It is not as straightforward in Xray as compare to metal detector. Many will depend on your product, scanning length and type of packaging.
Are you familiar with the Xray's gray scale? Depending on the brand, you can see "value" of the gray scale when you put the cursor on the image. The difficulty of the contaminant being detected increases when the gray scale of the product is nearing to the value of contaminant (test piece in this case).
If you have rigid container and you have single beam Xray, you may find "blind spot" which you will not detect your contaminant at all.
Depending on your scanning length, you may have contaminant within your pouch but will be difficult for your Xray to see. This can be true if you have long pouch and have large airspace inside the packaging.
How will the image of the product is displayed in your Xray screen will tell a lot on where could be the most hardest to detect location.
In summary, while many say most of the "product effect" is felt in metal detector, the same can be true depending on how you validate Xray and must be aware of the "blind spot" conditions.