DOI:10.2214/AJR.07.2835
AJR 2008; 190:1105-1111
© American Roentgen Ray Society
Atrophy and Fatty Infiltration of the Supraspinatus Muscle: Sonography Versus MRI
Viviane Khoury1,
Étienne Cardinal2 and
Paul Brassard3
1 Department of Diagnostic Radiology, McGill University Health Center, Montreal
General Hospital, 1650 Cedar Ave., Montreal, QC H3G 1A4, Canada.
2 Department of Radiology, Hôpital St-Luc, Centre Hospitalier de
l'Université de Montréal, Montreal, QC, Canada.
3 Division of Clinical Epidemiology, McGill University Health Center, Montreal,
QC, Canada.
Received July 8, 2007;
accepted after revision October 31, 2007.
Address correspondence to V. Khoury
(viviane.khoury{at}hotmail.com).
Abstract
OBJECTIVE. The objective of our study was to compare sonography with
MRI for the evaluation of supraspinatus muscle atrophy and fatty
infiltration.
SUBJECTS AND METHODS. Forty-five shoulders in 39 patients who had
undergone shoulder MRI for the assessment of rotator cuff disease were
evaluated blindly with sonography. Supraspinatus muscle atrophy was
quantitatively assessed by calculating the occupation ratio (cross-sectional
surface area of the supraspinatus muscle belly divided by that of its fossa).
This was done by reproducing on sonography the equivalent of the
"Y" view on MRI. Fatty infiltration was assessed by evaluating
supraspinatus muscle echogenicity compared with that of the trapezius muscle
and pennate pattern. The occupation ratio and fatty infiltration of the
supraspinatus muscle on sonography were compared with these findings on
MRI.
RESULTS. Occupation ratios calculated on sonography images ranged
from 0.07 (severe atrophy) to 0.81 (normal) and correlated with the ratios
calculated on MRI (R = 0.90; 95% CI, 0.83–0.95). All 20
shoulders with no fatty infiltration on MRI had normal echogenicity and a
pennate pattern on sonography. Eight of the 10 shoulders with mild fatty
infiltration on MRI had an effaced pennate pattern and mild hyperechogenicity
on sonography. In 13 of the 15 shoulders with moderate to severe fatty
infiltration on MRI, the pennate pattern was absent and marked
hyperechogenicity was present on sonography.
CONCLUSION. Our study suggests that there is a good correlation
between sonography and MRI for the assessment of supraspinatus muscle atrophy
and fatty infiltration.
Keywords: atrophy • fatty degeneration • fatty infiltration • MRI • rotator cuff muscles • shoulder • sonography
Introduction
Complete tears of the rotator cuff tendons may be accompanied by muscle
atrophy and fatty infiltration that are progressive and are probably
irreversible
[1–4].
These modifications are known to be important negative prognostic factors for
the anatomic and functional results after both tendon repair and shoulder
arthroplasty
[5–9].
The assessment of muscle quality using imaging is therefore a crucial element
in evaluating surgical indications and postoperative prognosis. However, few
studies in the radiology literature have addressed the evaluation of rotator
cuff muscles.
A classification system of fatty infiltration was established by Goutallier
et al. [6]. Although it is
widely used in the orthopedics literature
[7,
10–12]
and is highly reproducible, it is based on CT, whereas currently sonography
and MRI are the imaging techniques most used for the assessment of rotator
cuff abnormalities. MRI has been shown to be a valuable method for the
evaluation of supraspinatus muscle atrophy by assessing muscle size
[4,
13]. Sonography has also been
shown to be useful for assessing rotator cuff muscle atrophy
[9]. There is, however, no
sonography quantitative method or grading system for evaluation of muscle
atrophy and fatty degeneration, and MRI remains the standard of reference for
the assessment of these changes in routine clinical practice
[4,
9,
14–16].
Studies that compare the performance of sonography with MRI for the evaluation
of supraspinatus muscle atrophy are rare
[17].
The purpose of our study was to compare sonography with MRI for the
evaluation of supraspinatus muscle atrophy and fatty infiltration and to
establish a quantitative method for the sonographic assessment of muscle
atrophy.
Subjects and Methods
Forty-five shoulders in 39 patients who had undergone shoulder MRI were
included in this study. A musculoskeletal radiologist reviewed all shoulder
MRI examinations in the PACS database of our institution and selected study
cases. Patients included in the study had undergone an MRI examination for
rotator cuff disease (evaluation of tendinopathy or tear). Patients who had a
history of shoulder surgery, who had a poor-quality MR study (motion
artifacts, poor "Y" view delineation), or who could not undergo
sonography within 3 months of MRI were excluded. The mean age of the 19 men
and 20 women was 61 years (range, 44–77 years). There were 28 right and
17 left shoulders. Included were 13 shoulders with no or mild supraspinatus
atrophy on MRI, 13 with moderate atrophy, and 19 with severe atrophy.
Muscle atrophy was defined by calculating the occupation ratio according to
the method of Thomazeau et al.
[18]. The occupation ratio is
the ratio between the cross section of the muscle belly and that of its fossa
on the Y view. The Y view is the oblique sagittal plane that crosses the
scapula through the medial border of the coracoid process, where the
supraspinatus fossa is mostly limited by bone. When the ratio is between 1.00
and 0.60 (stage I), the muscle is considered normal or slightly atrophied; be
tween 0.60 and 0.40 (stage II), moderately atrophied; and below 0.40 (stage
III), severely atrophied.
Patients were evaluated using sonography by a musculoskeletal radiologist
who was blinded to the MRI findings. Both radiologists have been fellow
ship-trained: the MRI reader had 4 years of MRI experience at the time of the
study and the radiologist per forming sonography had 13 years of experience
with sonography at the time of the study. Written informed consent was
obtained from all patients and the study was approved by our institutional
review board.
MRI Assessment of Muscle Atrophy
MRI was performed on one of two 1.5-T scanners (LightSpeed, GE Healthcare;
or Avento, Siemens Medical Solutions). The shoulder was placed in a dedicated
receive-only shoulder coil (GE unit) or in a large surface flex coil (Siemens
unit). The arm was placed alongside the body in a neutral position. Complete
MRI examinations included a sagittal oblique (parallel to the glenoid fossa)
T1-weighted turbo spin-echo sequence (TR range/TE range,
525–535/10–15; 15–17 slices; field of view, 18 cm; image
matrix, 512 x 192 [GE unit] or 208 x 320 [Siemens unit]; section
thickness, 4 mm) and a coronal oblique T1-weighted turbo spin-echo sequence
(600–620/10–14; 16–18 slices; image matrix, 512 x 256;
section thickness, 4 mm).
Supraspinatus muscle atrophy was measured by calculating the occupation
ratio of the supra spinatus fossa according to the method of Thomazeau et al.
[18] as described earlier. On
the sagittal T1-weighted sequence, the cross-sectional surface area of the
supraspinatus muscle was divided by that of its fossa. The sagittal image used
for the measurement was selected, according to Thomazeau et al.
[18], where a Y view is formed
by the bone landmarks of the scapular spine, the coracoid process of scapula,
and the distal clavicle. On the PACS workstation, the boundary of the supra
spinatus muscle was drawn by tracing a line as close as possible to its outer
edge, the boundary of the supraspinatus fossa was drawn along the inner bone
margins of the Y, and the superior limits of the supraspinatus fossa were the
distal clavicle and a line drawn between the distal clavicle an teriorly and
the scapular spine posteriorly (Figs.
1A and
1C).
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Fig. 1A —Calculation of occupation ratio by MRI in two shoulders. Two
methods were used in each shoulder: using either lines (A and C)
or ellipse tool (B and D) on PACS console to outline contours of
supraspinatus muscle and those of its fossa, as shown on these sagittal
oblique T1-weighted images. Normal supraspinatus muscle (no atrophy) in
56-year-old woman. Using lines, occupation ratio is 3.45/5.61 cm2 =
0.61.
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Fig. 1C —Calculation of occupation ratio by MRI in two shoulders. Two
methods were used in each shoulder: using either lines (A and C)
or ellipse tool (B and D) on PACS console to outline contours of
supraspinatus muscle and those of its fossa, as shown on these sagittal
oblique T1-weighted images. Supraspinatus with severe atrophy in 68-year-old
woman. Using lines, occupation ratio is 2.04/5.53 cm2 = 0.37.
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A modified method of occupation ratio calculation was also performed in
which the cross-sectional area of the muscle and fossa was estimated by
drawing the best circle or ellipse around each and using the
"ellipse" tool provided by the PACS manufacturer (Figs.
1B and
1D).
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Fig. 1B —Calculation of occupation ratio by MRI in two shoulders. Two
methods were used in each shoulder: using either lines (A and C)
or ellipse tool (B and D) on PACS console to outline contours of
supraspinatus muscle and those of its fossa, as shown on these sagittal
oblique T1-weighted images. Using ellipse method in same shoulder as in
A, occupation ratio is 3.41/5.72 cm2 = 0.60.
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Fig. 1D —Calculation of occupation ratio by MRI in two shoulders. Two
methods were used in each shoulder: using either lines (A and C)
or ellipse tool (B and D) on PACS console to outline contours of
supraspinatus muscle and those of its fossa, as shown on these sagittal
oblique T1-weighted images. Using ellipse method in same shoulder as in
C, occupation ratio is 2.11/6.08 cm2 = 0.35.
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MRI Assessment of Fatty Replacement
Supraspinatus fatty infiltration was evaluated subjectively by examining
the proportion of fat in the muscle on the sagittal and coronal T1-weighted
images. Fatty infiltration was graded according to the method of Goutallier et
al. [2]. According to that
method, normal muscle that contains no fatty streaks is grade 0. In mild
muscle fatty infiltration (grade 1), there are a few fatty streaks; in
moderate muscle fatty infiltration (grade 2), there are about equal amounts of
muscle and fat; and in severe muscle fatty infiltration (grade 3), there is
more fat than muscle (Figs. 2A,
2B,
2C, and
2D).
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Fig. 2B —Evaluation of supraspinatus fatty infiltration on MRI.
Sagittal oblique T1-weighted images show mild (B), moderate (C),
and severe (D) fatty infiltration in 75-year-old man, 67-year-old man,
and 68-year-old woman, respectively.
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Fig. 2C —Evaluation of supraspinatus fatty infiltration on MRI.
Sagittal oblique T1-weighted images show mild (B), moderate (C),
and severe (D) fatty infiltration in 75-year-old man, 67-year-old man,
and 68-year-old woman, respectively.
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Fig. 2D —Evaluation of supraspinatus fatty infiltration on MRI.
Sagittal oblique T1-weighted images show mild (B), moderate (C),
and severe (D) fatty infiltration in 75-year-old man, 67-year-old man,
and 68-year-old woman, respectively.
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Sonography Assessment of Muscle Atrophy
The sonography examination was performed using a 5-12–MHz transducer
(ATL 5000 HDI, Philips Medical Systems). A complete standard shoulder
examination was performed with the patient sitting on a stool. In addition,
the supraspinatus muscle was examined with the arm alongside the body
(shoulder in neutral position, elbow extended) in the coronal oblique and
sagittal oblique planes.
To calculate the occupation ratio, the Y view of MRI was reproduced with
sonography by locating the suprascapular notch in the coronal oblique plane
(in the plane of the scapula) (Fig.
3A) and then rotating the transducer 90° to that plane. The
occupation ratio was then calculated using the same method as that used for
MRI—that is, by dividing the cross-sectional surface area of the
supraspinatus muscle by that of its fossa. The cross-sectional area of each
was measured on the sonography unit during the examination using the ellipse
tool provided by the manufacturer. The ellipse that was used to measure
supraspinatus muscle area was drawn around the boundaries of the muscle, which
is round or oval. The boundaries for the ellipse that were used to measure the
supraspinatus fossa were as follows: the hyperechoic line of the cortical bone
of the suprascapular fossa inferiorly and the border of the acoustic shadow of
the scapular spine posteriorly and that of the clavicle anteriorly. The
superior boundary of the ellipse was between the hyperechoic cortical bone
borders of the scapular spine and clavicle, where there is a change in
echogenicity between the fat of the suprascapular fossa and overlying
trapezius muscle (Fig.
3B).
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Fig. 3A —Calculation of occupation ratio by sonography in healthy
50-year-old man (no atrophy or fatty infiltration of supraspinatus). SS =
supraspinatus muscle, t = trapezius muscle. Suprascapular notch
(arrow) is located in coronal oblique plane.
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Fig. 3B —Calculation of occupation ratio by sonography in healthy
50-year-old man (no atrophy or fatty infiltration of supraspinatus). SS =
supraspinatus muscle, t = trapezius muscle. At notch (arrows),
transducer is rotated 90°, reproducing Y (sagittal oblique) view of MRI.
Ellipse tool on sonography unit was used to calculate occupation ratio in same
fashion as for MRI. C = clavicle, Sc = scapular spine.
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Sonography Assessment of Fatty Replacement
Supraspinatus muscle fatty infiltration was assessed by evaluating its
echogenicity and echostructure. The echogenicity of the supra spinatus muscle
was compared with that of the trapezius muscle to determine whether the
supraspinatus muscle was isoechoic, mildly hyper echoic, or markedly
hyperechoic. The echostructure of the supraspinatus muscle or pennate pattern
of the muscle was evaluated as normal (homogeneously distributed well-defined
hyperechoic streaks), effaced (slight loss of pennate pattern with blurring of
the margins of hyperechoic streaks), or absent (loss of pennate pattern with
very poor or no visibility of the streaks).
Results
Occupation Ratio of Supraspinatus Muscle with MRI: Thomazeau Versus Modified Thomazeau Methods
For the 45 shoulders examined on MRI, the occupation ratio of the
supraspinatus muscle according to Thomazeau et al.
[18] ranged from 0.11 to 0.84.
With the modified Thomazeau method (i.e., drawing an ellipse instead of
drawing lines around the supraspinatus muscle and fossa), the occupation ratio
values ranged from 0.11 to 0.87. The correlation coefficient between the two
methods is 0.98 (95% CI, 0.96–0.99).
Occupation Ratio of Supraspinatus Muscle: MRI Versus Sonography
For the 45 shoulders, the occupation ratio of the supraspinatus muscle on
sonography ranged from 0.07 to 0.81, compared with 0.11 to 0.87 on MRI, using
the ellipse tool to outline the areas of muscle and fossa for both techniques.
The correlation coefficient was 0.90 (95% CI, 0.83–0.95).
Echogenicity and Pennate Pattern of Supraspinatus Muscles
All 20 shoulders without fatty infiltration on MRI had normal echogenicity
and a normal pennate pattern on sonography. Eight of the 10 shoulders with
mild fatty infiltration on MRI had an effaced pennate pattern and mild
hyperechogenicity on sonography (Figs.
4A,
4B,
4C, and
4D). Thirteen of 15 shoulders
with moderate to severe fatty infiltration on MRI had absence of the pennate
pattern and marked hyperechogenicity on sonography (Figs.
4E and
4F). Two shoulders that were
found to have severe atrophy and fatty infiltration on MRI were assessed as
having a less severe grade on sonography; both had a full-thickness tear of
the supraspinatus tendon that was retracted approximately 4 cm. Two other
shoulders showed discordance between the two sonography features of fatty
infiltration: one with moderate fatty infiltration on MRI had an absent
pennate pattern but mild hyperechogenicity on sonography; another with mild
fatty infiltration on MRI had an effaced pennate pattern but marked
hyperechogenicity on sonography. For these two shoulders, there was severe
atrophy and tendon retraction of > 2 cm. The relationship between atrophy
and echogenicity and pennate pattern is shown in Figures
5A,
5B,
5C, and
5D.
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Fig. 4A —Sonography and MR images of shoulder in 75-year-old man
(A and B, respectively), 59-year-old woman (C and
D, respectively), and another 75-year-old man (E and F,
respectively). Coronal sonograms (A and C) show muscle
(arrows, A and C) being mildly hyperechoic relative to
trapezius muscle (t) and pennate pattern that is effaced. Coronal T1-weighted
MR images (B and D) show mild fatty infiltration.
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Fig. 4B —Sonography and MR images of shoulder in 75-year-old man
(A and B, respectively), 59-year-old woman (C and
D, respectively), and another 75-year-old man (E and F,
respectively). Coronal sonograms (A and C) show muscle
(arrows, A and C) being mildly hyperechoic relative to
trapezius muscle (t) and pennate pattern that is effaced. Coronal T1-weighted
MR images (B and D) show mild fatty infiltration.
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Fig. 4C —Sonography and MR images of shoulder in 75-year-old man
(A and B, respectively), 59-year-old woman (C and
D, respectively), and another 75-year-old man (E and F,
respectively). Coronal sonograms (A and C) show muscle
(arrows, A and C) being mildly hyperechoic relative to
trapezius muscle (t) and pennate pattern that is effaced. Coronal T1-weighted
MR images (B and D) show mild fatty infiltration.
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Fig. 4D —Sonography and MR images of shoulder in 75-year-old man
(A and B, respectively), 59-year-old woman (C and
D, respectively), and another 75-year-old man (E and F,
respectively). Coronal sonograms (A and C) show muscle
(arrows, A and C) being mildly hyperechoic relative to
trapezius muscle (t) and pennate pattern that is effaced. Coronal T1-weighted
MR images (B and D) show mild fatty infiltration.
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Fig. 4E —Sonography and MR images of shoulder in 75-year-old man
(A and B, respectively), 59-year-old woman (C and
D, respectively), and another 75-year-old man (E and F,
respectively). Sonogram shows that supraspinatus muscle (arrows) is
markedly hyperechoic relative to trapezius muscle (t) and that its pennate
pattern is lost.
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Fig. 4F —Sonography and MR images of shoulder in 75-year-old man
(A and B, respectively), 59-year-old woman (C and
D, respectively), and another 75-year-old man (E and F,
respectively). Coronal oblique T1-weighted MR image of same patient as in
E shows severe fatty infiltration.
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Fig. 5A —Supraspinatus atrophy and fatty infiltration of different
grades. In both patients, 60-year-old woman (A and B) and
75-year-old woman (C and D), there is severe atrophy, with
occupation ratio of < 0.4. On sonograms, inner circle represents
supraspinatus muscle, and outer circle represents supraspinatus fossa.
Sagittal sonogram (A) shows normal echogenicity and pennate pattern,
and no fatty infiltration is present on MRI image (B).
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Fig. 5B —Supraspinatus atrophy and fatty infiltration of different
grades. In both patients, 60-year-old woman (A and B) and
75-year-old woman (C and D), there is severe atrophy, with
occupation ratio of < 0.4. On sonograms, inner circle represents
supraspinatus muscle, and outer circle represents supraspinatus fossa.
Sagittal sonogram (A) shows normal echogenicity and pennate pattern,
and no fatty infiltration is present on MRI image (B).
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Fig. 5C —Supraspinatus atrophy and fatty infiltration of different
grades. In both patients, 60-year-old woman (A and B) and
75-year-old woman (C and D), there is severe atrophy, with
occupation ratio of < 0.4. On sonograms, inner circle represents
supraspinatus muscle, and outer circle represents supraspinatus fossa.
Sagittal sonogram (C) shows marked hyperechogenicity and lost pennate
pattern; these findings confirmed severe fatty infiltration on MR image
(D).
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Fig. 5D —Supraspinatus atrophy and fatty infiltration of different
grades. In both patients, 60-year-old woman (A and B) and
75-year-old woman (C and D), there is severe atrophy, with
occupation ratio of < 0.4. On sonograms, inner circle represents
supraspinatus muscle, and outer circle represents supraspinatus fossa.
Sagittal sonogram (C) shows marked hyperechogenicity and lost pennate
pattern; these findings confirmed severe fatty infiltration on MR image
(D).
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Discussion
Sonography is a widely used and accurate technique in the evaluation of
rotator cuff tendon tears [19,
20]. However, sonography is
sometimes viewed as insufficient compared with MRI for preoperative evaluation
of the rotator cuff because rotator cuff musculature quality, an important
prognostic element, is generally not routinely assessable on sonography
[21,
22].
Two distinct muscle abnormalities may be seen with rotator cuff tears:
atrophy (reduced cross-sectional area of muscle or reduced muscle bulk)
[3,
4,
13–16,
19,
23] and fatty infiltration or
"degeneration" (replacement of muscle fibers by fat)
[2,
6,
8,
14,
24]. Although these two muscle
modifications associated with rotator cuff disease are closely interdependent
[13,
24], they are not perfectly
correlated [25]. The two
phenomena are likely different manifestations of the same disease, although it
is currently not known exactly how the two are related. For this reason,
atrophy and fatty infiltration were evaluated separately in our study.
To our knowledge, only two studies have assessed sonography in the
evaluation of rotator cuff muscle quality. Sofka et al.
[26] used increased
echogenicity and decreased muscle bulk as indicators of muscle atrophy. A
limitation of their study was the lack of a standard of reference. In
addition, the degree of muscle atrophy (mild, moderate, or severe) was not
quantified. Strobel et al.
[17] reported that sonography
is only moderately accurate in grading fatty infiltration of the supraspinatus
and infraspinatus muscles. However, a limitation of their study was the use of
static images for interpretation by the examiners instead of a prospective
examination of patients, as in our study, based on dynamic scanning in
multiple planes, which likely gives a more global and thus more accurate
assessment of muscle echogenicity and bulk. In addition, Strobel and
colleagues used four parameters (visibility of outer contour, of central
tendon, and of pennate pattern, and echogenicity) to assess both atrophy and
fatty infiltration simultaneously, whereas quant ification of the degree of
muscle atrophy (muscle size) was not performed.
Fatty Infiltration
On sonography, fat replacement of skeletal muscles is generally hyperechoic
and is the main cause of hyperechogenicity of skeletal muscle
[27]. Fatty infiltration
should not be regarded as a true degenerative process, but rather as a
rearrangement of the tissue after macroarchitectural changes caused by
musculotendinous retraction
[28]. Rotator cuff muscle
fatty infiltration is considered quite specific for tendon tears
[2]. In the elderly population,
fatty infiltration may occur but is generally not more than mild and generally
homogeneous among different muscles of the shoulder
[23]. Therefore, the use of
the trapezius muscle as a standard of reference, as was done in our study,
appears reasonable.
Our study shows that two sonography features—echogenicity and pennate
pattern— correlate in most cases with the degree of fatty infiltration
on MRI. Using both echogenicity and pennate pattern to evaluate the degree of
fatty infiltration gives the operator increased confidence in grading this
parameter because both of these sonography features are interdependent.
Sonography has an advantage over MRI in that sonography can be used to assess
the internal architecture of muscle and its pennate pattern
[29]. On longitudinal
scanning, the hyperechoic fibroadipose septa of the perimysium surrounding
hypoechoic muscle bundles are responsible for the pennate pattern, appearing
as multiple parallel lines forming oblique angles with the echogenic
myotendinous junction [30].
With increased muscle echogenicity, the sharp contrast between muscle tissue
and its striations becomes less distinct, leading first to the effacement and
then to the disappearance of the pennate pattern. These sonography features
correlate with the MRI appearance of increasing fatty infiltration. Recent
studies have shown the utility of sonography in measuring the angles of the
pennate pattern of muscle
[31]. However, as other
authors have noted, visualization of the pennate angle becomes more difficult
with increasing atrophy
[17].
We found that when muscle has fatty infiltration and shows increased
hyperechogenicity, it can be difficult to distinguish from the surrounding fat
in the supraspinatus fossa with similar echogenicity. Although delineating the
contours of muscle was more challenging in such cases, it became easier with
practice. Muscle contours were more easily assessed on sonography first in the
coronal plane.
By grading echogenicity as either mild or marked and the pennate pattern as
either effaced or lost, we could not distinguish moderate from severe fatty
infiltration using sonography because marked hyperechogenicity and a lost
pennate pattern correlated with both moderate and severe fatty infiltration on
MRI. This distinction between moderate and severe infiltration may not be as
important clinically as distinguishing mild infiltration from moderate or
severe infiltration because many studies state that the likelihood of a
recurrent tear is the same when fatty infiltration is greater than mild, or
Goutallier grade 2
[6–8].
A sonography grading system for the evaluation of supraspinatus muscle
fatty infiltration may be used as follows: In the absence of fatty
infiltration (Goutallier grade 0), echogenicity is normal and the pennate
pattern is preserved. With mild fatty infiltration (Goutallier grades 1 and
2), there is mild hyperechogenicity and an effaced pennate pattern. With
moderate to severe fatty infiltration (Goutallier grades 3 and 4), there is
marked hyperechogenicity and an absent pennate pattern. Fatty infiltration and
atrophy should be assessed independently during sonographic evaluation of the
supraspinatus muscle at shoulder evaluation for rotator cuff disease.
Because MRI has replaced CT and CT–arthrography for the assessment
rotator cuff disease, it was not practical for our study to correlate
sonography with CT. Given that there is no standardized MRI grading system for
the assessment of fatty infiltration, we adapted the Goutallier system. One
study found that the correlation between MRI and CT was not satisfactory
[24]. Among the possible
reasons hypothesized were the use of different planes (sagittal for MRI and
transverse for CT) and the inability of CT to distinguish between fibrous
tissue and muscle. However, interobserver variability using each technique
individually was found to be good to excellent.
Muscle Atrophy
Several methods have been proposed to evaluate muscle bulk on MRI
[16,
25], although the method of
Thomazeau et al. [18] is
probably the most widely used. Because the Y view in this method uses osseous
(scapular) landmarks, we found that it could be easily reproduced on a
sonography study. We first showed on MRI that it is as accurate to use the
ellipse (modified Thomazeau method) as it is to draw lines as originally
described by Thomazeau et al.
[18]. Our study shows that
there is excellent correlation between the occupation ratios measured on MRI
and on sonography using the modified Thomazeau method. The task of measuring
cross-sectional areas is made easier today by the availability of elliptic
measuring tools on PACS consoles for MRI and sonography images and directly on
sonography units. We chose to use the ellipse around each area of muscle and
fossa because this method can be applied reasonably quickly in a busy clinical
practice. We may conclude therefore that the Thomazeau classification of
supraspinatus muscle atrophy can be transposed to sonography—that is, an
occupation ratio of > 0.60 is stage I (normal to mild atrophy);
0.40–0.60, stage II (moderate atrophy); < 0.40, stage I (severe
atrophy).
A limitation of our study is that reproducibility was not tested; it would
have been impractical and difficult and to justify having each patient return
for a second sonography examination. Another potential limitation is that an
experienced musculoskeletal radiologist performed the sonography examinations.
However, there is a learning curve with all musculoskeletal sonography, and we
believe that this learning curve applies to measuring the occupation ratio of
the supraspinatus muscle and classifying echogenicity and the appearance of
the pennate pattern.
In conclusion, our study suggests that there is good correlation between
sonography and MRI for the assessment of both supraspinatus muscle atrophy and
fatty infiltration.
Acknowledgments
We thank Assia Belblidia for her assistance.
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Abstract
Introduction
