Alliance Imaging Inc. said it has agreed to buy Los Angeles-based Mobile Technology Inc. for $100 million in cash and assumption of debts. The purchase will give Alliance 180 mobile magnetic-resonance-imaging systems nationwide, securing its rank as the nation's largest provider of MRI services. The Anaheim-based company said it hopes to complete the deal by the end of February. Alliance had been a darling of Wall Street, praised for its ability to recover from near-disaster to become a leader in the fragmented mobile MRI industry. But the company needed more cash than it had to make major acquisitions in the fast-consolidating field, as doctors and smaller companies increasingly opted to sell out. Looking for that extra cash, Alliance agreed in July to be acquired by Apollo Management.