The lecture presents three classic infectious scenarios:

1. 42-year-old man — toothache → neck swelling, fever, pain → dental abscess ^[1]
2. 10-year-old boy — fever, sore throat, neck swelling → pharyngitis with cervical adenitis ^[1]
3. 21-year-old woman — right neck pain worse with eating/swallowing → acute sialadenitis from sialolith ^[1]

Contrast-enhanced CT is the workhorse for suspected deep neck infection because it is fast, widely available, maps the airway and deep spaces, and can show an odontogenic or osseous source. ^[1]

Why CT with contrast and not without?

• IV contrast makes abscesses visible: the abscess wall enhances (picks up contrast) while the pus inside does not, creating the classic rim-enhancing fluid collection appearance. Without contrast, you cannot distinguish an abscess from surrounding oedematous tissue reliably.
• Contrast also helps delineate vascular structures and their relationship to the infection (critical for surgical planning).

Do not automatically substitute a noncontrast CT. The slide presents eGFR below 30 or a prior iodinated-contrast reaction as reasons to omit contrast, but current guidance is more nuanced. In acute kidney injury or eGFR below 30 mL/min/1.73 m², the radiologist and referring team should weigh diagnostic benefit against kidney risk and consider prophylactic isotonic fluid when appropriate. A prior severe reaction is a relative contraindication to the same contrast class; an alternative study/agent or a local premedication pathway may be used. A noncontrast scan can miss the distinction between phlegmon and drainable collection. ^[4]

1. Dental infection → submandibular abscess (Ludwig's angina if bilateral floor of mouth) ^[1]
2. Pharyngitis → cervical adenitis, retropharyngeal edema/abscess ^[1]
3. Sialolith (salivary gland stone) → acute sialadenitis ^[1]

CT has radiation — follow principle of ALARA. ^[1]

ALARA = As Low As Reasonably Achievable. In children, the thyroid is particularly radiosensitive, and lifetime cancer risk from radiation is higher because of more remaining years of life. For a superficial/localised paediatric lump, ultrasound can be an appropriate first test. If deep neck infection, airway compromise, complication or a potential drainage target is suspected, contrast CT is usually appropriate; use a paediatric low-dose protocol. ^[5]

Case 1: Submandibular abscess from dental infection

• 42 yo M, toothache 2 days → neck swelling, fever, pain
• Contrast CT shows a low-attenuation, rim-enhancing collection spanning the submandibular/floor-of-mouth region, with surrounding inflammatory change
• The bone-window image shows mandibular periapical lucencies: inspect the teeth as well as the soft-tissue window to identify the source

Case 2: Cervical adenitis and retropharyngeal edema from Streptococcal pharyngitis

• 10 yo M, fever, sore throat, neck swelling 2 days
• CT shows asymmetric cervical fat stranding and several enlarged enhancing/reactive nodes, with central retropharyngeal oedema
• There is no rim-enhancing drainable collection ^[1] — the visual distinction is cellulitis/adenitis plus oedema, not a mature abscess

Case 3: Acute submandibular sialadenitis

• 21 yo F, right neck pain worse with eating (classic for salivary gland obstruction — eating stimulates saliva production, which builds up behind the stone causing pain)
• Coronal CT shows an enlarged oedematous right submandibular gland; axial CT shows a small hyperattenuating calculus in the anterior floor of mouth along Wharton's duct

A new neck mass in adult without signs of infection should be considered malignant until proven otherwise (malignant UPO = Until Proven Otherwise). ^[1]

This is a deliberately cautious lecture rule. Current AAO-HNS guidance defines increased risk more precisely: no convincing infectious cause and persistence for at least 2 weeks without significant fluctuation (or uncertain duration), or a mass that is firm, fixed, larger than 1.5 cm, or associated with skin ulceration. Such patients need prompt targeted examination, contrast CT or MRI, and fine-needle aspiration (FNA) rather than open biopsy. ^[6]

CT or MRI neck with contrast for suspected neoplasm. ^[1]

The slide's suprahyoid/infrahyoid split is a useful heuristic, not an absolute rule. Choose the modality for the clinical question, patient and local protocol:

MRI is study of choice for: NPC staging, intracranial/perineural invasion, parotid tumours, oral cavity tumours (tongue), generally suprahyoid neck. ^[1] CT is good for: generally infrahyoid neck (larynx, hypopharynx). ^[1]

Case 4: Base of tongue cancer with nodal metastases

• 54 yo M, left neck mass 4 months, firm and fixed, tobacco + alcohol abuse
• CT: primary tumour at base of tongue with enlarged cervical nodes
• "In cases of malignant appearing cervical adenopathy, report should comment on primary" ^[1] — the radiologist must look for the primary tumour site
• "Imaging important for staging information (TNM)" ^[1]

Case 5: HPV-associated oropharyngeal cancer — THE CRITICAL PITFALL

A new cystic neck mass in adult should NOT be dismissed as branchial cleft cyst (BCC) or developmental lesion. Must exclude cystic nodal metastasis from HPV+ oropharyngeal SCC! ^[1]

This is highlighted repeatedly in the lecture for good reason. The scenario:

• 37 yo M, new left-sided cystic neck mass
• Outside hospital says "branchial cleft cyst"
• WRONG. In an adult, this is HPV+ oropharyngeal SCC with cystic nodal metastasis until proven otherwise.

HPV-associated OPSCC often arises in the palatine or lingual tonsil, may have a small/occult primary, and commonly presents with cystic cervical nodal metastases. p16 is a tumour-suppressor/cyclin-dependent-kinase inhibitor used by immunohistochemistry as a surrogate marker in the appropriate oropharyngeal setting; it is not a universal stand-alone HPV test for every head-and-neck cancer. [1,7]

Why are HPV+ nodes cystic? The tumour cells undergo cystic degeneration within the lymph node, producing a fluid-filled appearance that can be mistaken for a benign cyst on imaging.

25 yo Chinese woman with left neck mass 3 months → MRI is preferred for NPC staging. ^[1]

NPC is especially relevant for HKUMed because of its high prevalence in Southern Chinese populations. The 2023 MCQ Q64 directly tested NPC management.

Case: Large NPC with intracranial invasion (T4)

• 44 yo M, several months of right-sided hearing loss and ear fullness
• The preceding MRI case shows a nasopharyngeal primary with bilateral nodal metastases (T1N2 on the slide)
• Axial T2 fat-saturated and coronal post-contrast T1 fat-saturated images then show a larger nasopharyngeal mass extending to the skull base/intracranial compartment → T4 ^[1]
• The bright right mastoid fluid reflects Eustachian-tube obstruction and helps explain the unilateral hearing loss/ear fullness
• Demonstrates why MRI is good for intracranial and perineural disease ^[1]

Why MRI over CT for NPC?

• NPC arises in the nasopharynx, which is a deep suprahyoid structure surrounded by complex anatomy (skull base, cavernous sinus, cranial nerves)
• MRI provides superior soft tissue contrast to delineate tumour extent
• MRI good for intracranial and perineural disease ^[1]
• Critical for T-staging: detecting skull base invasion, intracranial extension (T4)

Radiation concern in young patients:

Radiation dose of neck CT: 3 mSv (Annual background radiation dose: 3 mSv). Thyroid gland is most radiosensitive organ in neck. ^[1]

These are broad teaching estimates; actual dose varies substantially with scanner, patient size, scan range and protocol. MRI is preferred for NPC primarily because it better answers the staging question (skull base, intracranial and perineural extension), with radiation avoidance as an additional benefit.

Case: Adenoid cystic carcinoma of parotid with CN-VII involvement

• 57 yo F, right-sided pain and facial paralysis
• The axial T1 image shows the right parotid mass; the accompanying diagram traces CN VII from the parotid back through the stylomastoid foramen and facial canal
• Coronal post-contrast fat-saturated T1 shows abnormal enhancement/thickening along that route toward the skull base
• "MRI to check for perineural tumour spread" ^[1]

Adenoid cystic carcinoma is notorious for perineural spread — tumour cells can track along nerve sheaths far from the primary tumour. CT may show foraminal widening or bone change, but contrast-enhanced fat-saturated MRI is more sensitive for nerve enlargement, enhancement and denervation change.

MRI superior to CT for intracranial or perineural invasion. ^[1]

The paired axial post-contrast MR images show how vessels help localise a carotid-space mass ^[1]:

1. Schwannoma — a smoothly marginated enhancing nerve-sheath mass that displaces adjacent vessels; the exact displacement pattern depends on whether it arises from the vagus or sympathetic chain.
2. Carotid body paraganglioma — centred at the carotid bifurcation, typically splaying the internal and external carotid arteries (the lyre sign) and containing serpiginous flow voids from marked vascularity.

Why flow voids? Rapidly moving blood may produce no signal and therefore appears dark. Larger paragangliomas can have the classic salt-and-pepper appearance, best appreciated on T2-weighted imaging: dark vascular flow voids ("pepper") interspersed with bright slow-flow/haemorrhagic foci ("salt").

Case: 58 yo M, painless, slow-growing neck lump for several years

• CT: "Typical fat attenuation on CT confirms diagnosis" ^[1]
• The mass measures the same HU as subcutaneous fat (-50 to -100 HU)
• On MRI: "Fat is bright on T1 (also: blood, melanin, protein, mineralization, gadolinium)" ^[1]
• "Confirm it is fat with fat suppression" ^[1] — when you apply a fat-saturation pulse sequence, the bright signal from fat disappears. If the lesion goes dark on fat-sat, it's confirmed as fat.
• A homogeneous fat-only mass with a thin capsule supports lipoma; thick septa, nodular non-fatty components or incomplete fat suppression require further assessment rather than assuming a simple lipoma.

"Look for a thorough radiology report in patient with suspicious cervical adenopathy — primary tumor, staging information." ^[1] "T1/T2 signal characteristics can help us (eg, T1 bright lesions)." ^[1]

Start with ultrasound if suspected congenital mass. ^[1]

Why ultrasound first?

• No radiation (critical in children)
• Determines solid vs cystic nature
• Assesses vascularity with Doppler
• Cheap, readily available, no sedation needed
• If US is unclear or the lesion is deep/extensive → MRI

MRI can be complementary to US — visualize full extent of lesion (deep, multifocal). May require sedation. ^[1] CT/MRI offers better anatomic localization and may be helpful as next step if US is unclear. ^[1]

Case 6: Second branchial cleft cyst

• 6 yo boy, painless slowly enlarging lump, recent increase after viral URI
• Grey-scale US shows a well-circumscribed, predominantly anechoic/hypoechoic lesion with posterior acoustic enhancement, establishing its cystic rather than solid nature
• Doppler shows no internal vascularity; peripheral colour signal belongs to adjacent vessels, an important visual distinction
• In a child this age and location, a branchial cleft cyst is a plausible leading diagnosis; an adult with the same cystic appearance still requires malignancy exclusion before accepting that label

Why does it enlarge after viral URI? Branchial cleft cysts are lined by lymphoid tissue. During an upper respiratory infection, this lymphoid tissue becomes reactive and inflamed, causing the cyst to swell.

Case 7: Venous malformation

• 5 yo boy, soft compressible lump, bluish hue to skin
• US and MRI both helpful for suspected pediatric vascular anomalies ^[1]
• MRI to visualize full extent (deeper or extensive lesions) ^[1]
• Doppler US shows compressible venous spaces with slow flow; flow can be sparse unless elicited by compression or dependent positioning
• Rounded echogenic foci with shadowing are phleboliths (calcified thrombi), a strong clue to a venous malformation
• MRI shows lobulated, very T2-bright venous channels that can cross tissue planes; dark internal foci may represent phleboliths or thrombi. MRI maps the trans-spatial extent before treatment.

Why is it compressible and bluish? Venous malformations are low-flow vascular anomalies made up of dilated venous channels. They compress because the blood can be squeezed out (like squeezing a sponge). The blue colour is venous blood visible through thin overlying skin.

The most important history is a previous allergic-like or unknown-type reaction to the same class of contrast. A prior severe reaction is a relative contraindication to reusing that class. When contrast is still necessary, involve radiology, identify the culprit agent if possible, consider a different agent within the class, and follow the current local premedication pathway. Premedication reduces but does not eliminate repeat reactions; breakthrough reactions still occur. ^[4]

The protocols pictured on the slide are institutional examples, not universal prescriptions: ^[1]

Current ACR guidance notes that regimens shorter than 4–5 hours have not been shown effective. In a genuine emergency, delaying a time-critical scan may cause more harm than the theoretical benefit of premedication; the radiologist and treating team should select the safest diagnostic plan and ensure readiness to treat a reaction. Gadolinium reactions are uncommon but can still be severe, and MRI with gadolinium is not an automatic substitute merely because the prior reaction involved iodinated contrast. ^[4]

The lecture prefers MRI and advises against gadolinium. The current, safer formulation is: ^[8]

• Ultrasound and noncontrast MRI are preferred when they can answer the question; available evidence does not show fetal harm from clinically indicated MRI in any trimester, and no special first-trimester restriction is recommended.
• Do not withhold CT when it is clinically necessary. For head/neck CT the fetus is outside the primary beam, so fetal exposure is low; protocol optimisation and radiology discussion are still appropriate.
• Iodinated contrast may be used when it is needed for diagnosis and will affect care.
• Gadolinium should not be routine, but it is not an absolute prohibition: use only when it substantially improves diagnostic performance and is expected to improve maternal or fetal outcome.

Gadolinium crosses the placenta and can recirculate through amniotic fluid, which is why the threshold for its use is high. The key comparison is not "radiation versus no radiation" in isolation, but the risk of the proposed examination versus the risk of delayed or missed diagnosis.

The slide lists orbital metal, reflux devices, insulin pumps, temporary pacing leads, aneurysm clips, shrapnel, infusion pumps, catheters, neurostimulators, cochlear implants and cardiac devices. Many contemporary devices are MR Conditional, while some older, temporary, damaged, abandoned or unidentified devices may be unsafe. Never infer safety from the device category alone. ^[9]

The hazards are displacement/torque, radiofrequency heating, induced currents, device malfunction and image artefact. Final clearance belongs to the MR safety team under the current institutional policy. ^[9]

GC 218 covers the clinical approach to neck masses (history, examination, differential diagnosis). This imaging lecture is the direct companion — once you've clinically assessed the mass, this lecture tells you what scan to order.

Deep neck space infections and pharyngeal/oral cavity cancers are the primary pathologies imaged in this lecture.

The adenoid cystic carcinoma case with CN-VII involvement directly connects to salivary gland malignancies and facial nerve palsy.

CT brain imaging principles (density, Hounsfield units) are the same as taught in this lecture for neck CT.