Chapter: Thyroid

Background Method

Thyroid & Submandibular Glands

IMBUS examination of the thyroid an submandibular glands is important for patients with lumbs in the anterior neck. It is important to understand the cautions and benefits of these examinations.

THYROID

Thyroid US raises some issues for primary care. If is fairly easy to perform, but requires experience to interpret and integrate the findings into clinical decision-making.

Fatal thyroid cancer is rare but thyroid nodules are common and often found incidentally when CT, MR, and carotid US exams include the thyroid in the field. Of these accidentally discovered nodules, only about 10% turn out to be malignant. The great majority of the malignancies are small, low-grade papillary cancers which probably don’t need resection because there has been no detectable change in thyroid cancer death rate despite a large increase in thyroid cancer resections. All of this information indicates that routine screening of all patients for thyroid cancer is hard to support.

Yet, some patients come to clinic thinking that they have seen or felt something in their lower neck. Others come because their unrelated imaging study has shown something in the thyroid. Some clinic physicians examine the neck routinely as part of an annual physical and something may be detected. Competence at thyroid IMBUS may avoid sending some patients for formal ultrasound studies or biopsy. The following approach is supported by recommendations from several national groups.

Patients at increased risk: family history, radiation as a child, genetic disease with an increased risk of medullary carcinoma.

Worrisome Features of a Thyroid Nodule:
Microcalcification
Rim calcifications
Irregular margin
Deeper than Wide
Extra-thyroidal spread

Purely Simple Cysts are unusual, but regardless of size, they are benign and no biopsy is recommended. Symptomatic cysts > 4 cm may still be resected. A purely simple cyst found with IMBUS could be followed once in clinic in 6-12 months for reassurance, but then needs no, or very infrequent, imaging.

Spongiform OR partially cystic nodules WITHOUT any worrisome feature are very low suspicion for malignancy (< 3% chance). TSH is recommended, probably for the small chance that a nodule could be hyperfunctional. Many of these are colloid nodules composed of irregularly enlarged follicles with abundant colloid. Inspissated colloid has a characteristic feature of short comet tails below multiple hyperechoic blots. These are not microcalcifications. IMBUS follow up for size change is used for nodules smaller than 2 cm. FNA is recommended for nodules greater than 2 cm unless they are classic colloid nodules, which can be watched longer.  The following clip shows a large colloid nodule with multiple short comet tails.

 

Isoechoic/hyperechoic solid OR partially cystic nodules with eccentric solid areas WITHOUT any worrisome feature are low suspicion for malignancy (5-10% chance). IMBUS follow up for size change is used for nodules smaller than 1.5 cm and FNA is recommended for nodules greater than 1.5 cm.

Solid hypoechoic nodules with smooth margins WITHOUT any worrisome features are intermediate suspicion for malignancy (10-20% chance). IMBUS follow up for size change is used for nodules smaller than 1 cm and FNA is recommended for nodules greater than 1 cm.

Solid hypoechoic nodules OR solid hypoechoic components of partially cystic nodules WITH one or more worrisome features are high suspicion for malignancy (> 70-90% chance). IMBUS follow up is still used for nodules smaller than 1 cm with FNA being performed at 1 cm or greater.


TECHNIQUE AND FINDINGS

Use the linear probe, with settings optimized for the thyroid. Most patients can keep their clothes on and be draped with washcloths for the exam. The patient should lie supine and attempt to extend the neck, but should not be uncomfortable. The exam can also be performed with the patient standing, particularly if the patient has mobility issues. The head can be slightly rotated contralateral to the side being examined.

It makes sense to cover the area with gel before the exam. Start with the right side and place the probe transverse, indicator physician left, above the clavicle as for a carotid/jugular vein exam. The probe is angled slightly medial to get a good image. The carotid is almost always at the lateral border of the thyroid. Optimize the depth, focus position, and gain.

If the thyroid lobe is already visible just above the clavicle, be sure to fan down and look for a retrosternal thyroid. If no thyroid is visible right above the clavicle slide cephalad until the thyroid is seen just lateral to the trachea.

Size the right thyroid lobe before evaluating its composition. Quickly slide up and down in the transverse plane and identify the location where the thyroid appears the largest and FREEZE the image. Use calipers to measure the transverse width (to the beginning of the isthmus) and the anterior/posterior depth. A normal thyroid lobe is less than 2 cm in width and depth. Here is a right lobe of the thyroid with a width and depth at the upper limits of normal.

After transverse sizing of the right lobe, slowly move up and down the lobe in the transverse plane again, looking at the parenchyma. It makes sense to briefly apply Color or power Doppler to the lobe, at least to be able to learn what normal looks like. Here is a clip from a patient with a very abnormal right thyroid.

This was a very large cystic nodule with separate solid elements within the cyst. Only a hint of colloid was seen at the end of the clip in the more anterior solid component at about 1:00. The solid components showed active blood flow with Color. This nodule was at least intermediate suspicion for malignancy and required formal ultrasound and biopsy. The final diagnosis was large colloid nodule.

After the transverse evaluation of the right lobe, rotate the probe to the parasagittal plane, indicator cephalad. Normal cephalad/caudad length is greater than the aperture of our linear probe, so it is impossible to accurately measure the length of the lobe. Fortunately, it is rare for a thyroid to be enlarged only because of its length so measuring just the width and depth is adequate in most patients. Here is a long axis of a normal right thyroid lobe. There is some remaining lobe off the screen.

To switch to the left lobe of the thyroid, cross over the trachea and evaluate the isthmus.  A normal isthmus should be less than 0.5 cm in depth but this is almost never increased in isolation so it does not need routine measurement if the lobes are normal. Here is a transverse isthmus in a normal adult.

The left thyroid lobe is examined in the same order as the right: transverse evaluation, followed by the longitudinal. The probe indicator stays physician left, which shows the anatomy in correct position, with the carotid artery lateral to the thyroid. Here is a transverse view of a left thyroid lobe. There appears to be a cystic/solid nodule in the posterior area of the lobe, but this is actually part of the cervical esophagus. This will be discussed a bit more below.

The following clip is from a screening exam of a normal patient. This nodule was too anterior to be esophagus and was mixed solid and cystic. It measured 1.0 x 0.5 cm, and had no worrisome features. This nodule was very low suspicion for malignancy and could be followed periodically watching for size increase over 2.0 cm before formal ultrasound is obtained.

Here is a little larger left thyroid nodule that was solid and isoechoic. It was 1.25 x 1.0 cm and had a thin hypoechoic halo, which may be a good sign in the thyroid. If there were no other worrisome features, this nodule would also be low suspicion for malignancy and would be followed with IMBUS until it reached 1.5 cm. However, this nodule was modestly deeper than it was wide, so the follow-up interval was shortened to 6 months to be sure that change was detected.

Here is the longitudinal view of the normal left thyroid that was shown in transverse plane above with the cystic/solid lesion in the posterior lobe. The esophagus posterior to the left thyroid lobe is now more obvious  During the clip the patient gently swallowed a little saliva.



SUBMANDIBULAR GLAND

High anterior neck lumps can be submandibular gland (SmG) in origin and these may be asymptomatic or uncomfortable, depending on the cause. A competent IMBUS exam can localize a lump to the SmG and some of the etiologies can be clear and require no further diagnostic study. The probe and the exam type are the same as for the thyroid,

The SmGs are salivary glands that are smaller than the parotids but larger than the sublinguals. The SmGs lie transversely under the midpoint of the mandible as shown in the following image of the right upper neck. Nodules in the submandibular gland are less common than in the parotid, but more likely to be malignant. Pleomorphic adenoma is the most common of the benign lesions and these are sometimes resected. Nodules > 1 cm should be pursued; smaller nodules can be followed with IMBUS for size change. The larger the nodule, the more necessary to have a CT or MR to assess local invasion or dissemination before doing the biopsy. Ultrasound guided FNA or core needle biopsy is needed in most situations.

The key anatomic features are the digastric muscle (DigastricM) which lies directly caudal and a bit internal to the SmG, the parasagittal oriented facial vein (FacV) and artery (FacA) at the end of the SmG closest to the ear, and the submental vein (SubMV) and artery (SubMA) that run transverse with the SmG under the mandible. The peculiar mylohyoid muscle (MyloHyM) comes from above and partially splits the chin-end of the SmG, but this muscle is not always seen well with IMBUS.

An IMBUS exam of the SmG can be kept simple. A transverse probe placement under the mandible (indicator physician left for both sides of the neck) is usually all that is needed. The gland should have a homogeneous echotexture very similar to a normal thyroid gland. The hypoechoic DigastricM is posterior to the SmG and can suggest a fluid filled structure depending on the gain setting. An adult SmG should not be much more than 3 cm in greatest diameter (length). Here is a normal right SmG. The digastric muscle with this gain did not appear like fluid.

An infrequent abnormality in adults is a branchial cleft cyst, which can occur posterior to the SmG and push the SmG anteriorly. Don’t confuse a normal hypoechoic DigastricM with a small branchial cleft cyst.  Varying the gain should help distinguish muscle from a fluid-filled structure and the cyst should have some posterior acoustic enhancement.

Arteries and veins appear in some scan planes, but are easily identified with color flow  or power Doppler. The SmG duct (Wharton) runs transversely through the gland to the floor of the mouth and is not seen unless it is enlarged. It may appear as a vessel but will not have Doppler flow. The whole duct can dilate with an obstruction near the floor of the mouth or only the distal duct will dilate if the stone is in the middle of the gland.

Here is a clip of a normal left SmG. Because the indicator remains physician left, the ear-end of the SmG is now at the right side of the screen. A few vessels are briefly seen.

The SmG can be affected by stones, cysts, infections, adenomas/ adenocarcinoma, and infiltrative disease like Sjogren syndrome and MALT lymphomas. Simple cysts can be diagnosed with IMBUS and reassurance given without additional imaging. Most solid lesions will need formal high-resolution US.

The next clip is from a clinic patient complaining of swelling under the left jaw. The SmG was enlarged and a substantial stone with posterior shadowing was seen. The duct distal to the stone (right side of the image) was dilated.

Infiltrative disease shows very heterogeneous gland tissue. The following composite image shows submandibular glands varying from normal (top left) through early and then more advanced Sjogren syndrome. The heterogeneity is easy to see even in the early stages

This last image is from a clinic patient who noticed a lump in the upper neck. The SmG lesion was solid with a small fluid-filled center. Biopsy diagnosed an adenocarcinoma and the patient had a resection.