Optimization of image acquisition techniques for dual‐energy imaging of the chest

Abstract

Experimental and theoretical studies were conducted to determine optimal acquisition techniques for a prototype dual-energy (DE) chest imaging system. Technique factors investigated included the selection of added x-ray filtration, kVp pair, and the allocation of dose between low- and high-energy projections, with total dose equal to or less than that of a conventional chest radiograph. Optima were computed to maximize lung nodule detectability as characterized by the signal-difference-to-noise ratio (SDNR) in DE chest images. Optimal beam filtration was determined by cascaded systems analysis of DE image SDNR for filter selections across the periodic table (Z(filter) = 1-92), demonstrating the importance of differential filtration between low- and high-kVp projections and suggesting optimal high-kVp filters in the range Z(filter) = 25-50. For example, added filtration of approximately 2.1 mm Cu, approximately 1.2 mm Zr, approximately 0.7 mm Mo, and approximately 0.6 mm Ag to the high-kVp beam provided optimal (and nearly equivalent) soft-tissue SDNR. Optimal kVp pair and dose allocation were investigated using a chest phantom presenting simulated lung nodules and ribs for thin, average, and thick body habitus. Low- and high-energy techniques ranged from 60-90 kVp and 120-150 kVp, respectively, with peak soft-tissue SDNR achieved at [60/120] kVp for all patient thicknesses and all levels of imaging dose. A strong dependence on the kVp of the low-energy projection was observed. Optimal allocation of dose between low- and high-energy projections was such that approximately 30% of the total dose was delivered by the low-kVp projection, exhibiting a fairly weak dependence on kVp pair and dose. The results have guided the implementation of a prototype DE imaging system for imaging trials in early-stage lung nodule detection and diagnosis.

Diagnostic Performance of a Prototype Dual-Energy Chest Imaging System

“…The system was modified for the purposes of DE imaging to include: 1) a flat-panel detector (Trixell Pixium-4600, Moirans, France) with a ~250 mg/cm 2 CsI:Tl scintillator and 143 µm pixel pitch, 2) a fingertip pulse oximeter to trigger x-ray exposure during diastole for reduction of cardiac motion artifacts, (44) and 3) a multi-position filter wheel within the collimator to change the added filtration between the low-kVp (2.5 mm Al) and high-kVp (2 mm Al + 0.6 mm Ag) exposures, as guided by previous studies of DE image quality performed in our laboratory. (44)(45)(46) The X-ray tube produces a polychromatic beam with spectrum of beam energies distributed around the pre set target tube potential. Therefore, a high-energy filter was selected to "harden" the high energy beam, to reduce spectral overlap between the low-and high-energy projections.…”

“…This was the subject of considerable investigation in our laboratory (45,46,47). Differential filtration between low-and high-kVp projections was selected and an extensive analysis of contrast, Noise Equivalent Quanta (NEQ) and nodule detectability for low-and high -kVp added filtration was performed.…”

“…Optimal acquisition techniques including kVp pair and dose allocation (Aέ) were identified in previous work by experimental measurements in phantoms (46) and theoretical analysis. (48) A chest phantom with simulated lung nodules was used.…”

Dual-Energy CT with Fast-kVp Switching and Its Applications in Orthopedics

“…Multiple studies [17][18][19] have suggested the CNR is maximized with ~30% flux allocation, defined as the percentage of entrance skin exposure of the low-kVp to that of the low-and highkVp combined. That is, roughly speaking, the exposure ratio between the low-and high-kVp is …”

Multiscale deformable registration for dual‐energy x‐ray imaging

“…The monitor recorded pulse oximetry and x-ray trigger signals to determine whether a given x-ray exposure was delivered synchronous to diastole or systole. Previous studies identified the optimal imaging techniques: 29 an LE beam at 60 kVp and an HE beam at 120 kVp ͑0.6 mm Ag+ 2 mm Al added filtration͒, with the radiation dose allocated such that approximately 1 / 3 of the total energy was imparted by the LE beam, and with total dose equivalent to that of a conventional PA chest radiograph ͑e.g., 0.11 mGy for average chest thickness͒.…”