Photon Counting CT Angiography of the Head and Neck: Image Quality Assessment of Polyenergetic and Virtual Monoenergetic Reconstructions

Visualization of the middle meningeal artery on photon-counting detector CT: Comparison with energy-integrating detector CT

PURPOSE

The middle meningeal artery (MMA) is the target arteries for the treatment of intracranial diseases. The purpose of this study was to investigate the ability of photon-counting detector CT (PCD-CT) to improve the visualization of the MMA compared to energy-integrated detector CT (EID-CT).

METHODS

We retrospectively analyzed head CT angiography images from patients who underwent PCD-CT with ultra-high-resolution (UHR) mode between April 2023 and July 2024. We compared the images reconstructed with a slice thickness 0.2 mm and the matrix sizes 1024 × 1024 ('PCD-1024') and 512 × 512 ('PCD-512') with the same-patient images previously obtained by EID-CT with 0.75- or 1.0- mm slice thickness and the matrix size 512 × 512 ('EID-512'). Quantitatively, a region of interest (ROI) was placed on the maxillary artery near the origin of the MMA, and the signal-to-noise ratio (SNR), contrast-to-noise ratio (CNR), and contrast ratio (CR) were measured. Image sharpness was evaluated by measuring the full width at half maximum (FWHM) and the maximum intensity from the MMA vessel line profile. Qualitatively, the overall image quality, sharpness, and artifacts were evaluated using four-point Likert scales. The same evaluation was performed using PCD-CT with 0.2 mm ('0.2 mm') and virtual monoenergetic images (VMIs) ('45 keV', '55 keV', and '70 keV').

RESULTS

30 patients (19 males, 11 females; median age 65 years) were included. Quantitatively, the SNR (mean ± SD) for PCD-1024, PCD-512, and EID-512 were 16.9 ± 5.6, 20.3 ± 6.2, and 23.2 ± 7.8, respectively (P < 0.01 for PCD-1024 vs. EID-512). The CNR were 16.6 ± 4.5, 20.2 ± 5.5, and 44.4 ± 13.9. The CR were 0.74 ± 0.06, 0.74 ± 0.06, and 0.68 ± 0.08. The FWHM were 1.41 ± 0.17, 1.48 ± 0.15, and 1.77 ± 0.12, and the maximum intensity were 407.5 ± 71.7, 386.0 ± 72.5, and 218.2 ± 58.2 (P < 0.001 for PCD-1024 vs. EID-512 and PCD-512 vs. EID-512, respectively). Qualitatively, significant differences (P < 0.05) were identified among the PCD-1024, PCD-512, and EID-512 groups regarding the respective scores (median [IQR]) for overall image quality (4 [3-4], 3 [3-4], and 2 [2-3]), sharpness (4 [3-4], 3 [3-3], and 2 [2-3]), and artifacts (3 [3-3], 3 [2-3], and 2 [2-2]). In VMI analysis, quantitatively, 45 keV showed the highest values for SNR, CNR, CR, and maximum intensity; however, there were no differences in FWHM among the groups. Qualitatively, 0.2 mm was superior to the other groups in all evaluated aspects (P < 0.05).

CONCLUSION

PCD-1024 visualized the MMA better than both PCD-512 and EID-512. The 0.2 mm reconstruction was visually superior to the VMIs.

Ultra-high resolution CT angiography for the assessment of intracranial stents and flow diverters using photon counting detector CT

BACKGROUND

The patency of intracranial stents may not be reliably assessed with either CT angiography or MR angiography due to imaging artifacts. We investigated the potential of ultra-high resolution CT angiography using a photon counting detector (PCD) CT to address this limitation by optimizing scanning and reconstruction parameters.

METHODS

A phantom with different flow diverters was used to optimize PCD-CT reconstruction parameters, followed by imaging of 14 patients with intracranial stents using PCD-CT. Images were reconstructed using three kernels based on the phantom results (Hv56, Hv64, and Hv72; Hv=head vascular) and one kernel to virtually match the resolution of standard CT angiography (Hv40). Signal-to-noise ratio (SNR) and contrast-to-noise ratio (CNR) measurements were calculated. Subjective image quality and diagnostic confidence (DC) were assessed using a five point visual grading scale (5=best, 1=worst) and a three point grading scale (1=best, 3=worst), respectively, by two independent neuroradiologists.

RESULTS

Phantom images demonstrated the highest image quality across dose levels for 0.2 mm reconstructions with Hv56 (4.5), Hv64 (5), and Hv72 (5). In patient images, SNR and CNR decreased significantly with increasing kernel sharpness compared with control parameters. All reconstructions showed significantly higher image quality and DC compared with the control reconstruction with Hv40 kernel (P<0.001), with both image quality and DC being highest with Hv64 (0.2 mm) and Hv72 (0.2 mm) reconstructions.

CONCLUSION

Ultra-high resolution PDC-CT angiography provides excellent visualization of intracranial stents, with optimal reconstructions using the Hv64 and the Hv72 kernels at 0.2 mm.

REGISTRATION

BASEC 2021-00343.

Ultrahigh-Resolution Photon-Counting Detector CTA of the Head and Neck: Image Quality Assessment and Vascular Kernel Optimization

BACKGROUND. Head and neck CTA requires fine-detail evaluation, including characterization of potentially very small vessels and intrastent lumina. Blooming artifacts also hinder evaluation. OBJECTIVE. The purpose of this study was to evaluate image quality of ultrahigh-resolution (UHR) photon-counting detector (PCD) CTA of the head and neck and to explore variation of image quality across body vascular (Bv) reconstruction kernels. METHODS. This prospective study included patients who underwent clinically indicated head and neck CTA from September 2023 to December 2023. Participants underwent PCD CTA in the UHR mode. Reconstructions for each examination included a reference reconstruction (reflecting clinical protocols) using a 0.8-mm slice thickness and Bv40 kernel and six UHR reconstructions using a 0.2-mm slice thickness and kernels of varying sharpness (Bv48-Bv80). Quantitative measures were recorded. Two radiologists independently evaluated qualitative measures using a Likert scale ranging from 1 (lowest quality) to 5 (highest quality). RESULTS. The analysis included 103 participants (mean age, 61.3 ± 13.0 [SD] years; 56 men, 47 women). Median vessel sharpness (in Hounsfield units per millimeter) was 100.9 HU/mm for the reference reconstruction, and for the UHR reconstructions, it varied from 110.0 HU/mm for Bv48 to 121.6 HU/mm for Bv76 and 134.7 HU/mm for Bv80. Median right internal carotid artery C2 luminal diameter was 3.8 mm for the reference reconstruction, and for the UHR reconstructions, it increased from 4.1 mm for Bv48 to 4.9 mm for Bv80. For both readers, median overall image quality for the reference reconstruction was 3, and for the UHR reconstructions, it was highest for Bv64 (score of 5); calcified-plaque blooming artifact for the reference reconstruction was 1, and for the UHR reconstructions, it was highest for Bv72 (score of 5) and Bv76 (score of 5); stent blooming artifact for the reference reconstruction was 1, and for the UHR reconstructions, it was highest for Bv76 (score of 5) and Bv80 (score of 5); soft-plaque delineation for the reference reconstruction was 1, and for the UHR reconstructions, it was highest for Bv76 (score of 5) or Bv80 (score of 5); small-vessel visualization for the reference reconstruction was 1, and for the UHR reconstructions, it was highest for Bv76 (score of 5) or Bv80 (score of 5). CONCLUSION. UHR PCD CTA yielded reduced blooming artifact from calcified plaques or stents and improved visualization of soft plaque and small vessels. These advantages were more pronounced for strongest kernels, although subjective image quality was better for a weaker kernel. CLINICAL IMPACT. The findings indicate benefits from the use of UHR PCD CTA for head and neck evaluation and may help guide such examinations' kernel selection.

High-Resolution Head CTA: A Prospective Patient Study Comparing Image Quality of Photon-Counting Detector CT and Energy-Integrating Detector CT

BACKGROUND AND PURPOSE

Photon-counting detector CT (PCD-CT) is now clinically available and offers ultra-high-resolution (UHR) imaging. Our purpose was to prospectively evaluate the relative image quality and impact on diagnostic confidence of head CTA images acquired by using a PCD-CT compared with an energy-integrating detector CT (EID-CT).

MATERIALS AND METHODS

Adult patients undergoing head CTA on EID-CT also underwent a PCD-CT research examination. For both CT examinations, images were reconstructed at 0.6 mm by using a matched standard resolution (SR) kernel. Additionally, PCD-CT images were reconstructed at the thinnest section thickness of 0.2 mm (UHR) with the sharpest kernel, and denoised with a deep convolutional neural network (CNN) algorithm (PCD-UHR-CNN). Two readers (R1, R2) independently evaluated image quality in randomized, blinded fashion in 2 sessions, PCD-SR versus EID-SR and PCD-UHR-CNN versus EID-SR. The readers rated overall image quality (1 [worst] to 5 [best]) and provided a Likert comparison score (-2 [significantly inferior] to 2 [significantly superior]) for the 2 series when compared side-by-side for several image quality features, including visualization of specific arterial segments. Diagnostic confidence (0-100) was rated for PCD versus EID for specific arterial findings, if present.

RESULTS

Twenty-eight adult patients were enrolled. The volume CT dose index was similar (EID: 37.1 ± 4.7 mGy; PCD: 36.1 ± 4.0 mGy). Overall image quality for PCD-SR and PCD-UHR-CNN was higher than EID-SR (eg, PCD-UHR-CNN versus EID-SR: 4.0 ± 0.0 versus 3.0 ± 0.0 (R1), 4.9 ± 0.3 versus 3.0 ± 0.0 (R2); all P values < .001). For depiction of arterial segments, PCD-SR was preferred over EID-SR (R1: 1.0-1.3; R2: 1.0-1.8), and PCD-UHR-CNN over EID-SR (R1: 0.9-1.4; R2: 1.9-2.0). Diagnostic confidence of arterial findings for PCD-SR and PCD-UHR-CNN was significantly higher than EID-SR: eg, PCD-UHR-CNN versus EID-SR: 93.0 ± 5.8 versus 78.2 ± 9.3 (R1), 88.6 ± 5.9 versus 70.4 ± 5.0 (R2); all P values < .001.

CONCLUSIONS

PCD-CT provides improved image quality for head CTA images compared with EID-CT, both when PCD and EID reconstructions are matched, and to an even greater extent when PCD-UHR reconstruction is combined with a CNN denoising algorithm.

Neurovascular Imaging with Ultra-High-Resolution Photon-Counting CT: Preliminary Findings on Image-Quality Evaluation

BACKGROUND AND PURPOSE

The first-generation photon-counting detector CT was recently introduced into clinical practice and represents a promising innovation in high-resolution CT imaging. The purpose of this study was to assess the image quality of ultra-high-resolution photon-counting detector CT compared with energy-integrating detector CT and to explore different reconstruction kernel sharpness levels for the evaluation of intracranial aneurysms.

MATERIALS AND METHODS

Ten patients with intracranial saccular aneurysms who had previously undergone conventional energy-integrating detector CT were prospectively enrolled. CT angiograms were acquired on a clinical dual-source photon-counting detector CT in ultra-high-resolution mode and reconstructed with 4 vascular kernels (Bv36, Bv40, Bv44, Bv48). Quantitative and qualitative image-quality parameters of the intracranial arteries were evaluated. For the quantitative analysis (image noise, SNR, contrast-to-noise ratio), ROIs were manually placed at standard anatomic intracranial and extracranial locations by 1 author. In addition, vessel border sharpness was evaluated quantitatively. For the qualitative analysis, 3 blinded neuroradiologists rated photon-counting detector CT and energy-integrating detector CT image quality for the evaluation of the intracranial vessels (ie, the aneurysms and 9 standard vascular branching locations) on a 5-point Likert-type scale. Additionally, readers independently selected their preferred kernel among the 4 kernels evaluated on photon-counting detector CT.

RESULTS

In terms of quantitative image quality, Bv48, the sharpest kernel, yielded increased image noise and decreased SNR and contrast-to-noise ratio parameters compared with Bv36, the smoothest kernel. Compared with energy-integrating detector CT, the Bv48 kernel offered better quantitative image quality for the evaluation of small intracranial vessels (P < .001). Image-quality ratings of the Bv48 were superior to those of the energy-integrating detector CT and not significantly different from ratings of the B44 reconstruction kernel. When comparing side by side all 4 photon-counting detector reconstruction kernels, readers selected the B48 kernel as the best to visualize the aneurysms in 80% of cases.

CONCLUSIONS

Ultra-high-resolution photon-counting detector CT provides improved image quality for neurovascular imaging. Although the less sharp kernels provided superior SNR and contrast-to-noise ratio, the sharpest kernels delivered the best subjective image quality on photon-counting detector CT for the evaluation of intracranial aneurysms.

Photon-Counting Computed Tomography Angiography of Carotid Arteries: A Topical Narrative Review with Case Examples

Photon counting computed tomography (PCCT) represents a paradigm shift from conventional CT imaging, propelled by a new generation of X-ray detectors capable of counting individual photons and measuring their energy. The first part of this narrative review is focused on the technical aspects of PCCT and describes its key advancements and benefits compared to conventional CT but also its limitations. By synthesizing the existing literature, the second part of the review seeks to elucidate the potential of PCCT as a valuable tool for assessing carotid artery disease. Thanks to the enhanced spatial resolution and image quality, PCCT allows for an accurate evaluation of carotid luminal stenosis. With its ability to finely discriminate between different tissue types, PCCT allows for detailed characterization of plaque morphology and composition, which is crucial for assessing plaque vulnerability and the risk of cerebrovascular events.

Low Dose Iodinated Contrast Material and Radiation for Virtual Monochromatic Imaging in Craniocervical Dual-Layer Spectral Detector Computed Tomography Angiography: A Prospective and Randomized Study

RATIONALE AND OBJECTIVES

To investigate the feasibility of virtual monochromatic imaging (VMI) of dual-layer spectral detector computed tomography (SDCT) to reduce iodinated contrast material (CM) and radiation dose in craniocervical computed tomography angiography (CTA).

MATERIALS AND METHODS

A total of 280 consecutively selected patients performed craniocervical CTA with SDCT were prospectively selected and randomly divided into four groups (A, DoseRight index (DRI) 31, iopromide 370mgI/mL, volume 0.8 mL/kg; B, DRI 26, iopromide 370mgI/mL, volume 0.4 mL/kg; C, DRI 26, ioversol 320mgI/mL, volume 0.4 mL/kg; D, DRI 26, iohexol 300mgI/mL, volume 0.4 mL/kg). 50-70 kiloelectron volts (keV) VMIs in group B were reconstructed and compared to group A to select the optimal keV. Then, the optimal keV in groups B, C and D was reconstructed and compared. Objective image quality, including vascular attenuation, image noise, signal-to-noise ratio (SNR) and contrast-to-noise ratio (CNR), was evaluated. Subjective image quality was assessed using a 5-point Likert scale. In addition, the effective dose (ED), iodine load and iodine delivery rate (IDR) were compared between groups A and D.

RESULTS

55 keV VMI was the optimal VMI in group B. The objective and subjective image quality of 55 keV VMI in group B were equal to or better than those of the CI in group A. The SNR, CNR and subjective image quality in group D were similar to those in group B (P > 0.05). The ED, iodine load and IDR of group D were reduced by 44%, 59% and 19%, respectively, when compared to those of group A.

CONCLUSION

Low dose iodinated CM and radiation for 55 keV VMI in craniocervical CTA using SDCT could still provide equivalent or better image quality than the conventional scanning protocol.

Minimizing Contrast Media Dose in CT Pulmonary Angiography with Clinical Photon Counting Using High Pitch Technique

RATIONALE AND OBJECTIVES

To evaluate the potential to reduce the amount of iodinated contrast media (CM) for computer tomographic pulmonary angiography (CTPA) with a novel photon-counting-detector CT (PCCT).

MATERIALS AND METHODS

Overall, 105 patients referred for CTPA were retrospectively included in this study. CTPA was performed using bolus tracking and high-pitch dual-source scanning (FLASH mode) on a novel PCCT (Naeotom Alpha, Siemens Healthineers). CM (Accupaque 300, GE Healthcare) dose was lowered stepwise following the introduction of the new CT scanner. Thus, patients could be divided into 3 groups as follows: group 1, n = 29, 35 ml of CM; group 2, n = 62, 45 ml of CM and group 3, n = 14, 60 ml of CM. Four readers independently assessed the image quality (Likert-scale 1-5) and adequate assessment of the segmental pulmonary arteries. Additionally, the pulmonary arterial contrast opacification was measured.

RESULTS

The subjective image quality was rated highest in group 1 with 4.6 compared to 4.5 (group 2) and 4.1 (group 3) with a significant difference between groups 1 and 3 (p < 0.001) and between groups 2 and 3 (p = 0.003). In all groups, almost all segmental pulmonary arteries could be assessed adequately without significant differences (18.5 vs. 18.7 vs. 18.4). Mean attenuation in the pulmonary trunk did not differ significantly between groups 321 ± 92 HU versus 345 ± 93 HU versus 347 ± 88 HU (p = 0.69).

CONCLUSION

Significant CM dose reduction is possible without a reduction in image quality. PCCT enables diagnostic CTPA with 35 ml of CM.

[A comprehensive review on photon-counting computed tomography: Principles, technical hurdles and analysis of clinical applications]

In recent years, photon-counting computed tomography (PCD-CT) based on photon-counting detectors (PCDs) has become increasingly utilized in clinical practice. Compared with conventional CT, PCD-CT has the potential to achieve micron-level spatial resolution, lower radiation dose, negligible electronic noise, multi-energy imaging, and material identification, etc. This advancement facilitates the promotion of ultra-low dose scans in clinical scenarios, potentially detecting minimal and hidden lesions, thus significantly improving image quality. However, the current state of the art is limited and issues such as charge sharing, pulse pileup, K-escape and count rate drift remain unresolved. These issues could lead to a decrease in image resolution and energy resolution, while an increasing in image noise and ring artifact and so on. This article systematically reviewed the physical principles of PCD-CT, and outlined the structural differences between PCDs and energy integration detectors (EIDs), and the current challenges in the development of PCD-CT. In addition, the advantages and disadvantages of three detector materials were analysed. Then, the clinical benefits of PCD-CT were presented through the clinical application of PCD-CT in the three diseases with the highest mortality rate in China (cardiovascular disease, tumour and respiratory disease). The overall aim of the article is to comprehensively assist medical professionals in understanding the technological innovations and current technical limitations of PCD-CT, while highlighting the urgent problems that PCD-CT needs to address in the coming years.

Photon-Counting Versus Energy-Integrating Detector CT Angiography of the Lower Extremity in a Human Cadaveric Model With Continuous Extracorporeal Perfusion

OBJECTIVES

Detailed visualization of the arterial runoff is mandatory for the assessment of peripheral arterial occlusive disease. This study aims to compare the performance of a first-generation photon-counting detector computed tomography (PCD-CT) to a third-generation energy-integrating detector CT (EID-CT).

MATERIALS AND METHODS

Computed tomography angiographies of 8 upper leg arterial runoffs were performed on human cadaveric models with continuous extracorporeal perfusion. For both PCD-CT and EID-CT, radiation dose-equivalent 120 kVp acquisition protocols (low-/medium-/high-dose: CTDI Vol = 3/5/10 mGy) were used. All scans were performed with standard collimation (PCD-CT: 144 × 0.4 mm; EID-CT: 96 × 0.6 mm), a pitch factor of 0.4, and a gantry rotation time of 1.0 second. Reformatting of data included the use of comparable vascular kernels (Bv 48/49), a slice thickness and increment of 1.0 mm, and a field of view of 150 × 150 mm. Eight radiologists evaluated image quality independently using a browser-based pairwise forced-choice comparison setup. Kendall concordance coefficient ( W ) was calculated to estimate interrater agreement. Signal-to-noise ratio and contrast-to-noise ratio (CNR) were compared based on 1-way analyses of variance and linear regression analysis.

RESULTS

Low-dose PCD-CT achieved superior signal-to-noise ratio/CNR values compared with high-dose EID-CT ( P < 0.001). Linear regression analysis suggested that an EID-CT scan with a CTDI Vol of at least 15.5 mGy was required to match the CNR value of low-dose PCD-CT. Intraluminal contrast attenuation was higher in PCD-CT than EID-CT, irrespective of dose level (415.0 ± 31.9 HU vs 329.2 ± 29.4 HU; P < 0.001). Subjective image quality of low-dose PCD-CT was considered superior to high-dose EID-CT ( P < 0.001). Interrater agreement was high ( W = 0.989).

CONCLUSIONS

Using cadaveric models with continuous extracorporeal perfusion allows for intraindividual image quality comparisons between PCD-CT and EID-CT on variable dose levels. With superior luminal contrast attenuation and denoising in angiographies of the peripheral arterial runoff, PCD-CT displayed potential for radiation saving of up to 83% compared with EID-CT.

Standardized assessment of vascular reconstruction kernels in photon-counting CT angiographies of the leg using a continuous extracorporeal perfusion model

This study evaluated the influence of different vascular reconstruction kernels on the image quality of CT angiographies of the lower extremity runoff using a 1st-generation photon-counting-detector CT (PCD-CT) compared with dose-matched examinations on a 3rd-generation energy-integrating-detector CT (EID-CT). Inducing continuous extracorporeal perfusion in a human cadaveric model, we performed CT angiographies of eight upper leg arterial runoffs with radiation dose-equivalent 120 kVp acquisition protocols (CTDI_vol 5 mGy). Reconstructions were executed with different vascular kernels, matching the individual modulation transfer functions between scanners. Signal-to-noise-ratios (SNR) and contrast-to-noise-ratios (CNR) were computed to assess objective image quality. Six radiologists evaluated image quality subjectively using a forced-choice pairwise comparison tool. Interrater agreement was determined by calculating Kendall's concordance coefficient (W). The intraluminal attenuation of PCD-CT images was significantly higher than of EID-CT (414.7 ± 27.3 HU vs. 329.3 ± 24.5 HU; p < 0.001). Using comparable kernels, image noise with PCD-CT was significantly lower than with EID-CT (p ≤ 0.044). Correspondingly, SNR and CNR were approximately twofold higher for PCD-CT (p < 0.001). Increasing the spatial frequency for PCD-CT reconstructions by one level resulted in similar metrics compared to EID-CT (CNR_fat; EID-CT Bv49: 21.7 ± 3.7 versus PCD-CT Bv60: 21.4 ± 3.5). Overall image quality of PCD-CTA achieved ratings superior to EID-CTA irrespective of the used reconstruction kernels (best: PCD-CT Bv60; worst: EID-CT Bv40; p < 0.001). Interrater agreement was good (W = 0.78). Concluding, PCD-CT offers superior intraluminal attenuation, SNR, and CNR compared to EID-CT in angiographies of the upper leg arterial runoff. Combined with improved subjective image quality, PCD-CT facilitates the use of sharper convolution kernels and ultimately bears the potential of improved vascular structure assessability.

Advanced Neuroimaging With Photon-Counting Detector CT

ABSTRACT

Photon-counting detector computed tomography (PCD-CT) is an emerging technology and promises the next step in CT evolution. Photon-counting detectors count the number of individual incoming photons and assess the energy level of each of them. These mechanisms differ substantially from conventional energy-integrating detectors. The new technique has several advantages, including lower radiation exposure, higher spatial resolution, reconstruction of images with less beam-hardening artifacts, and advanced opportunities for spectral imaging. Research PCD-CT systems have already demonstrated promising results, and recently, the first whole-body full field-of-view PCD-CT scanners became clinically available. Based on published studies of preclinical systems and the first experience with clinically approved scanners, the performance can be translated to valuable neuroimaging applications, including brain imaging, intracranial and extracranial CT angiographies, or head and neck imaging with detailed assessment of the temporal bone. In this review, we will provide an overview of the current status in neuroimaging with upcoming and potential clinical applications.

Dual Source Photon-Counting Computed Tomography-Part II: Clinical Overview of Neurovascular Applications

Photon-counting detector (PCD) is a novel computed tomography detector technology (photon-counting computed tomography-PCCT) that presents many advantages in the neurovascular field, such as increased spatial resolution, reduced radiation exposure, and optimization of the use of contrast agents and material decomposition. In this overview of the existing literature on PCCT, we describe the physical principles, the advantages and the disadvantages of conventional energy integrating detectors and PCDs, and finally, we discuss the applications of the PCD, focusing specifically on its implementation in the neurovascular field.