Digital infrared thermal imaging (DITI) is used to detect surface temperature because objects of absolute temperatures higher than zero would radiate electromagnetic waves of certain wave lengths according to Plank's law. DITI examination is non-invasive, non-contact and fast. Theoretically, temperature of the perfusion area of the occlusive arteries will decrease. Investigators will analyze DITI of PAD, and observe the relationship between DITI, ABI and vascular duplex examination. The dynamic change of DITI before and after percutaneous transluminal angioplasty of PAD will also be observed. Investigators would like to evaluate the possibility of the clinical application of DITI for PAD screening.