a 28-year - old man presented with swelling and redness on the right inguinal and buttock area of two weeks duration . initial laboratory findings showed elevated esr ( 66 mm / hr ) and crp ( 15.4 mg / dl ) levels . mri was performed to identify the infection focus and guide treatment planning , and showed a thin - walled fluid collection between the obturator internus tendon and the posterior surface of the ischium ( fig . however , diffuse enhancement of soft tissue was noted in the right inguinal area , the lesser pelvis , and along the sciatic nerve . diffuse pelvic infection with reactive bursitis of an obturator internus bursa was diagnosed based on clinical and mr imaging features . after eight days on antibiotics , his laboratory findings returned to the normal range , and follow - up mr 12 days later showed complete disappearance of the fluid pocket and of the soft tissue enhancement in the right pelvic area ( fig . a 33-year - old man was presented with a right buttock pain of two days duration , and had been febrile for two days , though this had been well controlled by antipyretics . a physical examination revealed tenderness , swelling , and local heating on the posterior region of the right buttock and paresthesia on the posterolateral aspect of his right leg . pyriformis and patrick tests were positive , and laboratory findings showed elevated esr ( 27 mm / hr ) and crp ( 12.6 mg / dl ) with mild leukocytosis ( 10,600/ui ) . mri was performed under the impression of septic arthritis of the hip , and revealed a boomerang - shaped fluid collection over the posterior surface of the ischium , and diffuse enhancement of the obturator internus tendon and inferior gemellus muscle ( figs . an accompanying multidetector ct ( mdct ) scan showed calcification along the obturator internus tendon ( fig . a 28-year - old man presented with swelling and redness on the right inguinal and buttock area of two weeks duration . initial laboratory findings showed elevated esr ( 66 mm / hr ) and crp ( 15.4 mg / dl ) levels . mri was performed to identify the infection focus and guide treatment planning , and showed a thin - walled fluid collection between the obturator internus tendon and the posterior surface of the ischium ( fig . however , diffuse enhancement of soft tissue was noted in the right inguinal area , the lesser pelvis , and along the sciatic nerve . diffuse pelvic infection with reactive bursitis of an obturator internus bursa was diagnosed based on clinical and mr imaging features . after eight days on antibiotics , his laboratory findings returned to the normal range , and follow - up mr 12 days later showed complete disappearance of the fluid pocket and of the soft tissue enhancement in the right pelvic area ( fig . a 33-year - old man was presented with a right buttock pain of two days duration , and had been febrile for two days , though this had been well controlled by antipyretics . a physical examination revealed tenderness , swelling , and local heating on the posterior region of the right buttock and paresthesia on the posterolateral aspect of his right leg . pyriformis and patrick tests were positive , and laboratory findings showed elevated esr ( 27 mm / hr ) and crp ( 12.6 mg / dl ) with mild leukocytosis ( 10,600/ui ) . mri was performed under the impression of septic arthritis of the hip , and revealed a boomerang - shaped fluid collection over the posterior surface of the ischium , and diffuse enhancement of the obturator internus tendon and inferior gemellus muscle ( figs . an accompanying multidetector ct ( mdct ) scan showed calcification along the obturator internus tendon ( fig . a bursa is a sac lined with synovial cells that typically forms in an area of tendon friction . recently , the mr imaging features of a number of uncommon bursae , such as , obturator externus bursa , subgluteus medius bursa , and subgluteus minimus bursa , have been described ( 4 , 6 ) . the obturator internus muscle originates widely from the margin of the obturator foramen , obturator membrane , iliac bone , and the base of the ischial spine , and covers most of the lateral pelvic wall ( 7 ) . it then passes through the lesser sciatic foramen , and finally , its fibers converge to form a tendon that inserts into the medial aspect of the greater trochanter of the femur , together with the tendons of the superior and inferior gemelli muscles ( 1 ) . in more detail , the obturator internus tendon makes a right - angled bend over the grooved surface of the ischium , between the ischial spine and tuberosity , and then passes horizontally across the posterior to the hip joint before it inserts into the greater trochanter . moreover , the obturator internus bursa is located between the obturator internus tendon and the grooved surface of the ischium ( 1 ) . a number of pathologic conditions involving the obturator internus muscle , tendon , or bursa have been described in the orthopedic literature ( 8 - 10 ) , although no imaging findings of obturator internus bursitis have been described . in fact , swezey ( 8) commented that obturator internus bursitis has been overlooked as a focus of myofascial irritability with lower back pain . because patients with obturator internus bursitis have vague symptoms , such as , nonspecific myofacial irritability , fever , and buttock swelling , clinical diagnosis is made by direct palpation over the anatomical locus of the obturator internus bursa . when a patient is lying on one side with the affected knee drawn toward the chest , the obturator mternus bursa may be palpated superiorly from the ischial tuberosity . several papers have described the clinical findings of obturator internus muscle abscesses in children , and haveemphasized the use of mri and ct to confirm the diagnosis ( 9 ) . moreover , obturator internus muscle abscesses are frequently mistaken for septic arthritis of the hip joint , because of their similar clinical presentations . the mri findings of obturator internus muscle abscesses are ; muscle swelling and a rim - enhanced fluid pocket in or adjacent to the obturator internus muscle . these abscesses usually have a thick enhancing wall and a larger cavity than infected obturator internus bursa . in our cases , there was no evidence of septic arthritis , i.e. , of joint effusion , synovial enhancement , or bone marrow signal changes in the hip joint . in one case report concerning obturator internus tendinitis , the patient was treated with a ct - guided injection of steroid and local anesthetic into the tendon sheath ( 10 ) . normally , the obturator internus bursa is in a collapsed state , and is only distended and visualized by mri when it is inflamed or infected . fluid collection in the obturator internus bursa is caused by primary bursal infection or is associated with a pelvic infection . in the described cases , we were able to characterize the locations and shapes of obturator internal bursae on mr images . these bursae were visualized as narrow and elongated thin - walled fluid pockets resembling " boomerangs " . the transverse plane provided best visualization , because the bursae were located parallel to the curved portion of the obturator internus tendon posterior to the ischium . furthermore , fat - suppressed gadolinium - enhanced t1-weighted mr images nicely demonstrate these lesions . in our cases , fluids within bursae were not drained . nevertheless , the clinical and radiological improvements achieved by antibiotic treatment confirmed in both cases a diagnosis of obturator internus bursitis . the majority of patients with obturator internus bursitis or obturator internus muscle abscesses have been reported to respond adequately to antibiotics without surgical drainage ( 8 , 9 ) . thus , knowledge of the mri features of obturator internus bursitis can avoid unnecessary surgery . in conclusion , a diagnosis of obturator internus bursitis should be considered when the " boomerang "- shaped fluid collection is observed between the obturator internus tendon and the posterior grooved surface of the ischium on mr images .