Imrie scored the only goal - with just 12 minutes remaining at Cappielow - as Morton upset Dundee 1-0.
He took his goal with some aplomb, side-footing high into the net after good work from Rowan Vine, but Dundee only have themselves to blame for losing their footing at the top after a series of missed chances.
Falkirk are still in the equation after beating Raith Rovers 2-1 at home, their fifth successive victory.
Rory Loy finished off a cross from Joe Chalmers after 19 minutes and they looked to be well in command after Conor McGrandles deceived Raith keeper Lee Robinson with a clever lob to make it 2-0 four minutes later.
Will Vaulks deflected a John Baird shot into the net for Rovers before half time but Falkirk were worthy winners.
Cowdenbeath can still beat the drop after a convincing 4-0 win over Livingston at Central Park.
John Armstrong broke the deadlock just before half-time when he headed in from a corner-kick.
Kane Hemmings made it 2-0 six minutes into the second half and then added his second shortly afterwards as Livingston were caught on the break.
Armstrong headed number four to make absolutely certain of the points.
Queen of the South took a big step towards the promotion play-offs with a 3-0 win at Dumbarton.
Iain Russell, who makes a habit of scoring against Dumbarton, made the breakthrough after 17 minutes with Derek Lyle adding in the second half.
Kevin Holt then made it 3-0 with a stunning volley.