A spiral CT demonstrates pulmonary vessel clots as a central filling defect outlined by contrast material. Disadvantages to Spiral CT for the diagnosis of PE is that lymphatic and connective tissue might sometimes appear to be outlined by contrast material, and that respiratory motion artifact can be significant problems. Although pulmonary angiography remains the "gold standard", it is associated with a 6% morbidity and 0.5% mortality, which is much higher then CT and VQ scanning. One study comparing VQ and CT demonstrated that interobserver agreement was "excellent" for CT, while only "moderate" for VQ scans. A problem with the VQ scan is with the categories of high, intermediate, and low probability, and one must analyze the scan together with "clinical suspicion" to come up with an overall percentage probability of PE, which was the basis for the PIOPED study. In one study, while CT picked up as positive 64 out of 67 pulmonary emboli (that were true emboli), VQ scanning labeled seven of these as low probability or normal, thus placing these true emboli is a "less likely" category. CT angiography is faster and more specific and sensitive then VQ scanning and is safer and more comfortable for the patient then pulmonary angiography.