Magnification error of femoral geometry using fan beam densitometers

Abstract

Hip axis length (HAL) has been reported as an independent risk factor for hip fracture. DEXA machines using fan beam techniques have become increasingly available. Errors in calculated hip axis length may be expected because of different degrees of magnification by the fan beam. The magnitude of this error on measurement of hip geometry was studied, using an anthropomorphic femur phantom with both fan beam (Lunar Expert and Hologic QDR-2000) and pencil beam (Lunar DPXL) densitometers. The clinical relevance of these findings was also examined using patient measurements of buttock soft tissue thickness. Femoral neck axis length (FNAL), which correlates well with HAL, was used as a measurement of hip geometry. There was a linear increase or decrease of FNAL with increasing distance of the phantom above the scanning table, when measured with the Lunar Expert or Hologic QDR-2000, respectively. There was no significant difference in FNAL at different heights using the pencil beam densitometer. The maximal difference in buttock soft tissue thickness in 30 women studied was 8.7 cm. From the equations, derived from the phantom studies, this difference would result in an 8.2% (1.4 SD) increase, or an 11.4% (1.9 SD) decrease in FNAL in the largest woman as compared with the smallest woman when measured using the Lunar Expert or Hologic QDR-2000, respectively. We conclude that there may be unpredictable degrees of magnification of FNAL in vivo, caused by differences in buttock thickness, when measured using a fan beam densitometers. Until these problems are resolved. FNAL, or related parameters of femoral geometry, should be measured using pencil beam instruments.

Bone densitometry: Clinical considerations

“…but precision error increases and the radiation dose to the patient doubles compared with the use of pencil-beam instruments (49)(50)(51)(52)(53). A change in the distance of the bone from the scanning table (as a result of different buttock soft tissue thickness, for example), can cause an increase or decrease in the values obtained, as a result of a different degree of magnification by the fan beam (53).…”

Axial and Total-Body Bone Densitometry Using a Narrow-Angle Fan-Beam

“…Fan-beam systems have substantial magnification which has direct effects on estimated BMC and area (about 3% per cm) and may even have some residual effect on BMD [9,[22][23][24]. This also would be the case for the PRODIGY without software corrections for the proper object plane.…”

“…In the case of osteoporosis, a direct measurement of area may prove useful since patients with small bone area at any BMD level preferentially fracture. In addition accurate measurement of hip axis length requires correction for magnification [23,24].…”

Assessment of bone mass following renal transplantation in children

“…The earlier DXA scanners used a pencil beam design that employed a well-collimated pencil beam X-ray coupled to a single detector that moved in a raster pattern across the patient, thus obtaining a true projection of the region scanned without significant magnification of the scanned bones [20,21]. More recently, pencil bean scanners have been replaced by a multiple detector array design coupled to a fan-shaped X-ray beam that shortens scanning time.…”

“…More recently, pencil bean scanners have been replaced by a multiple detector array design coupled to a fan-shaped X-ray beam that shortens scanning time. However, the fan beam technique causes an inherent magnification of scanned structures as the distance from the X-ray source, and hence from the apex of the fan beam, decreases [20,21]. For example, in the case of Hologic scanners, the apex of the fan beam is under the patient.…”