See here for the Prospect study published in the NEJM.
This multi-institutional study involved a follow up of 697 patients who presented with an acute coronary syndrome (ACS) treated with an invasive catheter procedure ( percutaneous coronary intervention or PCI). The patients underwent coronary angiography and coronary artery intravascular ultrasound imaging.
Over a three year period 20.4 % had another adverse coronary event . The authors analysis indicated that about half of these events were due to obstruction at the location of the culprit lesions ( i.e the obstructive lesion that was treated by PCI) and half were due to non-culprit lesions. Most of the non-culprit lesions were considered angiographically mild (typically with an obstruction less than 70%) and were described as "thin-walled fibroatheromas" with a large amount of plaque . I suppose if those lesions were not thought to be mild by the cardiologist that they would have been "fixed".
According to the authors, a lesion with a thin wall, a lumen area of less than 4 mm,and a plague burden of 70% has a three year risk of causing a coronary event of 17%.
Potentially useful information not provided in the article would include to what extent patient with these non-obstruction lesions were treated with medications thought capable of stabilizing plaques, e.g. clopidogrel, aspirin, and statins. You have to believe (hope?) that the percentage would have been higher if patients had not been treated with drugs that "pacify the platelets" and decrease inflammation.
The authors emphasize that the intravascular imaging techniques used in this study are not ready for prime time, everyday clinical use because the specificity of lesions with the most predictive risk characteristics is low and there were serious side effects (including rupture of a cornoary artery) .Further, data was only obtainable on the proximal portion of the major epicardial coronary arteries ( about 6-7 cm) so what was going on more distally is unknown.