Revenues
Total revenue for the nine months ended September 30, 2022 was $47.0 million, an increase of 17% compared to the nine months ended September 30, 2021 revenue of $40.3 million. During the nine months ended September 30, 2022, total U.S. revenue increased by 18% and international revenue decreased by 20% over the nine months ended September 30, 2021. The U.S. revenue growth was primarily driven by an increase in Treatment Session sales and the decline in international revenue was primarily driven by a decrease in international NeuroStar Advanced Therapy Systems sales.
U.S. NeuroStar Advanced Therapy System revenue for the nine months ended September 30, 2022 was $12.0 million, an increase of 72% compared to the nine months ended September 30, 2021 revenue of $6.9 million. For the nine months ended September 30, 2022 and 2021, the Company sold 153 and 92 systems, respectively, that were recognized as NeuroStar capital revenue during each period. Additionally, for the nine months ended September 30, 2022 and 2021, the Company executed 2 and 7 operating lease agreements, respectively, that contributed to operating lease revenue.
U.S. treatment session revenue for the nine months ended September 30, 2022 was $32.6 million, an increase of 6% compared to the nine months ended September 30, 2021 revenue of $30.7 million. The revenue growth was primarily driven by an increase in the number of accounts utilizing our PHQ 10 tool. As a result, our treatment session volume in our local per click accounts, those utilizing our PHQ 10 tool increased 36.3% compared to prior year period.
Cost of Revenues and Gross Margin
Cost of revenues increased by $3.0 million, or 37%, from $8.1 million for the nine months ended September 30, 2021 to $11.1 million for the nine months ended September 30, 2022. The increase was due to revenue growth versus the prior year period. Gross margin decreased from 79.9% for the nine months ended September 30, 2021 to 76.4% for the nine months ended September 30, 2022. The decrease was primarily a result of a change in the product mix of revenues versus the prior year period.
Sales and Marketing Expenses
Sales and marketing expenses increased by $10.6 million, or 38%, from $27.4 million for the nine months ended September 30, 2021 to $38.0 million for the nine months ended September 30, 2022. The increase was expected and primarily due to new marketing initiative related costs, including trade shows, digital paid media costs and market research, and sales personnel expenses related to salary, benefits, commissions and share-based compensation incurred versus the prior year period.
General and Administrative Expenses
General and administrative expenses remained materially consistent from $19.2 million for the nine months ended September 30, 2021 to $19.1 million for the nine months ended September 30, 2022.
Research and Development Expenses
Research and development expenses remained materially consistent at $6.2 million for the nine months ended September 30, 2022 and 2021.
Interest Expense
Interest expense remained constant at $3.0 million for the nine months ended September 30, 2021 and 2022.