Radiologic Imaging of Nasal Polyposis

Abstract

CT is the primary modality used to evaluate patients with NP, but MR does have an occasional role. Conventional radiographs are no longer within the standard of care for the evaluation of the paranasal sinuses. Modern helical multislice CT scanners can produce reconstructed images of diagnostic quality in any plane of imaging. Cystic fibrosis patients have preferential opacification of the paranasal sinuses, whereas non-CF patients with NP have preferential opacification of the nasal cavity. On MRI, fungal infection may have very low signal on T2-weighted images, and may thus mimic aerated sinuses.

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Core Messages

• CT is the primary modality used to evaluate patients with NP, but MR does have an occasional role.

• Conventional radiographs are no longer within the standard of care for the evaluation of the paranasal sinuses.

• Modern helical multislice CT scanners can produce reconstructed images of diagnostic quality in any plane of imaging.

• Cystic fibrosis patients have preferential opacification of the paranasal sinuses, whereas non-CF patients with NP have preferential opacification of the nasal cavity.

• On MRI, fungal infection may have very low signal on T2-weighted images, and may thus mimic aerated sinuses.

6.1 Introduction

Radiology plays several crucial roles in the evaluation of chronic rhinosinusitis (CRS) patients with nasal polyposis (NP), including establishing the diagnosis, evaluating progression of disease, surgical planning, and monitoring for recurrence. Computed tomography (CT) is the primary modality used to evaluate patients with NP, but magnetic resonance (MR) does have an occasional role.

Among sinus surgeons, endoscopy is considered the primary means of evaluating sinonasal cavity. However, radiologic modalities are used more frequently among primary caregivers, and in some instances, radiology may provide a more complete analysis of the nasal cavity and sinuses [7]. This is particularly true when the nasal cavity is completely filled with tissue, as may be the case in NP. The purpose of this chapter is to discuss the radiologic appearance of sinonasal polyps and specifically describe the findings that are associated with NP.

6.2 Radiologic Appearance of Sinonasal Polyps

Polyps appear radiographically as rounded nodules of soft tissue along the mucosal surfaces of the paranasal sinuses and nasal cavity. They are usually more radiodense than the surrounding mucosal thickening or secretions, which make them appear slightly brighter on CT (Fig. 6.1). This pattern may be reversed if the secretions become inspissated (Fig. 6.2). Sometimes, a thin pedicle is visible connecting the bulk of the polyp to the mucosal surface (Fig. 6.2). This sign may be helpful when it is present, but it is not seen in the majority of polyps.

Fig. 6.1

Hyperdense sinonasal polyps. Axial CT shows polyps (arrows) within the maxillary sinus. The polyps are denser than the surrounding secretions

Fig. 6.2

Hyperdense secretions. Axial CT shows innumerable polyps in the maxillary sinus in a patient with NP. Some of the polyps are pedunculated (arrow), distinguishing them from MRC. The inspissated secretions between the polyps have increased density, which reverses the normal density pattern seen in Fig. 6.1

Polyps do not erode into the surrounding bone, but pressure from a polyp may produce a benign local remodeling pattern that scallops the underlying bone (Fig. 6.3). This is distinct from a mucocele, in which the entire sinus expands. This bone remodeling will occasionally thin bony septations beyond the resolution of CT, giving the appearance of bone erosion, particularly in the ethmoid septations (Fig. 6.4).

Fig. 6.3

Bony remodeling from polyps. Coronal CT shows lobular remodeling of the maxillary sinus walls (arrowheads) in this patient with NP. This scalloped pattern suggests a benign etiology

Fig. 6.4

Remodeling of the ethmoid septa. Axial CT of a patient with NP shows thinning of the posterior ethmoid septa (arrow) so that they are no longer visible on CT. This is still a benign pattern

Polyps themselves do not enhance with contrast administration. However, the mucosa surrounding the polyp may enhance, giving the impression of rim enhancement (Fig. 6.5). This thin, uniform rim of enhancement is usually distinguishable from the complete enhancement of nonnecrotic tumors and the irregular enhancement of necrotic tumors.

Fig. 6.5

Mucosal enhancement around a polyp. Coronal contrast-enhanced T1-weighted MR image shows a low-signal polyp (asterisk) surrounded by a rim of enhancement. This rim of surrounding thickened mucosa should not be mistaken for the rim enhancement of a necrotic tumor

The location of the polyp can sometimes be predicted by the patient’s symptomatology [6]. Polyps under the cribriform plate may interfere with smell or taste; polyps obstructing the frontal sinus will cause frontal headaches, and polyps near the sphenoethmoidal recess will cause deep central headaches. These guidelines are not absolute, however, because pain may be referred to (or from) other areas of the head and neck. In most primary cases of NP without surgery, facial pain is an uncommon finding.

6.3 Special Types of Sinonasal Polyps

Solitary polyps that arise within the maxillary sinus (also called the maxillary antrum) may extend into the nasal cavity by remodeling and expanding the maxillary os and infundibulum. These lesions are called antronasal polyps. Sometimes, the polyp will be narrowed as it passes through the maxillary os, resulting in a dumbbell configuration on coronal CT. The medial wall of the maxillary sinus is usually bowed into the nasal cavity (Fig. 6.6). If the middle meatus becomes obstructed, secondary opacification of the ethmoid and frontal sinuses may mask the true source of the obstruction. In this scenario, the remodeled bone of the medial maxillary wall becomes an important diagnostic sign of antronasal polyp.

Fig. 6.6

Antronasal polyp. Axial CT shows a mass filling the right maxillary sinus and extending into the nasal cavity (asterisk). The medial wall of the sinus is remodeled (arrow), indicating a benign etiology

If an antronasal polyp becomes so large that it extends through the nasal cavity and across the choana into the nasopharynx, it is referred to as an antrochoanal polyp [2]. The most helpful radiologic feature is the mass itself extending into the nasopharynx (Fig. 6.7). Care should be taken, however, that a polyp of the posterior aspect of the inferior turbinate that extends across the choana is not mistaken for a true antrochoanal polyp. These polyps that arise in the nasal cavity and extend into the nasopharynx are called nasochoanal polyps. While the posterior aspect of the inferior turbinate may be enlarged and edematous, it is exceedingly rare for the inferior turbinate to undergo true polypoid changes, whereas the middle turbinate in patients with NP frequently undergoes polypoid changes. Radiographically, these changes of edema and polypoid transformation are indistinguishable.

Fig. 6.7

Antrochoanal polyp. Axial CT images show a low-density polyp (asterisk) extending from the antrum, across the nasal cavity, through the choana, and into the nasopharynx. Fungal colonization of entrapped secretions results in high density in the remainder of the antrum

Although inflammatory NP is frequently solitary, multiple sinonasal polyps can occur outside the setting of true NP. It is important to distinguish these entities for prognostic and therapeutic reasons. Complete pansinus opacification is more suggestive of NP (Fig. 6.8) [3]. Numerous convexities are usually visible along the inferior surface of the opacified nasal cavity in NP (Fig. 6.9). Severity of disease is also an indicator – there are usually many polyps in NP.

Fig. 6.8

Pansinus opacification in NP. Coronal CT shows complete opacification of the paranasal sinuses and near-complete opacification of the nasal cavity. The severity of disease suggests NP over multiple sporadic polyps

Fig. 6.9

Intranasal convexities in NP. Coronal CT shows numerous convex surfaces (arrowheads) along the aerated border of the nasal cavity. This finding suggests polyposis over other sinonasal diseases such as cystic fibrosis

6.4 Complications

If a sinonasal polyp arises in (or extends into) the middle meatus, it may obstruct outflow from the frontal sinus, the maxillary sinus, and the anterior ethmoid air cells. This can be referred to as a middle meatus syndrome (Fig. 6.10). Unilateral involvement of just these sinuses is highly suggestive of an obstructing mass. A chronically obstructed sinus may form a mucocele. On CT, the affected sinus enlarges and its walls become rounded outward (Fig. 6.11). This complication may be seen from NP itself, but is also commonly seen as a complication of polypectomy. An infected mucocele is called a mucopyocele, and it may be distinguished from an uninfected mucocele by MRI [1]. Sometimes, mucoceles will impinge upon surrounding structures such as the orbit (Fig. 6.12). In patients with NP, even if no mucocele is formed, the pressure from the polyps will often cause rarefaction of the ethmoid trabeculae (Fig. 6.13).

Fig. 6.10

Middle meatus syndrome. Coronal CT shows unilateral opacification of the maxillary, ethmoid, and frontal sinuses. Note the widened infundibulum (arrowhead) that indicates the offending antronasal polyp

Fig. 6.11

Mucocele. Axial CT shows an enlarged, opacified posterior ethmoid air cell (asterisk) with rounded convex borders. The surrounding sinuses are compressed

Fig. 6.12

NP with frontal mucocele causing mass effect on the orbit. Coronal CT shows complete opacification of the nasal cavity and sinuses, consistent with NP. The frontal sinus is expanding into the orbit (arrow). The underlying bone is thinned beyond the resolution of this CT

Fig. 6.13

Rarefaction of ethmoid trabeculae in NP. Axial CT shows thinning of the ethmoid septations (arrows) beyond the resolution of CT. Compare with Fig. 6.4

Fungal disease may coexist with NP, including a subgroup of CRS with NP termed allergic fungal sinusitis (AFS). On CT, the presence of hyperdense secretions between layers of thickened, hypodense mucosa is suggestive of AFS (Fig. 6.14). On MRI, fungi may have very low signal on T2-weighted images, and may thus mimic aerated sinuses (Fig. 6.15) [1]. Correlation with other pulse sequences is critical to avoid this diagnostic error. Unfortunately, the CT and MR findings of inspissated secretions overlap with those of fungal associated inflammation, and either may be seen in patients with NP.

Fig. 6.14

Allergic fungal sinusitis. Axial CT shows the nasal cavity and ethmoid air cells to be filled with hyperdense secretions between layers of thickened, hypodense mucosa

Fig. 6.15

NP complicated by allergic fungal sinusitis (AFS). On the axial T2-weighted image (a), areas of high signal (polyps) alternate with areas of low signal (arrows) indicating fungal infection. The corresponding postcontrast T1-weighted image (b) shows the characteristic flowing enhancement pattern of AFS

6.5 Radiologic Differential Diagnosis

6.5.1 Acute Rhinosinusitis

Acute rhinosinusitis may cause near-complete opacification of the nasal cavity and paranasal sinuses. However, rhinosinusitis lacks the multiple convexities seen in NP (Fig. 6.9). The presence of dense polypoid tissue (Fig. 6.1) is also an important distinguishing feature.

6.5.2 Mucus Retention Cyst

Mucus retention cysts (MRC) have a radiographic appearance that is almost identical to that of solitary polyps. If a pedicle is present (Fig. 6.2), the mass is more likely a polyp. If there is remodeling of underlying bone (Fig. 6.3) or expansion through the sinus osteum (Fig. 6.6), MRC is excluded. Usually, however, no distinguishing radiologic feature is present. But since solitary polyps and MRC both reflect chronic inflammation, the distinction is not usually of clinical importance. Thus, for small mucosal masses, radiologists may apply either term without fear of patient mismanagement.

6.5.3 Other Benign Masses

Inverted papillomas are often indistinguishable from polyps radiographically. Papillomas that occur in a characteristic location may be identifiable, such as inverted papillomas that arise in the medial wall of the antrum and extend both into the antrum and the nasal cavity (Fig. 6.16). A lobular (“cerebriform”) configuration is also suggestive of papilloma. Bony sclerosis and osteoneogenesis are frequently present at the site of attachment of inverted papillomas.

Fig. 6.16

Inverted papilloma. Axial CT shows a lobular mass (asterisk) centered in the lateral nasal wall, with extension into both the nasal cavity and the maxillary sinus. The location, configuration, and central calcifications are all indicative of inverted papilloma

Juvenile nasal angiofibromas are easily distinguished from other benign nasal mass by their characteristic location (centered at the sphenopalatine foramen), enhancement pattern, and remodeling of surrounding bony structures [4]. They rarely present a diagnostic dilemma. Encephaloceles and meningoceles can be surprisingly difficult to diagnose when imaged in axial plane. Coronal imaging is most useful to establish the communication with the cranial vault. While coronal T2-weighted MRI is useful to confirm this diagnosis and determine the amount of herniated brain tissue, the most important aspect of care is to consider the diagnosis in the first place [1].

6.5.4 Malignancy

Malignant intranasal masses that may mimic polyps include esthesioneuroblastoma, sinonasal undifferentiated carcinoma, squamous cell carcinoma, minor salivary malignancies, and malignant melanoma. These tumors will usually have an aggressive appearance, with erosion of underlying bone, rather than the benign remodeling (or no effect) seen with polyps (Fig. 6.3). Benign minor salivary tumors can arise from any mucosal surface, and may be seen in the same distribution as polyps. These tumors tend to remodel bone, but usually show pronounced focal remodeling, even when the tumor is small. Small polyps, in contrast, rarely cause remodeling.

6.5.5 Dense Secretions

Polyps, inspissated secretions, and fungal colonization can all result in material of greater-than-water density within the sinuses (Figs. 6.1, 6.2, and 6.14). Distinguishing between these entities can be difficult radiologically, even with MRI [1]. T2 signal dropout is classically associated with fungal infection, but can be seen also with inspissated secretions; it is unusual in polyps themselves. A rapid increase in the CT density of a polyp suggests fungal colonization of the polyp [4].

6.5.6 Cystic Fibrosis

There are few diseases that affect the sinuses as severely as NP. The other major contender is cystic fibrosis (CF). The clinical history of these diseases does not generally overlap, but there are also radiologic differences in the sinus manifestations (Fig. 6.17). CF tends to spare the nasal cavity, whereas NP is more likely to spare the sinuses themselves (Figs. 6.9 and 6.17). NP has more convex surfaces in the nasal cavity, whereas CF produces a greater degree of osteoneogenesis in the sinus walls.

Fig. 6.17

Cystic fibrosis. Coronal CT in a patient with cystic fibrosis shows fewer convex surfaces than in patients with NP, relative sparing of the nasal cavity, and extensive osteoneogenesis (arrowheads). Compare with Fig. 6.9

6.6 Radiologic Modalities

CT is the most frequently used radiologic modality for the assessment of NP. The detailed bony anatomy that is available on CT makes it more useful than MR, even though MR better characterizes soft tissue [1]. MR is mostly used after a mass is known, to differentiate between the mass and obstructed secretions or to limit the differential diagnosis. The multiplanar capabilities of MR are sometimes touted as an advantage over CT, which is intrinsically limited to the axial plane of imaging. However, modern helical multislice CT scanners can produce reconstructed images of diagnostic quality in any plane of imaging. Most of the diagnostic evaluation on CT is based on coronal images, but axial images are still useful to establish anatomic relationships. Multiplanar imaging with CT also allows for the use of sagittal CT images, particularly when evaluating the basal lamella of the middle turbinate or differentiating between anterior and posterior ethmoid cells. A CT scanner should have at least four data channels (preferably 16) for reconstructions to be of adequate quality.

Conventional radiographs are no longer within the standard of care for the evaluation of the paranasal sinuses. They may be used as a screening test for acute maxillary sinusitis, but evaluation of diseases such as NP requires cross-sectional imaging. Intravenous contrast is routinely used in MR imaging, but is usually not indicated with CT. Contrast is sometimes applied in the setting of a known tumor to evaluate the relationship to nearby vascular structures, or in the setting of a potential recurrence of an enhancing tumor. It is rarely useful in NP.

CT and MR are now routinely used for computer-assisted image-guided endoscopic sinus surgery for NP. CT may also have an important predictive value when preparing patients for sinonasal surgery – patients with greater opacification of the sinonasal regions on preoperative CT are at greater risk of complications during surgery [5].

Take Home Pearls

• Numerous intranasal convexities are the radiologic hallmark of NP.

• Spherical remodeling of sinus walls is the radiologic hallmark of a mucocele.

• The presence of hyperdense secretions between layers of thickened hypodense mucosa is suggestive of AFS.

• Polyps do not erode into the surrounding bone, but pressure from a polyp may produce a benign local remodeling pattern that scallops the underlying bone.

• Antronasal polyps show a dumbell configuration across the osteomeatal complex.

• Complete pansinus opacification is more suggestive of NP than of multiple sporadic polyps.