Diagnosis:
Sinus disease involving the left maxillary and sphenoid sinuses with the differential diagnosis lying between complicated sinusitis versus fungal disease
Adenoidal hypertrophy.

Findings:
MRI of the head and IACs was performed and demonstrates mucosal thickening and mucous within the left maxillary sinus. There is a rounded hypointense area within this mucous within the left maxillary sinus which could represent inspissated mucous with calcification and/or hemorrhage. Fungal disease such as aspergillosis is considered as well. Similar but less severe changes are noted in the sphenoid sinus also felt to represent either inspissated mucous or fungal disease. The patient may be immunocompromised.

Discussion:
The signal intensity on MRI is determined by the degree of desiccation of the nasal mucosal secretions that become entrapped within a paranasal sinus. A diagnostic dilemma can arise when semisolid or utterly desiccated secretions are present. When absolutely dried, inspissated secretions can produce signal voids on T2-weighted images. The MR finding of low signal can represent either desiccated secretions in a setting of chronic sinusitis, fungal sinusitis with mycetoma or a rhinolith. Refer to the gamuts below:

C-17
Chronic Nasosinus Hypointensity on MR*

Lesion - Chronic Secretions Paste-like consistency
T1 - Low
Proton Density - Lower than T1
T2 – Lower than proton density or signal void

Lesion - Mycetomas Cheesy consistency
T1 - Low
Proton Density - Lower than T1
T2 - Lower than proton Density or signal void

Lesion - Desiccated, rocklike
T1 - Signal void
Proton Density - Signal void
T2 - Signal void

Lesion - Acute hemorrhage
T1 - Low
Proton Density - Lower than T1
T2 - Lower than proton Density or signal void

Lesion - Air remaining in inflamed sinus
T1 - Signal void
Proton Density - Signal void
T2 - Signal void

Lesion - Tooth in dentigerous cyst
T1 - Signal void
Proton Density - Signal void
T2 - Signal void

* Adapted from Som PM, Dillon WP, Curtin RD, et al. Hypointense paranasal sinus foci: differential diagnosis with MR imaging and relation to CT findings. Radiology 176:777, 1990.

Nasosinus Disease on MR

1. Normal nasal cycle
a. Mucoperiosteal T1 isointensity and T2 hyperintensity limited to the nasal septum, nasal cavity and ethmoid sinuses (< 2-3 mm mucosal thickness)
b. Frontal, maxillary and sphenoid sinuses are spared
c. Alternating fullness of right and left turbinates during day (frequent unilateral prominence, transient)
2. True nasosinus inflammation: Isointense Tl and hyperintense T2 with frontal, maxillary or ethmoid sinus involvement (> 2-3 mm mucosal thickness)
3. Mucus retention cyst or mucocele
a. Serous type: Hypointense Tl, hyperintense T2, homogeneous
b. Mucoid type: Hyperintense Tl, hyperintense T2, homogeneous
c. Inspissated type: Hyperintense Tl, hypointense T2
4. Nasosinus polyp: Isointense Tl and hypointense center on T2, peripheral enhancement
5. Squamous cell carcinoma or other malignancy: Mixed T1 intensity, intermediate T2 isointensity, inhomogeneous enhancement
6. Rhinolith: Hypointense Tl, hypointense T2
7. Fungus ball/mycetoma: Intermediate Tl, hypointense T2

Resources:

Pomeranz SJ, Gamuts and Pearls in MRI, Second Edition. 1993. p 266-8
P. Som, H. Curtin. Head and Neck Imaging Fourth Edition, 2003. p.218