Intercalative Chest Roentgenography

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Abstract
As radiologists, we frequently make detailed analysis of the size and contour of the cardiovascular silhouette and pulmonary vessels from a single chest film. Yet it seems paradoxical that in the very patient submitted to fluoroscopy for abnormal or unusual pulsations, a film made at random in the cardiac cycle is employed to record the findings, and, similarly, randomly exposed films are utilized to evaluate the course of disease and response to therapy. The purpose of this paper is to describe a technic of precision timing of roentgenograms with the aid of electrocardiography to determine, in a group of normal subjects and patients with heart disease, whether the phase of the cardiac cycle alters the appearance of the heart, great vessels, and pulmonary vasculature. Materials and methods The circuitry of an existing x-ray system was modified to accept a cardiac programmer2 and an electrocardiograph. The simplified wiring diagram (Fig. 1) shows how the three basic units of x-ray, programmer, and electrocardiographic circuits are linked. The electrocardiographic leads from the patient are fed into the programmer which relays the impulses to the electrocardiograph. When the trigger button of the programmer is pressed, the unit selects the first R impulse which energizes the coil (R-1). As this event takes place, there is simultaneous completion and breaking of the x-ray circuit for the duration of the exposure via R-1a. Through R-1b, the exact time and duration of the x-ray exposure are recorded on the electrocardiogram. After the systolic film is obtained, the cassettes are rapidly changed by remote control, with a stereo changer (the tube motion having been disengaged). This is followed immediately by the diastolic film. The use of a radioelectrocardiograph3 makes positioning of the patient considerably easier and reduces interference with the tracing. The relationships of the different events in the cardiac cycle are well known (1). If we assume a relatively normal heart rate, the electrical impulse of ventricular contraction, the R wave, precedes mechanical systole by 150 to 350 milliseconds and precedes mechanical diastole by 600 to 1,100 milliseconds (Fig. 2). Correlating with the pressure changes in the heart and great vessels, the peak mechanical ventricular systole and diastole usually occur at the beginning of the T-wave and about the p-wave, respectively (2). It is at these points that the systolic and diastolic timing of chest films are chosen (Fig. 3). The initial step is to take a preliminary electrocardiographic tracing, usually lead 2. From this, one determines the proper time delays in milliseconds from the R-wave to apply to the cardiac programmer so that the x-ray exposures correspond to the peak mechanical ventricular systole and diastole. It is necessary to subtract the inherent circuitry delay of the x-ray generator, which in our case is 70 milliseconds.

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