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Diagnostic and Interventional Imaging (2012) 93, 351359

ICONOGRAPHIC REVIEW

Pitfalls in osteoarticular imaging: How to distinguishbone infection from tumour?

T. Mosera,, M. Ehlingerb, M. Chelli Bouazizc,M. Fethi Ladebc, J. Durckeld, J.-C. Doschd

aDepartment ofRadiology, Notre-Dame Hospital, Montreal UniversityHospitals, 1560,

Sherbrooke Est, Montreal, Quebec H2T1H1, Canadab Department ofOrthopaedic SurgeryandTraumatology, Hautepierre Hospital, Strasbourg

UniversityHospitals, avenue Molire, 67098 Strasbourg cedex, Francec Department ofRadiology, Kassab Orthopaedic Institute, KsarSaid2010, Tunisiad Department ofRadiology2, Hautepierre Hospital, Strasbourg UniversityHospitals, avenue

Molire, 67098 Strasbourg cedex, France

KEYWORDSBone infection;Osteomyelitis;

Bone tumour;Sarcoma;MRI

Abstract In this article examining pitfalls in osteoarticular imaging we examine the diffe-rential diagnosis of osteomyelitis from bone tumours. We describe the different features whichdifferentiate these two types of disease in radiology and CT and MRI scanning.

2012 ditions franaises de radiologie. Published by Elsevier Masson SAS. All rights reserved.

The appearances of bone infection and bone tumour are often similar on imaging. Thedifferential diagnosis of osteoarticular infection includes above all malignant tumours butalso some benign, inflammatory tumours (osteoid osteoma osteoblastoma) and pseudo-tumours (eosinophilic granuloma). The clinical and laboratory context does not alwaysdiscriminate between these and the diagnosis is occasionally only obtained on histologicalexamination. This confusion can lead to delays in treatment and inadequate management.

We describe here the imaging appearances of osteomyelitis, specifically those of sub-acute osteomyelitis. It is estimated that 50% of cases in children are initially confused withtumour [1].

Corresponding author.E-mail address: thomas.moser@umontreal.ca (T. Moser).

2211-5684/$ see front matter 2012 ditions franaises de radiologie. Published by Elsevier Masson SAS. All rights reserved.doi:10.1016/j.diii.2012.01.021

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Pitfalls in osteoarticular imaging: How to distinguish bone infection from tumour? 353

Figure 2. Gas present in the bone marrow cavity indicating reac-tivation of chronic osteomyelitis.

Figure 3. Compact periosteal reaction associated with sub-acutetibial osteomyelitis.

periosteal reaction with a Codman spur suggests osteosar-coma or Ewings sarcoma [13].

Sequestrum

Sequestrum is a fragment of dead bone surrounded by gra-nulation tissue and is seen in more than fifty per cent ofcases of chronic osteomyelitis. It appears on radiographs asa dense, often irregular fragment with a surrounding clear

Figure4. Sequestrum appearance in sub-acute tibial osteomyeli-tis. Note also the bone reaction which surrounds the focus ofinfection (involucrum).

zone (Fig. 4). It is important to diagnose as it is firstly verysuggestive of infection and secondly represents a reservoirof organisms, which are not readily accessible to antibiotics,and often requires surgical excision. The sensitivity of thissign to diagnose infection is poor with radiographs but betterfor CT scans [6,7]. It is not entirely specific as it is also seenin eosinophilic granuloma, fibrosarcoma and can occasio-nally be confused with a calcified nidus of osteoid osteoma[14].

Soft tissue abnormalities

The fatty planes, which are visible on radiography are clas-sically abolished by infection and displaced by tumours.Collections and fistulae are invisible on radiography unlessfistulography is performed. The first line investigation toexamine a subperiosteal or juxta-osseous collection is ultra-sound. Bone and soft tissues can be studied in great detailby MRI. Failing this, a CT scan with contrast injection andmulti-planar reconstructions can be used for a rapid urgentpatient assessments [15].

Soft tissues calcifications or ossifications are rare inosteoarticular infection due to the classic pyogenic orga-nisms and are more suggestive of a neoplastic (osteosar-coma) or pseudo-neoplastic (circumscribed myositis ossifi-cans) process. On the other hand it is a characteristic findingin tuberculosis and is also seen in hydatid disease and fungalinfections.

Clinical-radiological features and differentialdiagnosis

Acute osteomyelitis

The diagnosis is suggested from clinical evidence (infection,metaphyseal pain) and laboratory findings (acute inflamma-tory reaction, positive blood cultures). Initial radiography

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is normal. Ultrasound can be used to investigate for sub-periosteal abscesses and guide aspiration puncture [18]. Theinitial changes in bone marrow signal can only be seen on MRI(see below).

Sub-acute osteomyelitis

This is the most difficult situation, which raises the problemof the differential diagnosis from tumour. We propose thatthe Gledhill and Rombouts classification [16,19] be used asthe basis for description.Type 1 refers to the appearancesof a single osteolytic lesion, which may or may not be sur-rounded by a reactive bony reconsolidation line which spares

cortical bone. The most classical form of this is the Brodieabscess (Fig. 5) [16]. This is usually found in the metaphy-sis and may extend to the epiphysis along the epiphysealcartilage. It is usually 1 to 5 cm in diameter and is ovoidor funicular along the long axis of the bone. A periostealreaction or appearances of sequestrum are typically absent.Diaphyseal forms of disease have been described to havefar more variable appearances which may include cortical

thickening or a periosteal reaction and contains sequestrum[17]. The main differential diagnoses are eosinophilic granu-loma and osteoid osteoma (Fig. 6).Type 2 refers to a zone ofmetaphyseal osteolysis with cortical destruction. The main

Figure 5. Brodies abscess of the radial metaphysic (type 1 sub-acute osteomyelitis). Note the epiphyseal extension through the growthcartilage, which is far easier seen on CT.

Figure 6. Sub-acute tibial osteomyelitis (type 3 sub-acute osteomyelitis). Osteoid osteoma and stress fractures can be excluded by a CTscan.

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been achieved from the most recent work. MRI waspreviously performed second line after bone scintigra-phy and is now performed directly after radiographs andultrasound.

MRI signs

Abnormal bone marrow signal

Abnormalities in bone marrow signal are initially due to theoedema, exsudate, the presence of inflammatory cells andbone ischemia and then reflect the formation of collections

and bone healing. The trabecular bone signal abnormalitiesin acute osteomyelitis are homogeneous, poorly delineated,low or normal intensity on T1 and increased intensity on T2(oedema), simultaneously involving the bone marrow cav-ity, cortical bone and the adjacent soft tissues. The signalabnormalities are better delineated, greatly reduced inten-sity on T1 and increased intensity on T2 (collection) insub-acute osteomyelitis, occasionally with reduced intensity

sequestrum on T1 and T2 (Fig. 10). Signal abnormalities inchronic osteomyelitis are heterogeneous and involve a com-bination of reduced intensity T1 and increased intensity T2areas and reduced intensity T1 and T2 areas (sclerosis) [23].

Figure 10. a: weighted T2 MRI appearances of sequestrum and b: periosteal reaction correlated with CT scan findings.

Figure 11. Acute osteomyelitis of the humeral metaphysic (same patient as in Fig. 1). The necrosis and early intra-osseous collection areclearly seen after injection of contrast medium (a). On the T1-weighted sequence (b), fat can be seen in the focus of osteomyelitis.

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Pitfalls in osteoarticular imaging: How to distinguish bone infection from tumour? 357

The periphery of the bone collections can be highlighted byinjecting contrast medium, making them easier to detect(Fig. 11) [24].

The signal abnormalities seen with oedema are a sensi-tive but relatively non-specific sign of osteomyelitis. Somesigns which are variably present are more specific for thediagnosis.

Fat globules (acute osteomyelitis)Acute osteomyelitis causes septic necrosis of the bone mar-row, releasing fat particles which sediment with pus to formfat-fluid levels inside or outside of the bone [12]. The pres-ence of fat in soft tissues (Fig. 12) is believed to be dueto the fat passing through the Haversian canals and is anindirect sign of a breach in cortical bone [25].

Davies et al. [26] have studied the presence of fatin a focus of osteomyelitis. This sign is found in acuteosteomyelitis (Fig. 11) and is due either to persistent normalbone marrow within the oedema or to linear or to globularclumps of necrotic bone marrow.

Penumbra sign (Brodies abscess)The target appearance of the Brodies abscess on MRI wasinitially described by Marti-Bonmati et al. [27]. Four con-centric layers are seen: the centre (pus) which is reduced inintensity on T1 and increased intensity on T2, the internalring (abscess wall) which is of normal intensity on T1 andincreased intensity on T2, the external ring (reactive scle-rosis) which is reduced in intensity on T1 and T2 and theperiphery (bone oedema) which is reduced in intensity onT1 and increased on T2.

The penumbra sign (Fig. 13) described on T1-weightedsequences represents the internal ring which has a rela-tively high density signal compared to the other layers of

the target. Histologically, this represents the granulation tis-sue surrounding all abscess cavities (in bone, soft tissues

or any other organ) and appears to be due to the pres-ence of paramagnetic free radicals produced by activatedmacrophages. The penumbra sign is more apparent whenthe granulation tissue is young and represents direct annu-lar uptake of contrast. It appears to be very specific (99%)for abscess although its sensitivity varies from 27% to 75%depending on the study [28,29].

The double line sign described on T2-weighted sequences

is less useful. It represents the internal ring (increasedintensity) and external ring (reduced intensity). Its visibilitydepends on the window used and offers a sensitivity of only22% [28].

Periosteal reaction and sequestra

The periosteal reaction is often missed on MRI although itcan be seen before ossification and therefore earlier thanon a CT scan. The target appearance on a T2-weightedsequence (Fig. 14) represents the periosteal lamellae sepa-rated by granulation tissue [30]. The sequestra appear asreduced intensities on all sequences.

Soft tissue abnormalities

Sequences with contrast medium injection and abolition ofthe fat signal or useful to delineate collections and fistulatracts within inflammatory changes [31].

Texture sign (soft tissue mass)

It is not always easy to distinguish between inflammatory orneoplastic changes in a soft tissue mass. Classically, inflam-mation is diffuse and crosses the fascias whereas tumourforms a well defined mass which preserves the fascia. Thetexture sign proposed for use in following up treated sar-

comas can also be used. A visible muscular framework in aT1-weighted sequence supports inflammatory changes when

Figure 12. Fat present in soft tissues confirming the diagnosis of sub-acute osteomyelitis in a bone lesion with soft tissue mass. Thepresence of sequestrum (b) is also suggestive.

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Figure 13. Penumbra sign in a Brodies abscess centred on the tibial physis. On the T1-weightedsequences (a), an internal reducedintensity ring is silhouetted by the contents of the abscess on one side by the external ring of sclerosis and on the other by bony oedema.After injection of contrast medium and abolishing the fat signal (b), the internal ring representing the granulation tissue picks up contrastmedium intensely.

Figure 14. Appearances of a periosteal reaction on a T2-weightedMRI sequence. The unilamellar periosteal reaction and reducedintensity bone cortex are separated by increased intensity gra-nulation tissue.

signal abnormalities are present on a T1-weighted sequence[32].

Differential diagnosis by MRI

In the acute phase, the problem that arises with osteomyeli-tis is the differential diagnosis with bone oedema, which canhave many causes (trauma, necrosis, algodystrophy, inflam-matory joint disease, tumour, etc.). We will pay particular

attention to the aggressive presentation of benign inflam-matory bone tumours (osteoid osteoma, osteoblastoma andchondroblastoma), eosinophilic granuloma and stress frac-tures which cause bone and soft tissue oedema that is oftendisproportionate to the size of the lesion [33]. In these situa-tions, careful examination of radiographs is essential and ascan targeted on the abnormal area can occasionally revealsubtle abnormalities (nidus, fracture line, stress reaction).

The differential diagnosis between osteomyelitis andbone oedema as a reaction to neighbouring septic arthri-tis or cellulitis is also difficult. Bone would appear to takeup contrast medium less intensely than soft tissue in cel-lulitis with reactive bony oedema, whereas contrast uptakeappears to be equivalent in osteomyelitis [24]. Bone oedemaas a reaction to septic arthritis appears to be less intense onT1 sequences than in osteomyelitis [34].

An additional approach may be to use a contrastmedia containing supermagnetic iron particles (SPIO) whichshorten the T1 and particularly T2 relaxation time. Afterintravenous injection these particles are taken up by thereticulo-endothelial cells and accumulate in macrophagesand fibroblasts in the inflamed area. These contrast media

could help to delineate inflammatory reactions and distin-guish them from neoplastic disease [35,36].

Disclosure of interest

TM is a consultant for Arthrovision.

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