Download Categorical Course: MR imaging of infection and

Categorical Course: MR imaging of infection and autoimmune disease
Bone Infection (osteomyelitis & infectious spondylodiscitis)
A Mark Davies, MRI Centre, Royal Orthopaedic Hospital, Birmingham, UK
MR imaging is now firmly established as the imaging technique of choice for detecting
and determining the precise site and extent of bone infections be they in the extremities
or spine. Imaging protocols should include a minimum of T1-weighted and fluid sensitive
sequences (fat suppressed T2-weighted or STIR) in at least two orthogonal planes. The
main value of a gadolinium chelate is in distinguishing inflammatory oedema from true
abscess formation that can have implications on whether surgical drainage will be
required. It is well recognized that up to 50% of cases of subacute osteomyelitis,
particularly in children, can be confused with a bone tumour. The diagnosis of
musculoskeletal infection on MR imaging and distinction from tumour requires analysis
of the signal characteristics and the morphological features. Many of the morphological
features can also be assessed on radiographs and CT. The include assessment of the
extent of marrow oedema, growth plate involvement, cortical destruction and the abscess
cavity within bone. If the degree of marrow oedema greatly exceeds the size of the lesion
then this usually indicates a benign aetiology such as osteomyelitis, stress fracture,
osteoid osteoma, and chondroblastoma. In osteomyelitis in children growth plate
involvement is an early feature with a relatively small abscess and florid marrow oedema
extending from the metaphysis into the epiphysis. This contrasts with sarcoma where
growth plate involvement is a rather late feature. There are four components to an intra
osseous abscess (Brodie’s abscess). This comprises a central abscess cavity showing
decreased signal intensity on T1W and increased signal intensity on T2W and STIR
images. There is then an inner granulation lining which appears isointense to muscle on
T1 and hyperintense on T2 and STIR images. Outside this is a sclerotic rim showing
decreased signal intensity on all sequences with more peripherally a halo of oedema. .
The inner granulation layer frequently appears relatively hyperintense with respect to the
central abscess cavity and has been called the Penumbra sign (1). This is a useful sign in
distinguishing subacute osteomyelitis from sarcoma. Another useful sign recently
described is the marrow fat sign (2). In acute osteomyelitis the septic necrosis within the
medulla may lead to death of the adipocytes and the congregation of freed lipid. This can
be seen on MR imaging as globules of fat within both the medulla and the extra osseous
soft tissues if there is a cortical breach. Despite the undoubted value of MR imaging in
bone infection biopsy is mandatory to identify the precise causative organism.
(1) Davies AM, Grimer RJ (2005) Tips & Tricks in Radiology: The penumbra sign in
subacute osteomyelitis. European Radiology 15:1268-1270
(2) Davies AM, Hughes DE, Grimer RJ (2005) Intra- and extra-medullary fat globules
on MR imaging as a diagnostic sign for osteomyelitis. European Radiology 15:2194-2199
Categorical Course: MR imaging of infection and autoimmune disease
Bone Infection (osteomyelitis & infectious spondylodiscitis)
A Mark Davies, MRI Centre, Royal Orthopaedic Hospital, Birmingham, UK
The following are typical on MR imaging of subacute osteomyelitis in children
a. Extensive marrow oedema
TRUE
b. Diaphyseal location most common
FALSE
c. Early growth plate (physeal) involvement
TRUE
d. Normal MR imaging findings at presentation
FALSE
e. Approximately 50% cases may be mistaken for a bone sarcoma
TRUE