Principles and Application of MRI in ENT

1. PHYSICAL PRINCIPLES OF MRI

Basic Mechanism

• Cummings Otolaryngology, Ch. 8

Magnets Used in Clinical MRI

• Cummings Otolaryngology, Ch. 8

2. COILS AND TECHNICAL CONSIDERATIONS

Receiver Coils

• Standard head coil: Adequate for disease above the angle of the mandible; allows imaging of adjacent brain and orbits - useful when lesions extend intracranially
• Neck coil: Volume neck coil or anterior neck coil; covers skull base to clavicles; significantly improves signal/noise ratio
• Surface coils collect signal from a smaller body part, which improves resolution

• Cummings Otolaryngology, Ch. 8 & Ch. 107

Slice Thickness

3. PULSE SEQUENCES AND SIGNAL CHARACTERISTICS

T1-Weighted Images (T1WI)

• Short TR (500-700 ms) and short TE (15-40 ms)
• Fat = high signal (bright/white) - provides natural contrast in head and neck
• Air, rapid blood flow, cortical bone, CSF, vitreous = low signal (dark/black)
• Muscle = low to intermediate signal
• Bone marrow = bright (fat within marrow)
• The fundamental head and neck sequence: provides excellent soft tissue anatomy, high signal/noise ratio, minimal motion artifact (4-5 min acquisition)

T2-Weighted Images (T2WI)

• Long TR and long TE
• Fluid = high signal (bright)
• Fat = high signal
• T2WI is sensitive for pathology; postobstructive change with high fluid content is typically bright on T2, while tumors usually show intermediate brightness
• Particularly useful for differentiating tumor from retained secretions

Gadolinium-Enhanced T1WI

• IV gadolinium (a paramagnetic contrast agent) shortens T1 relaxation times and increases signal intensity in vascularized/inflamed tissues
• Enhances solid tumors, perineural spread, meningeal disease, infected mucosa
• Gadolinium substantially lower allergic reaction risk than iodinated CT contrast
• Contraindicated/used conservatively in renal dysfunction (risk of nephrogenic systemic fibrosis)

Fat-Suppressed Sequences (Fat-Sat)

• Selectively darken lipid-rich areas, allowing enhancing lesions to stand out
• Critical for distinguishing gadolinium enhancement from adjacent fat
• Applied to both T1WI (post-contrast) and T2WI
• Particularly important at the skull base and pterygopalatine fossa

Short T1 Inversion Recovery (STIR)

• Suppresses fat signal; highlights edema and fluid
• Useful for detecting marrow infiltration, lymph node disease, and inflammatory change

Diffusion-Weighted Imaging (DWI)

• Measures tissue cellularity and water diffusivity
• Restricted diffusion (bright DWI, low apparent diffusion coefficient [ADC]) indicates hypercellular or dense tissue (malignant tumors, cholesteatoma, epidermoid cysts)
• ADC values (sinonasal):
  • Benign lesions: ADC ≥ 1.5 (±0.5) × 10⁻³ mm²/s
  • Malignant lesions: ADC ≤ 0.98 (±0.5) × 10⁻³ mm²/s
• DWI is particularly useful in distinguishing epidermoid cysts (restricted diffusion) from arachnoid cysts (no restriction)
• Increasingly used for detecting residual and recurrent nasopharyngeal carcinoma (NPC) after treatment
• Cummings Otolaryngology, Ch. 8 & Ch. 47

Gradient-Echo Sequences

• Faster acquisition; useful for vascular imaging (MR angiography)
• More susceptible to metallic artifacts than spin-echo

4. MRI ARTIFACTS IN ENT

5. ADVANTAGES AND DISADVANTAGES OF MRI IN ENT

Advantages

• No ionizing radiation
• Superior soft tissue contrast and differentiation
• Direct imaging in any plane (axial, coronal, sagittal, oblique)
• Multiple sequences allow precise tissue characterization
• Better delineates perineural spread, intracranial extension, marrow invasion
• Distinguishes tumor from retained secretions (key in sinuses)
• Reduced artifacts from dental amalgam compared to CT
• Can fuse with CT/PET for radiotherapy planning
• Better detection of ectopic mediastinal parathyroid glands (sensitivity >80%)

Disadvantages

• Long scan duration (susceptibility to motion artifact)
• Small bore induces claustrophobia
• Contraindications: ferromagnetic implants, older cardiac pacemakers/defibrillators, ferromagnetic aneurysm clips
• Cochlear implants require special precautions (see below)
• No visualization of fine bony detail (CT is superior for this)
• Less useful in the thorax (motion, field distortions)
• Gadolinium contraindicated in renal impairment
• Cummings Otolaryngology, Ch. 107

6. ENT-SPECIFIC APPLICATIONS

6a. Paranasal Sinuses and Nasal Cavity

• Distinguishes tumor from postobstructive secretions (tumors: intermediate T2; secretions: bright T2)
• Detects perineural spread: nerve enhancement and enlargement on gadolinium T1 predict spread; retrograde involvement of Meckel's cave/cavernous sinus carries poor prognosis
• Assesses skull base invasion: dural thickening >2 mm, loss of hypointense zone, nodular enhancement are highly predictive of dural invasion
• Differentiates orbital periosteum from tumor: lamina papyracea enhancement indicates orbital involvement
• Detects encroachment into fat-rich spaces (periorbita, pterygopalatine fossa, parapharyngeal space) on precontrast T1WI - tumor appears darker against bright fat
• Fungal disease appears iso- or hypo-intense on T1, with signal void on T2 (characteristic finding)
• Post-obliterative frontal sinus: MRI can distinguish mucocele from fat necrosis and infection

• Cummings Otolaryngology, Ch. 94; Scott-Brown's Vol 1, Ch. 99

6b. Salivary Gland Tumors

• Parotid lesions are easily identified against the hyperintense fat on T1WI
• Post-contrast fat-saturated T1WI: fat becomes dark, mass stands out clearly
• Gadolinium helps assess perineural spread (enhancement along cranial nerves), intracranial extension, and bone marrow invasion
• Any irregular/ill-defined mass on US or suspicious cytology on FNA should be further evaluated with MRI

• Pleomorphic adenoma: Low T1, high T2 (similar to water); solid enhancement on fat-suppressed post-gadolinium T1WI; recurrent disease appears as single or multiple T2 hyperintense masses in parotidectomy bed
• Warthin tumor: Partially cystic lesion; common in smokers >40 years
• Adenoid cystic carcinoma: Infiltrative; classic perineural spread along V3 through foramen ovale (coronal gadolinium views)

• Cummings Otolaryngology, Ch. 82

6c. Nasopharyngeal Carcinoma (NPC)

• Delineates parapharyngeal extension, perineural spread, and marrow infiltration
• Differentiates tumor infiltration from retained secretions in paranasal sinuses
• Better defines optic chiasma, optic nerves, and brainstem - preserving organs-at-risk in radiotherapy planning
• MRI/CT image fusion enables precise radiotherapy targeting
• More useful than CT for monitoring disease and detecting recurrence after treatment
• DWI has increased diagnostic accuracy for residual/recurrent NPC; modern MRI with DWI is on par with PET-CT for detecting residual and recurrent disease
• Scott-Brown's Otorhinolaryngology (combined vol), Ch. on NPC

6d. Skull Base

1. Cribriform plate with periosteal covering
2. Dura mater
3. Subarachnoid space

• Epidermoid cysts: restricted diffusion (bright on DWI), distinguishable from arachnoid cysts (no restriction)
• Cholesteatoma: also shows restricted diffusion on non-EPI DWI sequences

6e. Temporal Bone and Ear

• Inner ear malformations: T2 FIESTA or constructive interference in steady state (CISS) sequences beautifully delineate the membranous labyrinth, endolymph, and perilymph
• Acoustic neuroma / Vestibular schwannoma: High T2 signal; gadolinium-enhancing lesion in internal auditory canal (IAC) and CPA angle
• Endolymphatic hydrops (Meniere's disease): Gadolinium-enhanced MRI (delayed, 4 hours post-IV) shows reduced endolymphatic signal in 3T MRI
• Cholesteatoma: DWI (non-EPI) shows restricted diffusion; used to detect residual/recurrent cholesteatoma without revision surgery

6f. Thyroid and Parathyroid

• Less common than ultrasound/sestamibi as a first-line localizing tool
• Sensitivity for abnormal parathyroid: 74-88%
• Useful for revision surgery (not degraded by surgical clips)
• Excellent for detecting ectopic mediastinal glands (sensitivity >80%)
• Parathyroid adenomas: intermediate to low T1 signal; high T2 signal
• Accuracy is reduced in concomitant thyroid disease (present in up to 40% of hyperparathyroid patients)
• Scott-Brown's Vol 1, Ch. 72

6g. Larynx and Hypopharynx

• Useful for staging laryngeal cancers and evaluating cartilage invasion (T4 disease)
• MRI slightly superior to CT for cartilage invasion but longer acquisition is a limitation
• Surface coil required; patients should refrain from swallowing during sequences
• Less useful in the thorax (respiratory motion artifact)

6h. Neck Spaces and Lymph Nodes

• MRI delineates fascial planes and neck space anatomy with high accuracy
• Useful for parapharyngeal space, retropharyngeal, and carotid space lesions
• DWI helps differentiate benign from malignant lymph nodes
• Preferred over CT for pediatric neck masses (no radiation)

6i. Fetal Neck Masses (Prenatal MRI)

• Superior soft tissue delineation for tracheal distortion and airway assessment
• Better differentiation between solid and cystic structures
• Less affected by raised BMI, poor fetal position, oligohydramnios
• Useful for identifying syndromes, craniofacial anomalies, and intracranial pathology
• In 50% of cases, MRI findings differ from ultrasound; MRI agreed with final histology in 73% of cases
• Relevant for planning EXIT procedure (ex utero intrapartum treatment) for airway management
• Scott-Brown's Vol 2, Ch. 36

7. MRI AND COCHLEAR IMPLANTS

1. Movement of stimulator/receiver or electrode array
2. Generation of noxious/injurious auditory stimuli
3. Generation of heat
4. Demagnetization

• Energies from ≤1.5 T fields do not produce sufficient heat to be troublesome
• Patients have not reported injurious auditory sensations at standard field strengths
• Movement is not problematic at field strengths <1.5 T; external stabilization can further limit risk
• Demagnetization does occur - up to 10% of magnetic strength lost per scan; degree depends on scan duration and field strength

• Advanced Bionics: Special magnetless implant (headpiece stabilized by special earpiece)
• Cochlear Corporation (Nucleus): Removable magnet - can be extracted under local anesthesia as outpatient, replaced later
• MED-EL Pulsar: Titanium silastic housing permitting magnet removal if MRI is necessary

• Shambaugh Surgery of the Ear, Ch. 6

8. CONTRAINDICATIONS TO MRI IN ENT

Summary Table: MRI vs CT in ENT

• Cummings Otolaryngology Head and Neck Surgery, Chapters 8, 47, 82, 94, 107
• Scott-Brown's Otorhinolaryngology Head & Neck Surgery, Vols 1 & 2, Chapters on sinuses, NPC, parathyroid, fetal neck masses
• Shambaugh Surgery of the Ear, Chapter on cochlear implants and MRI