million. For the three months ended September 30, 2023 and 2022, the Company sold 43 and 49 systems, respectively, that were recognized as NeuroStar Advanced Therapy System capital revenue during each period. Additionally, for the three months ended September 30, 2022 the Company executed 1 operating lease agreement that contributed to operating lease revenue.
U.S. treatment session revenue for the three months ended September 30, 2023 was $13.1 million, an increase of 10% compared to the three months ended September 30, 2022 revenue of $11.9 million. The revenue growth was primarily driven by an increase in treatment session volume and utilization over the prior year quarter.
Cost of Revenues and Gross Margin
Cost of revenues increased by $2.5 million, or 71%, from $3.6 million for the three months ended September 30, 2022 to $6.1 million for the three months ended September 30, 2023. This increase was primarily due to the recording of a $1.9 million inventory impairment for specialized component parts secured for discontinued NeuroStar Advanced Therapy Systems for which costs exceed net realizable value. Gross margin decreased from 78.4% for the three months ended September 30, 2022 to 65.8% for the three months ended September 30, 2023. The decrease in gross margin was driven by the one-time inventory impairment, higher operational costs related to our transition to a new third-party contract manufacturing partner and software amortization expense from the latest product release.
Sales and Marketing Expenses
Sales and marketing expenses increased by $0.5 million, or 4%, from $11.6 million for the three months ended September 30, 2022 to $12.1 million for the three months ended September 30, 2023. The increase was primarily due to increased spending on co-op marketing related to growth in that program.
General and Administrative Expenses
General and administrative expenses remained materially consistent from $6.4 million for the three months ended September 30, 2022 to $6.3 million for the three months ended September 30, 2023.
Research and Development Expenses
Research and development expenses decreased by $0.1 million, or 8%, from $2.3 million for the three months ended September 30, 2022 to $2.2 million for the three months ended September 30, 2023. The decrease was primarily due to higher software capitalization, related to the latest development of the NeuroStar system.
Interest Expense
Interest expense increased by $0.1 million, or 12%, from $1.1 million for the three months ended September 30, 2022 to $1.2 million for the three months ended September 30, 2023 due to interest rate and debt balance increases.
Other Income, Net
Other income, net decreased by $0.2 million, or 27%, from $0.9 million for the three months ended September 30, 2022 to $0.7 million for the three months ended September 30, 2023, primarily as a result of decreased interest income earned on the Company’s money market accounts which was partially offset by increase in notes receivable interest.