Generalized evaluations for improved high-resolution region-of-interest x-ray imaging systems
Gupta, Sandesh Kumar
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The requirements of higher image quality and resolution place stringent constraints upon high-resolution region-of-interest (ROI) systems utilized in minimally invasive Endovascular Image guided Intervention procedures (EIGIs), especially at higher frequencies to see the very small (about 100 microns or less) and low contrast objects for accurate and successful treatment of neurovascular diseases. Among all the factors, geometric unsharpness due to the finite size of focal spot, and different types and thicknesses of CsI(Tl) imaging phosphor plays a vital role in determining the image quality and resolution for a ROI imaging system at higher frequencies. Moreover, ROI imaging requires the use of a small focal spot with sufficient output to maintain spatial and contrast resolution. Furthermore, the imaging system needs to be evaluated using the generalized linear system metrics which provides the total system performance including the intrinsic detector performance and the effects of focal spot, scatter, and geometric magnification for a simulated clinical environment. The effect of the Line-Focus-Principle on the focal spot frequency distribution for different positions at the detector plane was evaluated, and consequently, the effect of these on the total system performances of the MAF without scatter was evaluated. The standard pin-hole method using a 10 micron pin-hole was used to for the focal spot Point Spread Function (PSF) measurement. Moreover, the effect of different types (HR and HL) and thicknesses (300 and 500 microns) of CsI(Tl) phosphor on the performance of the MAF was evaluated. Furthermore, we investigated three methods to increase the x-ray output while maintaining the focal-spot size for a small field-of-view (FOV) as needed for EIGIs: 1) increase in tube output made possible by reducing the anode angle and lengthening the filament for the small focal spot to maintain a constant effective focal-spot size, 2) increase of the image quality and resolution for a medium focal spot using asymmetric collimation to shift the beam to the anode side so that we can utilize the Line-Focus-Principle, 3) increase of the image quality for the existing small focal spot using recursive filtering and increased frame rate acquisition. Finally, we evaluate the total system performance for the high-resolution ROI detector SSXII. Three object magnifications (1.08, 1.13, and 1.20), two scatter fractions (0.28 and 0.33) characteristic of a standard head phantom, and three focal spots (small, medium, and large of 0.3 mm, 0.5 mm, and 0.8 mm nominal size, respectively) were used for generalized evaluations comparison. The radially averaged focal spot MTFs (average of MTFs in all directions) are found to be improved as we move from the cathode to the anode side at the detector plane for each focal spot in the tube axis direction. The improvement is higher for the small focal spot. The performances (GMTF and GDQE) of the MAF also improved as we move from the cathode side to the anode side; however, they degrade for large focal spot size and higher object magnification. The 300 HR MAF performs significantly better than the 300 HL and 500 HR MAFs at higher frequency in terms of the MTF and DQE. The 500 HR MAF performs similarly in terms of MTF and little better in terms of DQE than the 300 HL MAF. The GDQE and GMTF for all the MAFs and SSXII are degraded compared to the DQEs and MTFs, especially at low frequency because of the scatter blur due to the scatter radiation from the uniform head phantom, and at high frequency due to the focal spot blur due to the finite focal spot size. The degradation at higher frequencies is substantial for larger focal spot and/or higher magnification. Despite the above degradations, the MAFs and SSXII perform significantly better than the commercial Flat Panel Detector (FPD) at higher frequencies. Considering both increased anode-target area and increased inherent anode filtration, a net output increase of 4.0 (5.0 what's this?) times could be achieved with a 2-degree anode angle without the added filtration of 1.8 mm Al (0.2 mm Cu) with head phantom in the beam compared to the standard 8-degree target with an increase of 4 times in filament length. Moreover, the GNEQ, hence the image quality, can be increased for the MAF in the case of the medium focal spot using a view at the anode side resulting in higher output. Furthermore, the GNEQ, hence the image quality, can also be increased by 5 times in the case of the existing small focal spot using increased acquisition frame rates of 15 f/s with either a running average of 5 consecutive frames or with recursive temporal filtering with filtering weight of 3. Taking the advantage of the Line-Focus-Principle, higher resolution and dose efficiency, performances can be achieved using the MAF at the anode side of the x-ray tube. Despite significant degradation due to scatter and focal spot blurring, all the MAFs and SSXII provide superior performance over the FPD at higher spatial frequencies. Both substantially higher resolution and improved GDQE can be achieved for the MAF using the 300 μm HR phosphor instead of the 300 μm HL and using the 500 μm HR phosphor. The tube output, hence the image quality in terms of the GNEQ while maintaining the spatial resolution could be achieved in the region of interventional interest without the limitations of the small focal spot with these methods. As a result of the increased image quality and resolution, a patient could be diagnosed and treated in one session, which ultimately may save an additional procedure as well as provide decreased recovery time. Moreover, the higher image quality can help endovascular device deployment and verification after deployment which in turn could increase the success probability of procedures including complex ones.