MRI vs CT for Salivary Gland Pathology
A 45-year-old woman presents with a painless, slow-growing mass in the left parotid gland. She has no history of facial nerve weakness. On palpation, the mass is mobile and firm. She is referred for imaging. The clinician is deciding between MRI and CT. The patient has no contraindications to MRI. The mass is approximately 2 cm in diameter.
1. Compare the advantages and disadvantages of MRI and CT for evaluating a parotid mass.
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MRI advantages: superior soft tissue contrast, no ionizing radiation, ability to characterize tissue (e.g., T1, T2, fat suppression), and better delineation of the facial nerve and deep lobe involvement. It is the modality of choice for salivary gland tumors. Disadvantages: longer scan time, higher cost, susceptibility to motion artifacts, and contraindications (e.g., pacemakers). CT advantages: faster, better bone detail, lower cost, and widely available. Disadvantages: ionizing radiation, lower soft tissue contrast, and artifacts from dental restorations. For a parotid mass, MRI is preferred because it can differentiate benign from malignant tumors (e.g., pleomorphic adenoma vs. carcinoma) and assess perineural spread. CT is useful if MRI is contraindicated or for assessing cortical bone invasion.
2. What specific MRI sequences would you request, and what findings would suggest a malignant tumor?
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Request T1-weighted (for anatomy and fat), T2-weighted (for lesion characterization; pleomorphic adenomas are typically hyperintense on T2), STIR or fat-suppressed T2 (to highlight pathology), and post-contrast T1-weighted with fat suppression (to assess enhancement and necrosis). Diffusion-weighted imaging (DWI) can help differentiate benign from malignant. Findings suggesting malignancy: ill-defined margins, invasion of adjacent structures (e.g., muscle, bone, skin), perineural spread (enhancement along the facial nerve), lymphadenopathy (necrosis, rounded nodes), low T2 signal (high cellularity), and heterogeneous enhancement with necrosis. However, biopsy is required for definitive diagnosis.