For the purpose of making the density measurements, FDA interprets measurements taken at the "center" of the phantom to be equivalent to measurements taken on the center line as described later in this paragraph. As an alternative to repositioning the disk or replacement of the phantom cover, you may choose to keep the disk in place (as is currently recommended by ACR). You may then measure the background density at a different location along the center line of the phantom in the direction parallel to the chest wall, as long as this location does not obscure any of the phantom objects. In addition, you must measure the background density at the same location each time the test is conducted.
u should be aware that if you remove the disk, permanent defects may be produced in the cover plate. Such defects may produce "permanent artifacts" in the phantom image. As long as these "permanent artifacts" do not interfere with the scoring of the phantom (do not simulate masses, fibers or specks, and do not obscure the test objects), facilities may continue to use the phantom. The facility should not subtract these "permanent artifacts" from the phantom score. However, if these "permanent artifacts" interfere with the scoring of the phantom (simulate masses, fibers or specks, and/or obscure one or more of the test objects), the defective portion of the phantom must be replaced. The facility should also be aware that any artifacts on phantom images submitted for accreditation purposes may reduce the phantom image final score, which in some cases may result in a phantom image failure.