Oblique coronal images
are prescribed from the axial localizers. These images
are prescribed parallel to the long axis of the scapula.
Coverage should be from the coracoid process anteriorly
to the level of the quadrilateral space, posteriorly. If
the quadrilateral space is difficult for the
technologist to localize, the most posterior image
should be in the deltoid in order to ensure adequate
posterior coverage. The infraspinatus, supraspinatus and
teres minor muscles and tendons are best visualized in
the oblique coronal plane, whereas the subscapularis is
best seen on the axial images. It is important to
prescribe slices sufficiently posterior to the humeral
head on the oblique coronal images in order to obtain
optimal evaluation of the teres minor muscle and tendon.
The teres minor muscle is smaller than the
infraspinatus, and it is possible to miss the tendon
completely while still visualizing a portion of the
infraspinatus.
In patients with chronic cuff
disease, hypertrophy of the teres minor may compensate
for weakness and tears of the supraspinatus and
infraspinatus muscles, providing important information
for the referring clinician. Images are acquired with no
interslice gap, so as to minimize partial volume effects
when evaluating the long axis of the cuff tendons. This
is particularly important for the oblique coronal and
axial series.
Coronal
Oblique PD FSE
Coronal
Oblique T2 FSE FS*
TR/TE
3500-4000/36
3000/4000/60
ETL
8-12
10-15
Flip Angle
—
—
RBW
31.25
20.8
FOV
15cm
16cm
Matrix
512 x 384
256 x 224
Slice Thickness
3mm
3.5mm
Interslice Gap
0mm
0.5mm
NEX
2
2
Acquisition Time
5-6min
2-4min
Fat Sat
no
yes
Tailored RF
no
no
No Phase Wrap
yes
yes
Flow Compensation
no
no
Sat
left
left
Freq
S/I
S/I
The use of a larger FOV provides visualization of more regional anatomy, allowing assessment of the acromioclavicular joint and the axilla for occult soft tissue masses. A TEeff ranging between 30-36ms is used to maximize contrast between fluid, fibrocartilage, and articular cartilage. With this TEeff, the fibrocartilage of the labrum appears hypointense, as do the glenohumeral ligaments in their normal state. Articular cartilage is of more intermediate signal intensity, in contrast with the high signal intensity seen in the adjacent fluid. It should be noted that fluid is relatively bright on this pulse sequence despite a modest echo time due to the magnetization transfer effect inherent to the fast spin echo technique. An additional "bonus" of this pulse sequence is that it allows for effective evaluation of humeral cartilage on the oblique coronal images, and the glenoid cartilage on the axial sequences. Setting RBW to 31.25kHz for the ETLs shown in the above table should result in a TEeff in the desired range; however, TEeff should be verified immediately before the scan is initiated.
The repetition time for the higher resolution pulse sequences without fat suppression ranges between TR = 3500-5000ms, depending upon the number of slices required to obtain adequate coverage. Rarely, the axial images may require a TR of 5500ms. Minimizing the TR to save on scan time is NOT recommended; decreasing the TR to less than 3500ms will significantly affect tissue contrast and scan quality. Scan time can be minimized effectively by maximizing the ETL.
The latter diagnosis is confirmed by the presence of a "hole" in the tendon, which would be seen at arthroscopy. (Fig. 4.4, 4.5) The conventional approach, i.e. identifying rotator cuff tears by relying on "increased signal on both short and long TE pulse sequences" is more susceptible to imaging artifacts such as the Magic Angle phenomenon which may be seen at the anterior margin of the supraspinatus on short TE sequences.2
At HSS, high spatial resolution imaging for evaluation of the rotator cuff is emphasized. This approach places more diagnostic importance on morphologic changes of the tendon than on signal intensity changes alone. Individual oblique coronal images are assessed not just for areas of increased signal, but also for morphologic abnormalities such as fraying of the tendon (Fig. 4.3), or a frank partial thickness defect on either the articular or bursal side.
Figure 4.3 Oblique coronal FSE technique discloses intrasubstance fraying of the supraspinatus tendon without a discrete partial thickness defect on either of the articular or bursal sides.
The latter diagnosis is confirmed by the presence of a "hole" in the tendon, which would be seen at arthroscopy. (Fig. 4.4, 4.5) The conventional approach, i.e. identifying rotator cuff tears by relying on "increased signal on both short and long TE pulse sequences" is more susceptible to imaging artifacts such as the Magic Angle phenomenon which may be seen at the anterior margin of the supraspinatus on short TE sequences.2
Figure 4.4 High grade partial thickness tear articular side supraspinatus
tendon. There is retraction of an undersurface slip. Note the signal hyperintensity
in the remaining bursal margin of the cuff, indicating pre-existing cuff
degeneration and tendinosis.
Figure 4.5
Full thickness tear of the supraspinatus tendon with complete retraction is evident.
An additional problem with the conventional approach is that mucoid degeneration and the reparative fibrovascular tissue which occurs following intrasubstance partial tendon tears may account for T2 prolongation and increased signal intensity, without the presence of a defect that can be perceived at arthroscopy. These problems are minimized by focusing the diagnosis on morphologic changes detected on high resolution images.
A series of oblique coronal images with the same coverage as Series #2 is obtained using fat suppression and increased T2 weighting. By rescaling the contrast range, fat suppression allows for definition of small fluid collections in the subcoracoid or subdeltoid bursa, in addition to the normal recesses of the glenohumeral joint, inclusive of the biceps tendon sheath and subscapularis bursa. Small ganglion cysts are also made conspicuous on fat suppression. Due to the effectively high signal of fat displayed on the FSE technique, a cyst may have similar signal intensity to that of fat on images obtained in Series #2. If the radiologist is unsure as to whether a high signal seen in Series #2 is due to the presence of a cyst (such as a ganglion cyst) or fat, this dilemma may be resolved by using the fat suppressed images. SNR on this fat suppressed pulse sequence is comparatively poor. This series should therefore not be used as the primary sequence for determination of cuff integrity.
Extracapsular soft tissue edema, usually denoted in the setting of a capsular rupture or a muscle/tendon junction injury, is an important indicator of joint pathology. If the edema is centered around the axillary pouch and base of the glenohumeral joint, this should alert the radiologist to assess the integrity of the capsulolabral complex on axial images. (Fig. 4.6) Extracapsular soft tissue edema seen more anteriorly, adjacent to the biceps, particularly on the lower aspect of the field of view and close to the humeral shaft, generally denotes a biceps or pectoralis major tendon rupture.
Figure 4.6 (A) Oblique coronal fat suppressed T2-weighted image in a 16 year old patient discloses extracapsular soft tissue edema inferior to the axillary pouch, due to a detachment of the anteroinferior capsule. (B) The site of capsular detachment is noted on the axial high resolution sequence, off the humerus.
2 Erickson SJ, Cox IH, Hyde JS, et al. Effect of tendon orientation on MR imaging signal intensity: A manifestation of the "magic angle" phenomenon. Radiology, 181(2), 389-392, 1991.
