MAGNETIC RESONANCE IMAGING Whilst radiography is less sensitive than MR or nuclear medicine for the detection and eval- uation of suspected osteomyelitis in the diabetic foot, it is still the most appropriate initial imaging investigation.
In a complex foot infection, where deformity, previous amputations and neuropathic changes may also be present, plain films will facilitate an MR examination tailored to the individual.
Plain films were found to be essential or useful in the interpretation of up to 75% of musculoskeletal MR imaging studies.23 Many early papers on MR of the diabetic foot claimed 100% accuracy in differentiating infection from neuropathy, as fluid collections in the foot were always deemed abscesses and bone marrow oedema appeared in osteomyelitis but not in neuropathy.
Whilst often helpful, these observations are not pathognomonic.
More recently, it has been appreciated that bone marrow oedema on MR scanning occurs in neu- ropathy as well as infection.24 The plain film appearances may assist in differentiating acute neuropathic osteoarthropathy and osteomyelitis, as bone marrow signal changes on MR in
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Figure 19.10 Destruction of the great toe metatarsal head, with gas within it, indicating osteomyelitis. Periosteal reaction is present along the metatarsal shaft. The great toe phalanges have been amputated and there are also old and healing fractures of the second and third toes
osteoarthropathy can mimic those in osteomyelitis.25 The presence of fluid collections in the soft tissues on MR imaging is, however, highly suggestive of abscess formation, and its relation- ship to bone abnormality may then be very predictive of osteomyelitis, even in the presence of neuropathic changes.26,27 Characteristically, bone adjacent to a neuropathic joint is of low sig- nal intensity on both T1- and T2-weighted MR images (Figure 19.13), whilst marrow oedema is of low signal on T1-weighted but of high signal on T2-weighted images.28 In addition to T1- and T2-weighted images, sequences that suppress the signal from fat are increasingly used. These sequences may demonstrate marrow oedema with greater sensitivity than conventional T1- or T2-weighted sequences (Figure 19.14). Availability of MR also requires the appropriate technical and interpretative skills.29
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Figure 19.11 Gas in the soft tissues beneath the calcaneum, with underlying bone destruction at the base of the fifth metatarsal, indicating osteomyelitis. Note the vascular calcification
MR studies have reconfirmed the common sites of pedal osteomyelitis in the foot (fifth metatarsal, first metatarsal, and first distal phalanx in the forefoot and the calcaneus in the hindfoot), and also the usual presence of immediately adjacent cutaneous ulceration overlying osteomyelitis and the demonstration of associated septic arthritis in one third of patients with advanced pedal infection.30 Whilst the commonest indication for MR in the diabetic foot will be to confirm or exclude osteomyelitis, MR is the anatomic gold standard that can be used to define the extent of the pathological process in the foot,31 an information of value in determining the amputation level where this is necessary.
The frequency of tendon involvement by infection in the diabetic foot has also become more apparent with the greater use of MR imaging, the condition being present in approximately half of the patients who were scanned and then had surgery for pedal infection.32 The MR documentation of tendon involvement influenced the surgical procedure in only 6 out of 159 patients, it being noted that proximal migration of infection along tendon sheaths was rare.
The use of intravenous enhancement with gadolinium chelate allows demonstration of tissue necrosis on MR imaging, as such regions show no enhancement.
This was confirmed at surgery but it was noted that lack of enhancement in these regions could mask the coexistence of abscess and osteomyelitis.33 MR also demonstrates marked atrophy of the intrinsic muscles in neuropathic feet, with a 73% reduction in muscle bulk compared to non-diabetic controls.34
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Figure 19.12 Antibiotic pellets (a) when recently implanted; (b) smaller and irregular in appearance when partially absorbed (arrowhead)
Figure 19.13 Magnetic resonance (MR) images of a neuropathic foot: (a) Sagittal T1-weighted and (b) axial T2-weighted images both show low-signal-intensity bone and marrow spaces adjacent to the ab- normal articulations. (Courtesy of Dr JPR Jenkins, Consultant Radiologist, Manchester Royal Infirmary, United Kingdom)
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Figure 19.14 MR images of osteomyelitis: (a) T1-weighted, with low signal in the marrow space of the fifth metatarsal; (b) fat suppressed images post-contrast T1-weighted image demonstrates high signal (contrast enhancement) in the bone marrow, consistent with osteomyelitis. (Courtesy of Dr J Harris, Consultant Radiologist, Hope Hospital, Salford, United Kingdom)
CONCLUSIONS Plain radiography and MR imaging play crucial roles in the evaluation of the diabetic foot. Prompt and repeated evaluation of the foot by plain film radiography remains valuable in the management of diabetic foot complications, supported by further studies in equivocal cases utilising nuclear medicine, MR imaging and/or angiography as appropriate to confirm or exclude infection, ischaemia or avascularity and to guide amputation where indicated.