Single-heartbeat technology
Modern CT scanners are capable of capturing an image of the entire heart in a single heartbeat, which has dramatically improved diagnostic quality.
- Single-source systems (256- and 320-slice). Require a 180° gantry rotation. Scan time is about 125 ms.
- Dual-source systems (2x128- and 2x192-slice). The X-ray tubes are positioned at a 90° angle, so the gantry only needs to rotate by a quarter turn. This provides a record temporal resolution of up to 66 ms.
Instead of the old retrospective gating (scanning throughout the entire cardiac cycle), prospective gating is now used. The scanner takes images only at the end of diastole, when the myocardium is motionless. Combined with iterative reconstruction algorithms, this has reduced radiation exposure to 1 mSv.
Scanning conditions
To obtain high-quality images, the patient must meet several criteria:
- Regular rhythm and heart rate up to 65–70 bpm. If the heart rate is higher, $\beta$-blockers are prescribed.
- Body mass index (BMI) below 40 kg/m².
- Normal renal function (for safe intravenous administration of 50–80 ml of iodinated contrast).
- Adequate vasodilation. Achieved by sublingual administration of nitroglycerin immediately before the procedure.
Coronary calcium scoring and Agatston score
Quantitative assessment of calcification is most often performed using the Agatston score (Agatston score). The study is indicated for asymptomatic low-risk patients with a family history of early CAD, as well as for intermediate-risk patient stratification.
Historically, a calcium score >400 or >1000 units was considered a contraindication to CT coronary angiography: artifacts made the scans uninformative, and radiation exposure was unjustified. The modern approach has completely abandoned this tactic. Iterative reconstructions make it possible to obtain diagnostically valuable data even with massive calcification.
However, the "calcium paradox" exists: with a huge score (e.g., 1251), CT may show suspicion of multiple stenoses, whereas invasive coronary angiography will not reveal any hemodynamically significant narrowing.
Diagnostic value and plaque assessment
CT angiography is ideal for ruling out CAD (negative predictive value is 97%–99%). However, the positive predictive value is lower (64%–86%). The method tends to overestimate the degree of narrowing due to partial volume artifacts and blooming (blooming artifacts). This is due to the spatial resolution limit of the devices (0.5–0.6 mm).
In addition to the vessel lumen, the method allows for the analysis of atherosclerotic plaque composition. Based on density in Hounsfield units (HU), the following are distinguished:
- Lipid-rich plaques: 11–99 HU
- Fibrous plaques: 77–121 HU
Sometimes non-calcified plaques do not narrow the vessel lumen, but expand its outer contour. This phenomenon is called positive remodeling (positive remodelling).
Briefly: answers to topical questions
How does prospective ECG gating differ from retrospective gating in cardiac CT?
Prospective ECG gating differs from retrospective gating in data acquisition timing and radiation dose level.
| Characteristic | Prospective gating (Prospective gating) | Retrospective gating (Retrospective gating) |
|---|---|---|
| Data acquisition | During a small portion of the cardiac cycle in a single heartbeat | Continuously throughout the entire cardiac cycle over several heartbeats |
| Radiation dose | Low | High (12–21 mSv) |
| Administration | Modern systems, regular rhythm | Older systems, irregular rhythm, high heart rate |
| Reconstruction | Phase of least motion (mid/end diastole) | Any phase of the cardiac cycle |
What are the clinical indications for determining the Agatston calcium score?
Determination of the Agatston calcium score is used to assess coronary calcium and, in certain situations, aortic valve calcification.
Clinical indications supported by sources:
- Asymptomatic patients without established CAD — low-risk group in the presence of a family history of early CAD (premature IHD).
- CAD risk stratification — in patients from the intermediate-risk group.
- Confirmation of severe aortic stenosis — non-contrast cardiac CT with calcium scoring should be considered in patients with reduced stroke volume and low transvalvular gradients.
What pharmacological drugs are used to prepare a patient for CT coronary angiography?
Medications used for the pharmacological preparation of a patient for CT coronary angiography include heart rate-lowering agents, vasodilators, and radiocontrast media.
The following pharmacological drugs are used:
- Beta-blockers ($\beta$-blockers) — prescribed to lower the heart rate to target values below 65–70 bpm.
- Nitroglycerin — administered sublingually to achieve vasodilation of the coronary arteries.
- Iodinated contrast agent — administered intravenously in a volume of about 50–80 ml to enhance vascular contrast.
Coronary artery branches on images: LM (Left Main) — main stem, LAD (Left Anterior Descending) — descends anteriorly, LCx (Left Circumflex) — circumflex, RCA (Right Coronary Artery) — right.
Class and exam questions
Does a zero calcium score rule out coronary artery disease in a young patient?
No. Young symptomatic patients may have non-calcified yet hemodynamically significant atherosclerotic plaques.
Why is CT coronary angiography prone to overestimating stenosis severity?
Due to calcium blooming artifacts and partial volume effects, which arise from the physical limitations of the spatial resolution of CT detectors.
What is the advantage of photon-counting detectors (PCD)?
They directly convert X-rays into an electrical signal without optical baffles. This improves spatial resolution (up to 0.25 mm), reduces noise, and enhances stent visualization quality.
What else to study on this topic
- Magnetic resonance coronary angiography (CMR) in the diagnosis of congenital anomalies
- Principles of energy-integrating (EID) and photon-counting (PCD) CT detectors
- Functional imaging and myocardial viability assessment
- Aneurysm of the left anterior descending branch in Kawasaki disease
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