Target Audience: Clinicians and physicists who work for knee diseases

Background: Knee effusion occurs when excess fluid accumulates in the knee. Knee joint fluid may consist of hemorrhage, hypertrophic synovium, and exudation from inflamed tissues. Proton (1H) MR spectroscopy (MRS) provides information on molecular characterizations, and variations in metabolite signals could represent different pathologic conditions. Although some previous ex-vivo NMR studies reported the possibility of monitoring pathologic conditions in fluid-filled lesions (1,2,3), in vivo proton MRS has not yet been systematically used to study the joint fluids in human knees.

Purpose: To assess the ability of proton MR spectroscopy to identify the apparent heterogeneous characteristics of metabolic spectra in effusion regions in human knees using a 3T MRI system.

Eighty-four patients with effusion lesions who were referred for routine knee MR imaging underwent proton MRS with point-resolved single-voxel MR spectroscopy using a clinical 3.0 Tesla MRI system. Thirty-eight patients were confirmed to have the following: degenerative osteoarthritis, 21 patients (Group 1); traumatic diseases, 12 patients (Group 2); infectious diseases, 4 patients and an inflammatory disease, 1 patient (Group 3). Spectroscopy data were analyzed using the public jMRUI (http://www.jmrui.eu, Academy of Sciences of the Czech Republic, Brno, Czech Republic) free-ware software to obtain lipid metabolites. We defined three types of spectrums which are the CH2 and CH3 dominant (Type A), the CH=CH dominant (Type B) and both metabolite dominant (Type C). In order to quantify the three types of spectra, we calculated the lateralization index (LI) as LI = (PA-PB)/(PA+PB), where PA was the sum of the peak intensity of CH2 and CH3 and PB was that of CH=CH. Nonparametric statistical comparisons were performed to investigate any differences in metabolites among the three disease groups.Figure 1 shows the three typical types of spectra obtained from different patients in the different disease groups. Type A is the MRS spectrum that shows dominant peaks for the CH2 around 1.1–1.5 ppm and CH3 around 0.7–1.0 ppm lipids. Type B is the MRS spectrum that shows a dominant peak around 5.1–5.5 ppm for the vinylic CH=CH lipids. Type C is the MRS spectrum that shows both Type A and B metabolite peaks. Each patient had either a CH=CH lipid peak, CH2 and CH3 lipid peaks, or all three peaks. There were no significant differences among the three groups for the CH3 (p=0.9019), CH2 (p=0.6406), and CH=CH lipids (p=0.5467) and water (p=0.2853); none of the metabolites could differentiate between any of the three types of diseases. In addition, there were also no significant differences among the three groups in the sum of the CH3 and CH2 lipids (p=0.6886), in the ratio of the CH3 to CH2 lipids (p=0.1700), or in the ratio of the CH=CH lipids to the sum of the CH2 and CH3 lipids (p=0.3696) and LI (p=0.3696). The CH2 lipids in the 38 patients who had confirmed fluid characteristics were significantly correlated with CH3 lipids (rho=0.835, p<0.0001). The ratio of CH3 to CH2 was highest in the degenerative disease.The major metabolites were the vinylic CH=CH lipid around 5.1–5.5 ppm, CH2 lipid around 1.1–1.5 ppm, and CH3 lipid around 0.7–1.0 ppm. Each patient had a CH=CH lipid peak, CH2 and CH3 peaks, or both CH=CH and CH2 and CH3 peaks. None of the metabolites could differentiate between the degenerative and traumatic diseases. The ratio of CH3 to CH2 was highest in the degenerative diseases. To the best of our knowledge, this study was the first report of in vivo MR spectroscopy of the joint fluid area in human knees using a 3T MRI system.In both the degenerative and traumatic diseases, metabolite peaks of the vinylic CH=CH lipids around 5.1–5.5 ppm and of the sum of the CH2 and CH3 lipids around 0.7ppm–1.5 ppm were observed, but in the infectious disease, only a metabolite peak of the sum of the CH2 and CH3 lipids was detected, and none of the metabolites could differentiate between the three types of diseases. Additional studies must be performed with more patients.