The benefits of power injection systems in their ability to deliver higher flow rates and volumes in a more consistent manner than hand injections do not come without compromises. While it is obvious to view injection systems mainly along the lines of cost, features, accessories, and performance specifications (e.g., maximum flow rates and maximum pressure limits), the more important aspect when it comes to performance and the contribution to consistent image quality are the plastic disposables material and geometry combined with the mechanism of delivery. A number of injectors available in different brands and classifications on the market fall into one of three categories: piston-based, peristaltic pump, or hydraulic. Piston-based injectors load fluid into syringes or reservoirs which are then expelled through tubing sets into the patient by a piston/plunger (otherwise thought of as an automated hand syringe). Peristaltic pumps use rotational motion to pinch and un-pinch sections of a tube which draw fluid from a supply and inject it into the patient through an additional tubing set. Hydraulic injectors use fluid external pressure on a collapsible reservoir to compress the reservoir at a controlled rate and expel the fluid into the tubing sets to be delivered to the patient.

A problem common to all manufacturers regardless of delivery mechanism, is that the plastic disposables swell and stretch under the internal loads of the high injection pressures. This swelling and stretching is dependent upon temperature, injection pressure, reservoir volume or tubing volume, and material properties of the plastic. The loads at high injection pressures can be extreme for thin and flexible plastics to endure; for context, 300 psi of pressure in an example syringe can equate to 800 lbs or 363 kg of force pressing on every surface of the plastic. Figure 4 illustrates a mathematical simulation generated in this study of the non-linear expansion of the plastic disposables from an example piston-based injection system. In this simulation, the plastic disposables under high injection pressure can expand up to 10 mL in extra internal volume. The worst case as measured by the authors is on a hydraulic injection system with nearly 50 mL of additional volume swelling. By Conservation Law, this means that during an injection, while the expectation is that fluid is being delivered to the patient, the reality is that much of the volume is actually being injected into the expanding plastic disposables. This creates an unexpected delay in delivery of the bolus which is non-linear and dependent upon injection pressure. As the plastic disposables are elastically deforming under pressure but have not yielded into a new shape, when the pressure is relieved at the end of the injection the disposables decompress, expelling the extra trapped volume into the patient at a decaying rate.

Mathematical simulation illustrating the effect of expanding plastic disposables. “Compliance” is affected by both the tubing set volume and pressure.

The clinical implications of this factor are two-fold. First, the trapped volume from the expanding disposables is not a contributor to the main bolus of the injection, as this volume is only released when the injection pressure is relieved (partially during the saline flush and the remaining volume at the end of the injection). Therefore, the desired iodine delivery rate is not achieved for the intended duration, and the total effective iodine load (contributing to parenchymal enhancement) administered is likely less than intended. This phenomenon was measured using a real-time Coriolis density and flow meter (consistent with previous literature for assessing injected iodine concentrations), with the results from an example injection of CM with an iodine concentration of 370 mgI/mL (with a viscosity representing the middle of the range of available contrast-medium) at 4 mL/s for 40 mL as shown in Fig. 5. This was repeated with a piston-based injector and a peristaltic injector to simulate the two configurations with the most significantly different disposables designs. This figure displays the injected concentration over time entering the patient’s circulation. The magnitude of the shaded region at the beginning of the injection is a direct correlation to the expansion of the plastic disposables. The shape of the shaded region at the end of the injection is a combination of decompression of the plastic disposables as well as the efficacy of the saline flush in eliminating the contrast medium remaining in the tube. This latter contributor will be discussed in more detail below.

Concentration-time plot of ideal versus empirical injected flow rates for a single-phase injection. The shaded area represents the deviation between the ideal (dotted line) and empirical (solid curves measured from two different systems) injected flow rates as caused by confounding factors like plastic disposables expansion

The second clinical implication is that the enhancement achieved in a target body region will vary significantly even with the exact same injection protocol for the exact same patient as long as any variable is changed that affects injection pressure (and therefore disposables expansion) [4,5,6]. These variables include contrast-media viscosity, length and diameter of the tubing sets used, size/type of IV access device, etc. This holds true even if the exact same injection system itself is used. When adding in the different types of injection systems, the inconsistency may be even more substantial.

Figure 6 shows the contrast-medium and saline distribution profiles at two different injection protocols delivered across four different clinical setups with varying injection systems, contrast media viscosity, and IV catheter. The greyscale shading over time is directly correlated to the injected concentration of the fluid (equivalent method as described above). Each of the four different setups used a different injection system (two piston-based, two peristaltic-pump) configured each with a contrast-medium of constant concentration (370 mgI/mL) but different viscosity (5 cP, 12 cP, 17 cP, 22 cP), and each with a different IV catheter gauge and gravity flow rate (18 G 105 mL/min, 20 G 65 mL/min, 20 G 42 mL/min, 22 G 36 mL/min).

Contrast-medium and saline distribution profiles at two different injection protocols delivered on four different injection systems (two piston-based, two peristaltic pumps). The two injection protocols are shown in the header of the figure. The greyscale shading over time correlates with the injected concentration of the fluid as measured just proximal to the IV access device for each setup

The general expectation from clinicians would be that the delivered output of the CM to the patient would be roughly identical. As shown in the figure, even with an identical contrast-medium injection protocol, the output as delivered by the different setups is significantly altered from the expectation. Note that these variations are measured at the catheter site prior to the patient and represent variability in the input function. The variability introduced by patient-related factors has not yet even been added to the equation at this stage.

Because of the variable contrast-medium delivery caused by the plastic disposables expansion and injector mechanism, the seemingly same injection protocol would yield significantly different contrast enhancement. Unfortunately, there is no easy fix for this that can be attempted from the side of practicing clinicians. To compensate for this, the manufacturers of the disposables would need to significantly increase the strength of the materials used which adds substantial cost that would be economically unviable. The other alternative is to use mathematical modeling to predict the expansion of the disposables under given conditions and to compensate for this in real-time. In whichever way this previously undiscussed phenomenon is to be addressed, there is no clear-cut solution for everyday clinical practice at the moment.