Nuclear Medicine Final Exam Basic Gamma Camera Components verified answer Collimator NaI scintillation crystal light guide PMT array How does the collimator work verified answer Controls 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 verified answer Position and energy information could also contain time information Reflective Material Around Crystal verified answer Maximizes light output Aluminum Casing Around Crystal verified answer Hermetically seals crystal to protect from moisture hydroscopic Crystal Thickness is a Tradeoff Between verified answer Detection efficiency and intrinsic spatial resolution Relationship of light detected by PMT verified answer Amount 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 verified answer Utilize 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 verified answer Discriminate 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 verified answer Image is inverted and magnified Small image FOV Septa verified answer Prevent gamma rays from crossing from one hole to the next in the collimator Diverging Collimator verified answer Minified non inverted image Large image FOV Converging Collimator verified answer Magnified non inverted image Valid Event verified answer Gamma 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 verified answer Gamma 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 verified answer Gamma 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 verified answer Gamma 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 verified answer Limit of spatial resolution achievable by the detector and the electronics without the collimator What limits intrinsic spatial resolution verified answer Multiple 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 verified answer Lower 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 verified answer Thicker 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 verified answer Improves with thicker crystals at the expense of intrinsic spatial resolution Decreases with higher energy gamma rays Energy Resolution verified answer Determines the efficiency with which detector can perform pulse height analysis Depends largely on the statistical fluctuations in the number of light photons collected from a scintillation event good light collection efficiency is needed Advantages of improved energy resolution verified answer Narrower photopeak more efficient detection of unscattered photons within the chosen energy window Gamma rays scattered through large angles are rejected more efficiently thus improving image contrast Both result in improved image quality Performance at high counting rates verified answer Most gamma cameras have paralyzable systems Dead time losses can occur with high counting rates Effects of pulse pile up on gamma camera performance verified answer Counting losses and image distortion Image Nonlinearity verified answer Pincushion and barrel distortion Occurs when XY position signals do not change linearly with displacement distance of a radiation source across the face of the detector Image Nonuniformity verified answer Can be evaluated using flood Causes include nonuniform detection efficiency and nonlinearities Nonuniformity Correction Techniques verified answer Spatially varying energy corrections using intrinsic flood Tune PMT to ensure consistent performance over time Limiting factor in gamma camera performance verified answer Collimator Collimator Resolution verified answer Defined as the FWHM of the radiation profile from a point or line source of radiation projected by the collimator onto the detector PSF or LSF Collimator resolution improves with verified answer The ratio of septal hole diameter to effective length is made smaller Long narrow holes provide better resolution Collimator resolution degrades with verified answer Increasing source to collimator distance As this distance increases the max height of the PSF decreases but width increases so total area under the curve does not change although resolution is degraded Invariance of collimator efficiency and source to collimator distance verified answer Collimator efficiency is independent of source to collimator distance transmission decreases according to inverse square law but number of holes through which radiation can pass increase in proportion to this distance Effects cancel System