Nuclear medicine is a medical technique that can be used both in diagnosis and treatment. Nuclear medicine diagnosis techniques include organ imaging, functional analysis and in vitro radioimmunoassay. Radioactive tracers are taken internally by orally or intravenous injection, and emit radiation during the process of circulation and metabolism in the body. And external detectors (e.g.: gamma cameras) can capture and form images from the radiation emitted. Nuclear medicine therapy rely on tissue destruct power of the short-range ionizing radiation that emitted by radiopharmaceuticals.
Nuclear medicine imaging technique is simple, sensitive, specific, non-invasive, safety, easy to repeat, accurate, reliable, and can reflect organ function and metabolism. Therefore, it is widely used in clinical and basic research.
Introduction
Nuclear medicine is a medical technique that combines nuclear technology and medicine for diagnosis and treatment of diseases. The study of medical isotopes and nuclear radiation can be widely used in clinical and basic research, such as cancers, heart disease and certain other abnormalities within the body.
1.1 Diagnostic
Nuclear medicine diagnosis techniques include organ imaging, functional analysis and in vitro radioimmunoassay which can help physicians to visualize the morphology and function of an organ, tissue or some areas of the body.
Nuclear medicine diagnosis is noninvasive, painless and different from diagnostic X-ray that the external radiation need pass through the body in order to form an image. Radioactive tracers are general taken internally by oral or intravenous injection, and emit radiation during the process of circulation and metabolism in the body. And external detectors (e.g.: gamma cameras) can capture and form images from the radiation emitted. Through this way the patient’s organs morphology and function can be shown by digital, images, or photographs in the form of curves.
There are many techniques of nuclear medicine can be used for disease diagnose.
Scintigraphy: Scintigraphy is a diagnostic technique use radioisotopes within the body to create two-dimensional pictures.
Single photon emission computed tomography (SPECT): Actually SPECT is a gamma camera with a probe around the patient can be an organ for 360 ° rotation which we called 3D tomographic imaging technique. After data acquisition, it can provide true 3D information for disease diagnose.
Positron emission tomography (PET): Positron emission tomography (PET) is non-invasive Radionuclide imaging technique. It can uses antielectron to detect the distribution of radioactive isotope in vivo directly.
Medical imaging modalities include radiology (e.g.: X plain film, CT, etc.), magnetic resonance imaging (MRI), ultrasound imaging and radionuclide imaging and so on. But our traditional imaging such as radiology and MRI is anatomical imaging which focus on a particular section of the body. The radionuclide imaging is functional imaging that focus on tissue or organ specific. Vast majority of disease in its early course only have some functional changes (including blood flow, metabolism and receptors). The patient’s physiological function can be investigated in this way. Such as bone scan, brain scan, lungs scan and so on.
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In nuclear medicine procedures, radiopharmaceuticals composed of radionuclides and other compounds or pharmaceuticals. After administered to the patient, the radiopharmaceutical will localize to particular organs or cellular receptors. Because of the property of radiopharmaceuticals, the disease-process in the body can be imaged. In some diseases nuclear medicine researches, it can identify medical problems at an earlier stage than other diagnostic tests.
Nuclear Medicine Treatment
Nuclear medicine treatment of disease, based on radionuclides that emitted β-rays in the lesions and produce a series of biological effects of ionizing radiation. Radiation works on cells and transfers some or all their energy to the tissue. Radioactive energy can make cells loss their fertility, metabolism disordered, cells senescence or death to achieve the purpose of therapy. The sensitivity of radionuclide-ray is different between normal cells and diseased cells. Usually, the greater the cell division activity the more sensitive to the rays, and the ability of radionuclides uptake is stronger. Because of this, while rays destroy or inhibit lesions the normal tissues will be safe.
The common nuclear medicine therapies include Yttrium-90-ibritumomab tiuxetan and Iodine-131-tositumomab for refractory Lymphoma; 131I-sodium iodide for hyperthyroidism and thyroid cancer; Samarium-153 or Strontium-89 for palliative bone pain treatment; 131I-MIBG for neuroendocrine tumors. And the nuclear medicine department can also use implanted capsules of isotopes to treat cancer in some place.
Common Uses Of Nuclear Medicine
2.1 nuclear medicine techniques
Most Nuclear Medicine techniques can be used in clinical because of few side-effects, non-invasive, safe and effective diagnosis and treatment. And the most important thing is that it can show the functional changes of tissues which always happen in the early period of disease. Compared with Ultrasound, CT, magnetic resonance imaging (MRI) examination, nuclear medicine test can detect, determine the nature and extent of disease much earlier.
Nuclear medicine in the treatment for some diseases have unique advantages, such as hyperthyroidism, thyroid cancer, cancer metastasis to bone, no surgery of malignant pheochromocytoma all can be treated with nuclear medicine. This method is different from general radiotherapy (external irradiation of cobalt-60) which is guide the drug directly into the affected part. More direct role, the effect is more obvious.
Nuclear medicine diagnosis techniques include organ imaging, functional analysis and in vitro radioimmunoassay which can help physicians to visualize the morphology and function of an organ, tissue or some areas of the body.
Nuclear medicine imaging scans are performed to:
diagnosis
disease
characteristic
Myocardial perfusion imaging
miocardial
Sensitivity and specificity of about 90%,higher than the ECG
ischemia myocardial
Non-invasive, non-toxic side effects
Perfusion imaging of coronary small vessels
Good supplement for coronary angiography
Whole body bone scan
Whole body Bone Metastatic Tumor
Whole body bone were clearly visible, make up the weakness of CT, MRI imaging which can’t imaging the whole body only one time, is the best diagnostic method of whole body bone
Provide early detection than the X-ray for bone metastases: the sensitivity is better than X-ray, detection of disease almost 3 months early.
Lung perfusion / ventilation imaging
Pulmonary thromboembolism
Sensitivity and specificity are superior to X-ray examination. Because of its non-invasive it is more practical than pulmonary angiography.
It is the most simple and reliable method for pulmonary embolism diagnosis.
Renal imaging
Primary kidney disease caused by impaired renal function
Show renal shape, size, location, semi-quantitative evaluation of renal function
Evaluation patients renal function by semi-quantitative index, to make up the defect that endogenous creatinine clearance rate can only determine total renal function
Screening of renal dysfunction secondary to unilateral renal artery stenosis
Provide the most reliable function quantitative basis for Kidney surger
blood pool imaging
Show all parts of the body’s blood vessels
In many inspection methods, it is the diagnosis of vascular tumors with higher sensitivity and specificity. Used for liver, soft tissues, positioning of activity gastrointestinal bleeding (especially the small intestine)
Lymphoscintigraphy
Lymph node tumor, lymph node metastasis and malignant lymphoma
The most sensitive and reliable method of lymphatic system diseases diagnosis.
The only method for the systemic lymphatic checks in the imaging examination. It is is simple, easy to operate
Radionuclide in the decay can emits β-rays, the range of it only a few millimeters, and almost all rays can be absorbed by the diseased tissue. This process can destroy the diseased tissues effectively to achieve the therapeutic purposes, and there is no significant effect to other tissue and organ.
Radionuclide therapy has been applied in thyroid diseases first and used widely in this field. But nowadays, it can be used for more and more kinds of treatment.
The most typical nuclear medicine therapies include:
Radioactive iodine (I-131) therapy for hyperthyroidism, functional thyroid cancer metastases, functional autonomy of thyroid adenoma and so on.
Radioactive phosphorus for some blood disorders, such as P-32 used for Polycythemia disease and essential thrombocythemia; 188Re used for joint bleeding hemophilia.
99Tc-MDP (MDP) in the treatment of rheumatoid arthritis, hyperthyroidism exophthalmos
131I-MIBG for neuroendocrine tumors (mainly to pheochromocytoma).
Radioactive materials such as Sr-89 for therioma metastases of the bones.
2.2 Typical applications
2.2.1. Radioiodine (I -131) Therapy
Iodine is the main synthetic source of thyroid hormone by thyroid. Because of the same physical and chemical properties of radioactive iodine and stability iodine, radioactive iodine on thyroid also has a high degree of selective absorption and uptake capacity. Usually the concentration of iodine in thyroid up to 25 times plasma concentration, but in the case for lack of iodine the concentration may up to 500 times plasma concentration. As the speed and volume of the synthesis of thyroid hormones in patients with hyperthyroidism both increased, the ability to use iodine and the need of iodine also increased, it must be relative deficiency. The capacity of uptake radioactive iodine by thyroid is very strong and 80%–90% can be uptake after take orally. Radioactive iodine-131 stay longer in the thyroid gland, the effective half-life can up to 3.5 to 5.5 days, most of the more than 5 days. Radioactive iodine -131 emit β-rays (99%) andγ-rays (1%) during decay. Therefore, after uptake of iodine-131, there may be a long time concentrated β-ray irradiation, but only a few millimeters. Because of its short range it will not damage the surrounding organs and tissues. After radioiodine (I -131) treatment for several hours later, thyroid will swell because of β-ray irradiation, follicular cells may have vacuoles ,not normal nuclear, and died a few days later. Through this way, reduces the secretion of the thyroid gland to achieve a similar effect as subtotal thyroidectomy for hyperthyroidism treatment.
Radioiodine (I -131) therapy can be widely used for diagnostic and therapy in the field of unclear medicine such as hyperthyroidism, functional thyroid cancer metastases and so on.
131I treatment of hyperthyroidism: thyroid can uptake a mass of radioiodine (I -131), and hyperthyroidism in thyroid tissue would uptake more. In the process of 131I decay it will emit some β particles. Because of the short range (only 2-3mm) there must be no effect on surrounding normal tissue, just like partial resects thyroid tissue. This method is simple and safe which is the most effective method for the treatment of hyperthyroidism.
131I treatment of functional thyroid cancer metastases: Well differentiated follicular of thyroid gland and papillary thyroid cancer tissues can uptake 131I. After give a large number of 131I, the cancer tissues may be damaged with a sufficient amount of β-particle irradiation.
2.2.2. Positron emission tomography (PET)
Positron tomography (PET) is one of the most advanced radionuclide imaging techniques which also can be called PET scan. It produces a three-dimensional image for the whole body or the functional operation in the body.
The general approach is use one kind of substance, usually must be integrant substance during metabolism of biological life. Such as: glucose, proteins, nucleic acids, fatty acids, marked with short-lived radionuclides (such as F18, carbon 11, etc.) and inject into the human body. Through the metabolic activities of the substance the metabolic activities of life can be reflected. And we can also achieve the purpose of diagnosis. With no-influence of the environmental balance, do some research and early diagnosis of the human body pathophysiology and metabolic disorders.
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Before PET scan, people need take radioactive tracer isotope (such as fluorinated deoxyglucose, the radioisotope is fluorine -18, commonly used in tumor imaging) with shorter half-life. The decay process will emit positrons. All the positrons will be replaced into the easily metabolism molecules by chemical reaction, and then injected it in vivo (usually into blood circulation). People need to wait some time until the molecule get into the organism’s metabolic system (commonly used is fluorinated deoxyglucose (FDG), a kind of carbohydrate, usually need waiting for an hour) and focus on our interest organs. Then the subjects or patients can be in the image scanner.
When the radioisotope go through the positron emission decay (also known as β-decay positron), it will release a positron. After a few millimeters travel, the positron will encounter an electron in the organism and annihilation which will produce a pair of annihilation photons fired in two directions back to back. When they met the scintillation crystal substances of the detector, the light which produced by this process can be detected by light-sensitive photomultiplier tube or avalanche photodiode. The technique relies on the detection of concurrent events of a pair of photos.
PET not only medical tools but also research tools. There is a large number of applications in the research of medical imaging in oncology and proliferation of cancer.
PET scanning is non-invasive, but the patient may under exposure to radioactive isotopes. But to be honest, the total radiation is a fat lot that just seven cents unit Sv usually. Compared with it, the average annual environmental radiation in the United Kingdom is up to 2.2 mSv, chest X-ray radiation is 0.02 mSv, CT chest radiation is 8 mSv, the average annual radiation for air crew is 2 to 6 mSv¼Œenvironmental radiation in Cornwall every year can reach 7.8 mSv(Source of date: National Radiation Protection Association). PET generally used with CT in clinical applications. Because of the advantages of PET imaging of soft tissue can combined with sophisticated CT technology, PET / CT is the main form of commercial PET. Independent medical PET almost can not be sale in the market.
2.2.3. Gamma camer
Gamma camera, used for gamma radiation emitting radioisotopes imaging, is within the range of radiation nuclide imaging equipments. This imaging technique is called scintigraphy.
The major part of the equipment is a construction called “gantry”, equipped with an assembly named as “head”. The gantry is controlled by a system controller and computer, which is also used for data collection and image processing. Flat crystal detectors are placed in the head, as well as photomultiplier tubes for photon detection.
Gamma radiation absorbed by the crystals in “head” causes scintillations. When a gamma photon knocks into the crystal, it looses an electron from an iodine atom. When the excited electron comes to ground state level, it release optically detectable photons, which is captured by the Photomultiplier tubes (PMTs) and piled by the controlling system. The counts of the photons are used to reconstruct the image.
Spatial Resolution
A method is established to used detected photons to construction the relationship between them and point where they come from. This method is used to obtain spatial information. Spatial resolution is referred to the minimal distance between two points that can be distinguished.
For the best system, the spatial resolution can be 1.8cm, when the camera is 5cm away. But as the distance from the camera increases, the spatial resolution experiences a strong attenuation. This greatly reduces the image quality in clinical cases. For many cases, because of the thickness of tissues or organs, the range often exceeds 5cm, causing the detected images to be fuzzy, with blurring.
Imaging techniques based on gamma cameras
Scintigraphy: This is a basic application of gamma camera. In scintigraphy, gamma camera is used to capture the radiation that the internal radioisotopes emit, transforming them into fluorescent light and then creating 2D images.
SPECT: SPECT ( single photon emission computed tomography) is a technology using gamma camera to carry out computed tomography. Compared with scintigraphy, SPECT uses fewer detectors (normally one, two or three). When tomography is carried out, the heads are rotated around the patient at a speed much slower than normal.
If the configuration of the gamma camera system can detect simultaneous photons (detected in an extremely short period), it can be used for Positron emission tomography (PET). However, because of the low sensitivity of the crystal, gamma camera PET is much inferior to regular PET scanners. But, on the other hand, the cost is also reduced dramatically.
Benefits and risks
3.1 Benefits
Nuclear medicine test is non-invasive, safety, easy to repeat, accurate, reliable and painless.
It can identify medical problems at an earlier stage than other diagnostic tests.
Nuclear medicine is low cost but can get more useful information that routine exploratory surgery which can be used for diagnosis or appropriate treatment.
3.2 Risks
Because of the small doses of radiotracer administered, diagnostic nuclear medicine procedures result in low radiation exposure, acceptable for diagnostic exams. Of course, the radiation risk is very low compared with the potential benefits.
Nuclear medicine diagnostic procedures have been used for more than fifty years, and they do not have any long-term adverse effect from such low-dose exposure.
Allergic reactions to radiopharmaceuticals may occur. But it is extremely rare and always very mild. But you still need to tell the nuclear medicine personnel that any allergies you may have or other problems that may have occurred during nuclear medicine exam before.
Women must inform their physician or radiology technologist if there is any possibility that they are pregnant or if they are breastfeeding their baby.
Injection of the radiotracer may cause slight pain and redness which should be resolved rapidly.
limitation of Nuclear Medicine
Nuclear medicine procedures must be time-consuming work. It may cost hours to days for the radiotracer to accumulate in some part of the body under study or imaging may use several hours to perform. Although in some cases newer equipment is available and that can shorten the procedure time. You will be informed as how often and when you will need to return to the nuclear medicine department for further procedures.
The structures of the body’s resolution with nuclear medicine maybe not as clear as the other imaging techniques (e.g.: CT, MRI).But to be honest nuclear medicine scans are much more sensitive the other techniques. And the functional information gained from nuclear medicine exams is often unobtainable by any other imaging techniques.
Future
Nuclear medicine may be known as molecular medicine In the future. As our knowing of biological processes in the cells of living organism expands, specific probe may be developed to allow characterization, quantification, and visualization of biologic processes at the cellular and subcellular levels. Because of its emphasis on function and its utilization of imaging agents, nuclear Medicine is an ideal profession to adapt to the new discipline of molecular medicine.
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