The scanogram is a specialized, low-dose X-ray procedure designed to capture precise, full-length images of large body parts, such as the spine or lower limbs. Unlike a standard X-ray that captures only a small, localized area, the scanogram provides a comprehensive view of the body segment from one end to the other. This technique is engineered to assess skeletal length and alignment over a long distance. This broad perspective is crucial for physicians who need to calculate minute differences in bone length or evaluate the overall mechanical axis of the body.
How the Scanogram Image is Created
A scanogram image is created by moving the X-ray tube along the patient’s body, taking multiple sequential exposures in a continuous line. These individual images are then processed digitally and “stitched” together using specialized software to form one seamless, extended picture of the entire limb or spine.
This stitching process eliminates the magnification and distortion that would occur if a regular X-ray beam were used to try and capture the entire structure at once. To ensure the final measurements are accurate and properly calibrated, a radiopaque ruler or scale is often placed next to the patient during the exposure. This calibrated object allows the software to correct for minor discrepancies and produce a true-to-size representation of the bones. Modern digital radiography systems, sometimes referred to as slot-scanning digital radiography, perform this process very quickly, capturing the entire length in a single sweep while the patient remains still.
Key Diagnostic Uses
The primary function of a scanogram is to provide highly precise measurements, making it the gold standard for several orthopedic assessments. One of its most frequent applications is the evaluation of leg length discrepancy (LLD). The scanogram allows physicians to measure the exact length of the femur (thigh bone) and the tibia (shin bone) in both legs to determine the precise location and extent of the difference.
This degree of accuracy, often within a fraction of a millimeter, is necessary for planning corrective treatments like shoe lifts or surgical procedures such as limb lengthening or growth plate arrest. The imaging is also used to evaluate structural deformities and alignment issues in the lower limbs, such as genu varum (bowlegs) or genu valgum (knock-knees). By viewing the full mechanical axis from the hip to the ankle, the scanogram helps in assessing joint alignment and planning osteotomies, which are surgical bone cuts to correct angular deformities.
Scanograms are also widely utilized for assessing the overall alignment of the spine, particularly in conditions that affect spinal curvature. A full-spine scanogram is routinely ordered for the diagnosis and ongoing monitoring of scoliosis, which is an abnormal sideways curvature of the spine. Because the entire spine is captured in a single, unmagnified image, the angle of the curvature can be measured accurately using techniques like the Cobb method. This full-length view is also used to evaluate kyphosis (excessive outward curvature of the upper back) and to monitor the progression of these conditions.
Preparing for the Procedure
Preparation for a scanogram is generally straightforward, focusing on ensuring a clear path for the X-ray beam and optimal patient positioning. Patients are typically asked to wear comfortable, loose-fitting clothing without metal components like zippers or snaps, or they may be given a gown to wear. All external metal objects, including jewelry, belts, and body piercings, must be removed from the area being scanned as they can interfere with the image quality and distort the measurements.
The procedure itself requires the patient to remain very still for a short duration, usually only a few minutes, either standing or lying down, depending on the area being imaged. For leg scanograms, patients may be asked to stand with their feet a specific distance apart and their knees straight to simulate natural weight-bearing conditions. The radiation exposure from a scanogram is considered low-dose, comparable to that of a standard X-ray. Once the imaging is complete, the measurements are analyzed by a radiologist. The detailed report is then sent to the ordering physician, often an orthopedist, to guide diagnosis or treatment planning.