Bring a large pot of salted water to a boil. Add pasta and cook until al dete; drain.
Meanwhile, melt butter in a large heavy skillet over medium heat. Saute shrimp for 1 minute on each side. Stir in garlic and cook for 1 minute. Remove shrimp from skillet (some shrimp may not be cooked completely) and set aside.
Stir in flour and Cajun seasoning into the butter. Cook, stirring for 2 minutes. Gradually whisk in milk, then cook until thickened. Return shrimp to sauce and cook 1 to 2 minutes more until shrimp is done. Season with salt.
Top pasta with shrimp sauce and sprinkle with Parmesan cheese.