“Despite >700 CAC score in LAD, we could rule out coronary stenosis. High resolution and high keV spectral imaging make a difference!”
Prof. Pal Maurovich-Horvat, MD, PhD · Director of Medical Imaging, Semmelweis University, Budapest, Hungary
Coronary artery calcium scoring
CACS resulting from an unenhanced ECG-triggered CT is a well-established screening method to detect coronary artery calcifications. It is associated with excellent cardiovascular risk stratification and management as well as prognosis of clinical outcomes.
The four technological benefits outlined in the previous chapter and in Figure 1 have a direct effect on CACS. Through the use of virtual monoenergetic imaging, standardized calcium scoring is available regardless of the tube potential used for acquisition and using Quantum HD Cardiac may provide greater diagnostic confidence for CACS in patients with high calcium burden due to reduced blooming with higher spatial resolution.
On NAEOTOM Alpha, phantom studies indicate that CACS is independent of the applied tube potential down to 90 kV using a 70 keV monoenergetic reconstruction [9], with the potential to allow for up to 53% dose-reduced calcium scoring scanning in patients compared to conventional CT imaging [10].
CAD evaluation
A significant percentage of ICA procedures are not necessary procedures for patients with suspected CAD. As per CAD-RADS 2.0 sub-classification, a more accurate assessment of vessel stenoses is of improved benefit to patients and practices.
CCTA imaging by conventional CT is limited by its accuracy in the evaluation of calcified lesions where a high coronary calcium burden often leads to an overestimation of the degree of stenoses and hence to false-positive diagnoses in CT assessment. Delineation of the luminal stenosis can also be inaccurate in the presence of severe calcifications and partial volume artifact due to high-density calcifications averaged with density of other tissues in a single voxel combined with motion artifact can result in blurring of interfaces.
A 15% reduction in calcium blooming artefacts is achievable together with improved image sharpness and image quality when imaging coronary plaques in Quantum HD Cardiac (0.2 mm) mode compared to standard (0.6 mm) reconstructions [11], coupled with much improved visualization of spatial details, including potential identification of plaques with fibrous caps [12] (Figure 6), partially calcified plaques [11] (Figure 7), and differences in CAD-RADS classification [13] (Figure 5).
Figure 6. In the image reconstructed with standard resolution (Bv40, 0.6 mm) (Fig. 6a), only calcified plaques are visible in the distal segment 3 of the RCA. In the UHR image (Bv64, 0.2 mm) (Fig. 6b), a fibrous cap beneath the calcified plaque is visualized, due to reduced calcium blooming. OCT correlation (Fig. 6c) of the same plaque confirms the plaque composition. Courtesy of Johannes Gutenberg University Medical Center, Mainz, Germany.
“Ultra-high-resolution CCTA with PCD-CT enabled a remarkably precise characterization of partially calcified plaques discerning noncalcified and calcified parts, potentially improving the identification of plaques prone to rupture.”
Prof Dr Hatem Alkadhi · Head of Cardiovascular Imaging, University Hospital Zürich, Switzerland
Figure 7. Identification of a mixed plaque in distal left main coronary artery plaque in a patient with severe aortic stenosis. With Quantum HD Cardiac, improvement of the delineation of the partially calcified plaque is evident showing superior anatomic detail.
Left – cVRT, 0.2 mm slice thickness, Bv44, 512 matrix.
Middle – MPR, 0.6 mm slice thickness, Bv40, 512 matrix.
Right – MPR, 0.2 mm slice thickness, Bv72, 512 matrix.
Courtesy of University Hospital Zurich, Zurich, Switzerland.
Stent imaging
CCTA has emerged as a viable alternative to ICA and has demonstrated high diagnostic performance for the detection and exclusion of high-grade anatomic stenoses in native coronary arteries. However, CCTA on conventional CT systems exhibits a lower diagnostic performance for evaluation of intracoronary stents largely due to partial volume effects and beam hardening artifacts.
The presence of heavy calcifications in a stented segment further enhances metal artifacts as it contributes to beam hardening. Importantly, diagnostic performance is dependent upon stent size and type, with higher accuracy for larger over smaller diameters, and for polymer-based scaffolds over metallic stents due to metal artifacts (causing stent struts to appear to be thicker, in turn causing underestimation of the stent lumen).
Limited spatial resolution of conventional CT systems compounds imaging in implanted stents with characteristics unfavorable for stent imaging, such as thick-strut stents and stents with diameters less than 3 mm.
A key requirement for diagnostic imaging in cases where stents are present is the ability to visualize inside the stent to determine in-stent lumen visibility as an investigative tool for in-stent restenosis (ISR), which may occur due to developments such as neointimal hyperplasia, in-stent thrombosis, and stent fracture.
Although the incidence of ISR has declined due to the development of drug-eluting stents, it still occurs with a rate of approximately 3% to 20% of patients [12].
A major potential benefit of NAEOTOM Alpha through the use of Quantum HD Cardiac would be to offer interventional operators the ability to assess stenoses in stented segments of coronary arteries, in bypass grafts, and in native coronaries as accurately as with invasive coronary angiography and to position photon-counting CT of the coronaries as the method of choice for revascularization follow-up and thus therapeutic decision making (Figure 8) [15].
Phantom studies of commonly used coronary stents have demonstrated that NAEOTOM Alpha significantly improves coronary stent delineation and might overcome technical limitations of conventional energy integrated detector (EID) CT concerning in-stent lumen visualization [16].
With small stents of diameters between 2.5 mm to 4.0 mm, imaging with NAEOTOM Alpha described higher lumen visibility, lower in-stent CT attenuation, and lower image noise compared to conventional CT imaging at twice the radiation dose of NAEOTOM Alpha scans [16].
Figure 8. Severe in-stent restenosis at the ostium of the LADStent imaged in Quantum HD Cardiac mode (Bv72, 1024 × 1024, QIR 4, 0.2 mm slice thickness) on NAEOTOM Alpha, demonstrating in-stent restenosis. The patient underwent coronary stent implantation multiple times and had 8 stents implanted in total in the left and right coronary arteries. The orange arrow shows a focal, severe in-stent restenosis at the ostium of the LAD. Courtesy of Semmelweis University Hospital, Budapest, Hungary.
Structural heart planning
Transcatheter aortic valve replacement (TAVR) planning is used to plan for transcatheter aortic valve implantation in patients with severe aortic stenosis. CT imaging allows for a map of cardiac anatomy including the assessment of the aortic root and valve annulus to facilitate the safe deployment of appropriately sized aortic valve prostheses.
In traditional CT imaging, retrospective ECG-gated or prospective ECG-triggered scanning covers at least the aortic root in order to provide anatomical information of the aortic root, followed commonly by non-ECG-gated CTA data acquisition of the aorto-iliofemoral vasculature for assessment of the access vasculature. Typically, both acquisitions are combined in a comprehensive scanning protocol with a single contrast administration.
A new milestone with NAEOTOM Alpha in the planning for TAVR scans includes a new imaging protocol dedicated to high-pitch Turbo Flash TAVR imaging. This feature enables an ECG triggered Turbo Flash scan mode, which allows for an independent control of the z-position of the ECG target phase within the scan range.
This enables, for example, a high-pitch scan over the entire thorax from the aortic arch down to the iliac, while separately specifying the cardiac start phase at 30% at the level of the aortic valve in end-systole. As a result, it is possible to perform TAVR valve measurements (i.e., aortic diameter below valve plane, left and right sinus heights, sinus of Valsalva diameter and left main and right coronary artery heights) and to plan the access for TAVR procedures based on a single Turbo Flash scan range in under one second of scan time.
Figure 9: Post-procedure prosthetic valve imaging.
Left and Center – Cinematic VRT.
Right – Representative slice from 55 keV virtual monoenergetic image (VMI), Bv40 kernel, 0.8 mm slice thickness.
Courtesy of Erasmus Medical Centre, Rotterdam, The Netherlands.
Radiation dose and contrast media
Scanning with NAEOTOM Alpha with the imaging benefits brought by spectral imaging and reduced electronic noise also allows a potential reduction of the use of contrast media, which remains an important consideration for patients suffering from chronic kidney disease. Quantum Technology enables better weighting of low-energy quanta, potentially allowing for a reduction in contrast media volume.
Recently, phantom studies have evaluated imaging strategies to reduce contrast media volumes for CCTA, showing that in a dynamic cardiac vessel phantom, using VMI reconstructions at 40 keV with a reduction in contrast media concentration of 50%, could obtain diagnostic attenuation and objective image quality for CCTA imaging [8]. Recent clinical studies of thoracic CTAs have demonstrated that a reduction of 25% of injected contrast is possible when comparing VMI at 50 keV and conventional CT at a matched radiation dose, resulting in PCCT images above the boundaries of inferiority [17].
A recent case report presented an aortoiliac CTA case using NAEOTOM Alpha, performed with a total iodine dose of just 9.8 g, with roughly one third of the contrast media volume typically used by the authors on high-end conventional CT [18]. A comparable case from the same institute is shown below in Figure 10.
Furthermore, the pediatric population has a very strong focus on radiation dose. CTA examinations for suspected anomalous coronary arteries of young patients have demonstrated higher spatial resolution (a key requirement in younger patients), better image quality, and over 3 times lower radiation doses compared to conventional CT imaging [19].
Figure 10: A 74-year-old male patient, with a known history of an ascending aortic aneurysm, was referred for a CT follow-up. An aortoiliac CTA was performed using a QuantumPlus Turbo Flash mode. A total volume of 38 mL of contrast media, blended with 70% Ultravist370 and 30% saline, was injected followed by a saline chaser. A total iodine dose of 9.8 g, less than one-third of that used in the standard protocol, was applied – resulting in optimal contrast enhancement. The unique combination of acquiring spectral information with high temporal resolution using NAEOTOM Alpha provides great advantages for vessel evaluation. Courtesy of Freiburg University Hospital, Germany.