Final answer:
The term for aligning the IR's long dimension with the body's long axis is 'longitudinal'. It is important for proper imaging in medical diagnostics.
Step-by-step explanation:
The term that means the image receptor (IR) is oriented so its long dimension is aligned with the long axis of the body is 'longitudinal'.
This is in contrast to other orientations, such as sagittal, which would divide the body into right and left portions, or transverse, which would divide the body into upper and lower portions.
It's crucial to align the long dimension of the IR with the axis of the body for proper imaging and diagnostic purposes.