This will be two separate but identical studies: ALSE-A-02a, ALSE-A02b. Each study is designed to evaluate the diagnostic accuracy and safety of a single dose of 123I-ALTROPANE® in subjects with upper extremity tremor for less than three years. Currently, no radiopharmaceutical diagnostic imaging agent has been approved by the FDA in the U.S. for use in diagnosing Parkinson disease and related Parkinsonian syndromes. The diagnosis of Parkinsonian syndromes in the U.S. is based on clinical criteria only. The goal is to demonstrate that 123I-ALTROPANE® paired with SPECT imaging permits a more accurate early diagnosis of Parkinson disease than a clinical diagnosis by a general neurologist.