An efficient multi-slice inversion–recovery EPI (MS-IR-EPI) sequence for fast, high spatial resolution, quantitative T
1
mapping is presented, using a segmented simultaneous multi-slice acquisition, combined with slice order shifting across multiple acquisitions. The segmented acquisition minimises the effective TE and readout duration compared to a single-shot EPI scheme, reducing geometric distortions to provide high quality T
1
maps with a narrow point-spread function. The precision and repeatability of MS-IR-EPI T
1
measurements are assessed using both T
1
-calibrated and T
2
-calibrated ISMRM/NIST phantom spheres at 3 and 7 T and compared with single slice IR and MP2RAGE methods. Magnetization transfer (MT) effects of the spectrally-selective fat-suppression (FS) pulses required for in vivo imaging are shown to shorten the measured
in-vivo
T
1
values. We model the effect of these fat suppression pulses on T
1
measurements and show that the model can remove their MT contribution from the measured T
1
, thus providing accurate T
1
quantification. High spatial resolution T
1
maps of the human brain generated with MS-IR-EPI at 7 T are compared with those generated with the widely implemented MP2RAGE sequence. Our MS-IR-EPI sequence provides high SNR per unit time and sharper T
1
maps than MP2RAGE, demonstrating the potential for ultra-high resolution T
1
mapping and the improved discrimination of functionally relevant cortical areas in the human brain.