What is the role of MRI in the workup of allergic fungal sinusitis (AFS)?

Updated: Mar 16, 2018
  • Author: John E McClay, MD; Chief Editor: Arlen D Meyers, MD, MBA  more...
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Answer

Magnetic resonance imaging [18]

  • MRI also can provide information useful in preoperative identification of allergic fungal mucin, but it usually is not necessary when making the diagnosis unless the disease has extended into the intracranial cavity or confusion exists with the diagnosis. Som and Curtin have pointed out that protein concentrations exceeding 28% cause a decreased signal on T1- and T2-weighted MRI images because of protein cross-linking and slower macromolecular motion. This effect is more pronounced on T2-weighted images because of prolonged magnetic field relaxation times. The high protein and low water concentration of allergic fungal mucin, coupled with the high water content within surrounding edematous paranasal sinus mucosa, gives rise to rather specific MRI characteristics, as depicted in the image below.

    Coronal MRI showing expansion of the sinuses with Coronal MRI showing expansion of the sinuses with allergic mucin and polypoid disease; the hypointense black areas in the nasal cavities are the actual fungal elements and debris. The density above the right eye is the mucocele. The fungal elements and allergic mucin in allergic fungal sinusitis always look hypointense on MRI scanning and can be mistaken for absence of disease.
  • In a series of 10 patients with allergic fungal sinusitis (AFS), Manning et al demonstrated that hypointense central T1 signal, central T2 signal void, and the presence of increased peripheral T1/T2 enhancement were highly specific for allergic fungal sinusitis (AFS) when compared to other forms of fungal sinusitis (invasive fungal sinusitis, fungal ball) and mucocele. On MRI, the hypointense signal looks black and can be mistaken for absence of disease in the paranasal sinuses because a black signal in the sinuses on CT scan indicates the absence of disease. Compare the two images below. The combined CT scan and MRI findings provided a radiographic appearance that was highly specific for allergic fungal sinusitis (AFS).

    Coronal CT scan showing extensive allergic fungal Coronal CT scan showing extensive allergic fungal sinusitis involving the right side with mucocele above the right orbit and expansion of the sinuses on the right.
    Coronal MRI showing expansion of the sinuses with Coronal MRI showing expansion of the sinuses with allergic mucin and polypoid disease; the hypointense black areas in the nasal cavities are the actual fungal elements and debris. The density above the right eye is the mucocele. The fungal elements and allergic mucin in allergic fungal sinusitis always look hypointense on MRI scanning and can be mistaken for absence of disease.

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