Cook pasta according to package directions, omitting salt and fat. Drain and keep warm.
While pasta cooks, heat a large nonstick skillet over medium heat. Coat pan with cooking spray. Add mushrooms, and sauté 5 minutes or until mushrooms are tender and liquid evaporates, stirring frequently. Remove from pan, and keep warm.
Add butter to pan, and cook over medium heat 1 minute or until butter melts. Add flour, and stir with a whisk; cook 1 minute or until bubbly. Add milk, salt, and pepper; cook 3 minutes or until thick, stirring constantly.
Add reserved mushrooms, chicken, and artichokes to pan, stirring gently. Cook 3 minutes or until thoroughly heated. Add lemon juice and, if desired, green onions. Toss chicken mixture with pasta. Sprinkle with Parmesan cheese, if desired.