RF data in matlab

The RF data generated in matlab from data files is having number of samples as rows and number of transducer elements array as columns. The series SIG{1}, SIG{2} ... SIG{N} of ultrasound matrices have been generated by sending similar wavefronts with the same ultrasound array are represented as elements in the 3D array in MUST. In some documents I have seen that RF data is taken as number of samples by number of elements by number of scanlines. So how the data in matlab can be correlated to scanlines. Can anyone please help me on understanding these better. The data files i used are P4-2v, PWI_disk.mat, etc...