Quantitative assessment of synovial vascularity using contrast-enhanced power Doppler ultrasonography: correlation with histologic findings and mr imaging findings in arthritic rabbit knee model

Abstract

Objective: To validate contrast-enhanced power Doppler ultrasonography (PD US) for the evaluation of synovial vascularity in an arthritic rabbit knee model in correlation with MR and histological findings.

Materials and methods: Power Doppler ultrasonography was performed for carrageenin-induced arthritic left knee and control right knee of 13 rabbits, first without and then with sonic contrast agent enhancement (Levovist, Schering, Berlin Germany), followed by gadolinium-enhanced MR imaging. Synovial vascularity was quantitatively assessed by calculating the color pixel area in power Doppler sonography using a computer-aided image analysis program and by grading the enhancement on MR images: grade 1, enhancement of knee joint is less than one-third of the area; grade 2, one-third to two-thirds enhancement; and grade 3, more than two-thirds enhancement. Microvessel density (MVD) was measured on slides stained immunohistochemically for CD31 antigen for histological assessment.

Results: The mean area of color pixels in PD US changed from 4.37 to 16.42 mm(2) in the arthritic knee after enhancement (p < 0.05), whereas it changed from 0.77 to 2.31 mm(2) in the control knee (p < 0.05). Arthritic knees had greater power Doppler signal than control knees both before and after contrast administration (p < 0.05). The average MVD was 88 in arthritic knees and 46 in control knees. MVDs correlated with color pixel areas of contrast-enhanced power Doppler imaging in arthritic knees. In MR grading of arthritic knees, five were grade 2 and eight were grade 3. MVD and PD US revealed no significant difference between grade 2 and 3 arthritic knees (p > 0.05).

Conclusion: Sonic contrast-enhanced PD US improves the visualization of synovial vascularity and allows quantitative measurement in experimentally induced rabbit arthritic knees.

Fig. 1

Ultrasound images of carrageenin-induced arthritis of the rabbit. A, B. Power Doppler ultrasound examinations before (A) and after (B) ultrasound contrast injection in the arthritic left knee show expansion of the joint capsule and heterogeneous echogenicity in the posterior joint space (arrow in A). Power Doppler signals in the joint are faintly visible before contrast agent injection, except for the penetrating intraosseous signal (score = 0.45 mm²). After the contrast agent injection, Power Doppler signals increase in extent in the posterior joint space (double arrows in B) and signals of popliteal vessels (arrowheads in B) appear prominently (score = 30.78 mm²).

Fig. 2

Ultrasound images of control right knee of the rabbit. A, B. Power Doppler ultrasound examinations before (A) and after (B) ultrasound contrast agent injection in the control knee show faint power Doppler signals of the posterior joint space (arrow in A) before contrast agent injection (score = 0.74 mm²) and a few intra-articular signals (double arrows in B) after contrast enhancement (score = 2.57 mm²).

Fig. 3

MR images of carrageenin-induced arthritis of the rabbit. Sagittal scan of T1-weighted image (A), fat-suppressed, fast spin-echo T2-weighted image (B), three-dimensional fat-suppressed, fast spoiled gradient-echo (3D FS-FSPGR) image before contrast enhancement (C), and 3D FS-FSPGR image after contrast enhancement (D) show joint effusion with posterior joint capsular distension (arrow in A and B) and thick contrast enhancement at the posterior margin of the joint space on the 3D FS-FSPGR image (arrowhead in D, grade 2).

Fig. 4

MR images of the control right knee of the rabbit. Sagittal scan of T1-weighted image (A), fat-suppressed, fast spin-echo T2-weighted image (B), three-dimensional fat-suppressed, fast spoiled gradient echo (3D FS-FSPGR) image before contrast enhancement (C), and 3D FS-FSPGR image after contrast enhancement (D) show no abnormal intra-articular lesions. Note the prominent intra-articular fat on T1WI (arrow in A), a few high signals on T2WI (double arrows in B), and little contrast enhancement on 3D FS-FSPGR image (arrowhead in D, grade 1).

Fig. 5

Histology (Hematoxylin & Eosin staining, ×100) A. Arthritic knee shows marked synovial hypertrophy (arrow) and aggregation of foamy macrophages and lymphocytes in the subsynovial layer. B. Control knee shows an intact synovial membrane without hypertrophy (double arrows).

Fig. 6

Immunohistochemical staining for CD 31. A. Arthritic knee shows many vessels in synovial and subsynovial layers (in brown). B. Control knee shows a few vessels in synovial and subsynovial layers.