An apparatus for treating a body lumen, comprising: an elongate tubular member including a proximal end, a distal end sized for introduction into a body lumen, and a first lumen extending between the proximal and distal ends; an expandable balloon comprising a proximal end secured to the tubular member distal end, and a distal end comprising an outlet, the balloon comprising an interior communicating with the first lumen and the balloon outlet; an elongate member slidably disposed within the first lumen, the elongate member comprising a proximal end adjacent the tubular member proximal end, a distal end extending from the balloon outlet, and a guidewire lumen extending between the elongate member proximal and distal ends; and a sealing member on the elongate member distal end distal to the balloon distal end, the elongate member being movable between a first distal position wherein the sealing member is spaced from the balloon outlet such that fluid introduced through the first lumen passes through the balloon interior and out the balloon outlet, and a second intermediate position wherein the sealing member substantially seals the balloon outlet such that fluid introduced through the first lumen enters the balloon interior to expand the balloon; and a helical member including a first end coupled to the tubular member distal end and a second end coupled to the elongate member distal end, the helical member extending helically around the elongate member within the balloon interior, the elongate member movable to a third proximal position in which the elongate member distal end is directed towards the tubular member distal end to cause the helical member to expand radially outwardly to an expanded helical shape, thereby expanding the balloon to the expanded helical shape. an elongate tubular member including a proximal end, a distal end sized for introduction into a body lumen, and a first lumen extending between the proximal and distal ends; an expandable balloon comprising a proximal end secured to the tubular member distal end, and a distal end comprising an outlet, the balloon comprising an interior communicating with the first lumen and the balloon outlet; an elongate member slidably disposed within the first lumen, the elongate member comprising a proximal end adjacent the tubular member proximal end, a distal end extending from the balloon outlet, and a guidewire lumen extending between the elongate member proximal and distal ends; and a sealing member on the elongate member distal end distal to the balloon distal end, the elongate member being movable between a first distal position wherein the sealing member is spaced from the balloon outlet such that fluid introduced through the first lumen passes through the balloon interior and out the balloon outlet, and a second intermediate position wherein the sealing member substantially seals the balloon outlet such that fluid introduced through the first lumen enters the balloon interior to expand the balloon; and a helical member including a first end coupled to the tubular member distal end and a second end coupled to the elongate member distal end, the helical member extending helically around the elongate member within the balloon interior, the elongate member movable to a third proximal position in which the elongate member distal end is directed towards the tubular member distal end to cause the helical member to expand radially outwardly to an expanded helical shape, thereby expanding the balloon to the expanded helical shape.