2 Introduction to Imaging Introduction Rapid advancement of medical imaging in the last few decades has significantly enhanced the role of imaging in medicine. Imaging plays an integral role in the evaluation of many of the diseases confronted by the otolaryngologist. It is performed for diagnosis, staging of tumors, assessing efficacy of therapy, follow-up, and guidance for procedures. Imaging in the head and neck, like no other area of the human body, differs considerably based on the specific anatomic area of interest. In fact, unique imaging protocols have been designed for at least 9 specific areas of the head and neck. These include from superior to inferior the cerebellopontine angle-internal auditory canal (CPA-IAC), skull base, temporal bone, orbit, sinus and nose, suprahyoid neck, infrahyoid neck, pharynx, and larynx. Clinical history and physical findings in combination with high-resolution imaging available today create the possibility of a highly specific clinical-radiologic diagnosis. Computed tomography (CT) and magnetic resonance (MR) form the backbone of the imaging work-up of patients with diseases of the head and neck. Imaging Modalities In the past, radiography ("plain films" and "tomography") played a significant, but not very effective, role in imaging of the head and neck. With the emergence of advanced imaging, the role of radiography has virtually disappeared. Currently, CT, MR, and PET/CT are the most commonly performed imaging modalities for diseases of the head and neck. Modern multidetector, multiplanar CT now creates exquisite anatomical detail allowing the diagnosis of very small lesions, such as temporal bone otosclerosis, not thought possible even a decade ago. MR techniques, such as multiplanar thin-section enhanced, fat-saturated T1, allow the imager to identify subtle findings, such as perineural tumor spread along extracranial portions of cranial nerves. The use of a variety of MR sequences (T1, T2, FLAIR, GRE, enhanced T1 fat-saturated) permit the imager to differentiate tissue types, such as normal muscle from tumor. Other unique MR sequences can suggest specific diagnoses, such as the case of diffusion weighted imaging (DWI) MR sequence being highly specific for the diagnosis of epidermoid. Likewise, fluorodeoxyglucose positron emission tomography (FDG-PET)/CT imaging has a distinct role in the imaging of patients with squamous cell carcinoma (SCCa) staging and follow-up. Metabolically active primary tumor and malignant nodes mapped against an underlying contrast-enhanced CT can help differentiate normal tonsillar tissue and reactive nodes from invasive SCCa primary and metastatic nodes. Ultrasound and color Doppler are useful in evaluating the head and neck vasculature. Ultrasound is also used to evaluate neck masses and localize them for fine-needle aspiration. CT CT technology relies on the same physical principles as x-rays. The differential absorption of the x-ray beam by different tissues produces varied levels of density in the image (see Table 1), which on CT scans are measured in Hounsfield units (HU). This can be displayed in cross-sectional format or in multiple planes. Multidetector CT has enlarged the capability of CT with faster scans, greater spatial resolution, and multiplanar reformations. Nonenhanced CT (NECT) The role for NECT in head and neck imaging is small. A ductal stone, lesion chondroid (cartilaginous) or osteoid (bony) matrix, or radiopaque foreign body can be seen in the presence of contrast in the vessels of the neck. As a rule, contrast-enhanced CT (CECT) is done in all soft tissue neck imaging examinations if renal function and allergy history allow. CECT CECT is the workhorse in imaging the soft tissues of the extracranial head and neck. CECT is often preferred to MR because the images are acquired rapidly (fractions of seconds), whereas MR sequences require the patient to remain still for 3-5 minutes at a time (see Table 2). This is especially true in the infrahyoid neck area where movement artifact degrades MR images considerably. CECT is routinely used as a starting exam in most patients who need imaging of the head and neck to evaluate a deep tissue clinical question. Staging and follow-up of pharyngeal and laryngeal SCCa, abscess search, and suspected mass evaluation are but a few of the uses of CECT. Thin-Section Bone CT Temporal bone, skull base, and sinonasal CT evaluations begin with unenhanced, multiplanar, thin-section, and bone algorithm CT. Slice thickness varies by area (temporal bone: 0.6 mm; skull base: 1-2 mm; sinuses: 2-3 mm). If soft tissue questions remain after the CT is completed in these 3 areas, enhanced fat-saturated T1 MR is used to analyze tissue type and extent instead of repeating the CT with contrast at a later date. CT Angiogram (CTA) For many, CTA remains the study of choice for all emergent and nonemergent vascular conditions, such as carotid atherosclerotic stenosis, dissection, and pseudoaneurysm. It can also be used in the setting of a suspected dural arteriovenous fistula and carotid cavernous fistula. Other uses of CTA include assessing the integrity of the internal carotid artery when it is involved by dural tumors, such as meningioma or invasive neck SCCa. When significant neck trauma suggests the possibility of arterial injury, CTA is ideal to answer this clinical question. CTA is fast and less prone to artifact than MR angiography (MRA). A combined CTA of the head and neck, from the aortic arch to the cranial vertex, can be obtained with as little as 70 ml of IV contrast in < 15 seconds. CT Venogram (CTV) CTV is similar to CTA except for an added delay for optimal visualization of the venous system. It is a fast, reliable modality to exclude dural sinus thrombosis in an emergent setting. It is particularly helpful in the search for venous sinus thrombosis in the posterior fossa and skull base. MR The primary origin of the MR signal used to generate clinical images comes from hydrogen nuclei. Hydrogen nuclei consist of a single proton that is constantly spinning. A radio frequency pulse (RF pulse) emitted from the scanner results in some of the hydrogen protons being "knocked" out of alignment with the static magnetic field. As the energy from the RF pulse is dissipated, the hydrogen protons will return to alignment with the static magnetic field. The MR signal is derived from the hydrogen protons as they move back into
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Introduction to Imaging
IntroductionRapid advancement of medical imaging in the last fewdecades has significantly enhanced the role of imaging inmedicine. Imaging plays an integral role in the evaluation ofmany of the diseases confronted by the otolaryngologist. It isperformed for diagnosis, staging of tumors, assessing efficacyof therapy, follow-up, and guidance for procedures.
Imaging in the head and neck, like no other area of the humanbody, differs considerably based on the specific anatomic areaof interest. In fact, unique imaging protocols have beendesigned for at least 9 specific areas of the head and neck.These include from superior to inferior the cerebellopontineangle-internal auditory canal (CPA-IAC), skull base, temporalbone, orbit, sinus and nose, suprahyoid neck, infrahyoid neck,pharynx, and larynx. Clinical history and physical findings incombination with high-resolution imaging available todaycreate the possibility of a highly specific clinical-radiologicdiagnosis. Computed tomography (CT) and magneticresonance (MR) form the backbone of the imaging work-up ofpatients with diseases of the head and neck.
Imaging ModalitiesIn the past, radiography ("plain films" and "tomography")played a significant, but not very effective, role in imaging ofthe head and neck. With the emergence of advanced imaging,the role of radiography has virtually disappeared. Currently,CT, MR, and PET/CT are the most commonly performedimaging modalities for diseases of the head and neck. Modernmultidetector, multiplanar CT now creates exquisiteanatomical detail allowing the diagnosis of very small lesions,such as temporal bone otosclerosis, not thought possible evena decade ago. MR techniques, such as multiplanar thin-sectionenhanced, fat-saturated T1, allow the imager to identifysubtle findings, such as perineural tumor spread alongextracranial portions of cranial nerves. The use of a variety ofMR sequences (T1, T2, FLAIR, GRE, enhanced T1 fat-saturated)permit the imager to differentiate tissue types, such as normalmuscle from tumor. Other unique MR sequences can suggestspecific diagnoses, such as the case of diffusion weightedimaging (DWI) MR sequence being highly specific for thediagnosis of epidermoid.
Likewise, fluorodeoxyglucose positron emission tomography(FDG-PET)/CT imaging has a distinct role in the imaging ofpatients with squamous cell carcinoma (SCCa) staging andfollow-up. Metabolically active primary tumor and malignantnodes mapped against an underlying contrast-enhanced CTcan help differentiate normal tonsillar tissue and reactivenodes from invasive SCCa primary and metastatic nodes.Ultrasound and color Doppler are useful in evaluating thehead and neck vasculature. Ultrasound is also used to evaluateneck masses and localize them for fine-needle aspiration.
CTCT technology relies on the same physical principles as x-rays.The differential absorption of the x-ray beam by differenttissues produces varied levels of density in the image (seeTable 1), which on CT scans are measured in Hounsfield units(HU). This can be displayed in cross-sectional format or inmultiple planes. Multidetector CT has enlarged the capabilityof CT with faster scans, greater spatial resolution, andmultiplanar reformations.
Nonenhanced CT (NECT)The role for NECT in head and neck imaging is small. A ductalstone, lesion chondroid (cartilaginous) or osteoid (bony)matrix, or radiopaque foreign body can be seen in thepresence of contrast in the vessels of the neck. As a rule,contrast-enhanced CT (CECT) is done in all soft tissue neckimaging examinations if renal function and allergy historyallow.
CECTCECT is the workhorse in imaging the soft tissues of theextracranial head and neck. CECT is often preferred to MRbecause the images are acquired rapidly (fractions ofseconds), whereas MR sequences require the patient toremain still for 3-5 minutes at a time (see Table 2). This isespecially true in the infrahyoid neck area where movementartifact degrades MR images considerably.
CECT is routinely used as a starting exam in most patients whoneed imaging of the head and neck to evaluate a deep tissueclinical question. Staging and follow-up of pharyngeal andlaryngeal SCCa, abscess search, and suspected massevaluation are but a few of the uses of CECT.
Thin-Section Bone CTTemporal bone, skull base, and sinonasal CT evaluations beginwith unenhanced, multiplanar, thin-section, and bonealgorithm CT. Slice thickness varies by area (temporal bone:0.6 mm; skull base: 1-2 mm; sinuses: 2-3 mm). If soft tissuequestions remain after the CT is completed in these 3 areas,enhanced fat-saturated T1 MR is used to analyze tissue typeand extent instead of repeating the CT with contrast at a laterdate.
CT Angiogram (CTA)For many, CTA remains the study of choice for all emergentand nonemergent vascular conditions, such as carotidatherosclerotic stenosis, dissection, and pseudoaneurysm. Itcan also be used in the setting of a suspected duralarteriovenous fistula and carotid cavernous fistula. Other usesof CTA include assessing the integrity of the internal carotidartery when it is involved by dural tumors, such asmeningioma or invasive neck SCCa. When significant necktrauma suggests the possibility of arterial injury, CTA is ideal toanswer this clinical question. CTA is fast and less prone toartifact than MR angiography (MRA). A combined CTA of thehead and neck, from the aortic arch to the cranial vertex, canbe obtained with as little as 70 ml of IV contrast in < 15seconds.
CT Venogram (CTV)CTV is similar to CTA except for an added delay for optimalvisualization of the venous system. It is a fast, reliable modalityto exclude dural sinus thrombosis in an emergent setting. It isparticularly helpful in the search for venous sinus thrombosisin the posterior fossa and skull base.
MRThe primary origin of the MR signal used to generate clinicalimages comes from hydrogen nuclei. Hydrogen nuclei consistof a single proton that is constantly spinning. A radiofrequency pulse (RF pulse) emitted from the scanner results insome of the hydrogen protons being "knocked" out ofalignment with the static magnetic field. As the energy fromthe RF pulse is dissipated, the hydrogen protons will return toalignment with the static magnetic field. The MR signal isderived from the hydrogen protons as they move back into
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alignment with the magnetic field. The MR signal is thenbroken down and spatially located to produce images.
The movement of the neck, which occurs during swallowing,coughing, sneezing, scratching, and breathing, often degradesthe diagnostic quality of MR images of the neck. Enhanced fat-saturated MR is preferred over CECT in the suprahyoid neckwhere movement is less frequent and in the oral cavity whendental amalgam artifact obscures the CT image. MR is also apowerful imaging tool for mass lesions in the CPA-IAC, orbit,skull base, and sinonasal areas.
T1 and T2 are the fundamental parameters of MR anddetermine the contrast between tissues. MR sequences thatemphasize tissue differences in T1 relaxation are called T1weighted, and those that emphasize T2 relaxation are calledT2 weighted. Tissues with short T1 relaxation time, such asfat, melanin, and protein, produce high signal on T1-weightedsequences and appear "bright," whereas fluid is relatively dark.Fluid has a long T2 relaxation time and appears bright on T2-weighted sequences.
Spin-echo and gradient-echo are 2 basic sequences in MR. Allother sequences are variations of one of these sequences andare used to better characterize specific tissue types.
Fluid-attenuation inversion recovery (FLAIR) sequenceeliminates fluid signal, which thus appears dark. It is useful forhighlighting cisternal lesions. "Bright" CSF sulcal signal onFLAIR may suggest leptomeningeal disease with replacementof normal CSF by pus (meningitis), blood (subarachnoidhemorrhage), or tumor cells (leptomeningeal carcinomatosis).
Short-tau inversion recovery (STIR) sequence is used toeliminate signal from fat. In the head and neck area, STIRoften displays fluid-containing lesions (cysts, abscess) asconspicuous high-signal (bright) lesions. It is also used indiagnosing fat-containing lesions like lipoma and dermoid cyst.
Diffusion-weighted imaging (DWI) shows the molecularmotion or diffusion of water protons within tissue. DWIgenerates diffusion and apparent diffusion coefficient (ADC)maps. ADC is a measure of the magnitude of diffusion. Truerestricted diffusion will be bright on diffusion and dark on ADCmaps. Restricted diffusion can be seen in processes likepyogenic abscess, highly cellular tumor, and epidermoid cyst.Generally, the ADC value of malignant tissue is < the ADCvalue of benign tumors.
Gradient-echo sequence (GRE) is sensitive to small amountsof blood breakdown products as well as calcium and metallicdeposits, fat, and air. In lesions with blood products (lymphaticmalformation, cholesterol granuloma, paraganglioma) orcalcifications (osteosarcoma, chondrosarcoma, meningioma),the focal areas of blood will be low signal and bloom on GREsequences.
MRATime-of-flight (TOF) imaging is most commonly used for MRA.Signal in intracranial arteries is related to flow phenomenon,and thus no IV gadolinium is needed. TOF MRA can beperformed by both 2D and 3D techniques.
Contrast-enhanced MRA is often used to evaluate the neckvasculature. Contrast-enhanced intracranial MRA is useful inpatients with stents &/or coils.
MR Venogram (MRV)MRV can be performed with 2D/3D TOF techniques, which donot need administration of IV gadolinium. Contrast-enhanced
MRV is, however, more robust and is less susceptible toartifacts compared with the TOF techniques.
PET/CTPET/CT is the best imaging modality for staging, monitoring,and surveillance of advanced head and neck SCCa. It issuperior to PET, CECT, or MR imaging alone. It is also superiorfor identifying 2nd primary tumors.
A basic understanding of the core workings of PET is essential.Fluorodeoxyglucose (FDG) is transported into a cell in thesame manner as normal glucose. There, it becomes trappedand accumulates in cells with high glucose metabolism. FusedPET/CT offers the imager combined anatomic and physiologicinformation. It distinguishes extensive physiologic FDG uptakein the head and neck from true pathologic uptake.Standardized uptake value (SUV) provides a quantifiedmeasure of FDG uptake.
The clinical utility of PET/CT for SCCa of the head and neckdepends on the size of primary tumor. This technique is mostuseful for evaluating for nodal and distant metastases withlocally advanced T3 or T4 tumors or those with known nodalmetastases. It also supplies specific information about thepresence of 2nd primary tumors in the body. In the clinicalsetting of malignant metastatic nodes without a clinicallyidentified primary tumor ("unknown primary"), PET/CT attimes will locate the hidden primary tumor.
Pitfalls of PET/CT are multiple. False-positive uptake fromnormal physiology, inflammation, and posttreatment changesmust be guarded against. False-negative uptake from cysticlesions (necrotic primary or nodal metastases), small tumors,and non-FDG-avid tumors may be encountered. Reading headand neck PET/CT scans requires intimate knowledge of headand neck anatomy and tumor behavior for correctinterpretation to be rendered.
Ultrasound and DopplerGrayscale is used to for extracranial atherosclerotic diseaseand plaque morphology. Color Doppler detects turbulentblood flow. Doppler spectral analysis measures blood flowvelocity, which correlates with the degree of vascular stenosis.Ultrasound is also used to evaluate neck masses (especially inchildren) and localize them for fine-needle aspiration.
Digital Subtraction Angiography (DSA)DSA is still considered the "gold standard" in vascular imaging.However, DSA is an invasive procedure associated with risk ofcomplication, with 1% overall incidence of neurologic deficitand 0.5% incidence of persistent deficit. CTA and CTV havemade the use of DSA for diagnosis extremely limited. Instead,DSA is used to confirm the diagnosis of arteriovenous fistulawhile at the same time completing endovascular treatment. Itis additionally used for preoperative embolization of vasculartumors, such as paraganglioma and meningioma.
Selected References1. Dickerson E et al: Advanced imaging techniques of the skull base. Radiol Clin
North Am. 55(1):189-200, 20172. Corrales CE et al: Imaging innovations in temporal bone disorders.
Otolaryngol Clin North Am. 48(2):263-80, 20153. Srinivasan A et al: Biologic imaging of head and neck cancer: the present and
the future. AJNR Am J Neuroradiol. 33(4):586-94, 20124. Fortnum H et al: The role of magnetic resonance imaging in the
identification of suspected acoustic neuroma: a systematic review of clinicaland cost effectiveness and natural history. Health Technol Assess. 13(18):iii-iv, ix-xi, 1-154, 2009
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Table 1: CT Hounsfield Units
Tissue Type Hounsfield Unit RangeAir -1000
Fat -130 to -70
Water/cerebrospinal fluid 0-30
Muscle 40-60
Hemorrhage 70-80
Calcification 80-100
Bone +400
Table 2: Comparison of CT & MR in Head & Neck Imaging
Table 4: Summary of Preferred Imaging Studies in Head & Neck
Area of Imaging Preferred 1st Imaging StudyCerebellopontine angle-internal auditorycanal
MR: T1 C+ fat saturated with thin-section T2 (FIESTA, CISS, etc.) for surgical planning
Temporal bone Bone CT: 0.6-mm thick axial & coronal planesIf lesion extends regionally, add MR: 2- to 3-mm thick T1 C+ fat saturated axial & coronal; include DWI
Skull base Both MR (T1 C+ FS axial & coronal) & CT (bone only, no contrast) for complete lesion analysis
Orbit MR: T1 C+ fat-saturated; add bone CT for bone changes, calcifications as needed
Sinus & nose Sinusitis: Bone CT, multiplanar; in invasive fungal sinusitis need T1 C+ fat saturated axial & coronalTumor: MR, T1 C+ fat saturated axial & coronal; add unenhanced bone CT for surgical planning PRN
Suprahyoid neck CECT: Base of skull to clavicles; soft tissue & bone algorithmsIf lesion involves skull base, add enhanced MR for perineural tumor, dural, & vascular invasion
Infrahyoid neck CECT: Skull base to clavicles; extend to carina if left vagal neuropathy, parathyroid adenoma (PTA) atissue; ultrasound 1st for PTA, then Sestamibi scan, then CECT if anatomic localization needed
Pharynx in staging of squamous cellcarcinoma (SCCa)
CECT: Base of skull (BOS) to clavicles; soft tissue & bone algorithmsPET/CT controversial: Best for staging, monitoring, & surveillance of advanced stage SCCaAdd enhanced MR for perineural tumor, dural, vascular invasion or cartilage invasion
Larynx CECT: BOS to clavicles, quiet respiration; 2nd pass angled along hyoid bone plane with breath holdingAdd MR if cartilage invasion question on CECT to confirm CT impression
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(Left) Axial T1WI C+ FS MR ofthe CPA-IAC in a patient withleft sensorineural hearing lossshows a small enhancingvestibular schwannoma (VS)st within the IAC with a 3-mmfundal CSF cap lateral tothe tumor. Enhanced T1 FS MRis the gold standard ofimaging for CPA-IACpathology. (Right) Axial CISSMR in the same patient showsa VS defect st in the high-signal CSF of the IAC. Surgicalinformation like fundal CSFcap size & the relationshipto the cochlear nerve canal stare more readily seen with T2or T2-like CISS MR sequences.
(Left) Axial bone CT in apatient with a "dead" right earshows an ovoid petrous apexmass st with intramural airst & cochlear destruction .Cholesteatoma, cholesterolgranuloma, andchondrosarcoma are in thedifferential diagnosis. (Right)Axial diffusion-weighted MR insame patient showshyperintense restricteddiffusion st diagnostic ofcholesteatoma. When CTshows a lesion spreading intoadjacent structures, contrast-enhanced MR with specialsequences, such as MRA, MRV,and DWI, is recommended.
(Left) Axial 1-mm thick skullbase CT in a patient with rightcranial nerve VI palsy showsan erosive bone lesion in theright petrooccipital fissure stsuspicious for smallchondrosarcoma. (Right) AxialT1WI C+ FS MR in the samepatient reveals enhancingtissue within the rightpetrooccipital fissure st,enlarging the fissureposteriorly and pushing intothe prepontine cistern .There is involvement of thepetrous ICA wall st. MRshows true soft tissue extentof tumor, while bone CT showsassociated bony changes.
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Introduction to Imaging
(Left) Axial T1 FS MR inpatient with proptosis &decreased right eye visionreveals globular configurationof avidly enhancing opticnerve sheath meningioma(ONSM) st eccentricallysurrounding orbital segmentof right optic nerve. Note thetumor spares optic nerveimmediately posterior to globest, a common pattern. (Right)Axial orbital CECT showsanother enhancing ONSM stwith punctate & linearcalcification surrounding leftoptic nerve complex extendingthrough orbital apex. Noteadjacent hyperostosis .
(Left) Axial sinonasal bone CTin a immunocompromisedpatient with clinical suspicionof invasive fungal sinusitisshows left nose and maxillarysinus disease with loss ofmedial sinus wall st andtrabecular bone . Notepremaxillary soft tissueswelling st. (Right) Axial T1C+ FS MR in the same patientshows acute invasive fungalsinusitis in premaxillary softtissues st, retromaxillary fatpad , and masticator spacest. Bone CT alone is notadequate for evaluation ofinvasive sinonasal infection ortumor.
(Left) Axial CECT of thesuprahyoid neck shows acarotid body paragangliomast with avid, uniformenhancement. Note that thetumor sits in the notchbetween the ICA & ECA st.This location is diagnostic ofparaganglioma. (Right) AxialT1WI MR reveals a carotidbody paraganglioma in theleft carotid space st, between2 flow voids representing theICA and ECA st. Smallinternal foci of signal void("pepper") represent vascularflow of feeding vessels .This characteristic finding isonly present on MR, not on CT.
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Introduction to Imaging
(Left) Axial CECT reveals alarge, invasive anaplasticthyroid carcinoma with dense,focal calcification st, rightcommon carotid arteryencasement , & lateral wallof esophagus invasion st.CECT is the best 1st exam forinfrahyoid neck masses as it isless affected by motion thanMR. (Right) Axial CECT in achild with acute neck infectionshows a phlegmonous massst in the left neck involvingthe left thyroid lobe .Fourth branchial arch anomalyconnects the pyriform sinus toleft thyroid lobe. CECT is thebest 1st exam in this setting.
(Left) Four months followingcombined surgery, radiation,and chemotherapy for rightinferior palatine tonsil SCCa,this patient's axial CECT showsfullness in the right lateralpharyngeal wall st. (Right)Axial PET/CT in the samepatient revealshypermetabolic tissue in thearea of anatomic distortionseen on CECT only, diagnosticof recurrent SCCa. PET/CTprovides both anatomic andphysiologic information inadvanced stage SCCa of thehead and neck.
(Left) Axial CECT in a patientwith epiglottic SCCa showsinvolvement of the inferiorepiglottis st and leftaryepiglottic fold . Tumorinvolves preepiglottic fat stand bulges through thethyroid cartilage notch .CECT is the best 1st exam forthe larynx due to movementissues. (Right) Sagittal T1WIMR shows supraglottic SCCa inpreepiglottic space fat st thatspares the suprahyoidepiglottis st. Note the hyoid isinvaded . MR is used inimaging the larynx after CECTif specific issues like cartilageinvasion are present.
