Image quality of photon-counting detector CT virtual monoenergetic and polyenergetic reconstructions for head and neck CT angiography

We compared image quality of head and neck CT angiography (CTA) obtained with a photon-counting detector CT (PCD-CT), including virtual monoenergetic images and polyenergetic reconstructions, and conventional energy-integrating detectors CT (EID-CT) in three patients. PCD-CT monoenergetic reconstructions at 70 keV and lower provided excellent image quality, with improved signal-to-noise and contrast-to-noise compared to EID-CT and PCD-CT polyenergetic reconstructions. PCD-CT may enable radiation dose and iodinated contrast dose reduction for cerebrovascular imaging.

Photon-Counting Detector CT Angiography Versus Digital Subtraction Angiography in Patients with Peripheral Arterial Disease

RATIONALE AND OBJECTIVES

This study aims to compare the diagnostic confidence of photon-counting detector CT angiography (PCD-CTA) depending on the used vascular reformatting kernels with digital subtraction angiography (DSA) as diagnostic reference standard in peripheral arterial occlusive disease (PAOD).

MATERIAL AND METHODS

In 39 patients, 45 lower extremity PCD-CTA with subsequent DSA were analyzed. Advanced PAOD (Fontaine stage 4) was ascertained in 77.8% of patients. CTA post-processing comprised three vascular kernels (Bv36/48/56). Objective image quality assessment included vessel attenuation, image noise, contrast-to-noise (CNR) and signal-to-noise ratios (SNR). Subjective evaluation of calcium blooming, vessel sharpness, luminal attenuation and image noise was performed by three radiologists. Diagnostic performance and concordance to DSA were assessed.

RESULTS

The luminal attenuation remained kernel-independent constant. With sharper kernels, image noise increased substantially, while SNR and CNR decreased. Subjective reduction of calcium blooming and increased vessel sharpness were noted for the sharp Bv56 kernel. While sensitivity in stenosis quantification was comparable between kernels (81.6% vs. 81.5% vs. 81.0%, p = 0.797), specificity increased slightly higher sharpness (71.1% vs. 76.9% vs. 79.6%, p = 0.067). Diagnostic concordance of stenosis ratings compared to DSA increased likewise (Bv36 vs. Bv56, p = 0.002). Severe crural vessel calcifications had no influence on sensitivity, regardless of kernel selection. Contrarily, specificity was substantially worse in severely calcified tibial vessels but could be improved by using the sharp Bv56 kernel (Bv36 vs. Bv56 p = 0.024). Diagnostic confidence was highest for Bv56.

CONCLUSION

In lower leg PCD-CTA, sharp convolution kernels increase diagnostic confidence compared to DSA by improved vessel delineation and reduced calcium blooming with acceptable image noise.

Image quality assessment of coronary artery segments using ultra-high resolution dual source photon-counting detector computed tomography

PURPOSE

The study is intended to assess the image quality of ultra-high resolution (UHR) coronary computed tomography angiography (CCTA) performed on dual source photon-counting detector CT (PCD-CT).

METHOD

Consecutive patients, who underwent clinically indicated CCTA on PCD-CT (UHR 120x 0.2 mm collimation), were included. CCTA images were reconstructed at 0.2 mm slice thickness with Bv40, Bv44, Bv48 and Bv56 kernels and quantum iterative reconstruction level 4. Contrast-to-noise (CNR) and signal-to-noise ratios (SNR) were quantified from contrast-enhanced blood and subcutaneous adipose tissue. All reconstructions were scored per coronary segment (18-segment model) for presence, image quality, motion artefacts, stack artefacts, plaque presence and composition, and stenosis degree. Image quality was scored by two independent observers.

RESULTS

Sixty patients were included (median age 62 [25th - 75th percentile: 53-67] years, 45% male, median calcium score 62 [0-217]). The mean heart rate during scanning was 71 ± 11 bpm. Median CTDIvol was 19 [16-22]mGy and median DLP 243 [198-327]mGy.cm. The SNR was 9.3 ± 2.3 and the CNR was 11.7 ± 2.6. Of the potential 1080 coronary segments (60 patients x 18 segments), 255/256 (reader1/reader2) segments could not be assessed for being absent or non-evaluable due to size. Both readers scored 85% of the segments as excellent or very good (Intraclass Correlation Coefficient: 0.88 (95% CI: 0.87-0.90). Motion artefacts were present in 45(5%) segments, stack artefacts in 60(7%) segments and metal artefacts in 9(1%) segments.

CONCLUSION

UHR dual-source PCD-CT CCTA provides excellent or very good image quality in 85% of coronary segments at relatively high heart rates at moderate radiation dose with only limited stack artefacts.

Myocardial late enhancement and extracellular volume with single-energy, dual-energy, and photon-counting computed tomography

Computed tomography late enhancement (CT-LE) is emerging as a non-invasive technique for cardiac diagnosis with wider accessibility compared to MRI, despite its typically lower contrast-to-noise ratio. Optimizing CT-LE image quality necessitates a thorough methodology addressing contrast administration, timing, and radiation dose, alongside a robust understanding of extracellular volume (ECV) quantification methods. This review summarizes CT-LE protocols, clinical utility, and advances in ECV measurement through both single-energy and dual-energy CT. It also highlights photon-counting detector CT technology as an innovative means to potentially improve image quality and reduce radiation exposure.

Dual-Energy, Spectral and Photon Counting Computed Tomography for Evaluation of the Gastrointestinal Tract

The use of dual-energy computed tomography (CT) allows for reconstruction of energy- and material-specific image series. The combination of low-energy monochromatic images, iodine maps, and virtual unenhanced images can improve lesion detection and disease characterization in the gastrointestinal tract in comparison with single-energy CT.

Photon Counting Computed Tomography-Applications

Photon-counting detector CT (PCCT) is a new technology that has recently emerged as a powerful tool for a more precise, patient-centered imaging. Ever since the FDA approved the first Photon-counting system on September 30, 2021, this new technology raised much interest all over the scientific community and numerous studies have been published in a short period of time. By the end of 2022, the first results of phantom and in-vivo studies started showing the great potential of this new imaging modality, with benefits that range from neuroradiology to abdominal imaging and the promise to push previous limits of both patient size and age as well as image resolution. In this article, we will provide a brief explanation of how commercially available photon-counting detector CTs work and how they differ from energy-integrating detector CT systems. Then we will focus on the different clinical applications of this new technology with an in-depth systematic approach based on the most recent evidence. Because nearly every subspecialty of radiology has had impressive results, we will delve into each of these subspecialties and explain how every single domain can undergo significant transformation. This includes a wide range of possibilities, from the opportunistic screening of many different pathologies to the ability of seeing small structures with unprecedented precision, as well as a new kind of multi-energy imaging that can provide much more information on tissue characteristics, all while maintaining a lighter workflow and post-processing burden compared to what has been observed in the past.

Impact of Cardiac Motion on coronary artery calcium scoring using a virtual non-iodine algorithm on photon-counting detector CT: a dynamic phantom study

This study assessed the impact of cardiac motion and in-vessel attenuation on coronary artery calcium (CAC) scoring using virtual non-iodine (VNI) against virtual non-contrast (VNC) reconstructions on photon-counting detector CT. Two artificial vessels containing calcifications and different in-vessel attenuations (500, 800HU) were scanned without (static) and with cardiac motion (60, 80, 100 beats per minute [bpm]). Images were post-processed using a VNC and VNI algorithm at 70 keV and quantum iterative reconstruction (QIR) strength 2. Calcium mass, Agatston scores, cardiac motion susceptibility (CMS)-indices were compared to physical mass, static scores as well as between reconstructions, heart rates and in-vessel attenuations. VNI scores decreased with rising heart rate (p < 0.01) and showed less underestimation than VNC scores (p < 0.001). Only VNI scores were similar to the physical mass at static measurements, and to static scores at 60 bpm. Agatston scores using VNI were similar to static scores at 60 and 80 bpm. Standard deviation of CMS-indices was lower for VNI-based than for VNC-based CAC scoring. VNI scores were higher at 500 than 800HU (p < 0.001) and higher than VNC scores (p < 0.001) with VNI scores at 500 HU showing the lowest deviation from the physical reference. VNI-based CAC quantification is influenced by cardiac motion and in-vessel attenuation, but least when measuring Agatston scores, where it outperforms VNC-based CAC scoring.

Soft Reconstruction Kernels Improve HCC Imaging on a Photon-Counting Detector CT

RATIONALE AND OBJECTIVES

Hepatocellular carcinoma (HCC) is the only tumor entity that allows non-invasive diagnosis based on imaging without further histological proof. Therefore, excellent image quality is of utmost importance for HCC diagnosis. Novel photon-counting detector (PCD) CT improves image quality via noise reduction and higher spatial resolution, inherently providing spectral information. The aim of this study was to investigate these improvements for HCC imaging with triple-phase liver PCD-CT in a phantom and patient population study focusing on identification of the optimal reconstruction kernel.

MATERIALS AND METHODS

Phantom experiments were performed to analyze objective quality characteristics of the regular body and quantitative reconstruction kernels, each with four sharpness levels (36-40-44-48). For 24 patients with viable HCC lesions on PCD-CT, virtual monoenergetic images at 50 keV were reconstructed using these kernels. Quantitative image analysis included contrast-to-noise ratio (CNR) and edge sharpness. Three raters performed qualitative analyses evaluating noise, contrast, lesion conspicuity, and overall image quality.

RESULTS

In all contrast phases, the CNR was highest using the kernels with a sharpness level of 36 (all p < 0.05), with no significant influence on lesion sharpness. Softer reconstruction kernels were also rated better regarding noise and image quality (all p < 0.05). No significant differences were found in image contrast and lesion conspicuity. Comparing body and quantitative kernels with equal sharpness levels, there was no difference in image quality criteria, neither regarding in vitro nor in vivo analysis.

CONCLUSION

Soft reconstruction kernels yield the best overall quality for the evaluation of HCC in PCD-CT. As the image quality of quantitative kernels with potential for spectral post-processing is not restricted compared to regular body kernels, they should be preferred.

Quantum iterative reconstruction on a photon-counting detector CT improves the quality of hepatocellular carcinoma imaging

BACKGROUND

Excellent image quality is crucial for workup of hepatocellular carcinoma (HCC) in patients with liver cirrhosis because a signature tumor signal allows for non-invasive diagnosis without histologic proof. Photon-counting detector computed tomography (PCD-CT) can enhance abdominal image quality, especially in combination with a novel iterative reconstruction algorithm, quantum iterative reconstruction (QIR). The purpose of this study was to analyze the impact of different QIR levels on PCD-CT imaging of HCC in both phantom and patient scans.

METHODS

Virtual monoenergetic images at 50 keV were reconstructed using filtered back projection and all available QIR levels (QIR 1-4). Objective image quality properties were investigated in phantom experiments. The study also included 44 patients with triple-phase liver PCD-CT scans of viable HCC lesions. Quantitative image analysis involved assessing the noise, contrast, and contrast-to-noise ratio of the lesions. Qualitative image analysis was performed by three raters evaluating noise, artifacts, lesion conspicuity, and overall image quality using a 5-point Likert scale.

RESULTS

Noise power spectra in the phantom experiments showed increasing noise suppression with higher QIR levels without affecting the modulation transfer function. This pattern was confirmed in the in vivo scans, in which the lowest noise levels were found in QIR-4 reconstructions, with around a 50% reduction in median noise level compared with the filtered back projection images. As contrast does not change with QIR, QIR-4 also yielded the highest contrast-to-noise ratios. With increasing QIR levels, rater scores were significantly better for all qualitative image criteria (all p < .05).

CONCLUSIONS

Without compromising image sharpness, the best image quality of iodine contrast optimized low-keV virtual monoenergetic images can be achieved using the highest QIR level to suppress noise. Using these settings as standard reconstruction for HCC in PCD-CT imaging might improve diagnostic accuracy and confidence.

Liver Iodine Quantification With Photon-Counting Detector CT: Accuracy in an Abdominal Phantom and Feasibility in Patients

RATIONALE AND OBJECTIVES

To compare the accuracy of iodine quantification in liver parenchyma and lesions between dual-source photon-counting detector CT (PCD-CT) and dual-source energy-integrating detector CT (EID-CT) in a phantom and to demonstrate the feasibility of iodine quantification with PCD-CT in liver parenchyma and lesions in patients.

MATERIALS AND METHODS

An anthropomorphic abdominal phantom with a liver insert containing parenchyma and lesions was imaged on a clinical PCD-CT at 120kV and in the dual-energy mode on an EID-CT with kV-combinations of 80/Sn150kV, 90/Sn150kV, and 100/Sn150kV. Three patient sizes were imaged at three different radiation doses (CTDI_vol: 5, 10, 15mGy). Thirty patients with liver cysts, hemangiomas or metastases imaged with PCD-CT were retrospectively included. Iodine maps were reconstructed and iodine concentrations were measured in liver parenchyma and lesions. For the phantom, iodine error was quantified as the absolute difference to the vendor's specifications as reference.

RESULTS

Overall iodine error was 0.33 ± 0.29, 0.34 ± 0.32, 0.39 ± 0.37, 0.35 ± 0.39 mgI/mL for 80/Sn150kV, 90/Sn150kV, 100/Sn150kV of EID-CT, and PCD-CT, respectively, without significant differences between PCD-CT and EID-CT (p > 0.05). Radiation dose did not significantly influence error of PCD-CT (p > 0.05) nor EID-CT (p > 0.05). For both scanners, smaller patient sizes were associated with lower errors (p < 0.05). Iodine concentration and base material attenuation significantly influenced quantification for EID-CT (p < 0.05) but not PCD-CT (p > 0.05). In patients, iodine quantification was feasible in liver parenchyma, cysts, hemangiomas, and metastases.

CONCLUSION

Iodine quantification with PCD-CT is accurate in simulated liver parenchyma and lesions irrespective of radiation dose, iodine concentration, and base attenuation and is feasible in common liver lesions in patients.

Influence of heart rate and heart rate variability on the feasibility of ultra-fast, high-pitch coronary photon-counting computed tomography angiography

Coronary computed tomography angiography has become a mainstay in diagnosing coronary artery disease and is increasingly used in screening symptomatic patients. Recently, photon-counting computed tomography (PCCT) has been introduced into clinical practice, offering higher spatial and temporal resolution. As the applied radiation dose is highly dependent on the choice of scan mode and is lowest using the ultra-fast high-pitch (FLASH) mode, guidelines for their application are needed. From a retrospective study investigating the properties of a novel photon-counting computed tomography, all patients who underwent FLASH-mode PCCT angiography were selected between January and April 2022. This resulted in a study population of 46 men and 27 women. We recorded pre- and intrascan ECG readings and calculated heart rate (maximum heart rate 73 bpm) as well heart rate variability (maximum HRV 37 bpm) as measured by the standard deviation of the heart rate. Diagnostic quality and motion artifacts scores were recorded for each coronary artery segment by consensus between two readers. We found a highly significant association between heart rate variability and image quality (p < 0.001). The heart rate itself was not independently associated with image quality. Both heart rate and heart rate variability were significantly associated with the presence of motion artifacts in a combined model. Scan heart rate variability-but not heart rate itself-is a highly significant predictor of reduced image quality on high-pitch coronary photon-counting computed tomography angiography. This may be due to better scanner architecture and an increased temporal resolution compared to conventional energy-integrating detector computed tomography, which has to be addressed in a comparison study in the future.

Assessment of visibility of bone structures in the wrist using normal and half of the radiation dose with photon-counting detector CT

PURPOSE

To quantitatively and qualitatively assess the visibility of bone structures in the wrist on photon-counting detector computed tomography (PCD-CT) images compared to state-of-the-art energy-integrating detector CT (EID-CT).

METHOD

Four human cadaveric wrist specimens were scanned with EID-CT and PCD-CT at identical CTDI_vol of 12.2 mGy and with 6.1 mGy (half dose PCD-CT). Axial images were reconstructed using the thinnest possible slice thickness, i.e. 0.4 mm on EID-CT and 0.2 mm on PCD-CT, with the largest image matrix size possible using reconstruction kernels optimized for bone (EID-CT: Ur68, PCD-CT: Br92). Quantitative evaluation was performed to determine contrast-noise ratio (CNR) of bone/ fat, cortical and trabecular sharpness. An observer study using visual grading characteristics (VGC) analysis was performed by six observers to assess the visibility of nutrient canals, trabecular architecture, cortical bone and the general image quality.

RESULTS

At equal dose, images obtained with PCD-CT had 39 ± 6 % lower CNR (p = 0.001), 71 ± 57 % higher trabecular sharpness in the radius (p = 0.02) and 42 ± 8 % (p < 0.05) sharper cortical edges than those obtained with EID-CT. This was confirmed by VGC analysis showing a superior visibility of nutrient canals, trabeculae and cortical bone area under the curve (AUC) > 0.89) for PCD-CT, even at half dose.

CONCLUSIONS

Despite a lower CNR and increased noise, the trabecular and cortical sharpness were twofold higher with PCD-CT. Visual grading analysis demonstrated superior visibility of cortical bone, trabeculae, nutrient canals and an overall improved image quality with PCD-CT over EID-CT. At half dose, PCD-CT also yielded superior image quality, both in quantitative measures and as evaluated by radiologists.

An introduction to photon-counting detector CT (PCD CT) for radiologists

The basic performance of photon-counting detector computed tomography (PCD CT) is superior to conventional CT (energy-integrating detector CT: EID CT) because its spatial- and contrast resolution of soft tissues is higher, and artifacts are reduced. Because the X-ray photon energy separation is better with PCD CT than conventional EID-based dual-energy CT, it has the potential to improve virtual monochromatic- and virtual non-contrast images, material decomposition including quantification of the iodine distribution, and K-edge imaging. Therefore, its clinical applicability may be increased. Although the image quality of PCD CT scans is superior to that of EID CT currently, further improvement may be possible. The introduction of iterative image reconstruction and reconstruction with deep convolutional neural networks will be useful.

Improved visualization of the wrist at lower radiation dose with photon-counting-detector CT

OBJECTIVE

To compare the image quality of ultra-high-resolution wrist CTs acquired on photon-counting detector CT versus conventional energy-integrating-detector CT systems.

MATERIALS AND METHODS

Participants were scanned on a photon-counting-detector CT system after clinical energy-integrating detector CTs. Energy-integrating-detector CT scan parameters: comb filter-based ultra-high-resolution mode, 120 kV, 250 mAs, Ur70 or Ur73 kernel, 0.4- or 0.6-mm section thickness. Photon-counting-detector CT scan parameters: non-comb-based ultra-high-resolution mode, 120 kV, 120 mAs, Br84 kernel, 0.4-mm section thickness. Two musculoskeletal radiologists blinded to CT system, scored specific osseous structures using a 5-point Likert scale (1 to 5). The Wilcoxon rank-sum test was used for statistical analysis of reader scores. Paired t-test was used to compare volume CT dose index, bone CT number, and image noise between CT systems. P-value < 0.05 was considered statistically significant.

RESULTS

Twelve wrists (mean participant age 55.3 ± 17.8, 6 females, 6 males) were included. The mean volume CT dose index was lower for photon-counting detector CT (9.6 ± 0.1 mGy versus 19.0 ± 6.7 mGy, p < .001). Photon-counting-detector CT images had higher Likert scores for visualization of osseous structures (median score = 4, p < 0.001). The mean bone CT number was higher in photon-counting-detector CT images (1946 ± 77 HU versus 1727 ± 49 HU, p < 0.001). Conversely, there was no difference in the mean image noise of the two CT systems (63 ± 6 HU versus 61 ± 6 HU, p = 0.13).

CONCLUSION

Ultra-high-resolution imaging with photon-counting-detector CT depicted wrist structures more clearly than conventional energy-integrating-detector CT despite a 49% radiation dose reduction.