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T2 Mapping of Rat Patellar Cartilage¹

¹ From the Departments of Pharmacology (A.W., P.T.A.O., N.C.G., P.A.N., A.G.B., P.M.G., D.H.J.L.) and Clinical Rheumatology (D.H.J.L.), UMR 7561, CNRS-Université Henri Poincaré, Nancy I, Physiopathologie et Pharmacologie Articulaires, Faculté de Médecine, Avenue de la Forêt de Haye, BP 184, F 54505 Vandoeuvre Les Nancy, France; and the Department of Medical Magnetic Resonance Research (U2R2M), ESA CNRS 8081, Orsay, Paris, France (J.P.B.R., P.D.G., G.M.G.). Received May 5, 2000; revision requested June 17; revision received August 7; accepted September 6. Supported by grants from Projet Hospitalier de Recherche Clinique (1994 and 1998), the "Pole Européen de Santé," and Groupement de Recherches CNRS 2237. Address correspondence to P.A.N. (e-mail: netter@medecine.uhp-nancy.fr).

View larger version (135K): [in a new window]   Figure 1. Histologic sections obtained in 4-week-old rats (A, C, E) and old rats (B, D, F). HZ = hypertrophic zone, RZ = radial zone, SZ = superficial zone, and TZ = transitional zone. Cartilage thickness and cellularity decrease with cartilage maturation and aging. In sections stained with toluidine blue (proteoglycans constituent), the matrix was markedly stained in immature cartilage (A), whereas it was weakly stained in mature cartilage (B), without spatial variation with depth in both groups. (Toluidine blue stain; original magnification, x10.) C, The matrix was poorly stained with picrosirius red (collagen constituent) in immature cartilage, with a relative increase in staining in the superficial and transitional zones. (Picrosirius red stain; original magnification, x10.) D, Staining increases in mature cartilage, with thick bundles oriented perpendicular to the articular surface in the deep part of the radial zone. (Picrosirius red stain; original magnification, x10.) E, F, Polarizing light microscopy characterizes the collagen network organization. (Picrosirius red stain; original magnification, x10.) E, Immature cartilage manifests with a thick transitional zone, with a horizontal alveolar organization, and a thick hypertrophic zone constituted of two or three rows of cells. F, Mature cartilage manifests with a classic organization in four superimposed zones: superficial, transitional, radial, and hypertrophic.

View larger version (34K): [in a new window]   Figure 2. Graphs show glycosaminoglycan and hydroxyproline content of patellar cartilage during the maturation and aging process. Open circles represent the rat groups studied only by means of biochemistry. Black circles represent rat groups studied by means of both biochemistry and MR imaging. A, Biochemical analysis reveals a biphasic profile of proteoglycan content, which progressively decreases from 147.21 µg/mg ± 1.31 to 52.07 µg/mg ± 0.98 in 4-month-old rats before it increases to 71.63 µg/mg ± 0.44 in old rats (P < .001). B, Collagen content increases from 50.78 µg/mg ± 0.62 of cartilage to 76.71 µg/mg ± 0.48 in old rats (P < .001).

View larger version (114K): [in a new window]   Figure 3a. (a) Eight transverse spin-echo images of a rat patella and (b) the resultant T2 map. (a) Cartilage signal intensity decreases with the increase in echo time (TE). From an echo time of 5.5 msec to an echo time of 30 msec, the signal-to-noise ratio is divided by 10. (b) Resultant T2 map permits clear delineation of the cartilage structure. Arrows point to articular cartilage.

View larger version (129K): [in a new window]   Figure 3b. (a) Eight transverse spin-echo images of a rat patella and (b) the resultant T2 map. (a) Cartilage signal intensity decreases with the increase in echo time (TE). From an echo time of 5.5 msec to an echo time of 30 msec, the signal-to-noise ratio is divided by 10. (b) Resultant T2 map permits clear delineation of the cartilage structure. Arrows point to articular cartilage.

View larger version (59K): [in a new window]   Figure 4a. Transverse patellar cartilage T2 maps obtained with an 8.5-T microimager in (a) an 8-week-old rat and (b) an old rat. The T2 map displays on color-scale images the spatial distribution of the T2 values. Although the trabecular bone (thin straight arrow) is clearly identified as a structure with high signal intensity related to a high fat content, the cartilage (curved arrow) appears, whatever the age group, as a thin structure that has moderate signal intensity and is surrounded by two structures with low signal intensity, corresponding to the air (O) and to the subchondral bone (thick straight arrow).

View larger version (59K): [in a new window]   Figure 4b. Transverse patellar cartilage T2 maps obtained with an 8.5-T microimager in (a) an 8-week-old rat and (b) an old rat. The T2 map displays on color-scale images the spatial distribution of the T2 values. Although the trabecular bone (thin straight arrow) is clearly identified as a structure with high signal intensity related to a high fat content, the cartilage (curved arrow) appears, whatever the age group, as a thin structure that has moderate signal intensity and is surrounded by two structures with low signal intensity, corresponding to the air (O) and to the subchondral bone (thick straight arrow).

View larger version (23K): [in a new window]   Figure 5. Graph shows age- and site-related modifications of T2 in rat patellar cartilage. In each of the four groups, the mean T2 value calculated for the superficial layer is higher than the mean T2 value calculated for the deepest layer. The superficial () and deep () T2 values differ by 3 msec for groups 1, 3, and 4, whereas this difference is only 1.5 msec for group 2. However, because of the limited number of samples, we were unable to determine whether these differences were significant. With the maturation and aging process, there is a progressive and significant decrease in T2 values for the superficial (P < .05) and deepest layers (P = .01), respectively. Bars represent the mean.

View larger version (42K): [in a new window]   Figure 6. Bar graph shows age-related modifications of global T2 in rat patellar cartilage. With the maturation and aging process, there is a progressive decrease in the global T2 values from 12.10 msec ± 0.13 in 4-week-old rats to 7.95 msec ± 0.65 in old rats. In the immature groups (4 weeks and 8 weeks), the global T2 values range from 12.1 to 10.7 msec, whereas in the mature groups (4 months and old rats), these values are significantly lower, ranging from 8.3 to 7.9 msec (P < .01). Error bars = standard error of the mean (SEM).

View larger version (40K): [in a new window]   Figure 7. Bar graph shows histologic and MR imaging assessment of age-related variations in rat patellar cartilage thickness. Striped bars = 4-week-old rats, gray bars = 8-week-old rats, white bars = 4-month-old rats, and black bars = old rats. The T2 map shows cartilage thinning between the 4-week-old rats (334.36 µm ± 8.91) and the old rats (169.77 µm ± 24.00). There is good agreement globally between MR imaging and histologic data (r = 0.55, P = .02). Nevertheless, this Figure suggests that the correlation is less in the mature than in the immature groups, which may be because of the limited number of samples forming each group; however, this could not be confirmed statistically. Error bars = standard error of the mean (SEM).