Se följande beskrivning av termen vid panoramaröntgen:

In panoramic radiography, the x-ray source and the film are connected to each other. These two components rotate simultaneously around the patient to produce an image. The three-dimensional, horseshoe-shaped zone where images are sharp is called the focal trough, or image layer. The panoramic radiograph is composed largely of the anatomic structures located within this focal trough.

The focal trough is the area in which structures will appear most sharply and clearly. Structures, which fall in front of or behind, the focal trough, can be distorted, magnified or reduced. The size and shape of the focal trough varies between manufacturers.

(http://www.oralhealthgroup.com/news/panoramic-radiography-fo...

Inom kameraoptik brukar man ju prata om fokalplan eller bildplan och en sökning på bildplan + röntgen ger en del intressant material. Se t.ex. kompendierna i röntgenteknologi från Uppsala universitet. Särskilt följande har en bra bild: http://www.radiol.uu.se/utbildning/rtgsjuksk/Rtgteknologi/Rt...

I texten ovan kallas det ju även "image layer", så bildplan känns som att det skulle kunna funka i ditt sammanhang.