Normative Measurements of the Thyroid, Salivary Glands, and Tonsils on Imaging

Abstract

This chapter reviews the normative measurements of the thyroid, salivary glands, and tonsils. The objective imaging criteria that can help identify abnormalities are reviewed, along with an illustrated guide depicting how the measurements are performed. In addition, examples of conditions associated with abnormal anatomical dimensions are depicted.

7.1 Thyroid Gland

• The thyroid gland is located in the visceral space of the neck and consists of two lobes connected by an isthmus anterior to the trachea.

• The thyroid gland has a high iodine content, resulting in high attenuation of 80–100 Hounsfield unit (HU) on non-contrast computed tomography (CT) and an attenuation of 150–170 HU on post-contrast CT (Fig. 7.1).

• The normal dimensions of the thyroid in adults are 40–60 mm in craniocaudal and 13–18 mm in anteroposterior dimensions (Fig. 7.2).

• The mean thyroid volume in adults is 10.7 ± 2.8 mL (range: 5.7–17.1 mL) and the volume correlates with body size.

• The normal dimensions of thyroid gland in adult and pediatric groups are detailed in Tables 7.1 and 7.2.

• The pyramidal lobe is a remnant of the thyroglossal duct and is a superior extension of normal thyroid tissue. Its presence is relevant for thyroid cancer surgery.

• The mean anteroposterior diameter, transverse diameter, and length of the pyramidal lobe are 2, 6, and 21 mm, respectively (Fig. 7.3).

Fig. 7.1

Axial computed tomography (CT) images without (a) and with (b) contrast show the normally hyperattenuating and enhancing thyroid tissue

Fig. 7.2

Normal thyroid gland. Ultrasound images and Tables 7.1 and 7.2 show the dimensions of the thyroid gland and its components

Table 7.1 Thyroid measurements in adults

Table 7.2 Pediatric thyroid volumes by age

Fig. 7.3

Pyramidal lobe. Axial computed tomography (CT) image shows the dimensions of the pyramidal lobe

7.1.1 Practical Implications

• Imaging can be obtained to evaluate patients with goiter and associated thyroid nodules, substernal extension, and tracheal narrowing (Fig. 7.4).

• When the longitudinal length of the lobes and the width of the whole gland together measure 6.5 cm or more, the thyroid gland can be considered enlarged.

Fig. 7.4

Coronal computed tomography (CT) image shows goiter with tracheal narrowing

7.2 Parotid Gland and Ducts

• The parotid glands have a lobular morphology and can be divided into deep and superficial lobes, as well as an inferior projection referred to as the “tail,” which can be defined as the inferior 2 cm of the gland.

• Based on CT or magnetic resonance imaging (MRI), the maximum axial width can range from 26 to 67 mm; the depth can range from 33 to 86 mm; and the height can range from 38 to 80 mm (Fig. 7.5).

• Based on ultrasound, the parotid glands measure 46 ± 8 mm in the axis parallel to the mandibular ramus and 37 ± 6 mm in a transverse axis.

• The volumes of parotid glands among different demographic groups are listed in Table 7.3.

• Besides the actual size of the parotid gland, it can be helpful to consider symmetry of the glands, in which the glands typically measure within 10% on either side.

• The attenuation of the parotid glands normally decreases with age as the gland undergoes fatty infiltration (Fig. 7.6).

Fig. 7.5

Axial (a and b) and coronal (c) computed tomography (CT) images with the range of normal dimensions of the parotid gland

Table 7.3 Normal parotid gland volume

Fig. 7.6

Axial computed tomography (CT) images show higher attenuation of the parotid gland in a child (a) than in an adult (b)

• The mean size of the accessory parotid gland is 16 mm × 6 mm and the mean distance from the main parotid gland is 10 mm (Fig. 7.7).

• The mean parotid duct length is 50 mm.

• The mean width of the parotid duct is normally 1–2 mm (Fig. 7.8).

Fig. 7.7

Axial computed tomography (CT) image shows an accessory parotid gland

Fig. 7.8

Axial T2-weighted magnetic resonance imaging (MRI) shows a normal fluid-filled parotid duct

7.2.1 Practical Implications

• Enlargement of the parotid gland can be due to sialosis, infectious and inflammatory sialadenitis, and neoplasm (Fig. 7.9). Some of these conditions can also be associated with parotid ductal dilatation (Fig. 7.10).

• A small parotid gland can result from post-inflammatory atrophy or can appear reduced in size after partial parotidectomy (Fig. 7.11).

Fig. 7.9

Axial T2-weighted magnetic resonance imaging (MRI) shows enlarged parotid glands with multiple cysts related to Sjogren syndrome (a). Axial computed tomography (CT) image shows right parotid swelling related to parotitis with surrounding inflammation (b). Axial fat-suppressed post-contrast T1-weighted MRI shows diffuse infiltration of the right parotid gland from carcinoma (c)

Fig. 7.10

Parotid ductal enlargment. Axial T2-weighted MRI shows dilatation of the bilateral parotid ducts, left greater than right, due to post-inflammatory strictures

Fig. 7.11

Axial T1-weighted magnetic resonance imaging (MRI) shows a small amount of residual right parotid tissue following parotidectomy with scar tissue in the overlying subcutaneous tissues

7.3 Submandibular Gland and Ducts

• The normal submandibular gland measures 28 × 18 mm (±5 mm) in the axial plane (Fig. 7.12).

• The volume of submandibular glands in different demographic groups are listed in Table 7.4.

Fig. 7.12

Axial computed tomography (CT) image shows a normal submandibular gland

Table 7.4 Normal submandibular gland volume

• The mean length of the submandibular duct is 58 mm and the submandibular duct genu has a mean angle of 115° (Fig. 7.13).

• The mean width of the submandibular duct is 2–3 mm.

• An intra- or extra-glandular duct diameter of 3 mm or more indicates possible obstruction.

Fig. 7.13

Submandibular duct. Sagittal oblique magnetic resonance (MR) sialogram image shows the normal dimension of a normal submandibular duct

7.3.1 Practical Implications

As with the parotid glands, the submandibular glands can be enlarged due to sialadenitis or neoplasm (Fig. 7.14). On the other hand, the glands can be small after radiation therapy or chronic inflammation (Fig. 7.15). Enlargement of the submandibular duct can be congenital due to an imperforate submandibular duct, or result from obstruction by tumors, post-inflammatory strictures, and calculi (Fig. 7.16).

Fig. 7.14

Axial post-contrast T1-weighted MRI shows enlargement of the right submandibular gland due to an infiltrating carcinoma

Fig. 7.15

Axial CT image shows atrophy of the left submandibular gland due to chronic sialadenitis from calculi (encircled)

Fig. 7.16

Submandibular duct dilatation. Coronal T2-weighted magnetic resonance imaging (MRI) shows a dilated left submandibular duct (arrow) due to obstruction by tumor

7.4 Tonsils

• The tonsils comprise the lingual, palatine, and nasopharyngeal (adenoid) tonsils.

• The tonsils grow proportionally to the skeletal structures during childhood.

• The adenoids are largest in the 7–10 years age group with a mean of 15 mm and steadily decline to 5 mm by 60 years of age (Fig. 7.17).

• The lingual tonsils typically measure less than 10 mm in thickness (Fig. 7.18).

• The normal adult palatine tonsils measure up to 12 mm × 20 mm in axial section (Fig. 7.19).

Fig. 7.17

Sagittal T1-weighted MR images in a child (a) and adult (b) show normal adenoids

Fig. 7.18

Sagittal post-contrast T1-weighted magnetic resonance imaging (MRI) shows the normal lingual tonsils

Fig. 7.19

Axial fat-suppressed T2-weighted magnetic resonance imaging (MRI) shows normal palatine tonsils

7.4.1 Practical Implications

The tonsillar tissues can be enlarged due to benign lymphoid hyperplasia, infection, or neoplasm (Fig. 7.20).

Fig. 7.20

Axial post-contrast T1-weighted magnetic resonance imaging (MRI) shows a nasopharyngeal carcinoma (a). Axial computed tomography (CT) image shows lingual tonsil hyperplasia (b). Axial CT image shows right tonsillitis with peritonsillar abscess (c)