RADIONUCLIDE IMAGING OF THYROID AND PARATHYROID

THYROID SCAN

A radionuclide thyroid scan is routinely used for the diagnosis of and guiding the management of many thyroid conditions.

RADIONUCLIDES

I-123 is the most favorable because of its excellent image quality and relatively low radiation exposure.

I-131 is usually used for therapeutic purposes because of its beta emission and higher radiation dose.

Technetium-99m pertechnetate is used as a substitute for iodine because it is:

Trapped by the thyroid Low radiation doseReadily available and less time consumingInexpensive

INDICATIONS

Thyrotoxicosis

Thyroid goiter /nodules

Ectopic thyroid

Diagnosis and follow up of Thyroiditis.

Detection and follow up of thyroid cancer recurrences or metastases.

Suspected Occult thyroid malignancy.

Evaluation of congenital thyroid abnormalities.

Thyroid supplements and exogenous iodine source should be stopped.

PREPERATION

Use of radiographic contrast media may depress the uptake of iodine by the thyroid for up to a month, so imaging studies should be scheduled accordingly.

Antithyroid drugs need not be stopped, as they do not interfere with the uptake of iodide, only with its organification.

TECHNIQUEI-123 labeled sodium iodide orTc-99m sodium pertechnetate is given intravenously.

With I-123 images are obtained 2-3hrs later, while with Tc-99m images are obtained 20-30 minutes post injection, using a high resolution or a pinhole collimator.

Markers indicating the position of thyroid cartilage and sternal notch are helpful.

Additional markers may be used to indicate the site of palpable nodules.

Usually anterior and oblique views are taken.

Lateral view of the neck should be added if ectopic thyroid is suspected.

THYROID UPTAKE MEASUREMENT

Thyroid uptake indicates the level of functional activity of the gland by measuring the trapped proportion of ingested radioiodine at a certain time (2, 4 and/or 24 hrs) using a special probe.

Normal thyroid uptake is 10–35% in most laboratories at 4 and 24 h

Normal thyroid uptake of the gland

24-year-old woman with raised T4, T3 and low TSH values. The 24-hour RAIU was 84%.

Anterior with markerAnterior

RAO LAO

Grave’s disease

Thyroid images should be interpreted in association with clinical and lab data (TFTs) as well as the result of thyroid uptake especially in case of hyperthyroidism due to Grave’s disease since near normal images can be present in this condition.

Autonomous nodules

Anterior RAO LAO

Tc99m Pertechnetate

Multinodular gland

59-year-old woman with a very firm thyroid on examination. Lab data shows low T3 and T4 .The 24-hour RAIU was 7%.

ABSENT / REDUCED THYROID UPTAKE GENERALIZED o Sub acute thyroiditiso Ectopic thyroido Ectopic hormone productiono Thyroid supplements or exogenous iodine source Thyroid supplements or exogenous iodine source

LOCALIZEDo Colloid cyst o Non-functioning adenomao Multinodular goitero Carcinomao Local thyroiditis

(A) (B) (C)

“Hot” nodules (autonomously functioning thyroid nodules) are usually not malignant.

“Cold” nodules ( either hypo functioning or nonfunctioning) can be malignant in approximately 5-8% of cases.

ECTOPIC THYROID

Most commonly presents in childhood as a nodule or mass at the base of the tongue.

Retrosternal thyroids are usually extensions of a multinodular goiter in the neck.

For imaging I-123 is preferable to Tc-99m because it shows greater uptake in the thyroid tissue compared with salivary glands and mediastinum.

A 20-year-old female presented with a submandibular neck swelling. Her biochemical profile was suggestive of hypothyroidism.

PERCHLORATE DISCHARGE TEST

Disorders characterised by a failure to organify the trapped iodide(of which the most common is Pendred's syndrome).

Serial images of thyroid are taken giving first I-123 and 2hrs later sodium or potassium perchlorate which rapidly discharges the unbound iodide.

The normal gland will retain that proportion of the iodide which has been bound, whereas with an organification defect most of the thyroid activity will be eliminated.

THYROID CANCER Well differentiated papillary and follicular tumors retain

the ability to accumulate iodine although to a much lesser extent than normal thyroid tissue.

This property is used in the detection of local tumor recurrence as well as distant metastatic spread of thyroid cancer after surgery by the whole body iodine scan.

Post operative I-131 scan

Anterior

Post-op I-123 scan

Anterior

Residual neck thyroid tissue

Anterior Posterior

I-123whole body scan

I-123

(a) Initial post op scan. (b) Follow-up 1 year after I-131 ablation.

Other imaging studies are used particularly when I-123 or 131 study is negative such as thallium 201 and particularly in high-risk patients FDG-PET/CT study.

PARTATHYROID SCAN

Scintigraphy using Tc-99m MIBI(methoxy isobutyl-isonitrile) is currently the preferred nuclear medicine method for preoperative localization of hyperfunctioning parathyroid tissue.

it reduces operative time, cost, and operative failure rates.

Normally Tc-99m MIBI is taken by thyroid gland and it clears over time.

In the presence of abnormal parathyroid glands the radiotracer is retained in these glands and are seen as foci of tracer accumulation.

Tc99m MIBI

Early Delayed

Negative parathyroid study

Tc99m MIBI Tc99m Pertechnetate

Early

Early Delayed

Tc-MIBI

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