# Patient‐Specific Biases in Fat Fraction Estimates of Malignant Bone Marrow due to Relaxation Times Measured With STEAM at 3T

## TL;DR

The study finds that fat fraction estimates in bone marrow cancer imaging are biased by water relaxation times, suggesting more accurate methods like MRS could improve results.

## Contribution

The study introduces STEAM-based relaxometry to quantify patient-specific biases in fat fraction estimation for malignant bone marrow.

## Key findings

- Water T1 and T2 relaxation times vary significantly with disease state, affecting 2-point Dixon fat fraction estimates.
- MRS-derived proton density fat fraction (PDFF) shows closer agreement with imaging PDFF than 2-point Dixon methods.
- Monte-Carlo simulations confirm that relaxation times introduce substantial bias in fat fraction estimation.

## Abstract

Semiquantitative fat fraction estimation using 2‐point Dixon sequences is widely used in whole‐body (WB) MR imaging for malignant bone disease but is biased by relaxation times. Understanding this bias requires water‐ and fat‐specific relaxometry data in normal‐appearing marrow and lesions. This study measured bone marrow relaxation times in healthy volunteers and WB‐MRI patients using MRS at 3T. Five healthy female volunteers (mean age 38.0 ± 2.5 years) and 24 patients with malignant bone disease undergoing clinical WB‐MRI (13 male; mean age 67.7 ± 9.7 years; primary cancers: breast = 5, melanoma = 1, multiple myeloma = 8, prostate = 10) underwent variable inversion/echo time STEAM and 3D gradient echo fat‐water imaging. MRS water and fat peaks were fitted to determine T1, T2, R2* (from linewidths) and proton density fat fraction (PDFF). Scan‐rescan repeatability of MRS parameters was assessed in volunteers. Lesions were classified by disease state according to clinical reports and segmented in Dixon imaging data for comparison of fat fraction estimates with MRS. Repeatability was evaluated using coefficients of variation. Summary statistics (mean, standard deviation and range) were reported; exploratory inferential statistics were also determined with normality (Shapiro–Wilk) and variance (Levene's) tests before one‐way ANOVA and Tukey's comparisons (p < 0.05). Monte–Carlo simulations assessed relaxation bias on PDFF. All quantitative MRS parameters were repeatable (coefficient of variation < 10%). Water T1 and T2 were most sensitive to disease state in patients, ranging from 1121 to 2206 and from 15 to 71 ms respectively, and were demonstrated to substantially affect 2‐point Dixon fat fraction estimates with Monte–Carlo simulation. Imaging PDFF achieves closer agreement with MRS PDFF than 2‐point Dixon methods. These findings remain preliminary due to the small sample size, but they suggest value in future studies with larger cohorts evaluating water relaxation times or PDFF in malignant bone disease.

Water relaxation times measured with variable‐inversion and echo time STEAM show sensitivity to disease state in bone lesions.

In a volunteer cohort, these measurements are repeatable on a per‐subject basis in both L5 and the femoral head (average CoV < 10%).

Despite the small sample size, significant impacts on fat fraction estimation due to relaxation times using 2‐point T1w‐Dixon imaging recommended by international guidelines for whole‐body MRI.