Nuclear Medicine
Nuclear medicine is a medical specialty involving the application of radioactive substances in the diagnosis and treatment of disease. Nuclear medicine imaging, in a sense, is "radiology done inside out" or "endoradiology" because it records radiation emitting from within the body rather than radiation that is generated by external sources like X-rays.
In addition, nuclear medicine scans differ from radiology, as the
emphasis is not on imaging anatomy, but on the function. For such
reason, it is called a
physiological imaging modality. Single photon emission computed tomography (SPECT) and positron emission tomography (PET) scans are the two most common imaging modalities in nuclear medicine.
In nuclear medicine imaging, radiopharmaceuticals are taken internally, for example, through inhalation, intravenously or orally. Then, external detectors (gamma cameras) capture and form images from the radiation emitted by the radiopharmaceuticals. This process is unlike a diagnostic X-ray, where external radiation is passed through the body to form an image.
In nuclear medicine imaging, radiopharmaceuticals are taken internally, for example, through inhalation, intravenously or orally. Then, external detectors (gamma cameras) capture and form images from the radiation emitted by the radiopharmaceuticals. This process is unlike a diagnostic X-ray, where external radiation is passed through the body to form an image.
There are several techniques of diagnostic nuclear medicine:-
2D: Scintigraphy is the use of internal radionuclides to create two-dimensional images
3D: SPECT is a 3D tomographic technique that uses gamma camera data from many projections and can be reconstructed in different planes. Positron emission tomography (PET) uses coincidence detection to image functional processes
2D: Scintigraphy is the use of internal radionuclides to create two-dimensional images
- A nuclear medicine whole body bone scan. The nuclear medicine whole body
bone scan is generally used in evaluations of various bone-related
pathology, such as for bone pain, stress fracture, nonmalignant bone
lesions, bone infections, or the spread of cancer to the bone.
- Nuclear medicine myocardial perfusion scan with thallium-201 for the
rest images (bottom rows) and Tc-Sestamibi for the stress images (top
rows). The nuclear medicine myocardial perfusion scan plays a pivotal
role in the noninvasive evaluation of coronary artery disease. The study
not only identifies patients with coronary artery disease; it also
provides overall prognostic information or overall risk of adverse
cardiac events for the patient.
- A nuclear medicine parathyroid scan demonstrates a parathyroid adenoma
adjacent to the left inferior pole of the thyroid gland. The above study
was performed with Technetium-Sestamibi (1st column) and iodine-123
(2nd column) simultaneous imaging and the subtraction technique (3rd
column).
- Normal hepatobiliary scan (HIDA scan). The nuclear medicine
hepatobiliary scan is clinically useful in the detection of the
gallbladder disease.
- Normal pulmonary ventilation and perfusion (V/Q) scan. The nuclear
medicine V/Q scan is useful in the evaluation of pulmonary embolism.
- Thyroid scan with iodine-123 for evaluation of hyperthyroidism.
3D: SPECT is a 3D tomographic technique that uses gamma camera data from many projections and can be reconstructed in different planes. Positron emission tomography (PET) uses coincidence detection to image functional processes
- A nuclear medicine SPECT liver scan with technetium-99m labeled
autologous red blood cells. A focus of high uptake (arrow) in the liver
is consistent with a hemangioma.
- Maximum intensity projection (MIP) of a whole-body positron emission
tomography (PET) acquisition of a 79 kg female after intravenous
injection of 371 MBq of 18F-FDG (one hour prior measurement).
Nuclear medicine tests differ from most other imaging modalities in
that diagnostic tests primarily show the physiological function of the
system being investigated as opposed to traditional anatomical imaging
such as CT or MRI. Nuclear medicine imaging studies are generally more
organ-, tissue- or disease-specific (e.g.: lungs scan, heart scan, bone
scan, brain scan, tumor, infection, Parkinson etc.) than those in
conventional radiology imaging, which focus on a particular section of
the body (e.g.: chest X-ray, abdomen/pelvis CT scan, head CT scan,
etc.). In addition, there are nuclear medicine studies that allow
imaging of the whole body based on certain cellular receptors or
functions. Examples are whole body
PET scans or PET/CT scans, gallium scans, indium white blood cell scans, MIBG and octreotide scans.
Iodine-123 whole body scan for thyroid cancer evaluation. The study above was performed after the total thyroidectomy and TSH stimulation with thyroid hormone medication withdrawal. The study shows a small residual thyroid tissue in the neck and a mediastinum lesion, consistent with the thyroid cancer metastatic disease. The observable uptakes in the stomach and bladder are normal physiologic findings.
While the ability of nuclear metabolism to image disease processes
from differences in metabolism is unsurpassed, it is not unique. Certain
techniques such as
fMRI
image tissues (particularly cerebral tissues) by blood flow and thus
show metabolism. Also, contrast-enhancement techniques in both CT and
MRI show regions of tissue that are handling pharmaceuticals
differently, due to an inflammatory process.
Diagnostic tests in nuclear medicine exploit the way that the body handles substances differently when there is disease or pathology present. The radionuclide introduced into the body is often chemically bound to a complex that acts characteristically within the body; this is commonly known as a tracer. In the presence of disease, a tracer will often be distributed around the body and/or processed differently. For example, the ligand methylene-diphosphonate ( MDP) can be preferentially taken up by bone. By chemically attaching technetium-99m to MDP, radioactivity can be transported and attached to bone via the hydroxyapatite for imaging. Any increased physiological function, such as due to a fracture in the bone, will usually mean increased concentration of the tracer. This often results in the appearance of a "hot spot", which is a focal increase in radio accumulation or a general increase in radio accumulation throughout the physiological system. Some disease processes result in the exclusion of a tracer, resulting in the appearance of a "cold spot". Many tracer complexes have been developed to image or treat many different organs, glands, and physiological processes.
Iodine-123 whole body scan for thyroid cancer evaluation. The study above was performed after the total thyroidectomy and TSH stimulation with thyroid hormone medication withdrawal. The study shows a small residual thyroid tissue in the neck and a mediastinum lesion, consistent with the thyroid cancer metastatic disease. The observable uptakes in the stomach and bladder are normal physiologic findings.
Diagnostic tests in nuclear medicine exploit the way that the body handles substances differently when there is disease or pathology present. The radionuclide introduced into the body is often chemically bound to a complex that acts characteristically within the body; this is commonly known as a tracer. In the presence of disease, a tracer will often be distributed around the body and/or processed differently. For example, the ligand methylene-diphosphonate ( MDP) can be preferentially taken up by bone. By chemically attaching technetium-99m to MDP, radioactivity can be transported and attached to bone via the hydroxyapatite for imaging. Any increased physiological function, such as due to a fracture in the bone, will usually mean increased concentration of the tracer. This often results in the appearance of a "hot spot", which is a focal increase in radio accumulation or a general increase in radio accumulation throughout the physiological system. Some disease processes result in the exclusion of a tracer, resulting in the appearance of a "cold spot". Many tracer complexes have been developed to image or treat many different organs, glands, and physiological processes.