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Method and Phantom Design for the Evaluation of Material Quantification Accuracy of Contrast-Enhanced Spectral Computed Tomography (CT) Systems

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This regulatory science tool is a method and phantom design for the evaluation of iodine-specific quantitative measurements, specifically the accurate estimation of iodine concentration within contrast-enhanced spectral CT.

 

Technical Description

The accuracy of material quantification in contrast-enhanced spectral computed tomography (CT) systems is crucial for diagnostic precision and patient safety. The importance of this accuracy stems from the system's ability to discern between different tissue types and contrast agents, impacting clinical decisions and treatment strategies. The proposed method and phantom design, specifically aimed at the evaluation of iodine-specific quantitative measurements address this need by ensuring accurate estimation of iodine concentration within these CT systems.

The phantom, made from the widely accessible Lucite (PMMA), includes varying concentrations of iodine targets mixed with water, simulating clinical ranges. PMMA's similar x-ray properties to human tissue and its ease of cutting allow for diverse shapes and sizes in phantom construction (see Appendix A). The phantom is designed to accommodate a range of iodine concentrations, allowing users to tailor the iodine targets to their specific clinical applications and diagnostic claims. While our methodology does not specify exact values, it provides a flexible protocol in Appendix A for users to derive and validate their own concentrations based on the intended use and existing clinical guidelines or literature.

The method presented is cost-effective and straightforward, making the phantom design both accessible and practical for widespread application. Additionally, the design thoughtfully incorporates the evaluation of performance degradation factors like beam hardening, pulse pileup, scatter, the detector's energy response, and material decomposition algorithms.

The quantification performance is evaluated by comparing the measurements obtained from phantom CT images against a predetermined reference truth. This comparison utilizes metrics of accuracy and precision, as detailed in Part II: Evaluation Method and Quantitative Metrics.

The methodology is explained comprehensively in the paper:

'Assessing Spectral Efficiency in Quantitative Contrast-Enhanced Breast CT Using a CdTe Photon-Counting Detector: A Simulation Approach.' by Bahaa Ghammraoui, MU Ghani, JL Manus, SJ Glick – Submitted to the journal of Biomedical Physics & Engineering Express (BPEX). [Under Review]

Intended Purpose

The phantom and associated methods are intended to evaluate the accuracy of iodine-specific quantitative measurements with spectral contrast-enhanced CT systems. The phantom is constructed from Lucite (PMMA) (a material that is easily accessible), incorporates varying concentrations of iodine embedded at different peripheral locations to simulate clinical ranges. Coupled with our test methods (Part II: Evaluation Method and Quantitative Metrics), this setup serves to assess the impacts of all potential degradation factors on material decomposition accuracy such as beam hardening, scatter, and the energy response of detectors. This comprehensive evaluation approach is cost-effective, quantitative, and objective, offering the user an efficient methodology to incorporate within their device characterization needs.

Testing

Testing was conducted on the proposed phantom design and evaluation method in two distinct studies. The first study utilized a simulation approach to evaluate the performance of a GaAs-based Photon-Counting Detector in contrast-enhanced breast CT for quantitative iodine measurements, comparing it to an ideal detector [1]. The second study was experimental and performed on a custom-built photon counting breast CT benchtop and a physical phantom. This study aimed to evaluate the impact of different detector gain calibration methods on system performance for quantitative iodine measurements [2].

Our results demonstrated that the phantom design and evaluation metrics proposed are sensitive to variations in the detection mechanism, gain, beam hardening correction methods, material decomposition methods, and other significant parameters affecting the imaging process.

Limitations

The proposed method and phantom design assume a uniform background phantom, which may not reflect real-world clinical scenarios in terms of anatomical background. In addition, the use of liquids (water) to simulate tissues with iodine presents challenges when using scanners with sensitive electronics. Therefore, meticulous care must be taken to seal the iodine inserts securely to prevent leaks that could potentially damage the scanner equipment.

It is pertinent to note that the employment of a solid iodinated phantom material can serve as an alternate and effectively eliminate the problem, as described by Hill et al. [3].

Supporting Documentation

  1. Assessing Spectral Efficiency in Quantitative Contrast-Enhanced Breast CT Using a CdTe Photon-Counting Detector: A simulation Approach. Bahaa Ghammraoui, MU Ghani, JL Manus, SJ Glick - Physics in Medicine and Biology [Under Review]
  2. Assessing Spectral Efficiency in Quantitative Contrast-Enhanced Breast CT Using a CdTe Photon-Counting Detector: An Experimental study – Proceeding – CERN meeting workshop, Geneva, May 2024
  3. Hill, M. L., Mainprize, J. G., Mawdsley, G. E., & Yaffe, M. J. (2009). A solid iodinated phantom material for use in tomographic x-ray imaging. Medical physics, 36(10), 4409–4420. https://doi.org/10.1118/1.3213516

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