Scintillation, Anger, Gamma Camera & Quality Control :Scintillation, Anger, Gamma Camera & Quality Control


: The performance of a scintillation camera must be assessed everyday it is used to assure the acquisition of diagnostically reliable images.


Scintillation, Anger, and Gamma cameras :Scintillation, Anger, and Gamma cameras Cameras come in a variety of configurations, single, double, and triple head models Some allow for whole-body imaging, portable use and tomographic imaging ( SPECT single photon emission computed tomography) Planar imaging is appropriate for many studies, SPECT imaging allows for greater sensitivity and resolution of imaging deep tissues.


Almost anything can affect proper function and performance of a scintillation camera. :Almost anything can affect proper function and performance of a scintillation camera. Changes or failure of individual system components or subsystems Environmental conditions electrical power supply fluctuations physical shock temperature changes humidity dirt background radiation


Crystals :Crystals Crystals used in anger (gamma) cameras are extremely sensitive to moisture and are sealed in a aluminum housing. They are also sensitive to temperature (rapid changes in temp.) can produce fractures of the crystal.


Crystals :Crystals Vary in size from 7” to 25” Either circular or rectangular Crystals are ¼” to ½” thick with 3/8” being the most common. Thicker the crystal the greater the probability the incoming photon will interact, deposit it’s energy, and be detected.


Crystals :Crystals The thicker the crystal the better the sensitivity will be……however The thicker the crystal the poorer the spatial resolution will be…..because of the complex interaction b/w crystal, photomultiplier tube, and light pipe that is used to optically couple the two.


Positioning logic :Positioning logic Anger cameras have an array of PMT (photomultiplier tubes) attached to the back of the scintillation crystal # of tubes is determined by size & shape of both the crystal and individual PMT Common for cameras to have either 37, 55, or 61 PMT


Positioning logic :Positioning logic When a scintillation event occurs each PMT produces an output pulse Amplitude of pulse from a PMT is directly proportional to the amount of light (scintillations) the photocathode has received. PMT’s closest to scintillation event produce largest output.


Energy discrimination :Energy discrimination Pulse Height Analyzer is used in Anger (gamma) cameras to select a centerline and window Photopeak energy is used as the centerline and size of window is determined by percentage of the centerline energy ex:10% of 140Kev = to a range 126Kev – 154Kev +/- 10%


Energy discrimination :Energy discrimination Wider the window the greater the sensitivity…. However if the window is to wide it will allow compton scatter and resolution will decrease. Window width ie: 10%-20% etc is selected based on resolution and sensitivity requirements of study.


Image formation :Image formation Image can be formed in two ways analog and digital Virtually all cameras (analog & digital) form images via digital acquisition Virtually all cameras are interfaced with a computer Counts from the radioactivity that are collected are stored in the computers memory


Image formation :Image formation Camera crystals are electronically divided into a matrix consisting of many small areas called pixels (picture elements) Each pixel is assigned a separate storage location on the computers memory. At the beginning each storage locations are set at zero….


Image formation :Image formation Once a count (radioactivity) is detected on the crystal the storage location responds by assigning it a 1 At the conclusion of an acquisition, each storage location contains a number of counts (radioactivity) that have been registered at that location on the crystal.


Image formation :Image formation Matrix size determines amount of pixels used. If a 64 x 64 matrix is used than 4096 pixels are used in acquisitions Matrix size influences spatial resolution of an image. The larger the matrix size the better the spatial resolution


Image formation :Image formation Acquisitions can be acquired in either frame mode (most common) or list mode. Framing rate is the amount of each image is acquired Frame mode all counts are collected into a storage matrix in a given time period. Matrix size and framing rate are preselected.


Image formation :Image formation One frame of data is acquired by either number of counts acquired or a fixed length o time Dynamic frame mode is the framing rate in sec/frame for a determined amount of time ex: 20 sec per frame for 30mins. Matrix and framing rate cannot be changed after acquisition.


Image formation :Image formation Two factors are considered when selecting dynamic framing study Purpose of study and counting rate. Quantitative study usually requires a shorter framing rate to provide more data ex: renal scan. Qualitative study requires more counts per frame to provide better resoultion


Image formation :Image formation Higher the counting rate, the shorter the framing rate. Ex: 0.2-0.5sec/frame more frames per time than 2-5sec/frame Most common matrix sizes are 64x64, 128x128, 256x256, larger that matrix better the spatial resolution.


Image formation :Image formation Zoom mode may be used when a small organ occupies only a portion of the field of view. Zoom mode increases resolution prior to acquisition, postacquisition zoom makes image larger yet decreases resoultion.


Image formation :Image formation Information density= number of counts per square centimeter, directly affects resolution Must obtain adequate counts in a frame resolve activities b/w adjacent pixels= contrast. Increasing counts decreases signal-to-noise ratio and improves quality of image.


The most useful tests to help determine the proper function of a camera are reflected in the parameters of field uniformity, spatial resolution, linearity and sensitivity. :The most useful tests to help determine the proper function of a camera are reflected in the parameters of field uniformity, spatial resolution, linearity and sensitivity. These are all measured at the time of installation: confirm specifications provide standard for all subsequent performance evaluations/test Initial measurements are part of acceptance testing upon receipt of a new camera as well as after a camera has been serviced.


UNIFORMITY :UNIFORMITY The most basic measurement of camera performance is a flood-field uniformity - this is the ability of the camera to depict a uniform distribution of activity as uniform (full-field activity should cause a scintillation event across the entire camera head, the source should cover the entire camera head) It is assessed by “flooding” the field of the camera with a uniform field of radiation and then assessing the uniformity of the image that it produces. Flood fields are examined daily for homogeneity.


Inhomogeneous flood fields can result from: :Inhomogeneous flood fields can result from: Inadequate mixing of Technetium flood phantom Computer correction turned off Loss of coupling between crystal and PMTs Bad PMT voltage adjustment Camera pulse height analyzer off-peak for the radionuclide


Slide 25:Most of the nonuniformity in a camera detector occurs as a result of spatial distortion (the mispositioning of events) To correct distortion, reference images are acquired and digital correction maps are generated and stored. Each map contains values that represent x,y coordinate shifts. Microprocessor circuitry repositions each count in real time during acquisition of the study using these shifts. With many new cameras it is best to create correction maps with the same radionuclide that is used for patient imaging. In some cameras several correction maps are maintained for all the possible radionuclides that might be used and it is up to the technologist to select the appropriate one.


Slide 26:Variation in the position of a pulse from different areas of the camera within the pulse height window may also produce nonuniformities. This spatially dependent energy variation can also be corrected by microprocessor circuitry. The combination of energy variation and spatial distortion is responsible for the loss of spatial resolution and imperfect linearity and uniformity. In more advanced cameras all of these things are accounted for through really long boring math that no one wants to know. Some systems may even account for collimator-specific uniformity corrections.


Uniformity :Uniformity Field uniformity may be done either as intrinsic or extrinsic Intrinsic=collimators off. Monitors the condition of sodium iodide crystals and electronics. Extrinsic=collimators on. Monitors the camera as it is used clinically.


Uniformity :Uniformity Uniformity images must be inspected daily for nonuniformity and compared to previous flood (uniformity) images.


SPATIAL RESOLUTION :SPATIAL RESOLUTION Can be defined in terms of the amount by which a system smears out the image of a very small point source or a very thin line of radioactivity. It can be thought of as the distance by which two small point sources must be separated to be distinguished as a separate in the reconstructed image.


Slide 30:It is usually measured through the use of a transmission phantom (bar phantom) It consists of alternating patterns in lead to produce closely spaced areas of differing activity levels, which will allow for the analysis of resolution. The better the spatial resolution, the better the ability to detect small abnormalities which will present themselves as different radionuclide concentrations in clinical images.


: Resolution patterns should be used without a collimator to measure intrinsic performance. This can be difficult or impossible along with time consuming with some multihead cameras. It can also be useful to assess the resolution with a point or line source. The spread of the point or line is an indication of the degree of blurring, or the loss of the resolution in the camera. Resolution should be checked weekly.


LINEARITY :LINEARITY The ability to reproduce a linear activity source as linear in the image. A phantom with either an arrangement of bars or holes is usually used. The image produced should look exactly like the phantom that was used. For example: straight lines should be reproduced as straight lines and holes should be reproduced as holes


Slide 33:Linearity should be checked weekly Linear defects in a flood field are usually the result of a cracked crystal and less likely the result of a collimator defect. Linearity is checked along with spatial resolution with the bar phantoms


Florida Administrative Code: 64E-5.613Quality Control of Diagnostic Instrumentation :Florida Administrative Code: 64E-5.613Quality Control of Diagnostic Instrumentation Each licensee shall establish written quality control procedures for all equipment used to obtain images or information from radionuclide studies. The procedures shall be recommended by equipment manufacturers or be approved by the department. The Licensee shall perform quality control as specified in written procedures and retain a copy of the quality control results for 3 years.


NEMA standards :NEMA standards National Electrical Manufactures Association (NEMA) is a trade association for the manufactures of electrical products. Nuclear section on diagnostic imaging and therapy systems division developed the NEMA standards publication for performance measurements of scintillation cameras


NEMA standards :NEMA standards Document defines standards by which scintillation cameras may be measured. Published standards is to define methods by which scintillation cameras performance can be measured A comparison can be made between performance claims by different manufactors.


NEMA standards :NEMA standards American Association of Physicists in Medicine has published information on scintillation camera quality control Published information can provide additional guidance in designing quality control procedures.


NEMA standards :NEMA standards American Society of Nuclear Cardiology has also published guidelines for: Instrumentation quality assurance, performance, and quality control procedures for transmission emission tomographic systems (SPECT)


Slide 39:Question: The most basic measurement of camera performance is: A) Linearity B) Spatial Resolution C) Flood-Field Uniformity D) Sensitivity


Slide 40:Question: The most basic measurement of camera performance is: A) Linearity B) Spatial Resolution C) Flood-Field Uniformity D) Sensitivity


Slide 41:Question: How often should spatial resolution be checked? Daily Weekly Quarterly Annually


Slide 42:Question: How often should spatial resolution be checked? Daily Weekly Quarterly Annually


Are you ready…….. :Are you ready…….. To explode!!!!!!! Turn on the lights!!!!! ANY QUESTIONS………………