It works. Great. Thanks. fslmaths gives similar results as imcalc in SPM but mricron really saves identical image as hdr/img.

I just found that t1 images in hdr/img coregistered to native DTI images are oriented in ASL thus viewed strangely only in fsleyes. But mricron and SPM could view them in normal manner. The converted t1 images will be analyzed by fdt in fsl. Therefore, Do I have to convert the orientation of my t1 images from ASL to RPI (standard images in fsl)? Which tools would you recommend to perform these conversion to reliably set the 'qform or sform' matrix etc.? Thank you again.

Best regards,

Yong