**3. Retention cysts and polyps**

Retention cysts, mucinous or serous, are common findings and appear as smooth, convex soft tissue masses from the mucosal lining (**Figure 1a** and **b**) and should not be mistaken as fluid (**Figure 2**). The floor of the maxillary sinus is the most common site for retention cysts.

Mucinous retention cysts are due to obstruction of a mucinous gland, while serous retention cysts are caused by accumulation of fluid in the submucosal layer. Hence, they are not "true" cysts and therefore are also referred to as pseudocysts [4]. Retention cysts have no clinical significance and usually show no significant change in size by time [5].

Odontogenic cysts may mimic retention cysts. However, CT with bone algorithm and thin slices may reveal a tiny, peripheral calcification that will differentiate these from retention cysts (**Figure 3a** and **b**).

#### **Figure 1.**

*(a) Axial CT shows maxillary sinus retention cyst bilaterally from the posterior wall with dome-shaped appearance with upward convexity. (b) Coronal CT clearly reveals the cystic appearance.*

**13**

**Figure 4.**

*Imaging in Sinonasal Disorders*

**Figure 2.**

**Figure 3.**

*DOI: http://dx.doi.org/10.5772/intechopen.90773*

*concavity due to the fluid that settles along the sinus walls.*

*Odontogenic cysts may persist, despite dental treatment.*

*nasal cavity and choana forming an antrochoanal polyp.*

*Axial CT demonstrates bilateral maxillary sinus fluid. In contrast to retention cysts, fluid has upward* 

*(a) Coronal CT and (b) sagittal CT show an odontogenic cyst with characteristic peripheral calcification.* 

*(a) Coronal CT and (b) sagittal CT reveal a large pseudocyst growing out of the left maxillary sinus into the* 

#### **Figure 2.**

*Sino-Nasal and Olfactory System Disorders*

nant disorders.

Reconstruction with bone algorithm should be done with slices no thicker than 1 mm. Thin slices are important for evaluation of periapical tooth lucencies as seen in odontogenic sinusitis and for erosion and destruction of bone in case of malig-

Additional reconstruction with soft tissue algorithm, with 2.5-mm-thick slices, may be extremely valuable and mandatory in case of soft tissue pathology inside or outside the sinus walls. In the case of pyocele or allergic fungal sinusitis, the attenuation usually is higher than mucus [3], and a fungus ball (mycetoma) has typical scattered microcalcifications that are better presented on soft tissue algorithm. In the case of invasive fungal sinusitis, this can be diagnosed by obliteration of the fat

Magnetic resonance imaging (MRI) is complementary to CT when complica-

Low-dose CT (<20 mAs), without intravenous contrast medium, usually will be sufficient for "screening." However, if complications or malignant disease are suspected, CT should be performed with at least 50 mAs and with intravenous

Retention cysts, mucinous or serous, are common findings and appear as smooth, convex soft tissue masses from the mucosal lining (**Figure 1a** and **b**) and should not be mistaken as fluid (**Figure 2**). The floor of the maxillary sinus is the

Mucinous retention cysts are due to obstruction of a mucinous gland, while serous retention cysts are caused by accumulation of fluid in the submucosal layer. Hence, they are not "true" cysts and therefore are also referred to as pseudocysts [4]. Retention cysts have no clinical significance and usually show no significant

Odontogenic cysts may mimic retention cysts. However, CT with bone algorithm and thin slices may reveal a tiny, peripheral calcification that will differentiate these

*(a) Axial CT shows maxillary sinus retention cyst bilaterally from the posterior wall with dome-shaped* 

*appearance with upward convexity. (b) Coronal CT clearly reveals the cystic appearance.*

planes outside the sinus walls that is best seen on soft tissue algorithm.

tions to infection and neoplasms must be assessed.

contrast medium administration.

**3. Retention cysts and polyps**

most common site for retention cysts.

from retention cysts (**Figure 3a** and **b**).

change in size by time [5].

**12**

**Figure 1.**

*Axial CT demonstrates bilateral maxillary sinus fluid. In contrast to retention cysts, fluid has upward concavity due to the fluid that settles along the sinus walls.*

#### **Figure 3.**

*(a) Coronal CT and (b) sagittal CT show an odontogenic cyst with characteristic peripheral calcification. Odontogenic cysts may persist, despite dental treatment.*

#### **Figure 4.**

*(a) Coronal CT and (b) sagittal CT reveal a large pseudocyst growing out of the left maxillary sinus into the nasal cavity and choana forming an antrochoanal polyp.*

When more fluid is accumulated in the retention cysts, they may grow out of the maxillary sinus through the ethmoid infundibulum or the accessory maxillary ostium into the nasal cavity and choana and then be referred to as an antrochoanal polyp [4] (**Figure 4a** and **b**).

Pseudocysts are often referred to as a polyp in the radiological report. However, polyps contain more fibrous connective tissue than pseudocyst and tend to be more fibrotic. Hence polyps can surgically be removed intact, but not pseudocysts [4]. Pseudocysts may also rupture spontaneously or traumatically, causing unilateral rhinorrhea [6].
