Does kV Still Matter in Photon-Counting CT?

In conventional CT, kV matters enormously. Changing tube voltage changes iodine and calcium attenuation, making HU measurements less directly comparable across examinations.Photon-counting computed tomography (PCCT) may fundamentally change this relationship.

A new Investigative Radiology study tested 70, 90, 120 and 140 kV** on NAEOTOM Alpha (Siemens Healthineers) across multiple phantom sizes and virtual monoenergetic imaging (VMI) energies, complemented by a clinical cerebral computed tomography angiography (CTA) cohort.¹

In clinically relevant VMIs, attenuation became remarkably stable.
For iodine and calcium, CT values remained within the predefined 10 percent tolerance across all kV settings at 40–70 keV (ΔCT ≤ 8.4 percent). Overall, the authors conclude that 45 to 70 keV VMIs provide stable CT values regardless of tube voltage and patient size.¹

In other words, once keV is defined, kV largely stops determining the CT value. That is potentially important for longitudinal quantitative CT.

Why kV Still Matters

However, kV still matters, not primarily for attenuation but for dose efficiency.

In small phantoms at low VMI energies, 70–90 kV provided a 10 to 13 percent higher dose-corrected contrast-to-noise ratio (CNR) than 120 to 140 kV, corresponding to an approximately 20 percent potential dose reduction for the same CNR.¹

The exploratory clinical CTA data point in the same direction. At 55 keV, the mean contrast-to-noise ratio at unit dose (CNRD) was 23 percent higher at 90 kV than at 140 kV.¹

However, this advantage disappears in larger patients in whom higher kV remains preferable.

Does PCCT Redefine the Role of keV and kV?

This is a subtle but important conceptual shift. With PCCT, keV increasingly defines contrast and kV increasingly becomes an optimization parameter.

That separation could make CT measurements more reproducible across different acquisitions while allowing kV to be tailored primarily to patient size and dose efficiency.

There is an important caveat. 70–90 kV currently limits the spectral toolbox. If iodine maps or virtual non-contrast images are required, 120 to 140 kV remains necessary.

So, does kV still matter?

Absolutely. But perhaps it’s no longer for the reason we were used to, and that is another step toward truly quantitative PCCT.

Dr. Cademartiri is the director of advanced cardiovascular imaging and photon-counting CT at the Scientific Institute for Research, Hospitalization, and Healthcare Synlab Diagnostic Network in Naples, Italy. He is also a consultant in advanced cardiovascular imaging at CDI/Centro Diagnostico Italiano in Milan, Italy.

Reference

1. Jeukens CRLPN, Pinckaers FME, Martens B, Jasper S, Wildberger JE, Flohr T. Influence of X-ray tube voltage on computed tomography values and contrast-noise ratios in virtual monoenergetic images in routine clinical photon counting detector computed tomography: Does kV still matter? Invest Radiol. 2026 Sep 15. doi: 10.1097/RLI. 0000000000001312.

(Editor’s note: This blog is adapted with permission from Dr. Cademartiri’s original LinkedIn post at https://www.linkedin.com/feed/update/urn:li:activity:7505757123620958208/