Knee subchondral bone perfusion and its relationship to marrow fat and trabeculation on multi-parametric MRI and micro-CT in experimental CKD

Abstract

The pathogenesis of chronic kidney disease (CKD) is multifactorial. In the progression of CKD arthropathy, arteriosclerosis may alter the knee subchondral bone marrow by altering blood flow through the bone vasculature. Herein, multi-parametric MRI assessment, including dynamic contrast enhanced magnetic resonance imaging (DCE-MRI), magnetic resonance spectroscopy (MRS), MRI T2*, contrast enhanced MR angiography (CE-MRA), and micro-CT were applied in a rodent nephrectomy model to: 1) investigate the blood perfusion of subchondral bone marrow and its relationship to fat water content and trabeculation pattern in CKD and 2) demonstrate the feasibility of using multi-parametric MRI parameters as imaging biomarkers to evaluate the disease's progression. Two groups of rats in our study underwent either 1) no intervention or 2) 5/6 nephrectomy. We found that in the CKD group, perfusion amplitude A and elimination constant k _el values were significantly decreased, and vascular permeability k _ep was significantly increased. MRS showed that fat fraction (FF) was significantly lower, water fraction (WF) was significantly higher in the CKD group. Micro-CT showed a significant loss of trabecular bone. Knee subchondral bone marrow perfusion deficiency in experimental CKD may be associated with decreased fat content, increased water content, and sparse trabeculation.

Figure 1

A schematic diagram of the experimental design. The chart shows the time points of MRI and MRS studies (indicated by ↓); histologic examination (indicated by ⇩), and μCT examination (indicated by ).

Figure 2

Representative DCE-MRI, MRI T2*, and CE-MRA images. Exactly 44 weeks without intervention (control rats) or after 5/6 nephrectomy (CKD rats): (a) DCE images (amplitude A map) of the lateral femoral and tibial subchondral bone marrow of the right knee show significant hypoperfusion in the CKD group (lower) but not in the control group (upper); (b) MRI T2* maps of the lateral femoral and tibial subchondral bone marrow of the right knee show significantly elevated T2* values in the CKD group (lower) but not in the control group (upper); and (c) coronal maximum intensity projections (MIPs) of the CE-MRA images of popliteal arterieheas show a significantly lower signal intensity and smaller vessel diameters in the CKD group (lower). White arrows indicate the popliteal arteries.

Figure 3

Plots showing the time courses of three perfusion parameters and the MRI T2* values (mean ± SD) for the femoral and tibial subchondral bone marrow (SCBM) of the control and CKD groups. (a) Amplitude A (unit: au). (b) Elimination constant k _el (unit: min⁻¹). (c) Permeability rate constant k _ep (unit: min⁻¹). (d) T2* (unit: msec). All values were measured in the right knees of all rats. Asterisks demonstrated significant differences (p < 0.05). A significant increase in the blood volume parameter A and washout parameter k _el can be observed in (a) and (b) at week 0, and was followed by a decrease beginning at week 8. Significant increases can be noted in the permeability rate constant k _ep (c) starting from week 36 and in MRI T2* value (i.e., edematous change) (d) starting from week 16.

Figure 4

¹H-MRS spectra of subchondral bone marrow in the control and CKD groups. (a) Each spectrum includes a water peak (arrow head) at 4.7 ppm and the lipid peak (arrow) at 1.33 ppm (top row, left). Spectral analysis shows that the lipid peak height increases markedly between 0 and 44 weeks in the control group (top row), but only mildly between 0 and 44 weeks in the CKD group (bottom row). (b) Longitudinal MRS study found that the course of change in femoral and tibial SCBM fat-water content differed between the two groups after week 8. Fat content was increased at week 8 and thereafter significantly decreased in the CKD group, while water content is significantly increased in the CKD group after week 8.

Figure 5

μCT analysis of the subchondral bone marrow architecture in the control and CKD groups. Four ROIs including bone marrows of the lateral femoral subchondral bone (LFSCB), lateral tibial subchondral bone (LTSCB), medial femoral subchondral bone (MFSCB), and medial tibial subchondral bone (MTSCB) were assessed and four parameters including (a) trabecular thickness Tb.Th (unit: µm), (b) trabecular number Tb.N (unit: µm⁻¹), (c) bone volume divided by total volume BV/TV (unit: %), and (d) trabecular separation Tb.Sp (unit: µm) were measured at week 44 in the right knees of all rats. Subchondral bone marrows with significantly decreased Tb.Th, Tb.N, and BV/TV (a,b,c) and significantly increased in Tb.Sp (d) can be observed in the CKD group.

Figure 6

Histologic and immunohistochemical examination of the subchondral bone marrow in the control and CKD groups. Areas of subchondral bone marrow edema were statistically larger in the CKD group (b: arrow, H&E x40 and g) than in the control group (a: H&E x40 and g). Trabeculae were also thinner in the CKD group (b, arrowheads) than control (a, arrowheads). Images of subchondral marrow immunohistochemically stained with anti-alpha smooth muscle actin were analyzed using the dedicated software Image Scope (Aperio®, Leica Microsystems, Nanterre, France). The average vessel density was statistically significantly higher in CKD rats (d: green spots and h) than in control rats (c: green spots and h) and the average vessel diameter was statistically significantly smaller in CKD rats (f: arrows, x200 and i) than in control rats (e: arrows, x200 and i).

Figure 7

ROIs in the knee subchondral bone marrow. (a) ROIs (white) in the femoral and tibial subchondral bone marrow were manually selected using the DCE-MRI first-frame and MRI T2* first-echo sagittal image. (b) MR spectroscopy values were measured using a PRESS sequence, with two cube-shaped ROIs (1 mm each side) positioned at the center of the lateral and medial parts of the femoral and tibial subchondral bone marrow, respectively.