Medical Physics Option 9.6.3 2006

Syllabus - Contextual Outline The use of other advances in technology, developed from our understanding of the electromagnetic spectrum, and based on sound physical principles, has allowed medical technologists more sophisticated tools to analyse and interpret bodily process for diagnostic purposes. Diagnostic imaging expands the knowledge of practitioners and the practice of medicine. It usually uses non-invasive methods for identifying and monitoring diseases or injuries via the generation of images representing internal anatomical structures and organs of the body. Technologies, such as ultrasound, compute axial tomography, positron emission tomography and magnetic resonance imaging, can often provide clear diagnostic pictures without surgery. A magnetic resonance image (MRI) scan of the spine, for example, provides a view of the discs in the back, as well as the nerves and other soft tissues. The practitioner can look at the MRI films and determine whether there is a pinched nerve, a degenerative disc or a tumour. The greatest advantage of these techniques are their ability to allow the practitioner to see inside the body without the need for surgery. This module increases students’ understanding of the history of physics and the implications of physics for society and the environment.

Syllabus 9.6.1 The properties of ultrasound waves can be used as diagnostic tools

Syllabus 9.6.2 The physical properties of electromagnetic radiation can be used as diagnostic tools

Syllabus 9.6.3 Radioactivity can be used as a diagnostic tool

Syllabus 9.6.4 The magnetic field produced by nuclear particles can be used as a diagnostic tool

Ultrasound X-rays Medical Physics MRI Nuclear - PET Endoscopy

Brainstorm Radioactivity Individual 1 minute Group 2 minutes

Syllabus 9.6.3 Radioactivity can be used as a diagnostic tool

Radioactive Isotopes and Half-lives The atom Electrons Nucleus Protons neutrons Quarks outline properties of radioactive isotopes and their half lives that are used to obtain scans of organs

Radioactive Isotopes Atomic nuclei contain protons and neutrons The number of protons in the nucleus is called the atomic number The atomic number determines which element an atom is e.g. all fluorine atoms have 9 protons in the nucleus The number of neutrons in the nucleus of a particular element can vary Only hydrogen has isotopes with special names Atoms of the same element but with different numbers of neutrons are called isotopes outline properties of radioactive isotopes and their half lives that are used to obtain scans of organs

The atomic nucleus and isotopes Collectively, protons and neutrons are called nucleons The total number of nucleons in a nucleus is called the mass number Nuclei are represented by the symbol for the element, and the mass and atomic numbers Atoms of the same element but having different numbers of neutrons are called isotopes mass number 18 F fluorine 18 9 atomic number technetium 99 Tc 99 43 outline properties of radioactive isotopes and their half lives that are used to obtain scans of organs

Stable and unstable isotopes stable nucleus F 19 9 Isotopes having too many or too few neutrons, relative to the number of protons are unstable C 14 6 unstable nucleus F 18 9 unstable nucleus outline properties of radioactive isotopes and their half lives that are used to obtain scans of organs

Radioactive Decay Unstable isotopes may become more stable in several ways Alpha decay (decay) Beta decay (bdecay) Gamma () emission Positron emission (b+ decay) Each unstable isotope undergoes a particular type of change, which is on average constant and unique to that isotope outline properties of radioactive isotopes and their half lives that are used to obtain scans of organs

Radioactive Decay Unstable isotopes may become more stable in several ways Alpha decay (decay) Beta decay (bdecay) Gamma () emission Positron emission (b+ decay) outline properties of radioactive isotopes and their half lives that are used to obtain scans of organs

Radioisotopes Radioisotopes may be natural or they can be artificial Artificial radioisotopes are produced in nuclear reactors or using particle accelerators called cyclotrons Sydney has one medical cyclotron at Prince Alfred Hospital near Sydney University Lucas Heights produces a range of artificial isotopes for medical use outline properties of radioactive isotopes and their half lives that are used to obtain scans of organs

Radioisotopes outline properties of radioactive isotopes and their half lives that are used to obtain scans of organs

Radioactive Decay The time taken for 50% of a sample of the radioactive material to decay is called the half-life outline properties of radioactive isotopes and their half lives that are used to obtain scans of organs

P-210 has a half-life of 12 seconds Radioactive Decay Half-life 12 s P-210 has a half-life of 12 seconds outline properties of radioactive isotopes and their half lives that are used to obtain scans of organs

Review Question - Half-life What is the half-life of strontium-90? 28 years outline properties of radioactive isotopes and their half lives that are used to obtain scans of organs

Radioactive Decay When beta decay occurs, a neutron in the nucleus changes into a proton (which remains in the nucleus) and an electron (which is ejected from the nucleus at high velocity). In all nuclear reactions, mass and charge are conserved. Propose what may happen in the nucleus to produce positron emission. Answer A proton changes into a positron and a neutron, which remains in the nucleus and the positron is ejected from the nucleus. outline properties of radioactive isotopes and their half lives that are used to obtain scans of organs

Decay producing a gamma ray Technetium-99m nucleus Gamma decay does not change either the atomic number or the mass number The nucleus is left in a lower energy state as a result of losing energy in the form of gamma radiation Gamma rays are very important in nuclear medicine outline properties of radioactive isotopes and their half lives that are used to obtain scans of organs

Radioactive Isotopes and Half-lives outline properties of radioactive isotopes and their half lives that are used to obtain scans of organs

Beta+ Decay of Artificial Isotopes Positron (b+) decay occurs only in certain man-made isotopes + decay is medically very important (used in the process of PET scanning) Positrons are anti-electrons Positrons have the same mass as an electron, but the opposite charge (+1) outline properties of radioactive isotopes and their half lives that are used to obtain scans of organs

Penetration by Radiation outline properties of radioactive isotopes and their half lives that are used to obtain scans of organs

Review Define the term “isotope”. Outline the characteristic of an isotope that causes it to be unstable and give an example of an element having stable and unstable isotopes. Technetium-99 is an important medical isotope. Identify the type of radiation produced when Tc-99 decays. Fluorine-18 decays to produce a positron. Describe the main characteristics of positrons and identify the other main decay product from F-18. Isotopes are atoms of the same element, having different numbers of neutrons e.g. carbon-12 and carbon-14 Unstable isotopes are characterised by having too many or too few neutrons, relative to the number of protons. e.g. F-19 is stable however F-18 is unstable. Tc-99 produces gamma radiation when it decays. Positrons have the same mass as an electron but the same positive charge as a proton. When F-18 decays to produce a positron, an oxygen-18 nucleus is produced.

Properties of Radioactive Isotopes Alpha: highly ionising (high mass and charge), low penetration Not used medically Beta: less ionising (low mass and single charge), low penetration Not used medically Gamma: least ionising (photon, no charge), high penetration Used for imaging (gamma scan) outline properties of radioactive isotopes and their half lives that are used to obtain scans of organs

Properties of Radioactive Isotopes Source: ANSTO brochure outline properties of radioactive isotopes and their half lives that are used to obtain scans of organs

Properties of Radioactive Isotopes Source: ANSTO brochure outline properties of radioactive isotopes and their half lives that are used to obtain scans of organs

Metabolism of Isotopes - Organ Accumulation Source: ANSTO brochure describe how radioactive isotopes may be metabolised by the body to bind or accumulate in the target organ

Metabolism of Isotopes - Organ Accumulation Medical radioisotopes are metabolised to bind to specific organs either as elements or as a part of a molecule used by the body e.g. iodine-131 is used to monitor thyroid gland function because iodine is metabolised by this gland Iodine-131 decays with a half-life of 8.0197 days with beta and gamma emissions fluorine-18 is incorporated into a modified glucose (18 FDG) molecule to investigate a wide range of organ function, especially the brain Source: ANSTO brochure describe how radioactive isotopes may be metabolised by the body to bind or accumulate in the target organ

Metabolism of Isotopes - Organ Accumulation Factors influencing the choice of isotopes for medical use Half-life should be less than a few hours Should be easily attached to a pharmaceutical agent Can be delivered to hospital in a form to last several days describe how radioactive isotopes may be metabolised by the body to bind or accumulate in the target organ

Metabolism of Isotopes - Organ Accumulation Technetium-99m is a widely-used isotope in nuclear medicine It decays from excited to the ground state by emitting a g ray The parent isotope is molybdenum 99, having a half-life of 2.7 days Half-life of technetium-99m is 6.01 hours ® + 42 99 43 Mo  Tcm - + b Half life 2.7 days Half life 6.01 hours 43 99 Tc + (140 keV) m ® g describe how radioactive isotopes may be metabolised by the body to bind or accumulate in the target organ

Decays to the excited state of technetium 99 Milking the Cow Molybdenum-99 is delivered to hospital inside the generator (cow) Decays to the excited state of technetium 99 NaCl solution passed through cow 99Tcm removed in solution Mixed with pharmaceutical for labelling Injected into patient Radionuclide generator describe how radioactive isotopes may be metabolised by the body to bind or accumulate in the target organ

Comparison of Bone Scan and X-ray Images Inject patient with radiopharmaceutical labelled with radioactive isotope Detect where gamma rays are coming from within the body Use a gamma camera Local concentration in patient permits diagnosis ‘Hotspot’ may indicate tumour* * Tumours contain rapidly dividing cells… therefore metabolism occurs at a higher rate in the tumour… producing a higher than normal concentration of metabolites, in the cells. If tagged with radioisotopes this can be detected. perform an investigation to compare an image of bone scan with an X-ray image

Comparison of Bone Scan and X-ray Images Collimator restricts direction from which photons reach the camera Made of lead with parallel holes running through it Bigger, shorter holes means more photons, but more blurring and visa versa Details need not be memorised

Gamma camera Details need not be memorised

Comparison of Bone Scan and X-ray Images Detector made from single crystal of sodium iodide 250 mm to 250 mm diameter 6 mm thick Enveloped in thin aluminium can (light tight) Light guide takes all light directly to photomultipliers Glass plate with non-reflective coating Improves efficiency of light coupling Details need not be memorised

Delivery of Radioisotopes into the Body Radioisotopes are usually injected* into the body Short half-life isotopes are used to reduce risk Isotopes that are excreted rapidly are used Specific isotopes are chosen specific organs * sometimes they are inhaled or swallowed

Delivery of Radioisotopes into the Body Whole body bone scan made using a gamma ray camera. The image was produced using gamma emissions from phosphate tagged with technetium–99m. The tracer was injected intravenously and the phosphate is metabolised mainly in the bones. The test was done to determine whether cancer had spread to the patient’s bones. The image is normal, showing that metastasis has not occurred.

Bone Scan vs X-Ray perform an investigation to compare an image of bone scan with an X-ray image

Comparison of Diseased and Healthy Organs Question Compare a bone scan with an X-ray. Similarities Both x-ray and bone scans (gamma or PET) use high energy electromagnetic waves to produce the image Both types of image show 3-dimensions projected onto a 2-dimensional image Differences A radiograph uses x-ray radiation whereas a bone scan uses gamma radiation The source of x-rays for a radiograph is outside the body whereas the source of gamma rays for a bone scan is inside the body A radiograph has a higher resolution than a bone scan image [due to the need to use a collimator for the bone scan] A radiograph is a produces an image that shows structure whereas a bone scan produces an image resulting from functional differences (difference in metabolism - which may reflect structural differences) perform an investigation to compare an image of bone scan with an X-ray image

Comparison of Diseased and Healthy Organs What caused this hot spot? Answer The radioactive tracer was injected into the vein in the arm. Traces of the radioactive material remain at the injection site. gather and process secondary information to compare a scanned image of at least one healthy body part or organ with a scanned image of its diseased counterpart perform an investigation to compare an image of bone scan with an X-ray image

Comparison of Diseased and Healthy Organs Bone scan Recount how a bone scan is produced. Answer A radiopharmaceutical is put in the patient by injection, inhalation or ingestion. The patient waits while the radiopharmaceutical is metabolised. The radioisotope accumulates in the target organ. The patient lies down and remains stationary on or under a gamma camera. Gamma rays are emitted in all directions from the body, however the collimator allows only gamma rays following parallel paths to reach the gamma detector in the camera. Signals from the detector are processed by a computer to produce an image showing functional information. gather and process secondary information to compare a scanned image of at least one healthy body part or organ with a scanned image of its diseased counterpart

Positron Emission Tomography (PET) PET is used for bone imaging monitoring tumours monitoring the function of the heart monitoring blood-flow in the heart studying brain activity Overview A radiopharmaceutical that produces positrons is placed in the body, targeting a particular organ. The radioisotope accumulates in the desired organ. Positrons are emitted into the body as the isotope decays. Positrons annihilate electrons, producing a pair of gamma rays that travel in opposite directions. The pairs of gamma rays are detected by a ring of sensors. Signals from the sensors are processed by a computer to produce an image showing the location and concentration of the radioisotope in the body. The radioactive material is excreted or decays to form harmless products.

Radioactive Decay - Positron Production Phosphorus-30 and fluorine-18 are two artificial radioisotopes that undergo decay that produces positrons As a result of the radioactive decay of fluorine-18, a positron is emitted from the nucleus 18 F ® 18 O b + 9 8 30 P ® 30 Si + b + 15 14 Explanation - because there are too many protons in the nucleus for it to be stable, one of the protons decays, producing a positron and a neutron which remains in the nucleus identify that during decay of specific radioactive nuclei positrons are given off

Positron Annihilation - Gamma Ray Production A positron interacts with an electron, annihilating both and producing two g-photons [energy = 0.511 MeV] positron + electron => gamma rays The g-photons travel in opposite directions Gamma photons produced use in PET Gives high resolution compared with other gamma imaging technologies [but not as good as X-ray, CT, ultrasound, MRI] Typical isotopes used: 18F, 68Ga, 15O Isotopes produced using cyclotron Main advantage - the image produced is a functional image i.e. it shows HOW the body is working The most commonly used tracer is 18FDG (18F fluorodeoxyglucose) discuss the interaction of electrons and positrons resulting in the production of gamma rays

PET Scanner This imaging technology is called tomography because the image is obtained and usually viewed as slices perpendicular to the long axis of the body. Because the data are analysed by a computer, it is also possible to create a computer-generated 3-D image from the scans. describe how the positron emission tomography (PET) technique is used for diagnosis

Diagnosis Using PET describe how the positron emission tomography (PET) technique is used for diagnosis

How PET Scanning is Performed PET produces an image of a 2-D section through the body Tomography: “slice” or “section” Contrast this with a gamma scan, which produces a 2-D image of a 3-D volume Similar to CT - HOW? Many slices can be digitally combined to produce a virtual 3-D image describe how the positron emission tomography (PET) technique is used for diagnosis

How PET Scanning is Performed Positron + electron annihilate each other Two g photons are emitted in opposite directions PET uses annihilation coincidence detection (ACD)!! Ring of gamma ray detectors around body Time of arrival and intensity of gamma rays is recorded 2 simultaneous events give line of response (LOR) Image is built up from LOR data describe how the positron emission tomography (PET) technique is used for diagnosis

How PET Scanning is Performed Positrons emitted from the tracer annihilate electrons producing gamma ray pairs The gamma ray pair strikes detectors on opposite sides of the patient A flash of light is produced by the detectors arranged in a circular ring A computer compares the intensity of gamma ray pairs over time, and calculates the location of the positron-electron interactions to produce an image describe how the positron emission tomography (PET) technique is used for diagnosis

How PET Scanning is Performed The greater the depth of tissue through which the gamma rays travel, the greater the attenuation of the gamma rays - enabling the source point to be located by comparing intensities of gamma rays on opposite sides of the body. describe how the positron emission tomography (PET) technique is used for diagnosis

How PET Scanning is Performed describe how the positron emission tomography (PET) technique is used for diagnosis

How PET Scanning is Performed describe how the positron emission tomography (PET) technique is used for diagnosis

Diagnosis Using PET PET Scan gather and process secondary information to compare a scanned image of at least one healthy body part or organ with a scanned image of its diseased counterpart

Diagnosis Using PET gather and process secondary information to compare a scanned image of at least one healthy body part or organ with a scanned image of its diseased counterpart

Diagnosis Using PET gather and process secondary information to compare a scanned image of at least one healthy body part or organ with a scanned image of its diseased counterpart

Diagnosis Using PET describe how the positron emission tomography (PET) technique is used for diagnosis

PET Images of the Brain Heart with myocardial infarction. Arrows indicate diseased tissue Normal heart (left) gather and process secondary information to compare a scanned image of at least one healthy body part or organ with a scanned image of its diseased counterpart

PET Images of the Brain Normal brain (left) Brain of 9 year old girl suffering from epilepsy. Arrow indicates problem area, which was removed. Normal brain (left) gather and process secondary information to compare a scanned image of at least one healthy body part or organ with a scanned image of its diseased counterpart

Questions Identify the particle produced by radioactive decay that is used for to produce PET images. (1M) Identify the radiation detected to produce a bone scan. (1M) Compare the processes of taking an X-ray image to producing a PET image. (4M) Compare the type of images produced using ultrasound and PET. (2M) Positron Gamma rays The radiation source is outside the body in the case of X-rays but it is inside the body in the case of a PET scan. Both processes use e/m radiation to produce the image; X-rays and gamma rays, however the gamma rays are produced inside the body by positron-electron annihilation whereas X-rays are produced using an X-ray tube. Images produced using ultrasound are structural (showing anatomy) whereas PET produces functional images (showing physiology)

PET Images … or not?

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Answers to Questions Question Compare a bone scan with an X-ray. Similarities Both x-ray and bone scans (gamma or PET) use high energy electromagnetic waves to produce the image Both types of image show 3-dimensions projected onto a 2-dimensional image Differences A radiograph uses x-ray radiation whereas a bone scan uses gamma radiation The source of x-rays for a radiograph is outside the body whereas the source of gamma rays for a bone scan is inside the body A radiograph has a higher resolution than a bone scan image [due to the need to use a collimator for the bone scan] A radiograph is a produces an image that shows structure whereas a bone scan produces an image resulting from functional differences (difference in metabolism - which may reflect structural differences)

Answers to Questions