Radiopacity - in simple terms it is the whitish area on X-Ray films which denotes relative inability of X-rays to pass through the particular material.
Radiolucent - in simple terms it is the blackish area on X-Ray films which denotes relative transparency to X-Rays.
Hyperechoic tissues appear bright/whitish in ultrasound imaging (means there are many internal echoes ).
Hypoechoic structures appear dark/blackish in ultrasound imaging (means there are few internal echoes ).
Also homogeneous structures are darker than heterogeneous.
Hyperdense - relatively Whitish/Bright region on CT.
Hypodense - relatively Blackish/Dark region on CT.
Hyperintense - relatively Whitish region on MRI.
Hypointense - relatively Blackish region on MRI.