Abstract:
A cardiac phantom is provided for simulating a dynamic cardiac ventricle. The phantom comprises two concentrically-disposed, fluid-tight, flexible membranes defining a closed space between the walls of the membranes. A pump is operatively connected to the inner membrane for reciprocally delivering a volume of fluid to the inner membrane to inflate and deflate both membranes in simulating systole and diastole of a heart. A medium is disposed within the closed space defined between the walls of the membranes for maintaining a uniform distance between the walls of the membranes during inflation and deflation. The phantom is provided for evaluation of medical imaging systems. A method, computer system and a computer-readable medium are also provided for controlling the phantom.

Description:
BACKGROUND  
         [0001]    This invention relates generally to a medical imaging phantom, and more particularly to a cardiac phantom for use in medical imaging for diagnostic purposes.  
           [0002]    A number of different nuclear imaging instruments have been devised for diagnosing patient aliments and conditions. The field of use of such instruments is known as the field of nuclear medicine. Nuclear imaging instruments are advantageous in that they can produce images of conditions within soft tissue organs of a patient&#39;s body without exploratory surgery.  
           [0003]    In the practice of nuclear medicine, a low level of a tracer radioisotope is injected into a patient. The tracer radioisotope is carried in the patient&#39;s bloodstream to the patient&#39;s internal organs, such as the heart, liver, kidneys, or brain. The tracer radioisotope emits gamma rays, a portion of which pass from the patient&#39;s body and are detectable by nuclear imaging instruments and reflect, for example, blood flow or metabolic function in the organ.  
           [0004]    In order to calibrate and check the accuracy of nuclear imaging instruments, test structures, known in the field as phantoms, are utilized. Other prior nuclear imaging phantoms have been produced in forms which encase radioactive sources within a structure which simulate the shape and gamma ray attenuation properties of the human body. Such conventional test phantoms may employ concentrated radioactive sources which are imaged as “hot” spots or small non-radioactive structures within a homogenous radioactive source which exhibit “cold” spots in the image produced. The nuclear imaging instruments are used to produce images of the phantom. By examining the images produced, the degree to which the image conforms to the actual phantom structure and the degree to which irregularities exist in the image are ascertained.  
           [0005]    Nuclear imaging is a common non-invasive technique used in the evaluation of cardiac function and disease. Left ventricular ejection fraction and volume measurements are central to the objective characterization of cardiac performance. They are widely used as prognostic and therapeutic indicators in patients with different cardiac diseases. The importance of the measurement of the left ventricular ejection fraction and volume has been well recognized. Thus, the accuracy of the ejection fraction measurement and volume estimation performed by nuclear imaging devices with dynamic cardiac phantoms is important.  
           [0006]    Conventional cardiac phantoms of various sorts have been used extensively to study and validate measurements by nuclear and other medical imaging devices. Left ventricular models and casts have served as primary reference standards to validate imaging methods. However, cardiac phantoms are limited in applicability because they are static, or not anatomically realistic in shape. In particular, past cardiac phantoms have been particularly inadequate in simulating a beating heart. Moreover, substantial problems still complicate accurate measurements of left ventricular contractility. Realistic, expansible phantoms of the left ventricle to simulate the left ventricle of the beating heart are needed for use in left ventricular ejection fraction and volume measurement under various simulated clinical conditions.  
           [0007]    For the foregoing reasons, there is a need for a new, more realistic cardiac phantom. Specifically, the new phantom should be a beating phantom which allows real time nuclear imaging. In particular, the new phantom should allow medical imaging and measurement of the left ventricular region during all phases of the heart action.  
         SUMMARY  
         [0008]    According to the present invention there is provided an apparatus for simulating a dynamic cardiac ventricle. The apparatus comprises two concentrically-disposed, fluid-tight, flexible membranes defining a closed space between the walls of the membranes. A pump is operatively connected to the inner membrane for reciprocally delivering a volume of fluid to the inner membrane to inflate and deflate both membranes in simulating systole and diastole of a heart. A medium is disposed within the closed space defined between the walls of the membranes for maintaining a uniform distance between the walls of the membranes during inflation and deflation.  
           [0009]    Also according to the present invention, a phantom is provided for evaluation of a medical imaging system. The phantom comprises a closed container of fluid and a simulated dynamic cardiac ventricle disposed within the container. The simulated cardiac ventricle includes two concentrically-disposed, fluid-tight, flexible membranes defining a closed space between the walls of the membranes and a medium disposed within the closed space defined between the walls of the membranes for maintaining a uniform distance between the walls of the membranes during inflation and deflation. A fluid delivery system including a pump between the container of fluid and the inner membrane provides a reciprocating volume of fluid between the container and the inner membrane to inflate and deflate both membranes in simulating systole and diastole of a heart.  
           [0010]    Further according to the present invention, a method is provided for controlling an apparatus capable of simulating a cardiac ventricle. The method comprises the steps of determining data points for a ventricular volume versus time curve, sampling a position of a pump motor, determining the pump motor position and at least one subsequent position of the pump motor corresponding to the volume versus time curve, calculating velocity and acceleration to reach the subsequent position to simulate a cardiac ventricle, and moving the pump motor to the subsequent position based on the calculated velocity and acceleration.  
           [0011]    A computer system is also provided according to the present invention for controlling an apparatus capable of simulating a cardiac ventricle. The computer system comprises means for determining data points for a ventricular volume versus time curve, means for sampling a position of a pump motor, means for determining the pump motor position and at least one subsequent position of the pump motor corresponding to the volume versus time curve, means for calculating velocity and acceleration to reach the subsequent position to simulate a cardiac ventricle, and means for moving the pump motor to the subsequent position based on the calculated velocity and acceleration.  
           [0012]    Still further according to the present invention, a computer-readable medium is provided. The contents of the medium cause a computer system to control an apparatus capable of simulating a cardiac ventricle by performing the steps of determining data points for a ventricular volume versus time curve, sampling a position of a pump motor, defining the pump motor position and at least one subsequent position of the pump motor corresponding to the volume versus time curve, calculating velocity and acceleration to reach the subsequent position to simulate a cardiac ventricle, and moving the pump motor to the subsequent position based on the calculated velocity and acceleration. 
       
    
    
     BRIEF DESCRIPTION OF DRAWINGS  
       [0013]    For a more complete understanding of the present invention, reference should now be had to the embodiments shown in the accompanying drawings and described below. In the drawings:  
         [0014]    [0014]FIG. 1 is a perspective view of an embodiment of a cardiac phantom according to the present invention;  
         [0015]    [0015]FIG. 2 is an exploded perspective view of the cardiac phantom shown in FIG. 1;  
         [0016]    [0016]FIG. 3 is a sectional view of a left ventricular portion of an embodiment of the cardiac phantom according to the present invention;  
         [0017]    [0017]FIG. 4 is an exploded view of the left ventricular portion of the cardiac phantom shown in FIG. 4;  
         [0018]    [0018]FIG. 5 is a sectional view of another embodiment of a left ventricular portion of the cardiac phantom according to the present invention;  
         [0019]    [0019]FIG. 6 is a sectional view of still another embodiment of a left ventricular portion of the cardiac phantom according to the present invention; and  
         [0020]    [0020]FIG. 7 is a sectional view of a fluid tubing coupling for use in an embodiment of the cardiac phantom according to the present invention;  
         [0021]    [0021]FIG. 8 is a computer system for use in a method for using a cardiac phantom according to the present invention;  
         [0022]    [0022]FIG. 9 is a software sequence for use in a method for using a cardiac phantom according to the present invention; and  
         [0023]    [0023]FIG. 10 is a Wiggers&#39; Curve showing volume versus time of a normal left ventricle of a human heart. 
     
    
     DESCRIPTION  
       [0024]    Certain terminology used herein for convenience only is not to be taken as a limitation of the invention. For example, words such as “upper”, “lower”, “left”, “right”, “horizontal”, “vertical”, “upward” and “downward” merely describe the configuration shown in the figures. Indeed, the components may be oriented in any direction. Therefore, the terminology should be understood as encompassing such variations unless specified otherwise.  
         [0025]    Referring now to the drawings wherein like reference numerals designate corresponding or similar elements throughout the several views, FIGS. 1 and 2 show an apparatus for use as a cardiac phantom according to the present invention and generally designated at  10 . The cardiac phantom  10  comprises a thoracic portion  12  housing a left ventricular assembly  14  and a fluid delivery system  16  mounted to a support structure  18 . The support structure  18  comprises two generally horizontal plates  20  and a plurality of vertical support members  22  extending upwardly from the top surface of the thoracic portion  12 .  
         [0026]    The thoracic portion  12  comprises a fluid tight elliptical cylinder. In one embodiment of the present invention, the thoracic portion  12  is a thoracic phantom including an acrylic tank  22 , a top plate  24  and a bottom plate  26 . The top plate  24  and bottom plate  26  are fastened to the tank  22  using threaded fasteners  28 . Only the fasteners  28  for the top plate  24  are shown in FIG. 2. O-rings  30  are positioned between the plates  24 ,  26  and tank  22  to seal the interface between the edges of the tank  22  and the plates  24 ,  26  against fluid leakage. The thoracic phantom is designed to account for the low density and complex tissue distribution in the thorax. Accordingly, forms representing, for example, the lungs  32  and liver  34  within a human torso are integral with or mounted to the tank  22  wall and extend into the tank. The two lung inserts  32  may contain tightly-packed Styrofoam beads and are filled with water to simulate the density of lung tissue. Other pseudo-anatomic features may be built into the thoracic phantom simulating the spine  36 , the sternum (not shown), back muscles (not shown) and the like. A suitable thoracic phantom for use in the present invention is available from Data Spectrum Corporation, Hillsborough, N.C., under the name Anthropomorphic Torso Phantom, Model ECT/TORIP.  
         [0027]    The left ventricular assembly  14 , partially visible in FIGS. 1 and 2, is shown in detail in FIGS. 3 and 4. The left ventricular assembly  14  comprises a fluid delivery tube  38 , an inner tubular cap member  40  and an outer tubular cap member  42 . The fluid delivery tube  38  has an upper portion  44  and a lower portion  46 . The upper portion  44  of the fluid delivery tube  38  extends vertically through an opening in the top plate  24  of the thoracic portion  14 . As seen in FIG. 3, the opening in the top plate  24  is sealed using a close-fitting flange  48  and an o-ring  50 . The lower surface of the flange  48  has an annular groove  52  which receives the o-ring  50 . The flange  48  is secured against the top surface of the top plate  24  using threaded fasteners  28 . As the fasteners  28  are tightened, the o-ring  50  is pressed against the outer surfaces of the top plate  24  and fluid delivery tube  38  for sealing the interface. It is understood that all connections in the thoracic portion  12  and fluid delivery system  16  of the present invention may be sealed using this flange  48  and o-ring  50  arrangement. The lower portion  46  of the fluid delivery tube  38  forms an angle of between about 15 and about 30 degrees with the upper portion  44  to approximate the orientation of the heart in the chest cavity when the left ventricular phantom  14  is freely suspended from the end of the lower portion  46  of the fluid delivery tube, as seen in FIGS. 1 and 2.  
         [0028]    As best seen in FIG. 4, the tubular cap members  40 ,  42  are ellipsoid-shaped. The cap members  40 ,  42  are formed from flexible, resilient material allowing them to expand and contract like the heart muscle. In one embodiment, the cap members are latex balloons. The latex balloons are formed by preparing a solution by dissolving 50 parts of calcium nitrate tetrahydrate in 50 parts reagent alcohol. A latex dipping bath is also prepared by adding a wetting agent (0.1-0.25 pphr) and an antioxidant (1.0-3.0 pphr) to a pre-vulcanized, natural rubber latex. Aqueous dispersions of surfactants are acceptable as wetting agents, such as sodium lauryl sulphate, potassium oleate, or Darvan WAQ available from R. T. Vanderbilt Co., Inc., of Norwalk, Conn. and Octowet 70D available from Tiarco Chemical Co., of Dalton, Ga. Satisfactory aqueous dispersions of antioxidants of the non-discoloring, hindered-phenolic type, include Octolite 640 available from Tiarco Chemical Co., Bostex 24 available from Akron Dispersions of Akron, Ohio, Akrosperse W-19119 available from Akrochem Corp. of Akron, Ohio, and Agerite Superlite available from R. T. Vanderbilt Co., Inc. The latex is a medium modulus, pre-vulcanized natural rubber lattices such as Guthrie PVMM from Guthrie Latex, Inc. of Tucson, Ariz., Chemionics-960CX9949 from Chemionics Corp. of Tallmadge, Ohio, and Heveatex HA-1438/D710 from Heveatex Corp. of Fall River, Mass. The preferred components are Darvan WAQ, Agerite Superlite and Guthrie PVMM. Distilled water is added to the latex dipping bath until the total solids content is 55%. The compounded latex is matured at room temperature for about 48 hours, then filtered through a stainless steel 80-mesh sieve.  
         [0029]    A ventricle-shaped, aluminum former is heated in air at about 70° C. for 30 minutes, dipped in the prepared solution for less than about 10 seconds, then dried in air at room temperature for 15 minutes leaving the former coated with a uniform gel of coacervant. The former is heated to facilitate solvent evaporation. The coated former is submerged in the latex dipping bath where it remains dwelled for about 5 to about 10 minute. The former is withdrawn from the latex at a rate of 1-2 mm per second. The resulting opaque, rubber gel is dried in air at about 21-23° C. for up to about 30 minutes. The former is leached in distilled water at about 40° C. to about 50° C. for 2 to 3 hours to remove non-rubber constituents and then dried in air at 50° C. for several hours until the rubber cap material becomes transparent. The cap material is then wet-stripped from the former, leached in distilled water at 30° C. for up to 24 hours to further remove remaining hydrophilic materials, then dried in air at 50° C. for several hours until transparency is restored. Finally, the cap member is surface-treated by immersion in an aqueous chlorine solution (0.40 g Cl − /L water) at 21-23° C. for up to about 30 minutes. The cap member may optionally be neutralized in a KOH/water solution (pH=8.5) at 21-23° C. for 5 minutes. In either case, the cap member is rinsed in water at 21-23° C. for about 10 minutes, then dried in air at about 50° C. to about 70° C. for up to about 30 minutes. The resulting cap member is transparent, amber, glossy, and tack-free.  
         [0030]    Referring to FIGS. 3 and 4, the distal end of the lower end portion  46  of the fluid delivery tube  38  has a reduced diameter for receiving the open end of the inner cap member  40 . A rubber collar  54  fits snugly over the end of the tube  46  and cap  40 . The outer cap member  42  then slips over the collar  54 . The collar  54  has a circumferential groove  56  for receiving an o-ring  58  which fits over the outer cap member  42  and helps seal the outer cap member against the collar  54 . Finally, a clamp  60  is secured around the whole arrangement providing a fluid tight seal.  
         [0031]    Another embodiment of the left ventricular assembly, generally designated at  14   a , is shown in FIG. 5. In this embodiment, the inner cap member  40   a  and outer cap member  42   a  are open-ended. The cap members  40   a ,  42   a  may be formed from latex in the same manner as described above with the additional step of removing the closed ends. The upper ends of the cap members  40   a ,  42   a  are secured on the end of the lower portion  46  of the fluid delivery tube  38  in the same manner as described above in the previous embodiment. The lower free end of the inner cap member  40   a  fits snugly over a smaller diameter end of a headed plug  62 . The remainder of the structure at the lower free ends of the cap members  40   a ,  42   a  is the same as at the upper end, including a rubber collar  54 , o-ring  58  and clamp  60 . In this embodiment, a longitudinal guide assembly is also provided, comprising a ring  64  and at least one longitudinal guide member  66 . The ring  64  is secured on the lower portion  46  of the fluid delivery tube  38  adjacent the reduced diameter end. The guide members  66 , in one embodiment, are arcuate plates fastened at one end to the ring  64  and extending longitudinally from the ring at least to the head of the plug  62 . The guide members  66  may optionally be secured to the head of the plug  62 . This arrangement provides support to and guides the movement of the cap members  40   a ,  42   a.    
         [0032]    Still another embodiment of the left ventricular assembly, generally designated  14   b , is shown in FIG. 6. In this embodiment, the inner cap member  40   a  and outer cap member  42   a  are also open-ended. The upper ends of the cap members  40   a ,  42   a  are secured on the end of the lower portion  46  of the fluid delivery tube  38  in the same manner as described above in the previous two embodiments. The lower free end of the inner cap member  40   a  fits snugly over the end of a plug  68  which fits inside the end of the inner cap member  40   a . The remainder of the structure at the lower free ends of the cap members  40   a ,  42   a  is the same as at the upper end, including a rubber collar  54 , o-ring  58  and clamp  60 . In this embodiment, a disc  70  is secured inside the lower portion  46  of the fluid delivery tube  38 . The disc  70  has a plurality of openings to allow fluid to pass. A rod  72  a secured at the ends between the plug  68  and the disc  70 . The rod  72  provides support to the cap member assembly. In one embodiment, the rod  72  may be hollow and adapted to hold fluid, such as water or active solution for imaging.  
         [0033]    As seen in FIGS. 3, 5 and  6 , a space  73  is defined between the outer surface of the inner cap member  40 ,  40   a  and the inner surface of the outer cap member  42 ,  42   a . A longitudinal passage  74  is provided in the collar  54 . One end of a fluid line  76  fits into the passage  74  in the collar  54  and the other end of the fluid line  76  is attached to a fluid valve  78  mounted to the top plate  24 . The valve  78  allows the user to add water or active solution to the space  73  between the walls of the cap members  40 ,  40   a ,  42 ,  42   a . The space  73  represents the myocardium of the heart. The range of thickness of the space is adapted during expansion and contraction of the cap members to simulate the range of the left ventricular myocardial thickness usually encountered in clinical conditions.  
         [0034]    Another means for controlling the thickness and shape of the simulated myocardial wall is to use a layer of solid material such as foam, and in one embodiment an open-cell foam. The layer of foam is formed to the desired shape and thickness (about 1 cm.) of the myocardium. An adhesive is used to attach the outer surface of the inner cap member  40 ,  40   a  and the inner surface of the outer cap member  42 ,  42   a  to the inner and outer surfaces, respectively, of the myocardium-shaped foam. Alternatively, the foam myocardium could be made slightly thicker than the distance between the cap members  40 ,  40   a ,  42 ,  42   a . In this case, the foam would be self-positioning and no adhesive would be needed. In either embodiment, the foam has sufficient stiffness to provide adequate control of the thickness of the myocardial wall, yet be flexible enough to respond to the pump  84  of the fluid delivery system  16  to produce the desired ventricular volume curve, as will be described below. The foam returns to its original shape after thousands of “beating heart” cycles. The foam can also be shaped to simulate a defect of the heart. In one embodiment, the cells of the open-cell foam are of sufficient size so that liquid, radioactive tracer material can freely flow into and out of the foam.  
         [0035]    Referring now to FIGS. 1 and 2, the fluid delivery system  16  comprises tubing  80  and a pump assembly  82  including a pump  84  and motor  86 . The tubing  80  is formed of rigid acrylic material. The tubing  80  is rigid so that the tubing does not effect the transmission of the volume of fluid in the system  16 . The tubing  80  includes a front vertical portion  88  passing through the intermediate plate  20  and a curved crossover portion  90  joined to upper end of the front vertical portion  88  thereby approximating a j-shape. The lower end of the front vertical portion  88  opens into the thoracic portion  12  and is secured to the top plate  24  in a fluid tight manner, using a flange  48  and o-ring  50  arrangement as described above.  
         [0036]    The pump  84  is a positive displacement pump which is positioned in the fluid delivery system  16  for delivering water to the thoracic portion  12  while simultaneously removing the same volume water from the left ventricular assembly  14  and vice versa. A suitable pump  84  for use in the present invention is available from Bimba Manufacturing Co., of Monee, Ill., and is sold as part number PT-0981080-MX. This pump  84  includes two pistons (shown schematically in FIG. 9) disposed at each end of a metal cylinder  92 . A rack and pinion mechanism is positioned between the pistons. The pinion extends transversely from the midpoint of the cylinder  92  and is engaged by the motor  86  for reciprocating the rack, and therefore the pistons. In one embodiment the motor  86  is a servo motor. A suitable motor for use in the present invention is available from Quicksilver Controls, Inc., of Covina, Calif., and is sold under the trade name SILVERMAX, Part no. QCI-34H-1-E-01.  
         [0037]    The upper end of the pump cylinder  92  is sealingly secured to the rear outlet of the crossover  90  of the fluid tubing  80 . The lower end of the pump cylinder  92  is received within an enlarged-diameter cylindrical coupling  94  shown in FIG. 7. The upper end of the coupling  94  is sealed to the end of the pump cylinder  92  using the flange  48  and o-ring  50  arrangement described above. The lower end of the coupling  94  sealingly receives the distal end of the upper portion  44  of the fluid delivery tube  38  of the left ventricular assembly  14 . Due to the characteristics of the flange  48  and o-ring  50  arrangement used to join the parts of the apparatus, the fluid delivery tube  38  is axially and rotationally movable with respect to the coupling  94  and the top plate  24  of the tank  22 . This feature allows the left ventricular assembly  14  to be adjustably positioned within the thoracic portion  12  without disassembling the tank  22 .  
         [0038]    In use, the cardiac phantom  10  is filled with fluid beginning with the fluid delivery tube  38  and inner cap member  40 ,  40   a  of the left ventricular assembly  14 . This fluid is commonly referred to as the “blood pool”. Fluid is added through a fluid flow valve  96  provided in the wall of the coupling  94 . The pistons are positioned at their highest possible point during filling. Fluid may be added until the inner cap member  40 ,  40   a  occupies about 60 ml of volume which is the volume of an average human heart during diastole. Next, the space  73  representing the myocardium is filled with fluid through the valve  78  in the top plate  24 . The space  73  is filled with sufficient fluid so that the thickness of the space  73  is about 1.4 cm. during systole and about 0.8 cm. during diastole which is consistent with an average human heart. The sufficient amount of fluid is usually about 70 ml. Finally, the thoracic portion  12  is filled through a valve  98  in the top of the crossover  90  of the fluid delivery system  16 . The thoracic portion  12  is filled until an air volume of about 50 ml. remains. A syringe is used to remove the excess air through the valve  98  which provides a net negative pressure in the thoracic portion  12 . Thus, the entire volume of the thoracic portion  12 , left ventricular assembly  14  and fluid delivery system  16  is filled with fluid. Fluid surrounds the left ventricular assembly  14  and simulated non-cardiac thoracic structures  32 ,  34 ,  36 . All three volumes of fluid are physically separated from one another. Any of the fluid volumes may comprise an active solution having radioactive properties to simulate, for example, thoracic background. The valves  78 ,  96 ,  98  are positioned at the highest point of each of the contained fluid volumes for the removal of air bubbles.  
         [0039]    In operation, the motor  86  powers the pump  84  thereby reciprocally moving fluid within the left ventricular assembly  12  and, separately, the tank  22  in the thoracic portion  12  in response to movement of the pistons in the cylinder  92 . Thus, a “beating” left ventricle is created which is available for medical imaging. Moreover, since the system is closed, the same volume of water is always being moved by pump assembly and there is no net displacement of fluid in the cardiac phantom  10  during contraction and expansion of the ventricular portion  14  and there is no pressure change.  
         [0040]    The speed and volume delivery of the pump  84  is controlled by software via a control circuit which enables adjustable setting of systolic and diastolic phase lengths of the cardiac phantom  10  for the appropriate cycles, or beats, per minute. Software that can be used to implement the invention resides and runs on one or more computer systems, which in one embodiment, are personal computers, workstations, or servers. FIG. 8 illustrates further detail of a computer system  100  that is implementing part of the invention in this way. System bus  102  interconnects the major components. The system  100  is controlled by a microprocessor  104 , which serves as the central processing unit (CPU) for the system. System memory  106  is typically divided into multiple types of memory or memory areas, such as read-only memory (ROM), random-access memory (RAM) and others. If the computer system  100  is an IBM compatible personal computer, the system memory  106  also contains a basic input/output system (BIOS). A plurality of general input/output (I/O) adapters or devices  108  are present. Only two are shown for clarity. The I/O adapters or devices  108  connect to various devices including a fixed disk  110 , a diskette drive  112 , and a display  114 . The computer program instructions for implementing the functions of the cardiac phantom  10  are stored on the fixed disk  110 , and are partially loaded into the memory  106  and executed by the microprocessor  104 . The system also includes another I/O device, a communications adapter  116  for connection to other devices over an interface, such as a RS232, which in this case allows the computer system to be connected to the motor  86  of the pump assembly  82 . It should be noted that the system as shown in FIG. 8 is meant as an illustrative example only. Numerous types of general-purpose computer systems are available and can be used. Available systems include those that run operating systems such as Windows® by Microsoft® and various versions of UNIX®.  
         [0041]    Elements of the software program may be embodied in hardware or in software (including firmware, resident software, micro-code, etc.). Furthermore, the invention may take the form of a computer program product on a computer-usable or computer-readable storage medium having computer-usable or computer-readable program code embodied in the medium for use by or in connection with an instruction execution system. Such mediums are shown in FIG. 8 to represent the diskette drive  110  and the hard disk  112 . In the context of this specification, a computer-usable or computer-readable medium may be any medium that can contain, store, communicate, propagate, or transport the program for use by or in connection with the instruction execution system, apparatus, or device. The computer-usable or computer-readable medium may be, for example but not limited to, an electronic, magnetic, optical, electromagnetic, infrared, or semiconductor system, apparatus, device, or propagation medium. More specific examples, a nonexhaustive list, of the computer-readable medium would include the following: an electrical connection having one or more wires, a portable computer diskette, a random access memory (RAM), a read-only memory (ROM), an erasable programmable read-only memory (EPROM or Flash memory), an optical fiber, and a portable compact disc read-only memory (CD-ROM). Various memory types can be used, for example, to store portions of code at the mobile terminal that relate to the invention. Note that the computer-usable or computer-readable medium could even be paper or another suitable medium upon which the program is printed, as the program can be electronically captured, via, for instance, optical scanning of the paper or other medium, then compiled, interpreted, or otherwise processed in a suitable manner, if necessary, and then stored in a computer memory.  
         [0042]    In accordance with the present invention, the microprocessor  104  functions to provide a programmed sequence  120  (FIG. 9) which guides the cardiac phantom  10  through a simulated cardiac cycle based on a ventricular volume versus time curve shown in FIG. 10. Referring to FIG. 9, the program sequence  120  begins with the user selecting a number of data points, D(0) to D(n),  122  to be analyzed by a microprocessor. The number of data points which may be selected is not limited. Each data point selected corresponds to a particular ventricular volume at a distinct point in time during the cardiac cycle (FIG. 10), so that when the data points are sequentially connected a ventricular volume versus time curve results.  
         [0043]    In the next step, the software samples the position of the pump motor  124 . The software then defines a data point, D(i), on the ventricular volume versus time curve based on the position of the motor pinion at a given time. Therefore, the motor position is linked to ventricular volume at a given time, causing a data point, D(i), on the ventricular volume versus time curve  140  to be represented by motor position, (i), in the program sequence. In the same step  124 , the software defines a subsequent data point, D(i+1), based the motor position (i+1). This subsequent motor position, (i+1), represents a data point immediately subsequent to the prior data point, D(i), along the ventricular volume versus time curve. After the software  124  defines an initial pump motor position, (i), and a subsequent motor position, (i+1), the software calculates the velocity and acceleration at which the pump motor must advance to arrive at the position, (i+1)  126 , while following the curve.  
         [0044]    In the next step  128 , the software causes the pump motor to move towards the new position, (i+1), that represents the immediately subsequent data point, D(i+1), on the ventricular volume versus time curve. After moving the motor towards its new position, (i+1), the program sequence progresses to a decision step  130  that compares the actual motor position to its expected position at the new position. The decision step  130  determines if the motor moved to its correct position, (i+1), and if the time provided for the motor to reach that correct position has elapsed. If the answer to both questions is NO, the motor will continue to move towards its expected position (i+1). If the answer to either question is YES, the motor is in the correct position, (i+1), or the motor is not in the correct position but the time for it to reach that position has expired, the next step in the sequence considers the motor to have reached the correct position, (i+1)  132 .  
         [0045]    After the motor reaches its new position at (i+1), the next step  134  is a decisional step asking if i=n−1. If YES, the program continues to the next step  136  which defines the position of the motor, (i), as 0, and the sequence is started again with the motor at the initial position, i=0,  124  which correlates with the initial data point D(0). If NO, (i) does not equal 0 and represents some motor position between 0 and n. The program sequence begins again at step  124  where the new motor position, (i+1), becomes (i).  
         [0046]    It is understood that the velocity and acceleration parameters can be changed to mimic dynamic ventricular characteristics including changes in ventricular contraction rate and ejection fraction. The software can be programmed to follow Wiggers&#39; curve or similar normal heart rate curves. Moreover, the software allows the pump to be controlled to simulate in real time all pumping functions and possible clinical conditions of the left ventricular region of the heart, including variations in heart rate, ejection volume, or any abnormal cardiac curve. The cardiac phantom of the present invention is a highly versatile device for testing medical imaging instruments, including those used for SPECT, PET, MRI, x-ray CT, ultrasound, and the like for diagnostic purposes. The left ventricular phantom has a known volume and shape and may be used to calibrate, adjust and detect inaccuracies in virtually any commercially available type of medical imaging instrument or device. Image acquisition can be obtained throughout the normal real time functioning of the heart in diastolic or systolic or in dynamic diastolic-systolic conditions. In particular, the phantom can be used in numerous simulated cardiac situations for providing real time images. Simulated clinical conditions can be used to demonstrate changes in left ventricular ejection fractions and to quantify the left ventricular volumes. When active solution is filled between the walls of the cap members, myocardial abnormalities and defects of various sizes can be simulated, such as myocardial perfusion defects.  
         [0047]    The cardiac phantom of the present invention thus provides a useful tool for validating nuclear imaging devices in cardiac imaging studies over a wide range of simulated clinically relevant situations. The validation may be done in real time using the realistic, expansible dynamic left ventricular phantom to simulate the left ventricle of a beating heart. Therefore, the phantom of the present invention has considerable potential as a reference standard in assessing medical imaging devices for analysis of the heart, particularly during real time function.  
         [0048]    Although the present invention has been shown and described in considerable detail with respect to only a few exemplary embodiments thereof, it should be understood by those skilled in the art that we do not intend to limit the invention to the embodiments since various modifications, omissions and additions may be made to the disclosed embodiments without materially departing from the novel teachings and advantages of the invention, particularly in light of the foregoing teachings. For example, the cardiac phantom and method described herein for actuating the simulated ventricle can be used with other methods or cardiac phantoms and simulated ventricles, respectively, other than the embodiments disclosed herein. Accordingly, we intend to cover all such modifications, omission, additions and equivalents as may be included within the spirit and scope of the invention as defined by the following claims. In the claims, means-plus-function clauses are intended to cover the structures described herein as performing the recited function and not only structural equivalents but also equivalent structures. Thus, although a nail and a screw may not be structural equivalents in that a nail employs a cylindrical surface to secure wooden parts together, whereas a screw employs a helical surface, in the environment of fastening wooden parts, a nail and a crew may be equivalent structures.