Muskoskeletal radiology Measurements
Knowledge of anatomic of distances and angles in the skeleton is necessary and most important in pediatric radiology and traumatology. Tables detailing the normal maturation of the skeleton should be made available in any department of radiology.
Although measuring distances and angles is a classic radiographic discipline, the precise examination technique required to obtain adequate measurements is less well known. When measuring the distance between points A and B (Fig. 8), as in determining the length of the lower extremities, two conditions must be fulfilled:
(1) both points should be in the same plane and equidistant from the film plane, and (2) the x-ray magnification factor must be known.
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Figure 6.
Arthrography of the right hip joint using 4 ml of water soluble contrast medium.
a) AP view
b) Lauenstein projection. Note the
extension of the joint capsule along the femoral neck. The joint space is delineated clearly (open arrow), as is the acetabular limbus (arrow).
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b
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Figure 7.
Double contrast CT arthrography of the patellofemoral joint using 50 cc of air and 1 ml of water soluble contrast medium. As compared to Figure 15 the border of the patellar cartilage is clearly defined (arrowheads). The retinaculum has torn after a lateral patellar dislocation (arrow).
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Figure 8.
Magnification of an object and measurement of distances on radiography.
The length of the object O is measured using two separate exposures (R-a and R-b) of the same film. Both exposures are per pendicular to O and to the film (orthoradiographic technique). The length of O is equal to the distance that the x-ray table was moved between the exposures R-a and R-b. At a single exposure from R1 a magnification of the object O will occur, a1 - b1. If the distance between the tube R1 to the object O and the distance between R1 and the film plane (F) is known, the magnification can be calculated.
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Figure 9.
Measurement of rotational displacement.
When two fragments (a and b) with defined surfaces are displaced around a vertical axis (o), the displacement can be measured by two exposures, each of which is tangential to the surfaces. The angle of the direction of beams between the two exposures represents the displacement expressed in degrees.
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Figure 10.
Three-dimensional biplanar measurements of the anteversion angle of the femoral neck .If a dry femur is placed on a plate and examined in a transaxial view from the femoral candyles to the head, the femoral neck will be noted to have an anterior angle in relation to the plane of the plate (the plane consisting of the dorsal aspect of the two femoral candyles). This angle is defined as the anteversion angle (OH). The angle is calculated from two pelpendicular views of the proximal end of the femur and one lateral view of the femoral candyles. By measuring the distances a and b, respectively, from the central axis of the femoral diaphysis (o) to the centre of the femoral head (h) in both projections, the anteversion angle åf is calculated or measured directly on a drawing.
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Such measurements are performed with accuracy using orthoradiographic technique (Fig. 8). A prerequisite for direct measurement of angles is that both arms of the angle be positioned in the same plane parallel to the film plane. This condition rarely is fulfilled in the assessment of displaced fractures of long bones, and in such cases the correct angle is measured by three-dimensional biplane technique. The size of the measured angles is independent of magnification. Rotational displacement around longitudinal axes (e.g., fractures of long bones) can be measured directly if two points of any anatomic surface are defined in each fragment (Fig. 9). If two such anatomic landmarks are not available in one of the fragments (e.g., in a fracture of the femoral shaft or in hip joint replacement), a biplane technique as demonstrated in Fig. 10 can be used.
ACT examination provides a series of images. Each image corresponds to a volume in a three-dimensional coordinate system in which each anatomic structure in the different slices can be defined by X, Y, Z coordinates. By simple geometric calculations angles and distances between the different landmarks can be measured with high accuracy (Fig. II).
Niels Egund, Kjell Jonsson, Holger Pettersson and Donald Resnick