INTRODUCTION
- Paranasal sinuses (PNS) are air-filled structures located around the nasal cavity and formed with in the bones of the skull and face. The various functions are decreasing the weight of head, conditioning the inhaled air with moisture and heat and increasing the resonance of speech.
- There are four sets of sinuses on either sides namely, maxillary, frontal, ethmoid and sphenoid sinuses.
- Imaging of sinonasal pathology has evolved over the last three decades and advances in imaging have changed the way we look at diseases.
- The unprecedented detail in which the PNS and skull base anatomy are visualized in current generation imaging was not possible earlier.
- Parallel growth of new techniques and approaches, such as endoscopic and image guided surgeries have also increased the demand on imaging.
- Sinonasal inflammatory pathology is very common and usually secondary to infection or allergy.
- Most of them do not need imaging evaluation.
- There specific scenarios where imaging evaluation is indicated.
The consent statement also published the scenarios where imaging is not indicated:
- Uncomplicated sinusitis or upper respiratory tract infections.
- Responding to medical treatment.
To prevent misuse of imaging modalities American College of Radiology (ACR) has published its ACR appropriateness criteria in 2013, which rates the usefulness of an imaging modality in a given situation.2
IMAGING MODALITIES
The imaging modalities which are used to evaluate sinonasal diseases are:
- Radiographs
- Computed tomography (CT)
- Multidetector CT (MDCT)
- Cone-beam CT (CBCT)
- Magnetic resonance imaging (MRI)
- F-18 FDG positron emission tomography (PET).
Radiographs
Plain radiographs were used to evaluate PNS before the advent of CT scans.
Techniques
- The above standard set of views provide good assessment of PNS (Figs. 1.4A to D). Amongst these Water's view is most frequently employed.
- Abnormalities manifest as opacification of sinuses, bone destruction, soft tissue or displacement of structures.
- However, the sensitivity and specificity of plain radiographs are poor compared to newer modalities which make it a less favored modality.
- In a properly exposed radiograph, PNS density is identical to orbital density.
- Erect or sitting position used during radiography of PNS to identify presence or absence of fluid and to differentiate fluid and thickening caused by other pathology.
- The recent AAO-HNS guidelines are also not in favor of using plain radiographs for the evaluation of sinusitis.1
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CT Scan (Figs. 1.5A to C and Table 1.3)
- CT scan is the modality of choice and the workhorse for the evaluation of sinonasal diseases.
- The ability of CT scan to show the sinuses in much better detail coupled with good spatial resolution has practically replaced plain radiographs.
- Findings, such as sinus opacification, bone destruction, mucoperiosteal thickening and normal variations in the anatomy are not only valuable in the diagnostic point of view but also for the preoperative planning in these patients.
Figs. 1.5A to C: Computed tomography (CT) of paranasal sinuses (PNS). (A) Coronal soft tissue window; (B) Coronal bone window; and (C) Sagittal bone window sections used for PNS evaluation.
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Multidetector Row Computed Tomography (MDCT)
- Role of MDCT
- Axial scanning avoids the extension of head
- Lack of obscuration of important anatomy by spray artifact from dental restoration
- Allow more refined reconstructions in other plane than the primary scan plane.
- Role of Coronal Reconstructions
- Primary imaging orientation
- Best shows osteomeatal unit
- Relationship of ethmoid roof to brain
- Correlate with surgical orientation.
- Role of Sagittal Reconstructions
- Better visualization of frontal recess and frontal sinus, these are areas of persistent and recurrent sinus disease
- Better show sphenoid sinus and basal lamella and agger nasi cell, contributing the safer and accurate endoscopic procedure.
- Whenever CT scan images are reviewed, it is best to visualize all three planes simultaneously and they must be linked. So that when the cursor is kept on any plane the other two planes will align accordingly to the same point of interest (Figs. 1.6A to D).
Figs. 1.6A to D: Reviewing method of CT images. Images are viewed in all three planes simultaneously and they are linked to each other. Wherever cursor or cross-hair is placed on an image the other planes align to that point automatically.
- Demonstration of the pathology or the normal structures in multiple planes, such as coronal (most important), axial and sometimes sagittal is crucial for the surgical point of view.
- Multiplanar reformations (MPR) were not possible in older CT scanners, hence direct acquisition of images in both axial and coronal planes were performed. The axial acquisition is done in supine position and coronal acquisition is usually done in prone position with hyperextension at neck. This doubled the radiation dose and increased artifacts due to motion (patient discomfort due to awkward positioning) and dental implants.
- With the advent of MDCT, scenario changed and with the capability of rapid thin section acquisition it is now possible to acquire the entire sinonasal region in seconds. Once the volume data is acquired images can be reformatted in any plane. This technique negates the limitations of older scans.
- The volume data can be post processed to generate various types of 3D images such as volume rendering technique (VRT), surface shaded display (SSD), etc.
- The major limitation of CT scan is the radiation dose involved in the process. Roughly a standard dose CT PNS gives four times radiation dose than a standard three set plain radiographs.3 There are various methods available to reduce the radiation dose by maintaining the diagnostic quality. Two most important techniques which are widely used to reduce the radiation dose are—(1) tube current modulation, and (2) iterative reconstruction.
- Tube current modulation is a type of automatic exposure control in which the radiation dose is adjusted in accordance with the patient habitus, attenuation of body part and the desired image quality. The implementation of this technique varies with the vendors and it can bring down the radiation dose up to 60%.
- Iterative reconstruction is actually an older technique but the advances in computing have made it feasible only recently. Generally, it is added on to the existing filtered back projection method. It has the potential to bring down the radiation dose up to 85%.
Cone Beam CT
- CBCT is getting wide acceptance as a point of care imaging tool and it is compact, portable and cheap compared to main stream CT scanners.
- They use area detectors in contrast to multiple thin rows of detectors in conventional CT. The scanning of area of interest is completed in single rotation as compared to multiple rotations in conventional CT scanners (Table 1.4).
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Figs. 1.7A to C: Cone-beam computed tomography (CBCT) images in all three planes (A to C) showing deviated nasal septum with bony spur (arrow) and concha bullosa (asterisk) on the right side. Note that CBCT images are good for bony structures and but soft tissues are not well-visualized.
- In spite of the above advantages it is currently not able to replace conventional CT for routine evaluation of sinonasal diseases because of various limitations.
- The contrast resolution and signal-to-noise ratio (SNR) is poor in CBCT which is because of scattering effect on area detectors. Hence, it is good in evaluating only the high contrast structures, such as bones (Figs. 1.7A to C).
- The lower dynamic range and poor temporal resolution of the flat panel detectors also pose major limitations.
MRI (Table 1.5)
- Although CT is the preferred modality in the evaluation of sinonasal diseases, MRI has its own place in many situations and it is used as a complementary modality to CT.
- The main advantages of MRI over CT are as follows:
- Better contrast resolution and hence better characterization of soft tissues (differentiating tumor from secretions)12
Table 1.5 Magnetic resonance (MR) planning. - Coronal, axial and sagittal images
- Scan thickness 3–5 mm, interslice distance 1 mm
- Routine sequence T1W and T2W
- Contrast enhanced sequences (3D or 2D)
- T2W/FLAIR sequence for brain
- Fat suppressed used to evaluate complications of sinusitis or suspected neoplastic disease involve orbit and skull base
Figs. 1.8A to D: Magnetic resonance imaging (MRI) of paranasal sinuses (PNS) for sinusitis. (A and B) Axial T1W image and T2W image showing diffuse mucosal thickening in bilateral maxillary sinuses with hyperintense content. (C and D) Coronal and axial postcontrast T1W image revealing diffuse mucosal enhancement. - Better evaluation of tumor spread outside the bony outline of sinuses
- Perineural spread of tumor is best visualized on MRI
- Spread of pathology in to orbit or cranium are also better evaluated on MRI.
- Although there are overlapping features on DWI and perfusion imaging to differentiate benign and malignant lesions, they can be a valuable addition nonetheless.
F-18 FDG PET-CT
- PET-CT is another complementary modality mainly reserved for patients with malignant sinonasal masses to look for metastasis.
- It is also used to look for postoperative residual or recurrent lesions in patients with malignancy.
CONCLUSION
Imaging is not indicated in vast majority of cases of inflammatory lesions of sinus. However, imaging plays a vital role in the diagnosis, preoperative evaluation and intraoperative guidance for surgeons in neoplastic, traumatic, developmental and specific situations in inflammatory conditions. CT is the workhorse in the evaluation of pathology complemented by MRI. With the ever-growing technical advancements in imaging, the value it adds to the management of patient is becoming indispensable.
REFERENCES
- Setzen G, Ferguson BJ, Han JK, et al. Clinical consensus statement: Appropriate use of computed tomography for paranasal sinus disease. Otolaryngol-Head Neck Surg. 2012;147(5):808–16.
- Cornelius RS, Martin J, Wippold FJ, et al. ACR Appropriateness Criteria Sinonasal Disease. J Am Coll Radiol. 2013;10(4):241–6.
- Abul-Kasim K, Strömbeck A, Sahlstrand-Johnson P. Low-dose computed tomography of the paranasal sinuses: radiation doses and reliability analysis. Am J Otolaryngol. 2011;32(1):47–51.