F-18 Fluorodeoxyglucose positron emission tomography/computed tomography in tuberculosis of the hip: A case report and brief review of literature

INTRODUCTION

Tuberculosis (TB) is an enormous global health problem, which has been compounded by the emergence of new multidrug resistance (MDR) strains. Early diagnosis with institution of appropriate therapy is crucial for the successful treatment of this disease. Cross-sectional imaging modalities like ultrasound, computed tomography and magnetic resonance imaging (MRI) play an important role in the diagnosis, assessment of disease extent and response to treatment. Although they have superb resolution, they rely significantly on morphological changes and therefore fail to detect changes at a molecular level. Positron emission tomography-computed tomography (PET/CT), a unique molecular imaging technique, allows the most accurate correlation of anatomic and metabolic information. The role of PET/CT is gradually expanding to the evaluation of infectious processes as well. We report a case wherein PET/CT played a significant role and contributed valuable information in the evaluation of a patient with TB of the hip.

CASE REPORT

A 41-year-old male was admitted to a hospital 10 months ago with a history of moderate fever, anorexia, difficulty in walking and throbbing pain in the right hip. MRI scan of the right hip joint showed joint effusion with erosions on the articular surface of the acetabular cavity and right iliopsoas abscess. Pus culture revealed acid-fast bacilli. Hip biopsy revealed necrotizing granulomatous inflammation with Langerhans giant cells suggestive of TB. Chest X-ray was unremarkable. He was subsequently started on antitubercular therapy, but was repeatedly found to have pus collections at the hip joint, which were aspirated. Thereafter, he developed a discharging sinus. A repeat MRI scan of the right hip joint revealed extension of the disease in the right acetabulum with involvement of head and neck of femur and a large right iliopsoas abscess extending into the adjacent musculature. Drainage of abscess with debridement and girdlestone arthroplasty was performed, with modification of antitubercular regimen. He responded poorly to the above treatment, owing to extreme drug resistance, and was subsequently referred for a PET/CT scan.

Whole body PET/CECT scan (skull base to mid thigh) was performed on an integrated PET/CT scanner (Discovery STE 16, GE Healthcare) 60 minutes after injection of 10 mCi of F-18 fluorodeoxyglucose (F-18 FDG) following a 6-hour fast. The study revealed an intensely FDG avid organized collection in the right hip joint space, with extension of the inflammatory process into the surrounding muscle and soft tissue [Figures 1 and 2]. A large, relatively photopenic abscess was noted in the right iliopsoas muscle [Figure 3]. Destructive changes in right acetabulum with post-arthroplasty changes were also seen. An FDG avid sinus track extending from the right hip joint was clearly visualized on the MIP image. Additionally, active mediastinal lymphadenopathy was also noted [Figure 1].

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Figure 1 MIP image showing increased FDG uptake at right hip joint and surrounding soft tissue (thick arrows). FDG avid sinus track is well visualized (thin arrows). Mediastinal lymphadenopathy is also noted (dotted arrow)

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Figure 2 CECT (a) and PET/CT fusion images (b) show FDG avid heterogeneously enhancing organized collection at right hip joint (thin arrow) with a sinus track (thick arrow). Acetabular destruction with inflammatory changes in the adjacent muscles (dotted arrow) is also seen

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Figure 3 Right iliopsoas abscess is seen on CECT (a) and PET/CT fusion image (b) as a peripherally enhancing, relatively photopenic area (thin arrow). FDG avid inflammatory changes are visualized in right gluteal muscles (thick arrow). Increased radiotracer uptake is also seen in the left gluteal region owing to injection site granuloma (dotted arrow)

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DISCUSSION

PET/CT has emerged as a powerful tool in cancer imaging. The basic principle involved is the accumulation of the radiopharmaceutical FDG in areas of increased glucose metabolism. However, glucose is an important substrate in inflammatory cells as well. This uptake is further enhanced by a variety of cytokines and growth factors in inflammatory tissue.[1] The role of this modality is thus gradually expanding to the evaluation of infectious processes as well.[2]

There is a high incidence of osteoarticular TB in Asian countries, with vertebral involvement being the most common.[3] TB of the hip joint accounts for about 15% of cases of tubercular osteomyelitis.[4] Although the PET/CT appearances of pulmonary and extrapulmonary TB have been described in the past,[3567] there are limited reports on the role of PET/CT in the evaluation of TB of the hip joint.[8] Increased FDG uptake has been visualized in regions of active granulomatous inflammation in musculoskeletal TB, with cold areas in necrosed tissue containing pus.[9] The same findings were noted in our case too, with highest uptake (SUV 6.7) noted at the hip joint, and relatively low uptake in the region of cold abscess. Additionally, the sinus track extending from the hip joint to the skin surface was well-delineated in the present study – a finding which had not been demonstrated in previous imaging studies on the same patient. The ability of PET/CT to delineate the sinus track without the need for contrast instillation is thus an added advantage of PET/CT and has been described in the past.[10] With the incorporation of contrast-enhanced computed tomography (CECT) in the PET/CT protocol, the complete extent of bone, bone marrow and soft tissue involvement can be delineated. More-over, metabolically active disease can be distinguished from residual fibrotic tissue. In the present case, PET/CT could also demonstrate the presence of active mediastinal lymphadenopathy, which was clinically unsuspected, owing to the lack of localizing signs and symptoms. Thus, as in oncological imaging, PET/CT plays a useful role in determining multiple occult foci of involvement in a single study. It can also serve as a valuable baseline for monitoring response to treatment[11] and providing information on disease spread. It is also a useful tool to guide the site of biopsy or other interventional procedures.

However, the routine utilization of PET/CT cannot be justified owing to its limited availability and high costs. The exact role of FDG PET and PET/CT in TB and other inflammatory diseases is evolving and is not as yet clearly defined. With the development of newer and more specific radiotracers like positron-emitter labeled antituberculous drug molecules in the future, PET/CT may play a significant role in establishing an early diagnosis and effective monitoring of therapeutic response.[3]