DECT to the Edges—Photon-Counting CT in the ED

Written by

Logan Young

Dual-energy CT (DECT) has become a workhorse in emergency imaging, particularly for iodine mapping and virtual noncontrast applications. But as Aaron Sodickson, MD, reveals in a presentation now available in the ARRS Quick Bytes library, conventional DECT systems come with a quiet limitation: dual-energy information is not uniformly available across the entire field of view. Photon-counting CT (PCCT) changes that.

Left: What you see here is that on our existing high-end dual-energy scanners, we have a slight limitation, which is that we don’t get dual-energy information outside this yellow-dotted circle). Right: While we have iodine content centrally, we don’t have anything out in the periphery.

Conventional DECT’s Limitations: On many high-end dual-energy scanners, iodine maps are only reliable within a central circular region. Outside that area, dual-energy information is lost. In large patients or peripheral anatomy, this can mean incomplete iodine characterization and diagnostic uncertainty.

Here’s another example of the same thing, where you can see that we’ve lost our information out at the edges.

In practice, you may see clean iodine signal centrally, but nothing at the edges—simply because the system cannot acquire dual-energy data beyond that geometric constraint.

What Does PCCT Do Differently? PCCT acquires spectral information directly at the detector. Because it does not rely on paired detector geometries or source-based separation, dual-energy information is available across the entire field of view, meaning:

  • Iodine maps extend fully to the periphery
  • Large patients no longer “fall outside” the dual-energy zone
  • Edge anatomy benefits from the same spectral data as the center

In the ED, where patient size and positioning are unpredictable, this matters.

On the photon-counting scanner, all of this dual-energy information content is acquired at the detector, so we don’t geometrically need the two CT detectors that are being used in our conventional scanner. Thus, we can get dual-energy information all the way out to the edges, even in large patients outside that field of view.

Improved Noise Behavior in Iodine Maps: Another practical advantage of PCCT is cleaner iodine post-processing. On conventional DECT, iodine maps may show speckled signal in non-enhancing structures—algorithmic noise that can mimic low-level enhancement.

In this example below of a renal cyst, conventional DECT shows scattered orange signal that is not true enhancement. On PCCT, that noise is substantially reduced, allowing confident classification of a benign, non-enhancing complex cyst.

Looking at this renal cyst on a conventional scan, you see some noise in the iodine post-processing content (e.g., green arrow in orange speckle).
There isn’t actually enhancement within this cyst; this is noise in the algorithm. On the photon-counting scanner, we’re able to knock down that noise to see much more cleanly that this is a non-enhancing, benign, complex cyst.

Why This Matters in the ED: ED radiology rewards speed and certainty. PCCT improves both by:

  • Preserving iodine data across the entire image
  • Reducing false-positive enhancement
  • Increasing confidence in lesion characterization
  • Decreasing the need for follow-up imaging

Bottom Line: Photon-counting CT removes the “edge problem” of conventional DECT. By delivering full-field spectral data with improved noise characteristics, PCCT strengthens iodine-based interpretation—exactly where ED imaging needs it most.

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