Background: Osteoporosis is a worldwide health problem and a common cause of bone fractures; the most common type of osteoporosis is post-menopausal type. MRI has a role in the diagnosis of osteoporosis and can be used as a screening tool, so the purpose of our study was to define a quantitative MRI-based score (M-score) for the detection of lumbar spine osteoporosis and to evaluate the correlation between lumbar spine signal intensity measured by MRI and BMD (bone mineral density) in post-menopausal women. Our case-control study involved 100 cases (50 old post-menopausal females as a case group and 50 healthy females as a control group of matched age). Both groups were subjected to history taking, dual-energy X-ray absorptiometry (DEXA), and conventional lumbar MRI. DEXA was performed for the lumbar spine and all scores (T-score, Z-score, BMD) were calculated. Lumbar MRI was performed (sagittal T1WI and T2WI) from L1-L4 levels. SNR L1-L4 and M-score were calculated from T1W images. Results: All DEXA scores were significantly lower in post-menopausal females compared to the control group (P < 0.0001). Meanwhile, SNR L1-L4 and M-score were significantly higher among cases than controls (P < 0.0001). The diagnostic threshold of SNR L1-L4 and M-score for distinguishing osteoporotic from non-osteoporotic females was 104.5 for SNR L1-L4 with a sensitivity of 94%, specificity 60%, positive predictive values (PPV) 31%, and negative predictive values (NPV) 98%, and 3.5 for M-score with a sensitivity of 93.