Comparison of Dixon Sequences for Estimation of Percent Breast Fibroglandular Tissue.

OBJECTIVES:To evaluate sources of error in the Magnetic Resonance Imaging (MRI) measurement of percent fibroglandular tissue (%FGT) using two-point Dixon sequences for fat-water separation. METHODS:Ten female volunteers (median age: 31 yrs, range: 23-50 yrs) gave informed consent following Research Ethics Committee approval. Each volunteer was scanned twice following repositioning to enable an estimation of measurement repeatability from high-resolution gradient-echo (GRE) proton-density (PD)-weighted Dixon sequences. Differences in measures of %FGT attributable to resolution, T1 weighting and sequence type were assessed by comparison of this Dixon sequence with low-resolution GRE PD-weighted Dixon data, and against gradient-echo (GRE) or spin-echo (SE) based T1-weighted Dixon datasets, respectively. RESULTS:%FGT measurement from high-resolution PD-weighted Dixon sequences had a coefficient of repeatability of ±4.3%. There was no significant difference in %FGT between high-resolution and low-resolution PD-weighted data. Values of %FGT from GRE and SE T1-weighted data were strongly correlated with that derived from PD-weighted data (r = 0.995 and 0.96, respectively). However, both sequences exhibited higher mean %FGT by 2.9% (p < 0.0001) and 12.6% (p < 0.0001), respectively, in comparison with PD-weighted data; the increase in %FGT from the SE T1-weighted sequence was significantly larger at lower breast densities. CONCLUSION:Although measurement of %FGT at low resolution is feasible, T1 weighting and sequence type impact on the accuracy of Dixon-based %FGT measurements; Dixon MRI protocols for %FGT measurement should be carefully considered, particularly for longitudinal or multi-centre studies

A scan-specific quality control acquisition for clinical whole-body (WB) MRI protocols.

Acknowledgements: This study represents independent research funded by the National Institute for Health and Care Research (NIHR) Biomedical Research Centre at The Royal Marsden NHS Foundation Trust and The Institute of Cancer Research, London, and by the Royal Marsden Cancer Charity and the Cancer Research UK National Cancer Imaging Translational Accelerator (NCITA). The views expressed are those of the author(s) and not necessarily those of the NIHR or the Department of Health and Social Care. The authors thank Maria-Alexandra Olaru, Gregor Thoermer, and Mirko Appel at Siemens Healthineers, and David Higgins at Philips Healthcare for helpful discussions.Objective.Image quality in whole-body MRI (WB-MRI) may be degraded by faulty radiofrequency (RF) coil elements or mispositioning of the coil arrays. Phantom-based quality control (QC) is used to identify broken RF coil elements but the frequency of these acquisitions is limited by scanner and staff availability. This work aimed to develop a scan-specific QC acquisition and processing pipeline to detect broken RF coil elements, which is sufficiently rapid to be added to the clinical WB-MRI protocol. The purpose of this is to improve the quality of WB-MRI by reducing the number of patient examinations conducted with suboptimal equipment.Approach.A rapid acquisition (14 s additional acquisition time per imaging station) was developed that identifies broken RF coil elements by acquiring images from each individual coil element and using the integral body coil. This acquisition was added to one centre's clinical WB-MRI protocol for one year (892 examinations) to evaluate the effect of this scan-specific QC. To demonstrate applicability in multi-centre imaging trials, the technique was also implemented on scanners from three manufacturers.Main results. Over the course of the study RF coil elements were flagged as potentially broken on five occasions, with the faults confirmed in four of those cases. The method had a precision of 80% and a recall of 100% for detecting faulty RF coil elements. The coil array positioning measurements were consistent across scanners and have been used to define the expected variation in signal.Significance. The technique demonstrated here can identify faulty RF coil elements and positioning errors and is a practical addition to the clinical WB-MRI protocol. This approach was fully implemented on systems from two manufacturers and partially implemented on a third. It has potential to reduce the number of clinical examinations conducted with suboptimal hardware and improve image quality across multi-centre studies

Investigating the Influence of Flip Angle and k-Space Sampling on Dynamic Contrast-Enhanced MRI Breast Examinations

Rationale and ObjectivesTo retrospectively investigate the effect of flip angle (FA) and k-space sampling on the performance of dynamic contrast-enhanced (DCE-) magnetic resonance imaging (MRI) breast sequences.Materials and MethodsFive DCE-MRI breast sequences were evaluated (10°, 14°, and 18° FAs; radial or linear k-space sampling), with 7–10 patients in each group (n = 45). All sequences were compliant with current technical breast screening guidelines. Contrast agent (CA) uptake curves were constructed from the right mammary artery for each examination. Maximum relative enhancement, Emax, and time-to-peak enhancement, Tmax, were measured and compared between protocols (analysis of variance and Mann–Whitney). For each sequence, calculated values of maximum relative enhancement, Ecalc, were derived from the Bloch equations and compared to Emax. Fat suppression performance (residual bright fat and chemical shift artifact) was rated for each examination and compared between sequences (Fisher exact tests).ResultsSignificant differences were identified between DCE-MRI sequences. Emax increased significantly at higher FAs and with linear k-space sampling (P < .0001; P = .001). Radial protocols exhibited greater Tmax than linear protocols at FAs of both 14° (P = .025) and 18° (P < .0001), suggesting artificially flattened uptake curves. Good correlation was observed between Ecalc and Emax (r = 0.86). Fat suppression failure was more pronounced at an FA of 18° (P = .008).ConclusionsThis retrospective approach is validated as a tool to compare and optimize breast DCE-MRI sequences. Alterations in FA and k-space sampling result in significant differences in CA uptake curve shape which could potentially affect diagnostic interpretation. These results emphasize the need for careful parameter selection and greater standardization of breast DCE-MRI sequences

Bland-Altman plots for repeat high-resolution GRE PD-weighted measurements (HR GRE PD & HR GRE PD (R), respectively).

<p>a) FGT volume [cm³]; b) Total breast volume [cm³] and c) %FGT [%]. Mean differences and limits of agreement are represented by the central and outer dashed lines, respectively.</p

Low resolution water fraction images prior to signal correction for two subjects.

<p>A) a 26 year old volunteer with 64.6% FGT and B) a 50 year old volunteer with 18.4% FGT. GRE PD, GRE T₁ and SE T₁-weighted images are shown in i), ii) and iii), respectively—the water fraction values within the fat are much higher in the SE T₁-weighted datasets.</p

High resolution GRE T₁-weighted Dixon breast images of the right breast in a 31 year old volunteer, showing the effect of signal intensity correction.

<p>A signal intensity correction factor was derived from the maximum water and fat signal intensities within a centrally located region of interest (ROI) on the Water- and Fat-only images (<b>a</b> and <b>b</b>, respectively, each scaled to their maximum signal intensity) and was applied to the Water-only image. The |Water + Fat| and Water Fraction images prior to signal intensity correction are shown in <b>c</b> and <b>e</b>, respectively. The equivalent images following signal intensity correction are displayed in <b>d</b> and <b>f</b>, respectively; <b>d</b> shows clearly the normalized contributions of water and fat following correction of the Water-only image. Calculated %FGT was found to be 34.4%.</p