MR in Head & Neck Cancer

Michelle A. Michel, MD

Key Facts

Clinical Implications

MR is optimal imaging tool for precise delineation of tumor margins and identifying perineural tumor, vascular invasion, & marrow infiltration
Excellent soft tissue contrast resolution
Often better tumor characterization, such as parotid
No ionizing radiation & rare gadolinium reactions
Less affected by dental amalgam than CT

Utility of Sequences

Multiplanar imaging important for tumor extent
T1WI delineates anatomic detail of lesion
Fat-suppression techniques on long-TR sequences increase conspicuity and better characterize lesion
T1WI C+ FS further delineates & characterizes tumor and evaluates for perineural & dural/intracranial invasion
DWI & MR perfusion may add further information

Generic Neck Imaging Protocol

Skull base to at least supraclavicular fossa
Slice thickness: 4 mm; interslice gap: 0.5-1 mm
Axial & coronal T1WI
Axial & coronal T2WI FS or STIR
Axial & coronal T1WI C+ FS

Safety Considerations

Gadolinium associated with nephrogenic systemic fibrosis (NSF) in patients with severe kidney disease
MR compatibility of implanted devices & presence of foreign bodies should be determined prior to scanning
Claustrophobia or respiratory problems hinders MR for many H&N patients

Artifacts & Pitfalls

Susceptibility artifacts degrade image quality
Swallowing and motion make neck imaging difficult

(Left) Sagittal T1WI MR shows a large NPC

in a 16-year-old patient. There is invasion of the clivus

with replacement of normal bright fatty marrow signal. Trapped secretions are seen in the sphenoid sinus

. (Right) Axial T1WI C+ FS MR shows an enhancing buccal space mass

with well-defined margins, in this case a spindle cell sarcoma. The superior soft tissue differentiation of MR and lack of artifacts related to dental amalgam make it an excellent modality for oral and buccal lesions.

(Left) Axial STIR MR shows large B-cell lymphoma

centered within nasal cavity. High T2 signal trapped secretions within maxillary sinuses

are easily differentiated from tumor on MR. Relatively low signal in mass is consistent with highly cellular nature. (Right) Coronal T1WI C+ FS MR shows nasal cavity tumor

extending through cribriform plate

to intracranial cavity. There is no orbital involvement. Cyst formation at tumor margin

is distinguishing feature of esthesioneuroblastoma.

TERMINOLOGY

Abbreviations

• Magnetic resonance imaging (MR)

Definitions

• Fundamental imaging modality for evaluating head & neck cancer that does not employ ionizing radiation

CLINICAL IMPLICATIONS

Clinical Importance

MR is optimal imaging tool for precise delineation of tumor margins, orbital and intracranial extension, identifying perineural tumor spread (PNT), and determining vascular invasion & degree of marrow infiltration
- Better soft tissue differentiation & contrast resolution as compared to CT
- Preferred modality in suprahyoid neck (SHN)
  - Best for assessment of nasopharyngeal, oral cavity, sinonasal, & salivary neoplasms
  - Less affected by dental amalgam artifact than CT
  - Little motion present in SHN
- No ionizing radiation
- Preferred technique in patients with allergy to iodinated contrast

IMAGING APPROACHES

Staging

In setting of known malignancy, MR can be used to stage primary tumor and lymph nodes
Often obtained after initial CECT
MR evaluation of sinonasal and salivary primaries may be focused and not cover entire neck, as long as CECT covers entire neck for adenopathy

Equipment

Best images obtained with field strengths ≥ 1.5 T
Surface coils greatly improve signal:noise ratio and spatial resolution
Patients must be screened for presence of implantable devices & metallic foreign bodies prior to entering MR environment

Sequences

T1WI delineates fine anatomic detail of lesion, particularly if adjacent to fat
Employing fat-suppression techniques (chemical selective or STIR) on long-TR sequences increases lesion conspicuity
Axial & coronal T1WI C+ FS sequences are superior to CT for defining soft tissue extent, perineural tumor, & dural/intracranial invasion
- Fat-suppression increases conspicuity of enhancing lesions adjacent to otherwise hyperintense fat
- Fat-suppressed T2 and post-contrast T1 best for identifying nodal necrosis and extranodal spread
Number of signal averages, field of view (FOV), matrix size, & interslice distance adjusted to provide maximum detail and pixel width ≤ 1 mm

IMAGING PROTOCOLS

Neck

Coverage: Skull base to at least supraclavicular fossa
Sequences: 3 sequences, each at least 1 plane
- Axial & coronal T1WI
- Axial & coronal T2WI FS or STIR
- Axial & coronal T1WI C+ FS
- ± sagittal plane sequence
  - May be useful for nasopharynx (NP), oral cavity (OC), base of tongue (BOT), palate, & airway lesions
Parameters
- FOV: 20-22 cm
- Slice thickness: 4 mm; interslice gap: 0.5-1 mm
- Matrix: 192 × 256
- Surface coils improve image quality
- Saturation pulses reduce vascular flow artifacts

Sinonasal

Coverage: Axials (anterior cranial fossa through maxillary alveolus); coronals (nasal vestibule through cavernous sinuses)
Sequences
- Axial & coronal T1WI
- Axial & coronal T2WI FS or STIR
- Axial & coronal T1WI C+ FS
- Sagittal sequence optional
Parameters
- FOV: 16-18 cm
- Slice thickness: 3 mm; interslice gap: 0.5 mm
- Surface coil utilized (head coil)

Salivary Glands

Coverage: Top of petrous ridge through mandible
- Include course of CN7 for parotid mass evaluation
Sequences & parameters
- Similar to sinonasal protocol
- Diffusion images may aid in distinguishing benign & malignant lesions
- Surface coil utilized (head coil)

Special Techniques

Gauze padding: Oral cavity
- Improves visualization of small oral vestibule tumors obscured by apposition of buccal and gingival mucosa
- 2 × 2 inch rolled gauze inserted into oral vestibule has similar MR appearance to air
  - Analogous to “puffed cheek” CT technique
Neck padding/“water bags”
- Loss of fat suppression often occurs in lower neck/thoracic inlet due to variable width/thickness in extracranial head & neck
- Saline bags can reduce bulk susceptibility artifact and improve fat suppression

CLINICAL INDICATIONS & UTILITY

Roles

Multiple roles in H&N cancer patient
Much is due to better delineation of invasion of deep tissues and involvement of critical structures
- Preepiglottic fat infiltration
- Prevertebral fascia invasion
- Laryngeal cartilage penetration
- Marrow infiltration
- Perineural tumor spread
- Orbital fat invasion
- Dural and brain invasion
- Tracheal & esophageal involvement
- Arterial encasement
- Brachial plexus involvement
- Mediastinal infiltration
Staging
- As above, deep extent of tumor best delineated
- Tumor volume measurements correlate with local control & outcome for supraglottic, glottic, & pyriform sinus SCCa & NPC
Treatment planning
- MR important for determining resectability of primary lesion
- Delineation of true extent of tumor important for planning intensity-modulated radiotherapy (IMRT)
Treatment response & surveillance
- Baseline imaging after therapy used to assess for residual and as roadmap for future studies
- Recurrences occur most often in 1st 2 years after treatment

Nasopharynx

MR is superior to CT for detecting small NPC missed on endoscopy, determining deep extension (parapharyngeal space), skull base invasion, & intracranial spread
Skull base invasion may be direct, via PNT, or perivascular
- Direct extension through pharyngobasilar fascia or sinus of Morgagni around eustachian tube & levator palatini muscle
- Direct marrow infiltration of sphenoid bone, clivus, petrous apex, temporal squamosa
- PNT through foramen ovale, hypoglossal canal, &/or to pterygopalatine fossa (PPF)
- Perivascular spread along internal carotid artery (ICA) at foramen lacerum then into cavernous sinus
Nonenhanced T1WI best for evaluating skull base and parapharyngeal extension
- Tumor replaces normal high signal fat
T1WI C+ FS recommended for detection of PNT
- Important to evaluate entire anterograde and retrograde extent of involved cranial nerve

Oral Cavity & Oropharynx

Mucosal extent of oral cavity tumor often best determined on clinical exam
- Dental amalgam & dense mandibular bone do not cause prominent artifacts
Small tongue base and palatine tonsil tumors may be invisible on clinical exam
- These are frequently more readily detected with MR than CT
Tumor margins & thickness seen well with T2WI
- Margin delineation improved with T1WI C+ FS
Tumor > 2 cm with aggressive margins & sublingual space extension likely involves neurovascular bundle
- If lesion involves sublingual space, assess for contralateral extension under frenulum & posterior spread to submandibular space
Tumor thickness is prognostic factor for oral tongue SCCa
- ≤ 3 mm thickness has lower local recurrence rate & excellent disease-free survival
- ≥ 9 mm thickness has 24% probability of local recurrence & 66% 5-year disease-free survival
- Increased incidence of nodal involvement if tumor thickness > 9 mm
Marrow invasion of mandible shows ↓ T1 signal, ↑ T2 or STIR signal, & enhancement on T1WI C+ FS
- Reported accuracy for MR detection of mandibular invasion is ˜ 93%
- MR may overestimate the degree of marrow invasion
  - False-positive findings due to presence of inflammation or hemorrhage
Preservation of high T1 signal in retropharyngeal fat reliably predicts absence of prevertebral fascia invasion for posterior pharyngeal wall lesions

Larynx & Hypopharynx

Advantages of soft tissue differentiation of MR often outweighed by motion artifacts in larynx & hypopharynx
Cartilage invasion notoriously missed by CT
- On MR, look for loss of normal cartilage fatty signal; cartilage signal follows that of tumor
- ↓ T1 signal, intermediate T2 signal, contrast enhancement of cartilage marrow space
- MR specificity for cartilage invasion: Thyroid (56%), cricoid (87%), arytenoid (95%)
- Reactive inflammation, edema, & fibrosis may result in false-positive diagnosis of cartilage invasion
MR better for accurately predicting invasion of cervical esophagus
- Wall thickening, effacement of surrounding fat plane, ↑ T2 signal in wall
- These findings combined are ˜ 100% sensitive for esophageal involvement
- Circumferential mass > 270° is ˜ 100% specific