Air Cargo Security Improvement Act of 2018 (Sec. 2) This bill directs the Transportation Security Administration (TSA) to: (1) establish an air cargo security division, (2) submit to Congress a feasibility study regarding the use of computed tomography technology for the screening of air cargo transported on passenger aircraft operated by air carrier or foreign air carrier in air transportation, interstate air transportation, or interstate air commerce; (3) initiate a two-year pilot program to achieve enhanced air cargo security screening outcomes through the use of new or emerging screening technologies, such as computed tomography technology; and (4) report to Congress on actions to improve the Certified Cargo Screening Program. (Sec. 5) The Government Accountability Office must review the TSA's screening processes and procedures for air cargo entering the United States and assess its risk-based strategy for examining such cargo. (Sec. 6) The TSA must request the Air Cargo Subcommittee of the Aviation Security Advisory Committee to conduct a comprehensive review and security assessment of the known shipper program and recommend whether the program should be modified or eliminated in light of the full implementation of required 100% screening of air cargo carried on passenger aircraft.