# Bicomponent Mapping of Cortical Bone Using a New Interleaved UTE Imaging Sequence

## TL;DR

A new MRI technique improves the accuracy of measuring water in cortical bone by reducing scan inconsistencies.

## Contribution

An interleaved UTE dual-echo sequence is introduced to enhance bicomponent T2* mapping in cortical bone.

## Key findings

- The interleaved method reduced fitting errors and improved T2* quantification compared to conventional scans.
- Interleaved scans showed more stable and homogeneous parameter distributions with lower variability.
- Scan repeatability was significantly better with the interleaved sequence than with separate scans.

## Abstract

Ultrashort echo time (UTE) MRI enables direct imaging of cortical bone and quantification of its water compartments via bicomponent T2* modeling. However, conventional approaches require multiple separate dual‐echo scans due to limitations in gradient power. This approach is prone to inter‐scan inconsistencies such as motion and signal drift, which degrade fitting accuracy. This study proposes an interleaved dual‐echo acquisition sequence that acquires multiple echo time (TE) images in a single scan to improve bicomponent T2* quantification in cortical bone.

The proposed UTE sequence utilizes interleaved dual‐echo acquisitions with flexible TE spacings. This sequence was tested on five healthy subjects' tibial midshafts and compared to conventional separate dual‐echo scans with and without image registration. Bicomponent T2* modeling was performed, and fitting accuracy was evaluated using normalized‐root‐mean‐squared error (NRMSE). Three subjects were scanned three times to evaluate the scan repeatability.

The interleaved method significantly reduced NRMSE (3.2% ± 2.3% vs. 6.2% ± 3.1%, p = 0.0231) and yielded lower and more stable T2* (T2s*; 0.50 ± 0.10 ms vs. 0.76 ± 0.13 ms, p < 0.0001) and fraction (Fs; 78.2 ± 5.1 vs. 84.2% ± 7.1%, p = 0.0006) of short T2 components compared to separate scans without registration. Image registration had a minimal improvement on mapping results for separate scans. Parameter maps from the interleaved scans confirmed more homogeneous distributions of T2s* and Fs with lower fitting errors. The much lower coefficients of variance of the interleaved scans demonstrated improved repeatability compared with separate scans.

The proposed interleaved UTE dual‐echo sequence improves the robustness of bicomponent T2* mapping of the cortical bone by reducing inter‐scan inconsistencies.