Thermal therapy for osteoarthritis is becoming one of the options for pain-relief(1). In order to optimize the therapeutic effect, temperature visualization of the cartilage is indispensable, but rarely investigated to date. We have demonsrated the feasibility of noninvasive temperature imaging of knee joint cartilage under thermal therapy using water proton resonance freqrequency shift measured as phase shift(2). This technique is, however, useful only when the knee joint position and posture are unchanged during heating. The phase shift measument is suffered not only by the object displacement itself but also by the static magnetic field change due to the object displacement. When entire knee joint is heated like in warm bath treatment or kinesiologic approach, referenceless type of techniues (3, 4) based on interpolation of phase field is not applicable. In order to find a technique to measure temperature without an image subtraction, we have considered the use of spin-lattice relaxation time, T1, for mesuring absolute temperature of the knee joint cartilage. As a feasibility study, we have examined the temperature dependence of T1 in the knee joint cartilage in vitro in comparison with that of the water proton resonance frequency.Proton spectra of cartilage segment samples collected from porcine knee joints were observed in a 9.4 T NMR spectrometer. The sample was immersed in deuterium oxide (D₂O, 99%, Sigma-Aldrich) in a NMR sample tube of 5 mm in diameter. Trimethylsilyl propanoic acid (TSP) was added as an internal reference. After turning off auto field-frequency locking and shimming systems, the proton spectra were evaluated at various temperature ranging from room temperature, 30, 40, 50 and 60 ^oC. The sample temperature was controlled with an air blower system equipped with the spectrometer. Then a conventional inversion recovery sequence with the following conditions yielded T1 of water proton in the sample; TR, 30 s; TI, 0.1, 0.2, 0.4, 1.0, 2.0. 4.0, 8.0, and 16 s, observation bandwidth, 8.012 kHz, and number of spectral points, 65536. The area of water peak was obtained over 4ppm around the center frequency at each temperature, and used for T1 calculation. The spectrum of the cartilage sample exhibited a water signal with negligible (0.02-0.03 %) fractions of the other components as shown in Fig. 1. Thus the water proton resonance frequency as well as the peak area of the water signal was readily measured from the spectrum. The relationship between temperature and T1 in six independent measurements is plotted in Fig. 2a, while that between temperature and the water proton chemical shift measured from TSP is in Fig. 2b. The temperature coefficient of the water proton T1 was 36.9 ms/^oC (1.28%/^oC at 30^oC) for heating period and 36.0 ms/^oC (1.24%/^oC at 30 ^oC) for cooling. The two temperature coefficient did not have significant difference, and thus no hysteresis was recognized in the temperature range used in the present study. The correlation between the two temperature indicators is also plotted in Fig. 3, showing that the correlation was higher than 0.996.This work was straight forward examination to evaluate if T1 of the knee joint cartilage can be an index for absolute temperature measurement. As is seen from Fig. 2, the temperature dependence of the parameter is quite reproducible in a range from room temperature to 60^oC. Because no hysteresis was recognized, tissue denaturation seemed to have no effect on the T1 temperature dependence in this temperature range. In the imaging scheme for the particular tissue like knee cartilage, resonance frequency change measured with phase difference will be strongly suffered by tissue displacement and static magnetic field change. On the other hand, T1 can be mapped as its absolute value without difference calculation, although a quick T1 mapping method like with multiple flip angles needs care for B1 field homogeneity. These results support that T1 is a strong candidate for measuring absolute temperature in the cartilage, and thus temperature imaging unaffected by the tissue displacement is greatly expected.Feasibility of noninvasive and absolute MR thermometry for knee joint cartilage based on T1 was demonstrated. Although blood flow in the cartilage is poor and thus the in vitro results shown here should be valid in the in vivo tissues, similar careful study has to be made using living animals. Rapid T1 mapping technique with multiple flip angle approach to image the absolute temperature is under our current examination.