The CT localizer radiograph, also known as scanogram, scout and surview, is the name given to the initial images obtained at the start of a CT before the main volume is obtained. These initial images have two main functions: they ensure that the correct anatomy is being covered by the CT and permit the software to optimize the radiation used to obtain the images.
Terminology
The American Association of Physicists in Medicine (AAPM) publish a CT lexicon, which includes all the different vendor-specific terms for the CT localizer radiograph. Specifically, scout is used by GE, scanogram by Canon and Hitachi, topogram by Siemens and surview by Philips. Other less well-known manufacturers also use the same terms 1,2.
The scout radiograph has also been known by other terms over the fifty years that CT has been in clinical use, including scanned projection radiograph and pilot image.
Acquisition
The localizer radiograph is acquired before the main CT images are obtained. These radiographs are obtained in a lateral and/or a frontal projection, the latter either anteroposterior (AP) and posteroanterior (PA). When both are acquired, the lateral projection is generally obtained initially so that the patient is correctly centered in the vertical plane. If this is off, then the height of the CT table can be repositioned prior to the frontal projection localizers being acquired 1.
One of the key roles of the localizer radiograph is to ensure that the CT acquisition covers the correct anatomy. For example, that a CT of the abdomen covers the entire liver or a lung CT includes the top of the lung apices 1.
The localizers also permit the automatic exposure control (AEC) system of the scanner to modulate the radiation dose correctly 1.
The CT operator manipulates a transparent box digitally overlaid the localizer radiograph on the screen to optimize the anatomic field of view for the main CT acquisition. When the planned CT protocol includes multiple phases, each phase will have its own coverage box as the anatomy to be covered sometimes varies for each phase.
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