PF_R16_SINT does not seem to be supported when the UTexture2D with PF_R16_SINT is exposed to Material Blueprints. So I tried with PF_G16 and it seems to work well.
However this is not the best solution because PF_G16 = DXGI_FORMAT_R16_UNORM according to the source code, and the DXGI_FORMAT_R16_UNORM is “A single-component, 16-bit unsigned-normalized-integer format that supports 16 bits for the red channel” as described here ([DXGI_FORMAT (dxgiformat.h) - Win32 apps | Microsoft Docs][1]).
So I had to shift each value from short (-32768 to 32767) to unsigned short (0 ～ 65535) by adding 32768 to store each value to DXGI_FORMAT_R16_UNORM, and shift back in Material Blueprints.
There does exists DXGI_FORMAT_R16_SNORM which supports 16-bit signed-normalized-integer but Unreal Engine does not seem to support this pixel format.
In addition, I had to convert the automatically converted (calculated) sRGB value to linear RGB value using the formula described here ([A close look at the sRGB formula][2]), because the visualization of medical images does not use sRGB color space in general.
I looked for the option for not applying sRGB color to the final color output, but I could not find the option unfortunately. Is there anyone who knows the option for it?
And I would like to know if there are better solutions.
Anyway, here is the material I modified,
and the visualized CT image on Unreal Engine 4.
The image is exactly the same as the image visualized by ImageJ!