Although the Centers for Medicare and Medicaid is not hot on paying for Eli Lilly’s PET scan for all Alzheimer’s disease diagnostics, the scan is playing a pivotal role in the drug maker’s latest clinical trails of its experimental AD drug solanezumab.

The drug maker outlined its plans for this third round of trials on Friday. This time around, research will focus on patients with mild Alzheimer’s disease, and only candidates that have beta amyloid antibodies will be eligible for the trial, which means undergoing an Amyvid PET scan or undergoing a spinal fluid analysis.

This tack dovetails with CMS’s draft memo from earlier this week in which it outlined only two situations in which the Amyvid test would be relevant: clinical-trials to fulfill coverage with evidence development requirements and if it’s a close call for determining if a patient’s mental deterioration is due to Alzheimer’s or another condition. Beta amyloid is thought to be a possible cause of the brain disorder.

Unlike previous trials which focused on mild-to-moderate patients and eked out disappointing results, this round of testing focuses only on mild Alzheimer’s patients, an earlier phase of Alzheimer’s disease than the mild-to-moderate cases. Mild cases were a minor part of earlier testing (but the only population to show efficacy upon subgroup analysis).

Executives said they expect the screening requirements will mean a miss rate of about 50% of candidates. They anticipate a 22-month recruiting period, and that the trial of 2,100 patients will wrap by the end of 2016. This trial is sticking with the 400mg infusion levels of the previous two rounds. The AD team’s senior director, Eric Siemers, said Lilly is staying with this protocol so that the new data set can confirm what was found in the pooled results.