Abstract
By a directed chest exposure is meant a roentgen exposure of the chest at any phase of the cardiac cycle. This is accomplished by the synchronization of the exposure with a definite phase of the circulatory phenomena, heart sounds, pulse wave or electrocardiographic impulse.
McPhedran and Weyl 1 found the use of the heart sounds unsatisfactory for this purpose because of interference of the extraneous sounds, but utilized the pulse wave with success. A cannula is applied over the carotid artery and the pulse thrust is transmitted to a tambour. A light beam reflected from a mirror on the tambour enters a photoelectric cell at the appropriate point in the cardiac cycle. The photoelectric current is amplified and actuates the X-ray switch after an appropriate delay. The sudden outward thrust of the carotid pulse occurs simultaneously with systole (mechanically), and in order to obtain an exposure at the end of diastole, a delay of slightly shorter duration than the cardiac cycle is necessary. The method is, therefore, not feasible when there is an irregularity of rhythm.
Meek and Eyster 2 used the electrocardiograph as a control. They closed the X-ray switch manually by estimating the appropriate point from the pulse wave and checked the result by noting the disturbance produced by X-ray exposure in simultaneously recorded cardiogram. Thus it was possible to state at what point in the cycle any particular exposure had been taken, but this method of timing cannot be very accurate and there must of necessity be many exposures which are not properly timed.
Our method makes use of the cardiac action current. This method was considered by McPhedran and Weyl and discarded by them as being too cumbersome. As developed by us, we believe it to be simpler and more effective than any method based upon the pulse
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