Computed tomography (CT) is widely used to evaluate patients with suspected coronary artery disease (CAD). Coronary artery calcifications (CAC) scoring, of which Agatston score is the most widely used, has been utilized since the 1990s for cardiovascular risk estimation in asymptomatic patients. An AS of 0 strongly correlates with a lack of cardiovascular events over the following 5 years. In stenosis grading, conventional coronary CT angiography (CCTA) has high negative predictive value (NPV) but may overestimate stenosis severity due to limited spatial resolution and blooming artifacts. Photon-counting detector CT (PCD-CT) offers higher spatial resolution, reduced noise, and spectral imaging capabilities that enable virtual monoenergetic images (VMI), virtual non-contrast (VNC), and virtual non-calcium reconstructions, potentially improving diagnostic accuracy.

This thesis aimed to evaluate the reliability, advantages, and limitations of PCD-CT compared with conventional energy-integrating detector CT (EID-CT) in the evaluation of coronary artery disease. In study I and II the correlation and agreement of Agatston score (AS), was evaluated between the two CT systems. In study II and IV the image quality was compared between the two CT systems. Study III investigated how reconstruction parameters affect different plaque component quantification with PCD-CT using ultra high-resolution mode. In study IV the measured grade of stenosis was compared between EID-CT, PCD-CT, using standard resolution mode (SR), with invasive coronary angiography as golden standard.

Study I used data of cadaveric hearts, study II and IV, were based on clinical patient data, while study III used data of SCAPIS (Swedish CArdioPulmonary bioImage Study) study subjects.

The first study showed that the AS, measured on cadaveric hearts, correlated excellently between PCD-CT and EID-CT with high reproducibility, confirming the comparability of the two methods for calcium quantification. The study of AS included in study II confirmed this result. The second study demonstrated that both systems provide high image quality in CCTAs, but that PCD-CT offers sharper calcium delineation as well as lower radiation dose and contrast volume with preserved image quality. The third study showed that in CCTA, reconstruction parameters influence the measured volume of coronary plaques. The fourth study showed that PCD-CT, when using a standard-resolution scan mode, provides diagnostic accuracy equivalent to EID-CT for coronary stenosis assessment, but with superior image quality.

In summary, the thesis demonstrates that PCD-CT provides image quality and quantitative diagnostic results that match or surpass those of conventional CT in the diagnosis of CAD, with potential for improved image sharpness, dose reduction, and more reliable plaque characterization. The studies also highlight the need for careful parameter optimization in CCTA acquisition and reconstruction protocols to improve CAD evaluation as well as segmentation-based plaque analysis tools in PCD-CT.

Datortomografi (DT/CT) används i stor utsträckning för att utvärdera patienter med misstänkt kranskärlssjukdom (CAD). Konventionell kranskärls-DT- angiografi (CCTA) har ett högt negativt prediktivt värde men kan överskatta graden av stenos på grund av begränsad spatial upplösning och så kallade blooming-artefakter. Fotonräknande detektor-CT (PCD-CT) erbjuder högre spatial upplösning, minskat brus och spektrala avbildningsmöjligheter som möjliggör rekonstruktion av virtuella monokromatiska bilder (VMI), virtuella icke-kontrastbilder (VNC) och virtuella icke-kalkbilder, vilket kan förbättra den diagnostiska säkerheten.

Syftet med denna avhandling är att utvärdera tillförlitlighet, fördelar och begränsningar med PCD-CT jämfört med konventionell energi-integrerande detektor-DT (EID-CT) för kranskärlsdiagnostik.

I studie I och II utvärderades korrelation och överensstämmelse av Agatston score (AS), den vanligaste kalciumscore metoden, mellan de två DT-systemen. I studie II och IV jämfördes bildkvalitet mellan systemen. I studie III undersöktes hur rekonstruktionsparametrar påverkar kvantifiering av olika plackkomponenter med PCD-CT i ultrahögupplöst läge (UHR). I studie IV jämfördes graderingen av stenoser mellan EID-CT och PCD-CT, i standardupplösning, med invasiv kranskärlsangiografi som referensmetod. Studie II och IV, kliniska observationsstudier, baserades på kliniska patientdata, medan data från kadaverhjärtan användes i studie I och data från SCAPIS-deltagare i studie III, även dessa studier är observationsstudier. Den första studien visade att AS, uppmätt på kadaverhjärtan, överensstämde utmärkt mellan PCD-CT och EID-CT med hög reproducerbarhet, vilket bekräftar metodernas jämförbarhet för kalcium-scoremätning. Utvärderingen av AS i studie II bekräftade detta resultat. Den andra studien visade att båda systemen ger hög bildkvalitet vid CCTA, men att PCD-CT ger skarpare kalkavgränsning. PCD-CT ger möjlighet att använda lägre stråldos och kontrastmängd. Den tredje studien visade att val av rekonstruktion ”kernel” främst påverkar volymen av icke-förkalkade plackkomponenter, medan snitt tjockleken påverkar volymen av förkalkade plackkomponenter. Den fjärde, jämförande, studien visade att PCD-CT i standardupplösning ger lika hög diagnostisk noggrannhet som EID-CT vid bedömning av kranskärlsstenoser, men med bättre bildkvalitet.

Sammanfattningsvis visar avhandlingen att PCD-CT ger bildkvalitativa- och kvantitativa resultat som motsvarar eller överträffar konventionell DT vid diagnostik av kranskärlssjukdom, med potential för förbättrad bildskärpa, dosreduktion och mer tillförlitlig plackkarakterisering. Studierna betonar också behovet av noggrant val av parametrar för att förbättra CCTA-protokoll för bildtagning och rekonstruktion samt optimering av segmenteringsbaserade verktyg för plackanalys vid PCD-CT.

Background: Coronary CT angiography (CCTA) is a key non-invasive tool for evaluating coronary artery disease (CAD). While energy-integrating detector CT (EID-CT) offers high negative predictive value (NPV), its positive predictive value (PPV) is limited in heavily calcified vessels. Photon-counting detector CT (PCD-CT), with higher spatial resolution and reduced blooming, may enhance diagnostic performance. Current PCD-CT systems provide both standard-resolution (SR) and ultra-high-resolution (UHR) modes, but the clinical impact of these modes remains under investigation.

Objectives: To compare the diagnostic accuracy and image quality of SR-PCD-CT versus EID-CT in quantifying coronary stenosis, using quantitative coronary angiography (QCA) as reference. Materials and methods: In this prospective, single-centre study, 21 patients (5 women, mean age 71.5 years) with suspected CAD underwent CCTA with both EID-CT and SR-PCD-CT prior to QCA. A total of 301 coronary segments were assessed for stenosis severity, with ≥50 % stenosis deemed significant. Image quality was graded using a 5-point scale. 

Results: No significant differences in percentage diameter stenosis (%DS) were found between imaging techniques (p = 0.20). Both EID-CT and SR-PCD-CT showed good agreement with QCA (AUC: PCD-CT 0.89, EID-CT 0.86). Specificity and NPV were high for both; sensitivity and PPV were moderate. SR-PCD-CT yielded higher image quality compared to EID-CT (p < 0.001).

Conclusions: In standard resolution mode, PCD-CT offers excellent image quality for quantifying coronary stenosis at comparable diagnostic accuracy compared to EID-CT

The purpose of this study was to compare the correlation and agreement between AS derived from either an energy-integrating detector CT (EID-CT) or a photon-counting detector CT (PCD-CT). Reproducibility was also compared. In total, 26 calcified coronary lesions (from five cadaveric hearts) were identified for inclusion. The hearts were positioned in a chest phantom and scanned in both an EID-CT and a prototype PCD-CT. The EID-CT and PCD-CT acquisition and reconstruction parameters were matched. To evaluate the reproducibility, the phantom was manually repositioned, and an additional scan was performed using both methods. The EID-CT reconstructions were performed using the dedicated calcium score kernel Sa36. The PCD-CT reconstructions were performed with a vendor-recommended kernel (Qr36). Several monoenergetic energy levels (50-150 keV) were evaluated to find the closest match with the EID-CT scans. A semi-automatic evaluation of calcium score was performed on a post-processing multimodality workplace. The best match with Sa36 was PCD-CT Qr36 images, at a monoenergetic level of 72 keV. Statistical analyses showed excellent correlation and agreement. The correlation and agreement with regards to the Agatston score (AS) between the two methods, for each position as well as between the two positions for each method, were assessed with the Spearman s rank correlation. The correlation coefficient, rho, was 0.98 and 0.97 respectively 0.99 and 0.98. The corresponding agreements were investigated by means of Bland-Altman plots. High correlation and agreement was observed between the AS derived from the EID-CT and a PCD-CT. Both methods also demonstrated excellent reproducibility.

CASE PRESENTATION: A 26-year-old man presented with a 2-week history of productive cough and a 1-year history of effort-related dyspnea. His medical history was significant for hay fever and exertion-triggered asthma. He was not taking medicines regularly but was using inhaled salbutamol as needed. He was an ex-smoker, with a previous history of 2-pack years.