Dual energy rapid switching imaging system

A subtraction angiography method that uses a television (TV) camera to form images on its target corresponding to x-ray images and which camera produces vertical blanking pulses at constant periodicity to mark the beginning and end of TV frame times, said method including the steps of providing for:

• exposing an anatomical region to a low average energy x-ray beam from an x-ray tube for an interval beginning with occurrence of a vertical blanking pulse and ending within a frame time and extending over less or more than one frame time while a relatively low peak kilovoltage (kVp) is applied to the anode of the tube and a predetermined current (MA) is flowing through the tube to thereby form a nominally low energy mask image on the target in the TV camera;
• terminating said exposure with automatic exposure control (AEC) in response to a predetermined x-ray dose having been administered and then determining and storing the exposure time for the low energy mask image;
• after the exposure is terminated, reading out the TV camera target and storing the low energy mask image;
• exposing said region to a higher average energy x-ray beam for an interval beginning with occurrence of a vertical blanking pulse and ending within a frame time and extending over less or more than one frame time after the low energy exposure while higher kVp is applied to said anode and a predetermined MA is flowing through said tube to thereby form a nominally high energy mask image on the target in the TV camera;
• terminating said high energy exposure with AEC in response to a predetermined x-ray dose having been administered and then determining and storing the exposure time for the high energy mask image;
• after the high energy exposure is terminated reading out the TV camera target and storing the resulting high energy mask image;
• then in order to make a subsequent sequence of alternating low and high energy exposures wherein the exposures at one energy are terminated coincident with a blanking pulse that initiates the first available frame time for readout of said camera target and the exposures at the other energy are started coincident with the blanking pulse at the end of said readout frame, determining the time (T.sub.a) that elapsed between termination of the mask image exposure at said one energy and the next ensuing vertical blanking pulse demarking the end of the frame time in which said exposure terminated;
• initiating each of said subsequent exposures at said one energy after a delay of T.sub.a following occurrence of a vertical blanking pulse demarking the beginning of a frame time such that said exposures terminate coincident with a vertical blanking pulse ending a frame time;
• reading out the TV camera target during the interval between said last named blanking pulse and a following vertical blanking pulse;
• initiating said exposures at said other energy immediately after occurrence of said following blanking pulse and terminating the exposure at the same time that the corresponding mask image was terminated with AEC;
• and
• reading out the TV camera target beginning with the first blanking pulse following the frame in which the exposures at said other energy terminated.