Automatic Injection Devices
After completing this chapter, the reader will be able to perform the following:
Identify the need for automatic injection devices
List the basic components of an automatic injector
List and describe the advantages of a detachable injector head
List and describe the advantages of dual syringe capacity
Define the concept of flow rate
Identify the factors that affect the flow rate
Explain the injectors that are used for computed tomography (CT) and magnetic resonance imaging (MRI)
Many special radiographic procedures require an injection of contrast medium under specific controlled conditions. Performing these injections by hand would make it difficult to maintain a consistent flow rate. Maintenance of a sufficient dilution of contrast agent in the blood is also difficult to achieve with an injection made by hand.
Angiography requires the delivery of a specific amount and concentration of contrast agent to the target area. As the contrast agent enters the bloodstream, it is diluted. This dilution effect is dependent on several factors, such as the injection site, size of the vessel, and type and iodine concentration of the contrast agent. The target site determines the speed (rate of flow) of the injection. Vessel size also affects the concentration of the contrast agent. The larger the vessel, the greater the flow rate must be to maintain the proper concentration of contrast medium so that the desired anatomic features can be visualized radiographically. In some procedures, flow rates of 30 to 40 ml of contrast material per second are not uncommon.* These injections must be performed with a mechanical device in order to achieve consistency in the flow rates. The flow rate is controlled by many factors, such as the viscosity of the contrast medium, catheter length, catheter diameter, and injection pressure. The flow rate or injection speed can be increased or decreased by varying any of these parameters.
Most current injectors are designed for operation as either high- or low-pressure injectors, thereby broadening their usefulness in many types of angiographic procedures. The Medrad Avanta injector (Fig. 3-1, A) is a multimodality system that can be utilized in the cardiac and angiography arena as well as for larger vessels and peripherals. The Medrad Avanta is capable of injecting contrast material in variable mode at low flow rates and low pressures, fixed mode at high flow rates and high-pressure injections, and fixed mode for saline flushing. Dedicated automatic injection systems such as the Medrad Stellant CT injection system and Spectris Solaris EP MRI injector are also available (see Fig. 3-1, B and C). These systems can be configured with options specific to each of their respective modalities.
AUTOMATIC INJECTORS
Basic Components
All automatic injectors have certain components in common; each is equipped with a control panel, syringe, heating device, and high-pressure mechanism.
Control Panel
Each of the automatic injection devices has a control panel that is used to set the parameters of the injection sequence (Fig. 3-2). Depending on the unit and the optional equipment purchased, the control panel will appear more or less intricate. The injector systems are equipped with a touch screen display for ease of programming. Almost all of the units available are digitally controlled and ergonomically designed for ease of use. Some of the systems allow the control panel to be removed from the unit and taken to the control area during the procedure.
The controls and indicators present on the control panel vary with the type of options available with the system. These controls are clearly marked for ease of operation. In the Medrad ProVis Mark V injector the control panel is segmented and sequenced to guide the setup procedure. The system’s rotating and tilting console is easily read in a variety of lighting situations and throughout a range of distances. The Angiomat Illumena injector is also equipped with a tilting and rotating console that allows for viewing from a wide variety of angles.
Many of the systems come equipped with optional electroluminescent displays. This type of display can be easily read from a variety of angles and under a wide range of lighting conditions. Some of the systems also have graphical user displays for easy setup and system monitoring. The EmpowerCT provides a number of voice prompts (spoken cues) that can quickly alert the operator to potential problems.
Each of the systems is supplied with an instruction manual that should be referred to if questions concerning operation or troubleshooting arise. The manufacturers also make available product manuals and technical assistance on their websites.
Syringe
In all automatic injectors, the syringe is removable. Disposable syringes are available for use with all automatic injectors; these are presterilized and can be installed easily. The manufacturers also offer prefilled syringes for use with their systems. The syringes are completely disposable, thereby eliminating the possibility of cross-contamination from reusable parts. Syringe capacity varies with brand. The syringes come in a variety of sizes depending on the company and the automatic injector. These range from 60 ml to 200 ml with sizes of 125 ml, 130 ml, and 150 ml also available. The smaller syringes (60 ml) find wide usage in pediatric, neurologic, and coronary studies. The 130-ml and 150-ml syringes are mainly used in aortic, peripheral, hepatic, and left ventricular studies. The 200-ml syringe can be used when multiple studies are anticipated.
The syringe must be handled with care when preparing for a procedure because any abnormality can cause failure under operating conditions. It is wise to have extra syringes available in the special procedures suite so that if syringe failure or contamination occurs, a prolonged delay can be avoided. Figure 3-3 shows some syringes in use today. Latex-free syringes are also offered by the manufacturers to reduce the possibility of allergic reactions.
Heating Device
The heating system is an electronic device that heats and maintains the contrast medium at or near body temperature and reduces the viscosity of certain contrast agents, thereby facilitating the setting of certain flow rates from the injector. It is usually located on the injector head close to the syringe. The syringe temperature is thermostatically controlled and is usually preset at the factory to a nominal 37° C (approximately 98° F). Heating time varies with the injector being used. Most injection device heaters can only maintain the temperature of the contrast agent. Therefore, the contrast agent should be prewarmed when using injection devices equipped with this type of heater.
Injector Power Heads and High-Pressure Mechanism
The injector power head is generally attached to an articulating arm that allows for a wide variety of positions during the injection. Digital displays are available on the power head indicating the protocol used and the amount of contrast available. Various configurations are available from free standing, attached to the injector module, and table or ceiling mounted power heads. Figure 3-4 illustrates some of the configurations available.
The main type of high-pressure mechanism used on automatic injectors is the electromechanical system. This is simply an electric drive motor connected to a jackscrew that drives the piston into or out of the syringe. Figure 3-5 depicts a simplified version of an electromechanical high-pressure device. The high-pressure mechanisms of modern automatic injectors have several safety features built into them. The Medrad ProVis injector can prevent injections from exceeding their preset volume, flow rate, and pressure. There is also a mechanical stop that physically limits the injection volume to a preset amount. In the case of an electrical malfunction, the injection would still be limited by the device, which is not dependent upon electricity to function.
Optional Components
Double-Syringe Assembly
Also of interest is the double-syringe assembly, which allows preloading of two syringes. When one syringe has been emptied, the plunger is retracted and the assembly is turned 180 degrees to position the second syringe with minimum resulting time loss. The Medrad Mark V ProVis provides for varying configurations of the double syringe turret assembly. The syringe assembly can be configured as 60 ml/150 ml, 150 ml/150 ml, or 200 ml/200 ml on the same turret. Table 3-1 lists the syringe sizes and their applications. The E-Z-EM CT injector also is equipped with an injector head and display that swivels independently to allow for positioning the unit on either side of the CT gantry (see Fig. 3-6).
TABLE 3-1
Syringe Sizes and Their Associated Applications
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| Syringe Size | Application |
| 60 ml | Carotid arteriography |
| Coronary arteriography |