Patent Number: 056299710
Section: summary

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to the field of nuclear medicine. Particularly, the present invention relates to the field of transmission scanning to provide nonuniform attenuation correction within a gamma camera system. 2. Background of the Invention Non-uniform photon attenuation is an important factor that affects the quantitative accuracy of images collected using Single Photon Emission Computerized Tomography (SPECT) camera systems and can decrease the sensitivity of these systems for lesion detection. Nonuniform photon attenuation distorts images by interfering with and partially absorbing the radiation emitted from an organ containing a radio-pharmaceutical. Photon attenuation within SPECT systems tends to degrade images by also introducing image artifacts and other distortions that can result in false positive detection or the undetection of lesions. The effects of photon attenuation are especially complex in cardiac studies as a result of nonuniform attenuation attributed to the thorax. In transmission scanning, the source of radiation is directed toward the associated scintillation detector through the object of interest or patient. If the radiation field is significantly larger than the patient, the radiation source is allowed to directly radiate the detector, causing a high count rate in the scintillation detector. Those parts of the detector that become directly radiated are called unobstructed portions of the detector. It is not advantageous to allow large unobstructed detector areas because the resultant increase in count rate can lead to image degradation and in some cases the event detection electronics and processes can become overloaded (e.g., due to pulse pile-up) and temporarily terminate operation. These high count rates tend to reduce the imaging performance of the imaging system by loading down the signal detection and processing circuitry of the gamma camera. Transmission computed tomography (TCT) can be used as a method for generating a nonuniform attenuation correction distribution. The transmission image data is gathered using a known source (e.g., line, sheet, or flood) of radiation. If performed separately from the SPECT emission study, the collection of the transmission data requires additional data acquisition time and the collection of the transmission and emission data is susceptible to misregistration effects due to patient (e.g., "object") movement between the data gathering sessions. However, whether the transmission data is collected simultaneously with the collection of the emission data or not, the patient is generally exposed to additional radiation in order to collect the transmission data. Depending on the size and shape of the patient, different amounts of transmission radiation are required in order to collect the minimum required amounts of transmission data. Systems overexpose the patient with transmission radiation in order to obtain the minimum required amount without regard to the shape or size of the patient. What is needed is a system that can effectively minimize the exposure period of the patient to radiation utilized in collection of the transmission data by considering the particular size and shape contributions of the patient. The present invention offers such advantageous solutions. Accordingly, it is an object of the present invention to provide improvements within gamma camera systems utilizing nonuniform attenuation correction techniques for improved image quality generation. It is an object of the present invention to determine the minimum exposure time required for the most attenuating portion of a scanned object during a transmission study. It is an object of the present invention to provide the above while considering the size and shape contributions of a particular patient. It is another object of the present invention to provide a reduced radiation exposure period for generation of transmission data used in creating a nonuniform attenuation correction distribution. These and other objects of the present invention not specifically mentioned above will become clear upon discussions of the present invention herein. SUMMARY OF THE INVENTION A scan speed procedure and method is described wherein a minimum radiation exposure period is determined on an object by object basis for a transmission study. Two transmission scan phases are performed including a prescan followed by a second transmission scan phase. The first scan consists of a transmission prescan performed using a radiation source over the object. This prescan is of a rapid and predetermined duration (Tp). A resultant count density associated with the object is then generated and examined. The portion having the smallest count density (Cm) is determined and a value (Co) representing the minimum required number of counts for transmission study is given. From the above, a transmission period (Ts) is determined by the system and the second transmission phase (a multi-pass scan phase) is performed for the duration Ts. The computed duration Ts represents the minimum exposure duration required to collect transmission data to ensure that the count density distribution associated with the object will contain at least Cm count density over each portion of the object. Specifically, embodiments of the present invention include an mechanism for collecting transmission data associated with an object, the mechanism including: a scintillation detector for receiving radiation and reporting image information; a line source of radiation for emitting transmission radiation through the object to the scintillation detector; a processor coupled to receive information from the scintillation detector and coupled to control the line source of radiation, the processor for controlling the line source of radiation to perform a first transmission prescan phase over the object for a first duration and generating in response thereto a first count density information; the processor for computing a second duration for a second transmission scan phase based on the first count density information; the processor for controlling the line source of radiation to perform the second transmission scan phase over the object for the second duration and generating in response thereto a second count density information, the second duration representing a minimum duration of exposure of the object required for generating the second count density information; the processor for generating a set of attenuation correction factors based on the second count density information, the set of attenuation correction factors for use in correcting image data gathered by the scintillation detector during emission scanning; and a memory unit coupled to store and provide access to the first and the second count density information. Embodiments of the present invention include the above and wherein the second count density information is a distribution over different portions of the object and wherein the processor is for receiving, as input, a minimum count density wherein the distribution contains at least the minimum count density for each portion of the object, wherein the processor is for analyzing the first count density information and determining a minimum observed count density in response thereto wherein the minimum observed count density corresponds to a portion of the object that exhibits maximum attenuation and wherein the processor determines the second duration according to the procedure: ##EQU1## wherein Ts is the second duration, Co is the minimum observed count density, Tp is the first duration, and Cm is the minimum count density. Embodiments of the present invention include in a nuclear camera system having a pair of scintillation detectors for receiving radiation and reporting image information; a pair of radiation line sources each for emitting transmission radiation through an object to an associated scintillation detector; and a computer system, a computer implemented method of collecting transmission data associated with the object, the method including the steps of: obtaining a minimum count density value desired for each portion of a resultant count density distribution; controlling the pair of radiation line sources to perform a first transmission prescan phase over the object for a first predetermined duration and generating in response thereto a first count density distribution; computing a second duration for a second transmission scan based on the first count density distribution; controlling the pair of radiation line sources to perform the second transmission scan phase over the object for the second duration and generating in response thereto the resultant count density distribution, the second duration representing a minimum duration of exposure of the object required to generate the second count density distribution; and generating a set of attenuation correction factors based on the second count density information, the set of attenuation correction factors for use in correcting image data acquired during emission scanning.