The x-ray tube is the component of the radiographic system that produces the x-rays. It is made of Pyrex glass and encased in a sturdy, lead-lined, metal housing with large high-voltage electrical cables attached at each end. The x-ray tube’s primary components are the anode and the cathode (see Fig. 9-1). A tube stand or overhead tubecrane (OTC) suspension supports the x-ray tube and allows the radiographer to position it as needed over and around the patient.
Electrical energy is supplied to the x-ray tube through two large electrical cables. By design, x-ray tube tubes possess electrical polarity in that one side of the tube is positive (anode) and the other side is negative (cathode). This diode polarity is critical to its operation, as a large electrical potential is created between its poles during exposure. The potential difference, expressed as kilovoltage peak (kVp), causes the rapid acceleration of electrons to pass through the tube from cathode to anode, creating x-radiation. The tube converts this electrical energy into x-rays and heat in a manner similar to energy conversion in a light bulb.
The cathode filament is typically a tightly wound tungsten wire helix, similar to the filament in an incandescent light bulb. As electrical current passes through the filament wire, its temperature increases to the point of “boiling off” electrons through a process known as thermionic emission. At the precise moment of exposure, a large electrical potential is applied across the dipoles of the x-ray tube causing the liberated electrons to accelerate at tremendous speed from cathode to anode. As the electrons collide with the positively charged anode, the kinetic energy of the electron stream is converted into heat and x-radiation. In most modern-day x-ray tubes used for medical diagnosis, the anode assembly is a rotating disc of tungsten. Rotating anode tubes, as they are typically called, can withstand huge anode heat buildup and permit high milliamperage exposures with extremely short exposure times to minimize patient motion.
With an x-ray tube, the radiographer controls the number and energy of the x-rays produced by adjusting the amount of electrical energy going into the tube. This adjustment is made at the radiographic system’s control console.
Attached directly below the x-ray tube is an x-ray beam–limiting device called a collimator (Fig. 8-2). The collimator controls the size and shape of the x-ray field coming out of the x-ray tube. The radiographer determines the size of the x-ray field by adjusting two controls on the front or sides of the collimator, one for the length and one for the width, of the rectangular x-ray field.
Manipulation of the collimator blades is inherent in every radiographic procedure and becomes second nature. The process of “coning down,” as it is commonly called, implies increased collimation and a smaller x-ray field size. In addition, a retractable tape measure for measuring the source-image distance (SID) is conveniently built into the collimator cover, as is a tubehead angulation scale (goniometer).
Engineered into the collimator assembly is a high-intensity light bulb with a mirror. Turning the light field on involves a simple push of a button, which creates a projected light field onto the patient. The illuminated area is representative of the x-ray field exposure area. Keep in mind that the light bulb is high intensity and can become very hot to the touch. Manufacturers have protected the bulbs in heat shrouds, and the light turns off after a preset time interval. Newer collimator designs are now using light emitting diode (LED) light sources, which last much longer and do not get hot. Equipment designers have placed a Plexiglas “shadow shield” on the underside of the collimator to project a shadow of crosshairs marking the beam’s central point. In some cases, additional shadow lines indicating the position of exposure detectors, known as automatic exposure control (AEC), will also have been added.
Most x-ray machines are equipped with an automatic collimation system that performs positive beam limitation (PBL). This feature allows the x-ray unit to detect the size of the image receptor the radiographer is using and automatically collimates to a size not larger than the image receptor. Federal law no longer requires a PBL system, but permitting the x-ray beam to expose patient tissue larger than the image receptor is poor professional practice and an ethical violation of practice standards. In systems with flat panel DR detectors, oftentimes the collimation assembly is manually controlled without PBL. A good radiographer is diligent in the use of proper collimation and respected by colleagues.
The radiographic/x-ray table is the most obvious and recognizable component of the radiographic system. The size, shape, and location of the table controls vary among manufacturers. X-ray tables are classified as tilting or nontilting, having a free-floating or stationary tabletop, and having adjustable or non-adjustable height. Adjustable-height tables permit lowering of the tabletop to a comfortable and safe height for patients to get on and off the table (Fig. 8-3). To permit an x-ray exposure, typically the table must be raised to a working height. Care must be taken when lowering tables to ensure that nothing is under the top as it lowers. Many a technologist has caused tabletop damage through inattentiveness, costing the employer thousands of dollars in repairs. In addition, care must be taken when cleaning the tabletops, and only approved nonabrasive cleansers should be used.
Tilting tables are available in basically two types. A 90-90 table can tilt from the horizontal position to a complete vertical position in either direction. A 90-45 table can tilt to a complete vertical position in one direction and to a 45-degree tilt in the other direction. A footboard is usually attached to a tilting tabletop for patient safety. A tabletop in the upright position with a footboard attached is shown in Fig. 8-4. Table tilt is controlled by a switch located at table or floor level on the long side of the table. Depressing the switch tilts the table to a desired degree. Both tilting and nontilting tables may have additional features, including a moving tabletop, a receptor or cassette tray (often called a Bucky tray), and controls for tabletop movement and tilt (Fig. 8-5).
X-ray tabletops are radiolucent and movable in any combination of two directions. These movable tabletops may be motorized or free floating and permit the radiographer to move the tabletop rather than the patient. Floating tabletops have a switch that controls an electrical locking mechanism. If the locking mechanism is deactivated, then the radiographer can move the top manually in any combination of motions including diagonal or circular. Motorized tops have switches that drive the tabletop in the direction desired. The controls for the tabletop or table tilt are usually located at the end or center of the working side of the x-ray table. The Bucky mechanism is located directly beneath the tabletop and is designed to hold the image receptor stationary during the x-ray exposure and to keep it centered to the x-ray beam’s central ray. It also serves as the holder for the radiographic grid, which reciprocates back and forth during an exposure and is positioned underneath the tabletop, creating a top to the Bucky tray when it is pushed completely into the table (Fig. 8-6). The tray is pulled out from the table, and the cassette is placed in the tray and locked into place. The Bucky mechanism can be manually moved the entire length of the table and then locked into place. Newer digital detector systems have replaced the cassette tray, and some designs even permit cross-table lateral imaging by swinging the detector up into a vertical position, adjacent to the lateral edge of the table (Fig. 8-7). A beginning radiographer should take time to practice operating the various x-ray tables and Bucky trays before attempting to position any patients.
The x-ray generator is the workhorse of the total system. It consists of two main components: an electronics cabinet and operator console. The power of an x-ray system is rated in kilowatts and is expressed numerically, typically ranging from 30 kW to 100 kW. To the radiographer, the console is the electrical interface between the operator and the equipment. Generator console designs vary, but all have some key features (Fig. 8-8). All systems will allow the radiographer to turn the system on and off, select the x-ray exposure factors, initiate and terminate the exposure, and provide an audible and visual indication of x-ray exposure.
The console has five generic controls that the new student must learn: the main power, kilovolt peak (kVp), milliamperage (mA), timer, and rotor-exposure switch. The selection of kVp, mA, and time is collectively referred to as technique selection. On many units, mA and time are combined into a factor known as milliampere-seconds (mAs), which is simply mA multiplied by time in seconds (S). Additional controls typically include selections for anatomic body parts, AEC, image density, and x-ray tube focal spot size.
The main power switch supplies power to the radiographic system. Turning the power on does not activate x-ray production. The power device should be clearly marked on the control console and is usually either a switch or a push-button device. Some x-ray units require power to be activated both at the control console and at a main power box equipped with a high-voltage circuit breaker.
The penetration power of an x-ray beam is determined by its voltage and is expressed as kVp. As kVp increases, so does penetration; 1000 volts (V) of electrical power equal 1 kV, and the kilovoltages in radiography can range from 30 to 150 kVp. The kVp is user selectable and digitally displayed on the control console (Fig. 8-9). The correct kVp can vary based on patient thickness, body part, and examination type, and increasing or decreasing the kVp can be as simple as holding the kVp selection up or down or entering a numeric value on a keypad. The correct combination of mAs and kVp ensures the lowest patient dose and highest image quality, particularly with digital image detectors.
One milliampere is equal to one thousandth of an ampere (A). The milliamperage indicates the amount of current supplied to the x-ray tube. With most pieces of radiographic equipment, the radiographer can select the mA setting and the exposure time separately or can select the total mAs. On control consoles the mA setting generally ranges from 50 to 1000 mA and is selectable in increments of 100 (i.e., 200, 300, 400 mA) up to the maximum value. Most routine diagnostic radiography is done at 50 to 400 mA.
Just as with a digital camera, picture quality is determined by the amount of time for which the camera allows light into the detector. This is often controlled electronically, and the user gives it no thought when taking photographs. Similarly, radiography requires an x-ray exposure time that will be automatically terminated. Time is expressed in seconds and commonly in fractions of seconds or milliseconds (1/1000 seconds). The timing of an exposure is initiated by the operator, and the duration of the exposure is controlled by a sophisticated electronic timing circuit. Exposure technique selection may be done in the mAs mode or time mode. When the mAs mode is selected, the generator is programmed to use the highest allowable milliamperage to achieve the shortest exposure time; in select cases the time mode is used for breathing studies.
When the mA is multiplied by the total exposure time(s), the total quantity of x-ray exposure or mAs is the result. This mAs value directly affects the amount of x-ray exposure of the patient and ultimately the image receptor. Most generators will calculate mAs and display it. The radiographer still must understand the mAs calculation and what it means in terms of patient exposure and image quality.
After the x-ray exposure factors have been selected and the patient, x-ray tube, and other equipment for the examination have been properly prepared, an exposure can be made. The device that begins the exposure is called the rotor-exposure switch. It may be a push button or trigger-type switch as part of the control console. The rotor-exposure switch actually contains two different switches that are mechanically interlocked so that one must be activated before the other. The first switch that is activated is the rotor, or prep, switch. The rotor switch causes the anode to rotate and prepares the x-ray tube for the exposure factors that have been selected. The preparation process usually takes 1 to 2 seconds. After the tube has been properly prepared, the second switch is activated to begin the exposure. A sophisticated, electronic timer automatically ends the exposure. According to federal regulations, the termination of the exposure must be indicated both audibly and visually.
The duration of x-ray exposures may be established manually or automatically. Manual exposures are typically used with tabletop procedures such as extremity examinations, cross-table radiography, examination of patients on carts or in wheelchairs, and examination of small infants. The length of the exposure is a function of the total mAs chosen and the mA selected. Generally, the shortest exposure time possible is used to minimize motion on images. For many procedures, exposures are terminated using AEC circuitry. The technology of AEC has simplified exposure technique selection and improves the consistency of image quality. Using AEC requires that the selected anatomy be correctly positioned over the AEC detectors to achieve optimum image quality at the lowest patient dose. Most AEC systems use AEC detectors, with each detector activated separately or in any combination (Fig. 8-10). The radiographer will position patients properly using AEC technology and select the correct combination of detectors for a given examination. As more generators are integrated with DR technology, exposure technique selection is now done by way of a computer interface through the DR software. The basic principles of technique calculation and selection still apply, however. mAs controls total x-ray beam quantity, and kVp controls x-ray beam quality.
It is important to understand what is occurring when the x-ray tube is “prepped.” The anode begins to spin at a high rate of speed, and the cathode filament heats up to its selected mA. During the second step of the exposure sequence, a large potential difference (kVp) is applied across both electrical poles of the x-ray tube, causing electrons to travel from the cathode to the anode at high speed, producing x-radiation. Repeatedly prepping the tube unnecessarily can damage the x-ray tube and shorten its useful life. The radiographer clearly needs to understand proper exposure control operations and use extended “anode prep” times only when clinically appropriate. Much like a car has its own normal sounds of operation, x-ray units do also, and it is important to pay attention to unusual sounds and report them to appropriate personnel.