Task-based optimization of energy thresholds in PCD-CT directly translates into clinically meaningful improvements across a wide range of applications. Beyond enhancing image quality, spectral imaging fundamentally expands CT’s diagnostic potential by improving lesion conspicuity, enabling more accurate tissue characterization, and supporting more efficient and comprehensive imaging strategies.

Improved lesion conspicuity is one of the most immediate and clinically relevant benefits. In abdominal imaging, this is particularly important for hypovascular tumors, including pancreatic ductal adenocarcinoma, where low-keV images improve the contrast between tumor and normal parenchyma, enabling more accurate delineation of tumor extent and associated fibrotic stroma [38]. The advantages unique to PCD-CT are most pronounced in early pancreatic cancer detection (Fig. 2). Detecting early-stage pancreatic cancer may be challenging in the arterial phase; therefore, findings, including focal parenchymal atrophy and delayed enhancement, are important. PCD-CT is particularly advantageous for detecting these subtle features.

Cinematic volume-rendered images reconstructed from arterial-phase 40 keV VMI data (e, f) clearly depict focal atrophy of the pancreatic tail (arrowheads).

In the equilibrium phase, faint delayed improvement is suspected on 70 keV VMI (c), which becomes more conspicuous on 40 keV VMI (d) (yellow arrows).

In the arterial phase, 70 keV VMI (a) demonstrates focal atrophy and a subtle hypoattenuating area in the pancreatic tail. The lesion remains inconspicuous on 40 keV VMI (b), making its identification as a hypovascular tumor confidently challenging.

A 57-year-old female patient with suspected early-stage pancreatic cancer based on focal pancreatic atrophy on magnetic resonance cholangiopancreatography underwent dynamic contrast-enhanced PCD-CT.

Early-stage pancreatic cancer was evaluated using a photon-counting detector CT. A 57-year-old female patient with suspected early-stage pancreatic cancer based on focal pancreatic atrophy on magnetic resonance cholangiopancreatography underwent dynamic contrast-enhanced PCD-CT. In the arterial phase, 70 keV VMI (a) demonstrates focal atrophy and a subtle hypoattenuating area in the pancreatic tail. The lesion remains inconspicuous on 40 keV VMI (b), making its identification as a hypovascular tumor confidently challenging. In the equilibrium phase, faint delayed improvement is suspected on 70 keV VMI (c), which becomes more conspicuous on 40 keV VMI (d) (yellow arrows). Cinematic volume-rendered images reconstructed from arterial-phase 40 keV VMI data (e, f) clearly depict focal atrophy of the pancreatic tail (arrowheads). Detecting early-stage pancreatic cancer may be challenging in the arterial phase; therefore, findings, including focal parenchymal atrophy and delayed enhancement, are important. PCD-CT is particularly advantageous for detecting these subtle features

Early-stage pancreatic cancer is frequently challenging to identify because its hypovascular characteristics in the arterial phase are subtle, and detection frequently depends on minor findings, including delayed enhancement [39] or focal pancreatic atrophy [40]. PCD-CT has demonstrated improved conspicuity of these subtle features. Although dedicated pancreas-protocol CT remains the standard for pancreatic cancer evaluation, improved visibility of delayed enhancement and other indirect findings on routine contrast-enhanced CT may facilitate earlier suspicion in selected patients, particularly in real-world settings where CT is often performed for nonpancreatic indications. Furthermore, improved contrast facilitates the evaluation of tumor invasion depth, supporting more precise T staging in gastrointestinal malignancies, including colorectal and rectal cancers (Fig. 3). Although evidence remains preliminary, a recent pathology-correlated study of 51 patients demonstrated that optimized PCD-CT achieved diagnostic performance comparable to MRI for differentiating T3–T4 rectal cancers using 40-keV virtual monoenergetic imaging with QIR level 4 reconstruction [35].

Rectal cancer staging using photon-counting CT. A patient with rectal cancer underwent preoperative local staging. Rectal cancer staging has traditionally relied on magnetic resonance imaging (MRI); however, PCD-CT provided near–MRI-level depiction of mural invasion in this case. a, b Axial (a) and sagittal (b) T2-weighted MRI show a rectal tumor with suspected extension beyond the muscularis propria (yellow arrows). c, d Corresponding axial (c) and sagittal (d) virtual monoenergetic images reconstructed at 70 keV using PCD-CT demonstrate the rectal tumor; however, assessment of extramural extension is limited. e, f Axial (e) and sagittal (f) 40-keV virtual monoenergetic images substantially enhance tumor-to-wall contrast, clearly depicting disruption of the rectal wall and extramural tumor extension (yellow arrows), consistent with clinical T3 (cT3) disease. g Gross pathological specimen demonstrates focal tumor penetration beyond the muscular layer of the rectal wall (yellow arrows). h Histopathological examination (hematoxylin and eosin staining) confirms tumor invasion beyond the muscularis propria (yellow arrows), consistent with pathological T3 (pT3) disease. This case illustrates that low-keV photon-counting CT delineates subtle extramural invasion with high confidence, thereby supporting its role as a practical and reliable tool for preoperative T staging of rectal cancer, which approaches the diagnostic performance of MRI

Moreover, spectral imaging enables a more reliable assessment of contrast enhancement in cystic or complex lesions. PCD-CT improves differentiation between true and pseudoenhancement by increasing iodine-related attenuation at low keV and providing complementary iodine maps (Fig. 4).

Role of iodine maps in excluding enhancement within cystic lesions. An older woman in her 80 s underwent contrast-enhanced PCD-CT to assess malignancy risk in an intraductal papillary mucinous neoplasm. a, b Unenhanced computed tomography images demonstrating a low-attenuation cystic lesion without internal hyperdense components. c, d Contrast-enhanced virtual monoenergetic images reconstructed at 70 keV demonstrate no definite mural enhancement within the cystic lesion, although evaluation is limited. e, f Corresponding 40-keV virtual monoenergetic images increase iodine-related contrast, raising suspicion for possible mural or intramural enhancement within the cystic wall (yellow arrows). g, h Iodine maps clearly show the distribution of iodine, enabling a confident assessment of enhancement within the cystic lesion (yellow arrows) and facilitating differentiation between true enhancement and pseudo-enhancement associated with low-keV image contrast. The patient subsequently underwent surgical resection, and histopathological assessment confirmed IPMN with high-grade dysplasia

This is particularly valuable for assessing cystic renal masses, ovarian tumors, and pancreatic cystic lesions, including high-risk intraductal papillary mucinous neoplasms, where identifying enhancing mural nodules or septa is crucial for risk stratification and clinical management [41].

The clinical impact extends beyond oncology. Improved contrast resolution facilitates the detection of subtle inflammatory changes, vascular abnormalities, and small lesions that may otherwise be overlooked. For instance, drug-induced adverse events, including interstitial nephritis, pancreatitis, and colitis, may be detected with greater sensitivity due to improved contrast differentiation (Fig. 5).

Drug-induced interstitial nephritis detected by photon-counting detector CT. A 78-year-old female patient is undergoing follow-up CT after initiation of dabrafenib plus trametinib therapy for recurrent lung cancer postoperatively. On 70-keV VMI, coronal (a) and axial (b) images demonstrate subtle bilateral striated nephrograms. In contrast, 40-keV VMI (c, d) markedly increases the conspicuity of multiple heterogeneous hypodense areas in both kidneys (yellow arrows). Fusion images of iodine density maps and virtual noncontrast images (e, f) further improve the visualization of regions with decreased enhancement

Another important advantage is the potential to reduce contrast dose while maintaining diagnostic image quality [21]. By using low-keV imaging to amplify iodine attenuation, diagnostic-quality images were obtained with approximately half the contrast volume of conventional dosing (Fig. 6).

Feasibility of contrast medium dose reduction using photon-counting detector CT (PCD-CT). An 83-year-old male patient with alcoholic liver cirrhosis is undergoing follow-up for multiple hepatocellular carcinomas (HCCs). A prior dynamic contrast-enhanced CT performed 3 months earlier using a dual-source energy-integrating detector CT (SOMATOM Force) with a standard contrast dose (iomeprol 135 mL; 1.6 mL/kg) is presented (a, d, g, j). In the current examination, due to reduced renal function (estimated glomerular filtration rate [eGFR]: 43.5 mL/min/1.73 m²), a reduced-contrast dose protocol (iomeprol 67 mL; 0.8 mL/kg) was employed with PCD-CT. Images include 70 keV VMI (b, e, h, k) and 40 keV VMI (c, f, i, l). Images (a–f) correspond to the arterial phase, g–i portal venous phase, and j–l equilibrium phase. Despite the reduced contrast dose, arterial-phase hyperenhancement of HCCs is clearly demonstrated on 70-keV VMI b and further accentuated on 40-keV VMI c. On 40-keV images, early enhancement of HCCs is appreciable in the portal venous phase (i). Although low-keV VMI inherently exhibits increased image noise, the image quality obtained with photon-counting detector CT remains diagnostically acceptable. The visualization of peripheral vessels, including the pancreatic arcade, remains excellent (d–f), and contrast between parenchymal organs and the portal vein is preserved, indicating that substantial contrast dose reduction is feasible with PCD-CT while maintaining diagnostic confidence

This approach may be particularly valuable in patients with impaired renal function, for whom minimizing contrast exposure is clinically desirable. Although reduced contrast volume should not be equated with proven renal protection [42], low-keV PCD-CT may facilitate contrast-dose optimization in selected high-risk patients, although further outcome studies are needed.

In addition to reducing contrast dose, spectral imaging facilitates the generation of virtual noncontrast (VNC) images, potentially reducing or eliminating the need for true noncontrast (TNC) acquisitions in multiphase CT protocols. Although VNC imaging has already been established with DECT, PCD-CT may further improve attenuation stability and the accuracy of material decomposition. Several studies have reported that VNC images provide attenuation values and lesion conspicuity comparable to those of TNC images in many clinical scenarios, enabling the omission of the TNC and thereby reducing radiation exposure [43].

This benefit may be particularly important in oncologic abdominal and pelvic imaging, where patients often undergo repeated CT examinations. Abdominal PCD-CT has been reported to reduce radiation dose by approximately 30% compared with state-of-the-art dual-source dual-energy EID-CT while improving image quality and lesion conspicuity [44]. In addition, VNC imaging may further reduce cumulative exposure by allowing omission of TNC acquisitions in selected multiphasic protocols, with potential overall dose reductions approaching 50% while maintaining diagnostic performance [45]. However, important limitations must be recognized. Similar to conventional DECT, renal calculi, subtle hemorrhage, and hyperdense cysts may occasionally be underestimated or partially obscured on VNC images, although our experience suggests that these findings may be more stably preserved with PCD-CT than with prior EID-based DECT systems (Fig. 7a, b). Lipiodol deposition after transarterial chemoembolization may also appear reduced or absent on VNC images, similar to conventional DECT, thereby potentially affecting posttreatment assessment and representing an important diagnostic pitfall (Fig. 7c, d). In addition, although quantitative performance appears to be improved with PCD-CT, highly enhancing lesions may still exhibit artificially elevated attenuation values compared with TNC images. Therefore, caution remains necessary when applying VNC images to quantitative assessments, such as adrenal adenoma characterization using washout rate or relative enhancement ratio calculations (Fig. 7e, f).

Limitations and potential pitfalls of virtual noncontrast (VNC) imaging with photon-counting detector CT (PCD-CT) (a, b). A patient with renal calculi and a complicated renal cyst. a True noncontrast (TNC) image and b VNC image generated from contrast-enhanced PCD-CT data. The small renal calculus and hyperdense complicated cyst remain visible on the VNC image (arrowheads), suggesting better preservation of high-attenuation findings than on prior-generation dual-energy CT systems. c, d A patient with hepatocellular carcinoma after transarterial chemoembolization. c TNC image demonstrates dense Lipiodol deposition within the treated lesion (yellow arrow), whereas the corresponding VNC image d shows marked reduction or near-complete disappearance of the Lipiodol accumulation. Similar to conventional dual-energy CT, this represents an important pitfall of VNC imaging that may affect posttreatment assessment. e, f A patient with primary aldosteronism and adrenal adenoma. e TNC image demonstrates low attenuation of the adrenal lesion with a mean attenuation value of −2 HU, consistent with lipid-rich adrenal adenoma. However, on the corresponding VNC image (f), the attenuation value increases to 15 HU, exceeding the commonly used diagnostic cutoff of 10 HU. Although quantitative stability may be improved with PCD-CT, highly enhancing lesions may still demonstrate artificially elevated attenuation values on VNC images, potentially limiting the reliability of quantitative assessments such as adrenal adenoma characterization. These examples illustrate that, although VNC imaging may reduce radiation exposure and simplify multiphasic CT protocols, selective use of TNC imaging remains necessary in specific clinical scenarios

Conversely, calcifications are generally preserved on VNC images, although slight attenuation changes may occur at lesion margins [43]. Therefore, VNC provides a powerful tool for dose reduction and workflow simplification, whereas careful interpretation is required in specific clinical contexts, and selective use of TNC imaging remains necessary.

Spectral imaging has also been discussed in the context of “environmentally sustainable imaging,” a concept that refers to reducing the imaging burden through fewer examinations, lower radiation exposure, reduced contrast material use [46], and more efficient diagnostic workflows [47]. By improving lesion detectability and soft-tissue contrast within a single CT examination, spectral imaging may reduce reliance on additional imaging modalities in selected clinical scenarios. However, these concepts should currently be regarded as emerging possibilities rather than established universal standards. For example, clinical studies have suggested that contrast-enhanced PCD-CT performed as part of the metastatic workup may achieve lesion conspicuity approaching that of contrast-enhanced T1-weighted imaging in primary breast tumors [36]. Similar principles may potentially be applicable to selected pelvic malignancies, including uterine tumors (Fig. 8).

Complementary role of staging photon-counting detector CT (PCD-CT) in local assessment of endometrial carcinoma. A woman in her 60 s with endometrial carcinoma underwent pelvic MRI and contrast-enhanced staging PCD-CT for preoperative evaluation. a, b High-resolution T2-weighted images (T2WI) demonstrate disruption and irregularity of the junctional zone (yellow arrows), suggesting myometrial invasion. c Early-phase dynamic contrast-enhanced sagittal T1-weighted image (CE-T1WI) demonstrates irregular subendometrial enhancement (SEE) adjacent to the tumor (arrowhead), supporting focal myometrial invasion. d Delayed-phase axial CE-T1WI demonstrates that the abnormal SEE becomes less conspicuous. e, f Corresponding 70-keV virtual monoenergetic images (VMI) obtained with staging PCD-CT demonstrate the uterine mass, although the contrast between the tumor and surrounding myometrium remains relatively limited, making detailed local assessment difficult. g, h Corresponding 40-keV VMI images demonstrate improved tumor-to-myometrium contrast and clearer depiction of irregular SEE (arrowheads), similar to the findings observed on early-phase dynamic CE-T1WI. Notably, the enhancement abnormality is appreciable not only on sagittal but also on axial CT images. Overall, the imaging findings suggested myometrial invasion without serosal extension. Final histopathology confirmed invasion involving more than one-half of the myometrial thickness. Although MRI, particularly high-resolution T2WI, remains essential for local staging of endometrial carcinoma, this case suggests that low-keV staging PCD-CT may complement CE-T1WI by improving visualization of enhancement abnormalities and local tumor conspicuity during routine preoperative staging examinations

Nevertheless, MRI, particularly high-resolution T2-weighted imaging, remains the cornerstone for local staging of gynecologic malignancies [48], and current evidence does not support replacing MRI with PCD-CT. Rather, the potential role of low-keV PCD-CT may be complementary, particularly in situations where contrast-enhanced MRI is limited, incomplete, or unavailable. In such cases, staging CT examinations may simultaneously provide systemic and partial local diagnostic information, potentially reducing the need for additional contrast-enhanced imaging in selected patients. Although multimodality imaging remains important when maximum diagnostic accuracy is required, comprehensive MRI evaluation is not always practical across all stages of patient care. In this context, PCD-CT may provide complementary diagnostic information within routine CT examinations. For example, simplified combinations such as noncontrast MRI with contrast-enhanced staging PCD-CT may become feasible in selected settings, although further validation is required before broader clinical adoption.

Moreover, the ability to detect small lesions, including micrometastases in the brain and liver, on routine CT further highlights the potential for comprehensive evaluation within a single modality (Fig. 9).

Potential of one-stop imaging enabled by improved detection of metastatic lesions using photon-counting detector CT. A 57-year-old male patient with sigmoid colon cancer was detected during assessment for weight loss and positive fecal occult blood. Initial staging was performed using PCD-CT. The upper row demonstrates 70-keV VMI, and the lower row presents 40-keV VMI. On coronal images, the primary tumor is visualized as an enhancing mass, with improved conspicuity at 40 keV caused by higher lesion-to-background contrast. In the arterial phase, early enhancing hepatic nodules are inconspicuous on 70 keV VMI (c) but are clearly depicted on 40 keV VMI (d). These lesions demonstrate washout on 70-keV VMI in the delayed phase (e), which becomes more conspicuous on 40-keV VMI (f). On dynamic MRI, detecting the lesions is challenging in the early phase (g), whereas hepatobiliary phase imaging demonstrates decreased gadoxetic acid uptake, consistent with metastatic lesions (h).Contrast-enhanced MRI remains the reference standard for detecting liver and brain metastases; however, contrast-enhanced CT is increasingly capable of providing highly accurate local and metastatic evaluation. PCD-CT may enable comprehensive one-stop imaging with further advances

Spectral CT may reduce the need for additional imaging [49], and PCD-CT may further enhance this potential by providing high-resolution anatomical, spectral, and functional information in a single examination. However, the concept of “one-stop imaging” should still be interpreted cautiously. Although PCD-CT may reduce the need for additional imaging examinations in selected situations, the extent to which it reduces repeat MRI or CT examinations has not yet been sufficiently validated. Future studies should evaluate its impact on workflow, cost-effectiveness, patient burden, and sustainability.

Beyond improved visibility, spectral imaging enables quantitative assessment of tissue perfusion and vascularity. Iodine maps provide semi-quantitative information that can be used to assess disease activity, for example, in inflammatory bowel disease, including Crohn’s disease, where iodine uptake correlates with inflammatory activity [50, 51]. In oncology, quantitative iodine density is considered a promising imaging biomarker that reflects tumor perfusion, differentiation, disease stage, treatment response, and prognosis [52,53,54]. Further, it may be useful for assessing residual disease after neoadjuvant chemotherapy (Fig. 10).

Iodine density imaging with photon-counting detector CT as a potential biomarker for treatment response. A 77-year-old male patient with rectal cancer. Pre-treatment delayed-phase contrast-enhanced CT (a), T2-weighted MRI (b), diffusion-weighted imaging (c), and FDG PET/CT (d) demonstrate a circumferential tumor in the lower rectum (Rb) with intense FDG uptake (SUVmax: 18.66). The tumor was clinically staged as cT4N3M0. Post-chemoradiotherapy, follow-up delayed-phase PCD-CT demonstrates tumor shrinkage on 70 keV VMI (e). However, residual wall thickening is more clearly depicted on 40 keV VMI (f), predominantly on the right side (yellow arrow). Iodine density imaging (g) illustrates higher iodine concentration in the corresponding region, indicating residual tumor viability. Consistently, FDG PET/CT (h) shows persistent uptake in the same area (SUVmax: 4.73) (yellow arrow). As previously reported, iodine density imaging may serve as a useful imaging biomarker for assessing treatment response

Metastatic lymph nodes in the abdomen frequently show decreased iodine density during the venous phase; however, this pattern is not consistent across all tumor types. Considering the influence of contrast timing and acquisition protocols, normalization approaches (e.g., lesion-to-aorta ratios) and robust cutoff values are crucial for reliable interpretation [53].

The clinical utility of low-keV virtual monoenergetic imaging was established with EID-based DECT; however, increased image noise and the need for dedicated DECT acquisition limited its routine use. By enabling diagnostically acceptable low-keV imaging and on-demand spectral analysis from every examination, PCD-CT may facilitate a transition from spectral CT as a specialized technique to a routine clinical platform.