Radionuclide Imaging MII 3073 Detection of Nuclear Emission
Nuclear radiation detectors Detectors that are commonly used in nuclear medicine: 1. Gas-filled detectors 2. Scintillation detectors 3. Semiconductor detectors 4. Film badge and Thermoluminescent dosimeters (TLD) Gas-filled detectors: 1. Geiger-Mueller (GM) counter 2. Ionization chamber 3. Dose calibrator Scintillation detectors: 1. Sodium iodide well counter 2. Single probe counting system 3. Dose calibrator
Gas-filled detectors Operational principle: measuring the ionization that radiation produces within the gas. Commonly for monitoring α and β radiations. Typical gases used are argon and helium. The central electrode is an anode, that has been insulated from the chamber walls and the cathode. A voltage is applied to the anode and the chamber walls. As a charged particle passes through, it ionizes some of the gas (air). The positive anode attracts the electrons, or negative particles. The detector wall, or cathode, attracts the positive charges. This movement of ions/charges is an electric current, which can be detected by a sensitive meter. The current between the electrodes is a measure of the amount of incoming radiation.
Gas-filled detectors
Gas-filled detectors The amount of current produced depends on several factors: 1. The applied voltage between the two electrodes 2. Distance between the two electrodes 3. Type of gas 4. Volume, pressure and temperature of the gas 5. Geometry and shape of the electrodes Typically for a gas-filled detector, the amount of the current produced by a single radiation is a function of the applied voltage. Their relationship can be divided to 5 distinct regions.
Gas-filled detectors
Gas-filled detectors Region I: recombination The voltage is low, some ion pairs are still able to recombine and form neutral atoms or molecules. Incomplete collection of primary ion pairs by the electrodes. As the voltage increases, more primary ion pairs are collected and more current flows. Region II: ionization plateau The voltage is sufficiently high to attract all primary ion pairs. Region III: proportional The higher voltage is able to attract all primary ion pairs and sufficient to provide energy to some primary ion pairs for producing secondary ion pairs through collisions with neutral atoms and molecules of the gas (gas amplification). The amount of secondary ion pairs produced depends on the energy acquired by primary ion pairs. The amount of current produced by a radiation increases with voltage increasing.
Gas-filled detectors Region IV: Geiger Muller As the voltage is increased, a point is reached at which most of the gas within the detector is massively involved in the multiple, successive ionizations (no more gas amplification). The pulse of current is larger but becomes independent of number of primary ion pairs produced. Region V: Continuous discharge The voltage is so high that radiation is not necessary to produce discharge. Under this high electric field, the electrons are pulled out form the atomic shells, the atoms and molecules become ionized and a discharge may be established even without radiation (spontaneous and continuous ionization). This interaction stops only when voltage is lowered.
Dose calibrator A dose calibrator (activity meter) consists of: a cylindrically shaped, gas filled sealed chamber with a well, high voltage supply applied to electrodes, specific energy settings for different radionuclides, an activity readout (e.g. in MBq, GBq, etc). How to use dose calibrator? 1. Turn on the main power and wait for any self checks or warm-up up to complete. 2. Place the syringe or vial holder in the detector well. 3. Select appropriate (nuclide, energy) settings. 4. Zero the dose calibrator. 5. Measure the activity of the radionuclide in the syringe or vial. 6. Read the activity from the display console and record.
Scintillation detectors Scintillators are materials that emit visible or UV light following the interaction of ionizing radiation with material. The most widely used crystals are made of sodium iodide (NaI); clear glass-like structure, fragile and sealed in an airtight aluminum container. NaI crystals are doped with small amounts of stable thallium (Tl); improve response to gamma ray photons. When an incoming x or gamma ray hits the scintillation detector, it will interact with an electron (from the valence band) in the crystal, by either a Compton or PE process (energy transfer). Each of these energetic electrons distributes its energy among electrons in the crystals, leaving them in ionized and excited states.
Scintillation detectors These electrons may move to higher energy levels, known as the conduction band, until they fall into certain impurity centers, which act as energy traps. These traps are produced by the addition of chemical impurities into the crystal at the time of manufacture, called activators. For NaI, small amounts of thallium produce the trapping centers; (thallium-activated sodium iodide). For returning to the original state, the trapped electron may give up its energy in the form of a light photon. This light photon will then be detected and converted into electrical signal by photomultiplier tube (PMT).
Scintillation detectors The desirable properties of a scintillator are: 1. The conversion efficiency: the fraction of deposited energy that is converted into light should be high. (The conversion efficiency should not be confused with detection efficiency) 2. For many applications, the decay times of excited states should be short. (Light is emitted promptly after an interaction). 3. The material should be transparent to own emissions. (Most emitted light escapes reabsorption). 4. The frequency spectrum (color) of emitted light should match the spectral sensitivity of the light receptor (PMT, photodiode or film). 5. If used for x- and gamma-ray detection, the attenuation coefficient (µ) should be large, so that the scintillation detectors have high detection efficiency. (Materials with large atomic numbers and high densities have large attenuation coefficients). 6. The material should be rough, unaffected by moisture and inexpensive to manufacture.
Photomultiplier tube (PMT) The amount of light produced in Nai(Tl) crystals or any other scintillator is very small in volume. PMT is a light sensitive device that converts light into measurable electronic pulses. It consists of a photocathode facing the window through which light enters, a series of metallic electrodes known as dynodes arranged in special geometry and pattern, and an anode. All of these are enclosed in vacuum in a glass tube. Photocathode is a clear photosensitive glass surface that has been coupled with a light-conductive transparent gel to the surface of the crystal. The transparent gel has the same refractive index as the crystal and the PMT window.
PMT When the light photon hits the photocathode, it produces an electron of low energy through PE interaction; called photoelectron. This photoelectron is accelerated by a potential difference ( range of 50-100 V) between the emitting surface and the 1 st dynode. Upon collision with the dynode, the electron acquires sufficient kinetic energy to create a number of secondary electrons. These secondary electrons are then accelerated toward a 2 nd dynode, with a similar electron multiplication. Eventually, at the last dynode (generally 10 th ) the total electron gain of about 10 5-10 8 is produced. These electrons generate a current pulse of a few microamperes in amplitude and less than a microsecond in duration at the anode.
Anode Dynode Photo-electron Photo-cathode Visible light photon
Sodium iodide well counter Well counters are common in nuclear medicine laboratories, for performing in vitro studies as well as QC and QC procedures. Many NaI well counters are designed for counting radioactive samples in standard test tubes. Generally, there is a solid cylindrical NaI crystal with a cylindrical well cut into the crystal, into which the test tube is placed. PMT is optically coupled to the crystal base. Radiation from the sample interacts with the crystal and is detected by the PMT, which feeds into a scalar. The scalar readout directly reflects the amount of radioactivity in the sample and is usually recorded in counts for the period of measurement.
Sodium iodide well counter
Single probe counting system (thyroid probe) A thyroid probe has a single NaI crystal, a PMT at the end, and a single-hole collimator. Single probe counting systems using only 1 crystalline detector are useful for measuring not only thyroid uptake of radioactive iodine but also cardiac output. The probe used for thyroid counting is actually similar to the standard well counter, although it does not have the central hole in the NaI crystal. The typical crystal is 5 cm in diameter and 5 cm in thickness, with a cone-shaped collimator. Again, a PMT is located at the crystal base. When this probe is used, it is important for quantitative consistency to maintain a fixed distance from the object being measured to the face of the crystal and to eliminate all extraneous sources of background radiation.
Thyroid probe
Thyroid probe
Semiconductor detectors In metals, the valence band is partially filled. However, in semiconductors and insulators, the valence band is completely filled and the conduction band is completely empty. The energy gap between the valence and the conduction bands of semiconductors is smaller than that of insulators. Thus, in semiconductors, electrons (in valence band) can be easily excited to the conduction band. When a photon enters a semiconductor, the energy of the photon is absorbed (PE, Compton or PP). The electrons produced by the primary interaction of photons with the semiconductor will transfer their energy to the valence electrons, thus elevating them into the conduction band.
Semiconductor detectors This leaves equal numbers of holes in the valence band. These holes act as positively charged particles. If a voltage is applied across the semiconductor, the electrons in the conduction band will move towards the positive electrode and the holes in the valence band are for the negative electrode. Since the number of electron-hole pairs produced is proportional to the energy of the incident photon, the collection of charges on the respective electrodes results in a pulse whose height is proportional to the photon energy. This pulse can be amplified and energy-discriminated for counting purposes.
Film badge External radiation monitoring system. Film badge is the most common and economical, although not the most accurate. It consists of a small film enclosed in a plastic container with 4 windows of the covered with different radiation filters to identify the nature and energy exposing radiation. When the badge is exposed to ionizing radiation, the film emulsion darkens in proportion to the degree of radiation exposure received. The resultant optical density can be measured with a densitometer and calibrated to the degree of radiation exposure received. Film badge is capable of measuring exposures ranging from 0.1-20 msv. The film is normally changed each month.
Thermoluminescent Dosimeter (TLD) Contains small chips of a thermoluminescent material, usually lithium fluoride (LiF). When exposed to radiation, a portion of the absorbed energy is stored in the crystal structure of the LiF chips in metastable states. If the LiF chips are heated, the absorbed energy is released as visible light. The heating and measurement of LiF chips are carried out in a device called a reader. The amount of measured light is proportional to the absorbed radiation dose.
Collimators The collimator is made of perforated or folded lead and is interposed between the patient and the scintillation crystal. It allows the gamma camera to accurately localize the radionuclide in the patient s body. Collimators perform this function by absorbing and stopping most radiation except that arriving perpendicular to the detector face.
Collimators The collimator is made of perforated or folded lead and is interposed between the patient and the scintillation crystal. Nuclides emit gamma ray photons in all directions. The collimator allows only those photons travelling directly along the long axis of each hole to reach the crystal. Photons emitted in any other direction are absorbed by the septa between the holes. Without a collimator in front of the crystal, the image would be indistinct. Collimator is the rate limiting step in the imaging chain of gamma camera technology. Thus, by appropriate choice of collimator, it is possible to magnify of minify images and to select between imaging quality (resolution) and imaging speed (sensitivity).
Collimators Four types of collimators are commonly used with the gamma camera: 1. Parallel-hole 2. Pinhole 3. Converging 4. Diverging A parallel-hole collimator is made of a large number (many thousands) of small holes in a lead disc. The diameter of the lead disc is the same as the scintillation crystal used. Thickness of the lead disc and diameter of the holes depend on the desired spatial resolution and sensitivity of the collimators.
Collimators Pinhole collimator consists of a single hole, usually 2-4 mm in diameter. The image is projected upside down and reversed right to left at the crystal. However, it is usually corrected electronically on the viewing screen. A pinhole collimator generates magnified images of a small organ like the thyroid or a joint. In converging collimator, the holes are angled inward, toward the organ/patient. All holes focus at an axial point, outside the collimator. Therefore, the organ appears larger at the face of the crystal. A converging collimator may be used for examination of small areas.
Collimators A diverging collimator, has holes and septa that begin to diverge from the crystal face. Generally, use of a diverging collimator increases the imaged are by about 30% over that obtained with a parallelhole. However, the image itself is slightly minified. Diverging collimator is used particularly on cameras with small crystal faces to image large organs, such as the lungs. Commercially, the collimators are also classified according to their spatial resolution or sensitivity as high sensitivity (for dynamic studies), all purpose (for most clinical applications), or high spatial resolution (for fine details) collimators and according to the energies of rays low (0-200 kev), medium (200-400 kev) and high (400-600 kev)
Effect of septal length on collimator sensitivity and resolution Effect of different source-to-camera distances