embodiments of the present invention provide a method for positron emission tomography (“ pet ”) scanning with compensation for patient respiratory motion . other embodiments of the invention also provide for improved gating ( referred to hereinafter as “ optimal gating ”) of medical images . optimal gating , as disclosed herein , can be applied to radionuclide imaging modalities ( e . g ., pet and spect ). the present application incorporates by reference all of the material in u . s . patent application ser . no . 11 / 714 , 405 filed on mar . 6 , 2007 . embodiments of the invention can be performed using a pet scanner that can acquire image data in list mode . fig2 shows one example of a combination pet / ct apparatus that can be used with the present invention . the ct scanner provides a three dimensional image of patient anatomy , which is used to estimate the attenuation of the annihilation radiation imaged by the pet scanner , a well - understood procedure . the apparatus includes a ct scanner 16 a ( having detectors 12 ) and a pet scanner 16 b ( having detectors 14 ) in a common gantry ( although not shown , it is appreciated that in other embodiments of the invention , the ct scanner 16 a and the pet scanner 16 b can be in separate gantries ). a patient 17 lies on a patient bed 18 , that is movable between the ct 16 a and pet 16 b scanners . the patient &# 39 ; s respiration is monitored by a strain gauge 19 which is held against the patient &# 39 ; s thorax with a belt 20 . electrical signals from the strain gauge 19 are communicated through a cable 21 to a respiratory monitoring system 22 . the respiratory monitoring system is also shown as processor 510 in fig5 . the processor periodically samples and digitizes the strain measurements and inserts the digitized measurement into the pet data stream . synchronization of the respiratory amplitude measurements with the medical image measurements is essential . the latency associated with measurement insertion must be a small fraction of the respiratory cycle . this condition is realized routinely in modern pet and pet / ct systems , where the latency is a small fraction of one second . after the pet list mode data and strain level measurement data are acquired , the procedure advances to a computational process as shown in fig3 . in step 301 , a histogram of respiratory amplitude measurements is compiled , as shown in fig4 . at step 303 , a specified fraction parameter is defined , which is some major fraction of the entire histogram . next , at step 305 each lower respiratory amplitude level is considered , and a respiratory amplitude upper level is determined such that the interval defined by the lower and upper levels contains the predefined fraction of the entire histogram . this process is repeated for all lower strain levels . at step 307 , the lower and upper level pair is selected that has the narrowest range of level values , i . e ., the lower - upper level combination that minimizes the difference between the two levels . this process leads to an automatic recommendation of a strain levels pair that encompasses a high fraction of the total pet acquisition time , while at the same time corresponding to a relatively small amount of chest excursion . for example , equation ( 1 ) below represents an equation used in an algorithm that considers upper and lower strain levels . the algorithm considers all possible l values , and for each one it chooses a u ( l ) value which makes the sum between l and u ( l ) as close as possible to f . that is where f represents the user selected percentage and h ( s ) represents a respiratory signal level ( i . e ., amplitude ) in a histogram . this is illustrated in the pet scan respiration traces shown in fig4 . at step 309 , the operator is allowed to modify the recommended strain levels pair , by adjusting if desired either the lower level , upper level , both lower and upper level , or no level adjustment . at any point in time , the gate is either open or closed depending on the value of s ( t ). the gate is open if l ≦ s ≦ u , closed otherwise . medical image measurements made when the gate is open ( i . e ., pet events that occur when lsu ) are used to form the optimally gated medical image . the optimally gated medical image can be one of three types . the first type of optimally gated image is a static respiratory gated image , which uses events acquired when the respiratory gate is open but no other requirements are imposed . this type of image is made with no need to issue breathing instructions to the patient . the second type of optimally gated image additionally is a cardiac and respiratory gated image . this type of image is based on cardiac trigger signals present in the data list . it is well known in the state of the art that cardiac gated images can be formed by dividing the interval from one trigger to the next into several cardiac gates . data from a large number of heartbeats are combined into a single image . although this state of the art provides the ability to select just one phase of cardiac motion , all states of respiratory motion are present , which causes image blurring . it is possible in principle to arrest the respiratory motions by imaging only during a breath hold , but patients cannot hold their breath long enough to make a good image by pet or spect . by combining cardiac and respiratory gating criteria , a good image can be made in which image blurring due to breathing motions is largely eliminated , and breathing instructions are not needed . the third type of optimally gated image is a dynamic respiratory gated image . in the state of the art , dynamic imaging is used to separately frame data acquired in different time periods of the acquisition , so that one can observe changes in tracer distribution from the beginning to the end of the scan . for example , in a ten - minute scan with two - minute time frames , the first frame is based on all events acquired in the first two - minute period of the scan , the second frame is based on all events acquired in the second two - minute period of the scan , and so on . however , in this state of the art , each dynamic frame is degraded by breathing motion . by combining time framing and respiratory gating criteria , a good image can be made in which image blurring due to breathing motions is largely eliminated . fig5 depicts an embodiment of a high - level block diagram of a general - purpose computer architecture 500 for providing optimal gating in accordance with embodiments of the invention . the general - purpose computer of fig5 includes a processor 510 as well as a memory 504 for storing control programs and the like . in various embodiments , memory 504 also includes programs ( e . g ., depicted as an “ optimal gating module ” 512 for creating pet images ) for performing the embodiments described herein . the processor 510 cooperates with conventional support circuitry 508 such as power supplies , clock circuits , cache memory and the like as well as circuits that assist in executing the software routines 506 stored in the memory 504 . as such , it is contemplated that some of the process steps discussed herein as software processes can be loaded from a storage device ( e . g ., an optical drive , floppy drive , disk drive , etc .) and implemented within the memory 504 and operated by the processor 510 . thus , various steps and methods of the present invention can be stored on a computer readable medium . the general - purpose computer 500 also contains input - output circuitry 502 that forms an interface between the various functional elements communicating with the general - purpose computer 500 . although fig5 depicts a general - purpose computer 500 that is programmed to perform various control functions in accordance with the present invention , the term computer is not limited to just those integrated circuits referred to in the art as computers , but broadly refers to computers , processors , microcontrollers , microcomputers , programmable logic controllers , application specific integrated circuits , and other programmable circuits , and these terms are used interchangeably herein . in addition , although one general - purpose computer 500 is depicted , that depiction is for brevity on . it is appreciated that each of the methods described herein can be utilized in separate computers . in other embodiments of the invention , the respiratory amplitude is measured by an instrument other than a strain gauge . for example , devices commonly used in radiation therapy use digital cameras to optically track of the position of a marker placed on the patient &# 39 ; s abdomen . associated circuitry and computers in these devices supply a respiratory amplitude measurement which is communicated to the imaging system . another example , commonly used in standalone magnetic resonance imaging scanner examinations ( mri ), is a pneumatic device which generates an electronic signal corresponding to the air pressure in a flexible bladder held against the thorax and held tightly in place with a strap . another example occurs in the case of a pet scanner operating in the field of view of an mri scanner . in this case , routinely available mri imaging sequences and image processing hardware software provide many images per second of anatomical landmarks that move with the patient &# 39 ; s respiration . computerized methods identify the positional coordinates of the anatomical landmark in each image and convert this information to a respiration - correlated amplitude measurement that varies with time as the patient breathes . a familiar version of this technology is the so - called mri navigator , which follows the respiratory motions of the patient &# 39 ; s diaphragm . while the foregoing is directed to embodiments of the present invention , other and further embodiments of the invention may be devised without departing from the basic scope thereof , and the scope thereof is determined by the claims that follow .
methods and computer - readable mediums are provided for obtaining an optimally gated medical image . for example , in one embodiment , a method is provided that acquires medical images in list mode . the method also acquires a respiration correlated signal s . thereafter , a final upper strain threshold value and a final lower strain threshold value pair that has a narrowest interval are selected . the medical images are synchronized with the respiration correlated signal s . the synchronized images and signal are used to create an optimally gated medical image . in various embodiments , the disclosed optimal gating can be utilized in pet systems and in other embodiments the disclosed optimal gating can be utilized in spect systems . in yet other embodiments , the optimally gated images can be matched to mri systems and in still other embodiments , the optimally gated images can be matched to ct systems .