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 neck 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 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 can’t help themselves and examine the thyroid routinely as part of an annual physical. Competence at thyroid IMBUS may avoid sending some patients for formal ultrasound studies. 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.

Hypoechoic solid 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 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 examiner 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 normal right lobe of the thyroid with the width and depth measured.

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 flow Doppler to the lobe, at least to be able to learn what normal looks like. Since directionality of flow is not an issue with the thyroid, a good case can be made for using power Doppler instead of color flow. Here is a clip from a patient who had a carotid flow time performed, during which the right thyroid looked abnormal.

The largest nodule, with the hyperechoic walls, was about 1.5 cm in largest dimension. While it had cystic elements, it was not purely cystic. Cystic-appearing lesions should always have color flow/power Doppler applied to be sure they are not vascular structures. The smaller nodule more lateral is under 1 cm with mixed density. If only this smaller nodule had been seen, it could have been followed in clinic for a size increase without doing a formal ultrasound. The larger nodule needed a formal ultrasound and biopsy.

After the transverse evaluation of the right lobe, rotate the probe to the parasagittal plane, indicator cephalad. Normal cephalad/cauda length is greater than the aperture of most standard linear probes, 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 measurement of a normal right thyroid lobe. There is some remaining lobe off the screen on the cephalad side.

Slide slowly medial to lateral (and up and down) through the lobe, looking for nodules. Here is the longitudinal view of the incidental thyroid nodule shown above. The depth adjustment could probably have been brought up one notch, but the examiners wanted the lesion in the middle of the sector for optimum focus. The complexity of the lesion is obvious.

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, smaller adult that also showed both lobes of the thyroid.

The left thyroid lobe is examined in the same order as the right: transverse evaluation, followed by the longitudinal. The probe indicator stays examiner left, which shows the anatomy in correct position, with the carotid artery lateral to the thyroid. Here is a transverse view of a normal left thyroid lobe. The cervical portion of the esophagus can be seen near the posterior left thyroid lobe so don’t interpret this as a thyroid lesion. A patient can sip some carbonated water while the thyroid is imaged to help define the esophagus. Esophageal diverticula can more prominently masquerade as left thyroid nodules.

The following clip is from a patient who was having follow-up for an abnormal thyroid seven years before. This is a fairly large and complex lesion that was different from the seven years before, so this patient could not be considered stable and needed formal US and biopsy.

Finally, here is the longitudinal view of a normal left thyroid, again with some thyroid still above and below the field.



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 examiner 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 very hypoechoic DigastricM is seen posterior to the SmG and usually suggests a fluid filled structure to beginners. The following still image of a normal right SmG is typical in showing just the gland and the hypoechoic DigastricM posterior to it.

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.

A typical SmG length, measured transversely from ear to chin, is about 3 cm, as in the image above. The next clip of a normal right SmG shows three vascular structures coming in and out of view above the DigastricM. These are the FacV and FacA in cross section at the ear-end of the gland with the longitudinal SubMV briefly appearing as it runs transverse with the gland.

Arteries and veins appear in some scan planes, but are easily identified with color flow 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 color flow. The duct can dilate in whole with a distal obstruction or only in the part of the gland toward the ear if an obstruction is in the beginning part of the gland.

Here is a clip of a normal left SmG showing a prominent DigastricM but no vessels in the scan plane. Because the indicator remains examiner left, the ear-end of the SmG is now at the right side of the screen.

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.

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.