Radiology An overview of radiological investigations used to identify high risk foot pathologies

What radiology is available  Convential radiography (X-ray)  Sonography (US)  Computed tomography (CT)  Magnetic resonance imaging (MRI)  Nuclear medicine imaging

 X-ray first line modality, provides overview of anatomy and pathology of bone  Ultrasound more useful for diagnosis of fluid collections, periosteal involvement and surrounding soft tissue abnormalities  CT provides a method to detect early osseous erosion and presence of sequestrum, foreign body, or gas formation. CT is however less sensitive than other modalities in identifying osteomyelitis  MRI The most sensitive and specific imaging modality for osteomyelitis providing more accurate information regarding the extent of the infection process the bone and soft tissues involved.  Nuclear medicine imaging Useful in identifying multifocal osseous involvement.

Diagnosis of osteomyelitis and Charcot Osteoarthropathy  Diagnosis of these high risk foot pathologies is primarily through clinical diagnosis  Imaging modalities are essential to confirm the assumed diagnosis along with further detail as to the extent and exact location of the pathological process  Imaging is also very helpful to medical planning and if needed surgical interventions

Plain radiographs  Usually first line imaging for patients suspected of osteomyelitis or Charcot  However there is a delay present from initial infection to evidence on plain radiograph  first changes evident on radiograph indicating the infectious process has been ongoing for 2 to 3 weeks or possibly more  Generally osteomyelitis must extend at least 1 cm and compromise 30 to 50% of the bone mineral content to produce and noticeable changes on plain radiograph.

Early changes for osteomyelitis on plain radiograph  Include  Periosteal thickening  Lytic lesions  Endosteal scalloping  Osteopenia  Loss of trabecular architecture  New bone apposition

Periosteal thickening

Lytic lesions

Endosteal scalloping  Endosteal scalloping refers to the focal resorption of the inner margin of cortical bones,  This is usually seen in long bones due to slow growing medullary lesions.

Endosteal scalloping

Osteopenia  Is where the protein and mineral content of bone tissue is reduced

Osteopenia

Loss of trabecular architecture

New bone apposition

Other signs of osteomyelitis on plain radiography  Single or multiple radiolucent abscesses during subacute or chronic stages  Brodie abcesses are another sign of osteomyelitis however is more commonly found with children  A distinguishing sign of chronic osteomyelitis is necrotic bone, this can take plain radiographs many weeks to identify  Periostitis, involucrum formation and sinus tracts are due to subperiosteal abscess with lifting of the periosteum, new bone formation and soft tissue fistulas. All of these findings are in line with a longer duration of pathology

 Plain radiographs in identifying osteomyelitis have been found to be more specific than sensitive and as such additional imaging modalities such as MRI maybe required

Limitations to plain radiography  Bone marrow oedema (one of the earliest pathological features of acute osteomyelitis) is not present on plain radiography  The early signs of osteomyelitis that can be seen on plain radiography can also be seen in other pathologies such as stress fractures, bone tumours or soft tissue infections

 Plain radiography should still be considered first line imaging in osteomyelitis due to the ability to exclude some differential diagnosis  And provides a really important base line measurement to assess progression of infection with comparison between future plain radiography an important clinical tool

Ultrasound Can detect acute osteomyelitis several days earlier than plain radiographs with acute osteomyelitis recognised through an elevation of the periosteum by a layer of purulent material. In chronic osteomyelitis ultrasound can assess the surrounding soft tissue for abscesses which may surround any bony contours. Cortical erosions can also be identified through ultrasound.

Advantages of Ultrasound  Readily accessible  Performed quickly with minimal discomfort  Useful in those patients whom MRI is contraindicated  Has a lower cost when compared to MRI  Does not use ionizing radiation  Real time imaging  Can identify site and extent of infection  Can identify possible causative factors such as foreign body

Ultrasound summary  Ultrasound is limited through dependence on skill of operator, and can not beam across cortical bone  The main findings from ultrasound in identifing osteomyelitis is elevated periosteum, soft tissue abscess and any fluid collection  Ultrasound can also help guide any possible aspiration or biopsy

CT (Computed Tomography)  Is really good for multiplanar reconstructions of axial images  In chronic osteomyelitis CT shows  Abnormal thickening of the affected cortical bone  Sclerotic changes  Encroachment of the medullary cavity and  Chronic draining sinus

Although CT is less desirable than MRI this is due to decreased contrast with the soft tissue along with the exposure to ionizing radiation. The major role of CT in identifying osteomyelitis is the detection of sequestra (necrotic bone) it can be masked by the surrounding osseous abnormalities in plain film radiology

 The presence of sequestra identifies the presence of an infectious process  This detection can aid in the therapeutic options implemented  CT is superior to MRI for the identification of sequestra, cloacas, involucra or intraosseous gas  Sequestrum is the necrotic bone embedded in the infected tissue  Involucrum is the new bone laid down by the periosteum that surrounds the sequestra  Cloaca is the opening in the involucrum through which pus and sequestra make their way out

MRI (Magnetic Resonance Imaging)  Allows early detection of osteomyelitis as early as 3 to 5 days after the initial onset  Considered the most usefully imaging modality in evaluating the spread of infection  More sensitive and specific than plain radiography and CT

 The alteration of normal bone marrow is the first sign of ostemyelitis that MRI can detect  The findings found from MRI are generally related to the replacement of marrow fat with water secondary to edema, exudate, hyperemia and bone ischemia

Limitations  Image quality can be decreased through the presence of metallic implants that produce local artifacts  Availability is limited  Expensive  Not tolerated by patients whom are claustrophobic or morbidly obese  May require a referral through GP or specialist

Nuclear Medicine  Technetium-99m-labeled methylene diphosphonate  Gallium-67 citrate  Indium-111-labeled white blood cells

These are highly sensitive but low specificity as such it is difficult to differentiate osteomyelitis from other pathologies Nuclear medicine is more useful when used as an adjunctive imaging modality

So what about charcot????

Plain film radiography  Is the first line imaging modality in the initial onset of charcot  The earliest findings found on plain film radiography is focal demineralization along with flattening of the metararsal heads and subchondral or periartical changes in the midfoot with polyarticular distribution  Plain radiography has a low sensitivity and specificity rates in identifying the early stages of charcot

CT  Early changes of charcot such as bone marrow edema and micro fractures cannot be distinguished through CT, there is no need for CT in acute charcot

MRI  Is the most sensitive imaging modality in identifying charcot  With soft tissue edema, joint effusions, subchondral bone marrow edema of involved joints are most common findings found through MRI in acute charcot

Technetium-99m methylene diphosphonate bone scan  Is useful in identifying localised abnormal bone and has high levels of accuracy  In the presence of increased bone turnover such as infection, trauma or surgery specificity rates decrease  Tumors and degenerative changes can also cause false positive results

Plain film radiography  In chronic charcot, plain film radiography is really important it can show the degree of damage and identify any areas of bony prominence that could lead to ulceration  Chronic charcot can include pictures of joint distention, destruction, dislocation, disorganization, debris and increased bone density  There can also be pencil in cup appearance when the MPJ is involved  Involvement of the tarsometatarsal joints can also lead to the collapse of the longitudinal arch this can in turn increase the load on the cubic and rocker bottom deformity

CT  Chronic charcot is characterized through bone fragmentation and disorganization of the affected joints and as such is better seen through CT images due the to the three dimensional and multiplane views  This can be particularly useful if surgical intervention is being considered

MRI  Chronic charcot presents as subchondral cycts, bone proliferation, debris, intra-articular bodies, joint deformity with subluxation or dislocation

References  Fatma E, Saziye S, Ali O. Charcot foot in diabetes and an update on imaging, Diabetic Foot & Ankle 2013,4:21884  Yu Jin Lee et al. Imaging of Osteomyelitis, Quant Imaging Med Surg 2016,6(2):  Carlos Pineda et al. Radiographic Imaging in Osteomyelitis: The Role of Plain Radiography, Computed Tomography, Ultrasonography, Magnetic Resonance and Scintigraphy, Seminars in Plastic Surgery 2009,23:80-89