In general it is best to use high frequency X-ray systems to maximize image quality and to minimize patient exposure time. Older X-ray machines may be limited to low power such as 100 MA and 100KVp. These systems will stress the ability to record good quality images due to patient girth limitations.
What happens is that power limitations such at that listed above require that the patient be limited to a modest size, small physical girth, or else a multi-second exposure will be required. A long exposure time will require that the patient be as motionless as possible to avoid motion artifact.
Experience has shown that any time in excess of one second will require the patient to be as motionless as possible. This rarely works and image quality suffers.