Magnetic Resonance Imaging of the Stifle 157(A) (B)Figure 21.1 Sagittal (A) proton
density MR image and sagittal (B)
SE T2-weighted MR image of a
2-year-old, neutered female
Mastiff with a complete rupture
of the cranial cruciate ligament.
There is increased fluid within
the stifle joint. Compare the SI of
the ‘clean’ (arrows) and ‘dirty’
synovial fluid (∗)onthetwo
images.
fibrocartilaginous structures that partially
divide the joint cavity and provide structural
integrity during movement (Dyceet al. 2002).
Each meniscus has unnamed cranial-tibial and
caudal-tibial ligaments. The lateral meniscus
also has an attachment to the femur, the menis-
cofemoral ligament. The transverse ligament
connects the cranial-tibial ligaments of the
two menisci. The normal MRI appearance of
the meniscus is triangular or bow-tie shape
(sagittal and dorsal planes) with uniform low
SI, although some exceptions regarding SI exist
(Martiget al. 2006). Meniscal tears are common
in dogs secondary to CR: the medial meniscus
is affected more commonly (Blondet al. 2008).
Meniscal tears may be present at the time of
CR diagnosis, or they may develop weeks to
months after surgical treatment (Taylor-Brown
et al. 2014). The MRI appearance of a meniscal
tear is a primarily linear intrameniscal area
of high SI that extends definitively to one or
both articular surfaces, or abnormal meniscal
shape (Figure 21.2) (Rubin 2005; Blondet al.
2008).
A grading system for describing meniscal
injuries during MRI has been proposed in dogs,
ranging from 0 (normal) to 4 (Martiget al.
2006). However, the reported accuracy of MRI
signs for diagnosing meniscal tears has varied
from poor to good (Blondet al. 2008; Bottcher ̈
et al. 2012; Galindo-Zamoraet al. 2013). False-
positive results for meniscal tears may be due
to misinterpretation of the appearance of nor-
mal structures adjacent to the meniscus, or to
artifact (Bairdet al. 1998; Hashemiet al. 2004). A
truncation artifact appears as alternating bright
and dark bands at high-contrast interfaces such
as the meniscus and synovial fluid (Figure 21.3)
(Hashemiet al. 2004). It is due to an inability
to approximate exactly a step-like change in the
signal intensity due to a limited number of sam-
ples or sampling time, and usually occurs in
the phase direction (Hashemiet al. 2004). It may
be remedied by increasing the sample time to
reduce the ripples, or by decreasing the pixel
size by increasing the number of phase encodes
or decreasing the field of view (Hashemiet al.
2004).(A) (B)Figure 21.2 Sagittal (A) SE
T2-weighted MR image of a
2.5-year-old, neutered female
Saint Bernard and (B) sagittal SE
T2-weighted MR image of a
2.5-year-old, neutered female
Rottweiler. Compare the normal
appearance of the medial
meniscus (A) to the abnormal (B).
A full-thickness tear is present in
the cranial part of the medial
meniscus (arrow) and the caudal
portion is absent (∗).