Magnetic resonance imaging of the head and neck

• Head and neck abscess

• Olfactory neuroblastoma (esthesioneuroblastoma)

• Pleomorphic adenoma

• Sinus mucocele

• Adenoid cystic carcinoma

• Internal derangement of the temporomandibular joint

• Obstructive sleep apnea

• References

• Copyright

Computed tomography (CT) and magnetic resonance imaging (MRI) are used to detect pathology of the head and neck. Although CT still has a superior role in detecting calcifications, bony pathology, fractures, and acute hemorrhage [1], MRI is now the preferred imaging method for head and neck pathology. MRI allows superior tissue discrimination and fat- and water-suppression imaging. Fat-suppression imaging has greatly improved the ability to detect abnormal contrast enhancement in the head and neck region. Without this technique, abnormal contrast enhancement appearing as a bright area may be hidden by the very bright signal of fat. Very rapid MRI techniques now allow angiographic images and kinescopic (real-time) depiction of movement. Injectable contrast agents (gadopentetate) aid in the differentiation of certain tumor types and vascular lesions. Albumin-tagged agents which allow direct blood-volume calculations have been developed and are now pending FDA approval. Injectable MR agents, specifically for head and neck imaging, are also currently under development.

MRI, which uses nonionizing radiation from the radio-frequency (RF) band of the electromagnetic spectrum, functions by taking advantage of the weak magnetic properties of human tissue. The patient is placed inside a large magnet with a uniform magnetic field. When the hydrogen nuclei in the patient's body are exposed to this magnetic field, they align with the field. A sequence of RF pulses is then applied to the area of the body being evaluated; this signal must be at the resonant frequency so that the body's proteins can absorb it. These pulses create a transient magnetic field that is perpendicular to the main field; when exposed to this field, the hydrogen nuclei in the body absorb energy and change the direction of their axis of rotation. When the RF pulse is terminated, the nuclei again realign themselves with the external magnetic field. Energy is released from the tissue as weak RF signals, which are received by coils in the MRI device. The signals are rendered detectable by the application of an RF pulse that tips the energized proton down by 90° to be detected as a current by the receiving coil [2], [3], [4]. Subjecting these signals to a series of computer operations translates the signal into an image.

The signals emitted from the excited proton have two components, which are referred to as T1 and T2 relaxation times. T1 is defined as the spin-lattice relaxation time, reflecting the longitudinal axis, and decays at a faster rate than the T2 relaxation time. This is the energy expended by the excited proton to the lattice that contains the proton. As the excited proton is deflected by the 90° RF pulse toward the receiving coil and partially recovers to its free steady state during the early phase of read-out, the signal is dominated by the T1 relaxation time. T2 is defined as the spin-spin relaxation time and reflects the vertical axis. This is the energy expended between the various protons that have been energized.

Following the 90° pulse, the T2 signal decays faster than the predicted rate because of cancellation of signal vector by protons out of phase with each other. This results in the T2∗ (star) signal, which forms the basis for gradient echo imaging. By applying another 180° pulse (refocusing pulse), the protons reverse their direction and eventually come back into phase, generating a signal or echo that can be repeated with other 180° pulses with eventual total decay of signal. This echo or signal forms the basis of the standard spin-echo technique widely used for most conventional MRI. When the proton has nearly recovered, the T2 relaxation signal dominates. Thus, both T1-weighted and T2-weighted effects exist in all types of MR images, but one effect usually dominates [2], [3], [4].

A T1-weighted image is produced by a short repetition time (TR) between pulse sequences plus a short echo time (TE). Tissue with a short T1 decay produces an intense bright or white MR image. Tissue with a long T1 decay produces a low-intensity signal and appears dark on a T1-weighted image [5]. On T1-weighted images, fat is bright or white, cerebrospinal fluid (CSF) is dark or black, and gray matter is slightly hypointense to white matter. This level of contrast makes it possible for the MR image to render a high degree of anatomic detail. Thus, T1-weighted images are useful for depicting small anatomic regions such as the temporomandibular joint (TMJ).

A T2-weighted image is acquired by using a long TR and a long TE. Tissue with a long T2 decay produces a high-intensity signal and appears as a bright area on the image. Tissue with a short T2 decay produces a low-intensity signal and appears as a dark area on the image. On T2-weighted images, fat becomes less bright compared with its appearance on T1 images. CSF is bright or white, and gray matter is slightly hyperintense (brighter) to white matter, which appears dark or gray. In general, the T2 time of abnormal tissue is longer than that of normal tissue. Thus, T2-weighted images are ideal for showing inflammatory changes and tumors.

This article discusses several diseases of the head and neck for which MR imaging has significant advantages. Sample MR images accompany each description.

Osteomyelitis

Frequency/incidence

Osteomyelitis is an infection of bone that begins in the medullary cavity and extends to involve the cortical bone and periosteum. It is rare in the jaw in the absence of predisposing risk factors such as an immunocompromised state, trauma, and radiation treatment for cancer [6], [7].

Signs and symptoms

Pain and fever are the primary symptoms of both acute and chronic osteomyelitis. Swelling, paresthesia, loose teeth, and sinus tracks can also be present. The white blood cell count, erythrocyte sedimentation rate, and C-reactive protein concentration may be elevated, and blood cultures may also be positive [6], [7], [8].

Etiology/pathophysiology

Osteomyelitis can spread hematogenously, as is most common outside the mandible, or by direct spread from an adjoining structure. Odontogenic infections and trauma are the most common causes of osteomyelitis in the jaw [6], [7], [9]. Acute osteomyelitis most often results from infection with staphylococci or streptococci, although recent studies have shown positive cultures for anaerobic bacteria in these infections. Chronic osteomyelitis results from untreated or undertreated acute osteomyelitis [6], [7].

Image of choice for diagnosis

Bone scintigraphy (radionuclide scanning), plain films, and CT scans have been used to diagnose and manage osteomyelitis. Although scintigraphy is still the preferred imaging method for localizing osteomyelitis, MRI has the advantage of providing good contrast between normal and abnormal bone marrow, which aids in the early detection of osteomyelitis while offering a significant increase in the level of anatomic detail [10], [11].

Image hallmarks

On T1-weighted MR images produced with and without contrast enhancement, the normal hyperintense signal of bone marrow fat is altered by inflammatory edema, and the signal intensity of fatty tissue within the marrow is decreased (dark) when compared with a normal bone marrow signal (Fig. 1). In addition, MR images may also show nonossified periosteal reaction indicating bony response to the infection; this reaction appears as a thickened area of hypointensity surrounding the marrow [10].

Fig. 1. Osteomyelitis. (A) T1 noncontrast image showing decreased signal in the marrow of the left mandibular angle, thickening of the masticatory muscles, and loss of fat planes caused by edema. (B) T1 image with contrast showing moderate increase in signal of the bone marrow of the left mandible and masticatory muscles. (C) T1 noncontrast image showing loss of bone marrow signal. (D) T1 image with contrast showing heterogenous enhancement of the bone marrow with enlargement of the mandible. (From Reinert, S, Widlitzek H, Venderink DJ. The value of magnetic resonance imaging in the diagnosis of mandibular osteomyelitis. Br J Oral Maxillofac Surg 1999;37:459–463; with permission.)

Management

Appropriate antibiotic therapy, as indicated by culture and sensitivity, and surgical debridement with or without hyperbaric oxygen therapy are the treatments of choice.

Head and neck abscess

Frequency/incidence

Head and neck abscesses are relatively common and are often odontogenic in origin. Other sources include the sinuses, salivary glands, tonsils, traumatic wounds, and skin infections.

Signs and symptoms

The first symptoms of head and neck infections are pain and swelling. All infections involving the fascial planes of the head and neck can become life threatening, and symptoms may include trismus, stridor or airway compromise, inability to handle secretions, fever, and leukocytosis [6], [12].

Etiology/pathophysiology

Most head and neck infections are of dental, tonsillar, or salivary gland origin. These infections are most often caused by mixed flora indigenous to the oropharyngeal region.

Image of choice for diagnosis

Both CT images and MR images are of diagnostic benefit in the evaluation of infections of the head and neck. If the patient's airway is compromised, as may be the case with odontogenic or pharyngeal infections, CT is generally used because of the discomfort and potential complications that patients might encounter during the time required to generate an MR image. MRI, however, is the preferred imaging method for delineating abscesses of the midface and brain.

Image hallmarks

The affected tissues may demonstrate hypointensity on T1-weighted MR images and hyperintensity on T2-weighted images, but the images are not specific for any organism (Fig. 2). Likewise, abscess formation will be displayed as a hypointense area on T1-weighted images and hyperintense on T2-weighted images with an enhancing capsule or wall around the abscess or invasion of adjacent tissue [13]. Alternatively, fungal infections appear dark on the T2-weighted images because of their relative lack of water. Therefore, a fungal ball can be separated from normal sinusitis changes, which are very bright on the T2-weighted images.

Fig. 2. Peritonsillar abscess. (A) Axial T1-weighted image through the tonsillar area demonstrates a low-intensity mass in the left tonsillar area with mass effect. (B) Axial T2-weighted image at the same level demonstrates increased signal intensity of the abscess. (C) Coronal T2-weighted image shows a high-intensity abscess with a medial bulge of the pharyngeal wall. (From Weber AL, Siciliano A. Radiologic evaluation of the neck: CT and MR imaging evaluation of neck infections with clinical correlations. Radiol Clin North Am 2000;38:941–68; with permission.) Midface abscess is seen in (D) an axial T1-weighted image of an orbital/sinus abscess from an aspergillosis infection and (E) a coronal view of the same patient. The fungus has low signal and appears to be gray in contrast to the normal mucosa and normal inflammatory changes, which are bright.

Management

Abscess drainage and appropriate antibiotic management are required for treatment of head and neck infections. If the infection is life threatening, empiric antibiotic therapy should be given in conjunction with surgical drainage. As culture and sensitivity studies become available, the antibiotic spectrum should be narrowed.

Olfactory neuroblastoma (esthesioneuroblastoma)

Frequency/incidence

Unlike usual neuroblastomas, which occur predominantly in young children, olfactory neuroblastomas (ESTs) are rare lesions occurring in adults over a wide age range.

Signs and symptoms

The most common symptoms associated with ESTs are nasal obstruction, rhinorrhea, epistaxis, and pain [14]. This tumor may first appear as a polyp in the nasal roof or as a nasopharyngeal mass, but it is most commonly seen as an invasive lesion of the paranasal sinuses (especially the ethmoid) or of the anterior cranial fossa [14], [15].

Etiology/pathophysiology

An EST is a neuroectodermal neoplasm that is believed to arise from the olfactory epithelium, usually high in the nasal cavity close to the cribiform plate [14].

Image of choice for diagnosis

The most important aspect of MRI is its ability to accurately determine the extent of tumor and to distinguish it from normal mucosa and from other pathologic changes such as inflammation and retained secretions. T2-weighted spin-echo sequences and gadolinium-enhanced T1-weighted MR images are best for demonstrating such distinctions [16] and are the images of choice for all paranasal sinus tumors.

Image hallmarks

Fig. 3 shows a paranasal sinus tumor with a presentation typical of tumors in this region [14]. The tumor appears as a hypointense mass on gadolinium-enhanced T1-weighted images. The degree of enhancement appears to correlate with the degree of vascularity of the tumor. The tumor is further discriminated from associated sinusitis by T2-weighted spin-echo sequences, on which the EST appears less intense than the highly intense changes associated with secondary sinusitis. Because ESTs tend to grow along the various tiny perforating olfactory nerves, the tumor commonly pierces the cribriform plate and becomes a subfrontal, extradural tumor. This involvement through the cribriform plate is characteristic of ESTs but not of other paranasal sinus tumors, such as squamous cell carcinoma [10].

Fig. 3. Olfactory neuroblastoma. (A) T1-weighted image showing tumor filling the superior nasal cavity and ethmoid sinus with extension into the anterior cranial fossa. (Courtesy of Pamela Van Tassel, MD, Philadelphia, PA). (B) T1-weighted image showing an anteroposterior view of a similar tumor.

Management

Surgical excision with adjunctive radiation therapy is the preferred treatment for all paranasal sinus tumors. Because the cribriform plate is often involved by olfactory neuroblastoma, this structure must be removed along with the tumor to prevent recurrence.

Pleomorphic adenoma

Frequency/incidence

Benign pleomorphic adenoma (BPMA) is the most common neoplasm of major and minor salivary glands. BPMA accounts for 53% to 77% of parotid tumors, 44% to 68% of submandibular tumors, and 38% to 43% of minor gland tumors [17]. Eighty-five percent of pleomorphic adenomas occur in the parotid gland, which is by far the most common site of occurrence. Roughly 60% of intraoral lesions are located on the palate [17], [18].

Signs and symptoms

Regardless of the site of origin, pleomorphic adenomas are typically painless, slow-growing, firm masses.

Etiology/pathophysiology

Pleomorphic adenomas are derived from a mixture of ductal and myoepithelial elements [17].

Image of choice for diagnosis

CT images and MR images are the studies of choice for diagnosing pleomorphic adenomas.

Image hallmarks

On CT images, BPMA appears hyperdense in comparison with the parotid; the tumor is smoothly marginated with very little contrast enhancement (Fig. 4). If the tumor is small, BPMA appears homogeneous. Larger tumors may contain low-density areas related to central necrosis, hemorrhage, or cystic degeneration. Calcifications are indicative of BPMA. On T1-weighted MR images, the signal intensity is low to intermediate. On T2-weighted images, the signal intensity is intermediate to high. When BPMA is associated with significant hemorrhage, the image may be hyperintense on T1- and T2-weighted images.

Fig. 4. Pleomorphic adenoma. Gadolinium-enhanced T1-weighted (A) axial and (B) coronal MR images showing bilateral synchronous pleomorphic adenomas of the parotid glands. (Ahn M. Familial mixed tumors of the parotid gland. Head Neck 1999;21:773; with permission.)

Management

Surgical resection is the treatment of choice. Superficial tumors are best treated with superficial parotidectomy; deep parotid tumors should be treated with total parotidectomy. Every effort should be made to spare the facial nerve, because the tumor does not invade the nerve. When BMPA is treated by conservative enucleation, it commonly recurs. If BMPA is left untreated for an extended length of time, malignant transformation occurs in approximately 25% of cases [18].

Sinus mucocele

Frequency/incidence

Sinus mucoceles are relatively common lesions that occur most frequently in the frontal and anterior ethmoid sinuses of persons between the ages of 13 and 80 years. Mucoceles occur less frequently in the maxillary, posterior ethmoid, and sphenoid sinuses [18], [19].

Signs and symptoms

The presentation of sinus mucoceles varies depending on their location. Because they occur in the sinuses, they may grow to large dimensions before they become symptomatic, and may be seen as incidental findings on images taken for other reasons. When they become symptomatic, headache, rhinnorhea, and epistaxis may be seen. The classic frontal sinus mucocele may present with proptosis of the globe.

Etiology/pathophysiology

Sinus mucoceles are accumulations of mucin that are completely encased by epithelium (ie, they are true cysts). They generally occur after trauma, sinus surgery, or obstruction of the sinus ostium.

Image of choice for diagnosis

CT and MRI are the preferred diagnostic procedures. Because mucoceles expand bone, CT imaging is necessary to determine whether the bone is completely eroded. Also, CT imaging of the nasal cavity may detect an anatomic lesion that is causing sinus obstruction. MRI distinguishes inflammatory tissue from neoplastic tissue. MRI can also detect breakthrough into the cranial vault.

Image hallmarks

Recently formed mucoceles contain a high degree of water and protein and produce a hypointense signal on T1-weighted images and a hyperintense signal on T2-weighted images (Fig. 5). Inspissated mucoceles with very low water content, as in cases of superimposed fungal infections, have been reported to produce hypointense signals on T1- and T2-weighted images [19].

Fig. 5. Sinus mucocele. (A) Axial T1-weighted image through the ethmoid sinus demonstrates expansion on the right with low intensity in the sinus consistent with mucocele. (B) Coronal view.

Management

Surgical treatment either by debridement and curettage or marsupialization is recommended [19].

Adenoid cystic carcinoma

Frequency/incidence

Adenoid cystic carcinoma (ACC) is a relatively common salivary gland tumor that occurs most frequently in the minor salivary glands (50%), the submandibular glands (25%), and the parotid gland (25%). The palate is the most common intraoral site. ACC occurs primarily in middle-aged adults and is relatively rare in persons younger than 20. The tumor occurs fairly evenly in men and women, although some studies have reported that it is slightly more likely to occur in women [17], [18].

Signs and symptoms

ACC usually occurs as a painful, slow-growing mass. The pain is often described as a dull ache that gradually increases in intensity, which usually occurs before the appearance of any noticeable swelling. Parotid tumors may cause facial nerve paralysis, and palatal tumors may cause ulceration.

Etiology/pathophysiology

ACC is a slow-growing tumor of salivary gland origin, composed of a mixture of ductal and myoepithelial cells. These cells can be arranged in a cribriform, tubular, or solid pattern, with the cribriform pattern being the most common [17]. Perineural invasion is common for ACC. This invasion is usually detected at the time of recurrence rather than at the time of initial diagnosis, and extension of the tumor is often beyond what can be detected radiographically. Though the tumors grow slowly, local recurrence is common, and prognosis is poor.

Image of choice for diagnosis

Either CT or MRI demonstrate the primary salivary gland tumor. These imaging methods play a complementary role in the evaluation of perineural tumor spread: MRI demonstrates tumor spread along nerves, and CT demonstrates bony erosion and expansion.

Image hallmarks

When the tumor involves the parotid gland, it may appear benign with sharp margins. When it involves the minor salivary glands, the tumor may appear to infiltrate the margins. CT and MR images show abnormal soft tissue in the gland (Fig. 6). ACC shows enhancement with contrast, whereas benign pleomorphic adenoma does not. The presence of adenopathy also indicates malignancy. The imaging characteristics of this tumor are not specific; the tumor's appearance may resemble that of the more common mucoepidermoid carcinoma of the parotid gland. On MR images, abnormal contrast enhancement of the mandible indicates perineural tumor spread along the V3 branch within the mandible. CT images may show bony erosion of the canal and even widening of the foramen through which the nerve enters.

Fig. 6. Adenoid cystic carcinoma. (A) T2 axial view. (B) A different patient's postresection of tumor showing recurrence along inferior alveolar nerve.

Management

The preferred treatment for ACC is surgical excision with or without adjunctive radiation therapy. ACC is a relentless tumor that is prone to local recurrence and eventual distant metastasis. Because metastasis to regional lymph nodes is uncommon, neck dissection is not recommended. Because the overall prognosis for patients with ACC is so poor, regardless of treatment, surgical morbidity should be carefully considered, especially when the tumor is large or evidence of distant metastasis already exists [17].

Internal derangement of the temporomandibular joint

Frequency/incidence

Temporomandibular joint (TMJ) disorders are relatively common; 35% to 72% of the general population display symptoms of this disorder. Epidemiologic studies have not shown a difference in the sex and age distribution of TMJ disorders; however, most patients who seek treatment are young women between the ages of 20 and 35 years [20].

Signs and symptoms

Symptoms of TMJ internal derangement include pain in the joint, joint sounds (popping, clicking, crepitus), and changes in mobility of the mandible (decreased incisal opening, open lock, closed lock).

Etiology/pathophysiology

Anterior disc displacement results from elongation of the capsular and discal ligaments and concomitant thinning of the articular disc. These changes may be associated with trauma to the joint, either macro- or microtrauma. Sources of macrotrauma are obvious (motor vehicle accident, assault) and are often reported in the patient's history. Sources of microtrauma are subtler and usually associated with parafunctional habits [21].

Image of choice for diagnosis

Closed- and open-mouth MR images are the images of choice for evaluating the TMJ.

Image hallmarks

T1-weighted images of the TMJ disk display a hypodense biconcave structure, which should be interposed between the hyperintense condylar head and articular eminence in the closed- and open-mouth positions. Displaced disks will often be seen anterior to the condyle in the closed position. If there is reduction of the disk on opening, then the open-mouth position will show the disk in a more anatomic position. If there is no reduction of the disk, then it will continue to be displaced anterior to the condyle when the mouth is open. A cinematic view of the TMJ and disk can be seen using rapid gradient echo techniques incrementally imaging as the patient opens and closes the mouth (Fig. 7).

Fig. 7. Temporomandibular joint (TMJ). A closed (A) and open (B) T1-image of a normal condylar-disk relationship. A closed (C) and open (D) image of the left TMJ of a patient with severe long-standing rheumatoid arthritis and with a degenerated disk, which is anteriorly displaced. A closed (E) and open (F) T2 image of a nonrheumatoid patient's anteriorly displaced disk without reduction.

Management

Management of internal derangement of the TMJ is aimed at reducing pain and increasing range of motion to allow for normal function. Goals for function include interincisal opening of 40 mm with lateral excursions of 5 to 7 mm. Physical therapy, a nonchew diet, occlusal splints, nonsteroidal anti-inflammatories, and muscle relaxers have all been used in the nonsurgical therapy and management of symptomatic internal derangement. When these treatments fail to improve symptoms, surgical treatments usually follow and might include arthrocentesis, arthroscopy, or open arthrotomy.

Obstructive sleep apnea

Frequency/incidence

Obstructive sleep apnea is one in a spectrum of entities known collectively as sleep-disordered breathing. Although patients must have a polysomnogram (sleep study) to be diagnosed with obstructive sleep apnea, it is one of the most common diagnoses given by sleep centers, and reportedly affects 2% of women and 4% of men older than 30 [22].

Signs and symptoms

Snoring, apnea, and hypopnea during sleep are signs associated with obstructive sleep apnea and can be evaluated by polysomnogram. In addition, physical characteristics may include obesity, retrognathia, and a long, soft palate. Patients may complain of snoring and daytime sleepiness. They may report falling to sleep very quickly (in bed, watching TV, or during other activities) and may report never feeling rested after a full night's sleep.

Etiology/pathophysiology

Obstructive sleep apnea is a complex disease process caused by collapse or obstruction of the airway at one or multiple various levels, including the nasal airway, nasopharynx, oropharynx, and hypopharynx.

Image of choice for diagnosis

Sagittal and axial MR images can depict an obstructed airway and help diagnose the level of obstruction. Although such images are not necessary for diagnosis, they can be helpful in deciding how to treat the patient with obstructive sleep apnea. Most practitioners use plain radiography, especially lateral cephalometric radiographs, in their evaluation and treatment planning.

Image hallmarks

The soft palate and the tongue of patients with sleep apnea are much larger than those of patients who do not have this condition (Fig. 8). The size of the nasopharyngeal airway is reduced. The upper airway is narrowed because the tongue is almost in contact with the back wall of the oropharynx. Large tonsils may also narrow this upper airway.