INTRODUCTION TO NUCLEAR MEDICINE
3
Linda K. Petrovich, MD, and
E. Scott Pretorius, MD
1. How is a nuclear medicine test performed?
A radiopharmaceutical agent, which is a radioactively tagged compound, is administered to a patient. Many
radiopharmaceutical agents act like analogues of natural biologic compounds and localize to specific organs. Photons
are emitted from the radiopharmaceutical agent in the
patient, and a gamma camera is used to detect the Photomultiplier tubes Computer
tracer distribution. An image is created by a computer
system (Fig. 3-1).
2. What is the difference between the
x-rays used in plain films and computed
tomography (CT) and the gamma rays
used in nuclear medicine?
X-rays are produced from the interaction of bombarding
Sodium
photons or electrons within an atom. Gamma rays are
iodide
produced when an unstable nucleus transitions to a
crystal
more stable state. Diagnostic x-ray imaging is referred
to as transmission imaging. This term is used because
images are formed as x-ray photons from an external
source traverse tissue and emerge to form the image.
Collimator Monitor
Nuclear medicine is referred to as emission imaging
because photons are emitted from inside the patient and
subsequently detected by the gamma camera imaging
system.
Source
3. How does nuclear medicine differ
from other imaging modalities used in
radiology, such as plain film, ultrasound
Figure 3-1. Line drawing showing the flow of information of a
(US), CT, and magnetic resonance
typical nuclear medicine study, such as a bone scan, obtained with
imaging (MRI)?
a gamma camera.
Plain films, US, CT, and MRI produce anatomic images
with very high spatial resolution. A viewer can see
anatomy very well, but function generally is not assessed. Nuclear medicine studies sacrifice spatial resolution, but in
return offer information about organ function.
Key Points: How Nuclear Medicine Differs from Other Imaging Modalities
1. Radiopharmaceutical agents are administered to the patient before imaging.
2. Images are indicative of the functional status of an organ.
3. The spatial resolution in nuclear medicine is less than that of x-rays, US, CT, and MRI.
4. Photons are emitted from the patient and detected by a gamma camera.
5. The total time for a nuclear medicine test may range from minutes to days, depending on the half-life and distribution
of the radiopharmaceutical agent.
4. List some common clinical indications for performing nuclear medicine imaging.
. .
" To rule out pulmonary embolism, a ventilation-perfusion (V/Q) scan may be performed.
" To evaluate for acute cholecystitis, a hepato-iminodiacetic acid (HIDA) scan may be performed.
" To detect a potential gastrointestinal (GI) bleed, a bleeding scan may be performed.
" To evaluate for osteomyelitis, a three-phase bone scan may be performed.
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20 INTRODUCTION TO NUCLEAR MEDICINE
. .
5. What radiopharmaceutical agents are used in a V/Q scan?
. .
A V /Q scan attempts to identify regions of lung that are aerated lung but not perfused (such regions are suspicious
for pulmonary embolism). The scan consists of two parts: a ventilation phase where the aerated lung is imaged, and
a perfusion phase that maps blood flow to the lungs. The two sets of images are compared for discrepancies. For the
ventilation portion of the study, xenon-133 and technetium (Tc)-99m diethylenetriaminepentaacetic acid (DTPA) are the
most commonly used agents. For the perfusion portion of the study, Tc-99m macroaggregated albumin (MAA) is used.
. .
6. How is a V/Q scan performed?
The ventilation portion is performed using either radioactive gas (xenon) or radioactive aerosol (Tc-99m DTPA). When
radioactive gas is used, the study is accomplished in three phases: single-breath/wash-in phase, equilibrium phase,
and washout phase. For studies that use radioaerosols, the radiopharmaceutical agent is placed in a special nebulizer
system, and the patient breathes through the mouthpiece until sufficient radioaerosol is delivered to the lungs. Tc-99m
remains in the lung long enough to obtain multiple views with a gamma camera. Tc-99m MAA is injected into a
peripheral vein to assess perfusion. The particles travel to the right side of the heart and then to the lungs, where they
are filtered or trapped in the pulmonary vascular bed. The emissions from the trapped particles are imaged with a
gamma camera.
. .
7. How are the results of a V/Q scan interpreted?
If an acute pulmonary embolism is present, the thrombus in the blood vessel prevents radiotracer from reaching the
portion of lung supplied by the vessel, and a perfusion defect results. An acute thrombus does not prevent air from
being distributed to the lung via bronchi, however, and the results of the ventilation scan are normal. This combination
. .
of a perfusion defect without a corresponding ventilation defect is called a mismatch. The results of a V /Q scan are
classified as low, intermediate, or high probability for a pulmonary embolism. The classification is based on the number
and size of defects, with higher numbers and sizes resulting in greater probability that an embolus is present.
8. What radiopharmaceutical agent is used in HIDA scan?
Tc-99m labeled iminodiacetic acid (IDA) compounds are used. They share biologic activity with bilirubin and are also
taken up, transported, and excreted by hepatocytes.
9. How is HIDA scan performed?
The patient is required to fast for at least 4 hours, but not longer than 24 hours. If the test is performed after a recent meal,
the gallbladder may still be contracted, and this could lead to false-positive test results. After a prolonged fast, the gallbladder
may be filled with concentrated bile, and this may also lead to false-positive test results by preventing tracer accumulation in
the gallbladder. Tc-99m IDA is injected into a peripheral vein, followed by immediate imaging of the right upper quadrant.
10. How are the results of HIDA scan interpreted?
Because the tracer behaves similar to bilirubin, it should be taken up by hepatocytes and excreted into the bile ducts.
The liver should be visualized first, followed by visualization of the bowel and gallbladder. The appearance of tracer in
the bowel and gallbladder by 60 minutes after administration is defined as normal. Nonvisualization of the gallbladder
by 60 minutes is diagnostic of acute cholecystitis because this implies a functional obstruction of the cystic duct. False-
positive results can be caused by chronic cholecystitis, hepatic insufficiency, and fasting for less than 4 hours or more
than 24 hours as previously described.
Key Points: Common Clinical Indications for a Nuclear Medicine Study
1. To rule out pulmonary embolism (ventilation-perfusion scan)
2. To rule out acute cholecystitis, bile duct obstruction, or biliary leak (HIDA scan)
3. To rule out lower GI bleed (Tc-99m sulfur colloid or Tc-99m labeled red blood cells)
4. To rule out infection (bone scan, indium-111 labeled white blood cells, gallium 67)
5. To evaluate for metastatic disease (bone scan, positron emission tomography (PET) scan)
6. To evaluate for thyroid pathologic conditions (iodine-123)
7. To evaluate for renal pathologic conditions, such as obstruction, hypertension, reflux, and transplant (Tc-99m labeled
compounds)
11. What radiopharmaceutical agents are used for GI bleeding scan?
Tc-99m sulfur colloid or Tc-99m labeled red blood cells (RBCs) are used. Tc-99m labeled RBCs are more difficult
to prepare, but have a longer plasma half-life and are able to detect a GI bleed over a longer period.
12. How do you tell whether the results of the bleeding scan are positive?
An abnormal radiotracer hot spot appears and conforms to bowel anatomy. The activity should increase over time and
move through the GI tract. Because blood acts as an intestinal irritant, movement can be rapid and bidirectional.
INTRODUCTION TO IMAGING MODALITIES 21
13. What are some indications for performing a renal scan?
Renal scans have various clinical applications, including evaluating renal transplants, differentiating between obstructed
and dilated collecting systems, and diagnosing reflux and renovascular hypertension.
14. What are the four major renal functions that can be evaluated with radionuclide imaging?
Renal scans are helpful in evaluating blood flow to the kidney, glomerular filtration, tubular function (resorption and
secretion), and drainage of the collecting systems.
15. What are the main radiopharmaceutical agents used in renal scans? How do they
differ?
Tc-99m DTPA is a glomerular agent, meaning that it is cleared primarily by glomerular filtration and is neither
reabsorbed nor secreted by the renal tubules. It can be used to measure the glomerular filtration rate. Tc-99m
mertiatide (MAG3) is a tubular agent, meaning that its mechanism of renal clearance is solely tubular secretion. Tc-99m
dimercaptosuccinic acid (DMSA) is a cortical agent, meaning that it binds to tubular cells in the renal cortex, making
possible static imaging of the renal parenchymal cortex.
16. What three main factors should be examined when interpreting a renal scan?
Blood flow, radiotracer uptake, and radiotracer excretion should be examined.
17. What radiopharmaceutical agents are used in thyroid imaging?
Iodine-123 is the diagnostic agent of choice for imaging the thyroid. Iodine is administered orally. It is absorbed from
the GI tract and trapped and organified (incorporated into thyroglobulin molecules) in the thyroid. Tc-99m pertechnetate
is used if imaging has to be performed within 1 hour or if the patient is unable to ingest orally. Iodine-131 is used for
whole-body scans after thyroidectomy for thyroid cancer and for therapeutic purposes.
18. Describe the appearance of Graves disease, thyroiditis, and tumor on a nuclear
imaging study.
Graves disease is manifested by an enlarged gland that shows diffusely increased uptake. Thyroiditis appears as
diffusely decreased uptake. Thyroid cancer tends to appear as a focal area of decreased activity (a solitary cold nodule)
rather than showing increased activity on a nuclear medicine study.
19. What radiopharmaceutical agents are used in bone scanning?
Tc-99m labeled diphosphonates are injected intravenously to perform a bone scan. The radiotracer distribution is
representative of osteoblastic activity and regional blood flow to bone. Dynamic blood flow imaging is performed immediately
after the injection of radiotracer. Static imaging is performed 2 to 4 hours after injection and is indicative of osteoblastic activity.
20. What are the two types of bone scans that can be performed, and what are the
indications for each?
Focal three-phase imaging and whole-body imaging are the two types of bone scans. Focal three-phase imaging is used
to differentiate cellulitis from osteomyelitis. The three phases are the flow phase (1 minute after injection), the blood pool
(5 minutes after injection), and the skeletal phase (2 to 4 hours after injection). Cellulitis and osteomyelitis have increased
uptake in the first two phases of the bone scan; however, only osteomyelitis shows increased activity in the third phase.
Whole-body imaging consists of static images obtained 2 to 4 hours after injection. It is used for detection of metastatic
and metabolic diseases and bone dysplasia.
21. If you are concerned about an infection outside the skeletal system, what nuclear
imaging studies can you perform?
Indium (In)-111 labeled white blood cells (WBCs) and gallium-67 citrate can be used to detect occult infection. To label
WBCs with In-111, the cells must be removed from plasma. Blood is taken from the patient, labeled, and reinjected.
Imaging is performed 24 hours after the injection. The normal distribution of WBCs is spleen, liver, and bone marrow.
Activity seen outside the normal expected distribution is evidence of a focus of infection. Gallium, the other agent useful
in locating sources of infection, binds to iron-binding molecules. Its normal distribution is liver and bone marrow. It is
excreted by the kidneys for the first 24 hours and through the large bowel after 24 hours. As with an In-111 WBC scan,
activity seen outside the expected normal organs of uptake is evidence of infection.
22. What is a PET scan?
PET stands for positron emission tomography. A tracer that emits positrons is injected into the patient. After the positrons
are emitted from the nucleus of an atom, they travel through surrounding tissue and collide with electrons. This collision
between positrons and electrons is called annihilation and produces gamma rays. The gamma rays are detected by a PET
22 INTRODUCTION TO NUCLEAR MEDICINE
scanner and analyzed by a computer to form an image. Because positronic decay produces two 511-keV photons that
travel in exactly opposite directions, acquisition of PET data is sometimes called coincidence detection only photons
of the correct energy that are detected simultaneously by detectors 180 degrees from each other are registered as true
events, and the rest are assumed to be noise.
23. What radiopharmaceutical agent is used in PET? What type of pathologic conditions
can it detect?
Fluorodeoxyglucose (FDG) is used in PET. This is a radionuclide combined with glucose, which is the currency of
metabolism for malignant and benign cells. Because malignant cells tend to grow and metabolize glucose faster than
healthy tissue, however, malignant cells use more of the tracer. PET uses the difference in metabolism to differentiate
normal from abnormal tissue.
Key Points: Main Radionuclides Used in Nuclear Medicine
1. Tc-99m (most common)
2. Iodine-123
3. Gallium-67
4. Thallium-201
5. In-111
6. FDG (for PET imaging)
24. What is SUV?
SUV is standardized uptake value. It is a very important concept in interpretation of PET and PET/CT scans, and is a
measurement of the activity seen in a region of interest relative to the body as a whole. By definition, if the activity of an
injected dose were normalized over the entire body, the SUV everywhere would be 1.
25. What types of cancer are best detected on PET?
PET scans can be used to detect malignant tumors, determine cancer stage, and judge the effectiveness of cancer
treatment. They are most often used in patients with head and neck tumors, colorectal cancer, lymphoma, melanoma,
and lung cancer.
26. Should PET/CT be done with or without an intravenous contrast agent?
There is increasing evidence that the use of an intravenous contrast agent for the CT portion of PET/CT increases
detection of liver lesions and allows for more accurate characterization of the lesions detected.
BIBLIOGRAPHY
[1] S. Badiee, B.L. Franc, E.M. Webb, et al., Role of IV iodinated contrast material in 18F-FDG PET/CT of liver metastases, AJR Am. J. Roentgeol.
191 (2008) 1436 1439.
[2] J.H. Thrall, Nuclear Medicine: The Requisites, second ed., Mosby, St. Louis, 2001.