Abstract:
A system for dual-mode medical imaging is provided. The system features components for PET imaging as well as for ultrasonic imaging, with an internal probe that has components to provide capability for both. The system provides cost-efficient PET imaging for smaller regions and organs of interest than conventional full body PET scanner apparatus.

Description:
TECHNICAL FIELD 
       [0001]    In general, the invention relates to nuclear medical imaging. More particularly, the invention relates to dual mode image acquisition using positron emission tomography (PET) scanning and ultrasonography. 
       BACKGROUND OF THE INVENTION 
       [0002]    PET imaging systems have been known and commercially available for many years. PET is currently one of the most effective ways to diagnose cancer recurrences, metastases of cancer, whether an early stage of cancer is present or not, and, if cancer has spread, its responding to treatment. PET is also used in diagnosing certain cardiovascular and neurological diseases by highlighting areas with increased, diminished, or no metabolic activity. 
         [0003]    A normal PET scanner&#39;s ability to image large regions of the body efficiently is directly due to its large imaging field of view. This is appropriate since the ability to do a PET study of the whole body is at present the major use of PET. However, when the clinical need is to image just a small region of the body, a normal PET scanner is larger and more expensive than is actually needed. (Examples of small regions of clinical interest include the prostate when there is a suspicion of cancer, plaque deposits near the arteries which are to be evaluated for inflammation, and the myocardium of the heart, whose perfusion with blood needs to be evaluated and whose viability is of concern.) Therefore, there is a need for a smaller PET scanner system that is less expensive and better suited for performing PET imaging of selectable, small regions of the body. 
       SUMMARY OF THE INVENTION 
       [0004]    According to the invention, a system for dual-mode medical imaging is provided which permits imaging of relatively small-scale regions of interest. The system features components for PET imaging as well as for ultrasonic imaging, with an internal probe that has components to provide capability for both. 
         [0005]    Other aspects and features of the invention will be evident from reading the following detailed description of the preferred embodiments, which are intended to illustrate, not limit, the invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0006]    The invention will become more clearly understood from the following detailed description in connection with the accompanying drawings, in which: 
           [0007]      FIG. 1  is a perspective view of components of a dual-mode imaging system according to the invention; 
           [0008]      FIGS. 2A and 2B  are perspective views of the probe shown in  FIG. 1  illustrating the dual ultrasonic and PET imaging modalities; 
           [0009]      FIG. 3  is a schematic section view illustrating the system shown in  FIG. 1  in use to image a prostate; 
           [0010]      FIG. 4  is a schematic view illustrating the use of a vascular embodiment of a dual imaging mode probe according to the invention; and 
           [0011]      FIG. 5  is a schematic view of an imaging probe similar to that shown in  FIGS. 1 ,  2 A, and  2 B but having multiple radiation (PET) detectors. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0012]    An imaging system  10  according to the invention is illustrated in  FIG. 1 . The imaging system includes a probe  12  that is sized and configured to be introduced into a patient&#39;s body and an external PET detector  14 . The external PET detector  14  is generally conventional in that it is essentially a pixelated array of gamma ray-sensitive material such as lutetium oxyorthosilicate (LSO) or bismuth germanate (BGO), as is known in the art. The external detector  14  is, however, considerably smaller than PET detectors otherwise generally used in the art since, as illustrated and explained more fully below, it is intended to be used in much closer proximity to the patient&#39;s body and therefore need not be as expansive to register gamma photons generated upon annihilation of positrons emitted by an organ of interest  16  (e.g., the prostate). The acquired image data from the probe  12  and external detector  14  are processed in a signal processing unit (which can use well-known image signal processing techniques and therefore will not be further described here) to develop image signals that can be stored in a memory and/or sent to a display  32 , which may be any suitable, known display device such as an LCD, LED, plasma discharge, CRT, etc. 
         [0013]    As illustrated in  FIGS. 2A and 2B , the probe  12  is configured for dual imaging modalities. More particularly, as illustrated in  FIG. 2A , the probe  12  includes an ultrasonic emitter  18  and an ultrasound receiver/transducer  20  in the head of the probe. Additionally, as illustrated in  FIG. 2B , the probe  12  further includes a nuclear radiation detector  22  in the head of the probe. Like the external PET detector  14 , the radiation detector  22  is sensitive to gamma radiation emitted upon annihilation of positrons emitted by the organ of interest. Unlike the external PET detector  14 , however, the radiation detector  22  in the probe  12  consists of just a single crystal (essentially a large “pixel”). This eliminates the need to know a specific position where the gamma radiation strikes the radiation detector in order to generate an image (as is the case when using two pixelated detector arrays), although it does reduce the imaging volume to a cone beam as shown in  FIG. 1  (as opposed to a cube, as can be imaged using two square pixelated detector arrays). 
         [0014]    As a result, the imaging system  10  of the invention does not permit true tomography and does not have high enough resolution for whole-body applications; however, for imaging small regions that are close in to the organ of interest, the system provides acceptable imaging capabilities. Furthermore, because the detectors  14  and  22  of the imaging system  10  of the invention are used in very close proximity to the organ of interest  16 —with probe detector  22  being placed internally (such as in the rectum) and therefore as close as possible to the organ of interest  16 —photon attenuation by the patient&#39;s body is minimized. Moreover, the smaller size of the device over prior art PET detectors significantly reduces its cost. 
         [0015]    Exemplary use of the imaging system  10  according to the invention—e.g., for prostate cancer detection—is illustrated in  FIG. 3 . First, radiation activity is introduced (e.g., injected) into the organ of interest. As is known, cancerous tissue (e.g., masses  26 ) will take up certain radiopharmaceutical tracer substances at a faster rate than non-cancerous tissue and therefore will exhibit a greater concentration of the material than non-cancerous tissue, and that increased concentration can be measured by recording events that occur simultaneously, i.e. in coincidence, in the detectors  14  and  22 , and incrementing the histogram image corresponding to the detector array  14 . As shown, the head of the probe  12  is inserted into the patient&#39;s rectum  24 , and the ultrasonic imaging capability of the probe  12  is used to locate the prostate (organ of interest)  16  and position the head of the probe generally near it. The external PET detector  14 , on the other hand, is brought close to the patient&#39;s body generally located opposite the organ of interest from the probe detector  22 , e.g., pressed against the pubic bone  28 . The external PET detector  14  and probe radiation detector  22  are then used for positron imaging, which may take up to several minutes. 
         [0016]    As noted above, the system of the invention does not permit true (i.e., three-dimensional) tomography. However, the system will produce two-dimensional projection images of the organ of interest, as indicated by the projection  30  shown against the external PET detector  14  in  FIG. 3 . By repositioning the external PET detector  14  and/or the probe radiation detector  22  to the extent possible, multiple two-dimensional projection images may be acquired, from which the medical practitioner may gauge the nature and extent of any unusual tissue masses. Once the PET-based images have been constructed, they may be displayed (e.g., on display device  32 ,  FIG. 1 ) superimposed with ultrasonically generated images. The superposition allows the collection of additional diagnostic information and facilitates the taking of biopsies. 
         [0017]    In the system  10  illustrated in  FIGS. 1-3 , the probe  12  is sized to be inserted into the patient&#39;s body via a natural orifice (e.g., the rectum). It is, however, possible to manufacture the probe considerably smaller, so as to be introducible into the body via an incision. For example, as illustrated in  FIG. 4 , a probe  12 ′ can be manufactured that is on the order of a few millimeters in length and diameter, with electrical signal leads  13 . This configuration permits the probe  12  to be introduced into the body percutaneously. Thus, as illustrated in  FIG. 4 , the probe  12 ′ can be introduced into the femoral vein and moved into position to provide images of plaque deposits around the descending aorta. Further, the probe could be deployed via endoscope, to permit use in transesophageal echocardiography, the staging of gastrointestinal tumors, or other applications. 
         [0018]    In an alternate embodiment shown in  FIG. 5 , a probe  112  includes multiple radiation detectors  22  (e.g., four as shown). As illustrated, the detectors  22  are aligned along the axis of the head of the probe  112 , although other arrangements of the detectors are possible. Such a multiple-detector probe is advantageous for several reasons. First, it provides increased sensitivity as compared to the probes illustrated in  FIGS. 1-3 . Second, it facilitates limited-angle longitudinal tomographic reconstruction of the projections, thus creating images with partial depth information. This procedure is also known as digital tomosynthesis. 
         [0019]    The foregoing description is meant to be illustrative of the invention and not limiting. Various modifications to the disclosed embodiments will occur to those having skill in the art. The scope of the inventory is defined by the following claims.