MRI in necrotizing fasciitis of the extremities

Abstract

Necrotizing fasciitis is a life-threatening soft-tissue infection of bacterial origin, which involves mainly the deep fascia. Early recognition of this condition may be hampered by the uncommon nature of the disease and non-specificity of initial clinical signs and symptoms in less fulminant cases, making the role of imaging important. MRI is the most useful imaging modality in the diagnosis of necrotizing fasciitis. The presence of thick (>3 mm) hyperintense signal in the deep fascia (particularly intermuscular fascia) on fat-suppressed T2 weighted or short tau inversion-recovery images is an important marker for necrotizing fasciitis. Contrast enhancement of the thickened necrotic fascia can be variable, with a mixed-pattern of enhancement being more commonly encountered. Involvement of multiple musculofascial compartments increases the likelihood of necrotizing fasciitis. It is important to remember that T2-hyperintense signal in the deep fascia is not specific to necrotizing fasciitis and can also be seen in cases such as non-infective inflammatory fasciitis or muscle tear. In this pictorial essay, we aim to review the MRI findings in necrotizing fasciitis, discuss its limitations and pitfalls and identify differentiating features from non-necrotizing soft-tissue infections, such as cellulitis and infective myositis/pyomyositis, conditions which may clinically mimic necrotizing fasciitis.

Figure 1.

Fascial anatomy. Schematic drawing shows the superficial and deep fascia in the leg. A, skin; B, membranous layer of superficial fascia; C, peripheral layer of deep fascia; D, intermuscular layer of deep fascia; DAT, deep adipose tissue; SAT, superficial adipose tissue.

Figure 2.

Necrotizing fasciitis of the thigh in a 50-year-old female. (a) Axial T₂ weighted MR image shows thick T₂-hyperintense signal (white arrows) within the deep intermuscular fascia in the anterior and medial compartments of the thigh. (b) Axial post-contrast T₁ weighted MR image shows absent to mild contrast enhancement (white arrows) in the deep intermuscular fascia at sites of the T₂-hyperintense signal. At surgery, necrotic fascia with turbid fluid was found, consistent with necrotizing fasciitis.

Figure 3.

Necrotizing fasciitis in the right upper limb of a 71-year-old male due to gas-forming organisms (Streptococcus anginosus). Radiograph shows faint air lucencies within the soft tissues (white arrows). (b) Axial T₂ weighted MR image shows thick hyperintense signal in the deep intermuscular fascia (white arrows). (c) Axial T₁ weighted MR image taken at a more distal level shows few tiny foci of signal void in the superficial fascia (white arrows). (d) Axial gradient-echo image shows blooming artefacts in the superficial fascia (white arrows) compatible with air foci—these are more apparent on the gradient-echo image than the T₂ weighted image. (e) Urgent fasciotomy and surgical debridement was performed. Cultures grew Streptococcus anginosus.

Figure 4.

Necrotizing fasciitis of the leg in a 21-year-old male with no underlying risk factors. (a) Axial fat-suppressed T₂ weighted MR image of the mid-calf shows thick hyperintense fluid signal in the intermuscular fascia abutting the gastrocnemius muscle (white arrows). (b) Corresponding axial post-contrast fat-suppressed T₁ weighted MR image showing mixed enhancing and non-enhancing areas, corresponding to T₂ hyperintensity (white arrows). At surgery, early necrotizing fasciitis involving the fascia between superficial posterior and deep posterior compartments was found.

Figure 5.

Pitfalls on MRI. (a) Axial T₂ weighted image shows a reticulated pattern of T₂-hyperintense signal within the superficial fascia (white arrowheads) with associated thin linear T₂-hyperintense signal in deep intermuscular fascia (white arrows). (b) Axial post-contrast T₁ weighted image shows moderate contrast enhancement within the superficial fascia (white arrowheads) and at the site of thin linear T₂-hyperintense signal in the deep intermuscular fascia (white arrows). Patient improved with intravenous antibiotics and was well at discharge.

Figure 6.

Pitfalls on MRI. (a) Axial T₂ weighted MR image shows hyperintense signal located predominantly on outer aspect of the peripheral layer of deep fascia (white arrows) and within superficial fascia (white arrowhead). (b) Axial post-contrast T₁ weighted MR image shows mild contrast enhancement in the deeper aspect of the superficial fascia, abutting the outer aspect of the peripheral layer of deep fascia (white arrows) and at sites of T₂-hyperintense signal within superficial fascia (white arrowhead). Patient improved with intravenous antibiotics and was well at discharge.

Figure 7.

Pitfalls on MRI. Axial T₂ weighted fat-suppressed MR image shows a tear of the medial edge of gastrocnemius (black asterisk) with fluid signal in the peripheral deep fascia (white arrows). Patient was managed conservatively.

Figure 8.

Cellulitis of the calf in a 42-year-old male. Axial fat-suppressed T₂ weighted MR image shows T₂-hyperintense signal with a reticulated appearance in the superficial fascia (arrows).

Figure 9.

Pyomyositis in a 61-year-old diabetic male. Axial post-contrast fat-suppressed T₁ weighted MR image shows a rim-enhancing collection compatible with abscess formation (white arrows) in the deep posterior muscle compartment. Contrast enhancement was also noted in extensor hallucis longus as well as peroneus group of muscles (white stars), compatible with myositis. Patient was managed with drainage and antibiotics.

Figure 10.

Infective myositis. Axial fat-suppressed T₂ weighted MR image shows patchy areas of T₂-hyperintense signal involving the soleus muscle (superficial posterior compartment) and also the deep posterior muscle compartment (white arrows). No thick (>3 mm) T₂-hyperintense signal was evident in the deep intermuscular fascia. Findings were compatible with myositis. Patient was managed conservatively with antibiotics.