Radio isotope imaging, Production, characteristics

About radioisotope imaging, radionuclides, ideal characteristics etc

1. Radioactivity and nuclear transformation - Radionuclide production and their characteristics - internal dosimetry - radiation detection and measurement-types of detector and Basic principle - Scintillation Camera – Nuclear imaging- emission tomography -Single Photon Emission computed Tomography (SPECT), Positron Emission Tomography (PET). Physics of PET and Cyclotron: Principles of PET, PET Instrumentations, Annihilation Coincidence Detection, PET Detector and scanner Design – fusion imaging: PET-CT.

2. What is nuclear medicine imaging? Nuclear medicine imaging is a method of producing images by detecting radiation from different parts of the body after a radioactive tracer is given to the patient. The images are digitally generated on a computer and transferred to a nuclear medicine physician, who interprets the images to make a diagnosis. Radioactive tracers used in nuclear medicine are, in most cases, injected into a vein. For some studies, they may be given by mouth. These tracers aren’t dyes or medicines, and they have no side effects. The amount of radiation a patient receives in a typical nuclear medicine scan tends to be very low.

3. Positioning of the radiation source within (rather than external to) the body is the fundamental difference between nuclear medicine imaging and other imaging techniques such as X-rays. Gamma imaging by either method described provides a view of the position and concentration of the radioisotope within the body. Organ malfunction can be indicated if the isotope is either partially taken up in the organ (cold spot), or taken up in excess (hot spot). If a series of images is taken  over a period of time, an unusual pattern or rate of isotope movement could indicate malfunction in the organ. A distinct advantage of nuclear imaging over X-ray techniques is that both bone and soft tissue can be imaged very successfully.

4. The main difference between nuclear medicine imaging and other radiologic tests is that nuclear medicine imaging evaluates how organs function, whereas other imaging methods assess anatomy (how the organs look). The advantage of assessing the function of an organ is that it helps physicians make a diagnosis and plan treatments for the part of the body being evaluated.

5. CERN, the European Organization for Nuclear Research, is one of the world's largest and most respected centres for scientific research.

6. Reactor radioisotopes Iodine-125 (60 d): Used in cancer brachytherapy (prostate and brain), also diagnostically to evaluate the filtration rate of kidneys and to diagnose deep vein thrombosis in the leg. It is also widely used in radioimmuno -assays to show the presence of hormones in tiny quantities . Iodine-131 (8 d)*: Radioactive iodine 1  is a byproduct of the fission of uranium atoms. Two processes that lead to the creation of radioiodine are the fission of uranium as fuel in nuclear reactors and its use as an explosive material in atomic bombs. Widely used in treating thyroid cancer and in imaging the thyroid; also in diagnosis of abnormal liver function, renal (kidney) blood flow, and urinary tract obstruction. A strong gamma emitter, but used for beta therapy. Iridium-192 (74 d): Supplied in wire form for use as an internal radiotherapy source for cancer treatment (used then removed),  e.g.  for prostate cancer. Strong beta emitter for high dose-rate brachytherapy. Molybdenum-99 (66 h)*: Used as the 'parent' in a generator to produce technetium-99m. Technetium-99m (6 h): Used in to image the skeleton and heart muscle in particular, but also for brain, thyroid, lungs (perfusion and ventilation), liver, spleen, kidney (structure and filtration rate), gall bladder, bone marrow, salivary and lacrimal glands, heart blood pool, infection, and numerous specialized medical studies. Produced from Mo-99 in a generator. The most common radioisotope for diagnosis, accounting for over 80% of scans. Xenon-133 (5 d)*: Used for pulmonary (lung) ventilation studies.

7. Cyclotron radioisotopes Carbon-11, Nitrogen-13, Oxygen-15, Fluorine-18: These are positron emitters used in PET for studying brain physiology and pathology, in particular for localizing epileptic focus, and in dementia, psychiatry, and neuropharmacology studies. They also have a significant role in cardiology. F-18 in FDG ( fluorodeoxyglucose ) has become very important in detection of cancers and the monitoring of progress in their treatment, using PET. Gallium-67 (78 h): Used for tumour imaging and locating inflammatory lesions (infections). Iodine-123 (13 h): I-123 is produced in a cyclotron by bombarding Xenon-124 (Xe-124) or Tellurium-123 (Te-123) with protons. I-123 has a gamma emission of 159 keV and half-life of 13 hours, decaying by electron capture to form Te-123. Increasingly used for diagnosis of thyroid function, it is a gamma emitter without the beta radiation of I-131. Iodine-124 (4.2 d): Tracer, with longer life than F-18, one-quarter of decays are positron emission so used with PET. Also used to image the thyroid using PET. Krypton-81m (13 sec) from rubidium-81 (4.6 h): Kr-81m gas can yield functional images of pulmonary ventilation,  e.g.  in asthmatic patients, and for the early diagnosis of lung diseases and function. Thallium-201 (73 h): Used for diagnosis of coronary artery disease other heart conditions such as heart muscle death and for location of low-grade lymphomas. It is the most commonly used substitute for technetium-99 in cardiac-stress tests.

8. A tellurium compound can be irradiated while bound as an oxide to an ion exchange column, with evolved 131I then eluted into an alkaline solution. More commonly, powdered elemental tellurium is irradiated and then 131I separated from it by dry distillation of the iodine, which has a far higher vapor pressure. The element is then dissolved in a mildly alkaline solution in the standard manner, to produce 131I as iodide and hypoiodate (which is soon reduced to iodide 131I is a fission product with a yield of 2.878% from uranium-235

9. Radiation Detectors Detection and measurement of nuclear radiation must be accomplished by suitable instruments, since these radiations are invisible and their presence generally cannot be sensed by human perception. All radiation monitoring devices consist of a radiosensitive detector and a means of recording the effects of radiation on the detector (i.e. the response of the detector). Detectors respond to radiation by producing various physical effects which can be measured. Ionization is one of these effects. The ion-pairs can be collected to give an electrical signal which is related to the intensity of the radiation. Some detectors will emit light pulses in response to radiation and by counting the pulses the intensity of the radiation can be detected. Others will store the effects of ionizing radiation over a long period and can then yield the information at a later time. All these devices, in one way or other, respond to the energy deposited in them by the radiation. Instruments can be designed to indicate either the rate at which the radiation is being received or the integrated amount of radiation over a certain time period. The following are the media generally used for radiation detection.

10. Gases (e.g. Ion chamber, Proportional counter, GM counter) Scintillators [ Na I (Tl), Anthracene, etc.] Solid state detectors [Semiconductors, Thermo-luminescent dosimeters etc.] Photographic emulsions [Film] The selection of the detector depends on a variety of factors such as type, energy, and the level of intensity of radiation to be detected in addition to other factors such as cost, size, availability, electronics needed etc. Therefore, an understanding of different types of detectors and their characteristics is an important prerequisite for their selection and optimum use in a given situation.

11. 1-Region of recombination 2-Ionizations chamber region 3-Region of Proportionality 4-Region of limited proportionality 5-Geiger-Muller (GM) region 6- Region of continuous discharge IONISATION, PROPORTIONAL AND GM regions are commonly used regions of radiation detectors .

12. Solid State detectors Scintillation Detectors All the detectors described so far register the ionization produced by radiation in a gas. The scintillation detector works on quite a different principle: it measures radiation by detecting tiny flashes of light which radiation produces in certain materials . These light flashes, called scintillation , are converted to electrical pulses and, when fed into suitable electronics, can discriminate between different types of radiation and even between different energies of the same radiation. There are several types of scintillation counters, but their detector systems always consist of two components that are optically coupled. The first is a scintillator. This is a solid or liquid that emits light pulses when radiation deposits energy in it. This is called the scintillation 'phosphor'. The second component is a photomultiplier tube (PMT) which converts this light pulse into a pulse of electric current. There are scintillation detectors for alpha, beta, gamma, and neutron radiation. Scintillators are made of plastic, organics, or inorganic materials. They can be solid, liquid, and gas. They can be made in all shapes and sizes. Scintillation detectors can be used with portable survey meters or fixed equipment. Incoming radiation interacts with a scintillating material and a portion of/or the total energy is transferred to the scintillating material. The excited scintillating molecules produce light photons during the de-excitation process. A NaI (Tl) detector is commonly used for gamma scintillation detection and gamma analysis. Due to the high sensitivity, NaI (Tl) detectors give high background radiation levels. The detector is shielded to reduce background radiation levels before use.

13. Thermo-luminescent dosimeters When certain solids are exposed to ionizing radiations, the electrons released in the ionization process are trapped at lattice imperfections in crystalline solids. These electrons remain trapped till they are released by thermal agitation at some elevated temperature. They emit light in the process and the quantity of light emitted as the material is heated may be measured and related to the absorbed dose in the material. The material thus exposed and heated can be reused as such or after an appropriate heating called annealing. Many thermo-luminescent materials like LiF , Al 2 O 3 , CaF 2 , CaSO 4 :Dy are being used to measure the radiation. Of these CaSO 4  : Dy has been found to be very useful for dosimetric purposes, due to its high sensitivity, low fading, indigenous production and many other useful characteristics. These dosimeters are used as personnel monitoring device. Advantages Useful for in treatment vault measurements. Good photon rejection Suitable for high dose rate No down time Small size Usable for personnel dosimetry Reusable Available in various energy ranges from thermal to fast. Disadvantages May have strong temperature dependence (5% per C). This can be reduced by adding volatile liquid to the chamber whose vapor pressure compensates for temperature sensitivity. Bubble overlap makes readout difficult in high dose measurements. May loose sensitivity over time due to medium degradation. Neutron bubble detector after measurement (top) and prior to measurement (bottom). Neutron bubble detector