The present invention provides a fixation mechanism (5) for fixing a flex coil unit of a MRI system onto a patient comprising an oblique-shaped clip (9) comprising a first portion (9a) and a second portion (9b) arranged to form an oblique angle therebetween, wherein the first portion (9a) is configured to be arranged with a strap (7) along which the oblique-shaped clip (9) is subject to a force when the flex coil unit is fixed onto the patient； and an elongated slot (11) located at a longitudinal side of a housing of a spine coil unit of the MRI system and formed at least partially by the housing of the spine coil unit, the elongated slot (11) being configured to receive the second portion (9b) of the oblique-shaped clip (9) and further configured to allow the second portion (9b) of the oblique-shaped clip (9) to abut against a side wall of the elongated slot (11) by the force along the strap (7). According to the present invention, it is very convenient for an operator to insert the clip into the slot or release the clip out of the slot at any position along the longitudinal direction of the housing.
