By Mark Guarino

 The engine of Madonna's 21 - year career is reinvention. Look back and her lineage of videos and concert tours is lined with shifting selves - - from disco boy - toy all the way up to children's book author.

By naming her current tour "Re - Invention," the 45 - year - old is not so much trying anything new as she is, for the first time, collecting all her former selves and seeing if they can co - exist together.

Some call it nostalgia, but Madonna has never been that obvious. At the United Center Sunday, the first of four sold - out nights, she tried to make sense out of everything she's done in the past, but in the exhilarating collage, she demonstrated some previous lives live up to the present and a few do not.

Some reinvention was musical and on these songs, Madonna and her eight - piece band and core of dancers celebrated their durability. "Into the Groove," an early hit, was remixed with a more complex beat, rapping interludes from a recorded Missy Elliott and, strangely, a live bagpiper and drum corps. "Like a Prayer," part of her disco folk set, swelled with spiritual uplift with the help of a recorded gospel choir.

Unlike her dark and condensed "Drowned World" tour in 2001, this outing joyfully interchanged past with present. The best moments blurred images and toyed with mixed messages.

 She and her dancers performed "Express Yourself," an infectious dance pop statement of individuality, dressed in military gear and twirling rifles. For "Burning Up," her earliest dance hit, and "Material Girl," Madonna posed as a serious guitar rocker, hitting chords and transforming the songs' adolescent whine into adult certitude.

The flow of imagery had its chinks when Madonna revisited weaker material - - notably "Hanky Panky," a vaudeville jazz send - up from "Dick Tracy." And no matter what you think of Andrew Lloyd Webber, his material ("Lament") doesn't sound good being sung when the singer is strapped to a fake electric chair.

Unlike the past, the show was not designed to provoke but was filled with more moments where she tried to present herself as a serious songwriter.

She slipped into that mode during the show's third act, a short acoustic set that ended with a cover of John Lennon's "Imagine." The choice may have been in protest, since Clear Channel Entertainment, her tour's producer and promoter, is the same company that banned the song from its 1,200 radio stations after Sept. 11.

But since she was singing in front of a backdrop of televised starving children, it's more likely she was using the song to signal her altruism. Her shrill rendition didn't do that. Instead, it felt like another reinvention, just that this one was empty and presumptuous.