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What we term "portable" x-ray machines are technically "mobile" rather than "portable," since they are not usually equipped with handles for the x-ray technician to pick it up and carry from place to place. The very few true portable x-ray units in existence are low-power units designed for transportation by automobile or light truck to nursing homes and long-term care facilities. Many of these same units have been adapted for use in military field medicine. However, for the sake of common terminology, the large, wheeled, mobile x-ray units found in brick-and-mortar hospitals will be referred to as "portable" x-ray machines. Likewise, a smaller family of "portable" x-ray machines also exists. These machines are frequently collapsible and would be moved from place to place in an automobile, mini-van, or full-sized van. These units are intended for use in nursing homes, extended care facilities, and other temporary-use locations outside of the healthcare facility.
In either case, today's portable x-ray machine is literally a radiographie (rad) room on wheels. After looking at a modern rad room, one wonders how it is possible to compact all this to get through a hospital doorway and light enough to not overload an elevator or put into the trunk of a car. By making some compromises, eliminating some nonessential subsystems, and clever design (including borrowing a technique from avionics), the reader will see how this is achieved.
Technical Aspects of Design and Function
The biggest compromise made is that of film quality. No matter how the portable x-ray unit is powered, nothing but dedicated, hard-wired, three-phase electrical power can provide sufficient wattage to make a high-quality 175 kVp, 500 mA exposure. That requires 44 to 88 kVA, needing either a huge (about the size of your upper arm) extension cord, or a truckload of batteries. Neither one is feasible, so the first compromise made is in the kVp. The maximum is usually 125, with only a handful of manufacturers making a unit on the 133 to 150 kVp range. Collapsible, convenience-outlet powered units are seldom capable of producing over 100 kVp. Tube current is equally decreased because of power constraints. Although today's portable x-ray unit will produce good quality films in some of the most impossible locations, they cannot compare to fixed...