40 Imaging in ENT
40.1 Computed Tomography
CT is primarily used to delineate bony anatomy.
1. Unenhanced CT scans are indicated for:
• Assessment of sinus anatomy (Fig. 40.1) and benign sinus disease prior to functional endoscopic sinus surgery.
• Assessing bone invasion in sinonasal or other head and neck tumours.
2. High-resolution CT (HRCT) of the temporal bone is used for assessment of bony anatomy in the middle and inner ear (Fig. 40.2 and Fig. 40.3).
3. CT can also be useful for staging head and neck cancer in patients who have contraindications to MRI.
Modern multi-slice CT scanners acquire data volumetrically, allowing multi-planar reconstruction of CT images. This is particularly useful in oncology and in complex anatomical evaluation of the temporal bone.
It is important to remember that CT uses radiation. Although the doses received are now much lower on modern scanners, it is essential to limit its injudicious use, particularly in children.
CT is contraindicated in pregnancy. The use of intravenous (IV) iodinated contrast media is contraindicated in those with a history of allergy to IV iodine (not topical iodine) and in strongly atopic patients. IV contrast can be given cautiously in these patients as long as there is medical backup. Steroid use pre-injection is no longer thought to confer any benefit and is not used. Patients with renal impairment should not be given IV contrast unless absolutely necessary and in those circumstances, there should be involvement of the renal team pre-scan and for follow up with urinary function tests.
40.2 Magnetic Resonance Imaging
MRI provides markedly superior definition of soft tissues in any imaging plane and is preferable, for example, in delineating the extent of any head and neck lesion.
Fig. 40.1 Normal anatomy of sinuses on CT. A, cribriform plate; B, lamina papyracea; C, inferior orbital nerve; D, temporalis muscle; E, hard palate; F, inferior turbinate; G, middle turbinate
Fig. 40.2 Normal anatomy of the middle/inner ear on CT. Red arrow, head of malleus; green arrow, short process of incus; blue arrow, internal auditory canal; yellow arrow, lateral semi-circular canal
Fig. 40.3 (a–d) Normal anatomy of the middle/inner ear on CT. (a[i]) Red arrow, stapes inserting into oval window; blue arrow, long process of incus; yellow arrow, external auditory canal, (a[ii]) red arrow, scutum; green arrow, malleus; (b) red arrow, cochlear; blue arrow, internal auditory canal; green arrow, vestibule (c) red arrow, head of malleus; green arrow, long process of incus; (d) red arrow, scutum; green arrow, epitympanum or attic region; blue arrow, head of malleus
There are several MRI weightings and sequences, which can be used to answer specific diagnostic problems.
• T1-weighted (T1W) images In these images, the fluid signal in cerebrospinal fluid (CSF) or in the globes appears dark, whereas fat appears bright or white. T1W images are useful to look at the extent of tumours as they tend to grow through and obliterate the fat planes. The loss of the normal fat in bone marrow in the mandible and within skull base foramina on a T1W image is particularly useful in staging.
• T2-weighted (T2W) images In these images, fluid appears to be of high signal and fat is also bright or white. T2W images are useful for looking for high signal fluid necrosis in metastatic lymph nodes from a squamous cell carcinoma (SCC) and for distinguishing a sinus tumour from surrounding fluid secretions.
• Diffusion-weighted imaging uses the diffusion of water molecules to generate contrast. It is useful to differentiate cholesteatoma from fluid secretions. The water molecules in cholesteatoma are not free to diffuse (i.e. they are restricted) and so appear bright on a diffusion-weighted scan. Diffusion-weighted imaging has also been used to differentiate tumour recurrence from post-radiation changes and has a role in imaging the brain of patients who have suffered an acute stroke.
• Gadolinium is a paramagnetic contrast agent used in MRI. It behaves in exactly the same way as iodine-based contrast media described in the CT section. Fat-saturated T1W images post-gadolinium are used to demonstrate enhancement (an increase in signal) and are useful for showing enhancement in the wall of a necrotic lymph node or for enhancing tumours. Fat saturated means that the bright fat signal is suppressed or removed from the image, leaving the pathology more conspicuous.
• Short-tau inversion recovery (STIR) sequence is a T2W scan with fat signal suppression. This shows pathology and fluid as ‘white’ in the image and therefore more conspicuous than in an ordinary T2W image. This sequence is of great value in the delineation of head and neck cancers.
MRI cannot be used in patients with cardiac pacemakers, iron-containing brain clips, certain cardiac prosthetic valves and implants, and in the first trimester of pregnancy. Gadolinium should also be used with caution in patients with renal impairment. Patients can have an anaphylactic reaction to IV gadolinium; however, this is less common than with IV iodinated contrast.
Table 40.1 shows patterns of tissue characterisation, where observation of signal on T1W and T2W images allows identification of tissue type, and it also relates the MR characterisation to CT appearance.
Ultrasound is a very useful technique for the assessment of masses in the neck, thyroid goitres and for suspected salivary gland pathology. It does not involve any radiation exposure. It can be linked with ultrasound–guided-fine needle aspiration (FNA), performed as an out-patient procedure at the time of diagnostic scanning.
A high frequency (7.5–10 mHz) hand-held probe is placed directly on the neck and the fine needle can be visualised directly entering individual lesions as small as 1 cm in diameter. As a technique, however, it is very operator-dependent requiring an ultrasonologist with specific technical skills who performs head and neck ultrasound regularly.
40.4 Positron Emission Tomography and Computed Tomography
Positron emission tomography–computed tomography (PET–CT) scan uses radioactive glucose in the form of fluorodeoxyglucose (FDG) as the tracer. The FDG is injected into the patient and it accumulates in areas of high metabolic activity. This will include normal structures such as the brain and heart. The tracer is excreted by the kidneys and therefore the urinary tract will also show increased activity. Increased activity is also a feature of tumours, referred to as PET positive lesions.
PET and CT images are acquired during the scan and the images are then fused by software to allow a map of functional activity onto structural landmarks. In head and neck, the two main indications are for investigation of a primary of unknown origin (Fig. 40.4) and for patients with suspected recurrent head and neck cancer. FDG PET/CT is expensive and confers a high radiation dose to the patient; however, the advantage over structural imaging alone outweighs this.
Fig. 40.4 FDG PETCT fused image at the level of the tongue base. Red arrow, T1 primary SCC left tongue base; green arrow, metastatic left level 2 lymph node
40.5 Sentinel Node Imaging
Sentinel node imaging for head and neck is indicated in patients with early (T1) oral cavity tumours and an N0 neck on conventional imaging.
The primary tumour is injected per orally with 40 Mbq of technetium 99m nano-colloid and then dynamic and static nuclear medicine images as well as delayed SPECT/CT images are taken for localisation.
The patient then undergoes surgery the day after the sentinel node imaging and the first order drainage node is identified with a gamma probe in theatre and removed. The whole of the node is sectioned and analysed for tumour involvement. If the node is positive for metastatic disease, the patient is offered a formal neck dissection at a later date. If negative, the patient undergoes 2 years of ultrasound neck surveillance, with three monthly ultrasound scans, to identify any nodal recurrence.
40.6 Plain Films
Following the increased use of complex diagnostic techniques such as CT and MR, plain films are rarely used for diagnostic purposes in ENT. The Royal College of Radiologists’ guidelines recommend that ‘plain films of the sinuses are not routinely indicated’. The radiation dose of a 4 projection sinus series is equivalent to 25 chest X-ray doses or 10 weeks of background radiation.
Lateral soft-tissue views of the neck are of limited value in isolation. They may demonstrate opaque foreign bodies, but many foreign bodies are likely to be non-opaque. Contrast swallows are more accurate in this situation.
40.7 Contrast Swallow
A contrast swallow is indicated in the investigation of dysphagia or symptoms suggestive of motility disorders.
Barium swallows are of superior sensitivity in demonstrating pharyngeal pouches, pharyngeal webs (the web is a linear line indenting the anterior aspect of the barium column at the level of C5/C6) and cricopharyngeus hypertrophy (indents the posterior column of barium at the C6/C7 level). Digital screening is preferred to reduce radiation dose. Image acquisition is preferably at the rate of 2 frames per second during a bolus swallow. Motility disorders are best assessed using dynamic video swallow fluoroscopy.
Water soluble contrast swallows (using non-ionic relatively inert contrast media) are indicated when the history suggests that aspiration into the lungs is a real risk (if barium is used in these circumstances it may lead to resistant chest infection or permanent lung damage), when perforation of the oesophagus is suspected or a surgical anastomosis is being evaluated (barium used in these circumstances will remain in the soft tissues if a leak is present, obscuring the area for further follow-up studies and being a viscous thick suspension it is less likely to demonstrate small oesophageal leaks than water soluble contrast).
Imaging in Otology
Congenital cholesteatoma presents as an avascular mass behind an intact tympanic membrane and usually occurs in children or young adults with a male predominance. HRCT of the temporal bones shows a well-defined mass in the middle ear cleft with or without ossicular erosion.
In acquired cholesteatoma imaging shows a mass in Prussak’s space in the attic region of the middle ear cleft (pars flaccida). Ossicular erosion is seen in 70% of pars flaccida cholesteatoma. There may also be erosion of the scutum and lateral epitympanic wall. Pars tensa cholesteatoma is less common and is typically seen in the posterior mesotympanum. Differential diagnosis for all types of cholesteatoma would include cholesterol granuloma, glomus tympanicum or fluid opacification of the middle ear cleft.
Fig. 40.5 Normal anatomy of the IAM on axial high-resolution T2W MRI. Red arrow, 7th or facial nerve; blue arrow, 8th or vestibulocochlear nerve
HRCT is usually used as the first line of investigation to look for a middle ear mass with associated bone erosion. MRI with diffusion-weighted imaging is helpful to differentiate cholesteatoma from fluid or post-surgical change. Cholesteatoma is typically avascular and does not enhance post-gadolinium; however, it may show peripheral enhancement due to granulation or scar tissue.
40.9 Vestibular Schwannoma
The typical appearance of a vestibular schwannoma is of an enhancing mass at the cerebellopontine (CP) angle or within the internal auditory meatus.
MRI is usually the first line of investigation in patients presenting with sensorineural hearing loss and/or tinnitus. High-resolution T2W images of the CP angles give excellent detail of the vestibulocochlear nerves (Fig. 40.5 and Fig. 40.6) and can be used as a screening tool for vestibular schwannoma. Larger tumours may show central areas of cystic degeneration (Fig. 40.7) shown on both T2W and post-gadolinium scans. Schwannomas can significantly increase in size on serial scans due to interval cystic degeneration. Large areas of cystic change make these tumours less suitable for radiotherapy treatment compared to solid tumours. Bilateral vestibular schwannomas occur in neurofibromatosis type 2 (Fig. 40.8a, b).