Nuclear Medicine Final Exam Questions with 100% Correct Answers

Nuclear Medicine Final Exam Questions with 100% Correct Answers Basic Gamma Camera Components - answerCollimator, NaI scintillation crystal, light guide, PMT array How does the collimator work? - answerControls which gamma rays are accepted by absorptive collimation, forming a projected image of the distribution on the surface of the crystal. What information do event signals contain? - answerPosition and energy information (could also contain time information). Reflective Material Around Crystal - answerMaximizes light output. Aluminum Casing Around Crystal - answerHermetically seals crystal to protect from moisture (hydroscopic). Crystal Thickness is a Tradeoff Between: - answerDetection efficiency and intrinsic spatial resolution. Relationship of light detected by PMT: - answerAmount of light detected by a particular PMT is inversely related to the lateral distance between the interaction site and the center of the PMT. How to improve positioning accuracy of PMT: - answerUtilize signal threshold; only use signals with a significant pulse amplitude, and use a smaller number of PMT's around the interaction location. Can then detect multiple events simultaneously in different areas without overlap. Purpose of energy discrimination: - answerDiscriminate gamma rays that have been scattered within the body and therefore lost their positional information. Want narrow energy window centered on the photopeak (usually 20 keV centered around 140 keV). What happens to the image when using a pinhole collimator: - answerImage is inverted and magnified. Small image FOV. Septa - answerPrevent gamma rays from crossing from one hole to the next in the collimator. Diverging Collimator - answerMinified, non-inverted image. Large image FOV. Converging Collimator - answerMagnified, non-inverted image. Valid Event - answerGamma ray is emitted parallel to the collimator holes, passes through a hole and interacts photoelectrically in the crystal, depositing all its energy in a single location. Detector Scatter Event - answerGamma ray is emitted parallel to the collimator holes, passes through a hole and interacts by compton scatter within the crystal. The scattered gamma ray can either interact a second time and deposit all its energy or escape the detector. Inaccurate position information. Object Scatter Event - answerGamma ray is not emitted towards collimator holes but is scattered within the body, then passes through a collimator hole and subsequently is detected. Gamma ray loses energy during interaction, and provides inaccurate position information. Leads to low spatial frequency background resulting in a loss of contrast. Septal Penetration Event - answerGamma ray is emitted towards collimator, but not parallel to it. Due to incomplete attenuation by septal walls, the gamma ray reaches the detector. Leads to image blurring, incorrect position information. Becomes important when using high energy gamma emitters or high resolution collimators with thin septa. Intrinsic Spatial Resolution - answerLimit of spatial resolution achievable by the detector and the electronics, without the collimator. What limits intrinsic spatial resolution? - answerMultiple scattering of gamma ray photons within the detector. Statistical fluctuation of distribution of light photons among the PMT's from one scintillation event to the next. Intrinsic spatial resolution worsens with higher or lower energy gamma rays? - answerLower; lower energy gamma rays produce fewer light photons per scintillation event, and smaller numbers of light photons result in larger statistical fluctuations in their distribution. Greater blurring with lower energies. How does intrinsic spatial resolution depend on crystal thickness? - answerThicker crystals result in greater spreading of scintillation light before it reaches the PMT. Also a greater likelihood of detecting multiple compton scattering events, particularly with higher energy radionuclides. Detection Efficiency - answerImproves with thicker crystals, at the expense of intrinsic spatial resolution. Decreases with higher en