Methods of Contrast Media Delivery
Administration of contrast media has two method of delivering these are the drip infusion and bolus techniques
Drip Infusion
In the drip infusion technique, an Intravenous line is initiated and contrast medium is allowed to drip in during a period of several minutes. CT scanning begins after most, or all, of the contrast agent is administered this roughly at about 2 to 3 minutes. Because this technique relies on gravity the actual flow rate delivered is quite variable and affected by many factors like; bottle height, contrast volume, tubing length, Intravenous (IV) catheter size, and contrast medial viscosity. This variability prevents the injection from being uniformly reproduced in subsequent follow up studies. This method is not recommended for CT scan of the neck, chest, abdomen and pelvis because all of the scans acquired with this method are taken in the equilibrium phase. The drip infusion technique is the least effective injection method for abdominal CT imaging, and in some respects, is even inferior to scanning without contrast enhancement. The drip infusion technique cannot produce peak enhancement of sufficient magnitude for CT angiography. However, it can be used for routine brain scanning, like enhancement for metastases or primary central nervous system tumors, because there is no need for a high injection rate and a scan delay of at least 4minutes is typical.
Bolus Method
The bolus technique of contrast enhancement uses scanning after a rapid injection of contrast material. A volume of contrast of 50 to 200mL is injected at a rate or a combination of rates of between 1 and 6mL per seconds; scanning begins after a short delay. The interval between the initiation of the injection and the start of scanning (scan delay) is critical. The contrast bolus can be delivered by hand using syringes or by a mechanical injection system.
Hand Bolus Technique
When contrast media is injected by hand the flow rate is subject to many factors, including syringe size, contrast viscosity, Intravenous (IV) catheter size, and operator strength. Smaller size syringes require less operator strength to inject but must be serially disconnected when empty and reattached with replacement syringes. This delay will cause a drop in the peak enhancement. Higher viscosity agents and smaller indwelling catheters will require more operator strength for injection. The advantages of the hand bolus method are that it is relatively inexpensive and does not require any special equipment to implement. In addition, it allows the injection site to be closely observed so that the injection can be immediately stopped should there be signs of extravasation of contrast into the soft tissue. As mentioned earlier, this is often the recommended method for injecting into standard PICC lines.
Disadvantages of the Hand Bolus Method
- The operator will be exposed to scatter radiation from standing in the room during the scanning process, and because someone must stay in the scan room this method requires two operators.
- The flow rate is variable because of the factors mentioned earlier and the scan delay cannot be precisely controlled.
These two factors result in inconsistent images that are not readily reproducible in subsequent studies. So although the hand bolus method is an improvement over the drip infusion technique, significant disadvantage limit its use to special circumstances.
Mechanical Injection Systems
Mechanical injection systems are standard in CT scan suites because they deliver the precise flow rates and volumes specified by the operator or radiologic technologist, regardless of the viscosity of the solution and the game of the indwelling catheter. Injections are consistent and can be reproduced in subsequent examinations – providing parameters from studies are properly recorded and repeated. In addition, mechanical injector are programable, providing broad clinical utilities for a wide range of indications.
Mechanical Injection System are standard in CT scan suites because:
- They provide precise flow rates and volumes.
- Injections are consistent and can be reproduced in subsequent examinations.
- They are programmable, providing broad clinical utility for a wide range of indications.
Mechanical injectors, or also called the power injectors, are made by a variety of manufacturers, they come in different models, and offer various features.
Single Head Injectors
CT scan injectors may have a single head for affixing the syringe, or they may accommodate two syringes.
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| CT injectors may have a single head for affixing the syringe |
Dual Head Injectors
The dual head injectors are designed so that saline can be given immediately before or after contrast medial injection.
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| Dual-head injectors are designed so that saline can be given before or after the contrast injection. Photos courtesy of Medrad, Inc. |
Most models of mechanical injector include a programmable pressure limit. This allow the radiologic technologist to set an upper pressure limit, along with an injection rate. Contrast medium is then administered at the selected rate, unless the pressure reaches the maximum psi (pound per square inch) set. If the pressure reaches the selected limit, the injector reduces the flow rate to prevent exceeding the pressure limit and an alarm sounds to notify the radiologic technologist. Pressure limiting is designed to protect the integrity of the disposable components like the Intravenous (IV) tubing, used in the injection fluid path. Pressure is a result of the force required to overcome the resistance of pushing the contrast from the relatively large syringe barrel, through the patient connector tubing, any ancillary devices, and ultimately the catheter, at the required flow rate. Pressure is greatest at the point where the largest diameter merges to a far smaller diameter, in this case, the syringe tip. From the point to a point halfway down the length of the connecting tubing the pressure will drop by half, whereas at the tip of the catheter, pressure drops to near zero. A common reason for reading the pressure limit is when the intravenous (IV) tubing becomes kinked, restricting the flow of contrast media. Another common culprit for teaching the pressure limits is the use of components in the fluid path that are not compatible with power injectors and the flow rates and pressures they generate. Another key factor in pressure limiting is contrast viscosity. As mentioned previously, higher iodine concentration possess a higher viscosity, particularly at room temperature. Contrast viscosity can be dramatically reduced, in some cases by nearly half, by simply warning contrast to body temperature at 37 degrees.
Detection of Contrast Media Extravasation
When using the mechanical injector for intravenous (IV) contrast administration, precaution must be taken to reduce the risk of serious extravasation of the contrast media in the soft tissue.
Another special feature on some moder of mechanical injector devices designed to aid in the detection of contrast medium extravasation. Extravasation is the leakage of fluid from a vein into the surrounding tissue during intravenous (IV) administration. There is particular concern that mechanical injector may increase the severity of extravasation when extravasations occur. Because mechanical CT injector typically deliver contrast at fast flow rates and the radiologic technologist may not remain in the examination room throughout the injector to quickly intercede should signs of extravasation appear, there is worry that a large volume of contrast extravasation could more readily occur when a mechanical CT injector is used. However, when appropriate precautions are taken, the risk of serious extravasation detection feature available on some injector models is designed to augment, rather than supplant, such precautions.
Saline Test Injection
Another potential safety feature on some available power injectors is the ability to perform a saline test injection before delivering of contrast bolus injections. The saline may be programmed at the same flow rate as the contrast bolus, thereby more closely replicating an actual injection, and allowing the radiologic technician additional time to monitor the viability of an intravenous (IV) site.
Mechanical Injector Vs Air Embolism
Although it rarely occurs, when a mechanical injector is used, large air embolism can result from the incorrect preparation and inadequate connection of the injector syringe and tubing. Air embolism can occur during any intravenous injection:
Small quantities of air can be absorbed by the body, so small air emboli may never be detected of patients are asymptomatic. However, large air emboli can cause seizures, permanent neurologic damage, or occasionally death. These large air emboli occur only as a result of human error. Safeguards have been built into injection system that are successfully in preventing most errors of this kind. When mistakes occur, they usually are a result of a disruption in the routine of preparing the injector. For example, in preparing for the next patient, the radiologic technologist must dispose properly the empty syringe from the previous patient and replace it with a new syringe that is prefilled with the contrast media. In these injection systems, the plunger must be retracted before the empty syringe can be removed. The radiologic technologist does this, but before he can replace the empty syringe with full one, the phone rings and he is called away. His co radiologic technologist steps in to finish the process and, thinking that the syringe has already been replaced, he connects it to the patient’s IV line. Instead of injecting a 120mL of contrast media, the radiologic technologist injects a full syringe of air into the patient.
At least one injector manufacturer had incorporated automation to further reduce the possibility of air emboli. Injectors with this feature will automatically retract the injector position when the syringe is removed and returning it to the default position, before a new syringe injector is attached.
When a new, unused injector syringe is attached to the injector, it will automatically drive the piston forward to the load position, thus always avoiding an empty syringe being in place on the injector with the ability to inject air into the patient.
In addition, some syringe contain visual indicators that provide clear and immediate indications as to whether a syringe contains fluid or air.
Preparation of Mechanical Injector
The exact process of preparing the injector varies depending on the type of injector system, whether the facility uses prefilled syringe, and the specific injection protocol. Therefore, each facility should develop a clear protocol for preparing the mechanical injector used in the CT department. The protocol should clearly specify the steps taken to prepare the injector for use.
Read: Sample Clinical Department’s Mechanical Injector Protocol
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