Radioactive Tracers ( also known as Radiopharmaceuticals) Lesson Objectives: Describe the use of medical tracers Describe the gamma cameras A tracer is a radioactive material which is put into a patients body, given either orally (food/drink) or by injection. Tracers can be used to diagnose or treat. The tracers obtain information when radiation is emitted from inside the human body. The tracers trace the paths of various biochemical molecules in our bodies - obtain functional information about the bodies workings (i.e. physiology).

Considerations of Medical Tracers Must be gamma emitters – alpha and beta are too damaging to the body Patient will be exposed to some radioactivity (as well as family and radiographers) Half life of the tracer must be of a suitable time (long enough to obtain the source and carry out the test but no longer) Source must not be toxic/poisonous to patient It must be specific to the part of the body being imaged and possible to reach the place where it is needed Tracer must be able to be monitored

The Gamma Camera

Common Uses Heart/lungs/brain/liver Technetium is most commonly used tracer as it is absorbed by lungs, bones, heart, brain, liver. Has half life of 6 hours and can be produced in-situ. No alpha or beta decay. Gamma energies 140KeV ideal for gamma camera detection. Relatively cheap £ 10 –£30 per dose Thyroid Diagnosis - Patient is given iodine drink, taken up in thyroid, count rate measured and compared to a standard – phantom, if increased uptake – overactive thyroid etc. Treatment - Iodine 131 emits gamma and beta can be used but the beta does damage. Iodine 123 is preferred as it only only emits gamma at keV range, Iodine 131 can be used to treat patients with an overactive thyroid (or cancer) as the beta destroys part of the thyroid. Breast Cancer Irridium 192 – implanted behind breast to treat for breast cancer as it is a beta emitter. Body fluids / blood loss If patient in accident has lost lots of blood, Iodine 131 is injected in arm, after 15 mins blood sample taken from other arm. Same amount is also injected into known volume of water. Count rates are compared and patient blood volume is calculated.

PET ( Positron Emission Tomography) PET is an extension of gamma – ray imaging, used to detect abnormal cell activity. Radiotracers are injected alongside a substance used by the body e.g. glucose which will be absorbed by the cells. When a positron from the radiotracer is emitted it annihilates with an electron in the patients organ/cell, emitting two 511keV gamma photons in opposite directions. If two detectors receive two photons at the same time then the position of the annihilation must be along the line between the detectors. Hence computers generate a ‘map’ of the radioactivity in the patients body.

MRI ( Magnetic Resonance Imaging) First image produced in 1977

MRI An MRI scanner is a large, complicated piece of equipment – which creates a very large constant magnetic field (1.4 T). The magnetic field is produced by coils of wires which carry huge currents. These wires are kept at temperatures near absolute zero (-273ºC) by cooling system using liquid helium. At these temperatures the resistance of the wire is zero. How does it work? The coils transmit radio waves which excite hydrogen nuclei in the patients body. When the radio waves are switched off, the hydrogen nuclei relax and in doing so emit electromagnetic energy – MRI signal. The radio frequency coils receive this signal and send it to a computer

MRI SCANNER MRI scanners also contain additional very accurately calibrated magnets. Their job is to alter the strength of the magnetic field of the main magnet. This means different frequencies of radio waves can be used to target different areas in the body. The relaxation times then vary and hence can be detected.

MRI Signals Most biological molecules contain hydrogen. A hydrogen nucleus contains one proton, which spins on its axis – this gives it a very small magnetic property called a MAGNETIC MOMENT. When in a magnetic field,the radiofrequency pulses excite the hydrogen nuclei and makes it turn like a gyroscope to align with the magnetic field. This is known as PRECESSION. The frequency of precession is known as the LARMOR FREQUENCY. The Larmor frequency depends on the strength of the magnetic field. Larmor frequency = 4.25 x 10 7 x B (where B = magnetic flux density). Protons absorb energy if the Larmor frequency is equal to its natural frequency – this makes them resonate.

Once a proton has gained energy from a pulse of radio waves – it will quickly relax back to its lower energy level/state and in doing so emit electromagnetic energy – MRI SIGNAL. The time taken for the protons to return to their original state is called the RELAXATION TIME. Relaxation times for hydrogen in different tissues varies and it is this that therefore enables an image to be produced. Examples of relaxation times: Water = 2 s Brain tissue = 200ms Tumour tissues ~ 1 s MRI Signals cont.

Why MRI? Advantages Very high quality image No ionising radiation (to patient or staff) No known side effects Good distinction between different types of tissue Not blocked by bone – brain images are clear. Disadvantages No metal objects can be scanned as they heat up (no metal/pins in body, pacemakers) Noisy/claustrophobic/long Cost MILLIONS

MRI IMAGES

Non-Invasive Diagnostic Techniques Non-invasive techniques are ones which do not involve ionising radiation. There are many situations when ionising radiation is not able to be administrated to a patient and often problems do not warrant the risks/effects associated with this radiation. MRI is non invasive technique but is extremely expensive. Endoscopes and Ultrasound are also non-invasive. Endoscopes Optical fibres which are inserted into body openings: nose, mouth, rectum etc. Light travels down one set of optical fibres and reflected light passes back up through a different set of optical fibres. Control wires allow the endoscope to be positioned

Endoscope Uses Arthroscope: Joints Bronchoscope: Esophagus and lung Colonoscope: Colon and bowel Coloposcope: Vagina and cervix Cystoeurethroscope: Bladder and urethra Cytoscope: Bladder Duodenoscope: Small intestine Esophagogastroduodenoscope: Esophagus, stomach and small intestine Fetoscope: Womb Gastroscope: Stomach Hysteroscope: Womb Laparoscope: Abdomen Laryngoscope: Larynx Peritoneoscope: Peritoneum Proctosigmoidoscope: Lower part of the large intestine Sigmoidoscope: Large intestine Thoracoscope: Thorax Ureteroscope: Pelvis and ureter