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BLACK HOLE ARTIFACTS – A New Potential Pitfall for DXA Interpretation (The Hardest Thing to See is What You Can’t See)

S. L. Morgan MD, RD, CCD, R. Lopez-Ben, MD, CCD, M. Yester Ph.D., N. Nunnally RT (R), CDT, L .Burroughs RT (R), CDT

The University of Alabama at Birmingham, Departments of Nutrition Sciences and Medicine and Radiology,

The UAB Osteoporosis Prevention and Treatment ClinicInternal metallic objects, like orthopedic hardware appear white on properly exposed radiographs or in dual energy x-ray absorptiometry (DXA) scans, due to the increased penetration of the x-ray beam. Air within the lungs is radiolucent and black on images. We noted a lead bullet within a vertebral body, where the projectile scanned completely black (Figure 1).

We placed lead bullets and vascular hemoclips (Weck) made out of tantalum over the spine phantom and scanned them in array mode on a Hologic Discovery W scanner and scanned them in the array mode using 12.1:7 software. (Figures 4 and 5A and 5B)

•Approval was obtained from the UAB Institutional Review Board to publish clinical radiographs. •We stacked ½” x ½” copper squares on top of a Hologic spine phantom and scanned them in the array mode using 12.1:7 software on a Hologic Discovery W scanner. •When the pile reached a thickness of 10 mm, the opaque/white DXA image began to turn black and when the thickness was 13 mm the image was almost completely black. (Figures 3A – 3D).

•When scanning a dense object by DXA, the dual energy subtraction may be zero at a certain density threshold and the object will appear black on the DXA image. The image is visible on a single energy scan. •Unless this phenomenon is understood, small “black hole” artifacts, such as vascular clips may be overlooked or misinterpreted as bowel gas on PA DXA scans. It is unclear how this artifact affects the results of the DXA scan.•We plan future studies to catalog various types of clips and determine whether they appear “black” or “white” on the DXA scan. •We also plan to evaluate whether such “black hole” artifacts have a significant effect on BMD.

•The density of lead is 15.7 gm/cm3 and the Z is 82. •The density of tantalum is 16.6 gm/cm3 and Z = 73.•The usual orthopedic hardware consists of cobalt-chromium alloys which have lower atomic numbers. •Tantalum, like lead, exhibits a high degree of attenuation at both energies and thus the image ends up as “black” on DXA.

In addition, Weck tantalum vascular clips from an aorto-bifemoral bypass repair appeared radio opaque on radiographs and the single energy vertebral fracture analysis (VFA), while they scanned black on DXA images. (Figure 2A and 2B)

BACKGROUND MATERIALS AND METHODSDISCUSSIONDISCUSSION

CONCLUSIONSFigure 1Figure 1

Lead bullet in L2Lead bullet in L2

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Figure 2A- PA and lateral VFA showing vascular clips anterior to L2 and L3).

Figure 2B – DXA from patient in 2A. Notice the black areas overlying L2 and L3 (arrow).

Figure 3A – Scan of spine phantom with one ½” x ½” copper square on top of L3.

Figure 3B – Phantom with a 10 mm stack of ½” copper squares; the image is still white.

Figure 3C – Phantom with a 11.5 mm stack of ½” copper squares. Note that the image is becoming black in the center (arrow).

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Figure 3D –Phantom with a 13 mm stack of ½” copper squares. Note that the image has now become mostly black.

Figure 4 – Spine phantom with a .45 caliber lead bullet overlying L3 – the image of the bullet is black (arrow).

Figure 5A – Spine phantom with a small cassette of tantalum vascular clips overlying L3 (arrows). It is difficult to see the clip cassette.

Figure 5B – Spine phantom with a large cassette of tantalum vascular clips overlying L3 (arrows).↓↓ ↓↓ ↓↓ ↓↓

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