It is a superficial gland which should be homogeneous in it’s echotexture. There are 2 sides of the gland with an isthmus joining the left and right sides.
Isthmus scan plane-longitudinal view
Ultrasound image-The isthmus should be less than 10mm. It can be almost imperceptably thin.
Scan plane for longitudinal view Right lobe.
Ultrasound image- Normal Thyroid Lobe – longitudinal view of left lobe
Normal ultrasound of thyroid vascularity using colour doppler.
Flow should be readily seen scattered throughout, but not dominate the gland or have abundant aliasing.
Use a curvi-linear ultrasound probe to accurately measure or visualise a retrosternal thyroid.
The detail is reduced by the lower frequency, however measurements will be more accurate.
Ask the patient to swallow, or exhale to raise a retrosternal component into view.
The thymus is an important gland in the development/education of our T-cells (T lymphocytes) from the neonatal period to puberty when it atrophies.
The thymus lies superior to the heart and can easily be seen inferior to the left thyroid prior to puberty. It may still occassionally be seen in adulthood.
Pathology is rare. Lymphoma or a thymoma are the two most likely pathologies.
Anatomical diagram of the thymus gland in a neonate.
Ultrasound image- Normal thymus in a 12 year old girl.
Use of a curvilinear probe readily demonstrates the normal thymus inferior to the left lobe of the thyroid.
Ultrasound image- Longitudinal view of the thymus gland in a child.
Power doppler shows the normal flow.
Ultrasound image- Transverse view of the thymus gland in a child.
Role of Ultrasound
Ultrasound is a valuable diagnostic tool in assessing the following indications;
- Classification of a palpated lump. eg solid, cystic, mixed
- Evaluate adjacent structures
- Determining the location of a palpable lump (within or outside of the thyroid)
- Identifying a cause for Hyperthyroidism
- Identifying a cause for Hypothyroidism
- Post surgical complications eg abscess, oedema
- Multi Nodular Goiter (MNG): Follow up nodules
- Guidance of injection, aspiration or biopsy
- Relationship of normal anatomy and pathology to each other
The inferior most aspect of an enlarged thyroid with marked retrosternal extension will not always be visible on ultrasound.
- Low collared shirt
- Remove jewellery around the neck
- Towel across the shoulders/chest
- Lie the patient so their head is at the top of pillow and tipped right back.
- A pillow or towel can be placed under the shoulders
- A 7-18 MHz linear transducer
- Deep seated tumours, retrosternal thyroids or large patients may require a curvi-linear array transducer of 3.5-9 MHz
- Good colour / power / Doppler capabilities when assessing vessels or vascularity of a structure.
- Hashimoto’s Thyroiditis
- Graves disease
- De Quervain’s subacute thyroiditis
- Acute suppurative thyroiditis
- Malignancy occurs in approximately 1% of thyroid nodules.
- Papillary and/or mixed papillary/follicular carcinomas are by far the most common malignancy.
- The incidence is dramatically increased in post head/neck radiotherapy patients.
- Papillary Carcinoma 78%
- Follicular Lesion Carcinoma 17%
- Medullary Carcinoma 4%
- Anaplastic Carcinoma 1%
- Thyroid lymphoma – rare
- Metastases to the thyroid- rare
A ‘cold nodule’ on nuclear medicine increases the suspicion of malignancy however the likelihood is still low. Sonographic signs increasing the suspicion of malignancy:
- Solitary nodule
- Punctate calcification (rather than large or peripheral calcifications)
- Irregular surrounding halo
Commonly it is a MULTINODULAR GOITRE (MNG) but how to determine
Malignant V’s Benign
There is no single ultrasonographic feature to distinguish between the two. Features to assist in differentiating include:
- Cystic v’s solid
- Single v’s multinodularity
- Hypoechoic v’s Hyperechoic
- Macro v’s microcalcification
This is graded using TiRADS (Thyroid Imaging Reporting and Data System)
Importantly, a diagnosis of malignancy cannot be made without biopsy.
Again, keep in mind that the vast majority of thyroid nodules are benign.
Thyroid Imaging Reporting and Data System (TI-RADS).
- Nodule morphology
FNA criteria as outlined in the table below
- A ≥20% increase in at least two nodule dimensions, with an increase of >2 mm.
- A ≥50% or greater increase in volume of a nodule.
If there is no change in size for 5 years, the nodule can be considered as having a benign behavior, and further follow up is not needed. Comparison should be made with the oldest study available, and not only the last one.
If there is interval growth, without fulfillment of FNA criteria, the next follow-up should be after 1 year, regardless of the TI-RADS category.
- Number of nodules
When there are multiple nodules, there should be no more than 4 nodules classified. FNA is not recommended of more than 2 nodules.
In the case of multiple nodules, the nodule with the highest TI-RADS FNA criteria should be sampled, which is not necessarily the dominant or largest nodule.
If the patient presents with a neck lump/swelling for investigation, common possible diagnoses other than the thyroid are:
- Fat roll at the base of the neck
- Branchial cleft cyst (supero-lateral to the thyroid. May be transient)
- Thyroglossal duct cyst (midline superior to the thyroid)
- Parathyroid gland mass (Usually small and inferior on the thyroid)
- Sebaceous cyst
- Begin with a survey scan in transverse down the midline to assess for tracheal deviation and obvious pathology.
- Tilt the patients head slightly to the contralateral side and scan down in transverse.
- Rotate into longitudinal and scan from medial to lateral.
- Repeat this for the other side with the head tilted the other way.
- With the patients head/neck straight, scan the isthmus in longitudinal and transverse.
- Scan down each side of the neck in transverse for alternative pathology.
Basic Hardcopy Imaging
A thyroid series should include the following minimum images:
- Transverse images of the gland from superior to inferior.
- Longitudinal images of the gland with a length measurement.
- Measure the glands volume: Normal=7-11cc/lobe
(or at least the maximum length, thickness and width).
- Isthmus, trans and long with a maximum AP diameter.
- Colour Doppler entire thyroid.
- Left neck.
- Right neck.
- Assess the paracervical lymph nodes, carotid artery and jugular vein.
- Document the normal anatomy. Any pathology found in 2 planes, including measurements and any vascularity.