The pleomorphic adenoma, shown in the images below, is by far the most common benign salivary gland tumor, accounting for as many as 80% of all such tumors. Although pleomorphic adenomas most commonly occur in the parotid gland (about 85%), this tumor may be encountered in the submandibular, lingual, and minor salivary glands as well. Although almost one half of tumors found in the minor salivary glands are malignant, the pleomorphic adenoma is still the most common tumor in these glands. [1, 2]
Computed tomography (CT) scanning or magnetic resonance imaging (MRI) depicts the mass, and the findings may be essentially diagnostic in routine cases with typical features. Some authors have reported excellent sensitivity with ultrasonography, though this study typically does not help the surgeon to understand the 3-dimensional (3D) relationship of the tumor to the parotid gland and the facial nerve. [3, 4, 5, 6, 7, 8]
Limitations of techniques
MRI has the advantages of multiplanar imaging, and MRI results may suggest the tissue type on the basis of signal intensity characteristics. CT is often the first study ordered in patients with neck masses, and scans can show the mass and the retromandibular vein. Newer multisection CT scanners offer multiplanar capabilities rivaling that of MRI.
Disadvantages of CT include radiation exposure, the use of iodinated contrast material, and tissue distinction poorer than that of other studies. CT may also be problematic in cases of benign pleomorphic adenomas when the outer margin of the tumor appears indistinct and suggests malignant invasion of the surrounding tissue. MRIs show the well-defined outer borders in these instances. In many cases, however, CT may be the only study needed to guide the surgeon.
Sialography has been used in the past to diagnose parotid tumors, but it has been abandoned with the advent of CT and MRI. Pleomorphic adenomas appear as smoothly marginated masses that displace the parotid tissue and ducts.
Many parotid tumors and lymph nodes can have a similar appearance.
Pleomorphic adenomas, as shown in the images below, are typically smooth, well-marginated tumors, though nodularity along the outer surface is sometimes present. The attenuation values of the mass are usually homogeneous and higher than that of the surrounding gland, though lower attenuation masses that resemble cysts are occasionally seen. 
Tumor enhancement is variable and can result in a missed diagnosis if delayed images are not acquired. Pleomorphic adenomas are poorly enhancing in the early phase of contrast enhancement, though the amount of enhancement increases over time. Delayed images obtained at 5-10 minutes are often useful.
Lev et al described increased degree and homogeneity of tumor enhancement over time. Although the average delay was 24 minutes, in practice, a 5- to 10-minute delay is almost as effective.
When pleomorphic adenomas become large, they may develop a heterogeneous appearance with areas of necrosis, hemorrhage, cysts, and calcification. Large tumors commonly have a lobulated contour, which strongly suggests the diagnosis.
Degree of confidence
Although certain CT features (eg, lobulation, homogeneity, delayed contrast enhancement) can suggest the diagnosis of pleomorphic adenoma, these findings are not specific to the tumor.  In addition, CT results may falsely suggest invasion of the surrounding tissue; in these cases, MRI should be performed to ensure that a diagnosis of malignancy is not incorrectly made. MRIs can demonstrate a well-defined capsule, even when the border appears irregular on CT scans.
Magnetic Resonance Imaging
Pleomorphic adenomas, as shown in the images below, are usually well-circumscribed, homogeneous masses with low intensity on T1-weighted images and high intensity on T2-weighted images. They commonly have a rim of decreased signal intensity on T2-weighted images; this rim also appears hypointense on fat-suppressed T1-weighted images. This finding represents the surrounding fibrous capsule. [10, 11, 7, 8]
A well-defined parotid mass with increased signal intensity on proton density- and T2-weighted images is highly suggestive of the diagnosis. In fact, the increased signal intensity on T2-weighted images is so prevalent that any parotid mass with this finding should be viewed with suspicion, as this often indicates a carcinoma ex pleomorphic adenoma.
When it becomes large, the tumor may lose its homogeneous appearance on MRI, as with CT. It may have areas of fibrosis, necrosis, and hemorrhage. Inhomogeneous signal intensity should be apparent in these cases. Lobulation is commonly visualized and is, in itself, also suggestive of the diagnosis.
After gadolinium enhancement, the tumors are homogeneously enhancing unless they are large. Unlike CT, MRI has no role in delayed imaging.
Gadolinium-based contrast agents have been linked to the development of nephrogenic systemic fibrosis (NSF) or nephrogenic fibrosing dermopathy (NFD). The disease has occurred in patients with moderate to end-stage renal disease after being given a gadolinium-based contrast agent to enhance MRI or MRA scans.
NSF/NFD is a debilitating and sometimes fatal disease. Characteristics include red or dark patches on the skin; burning, itching, swelling, hardening, and tightening of the skin; yellow spots on the whites of the eyes; joint stiffness with trouble moving or straightening the arms, hands, legs, or feet; pain deep in the hip bones or ribs; and muscle weakness.
Degree of confidence
The diagnosis of pleomorphic adenoma can be at least strongly suggested in most cases. Special attention should be paid to the signal intensity of the tumor and to any possible invasion into adjacent soft tissues.
Kakimoto et al investigated the CT scanning and MRI features of 50 pleomorphic adenomas of the head and neck in 50 patients and found that tumor detectabilities were 77% on axial plain CT images, 90% on axial CE CT images, 86% on axial T1-weighted MRI, 88% on axial T2-weighted MRI, and 85% on axial CE T1-weighted MR images. The capsule could be hardly detected on CT images but could be detected in many cases on MR images. The authors therefore concluded that pleomorphic adenomas of the head and neck should be evaluated with MRI over CT. 
On sonograms, pleomorphic adenomas typically appear as smooth, round, hypoechoic masses with distal acoustical enhancement. Lobulations are commonly visualized. Large tumors appear more heterogeneous than small ones and are better imaged with CT or MRI than with sonography. [13, 6]
Degree of confidence
Although sonographic findings can suggest the diagnosis of pleomorphic adenoma in many small tumors, CT or MRI is needed to fully evaluate large tumors. Even in patients in whom the diagnosis is made by means of sonography and biopsy, CT or MRI is needed for preoperative planning.
Evidence suggests that dual-isotope imaging with technetium Tc 99m and thallium Tl 201 single-photon emission CT (SPECT) is accurate in distinguishing various tumors of the salivary glands, including pleomorphic adenomas.