Improved Visualization of Cartilage Canals Using Quantitative Susceptibility Mapping

<div><p>Purpose</p><p>Cartilage canal vessels are critical to the normal function of epiphyseal (growth) cartilage and damage to these vessels is demonstrated or suspected in several important developmental orthopaedic diseases. High-resolution, three-dimensional (3-D) visualization of cartilage canals has recently been demonstrated using susceptibility weighted imaging (SWI). In the present study, a quantitative susceptibility mapping (QSM) approach is evaluated for 3-D visualization of the cartilage canals. It is hypothesized that QSM post-processing improves visualization of the cartilage canals by resolving artifacts present in the standard SWI post-processing while retaining sensitivity to the cartilage canals.</p><p>Methods</p><p>Ex vivo distal femoral specimens from 3- and 8-week-old piglets and a 1-month-old human cadaver were scanned at 9.4 T with a 3-D gradient recalled echo sequence suitable for SWI and QSM post-processing. The human specimen and the stifle joint of a live, 3-week-old piglet also were scanned at 7.0 T. Datasets were processed using the standard SWI method and truncated k-space division QSM approach. To compare the post-processing methods, minimum/maximum intensity projections and 3-D reconstructions of the processed datasets were generated and evaluated.</p><p>Results</p><p>Cartilage canals were successfully visualized using both SWI and QSM approaches. The artifactual splitting of the cartilage canals that occurs due to the dipolar phase, which was present in the SWI post-processed data, was eliminated by the QSM approach. Thus, orientation-independent visualization and better localization of the cartilage canals was achieved with the QSM approach. Combination of GRE with a mask based on QSM data further improved visualization.</p><p>Conclusions</p><p>Improved and artifact-free 3-D visualization of the cartilage canals was demonstrated by QSM processing of the data, especially by utilizing susceptibility data as an enhancing mask. Utilizing tissue-inherent contrast, this method allows noninvasive assessment of the vasculature in the epiphyseal cartilage in the developing skeleton and potentially increases the opportunity to diagnose disease of this tissue in the preclinical stages, when treatment likely will have increased efficacy.</p></div

Main pre- and post-processing steps for SWI, QSM and QSM-WI.

<p>Main pre- and post-processing steps depicted for a single slice (in a plane parallel to <i>B</i>₀) from the distal femur of a 3-week-old pig at 9.4 T. Original GRE magnitude (A) and phase (B). Generation of segmentation mask was initiated with a single-slice manual ROI (C), which was extended to the entire 3-D volume automatically (D), generating a segmentation mask for further processing (E). In SWI post-processing, high-pass filtering of the phase was first done using homodyne filtering (F). The phase was converted to a negative phase mask (G) and the SWI data was generated by applying the phase mask to the original magnitude data (H). Finally the segmentation mask was also applied to the SWI data for further visualizations (I). For QSM post-processing, the phase was first processed using Laplacian and SHARP filtering (J) and, in turn, converted to a quantitative susceptibility map with k-space inversion (K) and masked with the segmentation and contrast-inverted to match the appearance of SWI (L). Finally, the susceptibility map was converted into an enhancing mask (M) and finally applied to the magnitude data to generate a QSM-WI dataset (N).</p

Three-dimensional reconstructions of cartilage canals using SWI and QSM.

<p>3-D reconstructions of the cartilage canals in the medial femoral condyle of an 8-week-old pig scanned at 9.4 T. The QSM processing (A) allowed visualization of the cartilage canals without artifacts. In the SWI post-processed data (B), the splitting artifact was seen. The red arrows point to a matching vessel identified in the two datasets.</p

Comparison of QSM, plain GRE, SWI and QSM-WI at 9.4 T.

<p>Comparison of QSM, SWI and QSM-WI post-processing as well as unprocessed GRE for the visualization of cartilage canals in a 1-month-old human cadaveric distal femur in 3 mm-thick minimum intensity projections in the main imaging planes with respect to the scanner geometry at 9.4 T (TE = 15.05 ms and bandwidth = 37 Hz/pixel). The first pane shows the axial view, perpendicular to <i>B</i>₀: both truncated k-space QSM (QSM) and QSM-weighted imaging (QSM-WI) results appeared nearly identical to the SWI result. The plain GRE appeared similar, but lacked some of the detail. The second pane shows coronal and sagittal views, parallel to the <i>B</i>₀ field. Both QSM visualizations demonstrate the vasculature without artifacts whereas, in the SWI data, the splitting of the vessels along the <i>B</i>₀ direction is noted. The plain GRE appeared similar to QSM and also did not show artifacts, but clearly lacked the definition seen with QSM. White arrows point to several matching vessels to aid comparison. The QSM contrast (first row) was inverted to match the contrast of the SWI data.</p

Quantitative susceptibility values of the cartilage canals.

<p>Relative susceptibility values of the cartilage canals with respect to the surrounding tissue in a 1-month-old human cadaveric distal femur scanned at 9.4 T (A) and at 7.0 T (B), and in a 3-week-old piglet scanned at 7.0 T <i>in vivo</i> (C) as a function of the truncation factor used in the k-space dipole inversion. Inset images in A-C depict single slices from the quantitative susceptibility maps at truncation factor values of 0.5, 5 and 20 at an intensity scale normalized with the intensity of the cartilage canals to facilitate visual comparison of the streaking artifacts. The second row shows the susceptibility histograms acquired for the corresponding cartilage canal ROIs for the respective specimens as a function of the truncation factor (D-F).</p

Comparison of QSM, plain GRE, SWI and QSM-WI at 7.0 T <i>in vivo</i>.

<p>Comparison of QSM, GRE, SWI and QSM-WI of a 3-week-old piglet scanned at 7.0 T <i>in vivo</i>. In the first pane, showing an axial plane perpendicular to B0, the datasets appeared visually similar. In the second pane, with views parallel to B0, artifactual splitting of the vessels was observed for the SWI data while both QSM datasets and the unprocessed GRE appeared artifact-free.</p

Protocols for MR relaxometry.

<p>Protocols for MR relaxometry.</p

Regression analysis of the percent differences of all parametric MRI relaxation constants and of the light absorption between the viable and necrotic epiphyseal cartilage.

<p>The solid line shows the linear fitting (R² = 0.39) between the percent differences of all MR relaxation constants (<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0140400#pone.0140400.t004" target="_blank">Table 4</a>) and the percent difference of the light absorption (<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0140400#pone.0140400.t002" target="_blank">Table 2</a>, column 4), resulting in the slope equal to 0.21.</p

Parametric MRI images of femoral condyle.

<p>Relaxation time maps for (A, E) T₂, (B, F) T_1ρ, (C, G) adiabatic T_1ρ, and (D, H) T_RAFF in the medial condyle of the distal femur at 5 weeks (top row, large lesion) and 6 weeks (bottom row, small lesion) post-surgically. Lesion locations are indicated by the arrows in the T₂ maps.</p

Percent difference in relaxation times between viable and necrotic epiphyseal cartilage using various MRI sequences.

<p>Large lesion (5 × 5 mm incision): 3, 5, 9, weeks post surgery</p><p>Small lesion (3 × 4 mm incision): 4, 6, 10 weeks post surgery</p><p>Due to technical problems, the T₂ measurement was invalid for the 3-week specimen and was not included.</p><p>Percent difference in relaxation times between viable and necrotic epiphyseal cartilage using various MRI sequences.</p