐ this type of motion artifact. Respiration and blood flow
weighted 3D GRE sequences. can cause multiple motion ghosting in anesthetized
PD images are easiest to evaluate ligament and ten­ patients (Figure 3.210).
don margins and symmetry. T2‐weighted images are Artifacts from magnetic field inhomogeneity lead to
useful for looking at fluid in soft tissues thanks to their image distortion or alterations in signal intensity and
high fluid contrast. However, T2‐weighted images show are more common in low‐field than high‐field magnets.
minimal shades of gray resulting in poor definition of Temperature fluctuations, the presence of ferrous metal,
soft tissue margins. STIR images show fluid in bone and and even some wound sprays or frog packing materials
soft tissues most easily because of their high fluid con­ like blue Play‐Doh may create field inhomogeneities.
trast, but they have a low resolution. T2*‐weighted or Magnetic susceptibility artifacts are a form of field inho­
spoiled T1‐weighted GRE sequences are used in 3D mogeneity artifact as they are caused by the presence of
acquisitions for evaluation of the fine anatomical detail materials with different susceptibility to magnetization,
in thin tissue slices. like ferrous metals or iron in hemoglobin and hemosid­
The choice of protocol is determined by the type of erin in tissues (Figure 3.211). 181,186 GREs are particularly
magnet (high or low field), the manufacturer’s sequence susceptible to these artifacts. Feet should be radio­
library, the region under scrutiny, and the preference of graphed routinely prior to MRI to ensure that all metal
the attending clinician. Consequently, protocols may fragments and debris have been removed from the hoof
differ between hospitals and clinicians. With an endless wall. 185
number of combinations of sequences, weightings, and Chemical shift artifacts are caused by the presence of
section planes available, it is tempting to continue scan­ fat and water adjacent to each other. Protons from water
ning every patient until the clinician feels all questions and fat have different precession frequencies that causes
have been answered. However, this approach lacks con­ the position of fat signal to shift in the frequency‐encod­
sistency and may cause problems. Sequences and section ing direction of the image relative to the position of
planes should be standardized to allow for consistent water signal. The shifting of fat signal to a different loca­
comparison between limbs and between horses. tion can results in spatial misregistration. This artifact is