MR Imaging of the Larynx

Advances in magnetic resonance imaging (MRI) and computed tomography (CT) scanning have improved the ability to visualize the larynx. MRI is better than CT scanning at delineation of soft tissue involvement and has the capability of multiplanar high-resolution imaging. ^{[1, 2, 3, 4, 5]}

MRI, however, has limitations. Because MRI cannot image solidly calcified structures, cartilaginous and bony imaging has been difficult. Furthermore, acquisition time in MRI is longer than in other radiologic techniques, most notably CT scanning. These challenges are complicated further by the degradation of images secondary to motion artifacts from swallowing, breathing, coughing, and carotid artery pulsations. See the image below.

Newer, faster MRI techniques have overcome some of these barriers, thus permitting assessment of deep laryngeal structures for accurate evaluation and staging of laryngeal disease. This allows for proper surgical management or conservative therapy. MRI is contraindicated in patients with cardiac pacemakers, ferromagnetic cerebral aneurysm clips, and metallic cochlear implants. Sedation may be necessary for claustrophobic patients because imaging is performed by placing the patient in the tight confines of the bore of a superconducting magnet.

For excellent patient education resources, see eMedicineHealth's patient education article Magnetic Resonance Imaging (MRI).

Discussion of the laryngeal anatomy can be divided into the tissue types that comprise it. The most prominent tissue types are the cartilages.

The thyroid cartilage is the largest structure. In the midline, a thyroid notch is present; extending laterally, a superior tubercle is present followed by the superior horn posteriorly. Inferiorly, an inferior tubercle and inferior horn are present. The inferior horn of the thyroid cartilage articulates with the cricoid cartilage. The size of the thyroid cartilage is sex linked (ie, men's thyroid cartilage is larger than women's).

The cricoid cartilage is the only complete cartilaginous ring and is thicker than the thyroid cartilage. It is joined to the trachea by the cricotracheal ligament. Sitting atop the posterior/superior border of the cricoid are the paired arytenoids.

The anterior aspects of the arytenoids comprise the vocal process; the muscular process is based laterally. The base of the arytenoids defines the level of the true cords. During quiet respiration, the cords are abducted. The hyoid consist of a lesser and greater cornu. The hyoid is joined to the trachea via the thyrohyoid membrane.

The epiglottis consists of elastic cartilage, which does not ossify. The thyroid, cricoid, and arytenoids consist of hyaline cartilage that undergoes endochondral ossification. Ossification usually begins near the midline and posterior borders and starts in puberty, continuing through the third decade of life. The hyoid bone undergoes ossification at infancy.

The connective tissue structures are important in preventing tumor extension. Extending from the superior margin of the cricoid to the free edge of the true vocal cords is the conus elasticus. It is a fibroelastic structure similar to the quadrangular membrane that extends from the false vocal folds to the aryepiglottic fold. The laryngeal ventricle separates the 2 membranes. The Broyle ligament (or ligament of the anterior commissure) provides attachment of the true cords to the thyroid cartilage. Because no overlying perichondrium exists, this is a potential area for spread of glottic lesions.

The superior laryngeal artery, the internal branch of the superior laryngeal nerve, the venae comitantes of the common facial vein, and the lymphatics pass through the thyrohyoid membrane. The external branch of the superior laryngeal nerve provides innervation to the cricothyroid muscle. The recurrent laryngeal nerve innervates the remainder of the laryngeal musculature.

Spread of laryngeal tumors depends on surrounding spaces. The lateral paraglottic spaces, the midline preepiglottic space, and the Reinke space are important. The preepiglottic space lies anterior to the epiglottis, thyroepiglottic ligament, and quadrangular ligament and posterior to the upper margin of the thyroid cartilage, hyoid, and thyrohyoid membrane. The paraglottic space is lateral to the preepiglottic space between the alae of the thyroid cartilage, conus elasticus, quadrangular membrane, and mucosa of the pyriform sinus. This space can allow extension to subglottic and extralaryngeal structures. The Reinke space is a potential space of the superficial lamina propria of the vocal cord that contains a loose fibrous matrix and has sparse lymphatics.

The extrinsic muscles that elevate the larynx consist of the stylohyoid, digastric, geniohyoid, and stylopharyngeus. The depressors include the omohyoid, sternohyoid, and the sternothyroid. The only extrinsic muscle in the laryngeal area proper is the cricothyroid muscle.

The intrinsic muscles control vocal cord tension and size and shape of the inlet. The most important intrinsic muscle is the thyroarytenoid, with its medial aspect known as the vocalis muscle. It is responsible for increasing vocal fold tension and vocal fold adduction. The remaining muscles include the posterior (only vocal fold abductor) and lateral (vocal fold adduction) cricoarytenoids and the transverse and oblique arytenoids (only unpaired muscles involved in adduction). The video below shows normal laryngeal examination findings by nasopharyngoscopy.

Laryngeal cancer comprises 1-5% of all malignancies diagnosed annually. Approximately 67% of laryngeal carcinomas are glottic; ^[6] 1-2% are subglottic. The prevalence and types of laryngeal cancer can be explained, to some degree, by the epithelial lining in that area.

The mucosae of the supraglottis and subglottis are lined with respiratory epithelium (pseudostratified ciliated columnar), whereas the glottis is lined with nonkeratinized squamous epithelium. The vast majority of neoplasms affecting the larynx are squamous cell carcinomas (SCCs). ^[7] Verrucous carcinoma, adenocarcinoma, sarcomas, and metastatic carcinomas make up a small percentage of other noted tumors in this region. Malignant tumors involving the glottis are typically low-grade, well-differentiated, keratinizing SCCs. The most frequently involved site is the anterior third of the true cord. Subglottic tumors tend to be poorly differentiated SCCs, and most are extensions of glottic tumors rather than primary lesions.

Technique

Atoms with an odd number of protons, such as hydrogen with one proton, can demonstrate the physical property of magnetic resonance. Quantum spin properties allow the protons of these atoms to assume different energy levels when placed in a strong uniform magnetic field. If external energy is applied as a pulse of radio waves of the appropriate frequency, a certain number of nuclei are raised to a higher energy level. When the radiofrequency pulse is no longer applied, nuclei in the higher energy state return to the resting energy state and emit energy in the form of a radio wave of the same frequency.

MRI is the result of measuring the energy emitted from the abundant hydrogen atoms in water and fat. By introducing gradients rather than uniform magnetic fields, certain slices of tissue can be selectively excited. Further use of gradients allows localization of the energy source within the image plane. Computer-based reconstruction algorithms, which use complex mathematical functions, use that localization to make images.

Upon the emission of energy, nuclei are said to relax, indicating that they assume a lower energy state by realigning with the applied magnetic field. The rate at which relaxation occurs is determined by two tissue properties, the T1 or longitudinal relaxation time and the T2 or transverse relaxation time. These relaxation times determine the amount of energy received from different tissues and are the basis for image contrast in MRI.

Imaging of the larynx is challenging given the function of this organ and its proximity to other structures (eg, esophagus, carotid arteries) that can cause motion artifacts. During imaging, patients are instructed to breathe quietly and not to swallow (or swallow as seldom as possible). Hyperextending the neck helps to prevent swallowing by making it difficult. The receiver coil should be separated from the chest wall to prevent movement; however, it must be near the neck. A number of flow compensation techniques have been attempted to assist in imaging (eg, respiratory and cardiac gating, presaturation pulse techniques attempted). These techniques have reduced but not eliminated motion artifacts from MRI. ^[8]

The studies are performed with the patient lying on his back and thus the patient's airway parallel to the table (see the image below). Following short localizer sequences, slice or volume acquisitions are obtained in coronal, axial, or sagittal planes. The axial plane is usually parallel to the true vocal cords, while the coronal plane is perpendicular to the defined axial plane. Both T1- and T2-weighted sequences are performed, with a slice thickness of 3-5 mm. Intravenous contrast is often used in MR imaging of the larynx.

The airway is visible as an area of no signal because air does not show resonance. Muscle and cartilage have an intermediate-to-low signal intensity. Where cartilage contains marrow or fat, signal intensity is high. Cortical bone shows very low signal intensity because of absence of resonance in the densely calcified bone. Fatty marrow demonstrates increased signal intensity on T1-weighted images (see the images below).

Sagittal slices well delineate the epiglottis, valleculae, and base of tongue (see the images below). This allows assessment of the preepiglottic space to evaluate for tumor invasion. T1-weighted images are useful for this region. In the coronal view, the thyroarytenoid muscle represents most of the true cord. The false cord contrasts with the true cord because of its fat content, giving it high signal intensity on T1-weighted images. Axial images allow evaluation of cartilaginous invasion. The level of the true cords can also be assessed with the contrast between the fatty paraglottic region at the level of the false vocal cords (high signal intensity) and the low signal intensity of the muscle at the cord level.

Carcinoma

For accurate staging and proper treatment, determining the degree of neoplastic cartilaginous invasion is important. Evidence indicates that cartilaginous invasion occurs preferentially where the collagen bundles attach to the perichondrium (Sharpey fibers). In the larynx, typical sites include (1) the anterior commissure, (2) the junction of the anterior quarter and posterior three quarters of the lower thyroid lamina, (3) the posterior border of the thyroid lamina, (4) the cricoarytenoid joint, and (5) the attachment of the cricothyroid membrane. ^[9]

Tumor angiogenesis factor (TAF) is thought to play a role in cartilaginous invasion of ossified cartilage, while TAF-inhibiting factor and collagenases are thought to inhibit invasion of nonossified cartilage. Laryngeal cartilage invasion by neoplastic tissue involves an osteoblastic phase in which hyaline cartilage is transformed into bone followed by an osteoclastic phase. However, the reaction is caused by proximity of the tumor and not by direct invasion. Therefore, increased osteoblastic activity and new bone formation become evident before tumor penetrates the perichondrium. Cartilaginous invasion of laryngeal cartilages can be thought of as occurring in 3 stages as follows: (1) inflammatory changes within the cartilage juxtaposed to the tumor, inducing osteogenesis prior to tumor invasion, (2) osteolysis, and (3) invasion.

With tumor invasion, hyaline cartilage displays higher signal intensity on T2-weighted images than normal cartilage. On T1-weighted images, invaded cartilage and fatty marrow display a low-to-intermediate signal intensity similar to tumor tissue (see the images below). However, the use of contrast permits enhancement adjacent to the tumor. If these signs are absent, cartilage invasion can be excluded.

However, the specificity of MRI to detect neoplastic invasion of the thyroid cartilage is only about 56% because of the high incidence of inflammatory changes in the thyroid with surrounding neoplasms. The specificity of the cricoid and arytenoid cartilages is roughly 87% and 95% respectively. The reported results of the sensitivity, specificity, and negative predictive value of MRI in laryngeal cancer are shown below. As shown, the negative predictive value is high, thus making MRI a useful imaging modality for evaluation and management of cartilage invasion.

• Castelijns, 1988 - Sensitivity 89%, specificity 88%, negative predictive value 92% ^[10]

• Becker, 1995 - Sensitivity 89%, specificity 84%, negative predictive value 94% ^[11]

• Zbaren, 1996 - Sensitivity 94%, specificity 74%, negative predictive value 96% ^[12]

An unfortunate complication of radiation therapy for laryngeal carcinoma is chondroradionecrosis. Changes include inflammation, cartilage necrosis, ulcerations, and abscess formation. In this case, neither MRI nor CT scanning is useful in differentiating recurrent carcinoma versus radionecrosis, much less biopsy. However, more recently, diffusion-weighted MRI (based on the principle that the rate of water diffusion in all tissues is a direct function of its physiological state) has been used to differentiate between persistent or recurrent tumor versus radionecrosis. The number of individuals in this study was low; however, the study does show some promise in the use of MRI in differentiating these pathologic states.

Paraganglioma

Paragangliomas are neuroendocrine tumors with a neural crest origin. They are highly vascular and benign and are usually observed by the otolaryngologist as a glomus vagale tumor. Laryngeal paragangliomas are much less common and are thought to arise from the superior laryngeal parasympathetic chain. Laryngeal involvement is most commonly supraglottic (82%), and patients usually present with hoarseness or dysphagia. Subglottic and glottic paragangliomas are less common, occurring approximately 15% and 3% of the time, respectively.

The tissue is composed of 2 main types of cells: chief cells and sustentacular cells. The chief cells contain neurosecretory granules; the sustentacular cells do not. Their arrangement has been described as a Zellballen pattern. The image below shows a sagittal view of a patient with a laryngeal paraganglioma. Preoperative angiography and embolization were performed, followed by a complete surgical resection.

Tuberculosis

Laryngeal tuberculosis (TB) has become rare since the advent of antibiotics, although in a patient with pulmonary TB, laryngeal involvement is a possibility. The first image below shows a sagittal view of a patient with laryngeal TB. The most common site of involvement in the larynx is the posterior aspect and epiglottis. The second image below (depicting a laryngeal schwannoma) shows invasion into the anterior longitudinal ligament.

Schwannoma

Schwannomas, similar to neurofibromas, are derived from the Schwann cells of peripheral nervous tissue. They are typically benign but have been reported to undergo malignant degeneration. Most commonly observed in the fifth to sixth decade of life, schwannomas occur more frequently in women. They have typically been described as a solitary, encapsulated, slow-growing tumor. In the larynx, the most common site of origin is the superior laryngeal nerve. The site of involvement in the supraglottis is on the aryepiglottic fold or false vocal cord. Patients typically present with symptoms of dysphagia, odynophagia, hoarseness, stridor, and/or globus pharyngeus.

The image below is a coronal T1-weighted image with intravenous contrast of a laryngeal schwannoma. They are typically hypointense to muscle on T1-weighted images and hyperintense on T2-weighted images.

Fast spin-echo techniques have improved the quality of MR images of the larynx by decreasing imaging time and, thus, the effects of motion artifacts. Fat suppression techniques have been used to eliminate high–signal intensity fat that might obscure borders with abnormal tissue on T2-weighted scans and enhancing tissue on T1-weighted scans. Proton magnetic resonance spectroscopy has shown some promise in distinguishing metastatic head and neck cancer from normal tissue and in distinguishing that the choline/creatinine ratio is higher for benign lesions than for SCCs and muscle. These techniques are being used and may contribute significantly to the diagnosis and management of laryngeal cancer.

Using cadaveric larynxes, a study by Kishimoto et al found that high-resolution MRI (45-117 μm² in-plane resolution) can resolve microanatomic structures such as the vocal fold superficial lamina propria, vocal ligament, and maculae flavae. Among the age-related tissue characteristics revealed were intramuscular fat deposition and cartilage ossification. ^[5]