Nuclear Medicine Physics Module Lecture 4 PDF

Nuclear Medicine Physics Module Dr Mohammed Saeed Alqahtani Lecture 4 Single-photon emission computed tomography (SPECT) SPECT/CT machine, which is used for daily nuclear medicine procedures Preclinical SPECT/CT machine, which is used for small animal studies 2 Positron emission tomography (PET) 18F...

Nuclear Medicine Physics Module Dr Mohammed Saeed Alqahtani Lecture 4 Single-photon emission computed tomography (SPECT) SPECT/CT machine, which is used for daily nuclear medicine procedures Preclinical SPECT/CT machine, which is used for small animal studies 2 Positron emission tomography (PET) 18F. ▪ The most common positron emitter utilised in PET is& - - ▪ When 18F emits a positive beta particle (positron), this travels for about 2 mm through the patient before being annihilated by a negative electron. ▪ Their combined mass (positron + electron) is converted into two energetic photons, each of exactly 511 keV, emitted simultaneously and in practically opposite directions. 3 Positron emission tomography (PET) ▪ A positron or PET camera comprises a ring, hexagon or other polygon surrounding the patient and composed of a very large number of solid scintillation detectors (10000-20000). ▪ These detectors often made of bismuth germinate (BGO). ▪ Other efficient scintillators, such as lutetium oxyorthosilicate (LSO) or gadolinium oxyorthosilicate (GSO), are also in use. 4 Positron emission tomography (PET) ▪ The ideal choice, from the previously mentioned scintillators, would be readily available, inexpensive to produce, and easy to manufacture into crystal blocks, with: A) High detection efficiency – to absorb and convert keV photons into optical light. B) Very short scintillation decay time C) Good energy resolution 5 Positron emission tomography (PET) ▪ As usual, a compromise is necessary; the following table shows a comparison with sodium iodide doped with thallium, which is normally used in gamma cameras: Property of the material BGO LSO GSO NaI(Tl) Effective atomic number Relative light output Decay time Typical energy resolution at 511 keV 75 15 300 ns 25 % 66 75 40 ns 25 % 59 25 60 ns 15 % 51 100 230 ns 10 % % 6 Positron emission tomography (PET) 7 Positron emission tomography (PET) ▪ Positron emission tomography detectors are commonly made in block format, coupled to photomultiplier tubes. ▪ Note that the detector material, configuration, number of detector blocks and number of photomultiplier tubes depend on the manufacturer, but the principles of operation are the same. 8 Positron emission tomography (PET) 9 Positron emission tomography (PET) ▪ If, as in the beside figure, the annihilation photons from the event (a) enter detector A and B, they produce simultaneous (coincident) pulses, which are then accepted and combined by the electronics. ▪ Any pulses that do not coincide in the time are ignored by the electronics, as are any single photons of background radiation. 10 Positron emission tomography (PET) ▪ These two detectors therefore measure only the sum of the activity present along a line AB, called the line of response (LOR). ▪ Each detector can operate in coincidence with perhaps half of the detectors that face it in the ring, so that the patient is criss-crossed by hundreds of LORs. 11 Positron emission tomography (PET) ▪ There are three types of coincidences that can occur: a) True b) Random – from independent events that may nor occur in the same plane (ring). c) Scattered – that may also be cross-plane. 12 Positron emission tomography (PET) ▪ To reduce the random and scatter events from adjacent rings, narrow lead or tungsten septa (b), which is about 1 mm thick by up to 10 mm deep. ▪ It can be used between each ring of detectors to act like a parallel antiscatter grid in radiography. 13 Positron emission tomography (PET) ▪ The main positron emitter used in PET imaging is 18F (half-life 110 min), primarily used to label deoxyglucose (FDG). ▪ Other useful radionuclides are 68Ga (68 min) and 82Rb (1 min), particularly as these two are produced by radionuclide generator. 14 Integrated SPECT/CT and PET/CT ▪ All gamma images give functional and physiological information. ▪ To be able to locate and visualise this information within the patient’s anatomy, PET (and SPECT) images are usefully fused with CT images or MRI images. ▪ Combining two sets of such digital images is often complicated by the need for adjustment of matrix size, matching rotational position, the transverse plane and coregistration to enable one to one spatial matching of the images. 15 Integrated SPECT/CT and PET/CT ▪ To overcome these complications, integrated SPECT/CT and PET/CT systems are used. ▪ Both detection systems are mounted on the same support, adjacent to each other, so that the single patient table moves along the central axis. ▪ Once the CT scan is complete, the patient table moves into position for the SPECT or PET data collection. ▪ Almost perfect matching of the images is obtained, except for involuntary patient movement. 16 Integrated SPECT/CT and PET/CT ▪ The combined images are particularly useful in oncology, both for diagnosis and for accurate tumour localisation and follow up. ▪ Carefully gating image acquisition to the cardiac cycle can also produce useful fused images in cardiology. 17 Dose to the patient 18 Dose to the patient ▪ Administrating a radioactive material to the patient necessarily gives the patient a radiation dose. ▪ the organ of interest that takes up the radionuclide (referred to as the source organ) will receive a dose, but it also acts as a source of irradiation for other socalled target organ and tissues in the body. 19 Dose to the patient ▪ An example of the source and target organs in lung ventilation studies is shown in the beside figure: 20 Dose to an organ ▪ The absorbed dose delivered to an organ by the activity within it increases in proportion to: → the activity administered to the patient → the fraction taken up by the organ → the effective half-life of the activity in the organ → the energy (keV) of beta and gamma radiation emitted in each disintegration 21 Dose to an organ ▪ It is also depends on how much of that energy escapes from the organ, and so does not contribute to the absorbed dose within the organ but will irradiate other tissues. ▪ Almost all the energy of beta ray is deposited inside the organ, and very little escapes. ▪ Some of the energy of gamma ray is deposited in the organ and some leaves it, to an extent depending on the size of the organ and how energetic the gamma ray is. 22 Effective dose to the body ▪ Unlike imaging with X-rays, the dose delivered by a radionuclide examination is unaffected by the number of images taken, neither it is confined to the region of diagnostic interest. ▪ After an intravenous injection, most tissues may receive some dose, but the organs of interest and the organs of excretion generally receive the highest doses. 23 Effective dose to the body ▪ The distribution of a dose is non-uniform and specific to the examination, but an average dose to the body as a whole can be calculated to give a measure of risk. ▪ This is termed the effective dose (E), which has the unit sievert (Sv). ▪ It is calculated using the differing sensitivities of the various organs and tissues to irradiation, by weighting each organ absorbed dose with a tissue-weighting factor (WT) to produce an equivalent dose. ▪ These are then summed to give the effective dose per MBq activity administered. 24 Typical activities and doses ▪ Most nuclear medicine investigations deliver an effective dose of less than 5mSv, which is of the order of the variation from place to place and individual to individual, in the annual dose of natural radiation. ▪ Some procedure, such as bone or static brain imaging, deliver doses in the region of 5mSv. ▪ A few examinations, such as tumour or abscess imaging with 67Ga, deliver higher doses and should be undertaken when other imaging modalities are inappropriate. 25 Typical activities and doses → Some typical radionuclide administrations for adults and consequent effective doses Site Agent Activity (MBq) Effective dose (mSv) Bone Technetium-99m diphosphonates 600 5 Lung ventilation Technetium-99m DTPA aerosol Krypton-81m gas 80 6000 0.5 0.1 Lung perfusion Technetium-99m HSA macroaggregates 100 1 Kidney Technetium-99m DTPA gluconate Technetium-99m MAG3 300 100 2 0.7 Infection Gallium-67 citrate Indium-111 leucocytes 150 20 15 7 Tumour Iodine-123 MIBG 400 5 Thyroid Iodine-123 iodide 20 4 Heart Technetium-99m MIBI Thallium201 chloride 400 80 3 18 Brain Fluorine-18 FDG 400 8 26 Typical activities and doses ▪ The activity should be checked and recorded before administration. ▪ The vial is placed in the ‘well’ of a large re-entrant ionisation chamber → the radionuclide or dose calibrator. ▪ The accuracy of the dose calibrator must be checked regularly using a reasonably long-lived source, such as 57Co. ▪ In order to minimise patient dose, patients should drink a good deal of water and empty the bladder frequently to reduce the dose to the gonads and pelvic bone marrow. 27 Typical activities and doses ▪ As fetal doses should also be constrained, various guides recommending that female patients should avoid conception for an appropriate period following administration of longlived (half-life more than 7 days) diagnostic radionuclides. ▪ Male patients do not need to be given any particular advice concerning diagnostic examinations. 28 Typical activities and doses ▪ If a female patient is or may be pregnant, certain examinations may result in a fetal dose greater than 10 mSv and should be avoided. ▪ Equally, if a patient is breast feeding, her infant will be exposed to a radiation dose from the activity secreted in her milk. ▪ An interruption in feeding may be recommended, and advice should be sought. 29 Precautions necessary in handling radionuclides ▪ When handling radionuclides, in addition to the external hazards from external radiation, there is a potential hazard from internal radiation due to accidental ingestion or inhalation of the radionuclide or its entry through wounds. ▪ It is therefore important to avoid contamination of the environment, the workplace and persons, and to control any spread of radioactive materials. ▪ Generally, the risk from contamination id greater than that from external radiation. 30 Precautions necessary in handling radionuclides – Segregation ▪ A nuclear medicine facility must have identified and preferably separate areas for: → the preparation and storage of radioactive materials → the injection of the patient → patients to wait (to allow uptake of the radiopharmaceutical into the organ of interest) → imaging → temporary storage of radioactive waste 31 Precautions necessary in handling radionuclides – Segregation ▪ Patients containing radioactivity are a source of external radiation. ▪ They should be spaced apart in the waiting area. ▪ Departmental layout should make use of the inverse square law to reduce the effect of background radiation from other patients and sources, particularly in the imaging area. 32 Precautions necessary in handling radionuclides – Personal protection ▪ Use should be made of distance, shielding and time. ▪ Staff should enter areas where there is radioactivity only when it is strictly necessary. ▪ All procedures must be carried out expeditiously and efficiently. ▪ Radionuclides are contained in shielded generators or in bottles inside lead pots. ▪ Where feasible, bottles and syringes are handled with long-handled forceps (tongs). 33 Precautions necessary in handling radionuclides – Personal protection ▪ Manipulations, such as the labelling of pharmaceuticals and the loading of syringes, are carried out with the arms behind a lead barrier that protects the body and face, and over a tray, lined with absorbent paper, to catch any drips. ▪ Syringes are protected by heavy metal, tungsten or lead glass sleeves (which can reduce radiation doses to the fingers by 75%), and are carried to the patient in special containers or on a disposable tray. 34 Precautions necessary in handling radionuclides – Personal protection ▪ Before injection, syringes are vente into swabs or closed containers and not into the atmosphere. ▪ To avoid accidental ingestion, waterproof (double latex) surgical gloves are worn when handling radionuclides. ▪ Cuts and abrasions must be covered first. ▪ There must be no eating, drinking or facial contact. ▪ Hands and work surfaces are routinely monitored for radioactive contamination, and the air in radiopharmacies should be also monitored. 35 Precautions necessary in handling radionuclides – Personal protection ▪ Staff may also be monitored for internal contamination. ▪ Swabs are taken from the radiopharmacy workstation to monitor for radioactive and bacterial contamination. ▪ Note that lead-rubber aprons are ineffective against the high-energy gamma rays of 99mTc. ▪ Hands should be washed regularly at special washbasins. ▪ Wherever there is a spillage, however slight, decontamination procedures must be followed. 36 Precautions necessary in handling radionuclides – Patient protection ▪ Every radionuclide should be checked for activity before administration, using a radionuclide (well) calibrator. ▪ The patient identity must be checked against the investigation to be made and the activity to be administered, and this must be recorded. ▪ Particular care should be taken to avoid contamination during oral administrations. ▪ Special circumstances apply for pregnant patients and those with babies whom they are breast feeding. 37 38