Skeletal measurements using a flying spot digital imaging device

A flying spot digital imaging unit was devised to measure skeletal length and angles. This device uses a spinning chopper wheel and fixed slit collimator in front of a conventional X-ray tube to produce a scanning pencil beam that passes through the patient and onto an electronic detector. The beam scans the patient transversely and the device moves longitudinally, creating a digital image with a skin dose of less than 2 mrad (0.02 mGy). Patients can be imaged both when upright and when recumbent. A large field of view makes it possible for the entire length of the spine or lower extremities to be included in a single image. Built-in computer algorithms are used to make measurements of skeletal lengths and angles. The accuracy of the digital device was tested by making measurements of known angles and lengths using protractors and rulers. Skeletal measurements were then made on a number of patients. Interobserver error was tested using both patient and nonpatient images. The following patient measurements were made: Cobb angle in 166 patients with scoliosis; knee joint angles in 120 patients undergoing knee surgery; lower extremity lengths in 78 patients with length inequalities; bony fragment angulation and displacement in 30 patients with healing fractures. These studies have shown the device to be accurate with low interobserver error, while delivering much lower patient dose than more conventional methods. The ability to manipulate window levels and widths enables visualization of thick and thin body parts on the same image. Our experience both in the laboratory and with patients has been encouraging, and the method appears to have several advantages over more conventional techniques.