1. Technical Field
This disclosure generally relates to radiographic imaging systems and methods, and more particularly to linkages used in such devices for holding support arms in desired positions.
2. Description of the Related Art
Various types of radiographic devices are generally known in the art. A known x-ray unit for podiatry is disclosed in U.S. Pat. No. 4,587,668, which is assigned to the same assignee as the present disclosure. Generally, such x-ray units include a platform upon which a patient places his or her feet to be x-rayed. The platform is raised above floor level to allow film cassettes to be positioned in a film well located below the platform. The platform may further include a slot for receiving a vertically oriented film cassette. A radiographic head is mounted on vertical mounting members, which serve to space the radiographic head a desired distance above the foot platform. The vertical mounting members are moveable in both the lateral and longitudinal directions so that x-rays of a patient's feet can be taken from many angles while easily maintaining the same source to image distance (SID).
Known radiographic devices typically use means, such as a spring assembly, to hold the mounting members and radiographic head in the desired position. The spring assembly is coupled to the mounting members by a linkage system. The '668 patent discloses a linkage system comprising a chain partially entrained about a rotatable hub. With the mounting members in a normal position, in which they are substantially vertically aligned, the spring system assumes a first or contracted position. When the mounting members are moved laterally, however, the spring assumes a compressed position to counteract the bending moment created by the weight of the radiographic head.
The aforementioned spring assembly and linkage adequately supported mounting members used in previously known radiographic devices, such as the device disclosed in the '668 patent. More recently, however, it has been proposed to lengthen the mounting members so that the radiographic devices can take a wider variety of images at a longer SID. More specifically, devices with shorter mounting members, such as approximately 28 inches or less, are typically useful for taking either foot or arm images, but not both. When the mounting members are lengthened to approximately 40 inches, however, the radiographic device may be easily adjusted to take images of both the foot and arm extremities. The increased length, however, creates a larger moment arm on which the weight of the radiographic head acts, thereby increasing the magnitude of the force that the spring assembly must counter to hold the mounting members in place. The previously known linkage assemblies do not adequately compensate for this increased force, and instead have demonstrated a tendency to at least partially return the mounting arms to the normal position when they are initially placed in the extreme lateral positions. Such “snap back” causes inaccuracies in the angle at which the radiographic head is placed and can require additional time for the radiograph technician to properly place the head.