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Scattered photons degrade nuclear medicine image contrast and resolution, and preclude simple attenuation corrections. Current scatter correction methods utilize detected events with energies below pulse height analyzer (PHA) window levels, making attenuation corrections source position dependent. This new scatter rejection technique analyzes only the photon signals occurring within the range of standard PHA windows. In real time, at each image location the PHA window energy distribution is analyzed, a scatter fraction determined, and a scatter corrected number of events are output. The method can be adapted to any imaging system which produces event location and energy signals. Attenuation corrections (u=0.15 cm(-1), 140 keV) are within 6% for 2-10 cm depths.

I ntroduction Objectives and scope Method of study and available information Description of the area and its climate Major aquifer units of western Oregon Genera I Pa Ieozoic-Mesozoic bedrock and granitic saprol ite of the Klamath Mountains Tertiary rocks of the Coast Range Columbia River Basalt Group Tertiary volcanic rocks of the Western Cascade Range Tertiary-Quaternary volcanic rocks of the High Cascade Range Tertiary-Quaternary sedimentary deposits 17 Descriptive aquifer-unit information by geographic area 20 Selected references 21 Explanation of we I I-number ing system 27 Tabulated hydrogeolog ic information 29 vii Storage coefficient. The volume of water an aquifer releases from or takes into storage per unit surface area of the aquifer per unit change in head. Transmfssfvity. The rate at which water of prevailing kinematic viscosity is transmitted through a unit width of the aquifer under a unit hydraulic gradient. It is equal to an integration of the hydraulic conductivities across the saturated part of the aquifer perpendicular to the flow paths. Water table. That surface in a groundwater body at which the water pressure is atmospheric. It is defined by the levels at which water stands in wells that peneterate the water body just far enough to hold standing water. vi 11 A DESCRIPTION OF AQUIFER UNITS IN WESTERN OREGON