Nuclear Medicine Resources Manual

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CHAPTER 5. GUIDELINES FOR GENERAL IMAGING

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organification. The presence of high concentrations of these radiotracers in the

thyroid gland provides excellent visualization of the gland by the gamma

camera.

5.7.1.2. Clinical indications

Thyroid scintigraphy may be required for any of the following purposes:

(a) To determine the size of the thyroid gland;

(b) For localization of thyroid nodules;

(d) To determine functional status of the thyroid gland;

(e) To evaluate presence of ectopic thyroid tissues, thyroglossal duct cysts

and substernal masses.

5.7.1.3. Radiopharmaceuticals

Details of the radiopharmaceuticals used in thyroid scintigraphy are

given in Tables 5.17 and 5.18.

Some centres have tried using other radiopharmaceuticals for evaluation

of the thyroid gland.

Thallium-201 is preferred by many specialists when

TABLE 5.17. CHARACTERISTICS OF THE RADIOPHARMA-

CEUTICALS USED IN THYROID SCINTIGRAPHY

Radioisotope

Property

I–131 I–123

Tc-99m

pertechnetate

Physical half-life 8 days 13 hours 6 hours

Mode of decay

–

Electron capture Isomeric transition

Photon energy (keV) 364 159 140

Abundance (%) 81 85 89

Dose 1.8–2.2 MBq

(45–55 µCi)

3–16 MBq

(80–400 µCi)

80–200 MBq

(2–5 mCi)

Method Oral Oral Intravenous

Timing of imaging 24 hours after

administration

3–4 hours after

administration

15–30 min after

administration

Thyroid dosimetry 78 rad (100 µCi) 7.7 rad (400 µCi) 0.13 rad/mCi

5.7. NUCLEAR MEDICINE IMAGING STUDIES IN ENDOCRINOLOGY

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thyroid replacement treatment cannot be discontinued and for looking for

cancer metastases in patients with high serum thyroglobulin but with negative

radioiodine scans. Other myocardial perfusion agents (

99m

Tc-sestamibi and

tetrofosmin) have also been utilized primarily to search for residual or

recurrent thyroid cancer, but their clinical usefulness has not yet been fully

assessed. Technetium-99m pertechnetate or low-dose radioiodine

131

I should be

used for routine thyroid scanning.

5.7.1.4. Equipment

A gamma camera with a pinhole collimator is preferred, to allow multiple

views of the thyroid and better resolution of thyroid nodules.

5.7.1.5. Patient preparation

Discontinuation or avoidance of medications or agents that interfere with

the thyroid uptake of the radiotracers (radioiodine and

99m

Tc-pertechnetate)

include:

TABLE 5.18. ADVANTAGES AND DISADVANTAGES OF THE RADIO-

PHARMACEUTICALS USED IN THYROID SCINTIGRAPHY

Radioisotope Advantages Disadvantages

I–131 Useful in delayed imaging,

particularly for thyroid

metastases and mediastinal

masses

High radiation dose to the thyroid and

unfavourable dosimetry

Longer time for imaging

I–123 Appropriate for visualization of

substernal thyroid tissues

More expensive

Longer time for imaging

May contain long lived radionuclidic

impurities that increase radioactive

burden. Consequently, doses which

are already 24 hours old cannot be

used

Tc-99m

pertechnetate

Less expensive and readily

available

More rapid examination

Provides lowest radiation dose/

unit of administered activity

Oesophageal activity can be mistaken

for ectopic thyroid tissue

Organification function cannot be

evaluated

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—Thyroid hormones (T3 and T4 for at least two and four weeks,

respectively);

—Anti-thyroid agents (for at least one week);

—Iodine-containing food, for example kelp (for at least one week);

—Iodine-containing medications (e.g. iodinated contrast agents for weeks,

and lipid soluble media and amiodarone for months).

5.7.1.6. Clinical contraindications

Radiopharmaceuticals are contraindicated in pregnant women. Enquiries

should be made about the menstrual history of female patients in the repro

ductive age group.

Discontinuation of breast feeding for nursing mothers (12 hours for

99m

Tc, permanently for current child with

131

I).

5.7.1.7. Procedure

The following procedure should be adopted:

(a) Patient position:

Supine with neck extended to elevate the thyroid.

(b) Timing of imaging:

—For

123

I: Imaging can be done 3–4 hours after oral administration.

Delayed images at 24 hours have lower body background but with a

lower count rate.

—For

131

I: Images are obtained 24 hours post-administration.

—For

99m

Tc-pertechnetate: Images are obtained 15–30 min after

intravenous administration.

—Obtain 100 000 counts or 5 min observation time, whichever occurs

first, with

99m

Tc, 20 000 counts or 10 min with

131

I, 50 000 counts or

min with

123

I for images in the following projections:

● Anterior view;

● 45° right anterior oblique view;

● 45° left anterior oblique view.

— Note that the image of the thyroid should occupy at least two thirds of

the FOV, necessitating adjustments in the distance between the

pinhole aperture and the neck.

—Radioactive markers may be used to identify anatomical landmarks

(e.g. sternal notches).

—Note the position of and mark the palpable nodules and surgical scars.

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—In cases of oesophageal activity in

99m

Tc-pertechnetate or

123

I due to

salivary excretion, ask the patient to rinse their mouth and drink a glass

of water.

5.7.1.8. Interpretation

A clinical examination is mandatory for adequate interpretation.

Note the size, shape and location of the thyroid gland: the thyroid is

normally a bilobed or a butterfly shaped organ with each lobe typically

measuring 4–5 cm by 1.5–2.0 cm. The right side is slightly larger than the left.

There is extreme variability in the appearance of the isthmus. The thyroid lies

superior to the suprasternal notch, though this is dependent on the degree of

neck extension present at the time of imaging.

Assess the tracer distribution in the thyroid gland: the tracer uptake in

the gland should be homogeneous and uniform. Intensely increased uptake in

the gland denotes a diffusely hyperplastic gland (e.g. Graves’ disease) and in

around 40% of cases may also show the pyramidal lobe. Uptake in only one

portion or one lobe is commonly seen post-surgery or in hyperfunctioning

autonomous adenomas. Diffusely decreased tracer uptake or non-visualization

may be seen in cases with concomitant anti-thyroid medication, in patients with

an increased iodine pool and in patients under thyroid suppression secondary

to thyroid replacement therapy. In early subacute thyroiditis (de Quervain’s

syndrome), there is very poor tracer localization in the thyroid gland rendering

visualization of the gland poor.

Make an evaluation for the presence of nodules and their functional

status. Correlate with the clinical findings on palpation: evaluation of the

nodules is one of the most frequent clinical indications of thyroid scanning.

Identification of these nodules is based on areas of altered uptake in

comparison with the rest of the gland and should always be interpreted in

correlation with the palpation findings. The presence of increased uptake

denotes a metabolically active nodule (‘hot nodule’), most often a result of a

benign process (autonomous adenoma) as may be seen in Plummer’s disease.

However, functioning nodules are not very common, occurring in less than

10% of all demonstrable palpable nodules. In comparison, the presence of

nodules with decreased to absent tracer uptake connotes a non-functioning

nodule (‘cold nodule’). Although the majority of nodules represent a benign

process (e.g. thyroid adenoma), the incidence of thyroid cancer in cold nodules

ranges from 5 to 15%, and most thyroid cancers present scintigraphically as a

solitary cold nodule. Solitary cold nodules are commonly due to an adenoma,

colloid cyst or primary thyroid carcinoma. Malignant thyroid lesions are very

rarely seen in association with hot nodules.

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Make an assessment for the presence of ectopic thyroid tissues or the

functional activity of the thyroglossal duct cyst and the substernal masses.

BIBLIOGRAPHY TO SECTION 5.7.1

BEIERWALTES, W., Endocrine imaging in the management of goiter and thyroid

nodules: Part I, J. Nucl. Med. 32 (1991) 1455–1461.

ERDEM, S., et al., Clinical application of Tc-99m tetrofosmin scintigraphy in patients

with cold thyroid nodules: Comparison with colour Doppler sonography, Clin. Nucl.

Med. 22 (1997) 76–79.

GUIFFRIDA, D., GHARIB, H., Controversies in the management of cold, hot, and

occult thyroid nodules, Am. J. Med. 99 6 (1995) 642–650.

KRESNIK, E., et al., Technetium-99m MIBI scintigraphy of thyroid nodules in an

endemic goiter area, J. Nucl. Med. 38 (1997) 62–65.

SUNDRAM, F.X., MACK, P., Evaluation of thyroid nodules for malignancy using

99m

Tc-sestamibi, Nucl. Med. Commun. 16 8 (1995) 687–693.

WILSON, M. (Ed.), Textbook of Nuclear Medicine, Lippincott-Raven, Philadelphia, PA

(1998) 153–187.

5.7.2. Thyroid uptake measurements

5.7.2.1. Clinical indications

Thyroid uptake measurements can be made for the following reasons:

(a) To determine the functional status of the thyroid gland;

(b) To calculate specific doses for the treatment of hyperthyroidism and

ablation therapy of thyroid cancer;

roidism and Graves’ disease).

5.7.2.2. Radiopharmaceuticals

The following radiopharmaceuticals are used:

(a) I-131: 0.04–0.4 MBq (1–10 µCi) orally. Choose the dose that is closest in

activity to the standard for that batch of in-house prepared doses.

(b) I-123: 3.2–4.8 MBq (80–120 µCi) orally.

5.7. NUCLEAR MEDICINE IMAGING STUDIES IN ENDOCRINOLOGY

305

Protocol for thyroid uptake tests

(a) Objectives

To measure the per cent uptake of a tracer dose of

131

I by the thyroid

gland. This is the simplest and most widely used test to evaluate thyroid

function. After oral administration of radioiodine, the 2, 24 and 48 hour uptake

measurements are done to see the rate of uptake, total buildup and discharge

of radioiodine by the thyroid gland.

(b) Equipment required

—Spectrometer;

—Flat field collimated scintillation crystal probe;

—Standard phantom;

—Standard lead shield 4 in × 4 in × 0.5 in (101.6 mm × 101.6 mm ×

12.7

mm);

—Marker;

—Carrier-free sodium iodide (

131

I) capsules (25 mCi).

—Switch on the main supply and the power switches on the spectrometer.

—After 1–2 min switch on the high voltage (HV).

—Increase the HV to the optimum value.

—Set the amplifier gain.

—Let the instrument stabilize for at least 30 min.

—Put the integral/differential switch on differential and the window on

1.0

—Keep the standard capsule in the phantom 30 cm away from the probe

and find out the photopeak for

131

I starting from a baseline of 300 V and

increasing by intervals of 0.5 V (i.e. five divisions), each time counting for

50 s or until the maximum counts are obtained (calibration procedures

may vary from instrument to instrument). Note the baseline reading.

—Set the window from 1.0 to 5 V and decrease the baseline setting by 20

divisions. At 1 V, the baseline is 360. Hence at 5 V, the baseline is 360

–

20 = 340.

—Count all the capsules and discard those that show a gross discrepancy in

counts.

—Keep one capsule as the standard; the remaining capsules are used in

patients to study their respective uptakes.

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(d) Procedure for uptake measurement

—Count the standard capsule by keeping it in a phantom 30 cm away from

the probe by means of a marker, take two readings of 2 min each and

calculate the counts/min (S1).

—Place the standard shield near the capsule and count the standard again

for 2 min and calculate the counts/min (S2).

—S1 –

S2 = net counts of the standard capsule.

—Administer the radioiodine capsule to the patient after screening.

—Two hours later measure the radioactivity in the region of the patient’s

neck, keeping a distance of 30 cm from the probe, take two readings of 2

min each and calculate counts/min (P1).

—Ask the patient to hold the standard shield in front of their neck, count

for 2 min and calculate counts/min (P2).

—P1 –

P2 = net counts of the patient’s thyroid.

Percentage uptake in the thyroid =

—Repeat the counting at 24 and 48 hours, and calculate the percentage

uptakes.

(e) Limitations

—The test cannot be performed on pregnant women.

—There is a possible radiation hazard to children, which, unless it is

essential, should be avoided.

(f) Disadvantages of the technique

—The prior administration of iodine containing drugs, thyroid hormones,

anti-thyroid drugs and several other compounds may invalidate the test

for a period of a number of weeks to several months.

—Serial readings are necessary for 3 days for a proper diagnosis.

(g) Advantages of the technique

—This is a simple test.

—The test provides dynamic functional information.

P1 P2

S1 S2

100.

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5.7.2.3. Interpretation

The normal range should be established locally. This is normally

determined by the dietary iodine intake, types of equipment, standard applica

tions and uptake phantoms.

Hyperthyroid individuals (with Graves’ disease, toxic adenoma or toxic

multinodular goitre) have elevated uptake values, while patients with subacute

thyroiditis or factitious hyperthyroidism will have low to normal uptake values.

A low uptake value has a lower precision, brought about by decreased

counting statistics. It is not a primary diagnostic criterion for hypothyroidism.

The interpretation of uptake should be made in conjunction with the

patient’s history and drug medication intake.

BIBLIOGRAPHY TO SECTION 5.7.2

BECKER, D., et al., Procedure guideline for thyroid uptake measurement: 1.0, J. Nucl.

Med. 37 (1996) 1266–1268.

PALMER, E., SCOTT, A., STRAUSS, W., Practical Nuclear Medicine, Vol. 311,

Saunders, Philadelphia, PA (1992) 311–341.

READING, C.C., GORMAN, C.A., Thyroid imaging techniques, Clin. Lab. Med. 13

(1993) 711–724.

SUNDRAM, F.X., Radioiodine (I-131) uptakes and hormonal (T4) levels in hyperthyroid

patients receiving radioiodine therapy while on anti-thyroid drugs and relation to incidence

of hypothyroidism at one year, Ann. Acad. Med. Singapore 15 4 (1986) 516–520.

SURKS, M.I., CHOPRA, I.J., MARISH, C.N., NICOLOFF, J.T., SOLOMON, D.H.,

American Thyroid Association guideline for use of laboratory tests in thyroid disorders,

J. Am. Med. Assoc. 263 (1990) 1529–1532.

WILSON, M. (Ed.), Textbook of Nuclear Medicine, Lippincott–Raven, Philadelphia,

PA (1998) 153–187.

5.7.3. Whole body imaging for differentiated thyroid cancer

5.7.3.1. Principle

Whole body scanning is primarily used for detection of thyroid metastases

or thyroid tissue with residual function. Radioiodine is extracted by the

residual thyroid tissue and by 75% of well differentiated thyroid cancers with

similar iodide physiology. For functioning thyroid cancers to be visualized by

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scanning, the thyroid remnant must be first destroyed or ablated. Metastatic

deposits may then be visualized by therapeutic doses of

131

5.7.3.2. Clinical indications

Whole body imaging can be used to:

(a) Determine the presence and extent of residual thyroid tissue after

surgery;

(b) Localize metastases of thyroid carcinoma.

5.7.3.3. Radiopharmaceuticals

Iodine-131 is the recommended agent, with a dose of 80–200 MBq (2–

mCi) taken orally. Most centres favour this dose range in order to avoid the

possibility of thyroid stunning. Other investigators have proposed conducting

diagnostic imaging coincident with the therapeutic dose of

131

I. In some cases,

small metastatic deposits can only be visualized after therapeutic doses of

131

Alternatively, the use of 185 MBq (5 mCi) of

123

I has been proposed.

Thallium-201 or

99m

Tc-sestamibi have also been utilized in detecting

residual thyroid tissues. However, the scintigraphic search should be confined

to the neck and chest as there is high background radioactivity in the abdomen.

There are advantages to using

201

Tl as it does not require the discontinu-

ation of thyroid hormone, as is the case with

131

I. In thyroid cancers that do not

concentrate radioiodine (medullary cancer and Hürthle cell cancer),

201

Tl is

very useful. It can also demonstrate metastases when the TSH level is normal

or suppressed. In patients with an oxyphilic subtype of differentiated thyroid

carcinoma, where there is negative immuno-histochemical staining for

thyroglobulin, a

201

Tl whole body scan is strongly recommended. Some centres

prefer

201

Tl in patients with high serum thyroglobulin but where the

radioiodine scan is negative. On the other hand, other investigators proposed

the use of FDG PET to detect lymph node metastases in conjunction with a CT

scan on such patients. European researchers have had promising results with

the use of

111

In-pentreotide somatostatin receptor scintigraphy to detect

recurrent thyroid cancer (both undifferentiated and medullary) in patients

without detectable iodine uptake.

Thallium-201, however, does not give information about the avidity of the

tumour to radioiodine, especially if ablation with

131

I therapy is being contem-

plated.

99m

Tc-sestamibi or

99m

Tc-DMSA (V) scan can be complementary to the

CT scan in patients with recurrent medullary thyroid cancer who have

5.7. NUCLEAR MEDICINE IMAGING STUDIES IN ENDOCRINOLOGY

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extremely elevated levels of serum calcitonin. At low levels, however, scan

usefulness appears to be limited.

5.7.3.4. Equipment

A gamma camera with a high energy collimator for

131

I is needed.

5.7.3.5. Patient preparation

Patient preparation should include:

(a) Discontinuation or avoidance of medications or agents that interfere with

the thyroid uptake of radioiodine:

(1) Thyroid hormones (T4 for 4–6 weeks, T3 for 2 weeks). Thyroid

hormones must be stopped to attain a high TSH level. Some centres

advocate replacement of T4 by T3 for 6–8 weeks to minimize the risk

of cancer progression during the time thyroid hormone is withheld

as T4 has a longer half-life of 1 week compared with 1.5 days for T3.

This replacement is then discontinued 2 weeks prior to scanning.

Ideally, TSH should be greater than 30 mIU/mL.

(2) Iodine-containing food (e.g. kelp, for at least 1 week). It is preferable

that the patient be on a low iodine diet for at least 1 week prior to

the study, to increase the sensitivity of the procedure.

(3) Iodine-containing medications (e.g. iodinated contrast, amiodarone

and iodine-containing expectorants for at least 2 to several weeks)

(b) An

131

I tracer dose is given when TSH levels are high. This reflects the

maximum stimulation of metastases to be seen in the scan.

history of female patients.

(d) Discontinuation of breast feeding by nursing mothers is essential for at

least two months, preferably completely.

5.7.3.6. Procedure

The following procedure should be adopted:

—The patient should be in the supine position.

—Timing of imaging: images are obtained at 2 and 4 or more days after

radioiodine administration. By this time, iodine initially extracted by the

salivary glands and gastric mucosa has already been cleared and excreted

via the urinary tract.