This is a feasibility trial studying anti-PD-1 therapy (Pembrolizumab) among patients with R/M HNSCC, delivered with ultrasound-induced microbubble cavitation, with the goal of optimizing delivery of Pembrolizumab and tumor response to Pembrolizumab. Patients will undergo 3 infusions of Pembrolizumab plus Definity 3 weeks apart. Following each infusion, ultrasound will be directed at the primary tumor site to induce microbubble cavitation, with the goal of tumor sonoporation. The primary endpoints will be feasibility, measured based on successful recruitment of 6 participants within 1 year of initiating recruitment, with successful completion of trial procedures among at least 80 percent of patients. Secondary endpoints will include proportion of serious adverse events, clinical/radiographic response, overall survival, and progression-free survival.