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1
Department of Radiology, Henry Ford Hospital, 2799 W. Grand Blvd., Detroit, MI
48202.
2
Department of Radiology, University of Michigan Medical Center, 1500 E.
Medical Center Dr., TC-2910G, Ann Arbor, MI 48109-0326.
Received June 20, 2000;
accepted after revision August 14, 2000.
Presented at the annual meeting of the American Roentgen Ray Society, San
Francisco, April-May 1998.
Abstract
 Top Abstract IntroductionMaterials and MethodsResultsDiscussionReferences |
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MATERIALS AND METHODS. The study group consisted of 36 consecutive knees in 36 patients evaluated with both MR imaging and sonography. Inclusion criteria included axial proton density—weighted or T2-weighted MR images, a sonography report that documented the evaluation of the popliteal region of the knee, and sonographic and MR images that were available for review. The MR images were retrospectively reviewed for the presence of Baker's cyst (fluid signal between the semimembranosus and medial gastrocnemius tendons). Sonography reports were compared with the MR imaging results. The sonographic images were also retrospectively reviewed to determine whether any characteristic findings on sonography were significantly associated with the presence of Baker's cyst on MR imaging.
RESULTS. Retrospective review of MR images revealed 21 Baker's cysts, one myxoid liposarcoma, one meniscal cyst, and 13 examinations with normal findings. The sonography reports revealed that the 21 Baker's cysts were correctly diagnosed, whereas the meniscal cyst and myxoid liposarcoma were misdiagnosed as Baker's cysts. Retrospective review of sonographic images showed a 100% sensitivity, specificity, positive predictive value, negative predictive value, and accuracy in the diagnosis of Baker's cyst when hypoechoic or anechoic fluid was present between the semimembranosus and medial gastrocnemius tendons. No other sonographic characteristics were significant.
CONCLUSION. Identification of fluid between the semimembranosus and medial gastrocnemius tendons in communication with a posterior knee cyst indicates Baker's cyst with 100% accuracy.
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionReferences |
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The diagnosis of Baker's cyst is effectively made with MR imaging because fluid distention of the gastrocnemiosemimembranosus bursa is well depicted on T2-weighted axial MR images [1]. The appearance of Baker's cysts on sonography has been described in the literature [4,5,6,7,8]. Fluid distention of the gastrocnemiosemimembranosus bursa is characteristic on sonography, similar to that on MR imaging [8]. However, studies determining the effectiveness of sonography in the diagnosis of Baker's cysts are limited by small sample sizes, lack of gold standards, and undefined sonographic criteria. In this study, we evaluate the ability of sonography to reveal Baker's cyst using MR imaging as a gold standard. In addition, we determine the sonographic characteristics critical to the diagnosis of Baker's cyst enabling differentiation from other posterior soft-tissue masses or cysts.
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionReferences |
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Retrospective review of the axial T2- or proton density—weighted MR images obtained on a 1.5-T scanner (Signa; General Electric Medical Systems, Milwaukee, WI) was then performed by one of the investigators who was unaware of the sonographic imaging and report results. MR images were evaluated for the presence of a popliteal or Baker's cyst. The criterion for a popliteal cyst was the presence of high-signal-intensity fluid between the medial gastrocnemius and semimembranosus tendons. Any other posterior soft-tissue mass or cyst not representing a Baker's cyst was noted, and patient records were reviewed for any surgical or pathologic follow-up in these cases. The MR imaging results were used as the gold standard for the sonographic data. Because the MR images were analyzed retrospectively, the parameters varied. However, the standard sequences used in this investigation included axial proton density— or T2-weighted fast spin-echo MR images with fat saturation (TR range/TE range, 2200-4000/56-102; 5-mm slice thickness; 0- to 2-mm slice gap; 256 x 192-256 matrix; 1-2 excitations; 16-18 x 16-18 cm field of view).
Sonography Report Analysis
The first part of the study compared the sonography reports with the
retrospective MR imaging results to determine the effectiveness of sonography
for diagnosis of Baker's cyst. Each sonographic examination of the knee
(models HDI 3000 and 5000, Advanced Technology Laboratories, Bothell, WA; or
model 5200, Acoustic Imaging, Phoenix, AZ) was completed by one of five
musculoskeletal radiologists with experience in musculoskeletal sonography
using 5-MHz curvilinear and 7- to 10-MHz linear transducers. It is not known
what information or correlative imaging studies were available at the time of
the sonographic examinations. The sonography report stated either that a
Baker's cyst was present or absent or, alternatively, that the sonographic
evaluation of the posterior knee showed unremarkable or normal findings. The
sonography reports were then classified on the basis of this information as
positive or negative for Baker's cysts. These data were compared with the
retrospective MR imaging data for calculation of sensitivity, specificity,
positive predictive value, negative predictive value, and accuracy.
Retrospective Sonographic Image Analysis
For the second part of the study, the Baker's cysts and other posterior
soft-tissue masses or cysts were retrospectively characterized on sonography
to determine whether any differentiating features existed. Establishing a
specific diagnosis was not included in this retrospective portion of the study
because this task would be limited without the benefit of real-time imaging
and by relying on static images from the sonographic study. Inclusion criteria
for this aspect of the study included a Baker's cyst or other posterior
soft-tissue mass or cyst present on MR imaging and axial sonographic images of
the medial popliteal region. If the medial gastrocnemius and semimembranosus
tendons were not visible on the axial sonographic images
(Fig. 1), the case was deemed
inadequate and discarded for the purpose of this retrospective part of the
study. This resulted in 19 cases. A retrospective review of the sonographic
images was then completed by a fellowship-trained musculoskeletal radiologist
with experience in musculoskeletal sonography. This individual was aware of
the project design but was unaware of the MR imaging findings and the
prospective sonography report results.
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All available sonographic images were assessed for anechoic or hypoechoic fluid extending posteriorly between the semimembranosus and medial gastrocnemius tendons (Figs. 2A,2B and 3A,3B) and for the presence of any soft-tissue mass or cyst of the posterior aspect of the knee. If an abnormal soft-tissue mass or cyst in the popliteal region was identified on the sonographic images, size (in centimeters), echogenicity (anechoic, hypoechoic, hyperechoic, or mixed echogenicity relative to muscle echogenicity), and internal septations (Fig. 3A,3B) were recorded. Evidence of rupture of a cystic abnormality (anechoic or hypoechoic area surrounding the mass or cyst) (Fig. 4A,4B) and intracystic intraarticular body (focal intrinsic echogenic object) (Fig. 5A,5B) were also recorded. The volume of the cyst was estimated by multiplying the three dimensions (anteroposterior, transverse, sagittal) if visible on the retrospective images. It was then determined whether there was a statistically significant association between any of the retrospective sonographic data and the MR imaging results using the Fisher's exact test. Sensitivity, specificity, positive predictive value, negative predictive value, and accuracy were also calculated for each of the sonographic characteristics.
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Sonographic Technique
Although the technique for Baker's cyst evaluation on sonography varied
among each of the five prospective examiners, a general description of
sonographic technique can be made. Examination began at the level of the mid
calf in the transverse plane for visualization of the medial gastrocnemius,
lateral gastrocnemius, and soleus muscles in the midline. After the most
medial aspect of the medial gastrocnemius muscle was identified, imaging in
the transverse plane was continued superiorly to the level of the knee joint.
Visualization of the semi-membranosus tendon medial to the gastrocnemius
tendon indicates the location where Baker's cysts are found (Figs.
1,2A,2B,3A,3B).
Because the medial gastrocnemius and semimembranosus tendons are not
completely parallel to each other, one of the normally hyperechoic tendons may
appear artifactually hypoechoic because of anisotropy
(Fig. 1). This occurs when a
tendon is not imaged perpendicular to the ultrasound beam
[9]. It is important not to
mistake this artifactual hypoechogenicity for hypoechoic fluid. Angling the
transducer in the cephalad and caudal directions will eliminate anisotropy and
allow the correct identification of the tendon. Imaging of a Baker's cyst in
the sagittal plane is then completed to determine its cephalocaudal extent and
to evaluate its inferior margin for rupture (Fig.
4A,4B).
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Retrospective analysis of the 36 MR imaging examinations of the knee revealed 21 patients with Baker's cysts (58%) and 15 patients without Baker's cysts (42%). Of these 15 patients, 13 had no soft-tissue mass or cyst in the popliteal fossa. The two remaining patients had posterior cystic abnormalities proved by surgery to represent a myxoid liposarcoma and a meniscal cyst with coexisting meniscal tear.
Sonography Report Analysis
Evaluation of the sonography reports
(Table 1) revealed that all 23
of the posterior soft-tissue masses or cysts shown on MR imaging were
identified on sonography. In the diagnosis of a posterior soft-tissue mass or
cyst (including a Baker's cyst) versus a normal popliteal region, sensitivity,
specificity, positive predictive value, negative predictive value, and
accuracy equaled 100%. All 21 Baker's cysts revealed on MR imaging were
correctly diagnosed on sonography. However, the two other posterior
soft-tissue masses or cysts (one myxoid liposarcoma and one meniscal cyst)
were misdiagnosed as Baker's cysts. In regard to the myxoid liposarcoma,
sonography was performed before the MR imaging examination (47-day interval).
In regard to the meniscal cyst, sonography was performed after the MR imaging
examination (27-day interval). The sonographic diagnosis was correct in the 13
cases without posterior soft-tissue abnormality or Baker's cyst. In the
diagnosis of Baker's cyst versus other findings (normal or other posterior
soft-tissue mass or cyst), this resulted in an overall sensitivity of 100%,
specificity of 87%, positive predictive value of 91%, negative predictive
value of 100%, and accuracy of 94% when comparing the prospective sonography
reports with the MR images.
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Retrospective Sonographic Image Analysis
Regarding the retrospective review of the sonographic images, four of the
23 cases representing Baker's cysts or other posterior soft-tissue mass or
cyst on MR imaging were discarded because the semimembranosus and medial
gastrocnemius tendons could not be identified on the sonographic images. Of
the remaining 19 cases, retrospective review of the sonographic images
revealed that all 19 of the posterior soft-tissue masses or cysts visible on
MR imaging were identified on the sonographic images. The 19 posterior
soft-tissue masses or cysts corresponded to 17 Baker's cysts, one meniscal
cyst, and one myxoid liposarcoma.
Of the 17 Baker's cysts, anechoic or hypoechoic fluid was identified between the semimembranosus and medial gastrocnemius tendons in all 17 (100%) (Table 2). This fluid communicated with the posterior cystic abnormality, creating a crescent shape that was concave toward midline as it wrapped around the medial gastrocnemius muscle and tendon (Figs. 2A,2B and 3A,3B). Of the two other posterior soft-tissue masses or cysts representing myxoid liposarcoma (Fig. 6A,6B,6C) and meniscal cyst (Fig. 7A,7B,7C), no fluid was identified between the semimembranosus and medial gastrocnemius tendons. There was a significant association (p = 0.006) between the presence of fluid between the semimembranosus and medial gastrocnemius tendons on sonography and the presence of Baker's cyst on MR imaging. This finding of fluid between the semimembranosus and medial gastrocnemius tendons on sonography resulted in a sensitivity, specificity, positive predictive value, negative predictive value, and accuracy of 100% in differentiating Baker's cyst from other posterior soft-tissue masses or cysts.
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In regard to other sonographic characteristics of the 19 soft-tissue masses (Table 2), 10 (59%) of the 17 Baker's cysts were anechoic, four (23%) were hypoechoic, three (18%) were of mixed echogenicity, and none (0%) were hyperechoic. The one meniscal cyst appeared to be of mixed echogenicity, whereas the one myxoid liposarcoma appeared hypoechoic. The average size of the Baker's cysts was 10 cm3 (range, 5.8-48 cm3), whereas the myxoid liposarcoma measured 22.9 cm3. Septations were identified in eight (47%) of the 17 Baker's cysts (Fig. 3A,3B) and in both the meniscal cyst and myxoid liposarcoma. An intraarticular body was present in one of the 17 Baker's cysts (Fig. 5A,5B). Evidence for rupture or leak (Fig. 4A,4B) was seen in three (18%) of the 17 Baker's cysts. Overall, besides fluid between the semimembranosus and medial gastrocnemius tendons, there were no sonographic characteristics significantly associated with the finding of Baker's cyst on MR imaging using the Fisher's exact test.
The sonographic features of Baker's cyst were then analyzed to determine their ability to indicate a diagnosis of Baker's cyst versus other soft-tissue masses or cysts. When the soft-tissue mass or cyst appeared anechoic, sensitivity was 59%, specificity was 100%, positive predictive value was 100%, negative predictive value was 22%, and accuracy was 63% in the diagnosis of Baker's cyst. If the soft-tissue mass or cyst appeared hypoechoic, sensitivity was 24%, specificity was 50%, positive predictive value was 80%, negative predictive value was 7%, and accuracy was 26%. If the soft-tissue mass or cyst appeared to be of mixed echogenicity, sensitivity was 18%, specificity was 50%, positive predictive value was 75%, negative predictive value was 7%, and accuracy was 21% in the diagnosis of Baker's cyst. The presence of septations resulted in a sensitivity of 47%, specificity of 0%, positive predictive value of 80%, negative predictive value of 0%, and accuracy of 42%. The presence of an intraarticular body resulted in sensitivity of 6%, specificity of 100%, positive predictive value of 100%, negative predictive value of 11%, and accuracy of 16%. If the soft-tissue mass or cyst appeared as a rupture or leak, sensitivity was 21%, specificity was 100%, positive predictive value was 100%, negative predictive value was 13%, and accuracy was 26% in the diagnosis of Baker's cyst.
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The gastrocnemiosemimembranosus bursa is a composite of two bursae [2]. The subgastrocnemius bursa between the medial gastrocnemius tendon and medial femoral condyle is the point of communication with the posterior joint capsule [2]. The posterior extension of a Baker's cyst represents the second bursa between the medial gastrocnemius tendon and semimembranosus tendon that commonly communicates with the subgastrocnemius bursa [2]. Both sonography and MR imaging depict these characteristic features of a Baker's cyst (Figs. 2A,2B and 3A,3B).
Retrospective review of the sonographic images showed that Baker's cysts extend posterior between the medial gastrocnemius and semimembranosus tendons in a crescent-shaped configuration (Figs. 2A,2B and 3A,3B). Anechoic or hypoechoic fluid can be identified wrapping around the medial gastrocnemius tendon in the axial plane concave toward midline. The presence of fluid between the semimembranosus and medial gastrocnemius tendons was the most helpful sonographic feature in the diagnosis of Baker's cyst. In the presence of a posterior soft-tissue knee mass or cyst, identification of fluid between the semimembranosus and medial gastrocnemius tendons communicating with the soft-tissue mass or cyst indicated Baker's cyst as the cause, excluding other soft-tissue masses or cysts (100% sensitivity, specificity, positive predictive value, negative predictive value, and accuracy).
Regarding the other sonographic features of Baker's cysts, 59% were anechoic, 23% were hypoechoic, and 18% showed mixed echogenicity relative to muscle. Although the cause of increased echogenicity within a Baker's cyst was not proven in this study, possibilities include hemorrhage and thickened synovium. Overall, we found no significant difference between Baker's cyst and other posterior soft-tissue knee mass or cyst on the basis of echogenicity or size or the presence of septations, intraarticular body, or rupture. However, a purely anechoic cystic structure was associated with 100% specificity and 100% positive predictive value in the diagnosis of Baker's cyst versus mass. In addition, the presence of an intraarticular body (Fig. 5A,5B) or cyst rupture (Fig. 4A,4B) revealed a similar 100% specificity and 100% positive predictive value. Given the low sensitivities of these sonographic characteristics, the finding of fluid between the semimembranosus and medial gastrocnemius tendons remains the most useful sonographic feature in the diagnosis of Baker's cysts with a sensitivity of 100%. Of note, the myxoid liposarcoma (Fig. 6A,6B,6C) was hypoechoic with septations, and the meniscal cyst (Fig. 7A,7B,7C) was of mixed echogenicity with septations. Neither showed communication with fluid between the semimembranosus and medial gastrocnemius tendons.
Comparison of the sonography reports and the MR images revealed that 23 of the 23 posterior soft-tissue masses or cysts were identified on sonography. Although 21 of the Baker's cysts were correctly diagnosed, two of the other soft-tissue masses or cysts, representing a myxoid liposarcoma and a meniscal cyst, were misdiagnosed as Baker's cysts on sonography. Retrospective review of the sonographic images in these two cases revealed that anechoic or hypoechoic fluid was not shown between the semimembranosus and medial gastrocnemius tendons. It is unclear if the examiner misinterpreted that fluid was present between the semimembranosus and medial gastrocnemius tendons or if the examiner did not even assess the fluid in this region. The results of this study show that noting the absence of fluid between the semimembranosus and medial gastrocnemius tendons would have indicated that the cystic masses were not Baker's cysts.
The reported incidence of Baker's cysts in the general population ranges from 10% to 41% [3]. The incidence of Baker's cysts in our study group is 58%. This finding may relate to a possible higher incidence of internal derangement in our patient population. This finding could also be associated with a higher threshold of the referring physicians in ordering MR imaging examinations, thus increasing the pretest probability of internal derangement.
Limitations of this study include inherent operator dependence in the acquisition of the sonographic data and images. Kappa statistics for interobserver variability were not calculated. Limitations also exist in the retrospective interpretation of sonographic images. The low number of soft-tissue masses or cysts not representing Baker's cysts may limit generalization of these results to larger populations. In addition, MR imaging was used as a gold standard and pathologic proof was not present in the diagnosis of Baker's cysts. Lastly, neither color nor power Doppler imaging was used in this study, so the ability of Doppler imaging to differentiate Baker's cyst from other abnormalities could not be assessed.
The results of this study show the effectiveness of sonography in the diagnosis of Baker's cysts. Analysis of the sonography reports revealed that all the posterior soft-tissue masses or cysts were depicted on sonography. Retrospective review of the sonographic images showed that when a cystic posterior soft-tissue mass is identified on sonography, visualization of communicating anechoic or hypoechoic fluid between the semimembranosus and medial gastrocnemius tendons indicates Baker's cyst and excludes other cystic soft-tissue masses or cysts such as tumor or meniscal cyst. This distinction is especially important when a complicated Baker's cyst appears heterogeneous and simulates a soft-tissue neoplasm.
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