Dual Energy Radiography

different attenuation between tissue and bone at different XR energies

intensity following log transformation of low energy = Ilo = μtloxt + μbloxb; μ = linear attenuation coefficient of tissue/bone; x = tissue/bone thickness

total intensity adding high and low energy images = I = kloIlo + khiIhi

weighting factors klo and khi multiplied to each image to cancel tissue or bone (ie set x=0), so for tissue cancellation

does not work perfectly due since spectra not mono-energetic

dual energy or spectral CT – multiple separations in the detected photon energies

Dual-Energy X-Ray Absorptiometry (DEXA)

originates from dual photon absorptiometry in nuclear medicine using 153Gd that emitted 44 and 100keV γ rays to determine bone mineral density

quantitative computer tomography (QCT) is less accurate and has higher dose than DEXA

kilovoltage and filtration switched (eg 70kVp/4mmAl to 140kVp/3mmCu) or just filters switched (eg 80kVp no filter and cerium/samarium filter) rapidly during image acquisition

XRT and scintillation detector (CdWO4 or NaI:Tl coupled to PMT) mounted on C-arm with pencil beam scanning (to reduce scatter) in rectilinear fashion and filter assembly used to switch filters; scan time 2-5min

second generation DEXA uses fan beam and detector array and rotating C-arm around patient to form a CT image and body composition parameters can be derived (bone mineral density and soft tissue composition)

Hybrid DSA

two mask and live images taken at diff kVp to generate tissue-suppressed images so live mages show just bone and opacified blood

feasible only when high and low kVp pairs (135 and 85kVp) are acquired almost simultaneously (within 16.7ms)

filters (brass for high kV, erbium for low kV) mounted onto wheel mechanism which spins synchronously

images have SNR half of conventional DSA due to additional manipulation steps, increasing dose and XRT loading