is a resident in Radiology,
is an Assistant Professor and Head of the Body Imaging and GI
Imaging Sections, and
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.
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.
Intravenous power injection was first applied to CT scanning in
Rome, as reported in 1980;
clinical trials were underway in the United States by 1984.
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.
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.
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;
two years later, a device manufactured by Medrad Inc. was approved
for use in the United States (figure 1).
Today there are approximately 650 MR injectors in use in the United
States, and a total of more than 1,500 in use worldwide.
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
Use of gadolinium increases the signal from flowing blood,
improving image quality and reducing the effect of such
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
and substantially improved signal-to-noise ratios have been
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
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
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
The MRI environment must exclude ferromagnetic missiles and
sources of radiofrequency (RF) interference, such as alternating
current and electrical motors.
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
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
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,
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
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
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.
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
MRI procedures use much lower volumes (15 cc on average,
injected at 1 to 3 cc/sec).
It also has been reported that the skin toxicity when using
gadolinium-based agents is far less than the toxicity of iodinated
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.
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
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.
Manufacturers of MR power injectors
Bruker, Bruker AGMedical, Industriestrasse 3, CH-8117
Fallenden, Switzerland, Phone: 41 (1) 8259111, Fax: 41 (1)
Daum,Daum GmbH,Hagenowe Strasse 73, D-19061 Schwerin, Germany,
Phone: 49 (385) 6344344, Fax: 49 (385) 6344152, E-mail:
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: email@example.com
Namoto (Liebel-Flarsheim), Liebel-Flarsheim Co., 2111 E
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)