RCSI MAP.18 Gamma Camera & Positron Emission Tomography (PET) PDF

MAP.18 Gamma Camera & Positron Emission Tomography (PET) PR ESE NTER : Dr. A ndy Ma Learning Outcomes Explain the requirement for radioactive measurement techniques. Explain the principle of use of; film badges, Geiger counters, photomultiplier tubes. Describe the structure of a gamma camera...

MAP.18 Gamma Camera & Positron Emission Tomography (PET) PR ESE NTER : Dr. A ndy Ma Learning Outcomes Explain the requirement for radioactive measurement techniques. Explain the principle of use of; film badges, Geiger counters, photomultiplier tubes. Describe the structure of a gamma camera and discuss their use in nuclear imaging. List the properties of an ideal radiopharmaceutical for radio-imaging. Describe the thyroid 24-hour uptake test. Discuss the principles of positron emission tomography (PET). Define coincident gamma photons and coincident lines, and explain how these are used to form PET images. Describe basic PET instrumentation.  Detecting Radiation Since our natural senses (sight, smell, sound, touch, taste) cannot detect radioactive decay directly, detection must be accomplished through indirect means. People who work in areas where there is a risk of accidental exposure (e.g. nuclear radiation workers, certain hospital workers) are required to wear detectors to determine possible exposure. What hospital based workers are at risk of radiation exposure?  Detecting Radiation The simplest form of detector is a Film Badge – these indicate cumulative exposure to radiation by the degree of blackening of photographic film.  Detecting Radiation It is typical for a single badge to contain a series of filters of different thicknesses and of different materials (the precise choice may be determined by the environment to be monitored). The use of several different thicknesses allows an estimation of the approximate energy/wavelength of the incident radiation. National protection agencies monitor a radiation workers dose each year. TLDs (thermoluminescence detectors) are reusable detectors and more commonly used in hospitals nowadays.  Detecting Radiation More complex ‘radiation detectors’ are usually based on the detection of the ionization caused by the radiation, rather than detecting the radiation itself. Two common forms of such a detector are: The Geiger-Muller (GM) Tube Scintillation Counters  Geiger-Muller Tube Electrode at high voltage (100’s of volts) compared to outer metal casing Radiation enters through a Tube filled with gas (e.g. helium, neon, mica window causing an argon) avalanche of ionization within the tube Geiger-Muller Tube  Geiger-Muller Tube 1. α/β/γ enter the tube & cause ionization of some of the gas molecules. 2. Resultant e- are accelerated towards the high voltage (+ve) wire electrode, and ionize other molecules as they do so. The +ve ions are accelerated towards the walls of the tube (-ve cathode), also creating further ionization. 3. This creates an avalanche of charged particles within the tube. 4. This results is a short, intense pulse of current which is amplified and measured, and indicates that an ‘event’ has taken place.  Geiger-Muller Tube After the event has been detected, it is important to ‘quench’ the discharge so that further ionization events can be detected. [This can be done electronically/chemically]. The period where quenching occurs (i.e. no further radiation can be detected) is called the Dead Time of the G-M tube, and is typically ~ 100 – 500ms. ‘Quenching’ is important, because a single particle entering the tube will cause a single discharge, and so the tube is unable to detect any further events until the discharge has been stopped.  Geiger-Muller Tube But what’s missing? The G-M tube does not normally distinguish between different types of radiation or give specific information (energy, type etc.) of the radiation.  Scintillation Counters Scintillation: Defined as the production of small flashes of visible light from certain materials (scintillators) as a result of the absorption of high energy radiation (e.g. g- rays). The scintillator consists of a transparent crystal (e.g. sodium iodide (NaI)) which fluoresces when struck by ionizing radiation. The visible flash (single or multiple photons) produced by the crystal is detected and amplified by a Photomultiplier tube (PMT). Since the number of emitted photons per MeV of incident energy is fairly constant, the intensity of the scintillation flash can be used to determine the original photon energy.  Scintillation Counters The visible photon emitted by the scintillation crystal is absorbed within the photocathode of the PMT tube to produce a photoelectron (photoelectric effect).  Scintillation Counters 1. This photoelectron(s) is then accelerated towards the first dynode (whose voltage is ~100 V w.r.t) the cathode 2. This dynode is coated with a material which emits several additional electrons for every electron that strikes it. 3. These electrons are then accelerated towards the second dynode which is held at a voltage ~100 V greater than the first, and further electrons are generated. 4. The geometry of the ‘dynode chain’ is such that a cascade occurs with an ever- increasing number of electrons being produced at each stage!!! Scintillation Counters Finally, the cascade of electrons reach the anode, where the accumulation of charge is registered as a pulse of current → indicating that a photon has been detected. Commercial PMT’s may contain up to 15 dynodes and typically operate at voltages of 1000-2000 V.  Scintillation Counters in Medical Imaging Scintillation counters are routinely used in Gamma Cameras – a common medical imaging device. Gamma Camera A typical gamma camera might consist of a large flat crystal of Sodium Iodide (~45 cm in diameter and 1 cm thick), with a bank of PMT tubes arranged in a hexagon fashion behind the crystal. The collimator typically consists of a thick sheet of lead with thousands of adjacent holes drilled through it. This allows spatial discrimination as to the source of the detected photons (i.e. relative position of the source of the photons).  Gamma Camera Crystal Crystal Collimator Patient Patient The collimator allows the specific location of the γ-rays to be determined, and hence a ‘map’ of activity as a function of position.  Gamma Camera The photomultiplier tubes (PMTs) are usually arranged in a close-packed array for minimal gaps between tubes. The PMTs convert light → electronic signal AND magnify the signal. The position logic circuits immediately http://www.scielo.br/img/revistas/cam/v25n2-3/a06fig02.gif follow the photomultiplier tube array and they receive the electrical impulses from the tubes. These circuits determine where each scintillation event occurred in the detector crystal.  Gamma Camera Imaging 1. A radiopharmaceutical (a radioactive emitter) is administered to the patient and this is preferentially absorbed into the organ to be studied (or introduced directly into it). 2. Typical Gamma Cameras are placed over the patient as shown. 3. The Gamma Camera detects the intensity of the radioactive activity within the organ under study and ‘plots’ the intensity as a function of position – i.e. a 2-d image of organ function is created. Gamma Camera Imaging Such imaging techniques allow for comprehensive studies of organ function (i.e. how does the activity vary with time) and can be very useful in assessing kidney function for example. Note: The activity in the right kidney is less than that in the left kidney, indicating a possible blockage (or problem with perfusion of the right kidney) Gamma Camera Imaging Gamma Cameras can also be used to conduct full body scans to assess the possibility of bone disease. 99mTcis the radionuclide incorporated into phosphate and administered to the patient. Abnormalities lead to an increase in blood supply to the diseased region, which increases the amount of uptake of the tracer in the area - clearly visible as ‘hotspots’ in the above diagrams.  Radionuclides Such cameras can also be used to conduct dynamic organ function studies of a number of organs, including the kidneys, lungs, heart etc. Many diagnostic nuclear medicine studies use radionuclides as tracers for specific organ function An ideal radionuclide must: 1. Be short lived (i,e, short radioactive and biological half lives). 2. Emit γ-rays with little more energy than required for detection (low patient dose). → Why not α and β? 3. Be non-toxic.  Radionuclides There are many radioactive species (radioisotopes) that are used routinely in diagnostic nuclear medicine. One common radiotracer is Technetium 99Tc. 99Tc has the following properties: It has a half life of 6 hours (why is this useful?). It emits a low energy γ-ray (140 keV). Its chemical structure allows it to be incorporated into many kinds of molecules (why is this useful?). It is a flexible ‘tracer’ which can be targeted to specific processes or organs. Radionuclides  24-hour Thyroid Uptake Test The thyroid gland uses iodine in the production of hormones that control the metabolic rate of the body. → an underactive thyroid will take up less iodine, whereas an overactive thyroid will take up more iodine.  24-hour Thyroid Uptake Test Approximately 8mCi of radioactive 123I is given orally to the patient. 24 hours later the intensity of 123I is measured using a scintillation counter and compared with the same amount of 123I prepared at the same time (i.e. a standard) The ratio of ‘thyroid counts’ to ‘standard counts’ gives the % 24 hour uptake.  24-hour Thyroid Uptake Test This simple test can determine whether the thyroid is overactive or underactive. 99Tc study showing a ‘cold’ nodule in the left lobe of the thyroid gland.  Positron Emission Tomography (PET) PET is an imaging technique that uses the co- aligned γ-rays formed during positron-electron annihilation to construct images of the form and function of various organs within the body. The aim is to form a 3-D image of the distribution of the radio‐labelled drug at some time post‐injection.  Positron Annihilation A positron e+ is a positively charged electron e-. Positrons can be produced in a number of ways, e.g. pair-production, β-plus decay etc. When a positron encounters an electron, positron- electron annihilation occurs. This produces TWO identical 511 keV γ-rays travelling in exactly opposite directions: → (i.e. 180 degrees to each other) Positron Annihilation β+ e- 511 keV γ-ray Annihilation between the b+ and e- particles occurs  Positron Annihilation The KEY to PET is the detection of the two γ-rays in coincidence, i.e. at the same time. Since the photons are emitted at 180° to each other, it is possible to localise their source by projecting back along their line of coincidence. Newer systems allow time-of-flight to be determined for each photon, so that the exact location of the ‘event’ can be determined. For older systems the data reconstruction is not as good as CT.  PET Imaging Reconstruction software determines the angular and linear coincidence events. By measuring the activity level (number of annihilation events) as a function of position, an image can be reconstructed → similar to that in CT. What radionuclides are used to generate the positrons used in PET and how are they introduced into the body?? Whole body image of a patient with lung cancer.  PET Radionuclides To conduct the scan, a short-lived radioactive tracer Brain Imaging isotope which decays by emitting a positron, and which has been chemically incorporated into a metabolically active molecule, is injected into the living subject (usually into blood circulation). There is a waiting period while the metabolically active molecule (most commonly fluorodeoxyglucose (FDG), a sugar, for which the waiting period is typically an hour) becomes concentrated in tissues of interest; then the subject is placed in the imaging scanner.  PET Radionuclides Radionuclides used in PET are typically isotopes with short half lives, such as: – Carbon → 11C (~20 minutes) – Nitrogen → 13N (~10 minutes) – Oxygen 15O (~2 minutes) – Fluorine18F (~110 minutes) Because of their very short half-lives, many of these isotopes have to made on-site using a cyclotron, which is very expensive. These radionuclides are then incorporated into compounds normally used by the body, such as glucose, water or ammonia and then injected into the body to trace where they are distributed. These ‘labelled’ compounds are normally referred to as Radiotracers. PET-CT PET scanners are very good at providing information on metabolism or molecular biology processes (rather than anatomy), and are increasingly constructed with integrated, CT scanners built in so that both PET and CT scans can be taken sequentially giving both anatomical and metabolic information.  PET-CT The two sets of images are ‘co- registered’ or can be superimposed digitally so that areas of abnormality on the PET scan can be perfectly correlated with anatomy on the CT scan. PET-CT Co-registered Images PET CT Co-registered  PET Imaging Applications PET scans can be very effective in determining post-treatment efficacy. Pre- 2 months post- 4 months post- chemotherapy chemotherapy chemotherapy Pre- 3 months 6 months treatment How does PET differ from MRI and CT?? PET `CT MRI How does PET differ from MRI and CT?? Feature PET CT MRI Ionising radiation ✓ ✓ X Main Feature: Functional information ✓ - - Main Feature: Structural bone information - ✓ - Main Feature: Structural tissue information - - ✓ External EM radiation source - ✓ - Internal radiation source ✓ - - Scan time Long Short Long Type of radiation involved γ rays X-rays Radio waves Learning Outcomes Explain the requirement for radioactive measurement techniques. Explain the principle of use of; film badges, Geiger counters, photomultiplier tubes. Describe the structure of a gamma camera and discuss their use in nuclear imaging. List the properties of an ideal radiopharmaceutical for radio-imaging. Describe the thyroid 24-hour uptake test. Discuss the principles of positron emission tomography (PET). Define coincident gamma photons and coincident lines, and explain how these are used to form PET images. Describe basic PET instrumentation. Thank you F O R M O R E I N F O R M AT I O N P L E A S E C O N TA N T D r. A nd y M a E MA IL: a ma @rc si.c om