Abstract:
A method for tomographic nuclear imaging determines the distance between a non-parallel hole collimator surface and a region of interest (ROI) by obtaining difference images between images acquired at different view angles of the ROI. The distance may be used in a nuclear image reconstruction algorithm to more accurately reconstruct an image of the ROI. The method takes advantage of the non-stationary Point Spread Function of a non-parallel hole collimator to determine depth information of gamma events emitted from the ROI.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of Invention 
         [0002]    The invention relates generally to imaging systems and more particularly to imaging systems for use in nuclear medicine. 
         [0003]    2. Description of Related Art 
         [0004]    In a nuclear medicine imaging device, such as a gamma camera for obtaining either planar images or Single Photon Emission Computed Tomography (SPECT) images, a collimator is mounted to the face of the imaging device. The collimator is constructed of a dense, high-atomic-number material, such as lead. The material is bored with numerous tiny straight holes that allow radiation (e.g., gamma rays) to pass through. If the radiation is not traveling along the path of the hole, then the material absorbs it and it will not reach the detector. The collimator thus collimates radiation, which is emitted from a distributed source (e.g., a radiopharmaceutical or radioisotope chosen for its affinity for a particular organ, tissue or region of the body) within a patient, before the radiation strikes a detector crystal. 
         [0005]      FIG. 2  is a block diagram of an exemplary SPECT or planar imaging device. A radiation source  302  within an object to be imaged  304  (e.g., human body part) emits gamma photons that emanate from the object  304 , pass through the collimator  308 , and are captured by a detector  306 , usually a large flat crystal of sodium iodide with thallium doping in a light-sealed housing, that converts the detected radiation into spatial projection data. The system accumulates counts of gamma photons that are absorbed by the crystal in the detector  306 . The crystal scintillates in response to incident gamma radiation. When an absorbed gamma photon releases energy, it produces a faint flash of light. This phenomenon is similar to the photoelectric effect. Photomultiplier tubes (PMT) behind the crystal detect the fluorescent flashes and convert them into electrical signals, and a computer  310  sums the fluorescent counts. The computer  310  in turn constructs and displays a two dimensional image of the relative spatial count density or distribution on a monitor. This image then reflects the distribution and relative concentration of radioactive tracer elements present in the organs and tissues imaged. The two dimensional images are also referred to as planar images because they are taken from only one angle and are similar to an x-ray radiograph. 
         [0006]    In order to obtain spatial information about the gamma emissions from an imaging object, a method of correlating the detected photons with their point of origin is required. Single Photon Emission Computed Tomography (SPECT) captures multiple images from multiple angles in order to reconstruct a three-dimensional representation of the region of interest (ROI). SPECT is usually performed using a parallel-hole collimator. The parallel-hole collimator does not provide any depth information as to the spatial origin of a gamma incident on its face. Thus, reconstructing the image in three dimensions requires processing multiple planar images of the ROI from multiple view angles in a manner well known in the art to yield a human-readable, three-dimensional image of the object. Because of the need to acquire planar images from multiple view angles sufficient to reconstruct tomographic images, the required scan time is relatively long. 
         [0007]    A varying focal-length or multi-focal collimator (MFC) has multiple focal points for axial and transaxial detector directions. The advantage of a MFC is that it enables the imaging of small organs or regions within a field of view (FOV) of a gamma camera detector to be imaged faster than with use of a parallel-hole collimator, as the acquired projection data can be limited to the ROI within the larger FOV. Tomographic reconstruction methods for MFC acquired projection data are known. However, such methods use such projection data in the same way that projection data are used in parallel-hole collimator imaging. 
         [0008]    In particular, MFC reconstruction methods fail to take into account that the Point Spread Function (PSF) of a detector with a MFC is non-stationary with respect to the position of the source relative to the MFC collimator surface, in contrast with parallel-hole collimators, where the PSF is static with respect to the source position relative to the collimator surface (i.e., in the axial-transaxial direction). The PSF describes the response of the detector to a point source of radiation. In a parallel hole collimator, the PSF of the detector is static with respect to the location of a point source vis-a-vis the collimator. In contrast, the PSF of a non-parallel hole collimator is non-stationary, meaning that the PSF of an MFC detector varies as the test point source is shifted in position with respect to the collimator, in all directions. 
         [0009]    Thus, the possibility exists for improvement in image quality by taking into account the depth information available from consideration of the fact that PSF in a MFC is non-stationary. 
       SUMMARY OF THE INVENTION 
       [0010]    An object of the invention is to take advantage of the non-stationary nature of PSF in multi-focal collimator nuclear detectors. By using non-parallel hole collimators, e.g., multi-focal, varying-focal length, fan beam or astigmatic collimators, the Point Spread Function (PSF) is no longer stationary, as it is in a parallel hole collimator, and depth information is thus encoded in difference images of the ROI in multiple locations. One aspect of the invention is to decode the depth information from the difference images. 
         [0011]    According to one embodiment, a subject is injected with, for example, a radioisotope and is imaged with a gamma camera. Using a non-parallel hole collimator, two or more images may be made of the subject at different locations by changing the position of the subject relative to the collimator surface. This may be done by either moving the subject or by moving the collimator. The two or more images may be combined to show the differences between them. This combination is referred to as a difference image. By examining the difference images, the depth of the gamma events may be determined, for example, by using a chi squared fit algorithm. 
         [0012]    As is known in the art, the depth information may then be used in a reconstruction algorithm to produce a more accurate tomographic image based on the projection data received during the nuclear imaging process. An accurate determination of the depth information will yield a more precise reconstruction of the image. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]      FIG. 1  is an illustrative representation of a patient inside a nuclear imaging apparatus. 
           [0014]      FIG. 2  is an illustrative block diagram of a nuclear system. 
           [0015]      FIG. 3  is an illustrative representation of the possible types of detector motion around a gantry center. 
           [0016]      FIG. 4  is an illustration of images taken of an object at different locations relative to a collimator surface, images taken after shifting the object in a plane parallel to the collimator surface, and resulting difference images, in accordance with an embodiment of the present invention. 
           [0017]      FIG. 5  is block diagram of a process for taking two images, forming a difference image, and calculating a distance to an ROI based on the difference image. 
           [0018]      FIG. 6  is a block diagram of a computer for processing images taken of an object at different locations relative to a collimator surface, images taken after shifting the object in a plane parallel to the collimator surface, and resulting difference images, in accordance with an embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
       [0019]    The following description is presented to enable any person skilled in the art to use a method to efficiently produce superior reconstructed images using, for example, planar imaging or Single Photon Emission Computed Tomography (SPECT). Various modifications to the preferred embodiments will be readily apparent to those skilled in the art, and the generic principles defined herein may be applied to other embodiments and applications without departing from the spirit and scope of the invention. In the following description, numerous details are set forth for the purpose of explanation. However, one of ordinary skill in the art will realize that the invention might be practiced without the use of these specific details. In order to more efficiently illustrate and describe embodiments of the invention, identical reference numerals are used in the specification and drawings to identify parts that are essentially the same in different stages, versions or instantiations of such parts shown in the drawings. Thus, the present invention is not intended to be limited to the embodiments shown, but is to be accorded the widest scope consistent with the principles and features disclosed herein. 
         [0020]      FIG. 1  depicts an exemplary embodiment of a patient  13  on a patient bed  15  inside a nuclear imaging apparatus  10 . Two collimators,  12  and  14 , are located around the patient  13 . The collimators  12  and  14  collimate radiation from the patient to be imaged. The collimated radiation is detected using a crystal (not shown) which scintillates in response to incident gamma radiation. The collimators  12  and  14  may rotate around the patient as shown by arrow  23 . The collimators  12  and  14  may also shift or swivel. In addition, the patient bed  15  may be capable of moving by rotating, shifting, or swiveling. The motion of the patient bed  15  and/or the collimators  12  and  14  facilitates taking images of the patient  13  from different angles and orientations. In accordance with the present invention, a non-parallel hole (e.g., multifocal) collimator is used to acquire image data of a patient at multiple orientations. 
         [0021]      FIG. 3  depicts examples of possible detector movements. Each of the three diagrams illustrates the motion of one detector with respect to the center of the gantry. The top example, labeled A, depicts the collimator/detector rotating around a gantry center. The center example, labeled B, depicts the collimator/detector swiveling around its central axis in addition to rotation about the gantry center. Finally, the bottom example, labeled C, depicts the collimator/detector shifting or translating the detector center with respect to the gantry center, in addition to rotation about the gantry center and swiveling about the detector central axis. The motions may occur independently or concurrently. 
         [0022]    A first image of a subject within the gantry may be taken with the multi-focal collimator in a first position with respect to the subject, and a second image may be taken with the collimator in a second position. The images may be combined to form difference images. For example, the imaging subject may lie at a distance z 0  from the surface of the collimator and a point (x 0 , y 0 , z 0 ). Next, the subject can be moved along one axis while keeping the subject at the same position along the other two axes by moving the subject to a point (x 0 , y 1 , z 0 ). The combination of the images taken at the two points reveals a difference image. Due to the unique characteristics of the PSF at the distance z 0  from the surface of the collimator, the difference image may then be examined to determine the distance of the subject from the collimator surface using, for example, a chi-squared fit algorithm. 
         [0023]    An embodiment of the present invention is based on the fact that in a multi-focal collimator, the PSF is dependent on the location of a gamma event with respect to the collimator surface (in the axial and trans-axial directions). Thus, depth information is available and is encoded in a difference image at different angular views of a target object. In accordance with an embodiment of the invention, difference images obtained as the object moves through the MFC FOV at z 0  are uniquely different from difference images obtained as the object moves through the MFC FOV at z 1 ≠z 0 . Thus, tomographic information can be extracted from the projection data as the projection views are being accumulated, and a tomographic image can be formed before the full detector orbit scan around the patient has been completed. 
         [0024]    A chi-squared fit algorithm may be used to compare an expected difference image at a certain distance with the actual obtained image. However, it should be appreciated by those skilled in the art that other types of algorithms can be used without departing from the scope of the invention. The expected image may be calculated because the PSF at each distance from the collimator surface is known and is unique with respect to different distances from the surface of the collimator due to the non-parallel orientation of the collimator holes. Because the PSF is unique at different distances from the surface of the collimator, the expected difference between images of the subject at the two points (x 0 , y 0 , z n ) and (x 0 , y 1 , z n ) can be determined for different distances from the surface of the collimator (i.e., at different z n ). Using this information, the difference between two images of the subject at different positions may be predicted at different distances from the collimator surface. Thus, the depth of an ROI may be estimated within 1 cm and thus eliminate trial-and-error, and also enable applications that require planar images with a MFC. 
         [0025]    The chi squared fit algorithm enables an iterative process of estimating the distance of the subject from the collimator surface until one iteration of the algorithm results in a best fit. In other words, a best fit can mean the expected difference between the images is approximately equal to the actual recorded difference. When a best fit is found, in the case of SPECT imaging, the distance of the ROI to the collimator surface may be used as an input into a reconstruction algorithm to produce a more precisely reconstructed image of the subject. This process may occur iteratively in the case of SPECT imaging because many pictures are taken to render a three dimensional reconstruction. Therefore, the distance calculation may be made several times in order to determine a more precise distance. 
         [0026]      FIG. 4  is an example of images of a subject at the same distance from the surface of a collimator, but at two different locations, and difference images. The subject is a cylindrical object, with open side pointing right, at two different distances from a collimator surface. Each of the six images contains a composite of fifteen images of the object at different locations. The left column depicts the object at 20 cm from the collimator surface and the right column depicts the object at 30 cm from the collimator surface. As can clearly be seen, the difference between the images in the same location but shifted up or down (e.g., left column versus right column) significantly changes the appearance of the object and is unique to the distance from the collimator surface. The top images depict the object in fifteen different locations but in the same plane parallel to the collimator surface. The second row depicts the same object, but shifted horizontally 4.8 mm. It is only important that the object is shifted; it is not important how much it is shifted. The third row depicts difference images generated by combining images from the top two rows in the same column. 
         [0027]    The difference images in the bottom row of  FIG. 4  may be used in the iterative chi-squared fit algorithm discussed above to determine the distance of the cylindrical object from the collimator surface. The distances may then be used as an input into a reconstruction algorithm to efficiently produce a more accurately reconstructed image. Many reconstruction algorithms exist in the art and are well known, and therefore will not be further described here. Many of these reconstruction algorithms use the distance between the subject and the collimator surface as an input. Thus, the method described above to yield an accurate distance measurement may be used in these algorithms to reconstruct a more precise image. 
         [0028]    To further illustrate the process used in one embodiment,  FIG. 5  describes obtaining two images of the reconstruction area using a multi-focal collimator (MFC). The ROI in the reconstruction area is determined and a difference image is obtained. The difference image is then used to determine the distance of the RIO from the surface of the collimator and used in a reconstruction algorithm. The process may occur iteratively, for example in a SPECT imaging process, until all necessary images are taken and the process can stop. 
         [0029]    More specifically, the process  500  is initiated at step  501  where during bed-in movement, MFC stationary projection data is acquired. At steps  502  and  503 , a ROI is determined in one image frame following the shape of the target organ. The process identifies the shape of the target based on the initial ROI. The process is repeated at steps at steps  504 ,  505  and  506 . The process creates a new shape based ROI based on the projected target organ in that frame, which is slightly distorted. This can be performed via a segmentation method. At step  507 , the difference between ROIs is compared. At step  508 , an improved determination of likely depth d is provided. The gradual shape deformation of subsequent translated projection images is used to compute the most likely depth given the known MFC characteristics, consistent with all shapes. This can be performed by some iterative scheme where some appropriate objective function, for instance the L2 measure of the difference images is minimized. The process proceeds to step  509  where a determination is made whether to repeat the process. If not repeated, the process is terminated at step  510 . 
         [0030]    Referring now to  FIG. 6 , according to an embodiment of the present invention, a computer system  601  for implementing the present invention can comprise, inter alia, a central processing unit (CPU)  602 , a memory  603  and an input/output (I/O) interface  604 . The computer system  301  is generally coupled through the I/O interface  604  to a display  605  and various input devices  606  such as a mouse and a keyboard. The support circuits can include circuits such as cache, power supplies, clock circuits, and a communication bus. The memory  603  can include random access memory (RAM), read only memory (ROM), disk drive, tape drive, etc., or a combinations thereof. The present invention can be implemented as a routine  607  that is stored in memory  603  and executed by the CPU  602  to process the signal from the signal source  608 . As such, the computer system  601  is a general purpose computer system that becomes a specific purpose computer system when executing the routine  607  of the present invention. 
         [0031]    The computer system  601  also includes an operating system and micro instruction code. The various processes and functions described herein can either be part of the micro instruction code or part of the application program (or combination thereof) which is executed via the operating system. In addition, various other peripheral devices can be connected to the computer platform such as an additional data storage device and a printing device. 
         [0032]    It is to be further understood that, because some of the constituent system components and method steps depicted in the accompanying figures can be implemented in software, the actual connections between the systems components (or the process steps) may differ depending upon the manner in which the present invention is programmed. Given the teachings of the present invention provided herein, one of ordinary skill in the related art will be able to contemplate these and similar implementations or configurations of the present invention. 
         [0033]    The particular embodiments disclosed above are illustrative only, as the invention may be modified and practiced in different but equivalent manners apparent to those skilled in the art having the benefit of the teachings herein. Furthermore, no limitations are intended to the details of construction or design herein shown, other than as described in the claims below. 
         [0034]    Those of ordinary skill may vary the data collection apparatus and methods for recording and processing the images without varying from the scope of the invention as defined in the appended claims.