Thyroid and parathyroids

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Thyroid and parathyroids

Methods of imaging the thyroid and parathyroid glands

1. Plain film (limited to crude assessment of superior mediastinal extension of thyroid goitre and any secondary tracheal displacement and narrowing)

2. Ultrasound (US)

3. Computed tomography (CT)

4. Magnetic resonance imaging (MRI)

5. Radionuclide imaging, including positron emission tomography (PET).

Ultrasound of thyroid

Indications

1. Palpable thyroid mass

2. Screening high-risk patients

3. Suspected thyroid tumour

4. ‘Cold spot’ on scintigraphy or increased avidity on PET

5. Suspected retrosternal extension of thyroid

6. Guided aspiration or biopsy.

Contraindications

Patient preparation

Equipment

High-frequency transducer – minimum 10 MHz (though in larger patients and assessment of larger goitres lower-frequency probe may be needed). Linear array for optimum imaging.

Technique

The patient is supine with the neck extended. Longitudinal and transverse scans are taken of both lobes of the thyroid and the isthmus. Nodules are assessed for echogenicity, margins and architecture. If there is retrosternal extension, angling downwards and scanning during swallowing may enable the lowest extent of the thyroid to be visualized. Cervical lymph nodes should be routinely assessed at the same time.

Fine-needle aspiration (FNA) is a frequent adjunct to ultrasound examination; US characterization of nodules is limited and ideally US should be undertaken with the facility to proceed to FNA where there is concern about the US appearance.

Ultrasound of parathyroid

Normal parathyroid glands cannot be visualized by US because of their small size and similar texture to surrounding adipose tissue. US is performed using a similar technique to thyroid for the detection of parathyroid enlargement by adenomas, hyperplasia and carcinoma. Scanning whilst the patient’s head is turned can reveal deeper glands. Scanning for ectopic glands should include the neck below the thyroid down to the superior mediastinum, superiorly to the hyoid region and laterally to the carotid sheaths.

Further Reading

Gharib, H, Papini, E, Paschke, R, et al. American Association of Clinical Endocrinologists, Associazione Medici Endocrinologi, and European Thyroid Association medical guidelines for clinical practice for the diagnosis and management of thyroid nodules. Endocrine Practice. 2010; 16(Suppl 1):1–43.

Johnson, NA, Tublin, ME, Ogilvie, JB. Parathyroid imaging: technique and role in the preoperative evaluation of primary hyperparathyroidism. Am J Roentgenol. 2007; 188(6):1706–1715.

CT and MRI of thyroid and parathyroid

Indications

1. In staging of known thyroid malignancy

2. To assess extension of substernal goitre and tracheal compromise.

As a result of its iodine content, normal thyroid is hyperdense relative to adjacent soft-tissues on non-contrast-enhanced CT. Other than for staging medullary thyroid cancer, CT of the thyroid is routinely performed without intravenous (i.v.) contrast which interferes with subsequent radionuclide thyroid imaging or treatment. Particular care must be taken if iodinated i.v. contrast is administered to hyperthyroid patients (see Chapter 2). For MRI, gadolinium-based i.v. contrast agents can be used without compromise.

In parathyroid disease, contrast-enhanced CT and MRI are used:

1. To detect ectopic or otherwise occult parathyroid adenomas in primary hyperparathyroidism

2. In patients with persistent or recurrent hyperparathyroidism following neck exploration.

Further Reading

Gharib, H, Papini, E, Paschke, R, et al. American Association of Clinical Endocrinologists, Associazione Medici Endocrinologi, and European Thyroid Association medical guidelines for clinical practice for the diagnosis and management of thyroid nodules. Endocrine Practice. 2010; 16(Suppl 1):1–43.

Johnson, NA, Tublin, ME, Ogilvie, JB. Parathyroid imaging: technique and role in the preoperative evaluation of primary hyperparathyroidism. Am J Roentgenol. 2007; 188(6):1706–1715.

Radionuclide thyroid imaging

Indications

1. Assessment of thyroid uptake prior to radio-iodine therapy for benign conditions

2. Follow-up assessment of treated thyroid malignancy

3. Assessment of neonatal hypothyroidism ¹

4. To locate ectopic thyroid tissue, e.g. lingual or in a thyroglossal duct cyst

5. Evaluation of toxic nodular goitre and thyroiditis.

Contraindications

Radiopharmaceuticals

1. ^99mTc-pertechnetate, 80 MBq max (1 mSv ED). Pertechnetate ions are trapped in the thyroid by an active transport mechanism, but are not organified. Cheap and readily available, it is an acceptable alternative to ¹²³I.

2. ¹²³I-sodium iodide, 20 MBq max (4 mSv ED). Iodide ions are trapped by the thyroid in the same way as pertechnetate, but are also organified, allowing overall assessment of thyroid function. ¹²³I is the agent of choice, but as a cyclotron product is therefore relatively expensive with limited availability. (¹³¹I-sodium iodide can also be used for imaging but is associated with a significantly higher radiation dose, so is generally used in the context of whole-body imaging post ¹³¹I ablation.)

Equipment

1. Gamma-camera

2. Pinhole, converging or high-resolution parallel hole collimator.

Patient preparation

None, but uptake may be reduced by antithyroid drugs, iodine-based preparations and radiographic iodinated contrast media.

Technique

^99mTc-pertechnetate

1. I.v. injection of pertechnetate

2. After 15 min, immediately before imaging, the patient is given a drink of water to wash away pertechnetate secreted into saliva

3. Start imaging 20 min post injection when the target-to-background ratio is maximum

4. The patient lies supine with the neck slightly extended and the camera anterior. For a pinhole collimator, the pinhole should be positioned to give the maximum magnification for the camera field of view (usually 7–10 cm from the neck)

5. The patient should be asked not to swallow or talk during imaging. An image is acquired with markers on the suprasternal notch, clavicles, edges of neck and any palpable nodules.

¹²³I-sodium iodide

The technique is similar to that for pertechnetate except:

1. Sodium iodide may be given i.v. or orally

2. Imaging is performed 3–4 h after i.v. administration or 24 h after an oral dose

3. A drink of water is not necessary, since ¹²³I is not secreted into saliva in any significant quantity.

Images

200 kilocounts or 15 min maximum:

2. LAO and RAO views as required, especially for assessment of multinodular disease

3. Large field of view image if retrosternal extension or ectopic thyroid tissue is suspected.

Analysis

The percentage thyroid uptake may be estimated by comparing the background-subtracted attenuation-corrected organ counts with the full syringe counts measured under standard conditions before injection.

Additional techniques

1. Whole-body ¹³¹I imaging is often performed after thyroidectomy and ¹³¹I ablation for thyroid cancer to locate sites of metastasis.

2. Perchlorate discharge tests can be performed to assess possible organification defects, particularly in congenital hypothyroidism.¹

Aftercare

Complications

Other techniques

Metastatic thyroid cancer can be evaluated using diagnostic ¹²³I or therapeutic ¹³¹I whole-body scans as above. When these scans are negative, but serum thyroglobulin is raised, FDG PET-CT is often positive.

Metastatic medullary carcinoma of the thyroid can be imaged with either pentavalent DMSA, indium-labelled octreotide, MIBG or, more recently, FDG PET-CT. These techniques are discussed in Chapter 11.

Reference

1. El-Desouki, M, Al-Jurayyan, N, Al-Nuaim, A, et al. Thyroid scintigraphy and perchlorate discharge test in the diagnosis of congenital hypothyroidism. Euro J Nucl Med. 1995; 22:1005–1008.

Further Reading

Martin, WH, Sandler, MP, Gross, MD. Thyroid, parathyroid, and adrenal gland imaging. In: Sharp PF, Gemmell HG, Murray AD, eds. Practical Nuclear Medicine. 3rd ed. London: Springer-Verlag; 2005:247–272.

Richards, PS, Avril, N, Grossman, AB, et al. Thyroid cancer. In: Husband J, Reznek R, eds. Imaging in Oncology. 3rd ed. London: Informa; 2009:642–679.

Sarkar, SD. Benign thyroid disease: what is the role of nuclear medicine? Semin Nucl Med. 2006; 36(3):185–193.

Radionuclide parathyroid imaging

Indications

Pre-operative localization of parathyroid adenomas and hyperplastic glands.

Contraindications

Radiopharmaceuticals

1. ^99mTc-methoxyisobutylisonitrile (MIBI or sestamibi), 500 MBq typical, 900 MBq max (11 mSv ED) and ^99mTc-pertechnetate, 80 MBq max (1 mSv ED). Both MIBI and pertechnetate are trapped by the thyroid, but only MIBI accumulates in hyperactive parathyroid tissue. With computer subtraction of pertechnetate from MIBI images, abnormal accumulation of MIBI may be seen. MIBI also washes out of normal thyroid tissue faster than parathyroid, so delayed images (1–4 h) can highlight abnormal parathyroid activity.

2. ^99mTc-tetrofosmin (Myoview) can be used as an alternative to MIBI, and is as effective if the subtraction technique is used, but not as good for delayed imaging since differential washout is not as great as for MIBI.

3. ²⁰¹Tl-thallous chloride (80 MBq max, 18 mSv ED) was previously used in conjunction with ^99mTc-pertechnetate, but is increasingly being replaced by the technetium agents due to their superior imaging quality and lower radiation dose.

Equipment

1. Gamma-camera (small field of view preferable for thyroid images, large field of view for chest images)

2. High-resolution parallel hole collimator. A pinhole collimator can also be used, but may result in repositioning magnification errors that compromise subtraction techniques

3. Imaging computer capable of image registration and subtraction.

Patient preparation

None, but uptake may be modified by antithyroid drugs and iodine-based medications, skin preparations and recent iodinated contrast media.

Technique

A variety of imaging protocols have been used, with either pertechnetate or MIBI administered first, with subtraction and possibly additional delayed imaging, and using MIBI alone with early and delayed imaging. Subtraction techniques are most sensitive, but additional delayed imaging may increase sensitivity slightly and improve confidence in the result. The advantage of administering pertechnetate first is that MIBI injection can follow within 30 min with the patient still in position to minimize movement, and if subtraction shows a clearly positive result, the patient does not need to stay for delayed imaging. With MIBI injected first, pertechnetate can only be administered after several hours when washout has occurred:

1. 80 MBq ^99mTc-pertechnetate is administered i.v. through a cannula which is left in place for the second injection.

2. After 15 min the patient is given a drink of water immediately before imaging, to wash away pertechnetate secreted into saliva.

3. The patient lies supine with the neck slightly extended and the camera is positioned anteriorly over the thyroid.

4. The patient should be asked not to move during imaging. Head-immobilizing devices may be useful, and marker sources may aid repositioning.

5. 20 min post injection, a 10-min 128 × 128 image is acquired.

6. Without moving the patient, 500 MBq ^99mTc-MIBI is injected i.v. through the previously positioned cannula (to avoid a second venepuncture which might cause patient movement).

7. 10 min post injection, a further 10-min 128 × 128 image is acquired.

8. A chest and neck image should then be acquired on a large field of view camera to detect ectopic parathyroid tissue.

9. Computer image registration and normalization is performed and the pertechnetate image subtracted from the 10-min MIBI image.

10. If a lesion is clearly visible in the subtracted image, the patient can leave.

11. If the lesion is not obvious, late MIBI imaging can be performed at hourly intervals up to 4 h if necessary to look for differential washout.

Additional techniques

1. In patients in whom pertechnetate thyroid uptake is suppressed, e.g. with use of iodine-containing contrast media or skin preparations, oral ¹²³I (20 MBq) administered several hours before MIBI may be considered.

2. SPECT and SPECT-CT may be used to improve localization and small lesion detection.

3. Dynamic imaging with motion correction may reduce motion artifact.

Aftercare

Complications

Further Reading

Johnson, N, Tublin, ME, Ogilvie, JB. Parathyroid imaging: technique and role in the preoperative evaluation of primary hyperparathyroidism. Am J Roentgenol. 2007; 188(6):1706–1715.

Kettle, AG, O’Doherty, MJ. Parathyroid imaging: how good is it and how should it be done. Semin Nucl Med. 2006; 36(3):206–211.

Palestro, CJ, Tomas, MB, Tronco, GG. Radionuclide imaging of the parathyroid glands. Semin Nucl Med. 2005; 35(4):266–276.

Related

Chapman & Nakielnys Guide to Radiological Procedures