Nuclear Medicine Imaging
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Nuclear Medicine Imaging
OverviewNuclear medicine: Therapeutic and diagnostic use of radioactive substancesRadioactivity:Naturally occurring radioisotopes (radioactive isotopes) discovered 1896 by BecquerelFirst artificial radioisotopes produced by the Curies 1934 (32P) Radioactivity, Radioactive
1947 - Kohman: Radionuclide = nucleus of measurable half-life 1935 - Hevesy uses 32P for metabolic studies with Geiger-Muller counter1949 - First radionuclide imaging by Cassen of 131I uptake in thyroid gland (scintillator+PMT, scanner, collimator,1/4 spatial resolution)1957 - Anger camera (planar imaging)1960 - Kuhl & Edwards construct Mark IV scanner (~10 years before x-ray CT)1977 Kayes & Jaszczak develop SPECT independently1950 first PET attempts 1976 First commercial PET (Phelps & Hoffman at CTI)
Radionuclide ImagingCharacteristics:The distribution of a radioactive agent inside the body is imagedProjection and CT imaging methodsImaging of functional or metabolic contrasts (not anatomic)Brain perfusion, functionMyocardial perfusionTumor detection (metastases)
Nuclear StabilityThe neutrons and protons which form the nucleus of an atom are held together by a combination of forces such as gravitational and electrostatic forces. The protons tend to repel each other. This means that, as bigger atoms are but together, it becomes more difficult for the nucleus to be stable as one collection of particles. The only reason that the nucleus is stable is that neutrons bind the other particles together, which is why the heavier atoms have more and more neutrons. As a general rule, there are about equal number of neutrons and protons in a nucleus. But, in heavier atoms, a greater proportion of neutrons have to be added to maintain the stability of the atom. The nucleus of many atoms is not stable. Nuclei with infavourable neutron/proton ratio will disintegrate or decay into stable nuclei by spontaneous emission of nuclear particles.Example: neutron rich nucleus :proton rich nucleus :electronneutrionpositron
Nuclear StabilityNeutron rich unstable elementProton rich unstable element
Isotope: Nuclides of same atomic number Z but different N (and A) same element
Nuclide: Species of atom characterized by the constitution of its nucleus (in particular N, Z)
Radionuclide: Nuclide of measurable half time
Radioactive decay : the process by which an unstable nucleus is transformed into a more stable daughter nucleus by emitting nuclear particles.
Examples of Radioactive Decay
Nuclear ActivityRadioactive decay is described by
N(t), N0: number of radionuclide at time t = 0 and t, resp.: decay constant [1/t]
Activity A = average decay rate [decays per second]
Nuclear activity is measured in curie: 1 [Ci] = 3.7 1010 decays/sec (orig.: activity of 1 g of 226Ra)Practical: 1 mCi, mCi. SI unit is becquerel [Bq] = 1 decay/second99mTc
Interaction of Nuclear Particles and MatterAlpha particlesHelium nucleus (4He++), mostly occurring for parent with Z > 82~ 3-9 MeV (accounts for the kinetic energy of the alpha particle + kinetic energy of the product nucleus)+ 2 charge large mass strong interaction (ionisation: attracts eectrons from other atoms which become cations)Mean range in air: Rm = 0.325 Ealpha3/2
Beta particlesCauses Bremsstrahlung (white, characteristic)Wiggly motion in matter (low mass)
Gamma raysElectromagnetic waves produces in nuclear processes ( < 0.1 nm, E > 10 keV)Identical to x-ray interaction (for E > 1.02 MeV: pair production and photo disintegration [emission of alpha, n, or p from nucleus])
Radionuclides in Clinical UseMost naturally occurring radioactive isotopes not clinically useful (long T1/2, charged particle emission)Artificial radioactive isotopes produced by bombarding stable isotopes with high-energy photons or charged particles
Nuclear reactors (n), charged particle accelerators (Linacs, Cyclotrons)
Radionuclide GeneratorOn-site production of 99mTc
99mTc is the single most important radionuclide in clinical use (gamma @ 140 keV)
Radiopharmaceuticals and their uptake in the bodyIn nuclear medicine imaging a radioactive isotope is introduced into the particular part of the body which is to be investigated.
Ex: in order to follow heart, introduce the activity into the blood stream.
Ex: In order to follow tyroid gland, introduce radioactive iodine (as tyroid absorn iodine)
In some cases, neither of the two methods are possible. attach the radioactive subtance to another chemical which is chosen because t is preferentially absorbed by part of the body. The chemicals towhich radipactive labels are attached are called radiopharmaceuticals.
Radiopharmaceuticals (cont.)If a chemical compound has one or more of its atoms substituted by a radioactive atom then the results is a radiopharmaceutical.
For more detailed information: see Belcher & Velter Radionuclides in medical diagnosis, 1971Selection of isotopes:1) choose an isotope so that the resultant radiopharmaceutical is in the correct chemical form which will allow it to be absorbed by the particular organ to be imaged.2) the energy of radiation must be suitable to the detectors to be used. Optimum energy range for gamma cameras is 100-300 keV. Efficiency drops beyond this range
Selection of isotopes (cont.)3) T1/2 must not be too short, otherwise it will decay before the radiopharmaceutical can be delivered. It must not be too long, otherwise the patient will be unnecessarily exposed to ionization.T1/2 (ideal) is a few hours. Exception: Se is used for pancreas scanning. T1/2 is 120 days. 4) radiation dose delivered to patient must be as low as possible
5) radiopharmaceutical must be available, it should be cheap.
The radionuclide that fulfills most of the above criteria is Technetium _ 99m (99m Tc), which is used in more than 90% of all nuclear medicine studies.
Properties of 99mTc:
T1/2 = 6 hradiates 140 keV gamma raythe short half time and absence of Beta emission allows low radiation dose to patient.The 140 keV gamma radiation allows for 50% penetration of tissue at a thickness of 4.6 cm.
Applications: 99mTc-Sestamibi (myocardial perfusion, cancer)99mTc-labeled hexamethyl-propyleneamine (brain perfusion)
Other gamma emitters:
123 I, 111 In, 67 Ga, 201 Tl, 81 Kr m
11 C, T1/2 = 20 min many organic compounds (binding to nerve receptors, metabolic activity)
13 N, T1/2 = 10 min NH3 (blood flow, regional myocardial perf.)
15 O, T1/2 = 2.1 min CO2 (cerebral blood flow), O2 (myoc. O2 consumption), H2O (myoc. O2 consumption & blood perfusion)
18 F, T1/2 = 110 min 2-deoxy-2-[18F]-fluoroglucose (FDG, neurology, cardiology, oncology, metabolic activity)
As long as the photons emanating from the radionuclide have sufficient energyto escape from the human body in significant numbers, images can be generated that portray in vivo distribution of the radiopharmaceutical.
Nuclear medical imaging may be divided into three categories:
1) conventional or planar medical imaging,2) Single photon emission computed tomography (SPECT),3) Positron emission tomography (PET).
Conventional or planar imagingThe three-dimensionally distributed radiopharmaceutical is imaged onto a planar ortwo-dimensional surface producing a projection image.
Detection of Gamma RadiationScintillation detectors most commonly usedCrystals: NaI(Ti), BGO, CsF, BaF2 Criteria: Stopping power, response time, efficiency, energy resolution Ion collection detectors (ionization chambers) not used because of low efficiency, slow response Semiconductor detectors (diodes): very high energy resolution, fast but small and high cost
Scintillation Camera (Anger Camera)Imaging of radionuclide distribution in 2DReplaced Rectilinear Scanner, faster, increased efficiency, dynamic imaging (uptake/washout)Application in SPECT and PETOne large crystal (38-50 cm Dia.) coupled to array of PMTEnclosureShieldingCollimatorNI(Ti) CrystalPMT
Anger Logic The Anger camera is a system for achieving a large number of resolvable elements with a limited number of detectors. It thus overcomes the previous difficulty of having the resolution limited by the number of discrete detectors.
The principle is based on estimating the position of a single event by measuring the contribution to a number of detectors. Cameras of this general type have a single crystal viewed by arrays of detectorswith the detected outputs followed by a position computer to estimate the position of each event.
Applications1. Thyroid imaging: The thyroid gland is situated in the lower part of the neck at a depth of about 1 cm. The purpose of thyroid is to secrete the hormone thyroxin which is carried in the blood stream and controls a number of body functions:
stimulate metabolism influence growth control mental development store iodine underactive thyroid :
mental dullness, low temperature decrease in metabolism
Imaging of thyroid can be useful for the following purposes:1. To determine the amount of thyroid tissue left after surgery or radiotherapy for thyroid disease,2. To detect thyroid metastases associated with thyroid cancer,3. To show the comparative function of different parts of the glands,4. To measure the size and position of the thyroid prior to surgery or other treatments of the disease.
To obtain images, the patient is given an oral d