Summary

Modern imaging modalities provide invaluable help in both the diagnostic process and the treatment planning in rhinology. The evolution of computed tomography (CT) technology provides high-quality submillimeter studies in less than a minute. These CT studies are then enhanced and reconstructed in three planes by modern software and produce an accurate three-dimensional (3D) imaging model of the anatomy and pathology, which allows for detailed diagnostic evaluation. Fine bone detail provided by the CT studies are also a valuable tool for the surgeon, who can conceptualize a virtual model of the anticipated situation during surgery and can set up a detailed operative plan before entering the operating room. The same CT studies can also be used by image guidance systems, when needed.

When soft tissue evaluation is needed, as in tumors and fungal disease, magnetic resonance imaging (MRI) can help in the differential diagnosis, providing information on the composition of the soft mass under evaluation, as well as on the possible extension into neighboring anatomical areas. Intravenous (IV) gadolinium further enhances differences between normal and pathologic tissue in MRI, and may also provide more information regarding etiology and spread.

Many pathologic entities have special imaging characteristics, which help in the diagnostic process, when identified. Nevertheless, we should always remember to treat patients, not imaging scans. In this respect, imaging studies must be carefully correlated with the patient′s history and clinical examination. No diagnosis should be based on imaging alone, and no important treatment decisions, especially in cases of tumors, should be made without histologic confirmation of the anticipated diagnosis.

Introduction

In modern rhinologic practice, the use of the endoscope in the outpatient clinic, which has gained wide acceptance over the past two decades, has eliminated the need for plain sinus radiographs. At the same time, improving understanding of sinonasal disease, continuously expanding indications of endoscopic sinus surgery, and wide application of image guidance technology have demanded more detail in morphologic information and therefore have led to increased use of more sophisticated, high-resolution, cross-sectional imaging modalities, such as CT.

Modern CT scanners, incorporating helical and multi-channel technology, can provide submillimeter slice studies within seconds, with minimal radiation delivered to the patient. The latest computer software packages provide excellent multiplanar as well as 3D representation of the anatomy and pathology, with image quality that would have been difficult to conceptualize not so very long ago. MRI is now commonly available to provide useful information when soft tissue evaluation is important.

Rhinologic surgeons work closely with radiologists and are increasingly involved in the interpretation of imaging studies, having realized that imaging is both a diagnostic tool and surgical instrument; it not only provides invaluable information on the extent and nature of the disease, but also reveals all necessary morphological details in the areas of the sinus complex, which are not accessible to endoscopy. Therefore, modern imaging modalities are an integral part of both the diagnostic evaluation of the rhinologic patient and the surgical procedure, when this is indicated.

This chapter discusses the clinical value of the imaging modalities currently in use. After a brief review of the imaging evolution and important technical details, information on anatomy and pathology provided by imaging is presented, highlighted by typical clinical examples. The distinctive characteristics of CT and MRI in different pathologic entities are discussed, along with the specific value of CT scan in preoperative planning.

From Plain X-Ray Films to Computed Tomography

Before and during the early days of endoscopy, plain X-ray films were the most widely used modality for the examination of the paranasal sinuses. Waters, Caldwell, lateral, and base views provided the best available information on the bony perimeter and mucosal lining of the large sinuses and orbit. As our knowledge of the physiology of the sinuses expanded and endoscopic sinus surgery gained acceptance, we needed more anatomical detail in areas such as the ostiomeatal channels and the ethmoid sinus, which were not adequately imaged by plain films. At the time that functional endoscopic sinus surgery (FESS) was introduced in Graz, Austria, Messerk-linger and Stammberger used X-ray polytomography to display the needed morphological information for the application of the new endoscopic techniques. When CT technology became available, Zinreich, Kennedy, and Stammberger showed that it is far superior to X-ray tomography in displaying anatomical detail.1 In 1986 CT eventually replaced the older tomographic techniques, being accepted as the imaging modality of choice for the nose and sinuses.2

Computed Tomography and Magnetic Resonance Imaging

CT scanners of various generations are in use today, ranging from the older, single-slice scanners, to the newest ones, with 312 channels (rows of detectors). Most scanners can easily provide a 4-mm scan thickness, which is adequate to show the gross regional morphology. More detail is required in cases with pathology in complex anatomical areas, such as the frontal recess, or if imaging information is to be used for image-guided surgery. In the latter case, the slice thickness depends on the equipment being used, but in general, slices are in the 1-mm thickness range. CT studies of higher or lower resolution can be acquired by all current spiral, multislice scanners.

For the scanning procedure, the patient is placed supine on the scanner bed, and the axial plane is adjusted to be parallel to the hard palate. The scanning area must extend from above the roof of the frontal sinus to the teeth of the upper jaw. If the examination is to be used on an image guidance system, it is important that the tip of the nose is included and that the earlobes are not distorted by headrests, pillows, or tapes. Initially, bone window and bone algorithm techniques are applied, which provide the needed bone detail. In cases where soft tissue pathology is suspected, such as neoplastic or fungal disease, additional soft tissue windows are acquired. IV contrast medium may be applied when necessary. It is important to note that MRI, with or without contrast, is superior to CT in soft tissue detail demonstration. Therefore, contrast-enhanced CT is considered only in cases where MRI is unavailable or contraindicated (i.e., claustrophobia, ferrometallic implants, and pacemakers).

The raw data collected by the scanner can then be used to produce multiplanar reconstructions (MPRs) in the axial, coronal, and sagittal planes, the latter preferably orthogonal to the hard palate. Several computer software packages available on the market or as free software on the Internet can provide in seconds MPR from the original imaging information. An axial study of good quality with resolution of 1 mm or less in digital form (Digital Imaging and Communications in Medicine [DICOM] images) is all that is needed for such software to produce an excellent MPR analysis as well as create virtual endoscopy and “run through” videos (see Video 3, Reconstruction of DICOM-CTA Using OsiriX 4.1 ® ). An additional, most helpful, feature of MPR function is the simultaneous point projection in three planes. The cursor (usually a cross) is placed by the user over a point in one plane, and the software projects the same point in the other two planes. Figure 7.1 demonstrates an example of MPR from 0.6 mm axial scans with three-plane point projection. Most MPR workstations also have the capability of oblique cuts in any desired angulation, which can be particularly helpful in the study of difficult anatomical areas.3

Studying the CT scan in three planes, either on the computer or on printed film, is the safest way to a clear understanding of both the anatomy and pathology. It greatly helps in the diagnostic analysis and therapeutic approach, but it is also the key procedure when setting up the surgical plan before an operation.

Newly developed flatbed cone beam CT technology has reduced the size of CT scanners, allowing their use in an outpatient rhinology clinic or private office setting. This technology delivers less radiation and provides adequate bone detail, but it has poor soft tissue resolution. An evaluation of the exact role and cost-effectiveness of these scanners needs to be undertaken by any practice considering its purchase.4

When digital technology is not available, single-slice coronal CT images can provide the most critical morphologic information. The coronal plane most closely simulates the information provided by endoscopy and is most useful in guiding the surgery. Should there be a need for additional information, the plane affording that information should be considered, and a separate set of images in that plane (axial or sagittal) should be obtained.

When a soft tissue mass is suspected from the CT, MRI will provide further information regarding the exact location, extension into adjacent compartments, and most probable etiology of the pathologic process. MRI can make an exquisite distinction between various soft tissues; it can distinguish solid from cystic lesions and can differentiate edematous mucosa from a solid tumor mass, the latter being impossible on non-contrast-enhanced CT. In malignant tumors, MRI may reveal clinically unsuspected information regarding perineural and perivascular spread of neoplasms and extension of disease beyond the bony margins of the sinus complex.5 T1- and T2-weighted sequences in the axial and coronal planes are initially acquired. Sagittal sequences may be obtained in midline lesions. IV administration of gadolinium with the chelating agent diethylenetriamine pentaacetic acid (gadolinium-DTPA) will enhance the differences between normal and diseased tissues, and may provide additional information with respect to etiology and pathologic spread. Fat suppression techniques are required to increase the sensitivity of the enhancement6 in areas containing a large amount of natural fat, such as the orbit.

Figure 7.2 demonstrates the value of MRI in discriminating between mucosal edema and solid tumor. The distinction is largely displayed on the T2-weighted image sequences. Edema is represented by a very bright signal intensity, whereas neoplasms often show an intermediate increase in T2 signal.7 In the same respect, entrapped secretions are differentiated from tumor.8 Typical clinical cases where MRI studies are necessary or useful for accurate evaluation include the following:

1. 
   

   Orbital or intracranial complications of sinusitis or sinus surgery

   
   
2. 
   

   Erosion/dehiscence of sinus bony margins or skull base on CT

   
   
3. 
   

   Unilateral/solitary disease on CT

   
   
4. 
   

   Complete opacification of sinuses on CT (pansinusitis) in the absence of clinical evidence of infection

   
   
5. 
   

   Clinical suspicion of tumor

   
   
6. 
   

   Previous osteoplastic obliteration of frontal sinus

Other Imaging Modalities

Fusion of CT and MRI series is possible with current software and is used in image-guided procedures in oncological surgery and radiotherapy. Some image-processing software applications can also fuse CT and MRI series for diagnostic purposes ( Fig. 7.3 ) (see Video 3, Reconstruction of DICOM-CTA Using OsiriX 4.1 ® ).

Image guidance systems use the high-resolution CT scan of the patient to aid in surgical navigation. Because they are based on a preoperative examination, they cannot follow intraoperative tissue alterations, they do not reflect dynamic tissue volume changes, and therefore they cannot assist effectively in procedures on tumors outside the sinus osseous margins. Intraoperative CT and MRI are available and already being used in clinical practice, but high costs and changes in the operating theater setup limit their use to specialized neurosurgical centers.

Positron emission tomography (PET) is a nuclear medicine imaging modality that evaluates metabolic aspects of disease by detecting accumulated radioactive markers in various types of cells. Morphologic resolution of PET is low; therefore, it is usually used in conjunction with CT (PET/CT evaluation). Scanners that combine PET capabilities with MRI technology (PET/MRI) are also available today. PET is helpful in the detection of nodal and distant metastases6 and is used in the staging of malignant neoplasms ( Fig. 7.4 ).

High-resolution CT studies allow for sophisticated computer elaboration and have led to the development of software applications such as virtual endoscopy that can be helpful in the demonstration of complex sinus outflow tracks.

Angiography is useful in the study of vascular lesions and is typically combined with embolization of the feeding vessels. A common clinical example for the application of this modality is the preoperative embolization of angiofibromas or paragangliomas.

Type A ultrasonography has been used from the early 1980s mainly for examination of the maxillary sinus, but it has been shown to have very low sensitivity and specificity scores10 and has not gained wide acceptance in clinical practice.

Rhinologists are often involved in the treatment of nasolacrimal system obstruction. Dacryocystography (DCG) is a technically simple radiologic examination in which iodinated contrast is injected via a small cannula in the inferior canaliculus into the lacrimal duct system. Dynamic images are obtained that can confirm the clinically suspected obstruction and reveal its exact anatomical level ( Fig. 7.5 ). An easy way to examine patency of the nasolacrimal passage is with CT DCG, as described by Zinreich et al.11 Function of the nasolacrimal system is better assessed with lacrimal scintillography.12

Evaluation of Pathology

When dealing with the common, uncomplicated, acute sinonasal infection, rigid endoscopy gives enough information to eliminate the need for radiologic imaging. In acute disease, CT scans will be necessary only when there are clinical signs of orbital or intracranial complications, or when the severity of the patient′s condition requires information beyond the surface view afforded by the endoscope.

This does not apply to chronic rhinosinusitis (CRS), where radiologic evaluation will be needed in most cases, especially when surgery is contemplated. Current imaging modalities are quite sensitive in exhibiting mucosal abnormalities; therefore, adequate medical treatment should precede imaging to avoid overestimation of mucosal disease.13 Before the CT examination, it is also recommended that patients blow their nose14 to clear secretions that could influence the interpretation of the scans.

It is a fundamental principle in clinical practice that we should treat patients, not CT scans. This axiom is underlined by the high prevalence of incidental radiologic findings in asymptomatic patients,15,16 as well as by the low correlation of radiologic findings with patients’ symptoms.17,18 CT findings are helpful but not obligatory in reaching the diagnosis of CRS, according to the European Position Paper on Rhinosinusitis and Nasal Polyps 2012.19 Nevertheless, today most rhinologists would agree that evaluation of the patient with CRS would be difficult without cross-sectional, high-definition imaging. A CT scan of good quality is the minimum requirement for this diagnostic process.

Coronal scans are preferably the first to look at, because they imitate the endoscopic image, although initial estimation of pathology on a CT scan can be easily done by browsing through the axial series. This rough estimation will give an idea of the extent of the disease and will allow easy scoring according to the preferred staging system. In the widely used Lund-Mackay system,20 a grade of 0 (clear), 1 (partial opacification), or 2 (complete opacification) is assigned to each of the sinuses (frontal, anterior ethmoid, posterior ethmoid, sphenoid, and maxillary), plus a score of 0 or 2 for the ostiomeatal complex on each side. This adds up to a maximum total of 12 for the completely opacified side. The Lund-Mackay score is simple and highly reproducible, correlating with some objective measures of disease severity, although not with symptom scores.21 However, it has its limitations, the most discussed being the inability to grade partial opacification. Nonsignificant mucosal thickening takes the same grade with subtotal opacification of the sinus; this is a significant limitation in recording the severity of the disease as well as its postoperative course. Furthermore, the Lund-Mackay system does not take into account the bony changes commonly seen in CRS (thinning or thickening, erosion, remodeling, and osteitis). Various improvements have been proposed, including subdivision of the Lund-Mackay scoring system into smaller categories and the inclusion of the patency of the main ostiomeatal channels in the scoring.22 More detailed, and therefore more complex, staging systems have been published, combining radiologic scoring with endoscopic and symptom assessment,23 but they have not gained wide acceptance to date. Recently, an interesting scoring scale was suggested, referring to bony thickness, irregularity, and extent of osteitis, which was shown to correlate well with the Lund-Mackay score and previous surgeries.24

Inflammatory Disease

Inflammatory disease on CT may appear as mucosal thickening inside and around the bony margins of the sinus or as complete occlusion of the sinus lumen. Secretions with or without air bubbles and air–fluid levels ( Fig. 7.6 ) are also common findings in the acute state. MRI is very sensitive in demonstrating mucosal inflammatory reaction, although the high T2 signal should not be confused with that demonstrated by the transient edema of the normal nasal cycle.25 Therefore, special care must be taken in MRI interpretation in cases of mucosal disease to exclude false-positive findings. However, a recent study showed that careful evaluation of the MRI can produce a CRS staging closely correlated to corresponding staging based on CT.26 CRS shows variable T2 signal intensity on MRI, depending on the protein concentration within the tissue. Som and Curtin classified possible variations of the MRI signal into four categories.27 The general rule is that high water content in tissue or secretions causes high signal on T2-weighted images and corresponding low signal on T1-weighted images.6 Very thick or inspissated secretions, which have lost their water content, within a sinus may impose a significant diagnostic problem on MRI, because they can mimic a normally aerated sinus due to lack of mobile protons. Secretions also show variable attenuation on CT, depending on their viscosity, increased attenuation corresponding to a high degree of proteins, or calcifications.

The shape of the mucosal thickening is taken into consideration in the differential diagnosis. Uniform, uninterrupted mucosal thickening around the periphery of a large sinus indicates, and is a major hallmark of, inflammatory disease. Well-defined, rounded masses in the large sinuses typically represent retention cysts. Rounded lining is also presented by inflammatory polyps. So, although a rounded focal lesion typically indicates benign, usually inflammatory, disease, a specific radiologic diagnosis cannot be assigned to it, as it could be either a polyp or a cyst. Antrochoanal or sphenochoanal polyps are imaged as opacification of the maxillary or sphenoid sinus (cystic part), from which a rounded, elongated mass encroaches into the middle or superior meatus, passing toward the nasopharynx (polypoid part).

Mucoceles, or mucopyoceles, are collections of thick mucus and/or pus, usually in the frontal and/or the ethmoid sinuses, which develop after obstruction of the sinus outflow track by disease or postoperative scar tissue. Clinical presentation varies and often includes symptoms or signs from the orbit. On CT, there is a symmetric, homogeneous, hypodense opacification of the involved sinus, with possible erosion of the lamina papyracea and extension of the encapsulated mucous mass into the orbit, without signs of periorbital infiltration. On MRI, the mucous content presents intermediate signal on T1- and T2-weighted images, depending on the water/protein relation. The peripheral mucosa shows lower signal than the mucocele content on T1-weighted images and enhances on contrast administration ( Fig. 7.7 ). Enhancement is never seen within a mucocele. Furthermore, as mentioned above, very thick secretions may appear as a void (lack of signal) in MRI, causing confusion in the imaging evaluation of suspected mucoceles. In these cases, correlation of the MRI with the CT scan will solve the diagnostic puzzle.

Bone involvement is common in CRS.28 It can be regarded as a marker of previous surgery24 or disease severity, but it also is an unfavorable prognostic factor when considering surgical management.29 On CT, thickening of bony structures, indicating neo-osteogenesis, can be expected in 36 to 52% of patients with CRS.30 In some cases, new bone formation can be excessive, disturbing sinus function and obstructing normal drainage ( Figs. 7.8 and 7.9 ). Increased attenuation of bone as measured in Hounsfield units on CT is found to correlate with histopathologic remodeling of involved bony structures.31 Bone involvement in CRS is uniform thickening without erosion; this represents a second imaging hallmark of inflammatory disease.

Fungal disease in the sinuses has various appearances, depending on the type of fungal involvement. Invasive fungal rhinosinusitis, either acute or chronic, is a life-threatening condition usually associated with immune deficiency induced by disease or drugs. Bone destruction, best seen on CT, is the characteristic finding, appearing in the later stages of acute disease or during the more prolonged progression of chronic disease. Soft tissue or brain infiltration, when present, is best visualized with MRI. Foci of increased attenuation on CT, found in > 70% of patients, are thought to indicate fungal colonies within the inflamed mucosa.32 The distinguishing findings that separate fungal disease from malignancy are the perisinus soft tissue induration and the obliteration of the peri-sinus fat planes without bony erosion.

Apart from invasive fungal disease, the types of fungal rhinosinusitis commonly associated with CRS are the fungal ball and allergic fungal rhinosinusitis.

The fungal ball, usually aspergilloma, is a mass of packed fungal growth within a sinus. It may produce symptoms of varying intensity and may be discovered incidentally. On CT, there is diffuse opacification of the involved sinus, often with central increase in density and thickening of the mucosal lining. Calcification foci and/or accumulation of ferromagnetic elements (iron and manganese) within the mass are diagnostic when present ( Fig. 7.10 ).3

Allergic fungal rhinosinusitis is an immunoglobulin E (IgE)–mediated (atopic) disease against fungi. CT findings may be nonspecific, although heterogeneity in an opacified sinus is considered as a typical radiographic finding of this condition.33 However, the dehydrated fungal concretions commonly found in the sinuses, which are rich in iron and manganese, give a characteristic signal of increased density on CT, while producing a decreased (dark) signal on T2-weighted MRI. This differentiates fungal disease from inflamed sinus mucosa, which gives bright signal on T2-weighted images.3,32 Bone erosion, when present, may be caused by chronic pressure from polypoid mucosal disease and/or allergic mucin or by locally secreted bone-destroying enzymes and should not be regarded as bone invasion by fungi.33

Complications of CRS or sinus surgery are imaged on the same principles as uncomplicated sinus disease. Orbital cellulitis and orbital complications of sinus surgery are initially assessed with a contrast-enhanced CT study, which should include the sinuses and the orbits as well as the brain; both bone and soft tissue windows must be analyzed. MRI may provide further information on the integrity of the periorbita, orbital fat, and muscles, while aiding in the evaluation of the optic nerve dural sheath. MRI without and with IV contrast is also indicated for evaluation of suspected skull base or intracranial complications, such as abscesses and venous thrombosis.

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