Premium
SU‐GG‐J‐116: Simplified Method for Acquiring CTN to RLSP Conversion Function
Author(s) -
Moyers M F
Publication year - 2010
Publication title -
medical physics
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.473
H-Index - 180
eISSN - 2473-4209
pISSN - 0094-2405
DOI - 10.1118/1.3468340
Subject(s) - imaging phantom , calibration , function (biology) , aluminium , atomic number , computational physics , materials science , nuclear medicine , physics , mathematics , optics , atomic physics , statistics , composite material , evolutionary biology , biology , medicine
Purpose : A major uncertainty that affects the margins in light ion beam planning is the conversion of x ray computed tomography numbers (XCTNs) to relative linear stopping powers (RLSPs). Determination of this conversion function has been difficult in the past because of several reasons. This work developed a simplified method of determining this function. Method and Materials : The new method was based on the premise that all scanners have their XCTNs for air and water calibrated daily. The XCTNs for high density / high atomic number materials are variable, however, for different scanning conditions. On the other hand, most previous conversion functions have been linear above an XCTN/RLSP of 1050/1.05. A function can therefore be established using a phantom consisting only of water and a single high density / high atomic number material. XCTNs and RLSPs were calculated for various tissues and aluminum 6061 alloy for various effective x ray energies and compared to literature values. A parameter derived from the scaled XCTN (SXCTN) of aluminum was determined that defines the slope of the function for different scanning conditions. A phantom was constructed and used to test the new method with different scanners at a variety of kV p s. Results : The RLSP for an SXCTN of 4095 is given by: RLSP 4095 = 16.616 − (1.0323×10 −2 * SXCTN A1 ) + (2.5181×10 −6 * SXCTN AL 2 ) − (2.1339×10 −10 * SXCTN AL 3 ) For soft tissues, the RLSPs calculated using the aluminum calibration method were between 0 and ± 1.5% of the values determined by different investigators on different scanners using traditional methods; deviations as great as 2.6% were found for hard bone. Due to difficulties with previous methods, it is not clear which values are most appropriate. Conclusion : The new method greatly simplifies acquisition of the conversion function, reduces ambiguity, maintains accuracy, and could promote standardization between facilities.