Magnetization Transfer MRI of Breast Cancer in the Community Setting: Reproducibility and Preliminary Results in Neoadjuvant Therapy

Abstract

Repeatability and reproducibility of magnetization transfer magnetic resonance imaging of the breast, and the ability of this technique to assess the response of locally advanced breast cancer to neoadjuvant therapy (NAT), are determined. Reproducibility scans at 3 different 3 T scanners, including 2 scanners in community imaging centers, found a 16.3% difference (n = 3) in magnetization transfer ratio (MTR) in healthy breast fibroglandular tissue. Repeatability scans (n = 10) found a difference of ∼8.1% in the MTR measurement of fibroglandular tissue between the 2 measurements. Thus, MTR is repeatable and reproducible in the breast and can be integrated into community imaging clinics. Serial magnetization transfer magnetic resonance imaging performed at longitudinal time points during NAT indicated no significant change in average tumoral MTR during treatment. However, histogram analysis indicated an increase in the dispersion of MTR values of the tumor during NAT, as quantified by higher standard deviation (P = .005), higher full width at half maximum (P = .02), and lower kurtosis (P = .02). Patients' stratification into those with pathological complete response (pCR; n = 6) at the conclusion of NAT and those with residual disease (n = 9) showed wider distribution of tumor MTR values in patients who achieved pCR after 2-4 cycles of NAT, as quantified by higher standard deviation (P = .02), higher full width at half maximum (P = .03), and lower kurtosis (P = .03). Thus, MTR can be used as an imaging metric to assess response to breast NAT.

Keywords: MT-MRI; MTR; NAT; repeatability; reproducibility.

Figure 1.

Multisite reproducibility of magnetization transfer ratio (MTR) in 3 normal subjects scanned at 3 sites. MTR values of fibroglandular tissue (FGT) are displayed in pseudo color and overlaid on top of an anatomical image.

Figure 2.

Repeatability of MTR maps of FGT in 5 women undergoing test–retest scanning with subject repositioning between scans (A). MTR maps of FGT are displayed in pseudo color and overlaid on top of an anatomical image. Bland–Altman plot of mean MTR repeatability in breast FGT (B). Bland–Altman plot of MTR standard deviation repeatability in breast FGT (C).

Figure 3.

Representative MTR maps pseudo-colored and overlaid on an anatomical image of a subject who experienced a partial response to neoadjuvant therapy (NAT) (A). Histograms of voxel distributions of tumor MTR from all participants at the first, second, third, and fourth magnetic resonance imaging (MRI) sessions show higher dispersion at later time points after the start of treatment (B).

Figure 4.

Mean tumoral MTR is similar across all subjects before therapy (first Scan) and at serial scans performed during the course of NAT (second, third, and fourth scans) (A). The standard deviation of tumor MTR values increases with longer duration of NAT (B). Full width half maximum (FWHM) of the distribution of tumor MTR values increases with longer duration of NAT (C). Kurtosis of the distribution of tumor MTR values decreases with longer duration of NAT (D).

Figure 5.

Example MTR maps from (A) a subject who achieved pathological complete response and (C) a subject who had residual disease at the conclusion of therapy at the first, second, and third scan sessions. Histograms of voxel distributions of MTR at the first, second, and third MRI scans for all patients who achieved pathological complete response (B) and all patients who had residual disease (bottom row) show increased heterogeneity in the patients who achieved pCR compared with histograms of those who had residual disease (D).

Figure 6.

Average tumoral MTR values are similar at the first (A), second (B), and third scan (C) sessions in patients who achieved pathological complete response (pCR) and those who had residual disease at the conclusion of therapy. The standard deviation of the distribution of tumor MTR values at the first (D), second (E), and third (F) scan sessions show higher standard deviation at the third scan session in patients who achieved pCR. The kurtosis of the distribution of tumor MTR values at the first (G), second (H), and third (I) scan sessions show lower kurtosis at the third scan session in patients who achieved pCR.