CASE 5
By: Dr. M. Salman and Dr. Shankar Raja
HISTORY: Middle age male with history of osteomyelitis since 2011, repeated episodes or recurrence.
Most recent MRI of left lower extremity is shown above, following course of I.V antibiotic therapy!
What are your findings?
What is the differential diagnosis?
What is the diagnosis?
FINDINGS:
Multi-planar MRI images showing intramedullary signal abnormality of left mid distal femur appearing low on T1, high on T2 fat sat sequences with post contrast Gad rim enhancement confined to medullary cavity, background changes of cortical thickening and irregularity without soft tissue component.
DIFFERENTIAL DIAGNOSIS:
Old fracture associated with surgery or fracture alone are difficult to differentiate.
Primary bone neoplasm.
Langerhans cell histiocytosis.
DIAGNOSIS:
Acute on chronic Osteomyelitis.
Follow-Up:
Patient received I/V antibiotic therapy for 3 months; follow up MRI showed minimal interval changes
Query: what other test would you consider evaluating the response to therapy?
Limited FDG PET-CT of bilateral hips and femurs.
What are the findings on FDG PET scan?
Limited FDG_PET of lower extremities shows no evidence of abnormal FDG accumulation corresponding to the hyperintense lesion of T2 FS and CT in LT. Femoral mid shaft. Indicating no active focus of inflammation/infection corresponding to the suspicious focus on MR and CT, and suggestive of either healed osteomyelitis, or good response to antibiotic therapy.
PEARLS AND DISCUSSION:
MRI:
Acute osteomyelitis will appear as abnormal signal intensity marrowT1, high T2 and STIR with or without cortical destruction. Fat-suppression sequences allow for better detection of bone marrow edema; however, infection and inflammation cannot be differentiated.
The disease process involves 5 stages:
Inflammation: This stage represents initial inflammation with vascular congestion and increased intraosseous pressure; obstruction to blood flow occurs with intravascular thrombosis.
Suppuration: Pus within the bones forces its way through the haversian system and forms a subperiosteal abscess in 2-3 days.
Sequestrum: Increased pressure, vascular obstruction, and infective thrombus compromise the periosteal and endosteal blood supply, causing bone necrosis and sequestrum formation in approximately 7 days.
Involucrum: This is new bone formation from the stripped surface of periosteum.
Resolution or progression to complications: With antibiotics and surgical treatment early in the course of the disease, osteomyelitis resolves without any complications.
MRI findings in osteomyelitis usually are related to the replacement of marrow fat with water secondary to edema, exudate, bone ischemia and necrosis.
Sequestrum of cortical bone appears low on T1, T2, and STIR sequences without gadolinium enhancement.
Sequestrum of cancellous bone is hyperintense relative to cortical sequestrum on T1, T2, and STIR images without gadolinium enhancement.
Involucrum is low on all 3 sequences and shows gadolinium enhancement.
Granulation tissue is low on T1, high on T2 and STIR images with gadolinium enhancement.
Draining sinuses and soft tissue inflammation are low on T1, and high on T2 and STIR images; however, it does show gadolinium enhancement.
PET:
MRI is very sensitive for the detection and evaluation of all phases of osteomyelitis including acute, chronic as well as acute on chronic. The sensitivity and specificity of MRI has been reported to be modest to high (92-100% and 89-100% respectively); however, its accuracy drops to 40-60% in traumatized bone (fractures, post surgical and hardware placement).
Furthermore, MRI is limited in evaluating the response to therapy usually showing response in weeks to month. While, emerging data suggests that functional imaging with PET can demonstrate response within one to two weeks as compared to MRI.
FURTHER READING:
1. Carek PJ, Dickerson LM, Sack JL. Diagnosis and management of osteomyelitis. Am Fam Physician. 2001 Jun 15. 63(12):2413-20.
2. Kruskal JB. Can USPIO-enhanced spinal MR imaging help distinguish acute infectious osteomyelitis from chronic infectious and inflammatory processes? Radiology. 2008 Jul. 248(1):1-3.
