Role of x-rays in Rhinology
Paranasal sinuses are air containing chambers surrounding the nasal cavity. Pathology involving the sinuses, may present on the radiograph as alterations in the translucency of the sinuses. Some pathological conditions affecting the para nasal sinuses may produce fluid hence effort should be made to demonstrate this fluid level. This is possible if the radiograph is taken in erect position allowing a clear air fluid level to be visualised in the radiograph.
Standard positions of sinus radiographs:
Radiographic positions to study the paranasal sinuses are standardised around three positions:
1. Two anatomical - namely cornal and sagittal
2. One radiographic - termed as radiographic base line. This line represents a line drawn from the outer canthus of the eye to the mid point of the external auditory canal.
The various radiographic positions used to study paranasal sinuses are:
1. Occipito-mental view (Water's view)
2. Occipital-frontal view (Caldwel view)
3. Submento-vertical position (Hirtz position)
4. Lateral view
5. Oblique view 39 Degrees oblique (Rhese position)
Occipito mental view: This is also known as the Water's view. This is the commonest view taken to study the paranasal sinuses. The patient is made to sit facing the
radiographic base line tilted to an angle of 45 degrees to the horizontal making the sagittal plane vertical. The radiological beam is horizontal and is centered over a point 1 inch above the external occipital protruberance. In obese patients with a short neck it is virtually impossible to obtain an angulation of 45 degrees. These patients must be made to extend the neck as much as possible and the xray tube is tilted to compensate for the difference in angulation. The mouth is kept open and the sphenoid sinus will be visible through the open mouth. If the radiograph is obtained in a correct position the skull shows a foreshortened view of the maxillary sinuses, with the petrous apex bone lying just beneath the floor of the maxillary antrum. In this view the maxillary sinuses, frontal sinuses and anterior ethmoidal sinuses are seen. The sphenoid sinus can be seen through the open mouth.
Image showing positioning of patient in x-ray sinus water's view
Image showing structures seen in Water's view
If the antrum in water's view demonstrates a loss of translucency which could be an indicator of fluid level, then another x ray is taken with a tilt of saggital plane to an angle of 30 degrees. This view will clearly demonstrate movement of fluid to a new position. In this view the fluid moves towards the lateral portion of the antrum where it can clearly be seen.
In x ray para nasal sinuses waters view the normal frontal sinus margins show scalloping. Loss of this scalloping is a classic feature of frontal mucocele. If frontal sinus is congenitally absent (agenisis) then a suture line known as the metopic suture is visible in the fore head area. Sometimes a pair of large anterior ethmoidal air cells may take up the place of frontal sinuses. Here too the metopic suture line is visible. This suture divides the two halfs of frontal bone of the skull in infants and children. This suture line usually disappears at the age of 6 when it fuses. If this suture is not present at birth it will cause a keel shaped deformity of the skull (trigonocephaly).
X-ray PNS showing metopic suture
Since hypoplastic antra are associated with sclerosis of its margins, it will be very difficult to perforate the medial wall of the antrum while performing antrostomy.
In conditions like malignancy involving the maxillary antrum X ray sinuses waters view shows the following features:
Expansion is characterised by increase in the size of maxillary antrum when compared to its counter part on the opposite side.
Erosion may occur in the medial wall of the antrum or in its antero lateral wall. The canine fossa area is the thinnest portion of maxillary antrum antero lateral wall. Erosion is hence common in this area.
Opacity is the term used to describe a maxillary sinus antra involved with malignant growth. This opacity is due to the periosteal reaction due to malignant growth.
X-ray PNS showing malignant growth maxilla right side
Occipito frontal view (Caldwell view): This position is ideally suited for studying frontal sinuses. In this position the frontal sinuses are in direct contact with the film hence there is no chance for any distortion or geometric blur to occur.
Image showing position of patient in Caldwel view
To get a caldwell view the patient is made to sit in front of the film with the radiographic base line tilted to an angle of 15 - 20 degrees upwards. The incident beam is horizontal and is centered 1/2 inch below the external occipital protruberance. This view is also known as the frontal sinus view.
Submentovertical view: is primarily taken to demonstrate sphenoid sinuses. Fluid levels in sphenoid sinuses are clearly shown in this view. To take an x ray in this position, the back of the patient is arched as far as possible so that the base of skull is parallel to the film. The x ray beam is centered in the midline at a point between the angles of the jaws. In elderly patients this view can be easier to achieve if carried out in the supine position with the head hanging back over the end of the table. This view also demonstrates the relative thicknesses of the bony walls of the antrum and the frontal sinuses.
Image showing positioning of patient in submento vertical view
This view helps in distinguishing the various pathologies involving the frontal sinuses. It helps in determining whether the loss of translucency is due to thickening of the anterior bony wall or infection of the frontal sinus per se. This view also demonstrates fluid levels in the antrum. This view also gives information on the naso pharynx and soft palate. This is infact a standard projection used to ascertain enlargement of adenoid tissue.
X-ray pns lateral view:
For this view the patient is made to sit with the sagittal plane parallel to the xray film and the radiographic base line is horizontal. The incident ray is horizontal and the incident beam is centered at the mid point of the antrum.
Image showing patient's position in lateral view
This view helps in demonstrating posterior ethmoid air cells and optic foramen. To obtain this projection the patient is made to sit facing the film. The head is rotated so that the sagittal plane is tured to an angle of 39 degrees. The radiographic base line is at an angle of 30 degrees to the horizontal. The incident beam is horizontal and is centered so that the beam passes through the centre of the orbir nearest to the film.
With the advent of CAT scan these modalities of imaging are slowly losing their relevance.