An External Lead Shutter System for Supervoltage Roentgenography

Abstract

Medical radiography in the supervoltage range has recently become an area of clinical and investigative interest for specialized diagnostic procedures. These include exploration of soft-tissue densities in the head, neck, and thorax, and portal verification in supervoltage therapy (1–5). In our Radiation Clinic, we employ a resonance transformer generator operating at 1,000 kv., with 1 mm. lead filter, h.v.l. 3.9 mm. Pb, TSD 70 cm., 3 ma, and an output of 60 r/min. A posteroanterior radiograph of the chest on industrial type C film requires a dose of 1.5 r, measured at the back of the patient, and an exposure time of three seconds. Conventional timers on therapy machines are regarded as unsatisfactory because they permit radiation exposure to the patient during the “build-up” time of the generator. Tuddenham, Hale, and Pendergrass (5) have suggested the use of an external lead shutter mechanism activated by a conventional impulse timer and out of direct contact with the generator. We have designed and constructed such a lead shutter system for use with our 1,000-kv. therapy machine. The use of this device reduces the dose to the patient from 1.5 r to 1.1 r, or 26.6 per cent. It may be easily constructed and adapted for use with other supervoltage or orthovoltage x-ray therapy equipment. In the horizontal position most x-ray heads, like ours, are perfectly counterbalanced. Any attempt to install a shutter system on the face of the port would cause an imbalance requiring considerable engineering for correction. Furthermore, repeated impact from constant use might injure the tube. For these reasons, a separate shutter mechanism had to be constructed out of direct contact with the generator. In Figure 1 are shown the relative positions of the shutter and the x-ray machine. Figure 2 is a schematic drawing of the system. Basically the shutter operates as follows: the lead shutter, weighing 25 lbs. (6 × 6 × 2 inches), is raised manually by a crank located on the top of the frame and connected to the shutter by wire supports (Fig. 2). An A.C./D.C. converter switch is thrown, activating a brake which holds the entire shutter mechanism at whatever height is selected. When peak x-ray voltage is reached, a push-button switch outside the therapy room releases the brake, allowing the shutter to fall freely by gravity until it makes contact with an actuator located near the bottom of the frame. The actuator makes contact with a microswitch, which again energizes the brake, stopping the fall of the lead shutter. In the event of a power failure, a spring system absorbs the fall of the shutter. Figure 3 illustrates the electrical construction. The cost of the parts is modest, and a skilled technician can readily assemble and wire the unit. Summary A simplified external lead shutter system for supervoltage roentgenography is described.