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Background
Photon-counting computed tomography (PCCT) is a promising new CT technology, but until recently has been experimental. Where available for clinical use, PCCT in abdominal imaging has shown immense potential and likely will ultimately improve the practiced of radiology. The aim of this exhibit is to demonstrate our initial experience with PCCT in abdominal imaging.

Educational Goals / Teaching Points
Conventional and dual energy computed tomography (CCT and DECT) utilize scintillation detectors (SD). SD convert incident x-ray photons into visible light via a scintillator, which is then converted to an electrical signal by a photodiode in a process known as indirect conversion. Electrical signal is proportional to the energy deposited by the visible light. Indirect conversion requires the use of detector septa and also introduces several intrinsic limitations ultimately leading to lower contrast, beam-hardening artifacts, and higher radiation dose. PCCT is named after its use of photon-counting detectors (PCD), which directly convert incident x-ray photons into an electrical signal via the use of semiconducting diode. Direct conversion allows for the measurement of the energy of individual incident x-ray photons and has smaller detector pixels as it does not require the use of detector septa. Each incident photon creates an electrical pulse that is directly proportional to the photon energy. Adjustable energy thresholds can then be used to organize and count the electrical pulses created by individual photons into separate energy bins. CCT images are constructed as a function of the total absorbed energy on a detector element. Therefore, higher energy incident photons, which have poor tissue contrast, will contribute to image formation disproportionately compared to lower energy photons. Energy binning can be used in PCCT to redistribute the photon energy weighting during image formation leading to improved tissue contrast and material decomposition.

Key Anatomic/Physiologic Issues and Imaging Findings/Techniques
A series of cases will be reviewed demonstrating the superior spectral separation and diagnostic performance of PCCT over CCT in the abdomen and pelvis with a focus on gastrointestinal pathology. One of the most promising features of PCCT is its capability to improve image noise and beam hardening artifacts that would otherwise render conventional scans nondiagnostic. An example of this is included in the case of an obese patient who's initial DECT scan was nondiagnostic due to beam attenuation and noise. This same patient was subsequently scanned on PCCT with markedly improved image quality, a result of using energy binning to select for higher energy photons. We have found PCCT to be particularly helpful in diagnosing bowel ischemia, where the diseased bowel wall may be hyper-dense due to necrosis and could be mistaken for enhancement and have included an example of this. Additional cases of the advantage of PCCT in evaluating the gastrointestinal tract will be provided.

Conclusion
PCCT exhibits numerous advantages over conventional CT and earlier iterations of DECT, particularly for abdominal applications.