Pre-FESS Imaging of Paranasal Sinuses and Nasal Cavity: Using Multi-detector Computed Tomography (MDCT) in Understanding Normal Anatomy and Anatomical Variations: Tips and Tricks

Abstract

With the advent of Multi-detector Computed Tomography (MDCT), imaging evaluation of paranasal sinuses (PNS) and nasal cavity has witnessed a paradigm shift. Submillimetric slice thickness with superior quality multiplanar reconstructions in orthogonal as well as non-orthogonal planes have resulted in better understanding of the intricate details of complex sino-nasal anatomy and their normal variations. Sagittal plane images have gained increasing acceptance among the sinus surgeons for understanding frontal sinus drainage pathway as well as lateral nasal wall. Analyzing the axial volume dataset in three dimensions has become a routine both for the radiologists as well as sinus surgeon. Besides default window settings, customized window settings enable better appreciation of air containing structures and their boundaries. Mandatory scan check list and template based structured reporting helps the sinus surgeon in better pre-operative planning. Regular mutual interaction between radiologists and sinus surgeon helps better understanding of the surgically relevant anatomy and anatomical variations.

Introduction

Rhino-sinusitis is a common clinical problem. Uncomplicated acute sinusitis is usually diagnosed clinically and often responds to medical treatment. However, patients with recurrent and refractory sinusitis may need surgical drainage procedure to alleviate their symptoms. Functional Endoscopic Sinus Surgery (FESS), which aims at establishing a drainage pathway with mucosal preservation, has evolved as a very effective and preferred mode of surgical treatment for chronic recurrent/refractory sinusitis. Although considered as minimally invasive, there can be serious or even potentially fatal complications following FESS. Most of these complications are attributed to unrecognized critical anatomical variations in the pre-operative imaging work-up. With the advent of Multi-detector Computed Tomography (MDCT), imaging evaluation of paranasal sinuses (PNS) and nasal cavity has witnessed a paradigm shift. Although literature is abundant with information about Pre-FESS CT scan evaluation of PNS and nasal cavity, there is paucity of information about practical applications of MDCT to the best advantage of the sinus surgeon. This review article deals with the tips and tricks of utilizing the full potential of MDCT in the evaluation of PNS and nasal cavity in the current context.

Role of Imaging

In today’s context, radiography of PNS has limited role for preoperative work-up of chronic rhino-sinusitis. CT scan, particularly MDCT, is the gold standard for Pre-FESS imaging workup of PNS and nasal cavity. Thin section high resolution CT (HRCT) scan helps in (a) understanding normal sino-nasal anatomy, (b) evaluation of extent of sinus disease, (c) identify anatomic variants that may narrow or obstruct sinus drainage pathways, (d) demonstrate anatomical variations that makes endoscopic access difficult or even predispose to surgical complications, (e) identify potential danger areas and alert the sinus surgeon pre-operatively as well as (f) navigation and guidance during surgery. Magnetic Resonance Imaging (MRI) has limited role and more often problem solving role in specific situations like evaluation of extent of soft tissue and intracranial involvement in infective and neoplastic pathologies.

Why MDCT for PNS Imaging?

Sino-nasal air spaces provide exquisite natural contrast with the bony outline, making CT scan an ideal modality for imaging of PNS. As compared to earlier generation CT scanners, MDCT has several advantages in PNS imaging as mentioned below:

1. Ability to carry out volume imaging in a comfortable patient position (mostly supine) without the need for mandatory direct coronal scanning and reconstruct images in any desired plane.

2. Ability to obtain thin sections (sub-millimetric) which enables multi-planar reconstructions in orthogonal as well as non-orthogonal planes with near isotropic resolution (Fig. 1) and helps generate excellent 3D images on post processing.

3. High speed scanning with significantly reduced suboptimal study due to motion blur.

4. Better contrast opacification and timing when contrast enhanced studies are needed.

Fig. 1.

a–c MDCT PNS in orthogonal MPR format. Note isotropic resolution of coronal as well as sagittal plane images when compared with axial plane. The data was acquired volume scan in axial plane. Note the clarity of course of infra-orbital nerve shown in the sagittal reformat (a)

Which CT Scan Machine?

An essential requirement of CT scan of PNS is thin section (≤ 1 mm) volume scan and image reconstruction in appropriate algorithm and window settings. In this context, it is important to understand that any present generation MDCT scanner starting from 16 row MDCT onwards can easily generate the required datasets. There is no need to rush for higher detector row/slice MDCT as far as PNS imaging is concerned.

CT Scan Technique

CT scans for PNS are best performed once episodes of acute sinusitis are over and are ideally deferred for 4–6 weeks after initiation of medical treatment. Patients are often requested to blow their nose before CT scan. Some institutions also use nasal decongestants prior to the study. A thin section (≤ 1 mm) volume acquisition in supine position suffices. Direct coronal imaging with tilted head is no longer required/ practiced in MDCT. Images are evaluated in all three planes. Traditionally, coronal plane images have played very important role in the evaluation of PNS. Most structures, especially the ostiomeatal complex is best evaluated in coronal plane. With the advent of MDCT sagittal images have been found to be increasingly relevant in the Pre FESS CT scan evaluation. Sagittal scan is particularly useful for the evaluation of frontal sinus drainage pathway, demonstration of obstructing anterior ethmoidal air cells as well as lateral nasal wall. Certain structures like optic nerves, pterygopalatine fossa, and fossa of Rosenmuller etc.are evaluated better on axial plane. Contrast enhanced studies are not performed routinely for pre FESS evaluation. However, post contrast studies may be required in cases of suspected/known neoplastic etiology.

CT scan protocols for image guided navigation includes: (a) Slice thickness < 1 mm, (b) No gantry tilt, (c) FOV to include both ears, entire maxilla, tip of the nose, chin and frontal sinuses, (d) MPR in all three orthogonal planes.

Viewing the Images

As with many other imaging modalities, CT scan images are also generally stored/ archived in DICOM (Digital Imaging and Communications in Medicine) format. Images are best viewed and analyzed in the dedicated workstation of the CT scan machine itself or through dedicated PACS. Stored images can also be viewed equally effectively and conveniently in any desktop/laptop with the help of DICOM viewing software. There are quite a few commercial free/paid DICOM viewing software available as on date. Such software varies in terms of requirement of operating system, processor/RAM, hard disc space, PACS/Cloud integration, and facility for multi-planar reconstructions (MPR), Maximum Intensity Projection (MIP), Image fusion and many advanced post processing applications etc. It is important to have one of these DICOM viewing software installed for viewing CT scan images. It is recommended to have a high resolution monitor for viewing DICOM images. In an MDCT, PNS images are usually acquired in axial plane in volume scanning mode. Therefore, the images need to be loaded in the MPR mode to analyze in coronal as well as sagittal plane besides axial plane. It is a good idea to always view images in all three orthogonal planes to understand the anatomy and pathology well (Fig. 2). Additionally, as needed, images can also be viewed in the oblique plane in the line of region of interest for better delineation of certain structures. 3 D rotation tool can be used to align region of interest in case of asymmetrical disposition of anatomical structures (Fig. 3).

Fig. 2.

a–c Viewing images in all three orthogonal planes in DICOM viewing software. Note bilateral concha bullosa. Left sided concha bullosa is depicted in all three planes highlighting value of analyzing images in MPR format

Fig. 3.

a–d Utility of 3D rotation tool available in DICOM software: Image dataset/ anatomical structures may not always be in perfect/symmetrical position. a, b depicts pneumatization of anterior clinoid process seen in two different slices/positions in coronal plane. Using the 3D rotation tool, the same is seen nearly symmetrical and in complete extent in both axial and coronal images (c, d)

Does Windowing Matter?

Conventionally, two sets of images are generated during PNS CT scan in single acquisition: one set in the bone window setting and another set in soft tissue window setting. Exact appearance of images may vary from machine to machine depending on the reconstruction kernel, algorithm and window settings used. It is important to remember that bony outlines will be better seen in bone window images as soft tissue details will be better appreciated in soft tissue window only. Attempt to evaluate soft tissue structures by modifying bone window images are not advisable as it results in unacceptable degradation of images (Fig. 4).

Fig. 4.

a–c Appearance of PNS CT scan images in the default bone (a) and soft tissue (b) window. Attempt to change the bone window image into soft tissue window image results in unacceptable level of noise in the resultant image (c)

Although air spaces in the PNS and nasal cavity provides excellent natural contrast for CT scan, many air cells and important structures are bound by very thin and nearly imperceptible walls if viewed only in the default bone window setting. Such structures are better seen if the window setting is customized by gradually lowering the window level (WL) also known as window centre. As the WL is decreased, air spaces become conspicuous with their outline. If only air spaces need to be evaluated, then a customized window setting similar to lung window can be used (Fig. 5). It is recommended that sinus surgeons customize the window settings depending on the structure/area of interest while viewing the CT scan images so as to obtain maximum information from the given images and avoid any potential pitfall.

Fig. 5.

a–d Effects of changing and customizing window levels. a depicts default bone window appearance of the image. As the window level is decreased to half (i.e. 300) keeping the window width constant (i.e. 2000), the thinner septae and linings start becoming visible (b). As the window level is further reduced to zero (c), the air spaces with intervening septae/ boundaries become dominantly visible as against the back ground. d Window settings similar to lung window and depicts mainly the air spaces and it would be difficult to comment on other structures

Are the Radiologists and Sinus Surgeons in Sync?

Unfortunately, answer to this critical question is- ‘possibly not’. It is well known that a pre-FESS CT scan is useful only when the scan report is clinically relevant. In an interesting nationwide survey in Canada, Deutschmann et al. [1] found that majority of the Otolaryngologists indicated that the radiology reports were lacking in clinically useful information and there were inconsistent reporting of predefined sinus CT items. It is extremely important that radiologists reporting Pre-FESS CT scans be aware of what the sinus surgeon needs, be in sync with the sinus surgeon and thereby make the CT scan report more relevant clinically and preoperatively. Irrespective of the experience of the radiologists, conventional reporting of CT scans tends to inadvertently overlook or miss out important and often relevant findings in the radiology reports. The solution lies in having a scan check list and mandatory structured reporting incorporating all relevant predefined sinus CT items [2, 3].

Scan Check List and Structured Reporting

A successful FESS requires precise preoperative anatomical roadmap. It is essential that radiologists reporting PNS CT scans are familiar with the relatively complex sino-nasal anatomy in all three planes. It is equally important for the radiologist to be aware of normal variants which may either make the endoscopic access to the drainage pathways difficult or even predispose to potential surgical complications. It is therefore important to have a scan check list for all surgically relevant anatomical landmarks including critical anatomical variants. It is also imperative that a CT scan report incorporates all such points in a structured format so that no surgically relevant finding is missed out inadvertently.

A comprehensive scan check list (adapted from Vaid et al. [2]) includes (but not limited to) the following: (a) Nasal septum- any deviation, pneumatization (Fig. 6); (b) Middle turbinate- attachments in all three planes (Fig. 7), concha bullosa, paradoxical curvature, inter-lamellar cell of Grunwald (Fig. 8); (c) Uncinate process: Orientation, superior attachment pattern, angle with lamina papyracea (Fig. 9); (d) Ostiomeatal complex & maxillary sinus- Haller Cells, accessory ostia, extent of pneumatization, intra-sinus septae, course & position of infra-orbital nerve (Fig. 10); (e) Frontal sinus drainage pathway- patency and size of frontal ostium; (f) Agger nasi- extent of pneumatisation; (g) Frontal sinus- Extent of lateral pneumatization (Fig. 11); (h) Anterior ethmoidal cells, Bulla ethmoidalis, Supraorbital cells- degree of pneumatization; (i) Basal lamella- pneumatization (Fig. 12); (j) Posterior ethmoid sinus- Height of post ethmoid cells from roof of maxillary antrum (Fig. 13); (k) Sphenoid sinus- Degree of pneumatization, intra-sinus sepate, Onodi cell (Fig. 14); (l) Anterior skull base- depth of olfactory fossa, slope of fovea ethmoidalis (Fig. 14); (m) Ant ethmoidal artery- presence and position (Fig. 15); (n) Lamina papyracea- dehiscence if any, position wrt maxillary ostium (Fig. 15); (o) Brain, orbit and nasopharynx- any abnormality.

Fig. 6.

a, b Depicts septal deviation (a) and pneumatization of bony nasal septum (b)

Fig. 7.

a–c Attachments of middle turbinate most anteriorly to the cribriform plate (a), in the mid portion to the lamina papyracea (b) and posteriorly to the medial wall of maxillary sinus (c)

Fig. 8.

a–c Concha bullosa (left) (a), Bilateral concha bullosa (b), Bilateral interlamellar cell of Grunwald (c)

Fig. 9.

a–c Uncinate process- superior attachment to the lamina papyracea (a), laterally positioned uncinate process with hypoplastic maxillary sinus (Rt) (b), pneumatization of uncinate process (c)

Fig. 10.

a–e Typical appearance of ostiomeatal complex in coronal plane (a). MM: Middle meatus, UP: Uncinate process, HSL: Hiatus semilunaris, BE: Bulla ethmoidalis, EI: Ethmoidal infundibulum, MO: Maxillary ostium. Haller cell (b), Accessory maxillary ostium (c), Intra-sinus septate Rt maxillary antrum (d) and hanging infra-orbital nerve with a bony mesentery in left maxillary antrum (e)

Fig. 11.

a–d Frontal sinus drainage pathways depicted in sagittal images (a, b), Agger nasi cell (b), inter frontal sinus septal cell (c), Vertical & posterior extent of pneumatization of frontal sinus shown (d)

Fig. 12.

a–c Bulla ethoidalis (a), Pneumatization of basal lamella shown in coronal and corresponding sagittal images (b, c)

Fig. 13.

a–d Measurement technique of height of posterior ethomoidal air cells shown (a), Various types of sphenoid sinus pneumatization pattern is shown- Conchal (b), Pre-sellar (c) and Sellar (d) type

Fig. 14.

a–d Extent of pneumatization of sphenoid sinus is shown (a, b), LR: lateral recess, Note vulnerable vidian nerve on the right side (arrow) (b), Onodi Cells (asterix, c), Measurement technique for height of olfactory groove (d)

Fig. 15.

a–c Anterior ethmoidal artery is shown in coronal (a) as well as sagittal images (b) (arrow). Supra-orbital cell when present can influence position of anterior ethmoidal artery (block arrows). Normally medial wall of orbit and maxillary ostium lie in the same plane as shown (d)

Need of the Hour

An optimum communication and coordination between the sinus surgeon and the radiologists remains the cornerstone for successful utilization of full potential of MDCT in the Pre-FESS imaging. The following needs to be adhered to:

For the Sinus Surgeon

(a) Inform the radiologist that the CT scan is meant for a pre FESS work up, (b) Inform the radiologist if image guidance CT protocol is needed, (c) Freely interact with the radiologist regarding any doubt/clarification about CT scan findings and (d) be ready to review the images at the CT scan console along with the radiologist.

For the Radiologist

(a) be aware about the requirements of the sinus surgeon, (b) be familiar with the normal anatomy and anatomical variations of PNS and nasal cavity, (c) ensure appropriate PNS CT protocol with ≤ 1 mm section is used, (d) provide images in all three orthogonal planes in bone as well as soft tissue windows, (e) use a mandatory scan check list before reporting, (f) always report in a structured format using appropriate template including all relevant CT sinus terms, (g) freely interact with the sinus surgeon regarding any doubt/clarification about surgical relevance of a particular CT scan finding.

Conclusion

MDCT plays an important role in Pre-FESS imaging work up. Use of correct MDCT PNS protocol & technique along with appropriately customized window settings while viewing images helps getting maximum useful information. Mandatory comprehensive scan check list with template based, structured reporting helps the sinus surgeon to understand the extent of disease, anatomy and surgically relevant variations for a better preoperative planning. This also helps to prevent/reduce instances of surgical complications during and after FESS.