X-ray-absorber step-wedge phantoms serve in projection radiography to assess a detection system's overall exposure-related signal-to-noise ratio performance and contrast response. Data derived from a phantom image, created by exposing a step-wedge onto the image receptor, are compared with predefined acceptance criteria during periodic image quality assurance (QA). For contrast-related measurements, in particular, the x-ray tube potential requires accurate setting and low ripple, since small deviations from the specified kVp, causing energy spectrum changes, lead to significant image signal variation at high contrast ratios. A K-edge-balanced, rare-earth-metal contrast phantom can generate signals that are significantly more robust to the spectral variability and instability of exposure equipment in the field. The image signals from a hafnium wedge, for example, are up to eight times less sensitive to spectral fluctuations than those of today’s copper phantoms for a 200:1 signal ratio. At 120 kVp (RQA 9), the hafnium phantom still preserves 70% of the subject contrast present at 75 kVp (RQA 5). A copper wedge preserves only 7% of its contrast over the same spectral range. Spectral simulations and measurements on prototype systems, as well as potential uses of this new class of phantoms (e.g., QA, single-shot exposure response characterization) are described.
The natural luminescence decay time of storage phosphors limits the scan speed of today's flying-spot scanners, since this 'afterglow' emission degrades the MTF along the fast-scan direction. Higher scan speeds can also decrease the total amount of signal captured at each point, that is, the read-out depth. One may reduce these problems by scanning entire lines at once, rather than points. However, this can create other problems related to stimulation and collection efficiency, system gain, sharpness, and noise. We have developed a new scanning engine containing a linear, laser-diode-based stimulation source and a light collection system with specially designed optics and multiple, linear, asymmetric CCDs. The stimulation/collection systems are housed in a compact, movable read head that scans quickly over a stationary image plate (IP). We evaluated this so-called 'ScanHead' with conventional powder (IPs) and with new CsBr:Eu<SUP>2+</SUP> needle-based IPs.We also compared it to a conventional flying-spot scanner. Image quality results differed depending on plate type. DQE values for conventional powder IPs in the prototype scanner were comparable to today's state-of-the-art commercial systems (DQE(0)~0.25-0-27). DQE values for needle IPs were significantly better than those for powder IPs (DQE(0) ~0.58-0-60) and comparable to those quoted for flat-panel DR systems based on CsI:Tl.