Computed tomography (CT) scans of the paranasal sinuses are routinely ordered by otolaryngologists evaluating patients with signs and symptoms of sinus disease. CT scans provide information about a patient's anatomy and disease, which may not be appreciated on physical examination, and are used to help otolaryngologists develop a treatment plan for patients. In cases in which patients undergo sinus surgery as a part of their treatment plan, CT scans are a critical tool that surgeons utilize to help them provide surgery that is complete, well tolerated and effective.
Although most otolaryngologists will review the images of any sinus CT they order, they also rely on radiologists’ interpretations to point out aspects they might have overlooked, as well as any incidental findings seen in the surrounding structures (i.e. intracranial abnormalities). Radiological interpretation and reporting is known to be widely varied; there is little consistency in radiology reports of sinus CT scan findings. We believe that a comprehensive, template-driven, methodical approach towards sinus CT interpretration would benefit radiologists and otolaryngologists. In this study, we present this approach towards interpretation of sinus CT scans [1▪▪].
HISTORY OF SINUS COMPUTED TOMOGRAPHY IMAGING
Compared with the other segments of the neck and head, the nasal cavity and paranasal sinuses are relatively easy structures to image. In fact, the main limitation to providing spatial information of the paranasal sinuses and soft tissue information of the adjacent facial and cranial spaces is regional artifact from dental repair and limitations of patient positioning. Image data contain both of these elements and they are usually competing with each other on a technical basis. Many of the disorders that afflict the patient population seen in a diagnostic radiology suite are inflammatory and allergic; but imaging techniques need to be comprehensive enough to depict the rare and sometimes ominous sources of symptoms and/or signs seen in an otolaryngology patient cohort.
CT is the workhorse of paranasal sinus imaging in the USA and has traditionally been performed on conventional CT platforms that spin an X-ray tube in a continuous circular arc around the patient. The X-ray beam is collimated both at the source and at the detector to reduce patient exposure and scatter radiation, which degrades image quality. These scanners are, by design, oriented upright resulting in a gantry opening that is perpendicular to the positioning table or couch. Direct, physiologically appropriate erect positioning of the head in this scanner design requires an uncomfortable hyperextension of the neck relative to the prone body position on the scanner bed to achieve a ‘direct coronal’ image. Alternatives to this include compact CT units and supine positioning, the latter of which is not physiologically appropriate [2,3].
Late model CT scanners provide a ‘block’ of imaging data that can be presented in the plane from which it was acquired: the ‘primary plane’. This block of ‘raw data’ can be reconstructed in the scanner's computer processor. The raw image data are available for manipulation into any tissue algorithm (bone, intermediate and soft tissue) or field of view (target), and can be reformatted into any ‘secondary plane’ of interest (primarily reformatted coronal images) as well as for three-dimensional renderings for surgical consultation. Because raw data are too large to archive permanently, any atypical image information that is needed should be specified by the ordering physician at the time of service.
Because the quality of bone detail has dramatically improved with multidetector row CT (MDCT) scanners, many radiology departments have transitioned to a more gentle, supine patient imaging position with the creation of reformatted coronal (and sagittal) series. The quality of the structural data is optimal because the images can be created with isotropic parameters (equal in three planes) beginning with a minimum of 16 detector rows. The comfort level of interpretation of images acquired in this mode varies amongst imagers. The redistribution of fluid collections and air, the repositioning of polypoid lesions and the shifting, albeit slow, of submucosal edema can confound accurate interpretation in cases of moderate and advanced disease. The benefit of single-plane image acquisition for reduction of the radiation exposure in the second plane is obvious and the primary driver of this approach.
COMPUTED TOMOGRAPHY INTERPRETATION – HOW WE DO IT
CT interpretation should be comprehensive yet concise. We find that a template-driven approach allows the radiologist to meet these goals in an efficient manner that satisfies the needs of the radiologist as well as the clinician who ordered the scan .
The authors recommend a methodical, step-by-step approach to sinus CT interpretation and reporting. We believe that a template-driven approach increases the likelihood for radiologists of recognizing abnormalities when present, as well as providing a consistent, user-friendly report for ordering physicians. We suggest a template similar to that in Fig. 1.
Imaging overview or impression provides a summary of the findings. Pertinent findings include the presence or absence of signs of infection, inflammatory disease, nasal/sinus masses, signs of prior surgery and skull base defects. A statement regarding septal deviation and turbinate hypertrophy is typically included in this section.
Comment on the nasal cavity typically covers the septum, inferior turbinates and middle turbinates. Presence or absence of septal deviation and/or perforation is noted here, as are comments on inferior turbinate hypertrophy. Commentary on middle turbinates should include the presence or absence of concha bullosa [5–8] as well as whether or not the turbinates are paradoxically curved . Paradoxically curved middle turbinates may impact septal and sinus development, and awareness of this anatomical finding can influence a patient's treatment plan [10▪].
As the sinus cavities are addressed individually, any inflammation, infection, mass or defect should be noted. Anatomic abnormalities that may predispose patients to sinus disease are noted when reviewing each sinus. Specifically, evaluation of the maxillary sinuses should comment on the presence of Haller cells and how they impact the sinus drainage pathway [11▪,12]. Maxillary retention cysts or polyps may be present and should be noted, especially in cases in which the cysts are large enough to affect sinus function. Hypoplastic maxillary sinuses are common and should be noted when present, as should any related impact on the location of the orbital floor [13▪,14]. Defects in the orbital floor/maxillary sinus roof may be noted in this section as well. It is useful to comment on the signs of prior sinus surgery, particularly as concerns abnormalities and scar tissue that might lead to persistent patient symptoms such as the presence of inferior meatal antrostomies and maxillary sinus recirculation [15,16].
The ethmoidal infundibulum is a critical structure whose status impacts the health of the surrounding sinuses. The CT scan will show the infundibulum to be open, narrowed or blocked, and this should be specifically commented upon. On occasion, the uncinate process, which partially defines the infundibulum, may be atelectatic, retracted laterally or partially resected. This should be noted.
Ethmoid sinuses are classically honeycomb in appearance. Due to the honeycomb nature of the sinuses, mucoceles are prone to form and should be noted, particularly as they may impact the adjacent orbit and skull base . Surgery on the ethmoid sinuses involves operating near the orbit and skull base and places these structures at an increased risk of injury. Certain anatomic abnormalities can increase risk and should be commented upon . Specifically, a missing or thinned medial orbital wall should be noted as should a thinned or defective skull base . A low-lying ethmoidal skull base should be commented upon when present. More anteriorly, an asymmetric or low-lying fovea ethmoidalis [19–21] should be described using the Keros classification system [22▪▪].
For those patients with frontal sinus disease, there are some anatomic abnormalities that can contribute to persistent blockage. Although many patients have agger nasi cells [23,24], which help to define the frontal sinus drainage pathway, others have cells that may block drainage. These ‘frontal cells’, as they have been called, may extend into the frontal sinus proper leading to constriction of frontal sinus drainage [25▪]. These are best appreciated in the sagittal plane and, when present, they should be commented upon .
Structures that impact function of the sphenoid sinuses include Onodi cells – or supraorbital ethmoid cells [27▪]. When present, these should be identified particularly in patients undergoing surgery. The sphenoid sinus abuts the carotid artery and the optic nerve. The relationship between these vital structures and the sinus is often worthy of comment .
The brain, intracranial vasculature, orbit, nasopharynx and oral cavity surround the paranasal sinuses and are imaged on sinus CT scan. Otolaryngologists rely on their radiologist colleagues to point out any and all abnormalities that might require further evaluation. It is imperative that comments on abnormalities of these structures be highlighted, preferably in the summary, for the ordering physician.
There is currently significant variation in the methodology by which sinus CT scans are interpreted and the findings are reported. We believe that it will benefit radiologists and otolaryngologists alike to use a simple, template-driven approach to describe the findings encountered on a sinus CT. In this study, we present one such approach.
Conflicts of interest
The authors have no conflicts of interest to report.
REFERENCES AND RECOMMENDED READING
Papers of particular interest, published within the annual period of review, have been highlighted as:
▪ of special interest
▪▪ of outstanding interest
Additional references related to this topic can also be found in the Current World Literature section in this issue (pp. 89–90).
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