Medical imaging
Because CT and MRI are sensitive to different tissue properties, the appearances of the images obtained with the two techniques differ markedly. In CT, X-rays must be blocked by some form of dense tissue to create an image, so the image quality when looking at soft tissues will be poor. In MRI, while any nucleus with a net nuclear spin can be used, the proton of the hydrogen atom remains the most widely used, especially in the clinical setting, because it is so ubiquitous and returns a large signal. This nucleus, present in water molecules, allows the excellent soft-tissue contrast achievable with MRI.
Necrotizing fasciitis
Imaging has a limited role in the diagnosis of necrotizing fasciitis. The time delay in performing imaging is a major concern. Plain radiography may show subcutaneous emphysema (gas in the subcutaneous tissue), which is strongly suggestive of necrotizing changes, but it is not sensitive enough to detect all the cases, because necrotizing skin infections caused by bacteria other than clostridial infections usually do not show subcutaneous emphysema. If the diagnosis is still in doubt, computed tomography (CT) scans and magnetic resonance imaging (MRI) are more sensitive modalities than plain radiography. However, both the CT scan and MRI are not sensitive enough to rule out necrotizing changes completely. CT scan may show fascial thickening, edema, subcutaneous gas, and abscess formation. In MRI, when fluid collection with deep fascia involvement occurs, thickening or enhancement with contrast injection, necrotizing fasciitis should be strongly suspected. Meanwhile, ultrasonography can show superficial abscess formation, but is not sensitive enough to diagnose necrotizing fasciitis. CT scan is able to detect about 80% of cases, while MRI may pick up slightly more.
A white blood cell count less than 15,000 cells/mm and serum sodium level greater than 135 mmol/l have a sensitivity of 90% in detecting the necrotizing soft tissue infection. It also has a 99% chance of ruling out necrotizing changes if the values have shown otherwise. Various scoring systems are being developed to determine the likelihood of getting necrotizing fasciitis, but a scoring system developed by Wong and colleagues in 2004 is the most commonly used. It is the laboratory risk indicator for necrotizing fasciitis (LRINEC) score, which can be used to stratify by risk those people having signs of severe cellulitis or abscess to determine the likelihood of necrotizing fasciitis being present. It uses six laboratory values: C-reactive protein, total white blood cell count, hemoglobin, sodium, creatinine, and blood glucose. A score of 6 or more indicates that necrotizing fasciitis should be seriously considered. The scoring criteria are: