Nuclear medicine (nuclear radiology) is a medical specialty involving the application of radioactive substances in the diagnosis and treatment of disease. Nuclear imaging is, in a sense, radiology done inside out, because it records radiation emitted from within the body rather than radiation that is transmitted through the body from external sources like X-ray generators. In addition, nuclear medicine scans differ from radiology, as the emphasis is not on imaging anatomy, but on the function. For this 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. Nuclear medicine procedures are the major clinical applications of molecular imaging and molecular therapy.Wagner Henry N. (2006), A Personal History of Nuclear Medicine. Springer. National Atomic Museum
Diagnostic medical imaging
Diagnostic 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 ("scint") is the use of internal radionuclides to create two-dimensional images.scintigraphy Citing: Dorland's Medical Dictionary for Health Consumers, 2007 by Saunders; Saunders Comprehensive Veterinary Dictionary, 3 ed. 2007; McGraw-Hill Concise Dictionary of Modern Medicine, 2002 by The McGraw-Hill Companies
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.
Nuclear medicine tests differ from most other imaging modalities in that nuclear medicine scans 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.
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.
Procedures Some of the most common clinical nuclear medicine procedures are: glucose metabolic imaging with F-fluorodeoxyglucose (FDG) for cancer, myocardial perfusion imaging for coronary artery disease, and skeletal imaging for both benign and malignant bone disease. Other non clinical, common procedures they perform include: brain perfusion and glucose metabolic imaging for seizure and dementia, blood pool imaging for myocardial function and gastrointestinal bleeding, gastric emptying studies for gastroparesis, hepatobiliary imaging for acute cholecystitis and gallbladder dysfunction, lymphoscintigraphy for sentinel lymph node biopsy, parathyroid imaging for hyperparathyroidism, pulmonary perfusion and ventilation imaging for pulmonary embolism, renal function imaging for various renal disorders, thyroid imaging for hyperthyroidism, thyroid whole body imaging for thyroid cancer, urinary tract imaging for vesicoureteral reflux, and white blood cell studies for infection. Less commonly performed procedures include: octreotide (pentetreotide) or NETSPOT (gallium 68) imaging for somatostatin receptors found on the surface of many tumors, meta-iodobenzylguanidine (MIBG) imaging for neuroendocrine tumors, heat-damaged red blood cell imaging for identifying ectopic splenic tissue, and gastric mucosa imaging for Meckel's diverticulum (especially in pediatric medicine).
Hybrid scanning techniques In some centers, the nuclear medicine scans can be superimposed, using software or hybrid cameras, on images from modalities such as CT or MRI to highlight the part of the body in which the radiopharmaceutical is concentrated. This practice is often referred to as image fusion or co-registration, for example SPECT/CT and PET/CT. The fusion imaging technique in nuclear medicine provides information about the anatomy and function, which would otherwise be unavailable or would require a more invasive procedure or surgery.
Practical concerns in nuclear imaging Although the risks of low-level radiation exposures are not well understood, a cautious approach has been universally adopted that all human radiation exposures should be kept As Low As Reasonably Practicable, "ALARP". (Originally, this was known as "As Low As Reasonably Achievable" (ALARA), but this has changed in modern draftings of the legislation to add more emphasis on the "Reasonably" and less on the "Achievable".)
Working with the ALARP principle, before a patient is exposed for a nuclear medicine examination, the benefit of the examination must be identified. This needs to take into account the particular circumstances of the patient in question, where appropriate. For instance, if a patient is unlikely to be able to tolerate a sufficient amount of the procedure to achieve a diagnosis, then it would be inappropriate to proceed with injecting the patient with the radioactive tracer.
When the benefit does justify the procedure, then the radiation exposure (the amount of radiation given to the patient) should also be kept "ALARP". This means that the images produced in nuclear medicine should never be better than required for confident diagnosis. Giving larger radiation exposures can reduce the noise in an image and make it more photographically appealing, but if the clinical question can be answered without this level of detail, then this is inappropriate.
As a result, the radiation dose from nuclear medicine imaging varies greatly depending on the type of study. The effective radiation dose can be lower than or comparable to or can far exceed the general day-to-day environmental annual background radiation dose. Likewise, it can also be less than, in the range of, or higher than the radiation dose from an abdomen/pelvis CT scan.
Some nuclear medicine procedures require special patient preparation before the study to obtain the most accurate result. Pre-imaging preparations may include dietary preparation or the withholding of certain medications. Patients are encouraged to consult with the nuclear medicine department prior to a scan.
Analysis The result of the nuclear medicine imaging process is a dataset comprising one or more images. In multi-image datasets the array of images may represent a time sequence (i.e. cine or movie) often called a "dynamic" dataset, a cardiac gated time sequence, or a spatial sequence where the gamma-camera is moved relative to the patient. SPECT (single photon emission computed tomography) is the process by which images acquired from a rotating gamma-camera are reconstructed to produce an image of a "slice" through the patient at a particular position. A collection of parallel slices form a slice-stack, a three-dimensional representation of the distribution of radionuclide in the patient.
The nuclear medicine computer may require millions of lines of source code to provide quantitative analysis packages for each of the specific imaging techniques available in nuclear medicine.
Time sequences can be further analysed using kinetic models such as multi-compartment models or a Patlak plot.
Interventional nuclear medicine
Radionuclide therapy can be used to treat conditions such as hyperthyroidism, thyroid cancer, skin cancer and blood disorders.
In nuclear medicine therapy, the radiation treatment dose is administered internally (e.g. intravenous or oral routes) or externally direct above the area to treat in form of a compound (e.g. in case of skin cancer).
The radiopharmaceuticals used in nuclear medicine therapy emit ionizing radiation that travels only a short distance, thereby minimizing unwanted side effects and damage to noninvolved organs or nearby structures. Most nuclear medicine therapies can be performed as outpatient procedures since there are few side effects from the treatment and the radiation exposure to the general public can be kept within a safe limit.
Radionuclide therapeutic procedures include things such as: treatment of hyperthyroidism, treatment of thyroid cancer, radioimmunotherapy with Y ibritumomab tiuxetan (Zevalin) & tositumomab (Bexxar) therapy of low-grade non-Hodgkin's lymphoma, and Re (Rhenium) as Rhenium-SCT for treatment of Non-Melanoma Skin Cancer (basal cell carcinoma or squamous cell carcinoma).
+Common nuclear medicine (unsealed source) therapies Substance Condition Iodine-131-sodium iodide Hyperthyroidism and thyroid cancer Yttrium-90-ibritumomab tiuxetan (Zevalin) and Iodine-131-tositumomab (Bexxar) Refractory lymphoma 131I-MIBG (metaiodobenzylguanidine) Neuroendocrine tumors Samarium-153 or Strontium-89 Palliative bone pain treatmentRhenium-188Squamous cell carcinoma or basal cell carcinoma of the skin
In some centers the nuclear medicine department may also use implanted capsules of isotopes (brachytherapy) to treat cancer.
+Commonly used radiation sources (radionuclides) for brachytherapy Radionuclide Type Half-life Energy Caesium-137 (137Cs) γ-ray 30.17 years 0.662 MeV Cobalt-60 (60Co) γ-ray 5.26 years 1.17, 1.33 MeV Iridium-192 (192Ir) β−-particles 73.8 days 0.38 MeV (mean) Iodine-125 (125I) γ-rays 59.6 days 27.4, 31.4 and 35.5 keV Palladium-103 (103Pd) γ-ray 17.0 days 21 keV (mean) Ruthenium-106 (106Ru) β−-particles 1.02 years 3.54 MeV
Instrumentation Several instruments are available for use by nuclear medicine physicians: Planar imaging Most radionuclides give off gamma-rays when they decay. A 2-dimensional image of the radionuclide distribution can be made with a gamma camera, often called an Anger scintillation camera after its inventor, Hal Anger. Single photon emission computed tomography (SPECT)