NMAC BTWN A B737 ON A 10 MI FINAL VISUAL APCH AND A CESSNA 172 NOT IN COMS WITH APCH CTLR IN CLASS B AIRSPACE. APCH CTLR SEPARATED TFC BY CLBING THE RPTR'S ACFT.

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JUST PRIOR TO OCCURRENCE WE WERE BEING VECTORED FOR ILS RWY 22R DTW. WE WERE ON A 090 DEG HDG AT 4000 FT 170 KTS ASSIGNED. WE WERE INSTRUCTED TO TURN R HEADING 200 DEGS DSND TO 3000 FT AND ACCELERATE TO 180 KTS; INTERCEPT THE LOC; WHICH WE DID. WE WERE ASKED DO YOU HAVE THE ARPT. WE SAID; RWY 22R IN SIGHT AT WHICH TIME WE WERE CLRED FOR THE VISUAL. THE APCH CTLR SAID YOU HAVE TFC AT 1 O'CLOCK POS AND 4 MI. WE DID NOT HAVE TFC IN SIGHT RPTED LOOKING. A FEW SECONDS LATER ATC INSTRUCTED US TO CLB TO 3500 FT TO AVOID TFC. HE STATED; HE WAS NOT IN COM WITH TFC AND IT WAS UNKNOWN TO HIM. THE TFC WAS A C172. WE HAD THE TFC IN SIGHT AS IT PASSED UNDERNEATH. AT THE MOMENT OF THE OCCURRENCE WE WERE WITHIN THE 11 DME ARC FOR DTW WHERE THE BASE OF CLASS B AIRSPACE WAS 3000 FT. I BELIEVE THE CESSNA ACFT HAD BEEN OFF COURSE SLIGHTLY AND ENTERED THE BOTTOM OF DTW CLASS B AIRSPACE. THE DTW APCH CTLR DID AN EXCELLENT JOB TO AVOID A POSSIBLE NEAR MISS/HIT OR A TCASII TA/RA. A POSSIBLE SOLUTION TO FIX THIS OCCURRENCE BESIDES ACFT (NON RADAR) STAYING OUT OF CLASS B AIRSPACE; WOULD BE TO HAVE ACFT APCHING DTW ARPT TO INTERCEPT THE GS AT A SLIGHTLY HIGHER ALT THAN THE BASES OF DTW'S CLASS B AIRSPACE.