Abstract:
A computer-controlled injector of the type having a motor which advances and retracts a plunger located within a syringe housing toward and away from a nozzle located in the front of the syringe to inject fluid into or out of an animal subject. Manual motion is induced by operating a manual motion control; the operator can manipulate the control to indicate the desired direction and velocity of motion. The manual motion control also has a locking mode in which manual motion of the plunger will continue once initiated without requiring the operator to continue manipulating the manual motion control. The injector performs injections in accordance with one of several pre-programmed protocols, and automatically tracks the fluid volume remaining. The injector compensates for plunger extenders found in some partially pre-filled syringes by applying a stored offset value to the computed plunger position.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   This application is a divisional application of pending application Ser. No. 09/307,633 filed May 7, 1999, which is a divisional of Ser. No. 08/919,610 filed Aug. 28, 1997, now U.S. Pat. No. 5,928,197, which is a continuation of application Ser. No. 08/494,795 filed Jun. 26, 1995, now U.S. Pat. No. 5,662,612, which is a continuation of application Ser. No. 08/157,823 filed Nov. 24, 1993, now abandoned, all entitled CONTROLLING PLUNGER DRIVES FOR FLUID INJECTIONS IN ANIMALS, all of said applications and patents being incorporated by reference herein in their entirety, including the microfiche appendix attached thereto. 

   BACKGROUND 
   Injectors are devices that expel fluid, such as radiopaque media (contrast fluid) used to enhance x-ray or magnetic images, from a syringe, through a tube, and into an animal subject. Injectors are typically provided with an injector unit, adjustably fixed to a stand or support, having a plunger drive that couples to the plunger of the syringe and may move the plunger forward to expel fluid into the tube, or move the plunger rearward to draw fluid into the syringe to fill it. 
   Injectors often include control circuits for controlling the plunger drive so as to control the rate of injection and amount of fluid injected into the subject. Typically, the control circuit includes one or more manual switches which allow a user to manually actuate the plunger drive to move the plunger into or out of the syringe; typically the user holds down a “forward” or “reverse” drive switch to move the plunger in the indicated direction. 
   To reduce the risk of infection, in a typical injection procedure the syringe is only used once, and is disposed after use. In some cases, the syringe is inserted into the injector empty. The empty syringe is filled by retraction of the plunger while the interior of the syringe communicates with a supply of the contrast fluid via an injection tube connected between the nozzle of the syringe and the supply of media. Then, bubbles are removed from the syringe, and the injection is performed. At the end of the procedure, the syringe plunger typically is forward, as is the plunger drive. 
   In some injectors, the syringe can only be removed or replaced while the plunger drive is fully retracted. As illustrated in  FIG. 1A , typically an empty syringe  10  is filled with sterile air, with the plunger  12  at the fully retracted position as shown. The plunger drive includes a jaw  18  designed to engage and disengage a button  14  on the rear side of the plunger while the plunger is in this fully-retracted position. Before an empty new syringe can be filled, it is necessary that the plunger be moved fully forward in the syringe so that the syringe can be filled by rearward retraction of the plunger. Thus, the reloading operation can involve fully retracting the plunger drive to allow removal and replacement of the syringe, then fully advancing the plunger drive and plunger to expel air from the syringe, and then retracting the plunger drive and plunger to fill the syringe. These lengthy, manual movements of the plunger and drive are time consuming. 
   The above-referenced patent application describes a front-loading injector in which a syringe can be replaced even though the plunger drive is not fully retracted. This injector substantially reduces the number of plunger drive movements necessary to prepare a syringe for a new injection; after an injection, the syringe can be removed and replaced without moving the drive from its fully-advanced position. (The plunger drive jaw  18  can engage and disengage button  14  regardless of the position of the plunger.) After the syringe is replaced, the drive is retracted, filling the syringe for a new injection. Thus, to ready the injector for a new injection, the plunger drive is manually moved once rather than three times. 
   Another recent development is the use of pre-filled disposable syringes. A pre-filled syringe also reduces the number of manual plunger drive movements necessary to prepare the injector for a new injection. After an injection, the plunger drive is fully retracted, the used syringe is removed and replaced with the pre-filled syringe, and the injector is ready for a new injection. Thus, again, the plunger drive is manually moved once rather than three times. 
   To prevent infection, contrast media remaining in a syringe after an injection must be discarded. However, contrast media is relatively expensive. For this reason, when preparing for an injection, an empty syringe is filled with only as much media as will be needed for the next injection. For the same reason, pre-filled syringes are sold in a number of capacities, e.g. ranging from 60 to 125 milliliters, allowing the operator preparing for an injection to select a syringe containing only as much media as is needed for the injection. 
   A typical pre-filled syringe is illustrated in  FIG. 1B . In many respects, the pre-filled syringe is identical to the empty syringe shown in  FIG. 1A . The barrels  10  and plungers  12  have the same size and profile in both syringes (injectors now in use accommodate only a few FDA approved syringe sizes, e.g., a 200 milliliter size and a 125 milliliter size, so all syringes use these sizes). Furthermore, both syringes have a button  14  which is initially located at the end of the barrel  10  (thus, both syringes are compatible with injectors which are designed to grip a button at the end of the syringe). The main difference is that in the pre-filled syringe of  FIG. 1B , the initial location of the plunger  12  is in the middle of the syringe (thus reducing the initial volume of the pre-filled syringe). An extender  16  is attached to button  14  of the plunger, and provides a second button at the end of the syringe which can be gripped by the injector. 
   SUMMARY 
   As noted above, at the present state of the art, preparing an injector for an injection requires at least one manual movement of the plunger drive into or out of the syringe barrel, and as many as three such movements. This operation is tedious and inefficient, not only because of the time consumed, but also because the operator must press and hold manual movement switches to produce the movement, and thus is physically tied to the injector and cannot use this time to make other preparations. 
   In accordance with one embodiment of the present invention, the plunger drive controller has a locked mode in which motion, initially requested by pressing a manual movement switch, will continue whether or not the operator continues pressing the switch, until the plunger drive reaches its fully-advanced or fully-retracted position. Thus, once the controller has entered the locked mode, the operator may release the manual switch and the desired movement, either advancement or retraction, will continue while the operator makes other preparations for the next injection. 
   In preferred embodiments, the operator causes the controller to enter the locked mode by pressing the manual movement switch for a predetermined period of time. For safety, the manual movement switch may comprise two buttons which must be simultaneously pressed to produce movement. Movement is initiated by pressing both buttons. While both buttons are held down, the plunger drive controller increases the velocity of movement until the velocity reaches a maximum, at which time the plunger drive controller enters the locked mode. If one button is released before the controller reaches maximum velocity and enters the locked mode, the movement will continue, but at a constant velocity. If the second button is released, the movement will stop. Alternatively, if the controller has reached maximum velocity and entered the locked mode, movement will continue even if both buttons are released; however, if thereafter either button is pressed, movement stops. The controller can provide visual feedback, for example via a light which blinks during motion and lights steadily when the controller is in the locked mode. This light may itself move in synchronism with the plunger drive to provide further feedback on the speed of motion. 
   As noted, the plunger drive controller is typically manually controlled by means of a switch which, when depressed, causes the plunger drive to move in one of two directions. In accordance with a second aspect of the invention, manual control is improved by providing an adjustment which allows the operator to adjust the rate at which the plunger drive moves or accelerates. This permits the operator to customize the operation of the plunger drive controller to enhance individual comfort. 
   In preferred embodiments, the manual control comprises a wheel which, when rotated, causes the plunger drive to move at a speed which is proportional to the speed of rotation. Alternatively, the manual control may be a forward switch and a reverse switch which cause the plunger drive to move in the indicated direction at a programmable velocity or acceleration. 
   To operate effectively, the plunger drive controller must determine the location of the plunger  12  relative to the ends of the syringe  10  so that, for example, the controller can determine the amount of contrast media remaining in the syringe. This can be done by a sensor which detects the location of the plunger drive jaw  20 , which is coupled directly to and moves with the plunger  12 . However, a pre-filled syringe may include an extender  16  which changes the relative location of the plunger  12  and the plunger drive jaw  20 , leading to malfunction in the plunger drive controller. In accordance with a third aspect of the invention, malfunction is avoided by storing an offset value representative of the length of the extender  16 , and applying this offset value to the computed drive jaw position. 
   In preferred embodiments, the offset value may be computed by querying the operator as to the capacity of the syringe and determining therefrom the appropriate offset value. The controller may be configurable so that this query is not made (for example, if the injector will not be used with pre-filled syringes, and therefore the offset value will not change). Alternatively, the offset value may be automatically computed by detecting physical indicia on the syringe or extender which indicate the length of the extender. 
   These and other aspects will be further illustrated in the following detailed description with reference to the attached drawings, in which: 

   
     BRIEF DESCRIPTION OF THE FIGURES 
       FIGS. 1A and 1B  are side, partial cut-away views of an empty syringe and a pre-filled syringe, respectively. 
       FIGS. 2A ,  2 B and  2 C respectively illustrate the console, powerhead, and powerpack of an injector. 
       FIGS. 3 ,  4 , and  5  are electrical and electrical-mechanical block diagrams of the powerpack, console and powerhead, respectively. 
       FIGS. 6A ,  6 B,  6 C,  6 D,  6 E and  6 F are illustrations of displays produced by the console in operation of the injector. 
       FIGS. 7A ,  7 B,  7 C,  7 D,  7 E,  7 F and  7 G are flow charts illustrating the software operating within the power pack. 
   

   DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
   Referring to  FIGS. 2A ,  2 B and  2 C, an injection system according to the invention includes three main components, a console  30 , a powerhead  40  and a powerpack  50 . 
   The console  30  comprises a liquid crystal display  32  of the type used in notebook computers (e.g., a display sold by Sharp Electronics Corp. of 5700 N.W. Pacific Rim Blvd., Camas, Wash. 98607 as part number LM64P62), coupled to an eight key keypad  34  within a housing  36 . As is further elaborated below, display screens presented on display  32  provide injection information and present the user with menus of one or more possible operations, each operation associated with one of the keys on keypad  34 . 
   The powerhead  40  includes a mount  42  (such as that described in the above-referenced patent application) which accepts a syringe  10  for an injection. The powerhead includes a plunger drive motor (not shown) for moving plunger  12  forward into and rearward out of syringe  10  during an injection in accordance with a preprogrammed sequence, or protocol, selected by the operator by operation of the console  30 . 
   The location and movement of the plunger drive is indicated by a light emitting diode (LED) which is mounted to the plunger drive and is visible to the operator through a graduated window  44  in the side of the powerhead  40 . As noted below, this LED flashes when the plunger drive is moving, and lights steadily when the plunger drive has been manually locked into forward or reverse motion in the manner described below. 
   The side of the powerhead  40  includes six pushbuttons: a start/stop button  45 , a forward manual motion button  46 , a reverse manual motion button  47 , and an enable/accelerate button  48 . The three enable/accelerate buttons  48  perform the same function; there are three buttons instead of one to improve operator accessibility. 
   The start/stop button  45  is used to start an injection protocol selected at the console, or to stop and restart an injection. During an injection, all of the eight buttons on the keypad  34  of the console  30  will perform an identical start and stop function. Furthermore, a remote handswitch (not shown) may be connected to the powerpack  50  (see below) to perform a start and stop function. (For this reason, the start/stop button  45  includes a picture of a handswitch.) 
   To manually move the plunger drive, the operator must simultaneously press a motion button  46  or  47  and an enable button  48 . This is a safety feature which reduces the risk of accidental movement of the plunger. If the operator presses the forward button  46  and any of the three enable buttons  48 , the plunger will begin forward motion; conversely, if the operator presses the reverse button  47  and any of the three enable buttons  48 , the plunger will begin reverse motion. Once motion is initiated in either direction, the operator may release one of the buttons; motion will be maintained at a constant velocity in the same direction so long as any one of the five buttons  46 ,  47  or  48  is held down. If, instead, after initiating motion in one direction, the operator continues to hold down an enable button  48  and a motion button  46  or  47 , motion will not only be maintained in the same direction, but will be accelerated in this direction until either the operator releases one of the buttons or a maximum velocity is achieved. At any time during the acceleration, the operator may release one of the buttons and hold down the other, and the motion will continue at the same velocity without acceleration. Thereafter, the operator can re-depress the released button, at which time acceleration will begin again. 
   If the velocity of motion increases to a maximum value, the plunger drive controller (described in more detail below) will enter a locked mode. In this locked mode, movement will continue at the maximum velocity in the same direction even if the operator releases all of the buttons. This frees the operator to perform other tasks when preparing for an injection without being forced to hold manual buttons on the injector until the plunger drive has made the lengthy transition to its fully-advanced or fully-retracted position. 
   For safety reasons, the locked mode can be terminated readily. If the operator has entered the locked mode and thereafter released all of the buttons, if at any time thereafter any of the buttons is pressed, the plunger drive controller will exit the locked mode and terminate motion. 
   Two lights  49 A and  49 B mounted on the rear of the powerhead  40  indicate the status of operation of the injector. Light  49 A is an injecting/fault indicator. This light glows while an injection is in process. It will flash if an error is detected. Light  49 B is an enabled indicator. It glows when the injector has been enabled and is ready to perform an injection protocol. 
   The rear end of the powerhead  40  (opposite mount  42 ) includes a jog wheel or switch (not shown in  FIG. 2B , see  163 ,  FIG. 5 ) used, in the manner described below, to manually activate motion of the plunger drive. 
   The powerpack  50  illustrated in  FIG. 2C  contains electronics which communicate with the console  30  and powerhead  40  to perform the functions described above. The powerpack is connected to the console  30  and powerhead  40  by standard computer communications cables (not shown). Signals carried on these cables are interfaced to circuitry inside of the powerhead, console, and powerpack in a manner described below. 
   As shown in  FIG. 3 , the circuitry in the powerpack includes a central processing unit (CPU)  52  which controls the operations of the powerhead  40  and console  30 . The CPU is preferably a programmable microprocessor such as the MC68332FN microprocessor, manufactured by Motorola, 2110 East Elliot, Tempe, Ariz. 85284. This microprocessor is a member of the 68000 family of microprocessors and features multitasking support; it is designed for use in so called “embedded” environments such as the circuit described herein, and therefore has more than the usual number direct-wired input-output ports. 
   The CPU connects to an address bus  54  for addressing a number of memory and communications components and a data bus  56  for retrieving and/or sending data from and to these components. Buffers  55  and  57  aid CPU  52  in interfacing to the address and data busses, respectively. Each of the elements connected to the address and data busses are briefly described below. 
   An erasable programmable read-only memory (EPROM)  58  connected to data bus  56  contains the program software which operates the CPU  52 . The EPROM contains an operating system, which performs low-level management of the CPU and its communications with other circuits, and a custom program for controlling the console and powerhead to perform injection protocols. In one embodiment, the operating system software is the USX68K operating system, a multi-tasking operating system for 68000 series microprocessors sold by U.S. Software of 14215 N.W. Science Park Drive, Portland, Oreg., 97229, and the custom program is written in the “C” programming language. This custom program is described below, and a copy of the “C” language source code for the custom program appears in the appendix to this application. 
   A second EPROM  60  connected to data bus  56  contains language information used by the program software in EPROM  56  when generating displays for presentation on the display  32  ( FIG. 2A ). As will be further elaborated below, the display screens presented on the display  32  include textual descriptions of actions being taken by the injector, and menu selections which the operator can select. The textual portions of these display elements are stored in the language EPROM  56 , from which they are retrieved and inserted into a template as CPU  52  is producing a display screen. Preferably, the language EPROM contains multiple versions of each textual insert, representing different languages, so that the operator can, through menu choices entered at the console keypad  34 , choose a preferred language in which to generate screen displays. An exemplary set of languages suitable for the North American and European markets would be English, German, French and Spanish. 
   A third, electrically erasable and programmable read only memory (EEPROM)  62  is attached to the data bus. EEPROM  62  stores data in a non-volatile manner (so that it will not be lost when the power is turned off). Among other things, EEPROM  62  stores preprogrammed injection protocols. These protocols are created and stored by the user as desired (details are reviewed with reference to  FIG. 6A , below). In addition, EEPROM  62  stores calibration information, used by CPU  52  in interpreting fluid pressure and plunger position information which it receives while performing an injection. Further, EEPROM  62  stores information on the most recently completed injection, such as the injection time and volume, so that this information may be retrieved by the operator. EEPROM  62  also stores operator preference data entered by the operator into the console (see  FIG. 6E , below). This includes the preferred display language, time, and date formats. Moreover, EEPROM  62  stores operating parameters such as a programmable pressure limit, and a flag (used in the manner described below) indicating whether the injector will be used with partially pre-filled syringes of the kind illustrated in  FIG. 1B . Finally, EEPROM  62  stores the registered name and/or number of the machine owner, to facilitate service and on-line customer support. 
   Data bus  56  is also connected to a random access memory (RAM)  64  which is used by the operating system to store a stack of register values generated during CPU operations and machine state information corresponding to currently inactive processes running on the CPU. The application software uses the remaining space in RAM  64  (as managed and allocated by the operating system) to store variables computed and manipulated during operation of the injector. 
   Most communications between CPU  52  and the powerhead  40  and console  30  flow through one of two universal asynchronous receiver/transmitters (UARTs)  66 ,  68  which are connected to the data bus. A UART is a communications circuit, generally available in integrated circuit form, which collects and buffers incoming and outgoing information to enable asynchronous communications between processors or computing systems over a data link. A suitable UART is the MC68681, sold by Motorola. The first UART  66  is responsible for communications with the powerhead circuitry (see  FIG. 5 , below), which pass through an interface  70  and a communications cable  71  connected to the powerhead. (However, pulses from the optical encoder  166  on the powerhead ( FIG. 5 , below) travel directly from interface  70  along line  71  to an interrupt input on the CPU  52 .) UART  66  also handles communications with an auxiliary interface  72 , which can be coupled through a communications cable  73  to a printer to allow CPU  52  to print records of an injection. Alternatively, interface  72  (or another, similar interface) can be used to attach CPU  52  to a remote computer or other external device to allow remote monitoring and/or control of the injector. 
   The second UART  68  is responsible for communication with the console  30  ( FIG. 2A ). Two consoles  30  can be connected to the powerpack via cables  75 ,  76 . 
   Cables  75  and  76  carry data representing keystrokes and screen activity between the powerpack  50  and console  30 . This data is encoded in a communications protocol and transmitted in accordance with the RS422 standard. The encoded data is carried via lines  75  and  76  to interface  74  which encodes and decodes transmissions for a second UART  68 . UART  68  routes keystrokes received by either console via interface  74  to CPU  52  via the data bus  56 , and further routes display information produced by CPU  52  to interface  74  for transmission to the consoles via lines  75 A and  76 A. 
   Cables  75  and  76  also include, on separate conductors, lines  75 B and  76 B, which carry logical signals corresponding to key  38  ( FIG. 2A ) of each console keyboard. As elaborated below, the software driving the console displays is written so that key  38  is the most frequently used key—depending on the screen being displayed, key  38  will function as an “Exit” key to depart the screen, an “Enter” key to accept a value or selection and depart the screen, or a “Disable” or “Cancel” key to terminate an operation. (Exemplary screens are discussed below with reference to  FIGS. 6A-6F .) Because key  38  is the most frequently used key, and because key  38  is used for time-sensitive input such as a cancel command, key  38  is connected to the CPU  52  differently than the other keys. Key  38  is connected directly to the CPU  52  via an interrupt line  79 ; when a keystroke is detected, a non-maskable interrupt interface (NMI)  78  (which essentially constitutes a RS422 transmitter and receiver which converts the signal on lines  75 B and  76 B to a clean logic signal on line  79 ) sets an interrupt on line  79 , which is immediately detected and subsequently serviced by CPU  52 . 
   A similar interface is used for the remote handswitch. The cable  81  leading from the handswitch connects to the handswitch interface circuit  80  which among other things, electrically isolates the handswitch from the powerpack ground, and “de-bounces” the handswitch (eliminates electrical noise created when the switch is pressed or released) so as to provide a clean logic signal indicating whether the handswitch button is being pressed or is released. This logic signal is connected, via line  82 , to a time processor unit (TPU) port on CPU  52 . CPU  52  reads the logic signal at this TPU port and responds appropriately according to the software in EPROM  58 . 
   The last component on the CPU data bus  56  is an analog to digital converter (A/D)  84 . This converter is used to generate a digital signal, readable through data bus  56 , which corresponds to an analog signal received on line  85 . A suitable A/D converter is the LT1094, sold by Linear Technology of 1630 McCarthy Blvd., Milpitas, Calif. 95035. A/D converter  84  is used by the motor servo control circuitry described below. The CPU has two additional interfaces to the motor servo control circuitry: an interface on line  87  to a digital to analog converter (D/A)  86  (which generates an analog signal on line  88  corresponding to a digital signal received on line  87 , for example the AD7245, sold by Analog Devices of One Technology Way, P.O. Box 9106, Norwood, Mass. 02062), and a second interface on line  90  to pressure limit control circuit  92 . These interfaces (lines  87  and  90 ) connect to synchronous peripheral interface (SPI) channels on the microprocessor, and are controlled in accordance with the software in EPROM  58 . 
   The D/A  86 , A/D  84 , servo control  94 , pressure limit control  92 , and pressure sense  96  circuits collectively form a motor servo control circuit which controls the operation of the motor  98  which drives the syringe plunger into and out of the syringe. (Motor  98  is shown for clarity, but it should be understood that motor  98  is physically located in the powerhead  40  ( FIG. 2B ,  5 ); lines  91  and  93  connect to the motor through several conductors of the computer interface cable connecting the powerhead  40  and the powerpack.) 
   Servo control circuit  94  responds to an analog voltage produced by D/A  86  on line  88  and produces a corresponding voltage between lines  99  and  100 . The voltage on lines  99  and  100  is transformed by transformer  102  to a level sufficient to drive motor  98  via lines  91  and  93 . Servo control circuit  94  contains a flyback transformer circuit which produces an output voltage related to the duty cycle of a switching FET. This duty cycle is produced by a UC3525 pulse width modulation (PWM) circuit—an integrated circuit which produces a 100 kHz digital output signal having a duty cycle which varies from 0% to 50% in response to an analog input voltage on line  88 . A suitable PWM circuit is the UC3525, sold by Unitrode of 7 Continental Boulevard, Merrimack, N.H. 03054. Thus, CPU  52  controls the speed and power output of motor  98  by writing a digital word representing a desired output voltage to D/A  86  via lines  87 ; this digital word is then converted to an analog signal, and the analog signal is converted to a pulse width modulated control signal in the servo control, resulting in the desired output voltage at the motor. 
   Pressure sense circuit  96  includes a current sense circuit of which detects the current flow through line  93  (i.e., through the motor) and produces analog signals on lines  104  and  85  proportional to the detected current. In essence, this current sense circuit comprises a low-value, high power rating resistor in series with line  93  which is attached to the motor  98 . A differential voltage amplifier (based on a low-noise, high common mode rejection op-amp) senses the voltage across the resistor and converts it to an analog voltage on lines  85  and  104 . The current flow through the motor is proportional to the force exerted by the motor and therefore to the injection pressure. Thus, the analog signals produced by pressure sense circuit  96  can be used to derive the injection pressure. 
   Pressure limit control circuit  92  uses the analog signal on line  104  to perform a hardware pressure control function. Pressure limit control circuit  92  contains a commercially available digital potentiometer, used to produce an analog comparison voltage. A suitable potentiometer is the DS1267, sold by Dallas Semiconductor of 4350 Beltwood Parkway South, Dallas, Tex. 75244. CPU  52  (via lines  90 ) programs this potentiometer to produce a comparison voltage corresponding to the maximum allowable pressure. Pressure limit control circuit  92  includes a comparator which compares the analog signal on line  104  produced by pressure sense circuit  96  to the comparison voltage. If the pressure exceeds the maximum allowable pressure (indicating a failure in the CPU  52 ), a digital signal is transmitted on line  105  to servo control circuit  94 , which in response ignores the analog signal on line  88 , and instead reduces the voltage on lines  99  and  100  to halt the motor. Thus, once the CPU  52  has programmed pressure limit control circuit  92  with the correct maximum pressure, the injector will not exceed this pressure even if the CPU  52  fails. 
   Under normal conditions, this hardware pressure limit will not be activated, because CPU  52  continuously obtains feedback on the performance of the motor and the pressure produced and controls the motor through D/A  86  to achieve the desired injection protocol. CPU  52  obtains feedback on an ongoing injection from three sources: (1) feedback on the injection pressure is obtained from A/D  84 , which produces a digital word on bus  56  corresponding to the analog voltage on line  85  produced by pressure sense circuit  96 ; (2) feedback on the motor speed is obtained from an optical encoder  166  physically coupled to the motor inside of the powerhead  40  (elaborated with reference to  FIG. 5 , below); and (3) feedback on the position of the plunger inside of the syringe is obtained from a linear potentiometer  168  physically coupled to the plunger (see  FIG. 5 , below). Using this information, CPU  52  carefully controls the injection pressure, volume and speed according to a pre-programmed protocol under control of software in EPROM  58 . 
   Power for the powerpack, powerhead, and console display is supplied by the AC power lines  107  and  108 . The AC line voltage is conditioned by a conventional power supply circuit  106  which includes a transformer which can be adjusted for use with non-United States line voltages, and a voltage sense circuit for selecting the appropriate transformer based on the detected line voltage. The power may be turned off by unplugging the injector, or preferably by a toggle switch which opens and closes a solid-state relay in remote on/off circuit  110 . 
   Referring to  FIG. 4 , the console circuitry is also built around a general purpose CPU  120 . A suitable microprocessor is the MC68332FN. The address bus  122  and data bus  124  connected to CPU  120  connect to a number of supporting circuits. Program ROM  126  contains the software which directs CPU  120 . (This software is written in assembly language, and is included in the attached appendix). Font ROM  128  includes font information retrieved by CPU  120  in producing fonts for text generated on the display screen. These fonts include foreign-language characters where necessary to support foreign language text. RAM  130  is used by microprocessor in performing display and retrieval operations. Battery-backed RAM  132  stores the current time of day, so that the powerpack may make a date and time-stamped record of an injection. 
   The primary function of the console circuitry is to generate screens on the display  32 , and to receive keystrokes from the eight-key keypad  34  ( FIG. 2A ) and relay the keystrokes to the powerpack. Displays are generated by a display controller  134 , such as the F82C455 VGA controller sold by Chips &amp; Technologies of 3050 Zanker Road, San Jose, Calif. 95134. This VGA controller interacts with CPU  120  via an address buffer  136  and data buffer  138 , and stores screen information in a dynamic random access memory (DRAM)  140 . Information is sent over lines  142  to the display  32 . 
   Keystrokes from the keypad are received by keyboard interface circuit  144  which “debounces” the keystrokes, producing clean logic signals on lines  146 . These logic signals are fed back to CPU  120  so that it may confirm keystrokes by producing an audible tone through speaker control circuit  150 . Speaker control circuit also generates unique audible signals to indicate other operations, such as the initiation of an injection, or to notify the operator that scanning should begin. A suitable controller is the MC3487, sold by Motorola. 
   CPU  120  communicates with the powerpack via an RS-422 interface circuit  148  which sends and receives digital signals over lines  75  and  76 . Interface circuit  148  also receives and forwards keystrokes directly from keyboard interface  144 . The eight keys on the console form a single, eight bit byte of information (where each bit indicates whether the key is pressed or released). This byte is coupled directly to CPU  120  via a “245” type logical buffer. 
   +28 Volt DC power is received from the power supplies in the powerpack via lines  152 . A power supply circuit  154  regulates this +28 Volt DC power line into a collection of supply voltages, as needed by the various circuitry in the console. Furthermore, a power inverter circuit converts +12 Volt DC power produced by the power supply circuit  154  into low-current 600 Volt AC power supplies for energizing the liquid crystal display. 
   Referring to  FIG. 5 , the powerhead also includes a circuit board  160  including microprocessor to perform communications with the powerpack  50  ( FIG. 2C ). A suitable microprocessor is the 68HC11E2, sold by Motorola, which is a low-cost, minimal functionality microprocessor in the 68000 family. The circuit board receives and forwards keystrokes from the buttons on the keyboard  162  (described above), and electrical pulses indicating movements from the manual knob  163  mounted on the rear of the powerhead. A suitable manual knob is the model 600 thumbwheel, sold by Clarostat of 1 Washington Street, Dover, N.H. 03820. The circuit board also lights and extinguishes the injecting/fault indicator light  49 A and the enabled indicator light  49 B. 
   The motor  98  is coupled to a gear box which translates rotary motion of the motor to linear translation of the plunger. One suitable motor is the CYMS A2774-2 motor, sold by Barber-Colman, P.O. Box 7040, Rockford, Ill. 61125. The rotation of the motor is detected by optical encoder  166  (encoder  166  essentially comprises a pinwheel which rotates between a light source and a light detector to produce electrical pulses, for example the HEDS-9100 encoder, sold by Hewlett-Packard of 3003 Scott Boulevard, Santa Clara, Calif. 95054). Encoder  166  sends electrical pulses to circuit board  160 , which relays them to powerpack  50 , allowing CPU  52  on the powerpack to monitor movement of the motor. 
   The position of the plunger is detected by a linear potentiometer  168 , for example the LCPL200, sold by ETI Systems of 215 Via Del Norte, Oceanside, Calif. 92054. The wiper  169  of potentiometer  168  is mechanically coupled to and moves with the plunger  12 . A DC voltage drop is placed across the potentiometer terminals  170  and  171 , and as a result, an analog voltage representative of the location of the plunger and wiper  169  is produced at the wiper  169 . An A/D converter on circuit board  160  converts this analog voltage to a digital signal which circuit board  160  forwards to the powerpack  50 . 
   Circuit board  160  also detects the output of two Hall effect sensors  172  and  174 . The powerhead has a removable face plate  42  ( FIG. 2B ). There are currently two different face plates having differently-sized apertures for accepting differently-sized syringes. Thus, although the face plate need not be removed to replace the syringe, it may be removed to use a different syringe size. Sensor  172  detects whether face plate  42  is open, and if so circuit board  160  sends a message to powerpack  50  which prevents any further injection procedures until the face plate is closed. Sensor  174  detects the size of the face plate in use. Currently, only one of the two face plates includes a magnet which triggers sensor  174 ; thus, circuit board can determine which face plate has been installed by determining whether sensor  174  has been triggered. This information is also forwarded to CPU  52  in the powerpack so that CPU  52  may compensate for the different syringe sizes when controlling motor  98  (as described below). 
   At the direction of CPU  52 , circuit board  160  also controls heater blanket  176 , which heats the contrast fluid in the syringe. Furthermore, circuit board  160  controls movement indicator board  178 . Movement indicator board  178  is mechanically coupled to the plunger  12  and includes two light emitting diodes LEDs  179  which are visible through window  44  on the powerhead ( FIG. 2B ). LEDs  179  provide the operator with feedback on the position of the plunger, by correlating the position of the diodes with the graduated scale on window  41 . The two sides of the window  41  contain different graduated scales: one calibrated for large syringes and one for small syringes. Depending on the syringe size detected by sensor  174 , the LED next to the appropriate graduated scale is illuminated. Furthermore, as discussed in more detail below, when the plunger is moving, CPU  52  directs circuit board  160  to flash the LED. Also, when the CPU  52  enters its “locked mode” (discussed above), CPU  52  directs circuit board  160  to steadily light the LED. Thus, LEDs  179  provide operator feedback on the plunger position, direction of motion, and the “locked mode”. 
   Referring to  FIGS. 6A-6F , an injection protocol will be described from the operator&#39;s perspective. The main operating screen is illustrated in  FIG. 6A . Box  200 , which is associated with an iconic representation  201  of the powerhead, identifies the current volume of contrast media in the syringe. Box  202 , which is associated with an iconic representation  203  of the syringe, identifies the total volume which has been dispensed during the currently selected protocol. Box  204  identifies the pressure limit pre-selected by the operator for the procedure, and box  206  identifies a scan delay (in seconds), which is the delay from the time the operator initiates an injection (either with the handswitch, a key on the console or a button on the powerhead) until the x ray or magnetic scan of the subject should begin (at the end of this delay, CPU  120  produces a tone indicating to the operator that scanning should begin; alternatively, scanning could be automatically initiated by a suitable electrical connection between the scanner and injector). In the illustrated situation, the syringe contains 180 ml of fluid, 30 ml of which will be used by the currently selected protocol, the pressure limit is 200 psi and there is no scan delay. 
   In the display illustrated in  FIG. 6A , the upper regions of the screen display stored injection protocols. Region  208  identifies protocols which the operator may select, and region  210  gives details of the currently selected protocol. As shown in region  210 , a protocol comprises a number of phases; during each phase the injector produces a pre-programmed flow rate to output a pre-programmed total fluid volume. The illustrated protocol “SERIO VASCUL” has only one phase; however, other protocols which can be selected by the operator have multiple phases. In region  208 , protocols are identified by name and by number of phases; thus, as illustrated, the “LIVER” protocol has 2 phases and the “ABDOMEN PI” protocol has 3 phases. 
   The user can select protocols, enable an injection, and otherwise navigate through display screens by pressing the buttons on the keypad  34  next to the display. Region  212  of the display is dedicated to identifying the functions available from the buttons on the keypad  34 . Thus, in this display illustrated in  FIG. 6A , the user may select the previous or next protocol in the list in region  208  by pressing the buttons next to the words “PREVIOUS PROTOCOL” and “NEXT PROTOCOL”, respectively, on the display. The user may also change and store the flow, volume and inject delay values for the current protocol by pressing the button next to “CHANGE VALUES”; doing so will alter the function of the keypad and region  212  of the display, so that the operator may select a value, increment and decrement the value, select characters to form or edit a protocol name, and then return to the display shown in  FIG. 6A . From  FIG. 6A , the operator may also enter a control panel display (see  FIG. 6E , below) to adjust operating parameters and other data. Also, the operator may enter a protocol manager in which the operator may rename or delete protocols, and may determine the order of the protocol list shown in region  208 . Finally, the user may also enable an injection from the display illustrated in  FIG. 6A  by pressing the button next to “ENABLE”. 
   As shown in  FIG. 6B , when the user enables an injection, as a safety measure, the injector first presents a text box  214  which asks the operator whether all of the air has been evacuated from the syringe. Region  212  of the display contains only the words “YES” and “NO”, indicating that the operator must answer the question as either yes or no. If the button next to “NO” is pressed, the injection will be cancelled. If the answer is “YES”, the injector will proceed to an enabled state, illustrated in  FIG. 6C . Here, region  208  of the display indicates the expected duration, and region  212  includes the word “START”, “AUTO ENABLE” and “EXIT”. If the operator presses the button next to “EXIT”, the injector will return to the state illustrated by  FIG. 6A . If the operator presses the button next to “AUTO ENABLE”, the injector will toggle into and out of the auto-enabled mode, as confirmed by a briefly-displayed box in the center of the screen. If the operator presses the button next to “START” the injection will begin and the injector will move to the state illustrated by  FIG. 6D . 
   While an injection is proceeding, the display shown in  FIG. 6D  is displayed. In this display, region  208  indicates the total injection time and the volume (in ml) delivered to the patient. Region  212  shows the word “STOP” next to each of the buttons on the keypad  34 , indicating that the operator may stop the injection by pressing any of the buttons (or by pressing the start/stop button  45  on the powerhead, or by pressing the handswitch). In addition, in box  200 , the total volume of fluid in the syringe counts down as fluid in injected into the subject. 
   After the injection protocol has completed, the injector will return either to the state illustrated by  FIG. 6A  or to the state illustrated by  FIG. 6C . If the operator put the injector in the auto-enable mode by pressing “AUTO ENABLE” at  FIG. 6C , the injector will return to the state illustrated by  FIG. 6C . However, if the operator did not put the injector into the auto-enable mode, the injector will return to the state illustrated by  FIG. 6A . Thus, by placing the injector in auto-enable mode, the operator can more easily repeat an injection protocol; this can be useful where, for example, the contrast media dissipates relatively rapidly, and multiple images will be taken on the same area of the subject. By using “AUTO ENABLE”, the operator may replenish the contrast media just before each image by pressing a single key (or the handswitch), without re-enabling the injector. 
   As noted above, injection operators may wish to use prefilled syringes for injections. However, prefilled syringes often include extenders which reduce the filled volume of the syringe (syringes of this type are known as “partial pre-filled” syringes). The injector described herein includes a feature for compensating for the reduced volume of partial pre-filled syringes, described below. 
   As noted above, to set up the injector, the operator may enter the “Control Panel”, illustrated in  FIG. 6E . In the control panel, the display identifies the current operational settings of the injector. Thus, the control panel includes a box  220  which identifies the current pressure limit, a box  222  which identifies the current language (as noted above, the operator may choose a language for the textual portions of the display), boxes  226  and  228  which identify the current time and date, and a box  230  which identifies the owners registration name and/or number. This information is entered using the keypad and region  212  of the display in the manner discussed above. 
   An additional box  232  on the “Control Panel” display is used to indicate whether partial pre-filled syringes will be used with the injector. Box  232  will include the word “YES” or “NO”, as selected by the operator (as shown in  FIG. 6E , when the user attempts to modify this box, region  212  of the display provides a menu with the choices “YES” or “NO”). 
   If the operator has modified box  232  to indicate that partial pre-filled may be used (i.e., box  232  has a “YES”), then the enable procedure described above is modified slightly. If partial pre-filleds may be used, after the operator enables an injection by pressing “ENABLE” at the display of  FIG. 6A , the injector presents the screen illustrated in  FIG. 6F , in which the operator must identify the pre-filled syringe size by pressing a button next to “50 ml”, “65 ml”, “75 ml”, “100 ml”, or “125 ml”. Once the operator has identified the pre-filled syringe size, the injector will continue to the display illustrated in  FIG. 6B . CPU  52  ( FIG. 3 ) will then compensate for the extender in the syringe, in the manner described below with reference to  FIG. 7B . 
   Referring to  FIG. 7A , the program operating in CPU  52  is initiated  240  when the power is turned on. The program first initializes  242  the hardware and software attached in powerpack  50 , powerhead  40  and display  30 . Then, CPU  52  performs  244  diagnostics to ensure that the injector is operating properly; essentially, this involves sending test data to various hardware elements and verifying that the appropriate responses are received. 
   After these diagnostics have passed, CPU  52  initiates a number of “threads”, or parallel processes; thereafter, these processes are time-multiplexed on CPU  52  under control of the above-described USX68K operating system. These threads communicate with the operating system and with each other by “messages” or semaphores—essentially, interprocess communications are placed in a globally accessible area, managed by the operating system, where they can be later retrieved by other threads. The operating system allocates processing time to the threads. Much of the time, a thread will be “inactive”, i.e., it will not have any pending operations to perform. The threads are generally written so that, if the thread is inactive, it will notify the operating system of this fact (“return time” to the operating system) so that the operating system can reallocate processing time to another thread. 
   The operating system allocates processing time to threads in a prioritized, round-robin fashion. Thus, the operating system will provide processing time to each thread generally in turn; if an active, low-priority thread uses more than a maximum amount of processing time, the operating system will interrupt the thread, and provide other, higher priority threads with an opportunity to use processing time. However, a high-priority thread will not be interrupted by lower priority threads, regardless of whether the high-priority thread uses more than the maximum amount of processing time. Under normal operation, most of the threads are inactive, and there is no conflict between threads for processing time; however, in those occasions where there is a conflict, this prioritized system allows the most important threads to continue uninterrupted where necessary. It should be noted, however, that even the highest priority thread (servo thread  254 ) occasionally returns time to the operating system (at those moments where an interruption can be tolerated), so that other threads are able to continue their operations even while the highest priority thread is active. 
   The threads operating in the CPU  52  generally fall into two categories: “communicating” threads which send information into and out of the powerpack  50 , and “operating” threads which generate or process the information sent or received by the powerpack. There are two operating threads: state machine thread  246  and servo thread  254 . 
   State machine thread  246  directs the console  30  to produce screen displays of the type shown in  FIGS. 6A-6E , and also processes button presses by the user. Thread  246  is essentially a state machine, where each “state” corresponds to a display screen, and each operator keystroke produces a state transition. The software in program EPROM  58  ( FIG. 3 ) essentially defines a state transition diagram, identifying specific states, displays associated with those states, and, for each state, the keystrokes or other activity which will cause a transition to another state. 
   As shown in  FIG. 7B , when initiated, thread  246  looks  270  for a message, for example a message from a communications thread indicating that console button was pressed, or a message from the servo thread indicating that the display should be updated to reflect recent injection activity. If no message has been received, the thread returns  272  time to the operating system. However, if a message has been received, the thread uses the software in program EPROM  58  to identify and transition  274  to the new state associated with the received keystroke or activity. In some cases, e.g. where the operator has pressed an invalid button, the new state will be the same as the old state; in other cases, the new state will be a different state. If the new state is a different state, the state machine thread sends messages to the appropriate communication thread to modify  276  the screen to reflect the new state. In addition, the state machine thread may send  278  messages to the servo thread, e.g. to notify the servo thread that the operator has pressed a button which starts a protocol. When this is completed, the state machine returns  280  to the operating system. 
   When a start message is sent to the servo thread, the thread sending the message initiates one or more global variables to indicate the kind of movement requested. Eight global variables (variables managed by the operating system and accessible by all threads), organized into four pairs, are used for this purpose. Each pair of variables identifies a desired new position for the plunger and a speed at which the plunger should move to that position. Four protocol phases can be described by the four variable pairs, and thus may be executed in one message to the servo thread. Thus, when the state machine thread sends  278  a message to the servo thread, it computes one or more desired ending positions and speeds from the selected protocol, and places the computed values into global variables. 
   Referring to  FIG. 7C , when initiated by the operating system, the servo thread  254  first checks  282  for a message telling the servo to start motion of the plunger. If no message is received, the servo thread returns  284  time to the operating system. If, however, a start message has been received, the servo thread starts  286  the motor to move to the desired position indicated by a global variable at the desired speed indicated by a global variable. At this point, the servo thread enters a loop; during each iteration the loop checks  288  if the plunger has arrived at the desired position (the plunger position is determined by the powerhead receive thread  260  as illustrated in  FIG. 7E , below), and if so, the loop terminates and the servo thread stops  290  the motor and returns. However, if the plunger has not arrived at the desired position, the servo thread checks  292  if the speed of the motor is correct (the motor speed is measured by an interrupt routine illustrated in  FIG. 7D , below). If the motor speed is incorrect, it is corrected  294  by adjusting the motor voltage. Once these steps are completed, the servo thread allows  296  the operating system three time slices (about 21 milliseconds) to operate other processes, after which it returns to step  288  to close the loop. 
   Referring to  FIG. 7D , as noted above, the motor speed is measured by an interrupt routine. When a pulse is detected from the optical encoder  166  ( FIG. 5 ) attached to the motor  98 , the processor in the powerhead circuit board  160  causes an interrupt to travel on line  71  to CPU  52 . When this interrupt is received  300 , the interrupt routine computes  302  the time elapsed from the previous count interrupt, and from this elapsed time computes  304  the plunger speed. This speed value is stored  306  in a global variable (where it can be accessed by the servo routine), and the interrupt is done  308 . 
   Referring to  FIG. 7E , the powerhead receive thread  260  is responsible for receiving messages from the powerhead and performing a number of tasks in response, including relaying manual movements of the plunger to the servo thread and (as noted above) relaying position measurements to the servo thread during movement of the plunger. 
   When the operating system initiates  260  the powerhead thread, the thread first checks  310  for any messages; if none have been received, the thread returns  312  time to the operating system. However, if the thread has received a message, it determines  312  what the message is and acts appropriately (this determination is illustrated for clarity as a multi-way branch, but in the code in the fiche appendix it is implemented as a series of individual tests performed on in sequence). The message may contain an error message  314 , a manual knob movement  316 , a linear potentiometer reading  318  (which are periodically generated by the powerhead), a fill button reading  320  (which is periodically generated by the powerhead), a start/stop button press  322 , or several others (multiple messages may be received at one time). 
   As shown in  FIG. 7E , if the message contains a linear potentiometer reading  318 , the reading is converted  324  into an equivalent volume (using calibration readings stored in EEPROM  62 ). Then, an offset value (which compensates for the presence of the extender in a partial pre-filled syringe), is subtracted  326  from the computed volume, and the result is stored in a global variable, where it can be later accessed by the servo thread at step  288  ( FIG. 7C ). The offset value used in step  326  is generated when the user identifies the partial pre-filled size in response to the display shown in  FIG. 6F ; if partial pre-filled syringes are not used, the offset is set to a constant zero value. Once the adjusted volume is stored, the powerhead thread returns  328  time to the operating system. 
   As shown in  FIG. 7F , when a fill button reading is received (i.e., the received message indicates the state of buttons  46 ,  47  and  48  on the keyboard  162  of the powerhead), the powerhead thread first determines  330  which button, or buttons, are pressed. 
   If a “fast” button  48  and the forward button  46  or reverse button  47  are pressed  332 , the thread first determines  334  whether the motor is at its maximum, latching speed (by reading the global variable indicating the motor speed, as produced by the interrupt routine illustrated in  FIG. 7D ). If not, the thread increases  336  the motor speed in the indicated direction—by increasing the value of the global variable identifying the desired speed, setting the global variable identifying the desired location to identify the end of the syringe (and sending a start servo message to the servo thread if the motor is not already running)—and returns  338  time to the operating system. If, however, the motor has reached its latching speed, then the thread determines  340  if buttons were pressed the last time a fill button reading was processed. If so, then the operator has accelerated the motor to its maximum speed and is continuing to hold down the buttons. In this situation, the motor should continue running at its maximum speed; therefore, the thread simply returns  338  time to the operating system. If, however, buttons were not pressed last time, then the operator latched the motor at maximum speed, released the buttons, and some time later pressed a button in an attempt to stop the motor. Thus, in this situation, the thread stops  342  the motor (by setting the global variable indicating the desired speed to zero), and returns  338  time to the operating system. 
   If the operator is pressing  344  the forward or reverse buttons alone, or any other combination of buttons, the thread first determines  346  if the motor is running (by checking the value of the global variable indicating the motor speed). If the motor is not running, then a single keystroke will not start it running, so the thread simply returns  338  to the operating system. If, however, the motor is running, then the thread determines  348  if buttons were pressed the last time a fill button reading was processed. If buttons were pressed last time, then the operator is merely trying to keep the motor running at its current speed by holding a button down; therefore, in this situation, the thread simply returns  338  to the operating system, allowing the motor to continue running. If, however, buttons were not pressed last time, then the operator latched the motor at maximum speed, released the buttons, and some time later pressed a button in an attempt to stop the motor. Thus, in this situation, the thread stops  342  the motor (by setting the global variable indicating the desired speed to zero), and returns  338  time to the operating system. 
   If no buttons are pressed  352 , the thread simply determines  354  if the motor is at its latching speed. If not, the thread stops  356  the motor and returns time to the operating system. Otherwise, the thread returns  338  directly, allowing the motor to continue running at the latching speed. 
   Referring to  FIG. 7G , manual motion can also be created by turning the manual knob  163  ( FIG. 5 ) mounted on the rear of the powerhead. As noted above, the powerhead CPU  160  regularly reports movements of the manual knob to the powerpack CPU  52 . This report identifies the direction of rotation and the number of electrical pulses received from the knob since the last report (more pulses indicating greater speed of rotation). When a manual knob message is received  316 , the powerhead receive thread first computes  340  a desired plunger speed from the number of pulses identified in the message, and computes  342  a desired end position from the number of pulses and the direction of rotation of the knob. These are then stored  344  in global variables accessible to the servo thread as described above. If the motor is not already running, the powerhead receive thread also sends a servo start message to the servo thread. Then the thread returns  346  time to the operating system. 
   The invention has been described with reference to a specific embodiment. However, it will now be understood that various modifications and alterations can be made to this specific embodiment without departing from the inventive concepts embodied therein. For example, the manual motion knob  163  may be replaced by any other control which allows velocity and direction control, for example by a button or knob which can be rotated or rocked to multiple positions corresponding to various velocities and directions of motions, or a set of buttons or knobs which allow the operator to separately select a desired velocity with one button or knob and a desired direction with another button or knob. As various changes could be made in the above-described aspects and exemplary embodiments without departing from the scope of the invention, it is intended that all matter contained in the above description shall be interpreted as illustrative and not in a limiting sense.