Dr. Brummett is a resident in Radiology, Dr. Fidler is an Assistant Professor and Head of the Body Imaging and GI Imaging Sections, and Dr. Stark is the Howard B. Hunt Centennial Professor and Chairman of the Department of Radiology at the University of Nebraska Medical Center in Omaha, NE. Dr. Stark is also a member of the editorial advisory board of this journal.

P ower injectors have been used for decades to administer iodinated contrast agents in angiography and CT. MRI is increasingly an alternative to these radiographic methods. Therefore, it is no surprise that analogous contrast-enhancement methods have become routine in MRI.

Surprisingly, little has been written about the adaptation of electromechanical devices to the problematic environment of MRI scanners. Magnetic field and RF shielding constraints have led to the development of MRI power injectors that bear only superficial resemblance to their x-ray ancestors.

Background

Intravenous power injection was first applied to CT scanning in Rome, as reported in 1980; ¹ clinical trials were underway in the United States by 1984. ² These early studies showed that sustained, uniform injection improved enhancement of vessels and abdominal organs such as the liver. Convenience, reduced staffing requirements, and relief from radiation safety concerns resulted in worldwide adoption of this x-ray CT technology by 1985.

In 1985, MRI contrast agents and MR angiography were undergoing rapid development. Specifically, paramagnetic contrast agents were gaining respect for increasing the sensitivity of brain MRI. However, these agents were less successful in MRI examinations of the abdomen, where CT remained king. ³

Although conventional time-of-flight "bright blood" MRA showed early promise for the evaluation of vascular disease in 1985, ultrasound and x-ray angiography seemed unassailable. ⁴

Prudent radiologists introducing MRI to their practice in 1985 understood it to be less invasive than CT, with inherently superior image contrast. Clinicians were reluctant to add the pain and cost of intravenous Gd-DTPA to MRI of the brain until around 1987-88, and many remained unconvinced of the need for contrast-enhanced MRA into the mid-1990s. Therefore, it was not until 1994 that Ulrich GmbH & Company was able to sell the first MRI-compatible power injectors in Germany; ⁵ two years later, a device manufactured by Medrad Inc. was approved for use in the United States (figure 1). ⁶ Today there are approximately 650 MR injectors in use in the United States, and a total of more than 1,500 in use worldwide. ⁶

Conventional MRA is a valuable part of many MRI examinations of the head, neck, body, and extremities. However, MRA suffers from certain deficiencies that reduce image quality in diseased vessels. For example, signal loss due to turbulence, in-plane flow, or magnetic susceptibility artifacts may obscure pathology and render MRA non-diagnostic. ⁷ Use of gadolinium increases the signal from flowing blood, improving image quality and reducing the effect of such artifacts.

CT injector usage taught radiologists the importance of correct bolus timing, contrast volume, and infusion rates. These same factors have been controlled to improve the quality of contrast enhanced MRA, ^7,8 and substantially improved signal-to-noise ratios have been achieved. ⁷

MR power injectors were initially used for cerebral and myocardial perfusion studies (figure 2). Today, MRA is increasingly used to look at arterial and venous blood flow throughout the body (figure 3). ^8-10 Pulmonary embolism, thoracic and abdominal aortic dissection or aneurysm, renal artery stenosis or transplantation, and lower extremity arterial insufficiency or deep venous thrombosis are now commonly studied by MRA. Solid organ enhancement and characterization of neoplasms also may benefit from power injection. ¹¹

Design of MR power injectors

CT power injectors generally are constructed with standard motors and screw driven pumps in a cast iron housing. This housing can be positioned close to the patient, minimizing tubing length. The entire apparatus can be supported on a steel frame or on rollers, or the device may be wall mounted. The motor is connected by wire directly to (a) the remote control panel and (b) 120-volt wall-socket power.

The MRI environment must exclude ferromagnetic missiles and sources of radiofrequency (RF) interference, such as alternating current and electrical motors. ¹²

MR power injectors as problem solvers

MR power injectors are designed to overcome many of the problems and satisfy many of the requirements inherent in traditional MRI studies (table 1), including:

I. Control panels must be accessible to the technologist seated outside the RF shielded room. Control signals must enter the RF shielded room to direct the timing, volume, and duration of the injection. Addition of RF shielded ports and electrical or fiber-optic cable is simple if anticipated at the time of room construction.

However, when an injector is to be added to an existing room, it may be less expensive to couple an infra-red or radiowave transmit/receive unit across the RF shield. Figure 1 illustrates such an installation at a corner of the technologist's observation window.

MRI scanner suite observation windows incorporate a fine copper mesh which is nearly transparent to light but opaque to RF at MRI frequencies (5 to 100 MHz). Radiowaves with frequencies in the giga-Hertz range (Medtron, see table) can freely penetrate RF shields designed for MRI frequencies, and do not interfere with the MR imaging process.

II. Electrical power to the injector motor can be 120 volt AC, provided that electromagnetic filters are used at the outlet and at the motor. If such a power source is not built into the scanner room, batteries composed of non-ferromagnetic nickel can be used to power a low-voltage DC motor.

III. Motors of conventional electrified rotor/stator design must be housed within metal (ferrous or non-ferrous) in a manner that contains spurious RF discharges. Motors generally are situated as far as possible from the magnet to reduce the risk of ferromagnetic components becoming a missile. However, remote positioning of the motor requires transmission of its propulsive power to a screw or piston located at the base of the syringe, near the patient.

Ultrasonic motors, comprised of piezoelectric crystals that acoustically pulse a ratchet forward or backward, can rotate a screw mechanism much the same as Canon Inc.'s family of ultrasonic camera lenses. These ultrasonic motors are sufficiently free of ferromagnetic materials and spurious RF noise so that they can be placed, unshielded, near the patient, in the bore of the magnet (Namoto, see table).

IV. Transmission of mechanical power from the motor to the pump or syringe can be accomplished in a number of ways. A rotational motor physically attached to the injector head may utilize a conventional transmission linkage, such as a gear, sprocket wheel, or belt.

If the motor is to be positioned a safe distance from the magnet, patient and pump, a flexible transmission (like a speedometer cable) can be used (Medrad, see figure 1). Transmission cables or gears can easily be manufactured from non-ferromagnetic alloys, so that no missile hazard exists as they enter high fringe-fields near the
magnet bore.

The greatest separation of the motor from the MRI system has been achieved using compressed air (Ulrich) which, via rigid tubing, provides both control and power to an air-driven non-ferrous pump at the magnet bore. Air eliminates all concerns about electromagnetic (RF) noise and missile hazards. However, the viscosity and compressibility of air may make control a bit more elastic or less precise than mechanical linkages.

V. Pumps drive fluid forward using either a piston or peristaltic mechanism. The direct action of a syringe plunger is that of a piston. Plungers can be advanced or retracted by a mechani-
cal rotating screw (Medrad), or by compressed air (Ulrich). These pump components are made of non-ferromagnetic materials. Peristaltic injectors are commonly used for therapeutic intravenous fluid delivery at the hospital bedside. These devices use a pair of mechanical fingers to trap fluid in a compressible hose, and then slide or rotate it forward. A rotational mechanism can be used to achieve uniform fluid movement. Peristaltic devices
typically have lower flow rates than syringe devices.

VI. Extravasation is of great concern in CT, where large volumes (50 to 200 cc) of hypertonic material are injected at rapid flow rates (3 to 10 cc/sec). Complications of extravasation range from transient pain to tissue necrosis requiring surgery. ^13-16 The occurrence of extravasation may be as frequent as in 0.6% of injections, and it is of such concern that one CT power injector manufacturer, EZ-EM, has developed a device to detect extravasation automatically. ¹⁴

MRI procedures use much lower volumes (15 cc on average, injected at 1 to 3 cc/sec). ¹⁷ It also has been reported that the skin toxicity when using gadolinium-based agents is far less than the toxicity of iodinated contrast media. ¹⁸ Therefore, complications of MR injector-driven extravasation are negligible. However, because the injectate volume is so low in MR procedures, the void volume of the tubing connecting the pump to the patient represents a significant fraction (3 to 10 cc) of the dose to be injected. For this reason, the distance from the pump to the patient must be kept as short as possible. Also, in order to limit the amount and cost (which can be up to $100 for 15 cc) of drug used, and to insure delivery of the entire amount into the patient during a continuous bolus, MR power injectors include a flushing mechanism. A second syringe (figure 1) or fluid reservoir is attached and controlled to sequentially flush residual contrast out of the tubing and into the patient.

Summary

MRI power injectors are increasingly being used. Early clinical data seem to show improved image quality and uniformity. Though presently available CT injectors are not MRI compatible, combination CT/MRI injectors may soon become commercially available from Medtron (Injektron MR CT2) and Ulrich (CT/MR Injector 2000).

In the United States, vendor choice is currently limited to Medrad. Namoto and Daum hope to secure FDA approval; Ulrich may follow. This is a rapidly developing market, and product specifications and availability will change. The information compiled here, from a variety of commercial sources, has not been independently verified by the authors. Safety and suitability of these devices for specific applications must be based upon FDA (510k) approval and proper, peer-reviewed clinical evaluations.

All indications are that vascular applications of MR will continue to develop, contrast media usage will increase, and the convenience of MR power injectors will lead to widespread adoption of this technology. AR

Manufacturers of MR power injectors

Bruker, Bruker AG­Medical, Industriestrasse 3, CH-8117 Fallenden, Switzerland, Phone: 41 (1) 8259111, Fax: 41 (1) 8259638

Daum,Daum GmbH,Hagenowe Strasse 73, D-19061 Schwerin, Germany, Phone: 49 (385) 6344344, Fax: 49 (385) 6344152, E-mail: medinfo@daum.de

Medrad, Medrad Inc., One Medrad Dr. Indianola, PA 15051-0780, Phone: (412) 767-2400, (800) 633-7231, Fax: (412) 767-4128

Medtron, Med-Tron Medizinische Systeme GmbH, Kreisstrasse 152, D-66128 Saarbruecken, Germany, Phone: 49 (681) 702860, Fax: 49 (681) 700956, E-mail: medtronk@aol.com

Namoto (Liebel-Flarsheim), Liebel-Flarsheim Co., 2111 E Galbraith Rd.

Cincinnati OH 45215-6305, Phone: (513) 761-2700, (800) 347-9730, Fax: (513) 761-2388

Ulrich, Ulrich GmbH & Co. KG, Buchbrunnenweg 12, D-89081 Ulm-Jungingen, Germany, Phone: 49 (731) 9654-0, Fax: 49 (731) 9654-199