Nuclear Medicine Physics Module PDF

Nuclear Medicine Physics Module Dr Mohammed Saeed Alqahtani Lecture 3 Preparation of radiopharmaceuticals 2 Preparation of radiopharmaceuticals ▪ The preparation usually involves the simple mixing or shaking at room temperature of the radionuclide (e.g. 99mTc as sodium pertechnetate) with the compou...

Nuclear Medicine Physics Module Dr Mohammed Saeed Alqahtani Lecture 3 Preparation of radiopharmaceuticals 2 Preparation of radiopharmaceuticals ▪ The preparation usually involves the simple mixing or shaking at room temperature of the radionuclide (e.g. 99mTc as sodium pertechnetate) with the compound to be labelled (e.g. MDP) and other necessary chemicals. ▪ Shielded syringes are used to transfer the components between sterile vails. ▪ The manipulations are carried out under sterile conditions in a workstation; for example, a glove box or a sterile laminar down-flow cabinet. 3 Preparation of radiopharmaceuticals Laminar down-flow cabinet A glove box 4 Preparation of radiopharmaceuticals ▪ All surfaces are impervious: continuous floors, gloss-painted walls, and formicatopped or stainless steel benches. ▪ Normal sterile procedures must be followed, and entry is via an air lock and changing room. ▪ With preparation time being necessarily short for PET radiopharmaceuticals, automated synthesis devices using microprocessor control are commonly used. → Using this technology reduce the radiation exposure of the staff 5 Preparation of radiopharmaceuticals Automated synthesis device for PET tracers’ production 6 Radiopharmaceuticals quality control ▪ Quality control includes testing for: A) radionuclide purity: → For example, testing for contamination with 99Mo, which would give an unnecessary dose to the patient, by measuring any gamma radiation from 99Mo after blocking off the gamma rays from 99mTc with 6mm lead. 7 Radiopharmaceuticals quality control ▪ Quality control includes testing for: B) radiochemical purity: → For example, testing for free pertechnetate in a labelled 99mTc compound using chromatography. C) chemical purity: → For example, the spot colour test for alumina, which may have come from the 99Mo column and would interfere with labelling. 8 Radiopharmaceuticals quality control ▪ Quality control includes testing for D) response of the radionuclide calibrator: → For example, testing the detector against a standard radioactive source with a long half-life. 9 Dose calibrator 10 Planar imaging 11 Planer imaging process ▪ The patient is given an appropriate radiopharmaceutical, usually by intravenous injection. ▪ The function of the radiopharmaceutical is, ideally, to concentrate in the organ or tissues of interest. ▪ The role of radionuclide is to signal the location of the radiopharmaceutical by the emission of gamma rays. 12 Planer imaging process ▪ The radionuclide most commonly used, 99mTc, emits 141keV gamma rays. ▪The emitted gamma photons are detected by a gamma camera, and an image of the radioactive distribution is produced on a monitor screen. ▪As gamma rays cannot be focused, instead of a lens a multi-hole collimator is used to delineate the image from the patient. 13 Large field of view (LFOV) gamma cameras ▪ Diversified hardware and connections are assembled together to form a medical gamma ray detection instrument, also known as the gamma camera. ▪ Each component plays an integral role in the identification of gamma photons and producing proper medical gamma images. ▪ These slides elaborate on the conventional gamma imaging system’s chief elements only and does not provide an explanation of every single component and their detailed functions. 14 LFOV gamma imaging system ▪ The major components observed in each gamma camera include a light guide, photomultiplier tube array, radiation shielding, scintillation crystal, collimator, electronics for energy positioning and differentiation. Additionally, there would be a display device and a computer for processing and demonstrating images and data 15 LFOV gamma imaging system 16 LFOV Gamma Camera Scintillators ▪ The process of converting gamma rays into optical light is performed using a scintillation crystal. ▪ As a part of a gamma camera design, several characteristics are considered before choosing the appropriate crystal for gamma ray detection. ▪ To attain enhanced imaging performance suitable for clinical use, the scintillator chosen must be inexpensive, have short decay duration, with high atomic number, and high density and light output. 17 LFOV Gamma Camera Scintillators ▪ the following table lists properties of common scintillators used in conventional gamma cameras: Properties Na(Tl) CsI(Na) CsI(Tl) Density (g cm-3) 3.67 4.51 4.51 Decay time (µs) 0.23 0.63 1 Refraction Index Photon yield (keV) 1.85 38 1.84 39 1.8 45 – 52 Hygroscopic Yes Yes Slightly Effective atomic number (Zeff) 50 54 54 420 540 Peak emission wavelength (nm) 415 18 LFOV Gamma Camera Scintillators ▪ Sodium iodide (NaI) scintillators are the most widely utilised for radiation detection in conventional gamma ray imaging systems. ▪ However, it has been found that NaI crystals have unwanted characteristics of moisture absorption and fragility; therefore, they are required to be sealed in airtight containers. ▪ To enhance the NaI crystals reaction to the incident gamma photons, they can be doped with thallium (Tl) atoms, which improves the scintillator’s response to the gamma photons. 19 LFOV Gamma Camera Scintillators ▪ As a result of photoelectric or Compton scattering interactions of the incident photons with the scintillator’s material, an energetic electron is released and moves within the scintillator and interacts with other atoms to produce more excitations and ionisations. ▪ The resultant electrons that are in an excited state subsequently return to their stable state through realising their energy in a form of optical photons. 20 LFOV Gamma Camera Scintillators ▪ Crystal design is an integral factor influencing the scintillation process. ▪ It has been shown that consistently high sensitivity levels are recorded while utilising thick crystals, as they can absorb most incident gamma photons; however, a reduction of spatial resolution is recognised. ▪ Conversely, gamma photons can escape thinner crystals easily, which will degrade the sensitivity of the detector. 21 LFOV Gamma Camera Scintillators ▪ For medical imaging purposes, the usual thicknesses for the manufactured scintillators of gamma cameras is in the range 6 to 12.5 mm, and these thicknesses match the detection of gamma photons in the 141 keV range (i.e. 99mTc isotope energy). 22 LFOV Gamma Camera Scintillators 23 Photomultiplier tube (PMT) ▪ There are two steps in the procedure of gamma detection using gamma cameras. ▪ The process commences with an interaction between the scintillation crystal and the incident gamma photons. ▪ Following this, the energy of these photons is deposited in the crystal and converted to visible optical photons. The transmitted light propagates and is guided towards an array of PMTs. 24 Photomultiplier tube (PMT) ▪ The PMT is a high-voltage, sensitive, optical detector, which can detect and convert optical photons into a measurable electrical signal. ▪ The PMT works on the directly proportional relationship between the electric current and the number of detected optical photons. ▪ the PMT is a vacuum tube with an entrance window, an anode, photocathode, dynodes (electron multiplier), and focusing electrodes. 25 Photomultiplier tube (PMT) ▪ The NaI scintillator transmits optical photons at a peak wavelength of 415 nm and these optical photons are received by the photocathode, which liberates photoelectrons (i.e. low energy phenomena: photoelectric effect). ▪ The photoelectrons, produced by the photocathode, are directed by a focusing electrode towards the first dynode. ▪ The collision between the primary electrons and the first dynode surface liberates more electrons. Consequently, the same step repeats between a series of dynodes with further stages of electron amplification. 26 Photomultiplier tube (PMT) ▪ This process does not end until the electrons reach the final dynode. ▪ This cascade procedure works as all the electrons are collected at the PMT’s anode. 27 Photomultiplier tube (PMT) 28 Gamma ray collimation principles ▪ The gamma image produced by a conventional gamma camera is formed on a pixelated grid. The number of detected gamma photons is recorded for each pixel within the camera’s field of view. ▪ Therefore, the formation of a gamma image is actually a true mapping for the radioactive agents in the targeted region of interest (i.e. the spatial locations of the detected gamma photons). 29 Gamma ray collimation principles ▪ To enhance the produced gamma image quality, scattered gamma photons should be minimised to reduce image noise. ▪ Additionally, improving the ability of the gamma camera to maximise the detection of primary gamma photons would improve the gamma imaging outcome. 30 Gamma ray collimation principles ▪ When the process of nuclear scanning takes place, there are several elements that influence the appropriate type of collimator for gamma ray detection. →Such as FOV dimension, targeted areas’ dimensions, and the desired level of sensitivity and spatial resolution. ▪ the gamma rays that are emitted from the examined region in the patient are mapped onto the scintillator and represent the targeted tissues’ respective locations in the image. ▪ The main drawback of this approach is that only gamma photons passing through the collimator’s holes can be registered and utilised to produce a gamma image; the other gamma photons will be absorbed by the collimator’s material. Therefore, the sensitivity of the gamma camera is affected. ▪ The following slides show routinely used gamma camera collimators in nuclear medicine clinics. 31 Gamma ray collimation principles 32 Parallel hole collimator ▪ The most common collimators for gamma image formation are the parallel hole collimators. ▪ This type of collimator consists of a high density material plate (e.g. lead is a commonly used material) with hexagonal structured holes arranged in a close-packed hexagonal array. ▪ Using this preferred hole structure and arrangement would maximise the exposed area of the gamma camera detector. ▪ However, these holes can be manufactured in other structures, such as triangular, square, or circular holes. 33 Parallel hole collimator ▪ Generally, the thickness of parallel hole collimators manufactured for traditional gamma cameras ranges between 25 and 80 mm, and they contain 3×104 - 9×104 holes. ▪ The structure of the parallel bore arrays is symmetrical, with identical holes, surrounded by lead septa. 34 Parallel hole collimator ▪ The parallel hole collimator allows incident gamma photons to pass through at a perpendicular angle. ▪ Parallel hole collimators provide a superior level of sensitivity compared to pinhole collimators. ▪ However, generally collimators impose considerable limitations on the FOV of the gamma imaging systems because most of the off-axis gamma photons are absorbed by the collimator body. 35 Parallel hole collimator ▪ In practice, the parallel hole collimator allows several incidence angles because the hole diameter is not infinitesimally small and the spatial resolution quality of the gamma camera degrades with larger dimensions of the collimator’s holes, as these will allow a wider range of acceptance angles. ▪ Using different gamma emitters also plays a significant role in the selection of the appropriate parallel-hole collimator; the energy of the gamma photons is the key factor to determine the septal thickness. ▪ This is because the scattered gamma photon penetration for the collimator body must be minimised using an appropriate septal thickness to avoid the projection of inaccurate data on the gamma image. 36 Non-parallel hole collimator ▪ Parallel hole collimators differ from diverging and converging collimators based on the field of view despite their corresponding exit plane dimensions. ▪ The converging collimator gives a smaller FOV (used to scan small targeted areas such as during brain or cardiac scanning procedures); whereas, the diverging collimator provides a larger FOV in comparison to the parallel-hole collimator. ▪ Diverging-hole collimators are utilised to collect gamma photons from a large region of interest (i.e. larger than a camera’s crystal) 37 Non-parallel hole collimator ▪ To satisfy the needs of nuclear medicine clinics with small organ imaging procedures, such as parathyroid and thyroid scanning, pinhole collimators are appropriate. ▪ The pinhole collimator has a cone-shaped structure made from metals, such as tungsten, lead or platinum. ▪ The aperture has a diameter range of 2 to 6 mm. ▪ It further demonstrates its effectiveness in examining bone joints, paediatric scans, and skeletal extremities nuclear scanning. 38 Non-parallel hole collimator ▪ The spatial resolution requirements are different (i.e. higher spatial resolution is required) for small animal gamma imaging systems as the small targeted structures require 1-2 mm pinhole diameter or even lower. 39 Non-parallel hole collimator ▪ The spatial resolution requirements are different (i.e. higher spatial resolution is required) for small animal gamma imaging systems as the small targeted structures require 1-2 mm pinhole diameter or even lower. 40 Non-parallel hole collimator ▪ The spatial resolution requirements are different (i.e. higher spatial resolution is required) for small animal gamma imaging systems as the small targeted structures require 1-2 mm pinhole diameter or even lower. 41 Types of collimator ▪ Low-energy collimator has thin septa (0.3 mm) and can be used with gamma rays of up to 150 keV (e.g. with 99mTc). ▪ A general purpose collimator might have 20000 holes each 2.5 mm diameter, a resolution (at 10 cm from the face) of 9 mm, and sensitivity 150 cps MBq-1. 42 Types of collimator ▪ A high-resolution collimator would have more and smaller holes and lower sensitivity; → It can be used where high resolution is required and the amount of radioactivity and the imaging times needed are acceptable. 43 Types of collimator ▪ A high-sensitivity collimator would have fewer and larger holes and poorer resolution. → It can be used in dynamic imaging where short exposure times are necessary and the poorer resolution must be accepted. 44 Types of collimator ▪ Medium-energy collimators, for use up to 400 keV (e.g. with 111In, 67Ga and 131I), have thicker septa (1.4 mm) and consequently fewer holes and lower sensitivity. 45 Tomography with radionuclides ▪ Conventional planar gamma imaging produces a twodimensional projection of a three-dimensional distribution of a radiopharmaceutical. ▪ The images of organs are superimposed, depth information is lost and contrast is reduced. ▪ These deficiencies are addressed in emission tomography. 46 Tomography with radionuclides ▪ There are TWO methods of emission tomography: A) Single-photon emission computed tomography (SPECT) B) Positron emission tomography (PET) 47 Single-photon emission computed tomography (SPECT) ▪ In its simple form, a gamma camera with a parallel hole collimator rotates slowly in a circular orbit around the patient lying on a narrow cantilever couch. ▪ Every 6°, the camera halts for 20 – 30s and acquires a view of the patient. ▪60 views are taken from different directions, each, however, comprising fewer counts than in conventional static imaging. 48 Single-photon emission computed tomography (SPECT) ▪ Some 3 million counts are acquired in an overall scanning time of around 30 mins. ▪ Image acquisition time is halved or the sensitivity improved by using a dual- or triple-headed camera. 49 Single-photon emission computed tomography (SPECT) ▪ The camera must move on a sufficiently large circular orbit to miss the patient’s shoulders. ▪ An elliptical orbit is sometimes used to minimise the gap between the collimator and the patient and so improve the resolution. ▪ Rotation of 180° is useful option (e.g. in cardiac tomography). 50 Single-photon emission computed tomography (SPECT) ▪ Some dedicated equipment uses three or four gamma cameras equally spaced around the rotating gantry, thus improving sensitivity. → The increase sensitivity can be used for faster patient throughput or to improve the resolution. 51 Single-photon emission computed tomography (SPECT) ▪ SPECT studies can be presented either as a series of slices or as a three-dimensional display. ▪ The 3-D display is particularly effective when the image is rotated continuously on the computer screen, as persistence of vision helps to reduce the effect of image noise. 52 Single-photon emission computed tomography (SPECT) ▪ Thallium studies of myocardial infarctions and ischaemia are major uses of SPECT, along with quantitative cerebral blood flow. ▪ Other applications, such as the detection of tumours and bone irregularities, are increasing in clinical usefulness, particularly when SPECT is combined with CT. 53 Single-photon emission computed tomography (SPECT) ▪ Gated acquisitions are also possible with SPECT as in planar imaging (e.g. MUGA studies). ▪ Cardiac gated myocardial SPECT can be used to obtain quantitative information about myocardial perfusion, thickness of the myocardium, left ventricular ejection fraction, stroke volume and cardiac output. ▪ Indeed, SPECT is probably the clinical standard for assessment of myocardial ischaemia, although PET mow offers more accuracy and better spatial resolution. 54 55