Cook pasta according to package directions, omitting salt and fat.
Preheat oven to 400°.
Cook sausage in a large nonstick skillet over medium-high heat until browned, stirring to crumble. Add onion, mushrooms, and celery; cook 6 to 8 minutes or until tender. Add tomatoes, pimiento, and garlic. Bring to a boil; reduce heat, and simmer, uncovered, 8 to 10 minutes or until almost all liquid has evaporated.
Combine sausage mixture and hot cooked pasta in a large bowl, mixing well. Pour into a 13 x 9-inch baking dish coated with cooking spray. Sprinkle with cheese. Bake, uncovered, at 400° for 15 minutes or until cheese melts.