Estimation of Gadolinium-based Contrast Agent Concentration Using Quantitative Synthetic MRI and Its Application to Brain Metastases: A Feasibility Study

Abstract

Purpose: Gadolinium-based contrast agents (GBCA) provide valuable information for assessing and differentiating lesions in the body. However, contrast enhancement evaluation on conventional MRI is qualitative because the signal intensity uses an arbitrary scale. An approach that allows more quantitative assessment of tissue enhancement that can be integrated into clinical use is desirable. This study aimed to provide a method that can estimate GBCA concentration in a clinically applicable scan-time.

Methods: Gadolinium-based contrast agent concentrations were quantified in phantoms containing water and nine different concentrations of Gadoteridol (Gd-HP-DO3A), ranging from 0.02 to 1.00 mmol/L, using quantitative synthetic MRI. Simple linear regression analysis between the estimated GBCA concentration and the reference values were performed to assess the accuracy. We performed region of interest analysis on each phantom, and recorded the mean and standard deviation. We evaluated the precision of the GBCA map by calculating the coefficient of variation (CV) for each concentration. The GBCA concentration quantification method was applied for 10 patients with metastatic brain tumors to demonstrate the feasibility of this method.

Results: For the phantom study, estimated GBCA concentrations were in a strong linear relationship (R² = 0.998) with reference values, with a slope and intercept on simple linear regression analysis of 0.98 and 0.02, respectively. On precision assessment, the CV was <5%, with the exception of concentrations under 0.07 mmol/L. In the range of 0.07-0.99 mmol/L, the mean CV was 1.5 ± 1.2%. For application to brain metastases, the maximum estimated GBCA concentration in the metastases was 0.73 mmol/L, which was under the upper limit evaluated in the phantom study (i.e. -0.99 mmol/L).

Conclusion: The concentration of Gd-HP-DO3A in the range of 0.07-0.99 mmol/L can be measured in a clinically applicable scan time using quantitative synthetic MRI, even though this study's results are only preliminary due to several limitations.

Keywords: brain metastasis; gadolinium-based contrast agent; quantitative imaging; relaxivity; synthetic magnetic resonance imaging.

Fig. 1

GBCA concentration map of phantoms containing contrast agent solutions. The change in longitudinal relaxation rate (inverse of longitudinal relaxation time T₁) is proportional to the concentration of GBCA. According to this relationship, the GBCA concentration was calculated using the absolute T₁ values of the pixel with and without GBCA. Calculations were performed for each pixel to obtain the GBCA concentration map. GBCA, gadolinium based contrast agents.

Fig. 2

Correlation plot comparing the estimated gadolinium-based contrast agent concentration to reference values. Error bars represent ±2 SDs. The regression line is superimposed in light blue. SD, standard deviation.

Fig. 3

Coefficients of variation on gadolinium-based contrast agent concentration estimation. The coefficient of variation is the standard deviation normalized by the mean values. GBCA, gadolinium based contrast agents.

Fig. 4

A representative apparent gadolinium-based contrast agent (GBCA) concentration map, additively overlaid on a non-enhanced T₁-weighted image. The estimated GBCA concentration is expressed using a color scale. The maximum apparent GBCA concentration value of the metastasis in the left hemisphere (arrow) was 0.73 mmol/L.