Triple negative breast cancer patient-derived xenographs were implanted in the 4th abdominal mammary fat pads of mice enrolled in a ~60 minute, multi-contrast, same-day (morning vs. afternoon), test-retest MRI protocol. Quantitative T1, T2, and ADC maps were acquired. Parameter distributions were characterized by standard statistical measures (mean, median, standard deviation, skewness, and kurtosis) and a Bayesian implementation of the maximum-entropy method-of-moments density function. TNBC PDX T2 maps were found to be markedly more robust to test-retest assessment compared to T1 and ADC maps. These results will inform studies employing MRI assessment of TNBC PDX response to docetaxel/carboplatin therapy.