Ultrasonographic echogenicity of normal salivary glands in adults: comparison of submandibular and parotid glands

Abstract

The normal submandibular gland (SMG) and parotid gland (PG) are thought to have similar homogeneous ultrasound (US) hyperechogenicity; however, this has not been extensively investigated. The aim of this study was to determine whether the normal SMG and PG have similar US echogenicity. Methods: We included 969 consecutive adult patients with normal salivary glands. The patients were categorized into three age groups: group 1 (19 to 29 years, n=27), group 2 (30 to 49 years, n=273), and group 3 (≥50 years, n=669). The echogenicities of the SMG and PG were prospectively evaluated by an experienced radiologist. Computed tomography (CT) attenuation in Hounsfield units (HUs) was quantitatively measured for the SMG, PG, and sternocleidomastoid muscle in 140 patients. Results: Relative to the PG, the echogenicity of the SMG was similar in 706 (73.0%) and homogeneously hypoechoic in 263 patients (27.0%). The frequency of SMG hypoechogenicity decreased with increasing age (group 1, 59.3%; group 2, 36.3%; group 3, 22.1%; P<0.001). The CT attenuation levels (in HUs) of the SMG and PG were significantly higher in patients with hypoechoic SMGs than in patients with SMG echogenicity similar to that of the PG (P<0.001). Conclusion: Hypoechogenicity of the SMG was observed in approximately one-third of this sample of adults with normal salivary glands. The SMG may be inadequate as a reference standard for evaluating thyroid nodule echogenicity in patients with diffuse thyroid disease with decreased echogenicity.

Can the ultrasound echogenicity of normal parotid and submandibular glands be used as a reference standard for normal thyroid echogenicity?

“…A previous study [ 15 ] that investigated the echogenicity of normal PG and SMG included only adult patients with normal PGs showing typical homogeneous hyperechogenicity and did not compare the echogenicity of normal PG and SMG with thyroid gland. The present study included consecutive patients of all ages with normal salivary glands and compared the echogenicity between the normal salivary glands with thyroid parenchyma in patients with or without diffuse thyroid disease.…”

“…Fat tissue can show variable echogenicity depending on the heterogeneity of its histologic architecture [ 24 ]. The computed tomography attenuation of the normal SMG was substantially higher than that of the normal PG in patients who had similar echogenicity of the SMG and the PG [ 15 ]. PGs with complete fatty changes after radioactive iodine ablation for the treatment of thyroid cancer usually exhibit hypoechogenicity [ 25 ], and there was no significant relationship between diffuse fatty changes and parenchymal echogenicity upon histologic examinations of PGs with incidental diffuse parotid disease in patients who underwent parotid surgery [ 18 ].…”

“…The normal parotid gland (PG) and SMG have been generally considered to have homogeneous hyperechogenicity comparable to the echogenicity of the normal thyroid gland [ 12 , 14 ]. A recent study [ 15 ] demonstrated that 27.0% of normal SMGs showed hypoechogenicity compared with normal PGs in adults and reported that the normal SMG may exhibit physiologic hypoechogenicity. However, in the previous study [ 15 ], the echogenicity of the normal PG and SMG was not compared to the normal thyroid gland, and a limitation of that study was that it could not prove whether normal echogenicity of the PG was similar to that of the normal thyroid gland.…”

“…A recent study [ 15 ] demonstrated that 27.0% of normal SMGs showed hypoechogenicity compared with normal PGs in adults and reported that the normal SMG may exhibit physiologic hypoechogenicity. However, in the previous study [ 15 ], the echogenicity of the normal PG and SMG was not compared to the normal thyroid gland, and a limitation of that study was that it could not prove whether normal echogenicity of the PG was similar to that of the normal thyroid gland. To the authors’ knowledge, no previous study has investigated whether the normal PG or SMG has similar echogenicity to that of the normal thyroid gland.…”