Metallic objects severely limit diagnostic CT imaging because of their high X-ray attenuation in the diagnostic energy
range. In contrast, radiation therapy linear accelerators now offer CT imaging with X-ray energies in the megavolt range,
where the attenuation coefficients of metals are significantly lower. We hypothesized that Mega electron-Voltage Cone-Beam CT (MVCT) implemented on a radiation therapy linear accelerator can detect and quantify small features in the
vicinity of metallic implants with accuracy comparable to clinical Kilo electron-Voltage CT (KVCT) for imaging. Our
test application was detection of osteolytic lesions formed near the metallic stem of a hip prosthesis, a condition of
severe concern in hip replacement surgery.
Both MVCT and KVCT were used to image a phantom containing simulated osteolytic bone lesions centered around a
Chrome-Cobalt hip prosthesis stem with hemispherical lesions with sizes and densities ranging from 0.5 to 4 mm radius
and 0 to 500 mg•cm<sup>-3</sup>, respectively. Images for both modalities were visually graded to establish lower limits of lesion
visibility as a function of their size. Lesion volumes and mean density were determined and compared to reference
Volume determination errors were reduced from 34%, on KVCT, to 20% for all lesions on MVCT, and density
determination errors were reduced from 71% on KVCT to 10% on MVCT.
Localization and quantification of lesions was improved with MVCT imaging. MVCT offers a viable alternative to
clinical CT in cases where accurate 3D imaging of small features near metallic hardware is critical. These results need to
be extended to other metallic objects of different composition and geometry.