With the increasing number of medical imaging modalities, compressing the digital medical images can facilitate their transmission and storage. This paper evaluates and compares two lossy compression methods : JPEG and wavelet, applied to the end diastolic phase slices of a cardiac MRI acquisition. First, the qualitative image quality was evaluated with clinical criteria and was performed by radiologists. The compression image quality was then not judged satisfactory over 22:1 for JPEG and 32:1 for wavelets. Secondly, we evaluated the effect of these lossy methods on physiological parameters, such as left ventricular mass and end-diastolic volume, calculated by an automated segmentation algorithm. The wavelet coding allows to accept a compression ratio of 67:1, while JPEG is limited to 30:1. Wavelet compression performed superior to JPEG at high and low compression ratios. We conclude that limiting the compression ratios to those defined by the psycho-visual assessment secures the validity of physiological parameters in cardiac MRI.