Abstract:
An asymmetric sampling scheme for use with a nuclear medicine gamma camera facilitates collection of a full set of higher resolution emission data and lower resolution transmission data with one complete 360° rotation of the gantry. The gantry ( 16 ) contains a plurality of radiation detectors ( 20   a-   20   c ) and at least one adjustably mounted radiation source ( 30   a ). During a scan, the gantry ( 16 ) is incrementally rotated about a subject receiving aperture ( 18 ) by a predetermined step size throughout a first 180° of a rotation (P 1 , . . . , P 6 ). The gantry ( 16 ) is then rotated about the subject receiving aperture ( 18 ) by one-half the predetermined step size (P 7  or P 8 ). The gantry ( 16 ) is then incrementally rotated about the subject receiving aperture ( 18 ) by the predetermined step size throughout the remaining 180° of the scan (P 8 , . . . , P 2 ). Emission data collected during the second half of the scan (P 8 , . . . , P 12 ) is interleaved into the data from the first half of the scan. Such sampling reduces overall scan time by cutting the number of steps approximately in half, therefore, eliminating 50% of the dead time while the gantry rotates and comes to rest.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to the arts of nuclear medicine and diagnostic imaging. It finds particular application in conjunction with gamma cameras and will be described with particular reference thereto. It is to be appreciated that the present invention is applicable to positron emission tomography (PET), whole body nuclear scans, and/or other like applications. 
     Diagnostic nuclear imaging is used to study a radionuclide distribution in a subject. Typically, one or more radiopharmaceuticals or radioisotopes are injected into a subject. The radiopharmaceuticals are commonly injected into the subject&#39;s blood stream for imaging the circulatory system or for imaging specific organs which absorb the injected radiopharmaceuticals. Gamma or scintillation camera detector heads, typically including collimators, are placed adjacent to a surface of the subject to monitor and record emitted radiation. Often, the detector heads are rotated or indexed around the subject to monitor the emitted radiation from a plurality of directions. In single photon emission computed tomography (SPECT), emission radiation is detected by a single collimated detector. In positron emission tomography (PET), data collection is limited to emission radiation that is detected concurrently by a pair of oppositely disposed heads. The monitored radiation data from the multiplicity of directions is reconstructed into a three dimensional image representation of the radiopharmaceutical distribution within the subject. 
     One of the problems with these imaging techniques is that photon absorption and scatter by portions of the subject or subject support between the emitting radionuclide and the detector heads distort the resultant image. One solution for compensating for photon attenuation is to assume uniform photon attenuation throughout the subject. That is, the subject is assumed to be completely homogeneous in terms of radiation attenuation with no distinction made for bone, soft tissue, lung, etc. This enables attenuation estimates to be made based on the surface contour of the subject. However, human subjects do not cause uniform radiation attenuation, especially in the chest. 
     In order to obtain more accurate SPECT and PET radiation attenuation measurements, a direct, transmission radiation emission measurement is made using transmission computed tomography techniques. In this technique, radiation is projected from a radiation source through the subject. Attenuated radiation rays are received by detectors at the opposite side. The source and detectors are rotated to collect transmission data concurrently or sequentially with the emission data through a multiplicity of angles. This transmission data is reconstructed into an image representation using conventional tomography algorithms. Regional radiation attenuation properties of the subject and the support, which are derived from the transmission computed tomography image, are used to correct or compensate for radiation attenuation in the emission data. 
     PET measurements are typically made at incrementally stepped locations. One difficulty resides in optimizing the sampling of both the PET and SPECT emission data, and the transmission data so as to reduce overall scan time. Typically, PET emission data is acquired with greater resolution than transmission data (i.e., approximately 2-4° per step for collecting emission data versus about 5-8° per step for collecting transmission data). The total time to perform such a scan is composed of the time to actually acquire data at each angular orientation and the time to mechanically move the gantry from one angular orientation to another and stabilize it at the new orientation. 
     In a PET scan, the data is typically collected at angular increments of the desired resolution over the first 180° of revolution. For 3° of resolution, the data is collected at 60 steps each 3° apart. When using transmission radiation in a fan beam pattern and a 360° reconstruction algorithm, transmission data can be collected at 30 additional steps each 6° apart (and the corresponding emission data discarded) or both can be collected at 60 additional steps each 3° apart. After each indexing, there is a significant wait time while the physical position of the heads stabilizes. A wait time for stabilization of only 2-4 seconds per step adds 4-8 minutes to a 120 step scan. 
     The present invention contemplates a new and improved data sampling technique for gamma cameras which collect emission data and transmission data simultaneously which overcomes the above-referenced problems and others. 
     SUMMARY OF THE INVENTION 
     In accordance with one aspect of the present invention, a method of diagnostic imaging using a nuclear medicine gamma camera includes placing a subject in a subject receiving aperture and injecting the subject with a radiopharmaceutical. At least one radiation source and a plurality of radiation detectors are positioned about the subject receiving aperture such that the radiation source is across the subject receiving aperture from a corresponding radiation detector. Radiation from the radiation source is directed toward the corresponding radiation detector. The radiation source and radiation detectors are incrementally rotated about the subject receiving aperture by a predetermined step size throughout a first 180° of rotation about the subject receiving aperture. The radiation source and radiation detectors are then offset by one-half of the predetermined step size and incrementally rotated about the subject receiving aperture by the predetermined step size throughout the remaining 180° of one complete rotation about the subject receiving aperture. Radiation emitted by the injected radiopharmaceutical and transmitted by the radiation source is detected by the plurality of radiation detectors at each angular orientation corresponding to each step of incremental rotation about the subject receiving aperture. Emission projection data and transmission projection data are generated and reconstructed into a volumetric image representation. 
     In accordance with another aspect of the present invention, a positron emission tomographic camera includes a rotating gantry which defines a subject receiving aperture. A plurality of radiation detectors heads are movably attached to the rotating gantry such that the detector heads rotate about the subject receiving aperture with rotation of the rotating gantry. A rotational drive steps the plurality of detector heads around the subject receiving aperture in even steps where the steps in one half rotation are offset from the steps in the opposite half rotation by one half of the even step. At least one radiation source is mounted to at least one detector head such that transmission radiation from the radiation source is directed toward and received by a corresponding detector head positioned across the subject receiving aperture from the radiation source. An angular position sensor senses the angular position of the gantry and each detector head attached thereto as the gantry rotates about the subject receiving aperture. A reconstruction processor reconstructs the emission and transmission data into a volumetric emission image representation. 
     In accordance with another aspect of the present invention, a method of sampling transmission radiation data and emission radiation data for use in diagnostic imaging includes selecting a rotation increment for effective emission data imaging resolution. A radiation source and radiation detectors are incrementally rotated about a subject receiving aperture by a predetermined step size over a first 180° of one complete rotation about the subject receiving aperture. Further, the radiation source and radiation detectors are incrementally rotated about the subject receiving aperture by the predetermined step size over a remaining second 180° of one complete rotation offset by a half step relative to the first 180° of rotation. Radiation emitted by the radiopharmaceutical and the radiation transmitted by the radiation source are detected at each stepped angular orientation corresponding to each incremental rotation about the subject receiving aperture. 
     In accordance with a more limited aspect of the present invention, the sampling method includes interleaving emission radiation data detected during the second 180° incremental rotation with emission radiation data detected during the first 180° incremental rotation. 
     One advantage of the present invention is that it reduces gantry dead time. 
     Another advantage of the present invention is that it reduces by approximately one-half the number of steps for emission and transmission data acquisition as conventional sampling methods. 
     Another advantage of the present invention is that it provides greater resolution for PET emission than is required for transmission data. 
     Other benefits and advantages of the present invention will become apparent to those skilled in the art upon a reading and understanding of the preferred embodiment. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention may take form in various components and arrangements of components, and in various steps and arrangements of steps. The drawings are only for purposes of illustrating preferred embodiments and are not to be construed as limiting the invention. 
     FIG. 1 is a diagrammatic illustration of a nuclear medicine gamma camera in accordance with aspects of the present invention; 
     FIG. 2 is a diagrammatic illustration of a preferred orientation of heads in a three head nuclear medicine gamma camera in accordance with the present invention; 
     FIG. 3 is a perspective view of a preferred orientation of two heads of a nuclear medicine gamma camera in accordance with the present invention; and, 
     FIG. 4 is a diagrammatic illustration of an asymmetric sampling scheme in accordance with the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     With reference to FIG. 1, a diagnostic imaging apparatus includes a subject support  10 , such as a table or couch, which supports a subject  12  (shown in FIG. 2) being examined and/or imaged. The subject  12  is injected with one or more radiopharmaceuticals or radioisotopes such that emission radiation is emitted therefrom. Optionally, the subject support  10  is selectively height adjustable so as to center the subject  12  at a desired height. A first or stationary gantry  14  rotatably supports a rotating gantry  16 . The rotating gantry  16  defines a subject receiving aperture  18 . In a preferred embodiment, the first gantry  14  is advanced toward and/or retracted from the subject support  10  so as to selectively position regions of interest of the subject  12  within the subject receiving aperture  18 . Alternately, the subject support  10  is advanced and/or retracted to achieve the desired positioning of the subject  12  within the subject receiving aperture  18 . 
     Detector heads  20   a ,  20   b ,  20   c  are movably mounted to the rotating gantry  16 . The detector heads  20   a - 20   c  also rotate about the subject receiving aperture  18  (and the subject  12  when received) with the rotation of the rotating gantry  16 . The detector heads  20   a - 20   c  are radially and circumferentially adjustable to vary their spacing on the rotating gantry  16 , as for example, in the manner disclosed in U.S. Pat. No. 5,717,212. Separate translation devices  22   a - 22   c , such as motors and drive assemblies, independently translate the detector heads radially and laterally in directions tangential to the subject receiving aperture  18  along linear tracks or other appropriate guides. Preferably, a single motor and drive assembly controls movement of all the detector heads  20   a - 20   c  individually and/or as a unit. 
     Each of the detector heads  20   a - 20   c  has a radiation receiving face facing the subject receiving aperture  18 . Each head includes a scintillation crystal, such as a large doped sodium iodide crystal, that emits a flash of light or photons in response to incident radiation. An array of photomultiplier tubes receive the light and convert it into electrical signals. A resolver circuit resolves the x, y-coordinates of each flash of light and the energy of the incident radiation. That is to say, radiation strikes the scintillation crystal causing the scintillation crystal to scintillate, i.e., emit light photons in response to the radiation. The photons are received by the photomultiplier tubes and their relative outputs of the photomultiplier tubes are processed and corrected to generate an output signal indicative of (i) a position coordinate on the detector head at which each radiation event is received, and (ii) an energy of each event. The energy is used to differentiate between various types of radiation such as multiple emission radiation sources, stray and secondary emission radiation, transmission radiation, and to eliminate noise. In SPECT imaging, a projection image representation is defined by the radiation data received at each coordinate. In PET imaging, the detector head outputs are monitored for coincident radiation. From the position and orientation of the heads and the location on the faces at which the coincident radiation was received, a ray between the peak detection points is calculated. This ray defines a line along which the radiation event occurred. The radiation data is then reconstructed into a volumetric image representation of the region of interest. 
     For SPECT imaging, the detector heads  20   a - 20   c  include mechanical collimators  22   a ,  22   b ,  22   c , respectively, removably mounted on the radiation receiving faces of the detector heads  20   a - 20   c . The collimators preferably include an array or grid of lead vanes which restrict the detector heads  20   a - 20   c  from receiving radiation not traveling along selected rays in accordance with the data type being collected. 
     With reference to FIG.  2  and FIG.  3  and continuing reference to FIG. 1, at least one radiation source  30   a  is mounted to at least one detector head  20   a  such that transmission radiation (represented by the arrows  32   a ) from the radiation source  30   a  is directed toward and received by the corresponding detector head  20   b  positioned across the subject receiving aperture from the radiation source  30   a . It is to be appreciated that radiation sources may be mounted to two or all three detector heads. In a preferred embodiment, a collimator is employed at least on the detector head  20   a  which receives the transmission radiation  32   a . That is to say, the collimator  22   b  restricts the detector head  20   b , (in the embodiment of FIG. 2) from receiving those portions of transmission radiation not traveling along direct rays in parallel planes normal to the radiation receiving face of the detector head. Alternately, other collimation geometries are employed for different applications and radiation sources, such as a point source. Additional collimation may take place at the source. 
     FIG. 2 illustrates a three-head embodiment, including a first detector head  20   a , a second detector head  20   b , and a third detector head  20   c  arranged on the rotating gantry  16  spaced from one another around the subject receiving aperture  18 . A first radiation source  30   a  is mounted to the first detector head  20   a  such that transmission radiation  32   a  therefrom is directed toward and received by the second detector head  20   b . Preferably, for positron emission tomography (PET), two detector heads  20   a ,  20   c  are used for coincidence detection of emitted radiation (represented by arrows  34 ), while the third detector head  20   b  is dedicated to receiving transmitted radiation from the radiation source  30   a.    
     In one embodiment, the radiation source  30   a  contains a radioactive point source  36   a  which adjustably mounted inside a shielded steel cylinder which is sealed at the ends. As shown in FIG. 3, the radiation source  30   a  rasters longitudinally across the field of view at each angular orientation of the rotatably gantry. The steel cylinder is adjustably mounted onto the corresponding detector head through a pivoting arm mechanism. Alternately, the radiation source  30   a  is a bar source, flat rectangular source, disk source, flood source, tube or vessel filled with radionuclides, or active radiation generators such as x-ray tubes. 
     With reference again to FIG. 1, as the gantry  16  rotates about the subject receiving aperture  18 , an angular position sensor  58  senses or indexes the angular position of the rotatable gantry, and therefore the angular positions of the respective detector heads  20   a - 20   c , when each set of data is collected. The discrete index of angular positions recorded by the angular position sensor  58  is used for indexing the recorded mission and transmission data. In one embodiment, the angular position sensor is an optical sensor which senses the position of the gantry by projecting light, such as a laser beam, through a particulate outer ring on the gantry and receiving the projected light. It is to be appreciated that conventional mechanical means, such as rotating gears, are also employed for detecting angular orientation of the rotatable gantry. 
     Running an imaging operation includes a reconstruction process for emission and transmission data. The reconstruction technique changes according to the type of radiation collected and the types of collimators used (i.e., fan, cone, parallel beam, and/or other modes). Emission radiation from the subject  12  and transmission radiation  32   a  from the radiation source  30   a  are received by the detector heads  20   a - 20   c , and emission projection data and transmission projection data are generated. The emission data normally contains inaccuracies caused by varying absorption characteristics of the subject&#39;s anatomy. A sorter  60  sorts the emission projection data and transmission projection data such as on the basis of their relative energies or the detector head which originated the data. The data is stored in a projection view memory  62 , more specifically in a corresponding emission data memory  62   e  and transmission data memory  62   t . A reconstruction processor  64   t  uses a fan beam reconstruction algorithm to reconstruct the transmission data into a transmission image representation or volume of attenuation factors stored in a memory  66 . Each voxel value stored in the memory  66  is indicative of attenuation of tissue in a corresponding location within the subject  12 . 
     An emission data trajectory processor  68  determines the trajectory of each emission data ray relative to the volume image representation in memory  66  and causes the attenuation values along the ray to be retrieved. Those skilled in the art will appreciate that in PET imaging, a projection at a given angular orientation and its 180 degrees-opposed counterpart are substantially equivalent. The nature of PET data and the appropriate sampling scheme for acquiring such data will be described below in greater detail. 
     An emission data correction means  70  corrects each emission data in accordance with the attenuation factors determined from the transmission data. More specifically, for each ray along which emission data is received, the emission data trajectory processor  68  calculates a corresponding ray through the transmission attenuation factors stored in the memory  66 . Each ray of the emission data is then weighted or corrected by an emission data correction processor  70  in accordance with the attenuation factors. The corrected emission data are reconstructed by an emission radiation reconstruction processor  72  to generate a three-dimensional emission image representation that is stored in a volumetric emission image memory  74 . A video processor  76  withdraws selected portions of the data from the image memory  74  to generate corresponding human-readable displays on a video monitor  78 . Typical displays include reprojections, selected slices or planes, surface renderings, and the like. 
     With reference to FIG.  4  and continuing reference to FIG. 1, the present invention utilizes an asymmetric sampling scheme which facilitates simultaneous or sequential collection of a complete set of emission and transmission data with one complete 360° rotation of the gantry about the subject. Artisans will appreciate that in coincidence detection for PET imaging, whenever two detector heads detect a radiation event concurrently, the event is mathematically assumed to have occurred on a ray  34  between the two detection points. In other words, a projection and its 180°-opposed counterpart are substantially equivalent. It will be appreciated that because the emission data is symmetric, the detector heads need only be rotated 180° for a complete sampling of emission data. To generate a complete fan beam data set, the transmission radiation data is collected over more than 180°, in the preferred embodiment over 360°. Optionally, the transmission data can be collected over 180° plus the fan angle. 
     For a given PET imaging sequence, emission data typically is typically sampled with a finer sampling (i.e., less angular rotation of the gantry per sampling step) than transmission data. In a preferred embodiment, the emission data has a 3° per step resolution, while the transmission data has a 6° per step resolution. Artisans will appreciate that because the transmission source is on the same gantry as the coincidence detector heads, it will have the same sampling as the emission data collection. For the purposes of illustration, FIG. 4 provides a sampling scheme for collection of both emission and transmission data during one complete 360° rotation around the subject. In this illustration, emission data resolution of 15° per step and transmission data resolution of 30° per step have been chosen. 
     In FIG. 4, each ray P 1 , . . . , P 12  represents an angular orientation at which an emission projection and transmission projection are sampled. For the first 180° of gantry rotation, emission data and transmission data are collected as described above at 30° intervals or steps P 1 , P 2 , . . . , P 6 . At the 180° point in the scan, the gantry is stepped at a single step of one-half the previous step size, 15° in this illustration. For the remaining second half of the scan, the gantry is stepped at the original step size of 30° P 7 , P 8 , . . . , P 12 . At this point, a complete transmission 360° fan beam data set exists with the desired 30° resolution. However, the emission data set over the first 180° is incomplete because it is sampled at 30° per step instead of the desired 15° per step. Because the emission data is symmetric, it is to be appreciated that the emission data collected during the second 180° of the scan P 7 , P 8 , . . . , P 12  is projected or interleaved into the first 180° P 7 ′, P 8 ′, . . . , P 12 ′, as shown. Therefore, the emission data set is provided by P 1 , P 8 ′, P 2 , P 9 ′, P 3 , P 10 ′, P 4 , P 11 ′, P 5 , P 12 ′, P 6 , P 13 ′. It is to be appreciated that this sampling scheme may be extended to situations where the desired transmission resolution is 1/n of the desired emission resolution. In such a case, data is sampled at steps corresponding to the transmission data resolution, using n 180° gantry rotations, with a step of 1/n at each 180° interval. For example, a scan having an emission data resolution of 15° per step and a transmission data resolution of 45° per step can be performed in 3 180° rotations having 45° steps, with a single one-third step of 15° after each 180° of rotation. Such an embodiment reduces artifacts due to patient motion and facilitates reconstruction of an image after 33%, 66% or 100% of the total acquisition time. Each additional 180° of rotation provides additional radiation counts and additional view angles, but each reconstructed image is complete. In the preferred embodiment, after one complete rotation, an operator collects a 180°, 3°/step emission data set and a 360° 6°/step transmission data set. Artisans will appreciate that the overall effect of this sampling scheme is to save approximately 50% of gantry dead time by requiring approximately one-half the number of steps. In addition, for PET and SPECT applications, the existence of two complete and semi-redundant acquisitions may provide means for estimating patient motion during a scan. It is to be further appreciated that the above sampling may be applied to SPECT applications by interleaving two 360° scans. This can be accomplished by performing two 180° rotations on a 180° opposed detector configuration, rotating by one half-step, and performing another 180° rotation. 
     The invention has been described with reference to the preferred embodiment. Modifications and alterations will occur to others upon a reading and understanding of the preceding detailed description. It is intended that the invention be construed as including all such modifications and alterations insofar as they come within the scope of the appended claims or the equivalents thereof.