Current Imaging Techniques for Head and Neck Tumors
Current Imaging Techniques for Head and Neck Tumors
ABSTRACT: Modern head and neck imaging has led to advances in both the diagnosis and treatment of head and neck cancers. Both computed tomography (CT) and magnetic resonance imaging (MRI) studies provide important information about the location and extent of neoplasm, particularly with respect to the interface of tumor with bone, fat, muscles and other soft tissues, air, blood vessels, dura, and brain. Conventional angiography can be used to assess tumor blood supply and vascularity and to perform therapeutic embolization. Magnetic resonance angiography (MRA) and CT angiography (CTA) are new tools for the noninvasive evaluation of blood vessels. Positron emission tomography (PET) shows promise for differentiating tumor from scar, edema, and other nonneoplastic soft tissues. [ONCOLOGY 13(5):697-709, 1999]
Modern head and neck imaging has advanced both the diagnosis and treatment of head and neck neoplasms. Computed tomography (CT) and magnetic resonance imaging (MRI) studies assess the interface of tumor with bone, fat, muscles and other soft tissues, air, blood vessels, dura, and brain. Conventional catheter angiography provides an assessment of tumor blood supply and vascularity, and at angiography, therapeutic embolization can also be performed. Magnetic resonance angio- graphy (MRA) and the newer CT angiography (CTA) provide tools for the noninvasive evaluation of blood vessels. Positron emission tomography (PET) scanning is just beginning to make contributions to the field of head and neck tumor imaging.
On CT without intravenous contrast, tumor is usually of intermediate density.[1,2] Tumor, blood vessels, muscles, and lymph nodes may all have the same density.
After the intravenous (IV) administration of iodinated contrast medium, tumors enhance to varying degrees: Paragangliomas, being very vascular, enhance intensely, whereas squamous cell carcinomas, being more cellular, may enhance intensely, or little or not at all. Foci of necrosis or prior hemorrhage are dark (hypodense) on CT. Lacking a blood supply, necrotic foci do not enhance after contrast administration.
On MRI, tumor signal is quite variable. Many head and neck neoplasms are isointense to (ie, have the same signal as) soft tissues (brain, muscle) on nonenhanced T1-weighted images, are isointense to hyperintense (have the same or brighter signal) on T2-weighted images, and enhance at least somewhat after IV gadolinium. Very cellular or fibrous neoplasms that contain little free water may be hypointense (dark) on T2-weighted image.[1,3] Necrosis produces a fluid signal on MRI; this is hypointense on T1-weighted images and hyperintense on T2-weighted images. As on CT, necrotic foci do not enhance on MRI images after contrast administration.
Some tumors have a distinctive appearance that allows a histologic diagnosis to be made (or suggested) on the basis of the preoperative imaging studies. This is usually based on a combination of factors, such as location, enhancement, and effect on adjacent structures. For example, a carotid body tumor arises at the carotid bifurcation (Figure 1) and enhances intensely on CT and MR studies. An esthesioneuroblastoma (olfactory neuroblastoma) involves the upper nasal cavity, anterior cranial fossa, and adjacent sphenoid and ethmoid sinuses (Figure 2a and Figure 2b).
Calcifications within a tumor are white on CT (Figure 3) and usually a signal void (black) on MRI. These may represent residual normal bone or tumor matrix. Calcified tumor matrix suggests a bone- or cartilage-forming tumor, such as a chondrosarcoma.
Most often, however, the importance of imaging is the accurate delineation of the extent of the tumor. This includes identifying structures that are spared by the tumor, as well as structures that are affected.
Tumor and muscle (sclera, mucosa) may have similar appearances on both CT and MRI studies (Figure 4). Tumor usually enhances more than any structures, except mucosa and the extraocular muscles. These muscles, unlike other skeletal muscles, enhance intensely on MRI studies (Figure 5). Interestingly, the enhancement is not apparent on CT images.
Scar tissue has a variety of appearances, depending on its age. Granulation tissue enhances intensely; fibrous tissue usually enhances less but cannot always be differentiated from tumor on CT or MRI.[1,3]
Consecutive studies are extremely helpful: With time, tumor grows (or occasionally remains stable), but scar tissue tends to contract and, thus, decrease in size. In the absence of prior studies for comparison, a CT- or MRI-guided biopsy of the abnormal soft tissue may provide the diagnosis (Figure 6).
Tumor has a very different density (on CT) and signal intensity (on MRI) than cortical bone.[3,4] Cortical bone is a dense white line on CT (Figure 7). Its density is much greater than that of tumor, making CT an ideal modality for the evaluation of erosion of bone.
Bone algorithms can be created from the initial CT scan data to emphasize bone detail. Even subtle erosion of thin cortical bone, such as tegmen tympani (Figure 7), cribriform plate, and lamina papyracea, is easily detected on CT bone algorithms.
On MRI, cortical bone is a signal void. This is a very different signal intensity from tumor, but subtle erosion of a signal void is difficult to detect, making MRI less useful than CT for evaluating cortical bone erosion by tumor.
In contrast, MRI is an excellent imaging modality for the evaluation of medullary bone and bone marrow. In adults, marrow signal in the head and neck is mostly attributable to fat. Recruitment of fatty or yellow marrow for hematopoiesis, as well as fibrosis of marrow, alters this signal. When hypointense tumor invades marrow, the different signals are easily seen on nonenhanced T1-weighted images. Because some tumors enhance intensely after IV gadolinium contrast administration, tumor may become isointense to marrow after gadolinium. Fat-suppression pulse sequences are special image acquisitions that decrease the high signal of fat without altering the high signal of enhancing tumor. Nasopharyngeal squamous cell carcinoma invading the clivus and oral cavity squamous cell carcinoma invading the mandible are two settings in which the extent of tumor invasion of the marrow can determine the extent of resection, or determine whether it is possible at all.
The appearance of the cartilage of the normal adult larynx is quite variable. Cartilage may mineralize and even ossify. Mineralized laryngeal cartilage is dense (white) on CT and hypointense on MRI. Ossified laryngeal cartilage has a cortex and marrow-containing medullary cavity (Figure 8a and Figure 8b).
Ideally, CT would be able to detect a squamous cell carcinoma of the true or false vocal cord that erodes the cortex of the ossified thyroid or cricoid cartilage. However, the normal laryngeal cartilage ossifies irregularly and discontinuously; consequently, on CT images, it may be impossible to determine whether a defect in the cartilage is erosion by tumor or a normal discontinuity (Figure 8a). Only if a laryngeal tumor extends to the extralaryngeal surface of the cartilage, invading or displacing strap muscles, can the discontinuity be presumed to be erosion. Magnetic resonance imaging, by contrast, is an ideal imaging modality for evaluating ossified laryngeal cartilage, as even small foci of marrow invasion by tumor are easily seen (Figure 8b).
Marrow edema secondary to tumor may have a similar or identical imaging appearance as the tumor itself. Although this is a potential limitation of imaging, in fact, marrow edema usually reflects tumor invasion nearby.
Tumor and fat have very different appearances on both CT and MRI studies and are readily distinguished.[1,3,4] On CT, fat is lucent (hypodense, dark) with respect to tumor (Figure 4). In general, only air is more lucent than fat. On T1-weighted MRI sequences, fat is hyperintense (white).
To differentiate tumor from fat on MRI, nonenhanced T1-weighted images are useful. After gadolinium administration, the white of enhancing tumor may blend with the white of normal orbital fat. Fat-suppression techniques darken the high signal of fat without altering the high signal of enhancing tumor (Figure 5), just as with marrow imaging. Both nonenhanced T2-weighted images and fat-suppressed enhanced images are essential to the evaluation of neoplasms that abut fat.
When it is impossible to differentiate tumor from extraocular muscle in the orbit, fat becomes a useful landmark: Identification of an intact fat plane between enhancing tumor and enhancing muscle on all images strongly suggests that the muscle is spared. Effacement of the fat may indicate frank invasion of muscle by tumor, or only tumor compression of fat (Figure 5).
As with marrow fat, edema at the periphery of a tumor may infiltrate fat and have an identical appearance to tumor in the fat. Edema secondary to radiation therapy is usually identified more easily as a reticular infiltrate in deep and subcutaneous fat planes. After radiation therapy, this is usually accompanied by thickening of the platysma muscle, intense enhancement of the major salivary glands (eg, the parotids), and low-density thickening (edema) of the aryepiglottic folds.
Cranial nerves exit the skull base surrounded by fat.[8-11] Replacement of this fat, on CT or MRI, by soft tissue strongly suggests perineural extension of tumor along cranial nerves. This is most easily appreciated just beneath the skull base at the neural foramina: stylomastoid foramen for the facial nerve (parotid tumors) and foramen ovale for the mandibular division and fora-men rotundum for the maxillary division of the trigeminal nerve (oral cavity tumors).
On CT (Figure 3 and Figure 9a), perineural tumor causes enlargement and erosion of the skull base foramina and canals that transmit the affected cranial nerves.[8-10] On MRI (Figure 9b), the corresponding abnormalities are intense enhancement and enlargement of the nerves. Magnetic resonance imaging does not show erosion of neural foramina very well. It is likely that MRI shows enhancement earlier than CT shows enlargement or erosion of the bone; therefore, MRI is a more sensitive test for the detection of perineural tumor growth.
Adenoid cystic carcinoma from major and minor salivary glands, squamous cell carcinoma, lymphoma, and melanoma all have a predilection for perineural tumor spread. In patients with these tumors, it is especially important to inspect the path that cranial nerves take to and from the tumor.