Basic principle of gamma camera

Upakar Paudel

1. BASIC PRINCIPLE OF GAMMA CAMERA Upakar Paudel ( B.Sc MIT) UCMSTH

2. INTRODUCTION Nuclear medicine is a branch of medicine and medical imaging that uses small amount of radiotracer/radiopharmaceuticals to diagnose disease and treat disease. Nuclear medicine differs from other medical imaging modalities in the sense that CT and MR scans are anatomically based but nuclear medicine looks at physiology of all the organ system. So, we can follow the physiological processes as they occur in the living humans using these radiopharmaceuticals through use of appropriate imaging system.

3. HISTORY 1895 : Discovery of x-ray by Roentgen 1896 : Discovery of radioactivity by Bequerel 1898 : Production of radium by Curie 1927 :Use of radon to measure the blood transit 1945 : Invention of nuclear reactor 1951 : Rectilinear scanner to acquire images 1958 : Invention of Anger camera 1964 : Use of Tc-99m (I-131 only prior to 1964) Tc-99m: metastable (T1/2 = 6.01 hr) pure γ decay (E = 140 keV), flexible for labeling. I-131: electrons and 364 keV photons, thyroid disorders only 1970 : Derivation of image reconstruction algorithm for tomography (CT, SPECT, PET)

4. The First Anger camera (1958)

5. Activity The quantity of radioactive material, expressed as the number of radioactive atoms undergoing nuclear transformation per unit time, is called activity (A). Decay constant Number of atoms decaying per unit time is proportional to the number of unstable atoms Constant of proportionality is the decay constant ( ) - dN / dt =  N A =  N

6. Half life Useful parameter related to the decay constant; defined as the time required for the number of radioactive atoms in a sample to decrease by one half. Physical half-life and decay constant are inversely related and unique for each radionuclide

7. Nuclear transformation When the atomic nucleus undergoes spontaneous transformation, called radioactive decay, radiation is emitted If the daughter nucleus is stable, this spontaneous transformation ends If the daughter is unstable, the process continues until a stable nuclide is reached Most radionuclides decay in one or more of the following ways: alpha decay Beta decay (  + or  - ) Gamma emission

8. Alpha Decay Beta minus decay Beta plus decay Electron capture decay

9. Gamma Emission Radionuclides emitting gamma radiation during their decay are potential imaging agents for nuclear medicine providing their gamma energies are between 80 and 200 KeV . This is the ideal energy range for gamma cameras since lower energies undergo tissue absorption and higher energies are not absorbed by thin NaI ( Tl ) detector used in gamma camera construction. Gamma emission results from any radioactive decay and radioactive decay followed by isomeric transition of metastable radionuclide to daughter product. Clinical Value : Gamma radiation within the energies 90 to 200 KeV is optimal for gamma camera imaging.

10. Isomeric Transition During radioactive decay, a daughter may be formed in an excited state. Gamma rays are emitted as the daughter nucleus transitions from the excited state to a lower-energy state. Some excited states may have a half-lives ranging up to more than 600 years. Clinical Value : Those isotopes that decay by isomeric transition provide a source of activity that is short lived yielding a pure gamma emitter with a minimal patient radiation dose.

11. Gamma Camera Developed by Hal Anger at Berkeley in 1957 therefore also called Anger camera An electronic device that detects gamma rays emitted by radio pharmaceutical ( e.g technetium 99m (Tc-99m) that have been introduced into the body as tracers. Once a radiopharmaceutical has been administered, it is necessary to detect the gamma ray emissions in order to attain the functional information. The position of the source of the radioactivity can be plotted and displayed on a TV monitor or photographic film. Either digital or analog.

12. Internal radiation is administered by means of a pharmaceutical which is labeled with a radioactive isotope / tracer / radiopharmaceutical, is either injected, ingested, or inhaled. The radioactive isotope decays, resulting in the emission of gamma rays. The Gamma camera collects gamma rays that are emitted from within the patient, enabling us to reconstruct a picture of where the gamma rays originated. From this, we can determine how a particular organ or system is functioning. The gamma camera can be used in planar imaging to acquire 2-dimensional images Gamma Camera

13. Components of Gamma Camera The collimator Detector crystal Photomultiplier tube array Position logic circuits Gantry

14. Collimator Made of perforated or folded usually lead or tungsten and is interposed between the patient and the scintillation crystal. Is about ½ - 2 inches thick slab. Allows the gamma camera to localize accurately the radionuclide in the patient’s body. Performs this function by absorbing and stopping most radiation except that arriving almost perpendicular to the detector face.

15. Of all the photons emitted by an administered radiopharmaceutical, more than 99% are wasted and not recorded by the gamma camera, less than 1% are used to generate the desired image. Thus it is rate limiting step in the imaging chain of gamma camera technology. Consist of single or many holes The lead walls between the holes are referred as septa. Collimator

16. Types of Collimator Pin hole collimator Multi hole collimator -parallel hole collimator -converging collimator -diverging collimator

17. Pin Hole Collimator Collimator consists of small pinhole aperture in a piece of Lead, (most common),Tungsten, platinum aperture is located at the end of a lead cone , typically 20-25cm from detector. Size of pinhole can be varied by using removable inserts. Radiation must pass through the pinhole aperture to be imaged and the image is always inverted on the scintillation crystal. Used primarily for magnification imaging of small objects Poor sensitivity. Are routinely used for very high resolution images of small organs , such as thyroid and for certain skeletal regions such as hips or wrists.

18. Increasing source-to-detector distance leads to : increasing FOV decreasing image size lower R lower sensitivity Pin Hole Collimator

19. Multi Hole Collimator Most widely used multihole collimator in nuclear medicine laboratories. Consists of thousands of parallel holes with la long axis perpendicular to the plane of the scintillation crystal. The lead walls between the holes – septa. Holes may be round, square, triangular or hexagonal. Septa absorbs most gamma rays that do not emanate from the direction of interest. For high energy gamma rays- thicker septa are used than for a low energy rays. Septa are designed in such a way that septal penetration by unwanted gamma rays does not exceed 10% to 25%.

20. Collimators are available with different lengths and different widths of septa. Longer the septa, the better is the resolution but lower is the count rate for ( Sensitivity) for a given amount of radionuclide. The count rate is inversely proportional to the square of the collimator hole length. There is an inherent compromise between the spatial resolution and the efficiency ( sensitivity) of the collimator. If length of the septa decreased, the detected count rate increases and resolution decreases. Size of object = size of image i.e. neither magnification nor minification gain. Multi Hole Collimator

21. Most common designs are Low Energy All-Purpose (LEAP), Low Energy High-Resolution (LEHR) of less than 140 KeV and Medium of less than 260 KeV and High Energy collimators of less than 400 KeV . LEAP collimators have holes with a large diameter. The sensitivity is relatively high & resolution is moderate (average sensitivity500,000 cpm for a 1-uCi source & resolution is 1.0cm at 10cm from the patent side of the collimator) LEHR collimators have higher resolution images than the LEAP. Holes are smaller & deeper. The sensitivity is approx. 185,000 cpm for 1-uCi source, and the resolution is higher 0.65cm at 10cm from the patient side of the collimator. Multi Hole Collimator

22. Medium Energy Collimators are used for medium energy photons of nuclides such as Krypton81, Gallium67, Indium111. High Energy Collimators are used for Iodine131 and F-18FDG. These collimators have thicker septa than LEAP and LEHR collimators (mainly used with Technetium 99m) in order to reduce septal penetration by the higher energy photons. Multi Hole Collimator

23. Converging Collimator Converging collimator have an array of tapered holes that converge at a point (usually 50 cm) in front of collimator (Focal point) This convergence forms a magnified image. Resolution(high at surface) and decreases with distance Sensitivity- slowly increases as source is moved from collimator face to focal plane and then decreases. Good for imaging smaller objects.

24. Diverging Collimator Has an array of tapered holes that diverge from hypothetical point behind crystal( 40-50 cm). Generally, the use of a diverging collimator increases the imaged area by about 30% over that obtained with a parallel hole collimator. The image so obtained is minified. Both the sensitivity and resolution worsens as the object of interest moves away from the collimator. Used particularly on cameras with small crystal faces to image large organs such as lungs.

25. Diverging Collimator

26. Specialized Collimator Fan beam collimator- hybrid of parallel and converging collimator ,used in SPECT Multiple pinhole collimator -50% more sensitivity than parallel collimator at same spatial resolution. Rotating slant hole collimator- variation of the Parallel hole ,has all tunnels slanted at a specific angle , generates an oblique view for better visualization of an organ, which is blocked by other parts of body & can be positioned close to the body for the maximum gain in resolution.

27. Scintillation Detector Uses Sodium iodide crystals activated with thallium (0.1-0.4 mole %) coupled to PMT as detector . The crystal surface may be circular and up to about 22 inches( 10- 21.5 inches) in diameter or it may be square or rectangular. Crystals are usually ¼ - 5/8 thick( usually 3/8 inch) The crystal has an aluminum housing that protects it from moisture, extraneous light, and minor physical damage. Larger the crystal surface diameter, the large is the field of view. Thicker the crystal, worse is the spatial resolution. With thinner crystal , the overall sensitivity (count) decreases by 10 % but approx. 30% increase in spatial resolution.

28. Properties of scintillation crystal High efficiency for stopping gamma rays. Stopping should be without scatter. High conversion of gamma rays into light. Wave length of light should match response of PMT’s. Crystal should be transparent to the emitted light. Crystals should be mechanically robust. Length of scintillation should be short. High iodine content (Z=53) & high density (3.67g/cm3)provides high QDE Is sensitive to photons of energy 50 to 300 keV

29. High conversion efficiency , 13 % of deposited energy is converted to light – best energy resolution. Sensitivity>85% at140 keV Transparent to its own scintillation emission Emits light very promptly, decay constant – 230 nsec Can be grown relatively inexpensively in large plates Spectral matching between the emitted light and the sensitivity of the PMT However they are fragile and crack easily if subjected to rapid temperature change Properties of scintillation crystal

30. What happens within crystals.. Unfortunately, only a small fraction of the energy lost by a gamma ray is converted into light, typically 10%. Interaction of the gamma ray with the crystal may result in ejection of an orbital electron (photoelectric absorption), producing a pulse of fluorescent light (scintillation event) proportional in intensity to the energy of the gamma ray. Some photons may pass through the crystal without causing light event. Some scatter out and deposit some energy ( Compton Scattering) In most thallium activated sodium iodide crystals, about 20-30 light photons are released for each KeV of energy absobed .

31. About 30% of the light from each event reaches the PMTs so amplification is necessary. Low-energy radionuclides do not show much difference in sensitivity between the two. As the energy of the radioisotope increases, the difference in sensitivity increases Modern camera have rectangular crystal (60x40 cm) which provides increased FOV Is surrounded by highly reflective material TiO 2 or Mag.Oxide to maximize light output At higher photon energies (>=300 KeV ) , needs large volume of NaI ( Tl ) for adequate detection efficiency What happens within crystals..

32. Light Guides While the front face and sides of the crystal are canned, usually with aluminum sufficiently thin so as not to attenuate the incoming gamma rays unduly, the rear crystal surface needs a transparent interface between the crystal and PMTs. This is usually provided by a Pyrex optical plate or light guide a few centimeters in thickness. Array of PMT is coupled optically to the back face of the crystal with a silicon based adhesive or grease. Light Guides(Lucite light pipe)- employed between the detector crystal and PM tubes. It increase light collection efficiency , improve the uniformity of light collection as a function of position.

33. Photo Multiplier Tube Is an electronic vacuum tube containing a light sensitivity photocathode, 10 to 12 electrodes called dynodes and an anode While PMTs with a photocathode diameter of 3 inches are used mainly, it is also necessary to use some 2 inch diameter tubes. It performs two functions-conversion of light photon to electrical signal & Signal amplification PMT is attached to the back of the crystal directly on the crystal, connected to the crystal by light pipes or optically coupled to the crystal with silicon like material. PMT detects and amplifies the signal.(10 6 -10 8 )

34. Gamma camera consists of array of PMT(1 st gamma camera – 7 PMT) Cameras with 37,55,61,75 or 91 tube are common ( 40-100 ). The greater the no. of PMTs, the greater is the resolution. Number is determined by the size & shape of both crystal & each individual PMT. More the number better is the spatial resolution & linearity With thicker crystal, PMTs are farther away from the scintillation point and are unable to determine the coordinates as accurately, thus reduced spatial resolution. Current tubes have hexagonal cross section to cover more area for efficient detection of scintillation photons Photo Multiplier Tube

35. When a photon is absorbed by the crystal, a no of PMT around the specific point will see the light & produce electrical pulse . Amplitude of pulse in a given PMT is directly proportional to amount of light received by photocathode PMT closest to scintillation event will give largest output pulse Localization of the event in the final image depends on the amount of light sensed by each PMT and thus on the pattern of PMT voltage output. Position and Summing Circuit

36. The summation signal for each scintillation event is then formed by weighing the output of each tube. This signal has three components: spatial coordinates on x- and y- axes as well as a signal (z) related to the intensity (energy). The x- and y- coordinates may go directly to instrumentation for display on the CRT or may be recorded in the computer. Z (energy) pulse is obtained by adding the signal from all PMTs & is proportional to total energy deposited in crystal. The signal intensity is processed by the PHA. Position and Summing Circuit

37. The basic principle of the PHA is to discard signals from background and scattered radiation or radiation from interfering isotopes so that only photons known to come from the photopeak of the isotope being imaged are recorded. The PHA discriminates between events occuring in the crystal that will be displayed or stored in the computer and events that will be rejected. The PHA can make this discrimination because the energy deposited by a scintillation event in the crystal bears a linear relation to the voltage signal emerging from the PMTs. The pulse height analyzer allows the operator to select only the signals from those gammas in which the height of the Z signal, that is, gamma ray energy, has a certain value or range of values. Pulse Height Analyzer (PHA)

38. If many useful gammas are not to be excluded from the image, a range of energies must be allowed through the PHA, and typically a window equal to 20% of the peak energy value is used For 99mTc with a gamma ray of 140 KeV those signals with energies between 126 and 154 keV are judged to be acceptable. When using radionuclides ( gallium 67) that emit gamma rays at different energies, multiple window analyzers need to be employed. Typically a maximum of three sets of windows is available. On newer cameras, the signal processing circuitry such as preamplifiers and PHAs is located on the base of each PMY so that there is little signal distortion between the camera head and the console. Pulse Height Analyzer (PHA)

39. Most gamma camera allow for a fine adjustment known as automatic peaking of the isotope. Occasionally, an asymmetric window is used to improve resolution by eliminating some of the Compton scatter. Image exposure time is selected by console control and usually compromises : a preset count, a preset time and a preset information density for the image a accumulation. Console Controls

40. Display The final x and y signals generated by the positional circuitry are accepted by either an analog film formatter or digitized to take part in computer display system. The analog film formatter uses a cathode ray tube that has an extreamly fine dot dimension. The light from the dot is recorded on the flm directly on the film to produce an image. Single dot represents a single gamma photon within the chosen energy window. A good quality image requires at least 1 million of these dots equivalent to 1 million of the accepted gamma events.

41. Summary A collimator accepts orthogonal gamma events from the patient. A large NaI crystals is the scintillation detector. PM tubes forms a hexagonal or rectangular array on its surface. The signals from those goes to charge amplifiers. The signals are logarithmically amplified and accepted by the positional circuitary which computes x and y axes of gamma events. PHA enables this signal if x and y signal is a photo peak event. The x and y signals forms the display.

42. References… Essentials of Nuclear Medicine Imaging, 5 th Edition, Fred A. Mettler & Milton J. Guiberteau . The Essential Physics of Medical Imaging, 2 nd Edition, Bushberg . The Physics of Diagnostic Imaging, 2 nd Edition, Dowsett , Kenny & Johnston Various websites.

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