The mixability of angiographic contrast with arterial blood

Abstract

Purpose: Angiographic contrast that is routinely injected into arteries is used not only to evaluate arterial geometry but also in many cases to assess perfusion. The authors conducted two experiments to examine the dispersion of angiographic contrast injected antegradely into an artery under conditions similar to those found in selective (carotid artery) or superselective (circle of Willis) angiography in order to determine the distance from the catheter tip at which the contrast can be considered fully mixed with the blood. A third experiment investigated whether the contrast once mixed with blood will separate from the mixture under the gravitational field due to a density mismatch.

Methods: Experiment I--Under high-speed angiographic acquisition, a bolus of contrast was injected through a catheter along the flow direction of a blood analog fluid flowing through a straight, long, cylindrical tube. The variation in grayscale intensity along the length of the tube was acquired and modeled as the step response to a second-order system. The distance from the catheter tip at which the contrast mixes with the working fluid, the mixing length, was determined as the length along the tube after which the step response settles to within 3% of the steady state value. Experiment II--A bolus of angiographic contrast was injected at rates varying from 0.1 to 1 cc/s through three different catheter sizes in the left common carotid artery of three rabbits. The average cross-sectional grayscale intensity over one cardiac cycle was calculated at four locations along the artery: Immediately distal to the catheter tip, at location of maximum grayscale intensity, and at 10 and 20 arterial diameters from the catheter tip. The status of mixing within 10 arterial diameters was assessed by differences between the grayscale value at this location and that at the maximum and 20 arterial diameter location. Experiment III--Angiographic contrast was premixed by agitation in three separate vials containing normal saline, canine blood, and glycerol/distilled-water mixture. The vials were then stationed vertically and angiographic images obtained every 5 min for 1 h. The average intensity of contrast along the vertical length of each vial was obtained for every time point to record any changes in the distribution of contrast over time.

Results: The first experiment shows that angiographic contrast completely mixes with steady flowing blood analog fluid within about eight tube diameters of the injection site. The second experiment shows that contrast completely mixes with blood within ten arterial diameters under appropriate injection parameters. The third experiment shows that angiographic contrast does not separate from, or settle out of, contrast-carrying fluid mixtures for a period of 1 h.

Conclusions: The results demonstrate that under typical injection conditions in the clinical setting, contrast issuing from the catheter completely mixes with the blood within ten artery diameters downstream of the catheter tip. Once mixed, it does not separate from the blood due to gravity.

Figure 1

Schematic of a catheter tip placed concentrically inside an artery. The discharge velocity through the catheter is much higher than the surrounding flow. The high velocity discharge tends to generate an ejector effect whereby the surrounding fluid is entrained into the jet flow enhancing both mixing and flow of the slow moving fluid. Q_b and Q_c are the flow rates of blood analog fluid and injected contrast, respectively; V_b and V_c are the corresponding velocities; D and d are the inner diameters of the artery and the catheter, respectively. The mixing length is the distance from the catheter tip at which contrast has formed a homogeneous mixture with the surrounding fluid.

Figure 2

Schematic of the experimental flow phantom apparatus.

Figure 3

Hemodynamic variables in the flow loop during the injection of contrast for one of the experimental cases (Exp I #3). The tube flow rate measured by the flow probe (Q_a) is shown as a solid line whereas the injection profile (Q_c) is shown as a dashed line. Flow rate of the working fluid only (Q_b, dotted line) is obtained by subtracting the injection profile from the tube flow rate. About 0.4 s into the injection, entrainment begins with a large starting transient (onset of entrainment) and eventually stabilizes at a value higher than the preinjection flow rate (entrainment level). After the injection is stopped, the pressure and the flow through the tube decay to preinjection levels.

Figure 4

Subtracted angiograms showing contrast injection in the LCCA for one case of rabbit 1. The four images were acquired at the start of injection and span the duration of about one cardiac cycle. The leftmost image shows pressure catheter in innominate artery and electromagnetic flow probe on distal LCCA. Contrast reflux can be observed in the rightmost image; also shown is the jet intact core penetration distance (x₀) for this injection.

Figure 5

Recorded flow and pressure traces for one case from experiment II, rabbit 2; nominal contrast injection rate of 0.6 cc∕s. Panel (A) shows the cardiac average flow (solid line) and cardiac average pressure (dashed line) over a 80 s acquisition period; arrows indicate start and stop of contrast injection. A vasodilatory response followed most injections. Vertical dashed lines indicate the time window showing in panel (B). Panel (B) shows the pulsatile flow (solid line) and pulsatile pressure (dashed line) recorded during contrast injection (dotted line).

Figure 6

Images of the flow tube (bottom) and the reference tube (top) for the three experimental cases at 0.633 s after start of injection after background subtraction: (A) Exp I #1; (B) Exp I #2; (C) Exp I #3; flow is from left to right. The distal tip of the catheter in the flow tube is at the pixel on the immediate left of each figure.

Figure 7

Second-order step response fits (solid lines) to the ratio of the cross-sectional average contrast concentration along the length of the flow tube to reference tube (dotted lines) for the three experimental cases at 0.633 s from start of injection. Mixing length for each case is indicated (dashed lines) as the distance after which the step response settles to within 3% of the steady state value. Inset shows the fit of a sigmoid function to the experimental data for one case (Exp I #2) to identify the jet intact core penetration length (x₀). D is the tube diameter.

Figure 8

Goodness of the fit between the step response function predicted values and the experimental values for three images (0.6–0.667 s from start of injection) from one experimental case (Exp I #3). R² is the coefficient of determination and the solid line is the line of identity.

Figure 9

Profiles of angiographic contrast intensity along the axis of the flow tube for case Exp I #3 at 0.633 s from start of injection (mixing length of 8.7D). Profiles are plotted as a fraction of the maximum contrast intensity in the reference tube. The profiles can be seen to become self-similar after the mixing length distance. D and R are the tube diameter and radius, respectively.

Figure 10

Average (solid lines) of contrast intensity profiles in the flow tubes after the mixing length distance for the three experimental cases at 0.633 s from start of injection. Profiles are plotted as a fraction of the maximum contrast intensity in the corresponding reference tube. The number of profiles averaged in each case is indicated by n; dashed lines are one standard deviation.

Figure 11

NACGI values for the five injections in rabbit 1, experiment II. Legend shows the nominal injection rates. ^*∧ Significant differences were noted between the grayscale values at 10 and 17 (nominally 20) arterial diameters for two injection rates.

Figure 12

Cross-sectional average grayscale intensity along the dimensionless vial length from top (0) to bottom (1) at the start of the experiment (t=0 min) and at the end of the experiment (t=60 min) showing no settling of the contrast in any of the fluids tested. Δ represents the percentage change in mean intensity along the length of the tube from the start to end of the experiment.

Figure 13

Parametric study of the factors affecting contrast mixing during selective angiography [6.4 mm artery, inner diameter of catheter=0.83×(catheter outer diameter)]. Q_b and Q_c are the flow rates of blood and contrast injection, respectively; V_b and V_c are the corresponding velocities at the catheter exit.

Figure 14

Parametric study of the factors affecting contrast mixing during superselective angiography [3 mm artery, inner diameter of catheter=0.72×(outer diameter)]. Q_b and Q_c are the flow rates of blood and contrast injections, respectively; V_b and V_c are the corresponding velocities at the catheter exit.