The use of intravenous ultrasound contrast agents can improve endocardial border delineation and can result in improved diagnostic accuracy.181 Myocardial contrast echocardiogra- phy also has been examined for determination of rest and stress myocardial perfusion, with the results showing compa- rability to myocardial perfusion SPECT findings in small patient series.182 However, the technique is currently in limited use in the United States. The diagnostic accuracy of all imaging modalities is influenced by technical factors that could be inherent in the technique (ie, variable correlation between perfusion and wall motion abnormalities and CAD extent and severity) or that result from physical characteristics of the patient that reduce image quality. For echocardiography, reduced image quality, defined as reduced LV endocardial visualization, has been reported for obese individuals and those with chronic lung disease, although the use of intravenous contrast enhance- ment results in sizeable improvement in endocardial border delineation. 2.2.3.3. Exercise and Pharmacological Stress Nuclear Myocardial Perfusion SPECT and Myocardial Perfusion PET Myocardial perfusion SPECT generally is performed with rest and (for exercise or pharmacological stress) with stress Tc-99m agents, with Tl-201 having limited applications (eg, viability) because of its higher radiation exposure.97 Pharma- cological stress generally is used with vasodilator agents administered as a continuous infusion (adenosine, dipyridam- ole) or bolus (regadenoson) injection. The diagnostic end- point of nuclear MPI is reduction in myocardial perfusion after stress. 