Abstract:
An apparatus and method are disclosed for controlling infusion of liquid into a patient using a peristaltic pump. The liquid infusion apparatus includes a liquid conduit having a proximal segment, a distal segment and an intermediate segment. The liquid infusion apparatus also includes a flow valve in fluid communication with the intermediate and distal segments. The flow valve includes a shuttle member slidably disposed within a housing about the distal segment, the shuttle member being configured for lateral movement relative to an elongated axis of the distal segment. The shuttle member is configured to operate in a first configuration to impede fluid flow through the liquid conduit and in a second configuration for which fluid flow through the liquid conduit is unimpeded. The shuttle member includes a resilient grasper for selectively engaging and disengaging a mating actuator resiliently received by the grasper.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 12/494,166, filed Jun. 29, 2009, which is a divisional of U.S. patent application Ser. No. 11/271,705, entitled “Liquid Infusion Apparatus,” filed Nov. 10, 2005, the entire contents of each of which are incorporated by reference herein. 
    
    
     FIELD 
     This invention relates to apparatus for infusing liquid into a patient, and more particularly to such apparatus suitable for operation in a magnetic resonance image (MRI) environment of high magnetic fields and required low radiofrequency interference. 
     BACKGROUND 
     It is desirable to carefully control the intravenous (IV) administration of liquids to a patient. Conventional gravity IV solution delivery via commonly-available IV administration sets is typically not sufficiently accurate for the delivery of many types of fluids and drugs. Various positive displacement pumping devices have been developed for carefully controlling such IV administration. Some types of IV pumps control flow within a standard IV administration set via peristaltic (either linear or rotary) pumping schemes directly on the tubing of a conventional IV infusion set. Other types may incorporate a proprietary volumetric cassette, and still other types utilize a syringe-like device. However, there currently exists no IV controller capable of completely safe operation within a MRI suite wherein a considerable need exists for the controlled delivery of medicinal liquids. Frequently, patients scheduled for MRI examination arrive at the MRI suite with IV solutions being administered and controlled by devices which must be disconnected as the patient is moved into the suite where high magnetic fields are present and no outside RF interference can be tolerated. 
     The basic characteristics of an infusion pump involve the delivery of medicinal or nutritional liquids, over time, into the venous system of a living subject. Certain physical limitations regarding the delivery rate and pressure are elemental in IV liquid-infusion control. IV fluids are pumped at pressures typically in the range of 0.2 to 10 PSI. The infusion device should include detection of over-pressure and operational limits at not more than about 20 PSI. Flow ranges typical of IV pumps are from 0.1 to 2000 ml/hr. Such specifications for conventional IV infusion apparatus are quite different from the specifications for injector devices which are often used in radiologic settings, including MRI, for purposes of very rapid bolus injection of image-enhancing contrast agents. Such devices ‘push’ contrast agents at pressures up to 300 PSI and in very short periods of time in contrast to IV drug delivery systems. Contrast agents are solely for image enhancement and have no medicinal value in a patient and are commonly delivered using piston or syringe-type pumps that provide the requisite high fluid pressures and rapid deliveries. 
     The high magnetic field surrounding MRI systems can negatively affect the operation of various devices (including conventional IV control devices), especially those devices that are constructed with magnetic materials, and can seriously jeopardize a patient&#39;s safety as a result of devices utilizing magnetic materials that can be attracted at high velocity into the magnetic field of the MRI system where patient or attendant personnel are located. 
     Conventional devices for infusing liquids into a patient are typically small portable units often attached to an IV pole holding both the infusion device and associated liquids to be infused. Such devices utilize either stepper-type motors or simple DC motors which include magnetic materials for providing the mechanical power required to drive the pumping unit. Further, some form of electronic control unit receives the manual inputs of prescribed infusion rate settings, and controls the pumping unit to deliver the desired quantity of liquid over time. Such control unit may emit spurious radio frequency signals as a result of poor electrical design or insufficient shielding and are therefore commonly installed outside an MRI environment and outside the shielding therefor. 
     With the advent of MRI procedures for the imaging of internal body structures, very special requirements must be satisfied in the design of medical devices intended to be used within the MRI environment. MRI systems exploit the physical phenomenon of nuclear magnetic resonance (NMR) by which RF stimulation of atomic nuclei within an associated magnetic field results in the emission of a small RF ‘spin echo’ from the nucleus so stimulated. In the case of patient imaging, hydrogen nuclei bound with water are the usual targets for magnetic resonance at selected frequencies. Other molecules and compounds can also be selected for study, as in Nuclear Magnetic Spectroscopy, by choosing resonance specific magnetic field strengths and associated radio frequencies. For simplicity the typical hydrogen atom-based MRI image-acquisition process is referred to herein, but it should be recognized that the subject invention is equally useful in MRI spectrographic studies at a plurality of field strengths and frequencies. 
     The typical MRI system includes several components, as shown in  FIG. 1 . For example, the operator&#39;s console  25 ,  27  and various processing  37 , display  29 ,  31  and radio frequency and magnetic gradient amplifying equipment  33 ,  35  are all located outside of the environment of the MRI scanning suite which must be configured to eliminate image-degrading radio frequency interference and field effects of metallic structures that can introduce field distortions and become safety hazards. The MRI scanning unit produces large magnetic and RF fields, and must be capable of receiving the extremely small RF nuclear ‘echoes’, and is therefore typically located within a shielded room  11 . Such rooms greatly attenuate outside RF noise and may also provide some containment of the scanner&#39;s magnetic fields that include both fixed high B field and dynamic fields due to high-field ramping gradients. 
     However, certain devices are required to be placed in the scan room either to assist with care of the patient being imaged or for the use of attending staff. Of particular interest are those devices which must be placed in the scan room during the time of image acquisition when the patient is present and the magnetic fields are ‘up’ and RF reception of the tiny nuclear ‘echoes’ must be cleanly acquired. Electrically passive metallic items such as oxygen bottles or ‘crash carts’ present safety hazards to the patient due to their potential to be strongly attracted by the magnetic field of the scanner. Such items can be ‘pulled’ into the imaging volume where the patient is located, creating potential for serious injury or death. Additionally, great effort is made during the manufacture and installation of the scanner/magnet to assure that the lines of flux within the imaging volume are highly homogenous to assure that acquired images have minimal spatial distortion. Thus, devices formed of magnetic material that are positioned within the magnetic field of the scanner can introduce distortions into this homogeneous field and the resultant images. The level of hazard and the degree of field/image distortion due to magnetic materials depends upon the composition and location with respect to the imaging volume. 
     The hazards due to ‘flying’ objects can be controlled to some degree by the use of non-ferrous devices such as the aluminum oxygen bottle. Additionally, the gravitational weight of some devices or their rigid fixation in the scanning room may be sufficient to overcome the force of magnetic attraction on the ferrous mass of such devices toward the imaging volume. However, such devices with some ferrous mass, though inhibited from being pulled into the magnetic field, may nevertheless introduce inhomogeneity in the magnetic field. Distortions in the homogeneity of the magnetic field within the imaging volume must be kept at such a level as to be of minimal consequence to the operator reading the resultant image or data. And, the possibility of field distortion is proportionally increased as devices with metallic materials are positioned closer to the imaging volume, with the most critical position being near the center of the imaging volume, essentially where the patient is positioned. Additionally, because of the extremely low levels of RF signals produced by the target image nuclei, great care must be taken to assure that devices with active electronic circuits do not emit spurious RF signals as forms of electronic noise. Such noise can so degrade the signal-to-noise ratio of signals received by the MRI sensor coils and receivers that image resolution is reduced or rendered completely unreadable. Active circuits must be carefully shielded to assure that their RF emissions are extremely low at the specific frequencies of the imaging process. Conversely, it is possible through careful design, to place a source of RF energy for signal transmission, therapy, or the like, within the MRI environment, but such signals must be chosen to avoid the discreet Lamar frequencies unique to the particular magnetic field strength of a given MRI scanner, and must be of such high spectral purity as to coexist with the MRI without causing any deleterious effects. The intense magnetic fields produced by the MRI scanner can cause detrimental effects on the performance of common DC and stepper motors in devices needed within the MRI scanning room, to the point of making their control difficult or causing their complete failure. The gradient or time-varying magnetic fields can induce changing (AC) currents in motors and associated circuitry which may also cause false motor operation. 
     For example, injectors of image-enhancing contrast agents are commonly required to inject such contrast agent during actual imaging acquisition, and such devices include motors that contain magnetic material and that must therefore be located at a sufficient distance to reduce interactive effects with the magnet of the MRI scanner for proper operation and safety. Controllers and consoles of electronics and displays that generate spurious RF signals are therefore located outside the MRI scan room to avoid interference with the sensitive RF receivers of the RF scanner. 
     Accordingly, it is desirable to provide a self-contained, MRI-compatible infusion pump for the relatively long term control and delivery of the various infusion solutions and drugs routinely delivered to a patient within the MRI environment during image acquisition. Such devices must not emit any significant RF emissions that might adversely affect image acquisition operation from within the MRI scan room and must not interact with the magnetic fields therein either to cause distortion of the field or to be influenced by these fields sufficiently to jeopardize reliable operation of such devices. 
     For various reasons, including cost, safety, convenience, and performance, it may be desirable to use the MRI-compatible pump only for short durations while the patient is in the MRI. In this case, the patient must be disconnected from a non-MRI-compatible pump and connected to the MRI-safe pump prior to the MRI, and later switched back. Switching a patient&#39;s IV set involves a health risk due to sterility concerns and a cost in medical personnel&#39;s time. Additionally, fluid may be wasted from a prescribed volume during the IV-switch procedure. 
     Therefore, it is also desirable to provide a method for substituting an MRI-compatible pump for a prior-connected, non-MRI-compatible pump for a short duration without removing the patient from the original IV set. The MRI-compatible pump may be connected in substitution for the original pump after the original pump is removed. The original pump may be similarly reconnected, and the MRI-compatible pump removed, after the MRI is complete. By easily interchanging pumps on the same IV set installed on a patient, the time and expense of interchanging pumps are minimized, and compromises of the sterility of an IV installation on a patient are minimized. 
     An IV set commonly includes a length of tubing to extend from a fluid connector at a source of a liquid to be infused into a patient to a fluid connector disposed at a distal end of the tubing for connecting to an intravascular needle. It is desirable to be able to rapidly transfer a patient that is begin infused with liquid via a pump that is non-MRI compatible to a pump that is MRI compatible in preparation of the patent for MRI procedures, without disconnecting the tubing or removing the needle from its intravascular function, or other actions which may compromise sterility or inconvenience the patient. 
     SUMMARY 
     In accordance with the illustrated embodiment of the present invention a safe and effective infusion device for use within the MRI scan room achieves reduction of magnetic material and accurate pumping control as well as reduction of RF emissions. In one embodiment, the infusion device includes an ultrasonic motor that eliminates magnetic materials and that does not produce any detrimental magnetic fields and that is not affected by external magnetic fields. The ultrasonic (U/S) motor drives a peristaltic or other suitable fluid pumping mechanism, and is driven by a multiphasic electronic signal specifically designed to produce very little RF harmonic noise in the spectral range of about 6 or 8 MHz to about 130 MHz in which MRI receivers are most sensitive. The drive power for the U/S motor is generated via circuitry which produces multiphasic drive signals of at least sine and cosine wareforms at related ultrasonic frequencies. These drive signals are produced as a sinusoidal wave to reduce high frequency harmonic components which may disturb the MRI&#39;s RF-sensitive responsiveness. One scheme for producing these multiphasic signals uses coreless or “Air Core” transformers constructed with inherent leakage inductance that interacts with the complex impedance of the U/S motor to convert lower voltage square wave signals at the primary winding into sinusoidal high voltage signals at the secondary windings suitable for powering the U/S motor and producing little harmonic RF interference. Alternatively, D.C. voltages of opposite polarities can be alternately switched to supply alternating voltages. The switched signals can be filtered into sinusoidal signals and applied to the inputs of high voltage linear amplifiers that are set for such gain as needed to produce resultant outputs of sufficient voltage and sinusoidal shape to drive the U/S motor. 
     Control electronics receive commands through an input keypad for setting prescribed fluid flow rates to be delivered, and such inputs are translated into signals to control the U/S motor and pumping mechanism. Various safety devices feed back operational information to the control electronics, including detection of motor speed and motion of pump elements, air bubbles in the fluid path, drip rate, high pressure, low fluid, low/no flow, overtime, and the like. The present infusion device includes battery power for portability, and is housed in one RF-shielded, non-magnetic housing for convenient location anywhere within the MRI scan room without introducing image degrading RF interference or producing distortions of the homogeneous magnetic field, and without being affected by the strong magnetic fields or RF energy produced by the MRI system. Such unrestricted placement of the device is of great importance to the safety and convenience of the attending MRI staff and imaging patient. Further, in the case of a linear peristaltic pump mechanism, the particular position of pump elements, along with the speed of motion of these elements, must be known to the controller. The degree to which the controller may modulate speed and control exact positions of the pump elements directly affects the resolution and accuracy of the fluid delivery system. To provide a high degree of speed and position accuracy, an optical encoder ( 801 ,  802 ) is installed along the main pump shaft. The encoder disk ( 802 ) has many small graticule marks about the circumference, and a single index mark. The optical sensor ( 801 ) detects the marks and produces output signals indicative of both the index and individual graticule marks. The index occurs only once each 360 degrees of rotation to facilitate the controller sensing an index to know the position of the pumping elements. The rate at which the graticule marks are sensed indicates the speed of the pump shaft as well as its fine position relative to the index mark. The controller responds to the optical encoder to modulate the speed of motion of the pump elements at specific positions of the pump shaft in order to reduce inherent non-linearities in fluid delivery of the linear-type peristaltic pump. In this way, highly accurate and linear fluid flow may be achieved. 
     In an additional embodiment, a method is employed to substitute an MRI-compatible pumping device for a prior-connected, non-MRI-compatible pumping device while preserving the patient&#39;s connection to a prior-connected primary intravenous (IV) infusion set. The patient is commonly connected to a primary IV set through a primary, non-MRI-compatible pump. Upon arrival at the MRI suite, a secondary, MRI-compatible pump attached to a secondary IV set connects to the primary IV set to continue actively-pumped IV fluid delivery. The primary pump is disengaged from the patient&#39;s installed primary IV set, and a flow preventer (to shut off flow) that is standard on most IV sets is activated to inhibit liquid flow through a segment of the primary IV set. The fluid-receiving or proximal end of the secondary IV set is connected to the upstream end of the primary IV set near the source of the IV fluid. The upstream connection may be conveniently formed by puncturing a conventional “Y” connector on the primary IV set with a hollow needle on the secondary IV set, or via a luer-type “Y” site connector. Air is flushed from the secondary IV set by flowing fluid from the upstream connection, and the fluid-delivery or distal end of the secondary IV set is then connected in similar manner at a downstream connection in the primary IV set. Pumping of the liquid may then be resumed using the MRI-compatible pump operating on the secondary IV set without having dislodged the intra-vascular needle or actually opening the primary IV circuit. The patient and MRI-compatible pump may then be moved close to the MRI scanner while maintaining the controlled IV therapy. The secondary IV set so employed may simply be disconnected and discarded after the MRI procedure, again without opening the original primary IV circuit, and the original primary IV set may be reinstalled into the original, non-MRI-compatible pump while preserving the patient&#39;s connection to the primary IV set. By not opening the fluid circuit of the primary set, minimal risk to the patient and sterile IV path are achieved as well as reducing medical waste and cost of replacing the primary IV set after the MRI procedure. 
     To facilitate rapid transitions between primary and secondary pumps and infusion sets, one embodiment of the present invention includes an infusion device that receives a liquid conduit for delivering liquid to a patient at volumetric rates that are controllable by the device. Peristaltic pumping of liquid through the conduit installed within the device is enabled only upon proper registration of a flow valve within a receptacle of the device for actuation upon closing of a safety door. Flow of liquid through the conduit is inhibited upon opening the safety door, and various sensors are incorporated into the device along the path of the conduit to detect inflow and outflow liquid pressures, available liquid supply, air bubbles in the conduit, and the like. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a pictorial plan view of a conventional MRI system showing typical placement of operational components; 
         FIG. 2  is a partial perspective view of an infusion device in accordance with one embodiment of the present invention; 
         FIG. 3  is a block schematic diagram of the infusion device of  FIG. 2 ; 
         FIG. 4  is a partial perspective view of the two pumping apparatuses in accordance with one embodiment of the present pump-substitution invention; 
         FIG. 5  is a flowchart illustrating a method of temporarily substituting a secondary IV pump for a primary IV pump without removing the patient&#39;s primary IV set in accordance with the present invention; 
         FIG. 6  is a flowchart illustrating a method of replacing a secondary IV pump with a primary pump in accordance with the present invention; and 
         FIG. 7  is a side view of a length of precision tubing in accordance with the present invention; 
         FIG. 8  is an exploded top view of valve apparatus in accordance with one embodiment of the present invention; 
         FIG. 9  is a top view of the valve apparatus of  FIG. 8  in one operating configuration; 
         FIG. 10  is a top view of the valve apparatus of  FIG. 8  in another operating configuration; 
         FIG. 11  is an exploded side view of operative components in one embodiment of the present invention; 
         FIG. 12  is a partial side view of the embodiment of  FIG. 11  in assembled, operational configuration; 
         FIGS. 13A, 13B and 13C  comprise a block schematic diagram illustrating operating components of the illustrated embodiment of the present invention; 
         FIG. 14  is a schematic diagram of drive circuitry for a multiphasic u/s motor in accordance with one embodiment of the present invention; 
         FIG. 15A  is a chart illustrating typical flow rate through a linear peristaltic pump operating at constant speed; 
         FIG. 15B  is a chart illustrating flow rate through a linear peristaltic pump operating in compensated manner in accordance with an embodiment of the present invention; 
         FIG. 16  is an exploded perspective view of a pump unit according to the present invention; 
         FIG. 17  is a partial sectional view of a gasket disposed between housing segments; 
         FIG. 18  is a partial cross sectional view of annunciator lights according to the present invention; and, 
         FIG. 19  is a front view of the pump unit of  FIG. 16 . 
         FIG. 20  is a schematic representation of an embodiment of an optical encoder disk. 
     
    
    
     DETAILED DESCRIPTION 
     Issued U.S. Pat. No. 7,404,809 entitled “Non-Magnetic Medical Infusion Device,” filed on Oct. 12, 2004 by R. Susi, and issued U.S. Pat. No. 7,267,661 entitled “Non-Magnetic Medical Infusion Device,” filed on Jun. 17, 2002 by R. Susi, are incorporated herein in their entireties by this reference thereto. Referring now to the plan view in  FIG. 1  of an MRI system, the scanning room  9  is disposed within shielding boundary walls  11 , with a control room  13  for operators or attendant personnel positioned outside the boundaries of the scanning room  9 . The scanning room  9  includes the image acquisition equipment including a source  15  of intense magnetic field  16  that emanates from the source in substantially homogenous array throughout the adjacent space and around a patient  17 . Various components of the system for performing the image acquisition operations, including gradient  19  and sensor  21  and RF coils  23  are disposed about the patient  17  for stimulating the nuclei ‘echos’ to map the positions thereof within the spatially-homogenous magnetic field  16  as the patient&#39;s body is scanned in conventional manner along multiple orthogonal axes. The shielding boundary walls  11  (and ceiling and floor) provide shielding against radio-frequency interference and, as fabricated with ferrous materials, may also establish outer limits of the magnetic field distribution around magnetic  15 . 
     The control room  13  is disposed outside the shielding boundary walls  11  and is equipped with computer input keyboard  25 , computer display  27 , monitor  29  of patient&#39;s vital life signs, controls  31  for liquid infusion apparatus, and the like. Such representative equipment is housed outside the shielding boundary walls  11  to inhibit intrusion of spurious magnetic and electrostatic and RF signals into the image acquisition operations within the scanning room  9 . Similarly, the gradient amplifiers  33  for amplifying signals from conventional gradient coils  19 - 21 , along X, Y, and Z coordinates and RF amplifiers  35  and the image-processing computer  37  are also located outside the shielding boundary walls  11  for the same reason. The thru-wall interconnections  39  between the components within the scanning room  9  and the electronic equipment  25 ,  27 ,  29 ,  31 ,  33 ,  35 ,  37  disposed outside the room  9  typically also includes RF filtering to diminish the sources and the portals by which and through which RFI signals may enter the scanning room  9 . 
     A high-pressure liquid-injection device  41  commonly resides within the scanning room  9  to administer IV injection into the patient  17  of liquid compositions, for example, that enhance image acquisition (e.g., contrast medium) or that otherwise provide diagnostic or therapeutic benefits to the patient  17  being scanned. Such conventional injection device  41  should desirably be positioned close to the patient  17  to facilitate IV liquid infusion, but must be positioned remotely to avoid disrupting the homogeneous magnetic field  16 , and to minimize RFI and operational failures of the infusion device  41  resulting from operating in the intense magnetic field adjacent the patient  17 . Control of such infusion device  41  may be via remote controller  31  disposed within control room  13 . 
     In accordance with the embodiment of the invention illustrated in  FIG. 2 , an improved liquid infusion device  43  is operable within intense magnetic fields and with negligible RFI to provide positive displacement of a liquid  45  such as saline or antibiotics, or sedative, or the like, in controlled volumes per unit time. The device does not include any ferrous or magnetic materials, and is substantially shielded against irradiating any RFI during operation. Specifically, the device  43  includes a pump in the lower chamber  47 , as later described herein. The pump chamber  47  receives therein the flexible, resilient tubing  49  that is pre-packaged and sterilized as a component of a conventional IV liquid infusion set that also includes a conventional drip chamber  51  as part of the infusion set. Controls for the pump in chamber  47  include an operator&#39;s input keypad  48  that is shielded against radiation of RFI for setting infusion parameters, and a drip detector  53  that may be disposed about the drip chamber  51  to detect flow of liquid from the supply  45 . A display  53  is positioned in the upper portion of the housing  55  which may be formed of non-magnetic, RF-shielding material such as conductively-coated plastic or aluminum, or the like. The housing  55  attaches with one or more clamps  57  to a rigid support  59  formed of non-magnetic material such as fiberglass or aluminum, or the like. 
     Referring now to the pictorial block schematic diagram of  FIG. 3 , there is shown a peristaltic-type positive-displacement pump  60  disposed within the pump chamber  47  of the housing  55  to operate with the length of tubing  49  that passes therethrough between the drip chamber  51  and the patient. The peristaltic pump  60  (linear or rotational) is driven by an ultrasonic motor  64  via appropriate mechanical linkage  65  to actuate a squeeze roller against the tubing  49  in known peristaltic pumping manner, or to actuate a series of elements  67  through a linear tubing-squeezing sequence to produce peristaltic pumping action in known manner. Various visual and audible annunciators  61  may be provided to signal operational conditions either within acceptable limits, or within error or failure conditions. 
     A conventional ultrasonic (U/S) driving motor  64  is powered in known manner by multiphasic signals applied thereto from the motor drive circuit  69 . A controller  71  for the device includes a central processing unit  73  with associated peripheral components including Random Access Memory (RAM)  75 , Read-Only Memory (ROM)  77 , Digital-to-Analog (D/A) converter  79 , and an Input/Output channel  81 . This controller  71  receives input control information from the operator&#39;s keypad  48 , and receives feedback information about pump speed from sensor  83  and about liquid flow from drip detector  85  disposed about the drip chamber  51 . In response to the inputs supplied thereto, the controller  71  operates on stored programs to actuate a display  53  of operating parameters (or other data), and to actuate the motor drive circuit  69  for energizing the ultrasonic motor  64  for rotation at a controlled speed. A power supply  63  is connected to the controller  71  and drive circuit  69  to supply electrical power thereto, and is connected to a battery  87  to receive electrical power therefrom during stand-alone operation, or to receive line voltage via plug  63 , as required. 
     In accordance with this embodiment of the present invention, no magnetic material is used in any of the components of the infusion device  43  including the ultrasonic motor  64 , pump  60 , power supply  63 , controller  71  and associated components. Additionally, none of such components is adversely affected during operation by a strong magnetic field. And, any RF energy that may be generated by electronic signals within the ultrasonic motor  64 , drive circuit  69 , controller  71 , power supply  63  or associated components is specifically shielded by conductive structures  91 ,  93  disposed around such components to inhibit radiation of RFI. Additionally, radio-frequency interference filters  95  are disposed about all through-shield conductors to inhibit radiation of RFI through such portals. 
     Referring now to  FIG. 4 , in an additional embodiment of the present invention, a method is employed to substitute an MRI-compatible pumping device  406  for a prior-connected, non-MRI-compatible pumping device  430  while preserving the patient&#39;s  450  connection to a prior-connected primary IV infusion set  432 . The patient is initially connected  450  to the primary IV infusion set  432  which is installed in a non-MRI-compatible primary pump  430 . The primary pump  430  controls the pumping action in response to information entered into the unit, and in response to a sensor  434  that monitors liquid flow. Fluid connectors are disposed in the primary IV set upstream  400  and downstream  410  of the primary pump  430 . Compatible fluid connectors are also disposed in the secondary IV set upstream  402  and downstream  408  of the secondary pump  406 . 
     Referring now to the flowchart of  FIG. 5 , a method is illustrated for substituting an MRI-compatible pumping device  406  for a prior-connected, non-MRI-compatible pumping device  430  while preserving the connection  450  of a prior-connected primary IV infusion set  432  to a patient. Initially, the patient is connected  500  to the primary IV set  432  and primary pump  430 , and liquid is infused  502  into the patient through the primary IV set  432  and primary pump  430 . Before entry of the patient into the MRI environment, the primary pump  430  is disabled from pumping liquid, and the flow of liquid through the primary IV set  432  is inhibited via a shut-off mechanism within the primary IV set. As shown in  FIG. 5 , the method can include removing the primary pump  430  from the primary IV set  432  (Block  504 ) and can optionally include disconnecting a flow sensor from the primary pump  430  (Block  506 ). 
     The primary IV set  432  is then removed  504  from the non-MRI-compatible pump  430 . A flow sensor may optionally be disconnected from the non-MRI-compatible pump  430  The secondary MRI-compatible pump  406  is configured to operatively receive a secondary IV set  404 . The secondary pump  406  may include a sensor for monitoring liquid flow to control the pumping action.  1   
     To transition a patient from a non-MRI-compatible pump  430  to an MRI-compatible pump  406  without altering the primary IV set  432  as installed on a patient, the upstream fluid connector  402  of the secondary IV set  404  is connected  508  to the upstream fluid connector  400  of the primary IV set  432 . After purging the tubing of air, the downstream fluid connector  408  of the secondary IV set  404  is connected  510  to the downstream fluid connector  410  of the primary IV set  432 . The secondary IV set is operatively installed  512  into the secondary MRI-compatible IV pump  406 . In one embodiment, a sensor  434  may be connected  514  to the pump  406  for measuring the liquid pumped from the liquid source. Liquid is infused  516  into the patient through the connection  450 A of the primary IV set  432  to the patient, and through the secondary IV set  404  and secondary pump  406 . Because the secondary pump  406  is MRI-compatible, the infusion may continue via the secondary pump  406  within the MRI environment. The primary IV set  432  remains installed on a patient who therefore does not have to be directly connected at  450 A to the secondary IV set  404 , but rather the secondary IV set  404  “bypasses” the section of the primary IV set  432  that remains occluded between connectors  400 ,  410 . 
     In an additional embodiment, the secondary pump  406  is connected to the primary IV set  432  before the primary pump  430  is operatively disengaged and removed from the primary IV set  432 . 
     Referring now to the flowchart of  FIG. 6 , a method is illustrated for removing the secondary pump  406  and reconnecting the primary pump  430  after the patient is removed from the MRI environment. The fluid connectors  402  and  408  of the secondary IV set  404  are disconnected  600  from the fluid connectors  400  and  410  of the primary IV set  432 . This prevents flow of liquid through the secondary IV set  404 . In one embodiment, the flow through the primary IV set  432  is prevented by a shut-off mechanism disposed in the primary IV set  432  between connectors  400  and  410 . Such shut-off mechanism may manually cut off fluid via hand-operated slide or roller clamp, or the like. The secondary IV set  404  is operatively re-installed removed  602  from the secondary pump  406  and is discarded. The primary pump  430  is operatively re-installed  604  on the primary IV set  432  to which the patient remains connected  450 . The flow sensor  434  may be reconnected  606  to control the rate of liquid infusion through the primary IV set  432 . Liquid is again infused  608  into the patient via the primary IV set  432 , the primary pump  430  and the original connection  450  of the primary IV set to the patient. 
     Referring now to  FIGS. 4 and 7 , there are shown perspective and side views, respectively, of operational aspects of the liquid-delivery systems in accordance with one embodiment of the present invention. Specifically, a liquid conduit  404  includes a fluid connector  402  at an input or proximal end, and includes a flanged connector  701  that couples the proximal segment of the liquid conduit  404  to an intermediate segment including a length of precision tubing  703 , as later described herein, that terminates in flow valve  705 . A distal segment of the fluid conduit includes a fluid connector  408  at the distal end thereof, and is coupled at its proximal end to the flow valve  705 . The entire assembly including proximal and distal segments of the fluid conduit  404 , fluid connectors  402 ,  408 , flanged connector  701 , precision tubing  703  and flow valve  705  is prepared and sterilized and packaged in an hermetically-sealed envelope in a conventional manner for immediate installation in the pumping device  406 , as needed to infuse liquid into a patient conveniently during an MRI procedure. 
     The precision tubing  703  may be formed as a thin-walled extrusion of a flexible, elastic material such as silicone rubber, or other biocompatible polymer that confines a selected liquid volume per unit length within the bore of selected cross-sectional dimension between the flanged connector  701  and the flow valve  705 . In this way, progressive peristaltic pumping by successive pinching and advancing of the pinch point along the tubing  703  toward the flow valve  705  administers a known volume of liquid to a patient. The length of tubing  703  between flanged coupling  701  and flow valve  705  may be slightly stretched into position within the pumping device  406  to provide resilient engagement of the flanged connector  701  and flow valve  705  within their respective mating receptacles  706 ,  708  disposed at opposite ends of the active peristaltic pumping mechanism of the device  406 . 
     The flow valve  705 , as illustrated in the exploded top view of  FIG. 8 , includes an outer housing  707  within which the precision tubing  703  passes. The mating receptacle  708  in the pumping device  406  for the housing  707  includes a complementary recess that receives the housing  707  in only one orientation and secures the properly-installed housing in place with the aid of slight tension exerted thereon by tubing  703 . A slide or shuttle element  710  is disposed within the housing  707  to slide laterally relative to the elongated axis of the tubing  703 , with the tubing  703  passing through a tapered aperture in the shuttle element  710 . Thus, with the shuttle element  710  fully depressed within the housing  707 , the tubing  703  passes through the portion of the aperture of maximum cross sectional dimension, leaving the bore of the tubing  703  fully open for unimpeded flow of liquid therethrough. In alternate position of the shuttle element  710  maximally protruding from the housing  707 , the tubing  703  is pinched within a portion of minimal cross-sectional dimension of the aperture, as shown, to inhibit liquid flow through the tubing  703 . Thus, as initially installed within the pumping device  406 , the flow valve  705  is configured to inhibit flow through the liquid conduit  404  to ensure no inadvertent dosing of a patient until the pumping device  406  is rendered fully operational. 
     In accordance with one embodiment of the present invention, the device  406  is inhibited from administering liquid to a patient until a liquid conduit  404  is properly installed and an access door  407  is fully closed and safely latched shut. The access door  407  carries passive components of interlocking elements that properly engage and interface with active components of the device  406  for proper operation only with the access door  407  fully closed and safely latched shut. The region of the device  406  that is accessed through the opened access door  407  includes a generally vertical channel for receiving the flanged connector  701  in a complementary receptacle  706  that is positioned above the peristaltic pumping mechanism  712 . A sensor may be disposed above the receptacle for the flanged connector to optically sense presence of liquid in the proximal portion of the conduit  404 , and operate to inhibit the pumping device  406  from further pumping activity in response to sensing an empty conduit. 
     The access door  407  carries an upper platen  716  that cooperates with a pressure sensor  717  disposed behind a flexible membrane  711  and intermediate the receptacle  706  for the flanged connector  701  and the peristaltic pumping mechanism  712  to position an initial length of installed tubing  703  between spaced platen  716  and pressure sensor  717 . In this way, the pressure at which liquid is supplied to the device can be tonometrically determined within the precision tubing  703 , or otherwise measured, for use in correcting calculation of pumping activity required to deliver a selected volumetric infusion rate of liquid to a patient. 
     Similarly, a platen  718  is carried on the access door  407  at a location aligned with another pressure sensor  719  disposed intermediate the pumping mechanism  712  and the flow valve  705 . In the manner, similar to operation of pressure sensor  717 , the pressure sensor  719  and platen  718  confine the precision tubing  703  to provide tonometric measurement, or other measurement, of outlet pressure. An upper limit of outlet pressure may be selected to trigger an alarm condition if such liquid outlet pressure exceeds the set limit as an indication of a clogged outlet conduit. 
     The access door  407  also carries a platen  721  positioned in alignment with the peristaltic pumping mechanism  712  to confine the precision tubing  703  therebetween to effect linear peristaltic pumping activity in the generally downward direction from inlet pressure sensor  717  toward outlet pressure sensor  719 . Neither pressure sensing nor pumping activity may proceed until the access door  407  is fully closed to position the associated platens about the precision tubing  703  for proper sensing and pumping operations. 
     The access door  407  also carries a detent element  723  that mates with a resilient clamp  725  carried on the shuttle element  710  of flow valve  705 . Specifically, these mating elements effect sliding movement of the shuttle element  710  from initially protruding position (i.e., tubing  703  pinched) toward fully open position (i.e., tubing  703  not pinched) as the access door is closed, as illustrated in  FIG. 9 . Additionally, the engaged detent element  723  and resilient clamp  725  remain engaged as the access door  407  is initially opened, thereby to pull the shuttle element  710  toward maximum protrusion from the housing  707  to pinch tubing  703  and inhibit further liquid flow therethrough, as illustrated in  FIG. 10 . The attachment of the resilient clamp  725  carried on the shuttle element  710  of flow valve  705 , and the detent element  723  carried on the access door  407  is overridden and resiliently released following maximum protrusion of the shuttle element  710  and further opening of the access door  407 . Of course, detent element  723  may be carried on the shuttle element  710 , and a resilient clamp  725  may be carried on the access door  407  to effect similar interaction and safety operation. 
     An ultrasonic or optical sensor may be disposed within the device  406  at a location thereon below the flow valve  705  and about the distal segment of the liquid conduit  404  to detect the presence of air bubbles in the outlet conduit (that is formed of ultrasonically or optically-transmissive material). This sensor may include a protruding U-shaped receptacle for receiving the conduit therein and for supporting optical elements in the protruding arms of the receptable to sense bubbles in liquid passing therebetween in the outlet flow of liquid within the conduit. A mating U-shaped element  407  is supported on the access door  711  in alignment with the U-shaped receptacle of the bubble detector to capture the liquid conduit  404  fully recessed therein in order to fully close the access door  407 . 
     Referring to the partial side view of  FIG. 12 , there is shown a partial side view of the components of  FIG. 11  assembled into operational configuration. Specifically, the access door  407  disposed in closed configuration positions the platens  716 ,  718 ,  721  on one side of the intermediate length of precision tubing  703  against the respective sensors  717 ,  719  and pumping mechanism  712 . The flow valve  705  is configured to open condition and liquid is pumped through the conduit  404 ,  703  in response to rotation of the cam shaft  727  of the peristaltic pumping device  712 . In this manner, pinch points along the precision tubing  703  progress downwardly as successive pump elements  729  of the pumping device  712  are manipulated by the rotating cam shaft  727  to provide the peristaltic pumping action in known manner. 
     Referring now to  FIGS. 13A, 13B and 13C  there is shown a block schematic diagram of the operational components of the fluid delivery system according to one embodiment of the present invention. The peristaltic pump includes pumping elements or fingers  729  that are manipulated in a pumping sequence in response to rotation of the shaft  727 . The output shaft of ultrasonic (U/S) motor  800  is coupled to the pump shaft  727  that carries an optical encoder disk  802 . The optical sensing element  801  detects peripheral marks and an index mark for producing outputs indicative of disk position and speed of rotation. These outputs are supplied to the controller  71  that also receives control signals from manual-entry keyboard  48  and from pressure sensors  717 ,  719 , bubble detector  718  and access door safety switch  716 . The controller  71  generates multiphasic drive signals via drive circuit  69  and, among other functions, also controls the display  53 , alarm indicators, and the like. 
     For proper operation, the linear peristaltic pump mechanism requires a high degree of control in order to assure accuracy and linearity of fluid flow rate. The operating speed of the pump shaft is modulated to overcome flow-rate non-linearities or discontinuities commonly experienced within a peristaltic pumping cycle, as illustrated in the chart of  FIG. 15A , of fluid flow rate over time at constant shaft speed. For this reason, the controller  71  requires signal information indicative of the exact location of pump elements during the interval of a pumping cycle in order to determine requisite speed modulation and when to apply the speed modulation during a pumping cycle.  FIG. 15A  shows the uncompensated flow output of the peristaltic pump according to one embodiment operating at a very slow RPM rate, over slightly more than one revolution (one cycle of 12 pump fingers) that takes about 31 minutes and delivers about 0.32 ml of fluid. It should be noted that there exists a no-flow “dead band” of approximately 11 minutes in the 31 minute cycle, including a small discontinuity. The discontinuity is dependent on very small mechanical tolerances such as the lengths of the fingers, the perpendicularity of the platen to the fingers, and the likes which vary pump to pump. However, the long 11-minute dead band is very similar pump to pump. 
     In accordance with the present invention, very fine control of pump-flow characteristics is established utilizing modulation of the rotational speed during each cycle of the peristaltic mechanism. The resultant flow, as illustrated in the graph of  FIG. 15B  resembles the smoothness and linearity of syringe-fine pumps, a desirable characteristic when infusing potent drugs or infusing small patients, i.e., babies. 
     Specifically,  FIG. 15B  shows the flow output of the pump resulting from speed ‘modulation’ applied to each rotation, in accordance with the present invention. The rotational speed modulation is accomplished using, for example, 8 discrete different speeds of the motor and pump during the dead band interval. To accomplish such speed modulation for flow correction, the drive motor  800  must be able to start and stop very quickly and in very small angular displacement typically in the range from about 3 to about 10 milliseconds, and within about 0.3 to about 0.9 degrees of arc. The encoder  801 ,  802  outputs of index and 1000 pulses per revolution indicate to the controller  71  the starting position of the dead band (index plus mechanical offset by number of pulses counted) for compensation and the exact (i.e., the rotational distance as pulses counted) to control timing and application of the discrete speeds. After compensation is applied in this way, according to the present invention, the flow output of the linear peristaltic pump is very linear in delivering very precise amounts of fluid of about 1 ml/Hr. The lowest pump rate (1 ml/HR) is a basis for compensation as at high speeds the dead band is inherently shorter and less consequential. 
     The optical encoder  801 ,  802  provides both fine and coarse output indications of the disk position and speed of rotation. Specifically, one index mark  803  is sensed to identify the exact angular position of the pump shaft  727 , and numerous peripheral graticule marks  813  (e.g.,  1000  about the periphery) provide fine indication of angular re-positioning of the shaft relative to the index mark. Of course, the frequency of recurrence of sensed graticule marks also indicates rotational or angular speed of shaft  727 . Thus, the controller  71  receives control signals from the optical encoder  801 ,  802  that facilitate modulation of motor speed in the manner as described above to overcome discontinuities or anomalies in a selected flow rate of fluid through the peristaltic pump as illustrated in  FIG. 15B , during portions of the pump cycle driven by the ultrasonic motor  800 . 
     In order to accomplish fine resolution of fluid flow rates through the peristaltic pump, the drive motor  800  must be able to start and stop very rapidly, typically within the range of about 3 to 10 milliseconds. The driving ultrasonic signals are generated by the drive circuit  69  at about 43 KHz with very low harmonic content in the range of about 6 or 8 MHz to about 130 MHz within which MR scanners are sensitive to RF signals. This is accomplished on the drive circuit  69 , as shown in the schematic diagram of  FIG. 14 , using a shift-register type of counter  70  that receives input from voltage-controlled oscillator  72  to generate high-voltage ultrasonic frequencies in sine and cosine relationship  74 ,  76 . Coreless or air core transformers  78 ,  80  are driven push-pull through field-effect power transistors that receive paired outputs from the register  70 . The primary inductance (through the turns ratio) and the leakage inductance of these transformers  78 ,  80  coact with the characteristic input capacitance  82 ,  84  of the ultrasonic motor  800  to produce substantially sinusoidal, high-voltage drive signals  74 ,  76  of low harmonic content. These sinusoidal drive signals also pass efficiently through the filters  95  from the electrically shielded controller section  86  to the electrically shielded motor section  88 , and exhibit concomitant low to negligible RF interference attributable to drive signal harmonics. 
     It should be noted that the ultrasonic motor  800  provides an AC signal  90  representative of the composite sine and cosine drive signals. This AC signal  90  is rectified and integrated or low-pass filtered to produce a DC voltage level  92  that is indicative of motor speed, and is distinguishable from the position and rotational speed indications digitally derived from the optical encoder  801 ,  802 . The analog DC voltage level  92  is applied via the operational amplifier  98  to the voltage-controlled osullator  72  in order to control the frequency of the motor drive signals. Specifically, the rotational speed of the ultrasonic motor  800  varies inversely with frequency of the drive signals. Accordingly, an applied ‘motor run’ signal  94  in combination with the DC feedback voltage  92  and the time constant of the R and C filter  96 , cause the drive circuit  69  to generate drive signals  74 ,  76  that sweep in frequency from a slightly higher initial frequency that is useful for starting the motor  800  from standstill to an appropriate running frequency that establishes a steady-state motor speed. 
     Alternatively, the drive signals,  74 ,  76  for the ultrasonic motor  800  may be generated from combined signals Q 1 /Q 3 , and Q 2 /Q 4  through suitable filtering to generate low voltage sinusoidal sine and cosine signals. These signals may then be amplified to sufficient level (typically about 100 Volts RMS) to drive the ultrasonic motor  800 . 
     Referring now to  FIG. 16 , there is shown an exploded perspective view of one embodiment of the pump unit  406  of the present invention in which a gasket  806  is disposed between mating segments  805 ,  807  of the housing. The gasket  806  is formed of a flexible and electrically conductive material to form a fluid-tight seal between the housing segments  805 ,  807  as shown in the sectional view of  FIG. 17 . The conductive gasket  806  also inhibits internally-generated RF noise signals from radiating out of the conductive housing segments  805 ,  807 . The conductive housing segments  805 ,  807  thus form an integral shield that prevents radiative electronic signals from emanating from internal circuitry, for example as illustrated in  FIG. 13 , and additionally protects such internal circuitry from fluid spills that might be detrimental to reliable operation. 
     Referring now to the sectional view of  FIG. 18 , there is shown one light source such as light-emitting diode  804  of a plurality of such light sources and different colors that are lineally disposed within the housing segment  805  near a top edge thereof. These light sources are positioned behind the door  711  of conductive material that is hinged  730  along an outer edge of the housing segment  805  to facilitate easy access to the peristaltic pumping structure that is supported therein. The door  711  includes a locking lever  733  for securely closing the door  711  in operational position against a length of tubing  703 , as illustrated and previously described herein with reference to  FIG. 7 . The door  711  also includes a clear or translucent window  810 , as illustrated in  FIGS. 18, 19 , in alignment with the light sources  804  to provide large-area illumination for easy visualization from a distant location of the light from a source  804 . A light-scattering element or light pipe  812  may be disposed intermediate the light sources  804  and the window  810  to provide more uniform illumination over the area of the window  810  in known manner. Thus, a light source  804  of green color may pulse on and off recurring during normal pumping operation, and a light source of red color may pulse on and off recurring to indicate an alarm condition, all for convenient visualization from a distant location. And the light sources  804  are sufficiently recessed within the conductive housing segment  805  to inhibit radiative RF noise signals from emanating from the housing. 
     Therefore, the liquid infusion apparatus of the present invention promotes easy replacement or substitution of pumping devices without interrupting patient connection or otherwise comprising sterility of an installed infusion system. An infusion set includes integral segments of a liquid conduit and operable components for interaction and operational engagement with associated components of a pumping device that is compatible with an MRI environment. Ultrasonic motor drive signals are generated with low harmonic content using efficient step-up transformer that co-act with the characteristic input impedance of the ultrasonic motor to shape signals as sinusoidal waveforms of low harmonic content.