1 Department of Biostatistics, University of North Carolina at Chapel Hill, Chapel Hill, NC 27599, USA.
2 Statistics and Operations Research, University of North Carolina at Chapel Hill, Chapel Hill, NC 27599, USA.
3 Department of Biostatistics, University of North Carolina at Chapel Hill, Chapel Hill, NC 27599, USA ; Biomedical Research Imaging Center, University of North Carolina at Chapel Hill, Chapel Hill, NC 27599, USA.