Revenues
Total revenue for the six months ended June 30, 2022 was $30.5 million, an increase of 15% compared to the six months ended June 30, 2021 revenue of $26.5 million. During the six months ended June 30, 2022, total U.S. revenue increased by 16% and international revenue decreased by 2% over the six months ended June 30, 2021. The U.S. revenue growth was primarily driven by an increase in NeuroStar Advanced Therapy System 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 six months ended June 30, 2022 was $8.0 million, an increase of 85% compared to the six months ended June 30, 2021 revenue of $4.3 million. For the six months ended June 30, 2022 and 2021, the Company sold 104 and 59 systems, respectively, that were recognized as NeuroStar capital revenue during each period. Additionally, for the six months ended June 30, 2022, the Company executed 1 operating lease agreement that contributed to operating lease revenue.
U.S. treatment session revenue for the six months ended June 30, 2022 was $20.8 million, an increase of 2% compared to the six months ended June 30, 2021 revenue of $20.4 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 14% compared to the prior year period.
Cost of Revenues and Gross Margin
Cost of revenues increased by $2.5 million, or 51%, from $5.0 million for the six months ended June 30, 2021 to $7.5 million for the six months ended June 30, 2022. The increase was due to revenue growth versus the prior year period. Gross margin decreased from 81.2% for the six months ended June 30, 2021 to 75.3% for the six months ended June 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 $8.7 million, or 50%, from $17.6 million for the six months ended June 30, 2021 to $26.3 million for the six months ended June 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 $12.8 million for the six months ended June 30, 2021 to $12.7 million for the six months ended June 30, 2022.
Research and Development Expenses
Research and development expenses decreased by $0.8 million, or 16%, from $4.6 million for the six months ended June 30, 2021 to $3.8 million for the six months ended June 30, 2022. The decrease was primarily due to a reduction in product development costs related to the capitalization of certain of the Company’s software project costs, which was partially offset by an increase in clinical development costs versus the prior year period.
Interest Expense
Interest expense remained consistent at $2.0 million for the six months ended June 30, 2021 and 2022.