The Effect of Contrast Material on Radiation Dose at CT: Part I. Incorporation of Contrast Material Dynamics in Anthropomorphic Phantoms

Abstract

Purpose To develop a method to incorporate the propagation of contrast material into computational anthropomorphic phantoms for estimation of organ dose at computed tomography (CT). Materials and Methods A patient-specific physiologically based pharmacokinetic (PBPK) model of the human cardiovascular system was incorporated into 58 extended cardiac-torso (XCAT) patient phantoms. The PBPK model comprised compartmental models of vessels and organs unique to each XCAT model. For typical injection protocols, the dynamics of the contrast material in the body were described according to a series of patient-specific iodine mass-balance differential equations, the solutions to which provided the contrast material concentration time curves for each compartment. Each organ was assigned to a corresponding time-varying iodinated contrast agent to create the contrast material-enhanced five-dimensional XCAT models, in which the fifth dimension represents the dynamics of contrast material. To validate the accuracy of the models, simulated aortic and hepatic contrast-enhancement results throughout the models were compared with previously published clinical data by using the percentage of discrepancy in the mean, time to 90% peak, peak value, and slope of enhancement in a paired t test at the 95% significance level. Results The PBPK model allowed effective prediction of the time-varying concentration curves of various contrast material administrations in each organ for different patient models. The contrast-enhancement results were in agreement with results of previously published clinical data, with mean percentage, time to 90% peak, peak value, and slope of less than 10% (P > .74), 4%, 7%, and 14% for uniphasic and 12% (P > .56), 4%, 12%, and 14% for biphasic injection protocols, respectively. The exception was hepatic enhancement results calculated for a uniphasic injection protocol for which the discrepancy was less than 25%. Conclusion A technique to model the propagation of contrast material in XCAT human models was developed. The models with added contrast material propagation can be applied to simulate contrast-enhanced CT examinations. ^© RSNA, 2017 Online supplemental material is available for this article.

Figure 1:

Diagram shows PBPK compartmental model used for simulating pharmacokinetics of contrast medium through the cardiovascular system. Model includes 37 compartments: Each ellipse represents a vessel compartment and each box represents an organ, with numbers corresponding to compartment numbers in Table 1. Red dashed lines show first path of contrast material, while blue dashed lines show return path of contrast material from organs to heart.

Figure 2a:

(a) Organ and (b) vessel compartments. Each organ is modeled with three subcompartments: Intravascular (IV), extracellular (EC), and intracellular (IC). C_IV and C_EC are intravascular and extracellular concentrations, and V_IV and V_EC are intravascular and extracellular volumes. Note that input and output blood flow rates (Q) are assumed to be equal.

Figure 2b:

(a) Organ and (b) vessel compartments. Each organ is modeled with three subcompartments: Intravascular (IV), extracellular (EC), and intracellular (IC). C_IV and C_EC are intravascular and extracellular concentrations, and V_IV and V_EC are intravascular and extracellular volumes. Note that input and output blood flow rates (Q) are assumed to be equal.

Figure 3:

Graphs show uniphasic (left) and biphasic (right) intravenous contrast agent injection protocols. Uniphasic injection was of 125 mL of contrast material (320 mg of iodine per milliliter [mgI/ml]) at 5 mL/sec and biphasic injection was of 50 mL of the same contrast agent at 2.5 mL/sec followed by 75 mL at 1 mL/sec.

Figure 4:

Coronal images show distribution of contrast material throughout five-dimensional XCAT model subjected to uniphasic injection protocol of 125 mL of contrast agent (320 mg of iodine per milliliter [mgI/ml]) at 5 mL/sec in the first 100 seconds after injection.

Figure 5:

Graph shows iodine concentration curves for different organs used to update the particular organ’s material as a function of time in male XCAT model. mgI/ml = milligrams of iodine per milliliter.

Figure 6a:

Graphs show distribution of peak value of simulated arterial and hepatic contrast-enhancement curves from 58 XCAT models for (a) uniphasic and (b) biphasic injection protocols.

Figure 6b:

Graphs show distribution of peak value of simulated arterial and hepatic contrast-enhancement curves from 58 XCAT models for (a) uniphasic and (b) biphasic injection protocols.

Figure 7a:

Graphs show simulated and clinical aortic and hepatic contrast-enhancement time curves with (a) uniphasic and (b) biphasic injections. Simulated results are shown with thin solid curves for the aorta (blue) and liver (red). Thicker black curves represent mean value of simulation data for each data set.

Figure 7b:

Graphs show simulated and clinical aortic and hepatic contrast-enhancement time curves with (a) uniphasic and (b) biphasic injections. Simulated results are shown with thin solid curves for the aorta (blue) and liver (red). Thicker black curves represent mean value of simulation data for each data set.