Explain Cardiac CT
Definition
Cardiac CT is an advanced CT examination used to assess the heart, coronary arteries and related cardiovascular structures. Because the heart is continuously moving, specialised acquisition techniques are required to reduce motion artefact and obtain high-quality diagnostic images.
Patient Preparation and Positioning
The patient is generally positioned supine on the CT table with the arms extended above the head, where the patient’s condition allows. ECG electrodes are applied appropriately to obtain a reliable ECG signal for cardiac synchronisation. The patient should receive clear breathing and breath-hold instructions before image acquisition. For contrast-enhanced cardiac CT, appropriate intravenous access is essential. A suitable large-bore IV cannula, commonly 18–20G, preferably in an antecubital vein, is used where possible. Cardiac CT angiography requires a relatively high contrast injection rate. Injection rates commonly around 5–6 mL/s may be used where appropriate, depending on the scanner, contrast concentration, patient factors and local protocol. The IV access and contrast injection must always follow the local departmental contrast policy.
ECG Synchronisation
The ECG is used to identify the cardiac cycle and synchronise image acquisition or reconstruction with an appropriate phase of cardiac motion. The aim is to acquire or reconstruct images during a phase when coronary artery motion is relatively low. Two important approaches are prospective ECG-triggered acquisition, where data are acquired during a selected phase of the cardiac cycle and this can substantially reduce radiation exposure in appropriate patients; and retrospective ECG-gated acquisition, where data are acquired over the cardiac cycle and the ECG signal is used to reconstruct images at different phases, providing greater flexibility for assessing different cardiac phases and selected functional information, but potentially resulting in greater radiation exposure.
Temporal Resolution
Temporal resolution is particularly important in cardiac CT. The heart and coronary arteries are moving structures, so faster CT acquisition reduces the time over which motion occurs during image acquisition. Modern cardiac CT systems use technologies designed to improve temporal resolution, including fast gantry rotation, large detector coverage, advanced reconstruction techniques, and ECG synchronisation. Modern CT scanners may have gantry rotation times in the region of 0.2 seconds, although the exact value varies between scanner technologies. A shorter rotation time and appropriate temporal resolution can help reduce motion artefact and improve coronary artery visualisation.
Spatial Resolution and Detector Coverage
High spatial resolution is important because the coronary arteries are relatively small structures. Cardiac CT systems may use large detector arrays, thin detector elements, wide anatomical coverage, and high-resolution acquisition and reconstruction. A wider detector can reduce the number of acquisitions required to cover the heart and can be particularly useful for cardiac imaging. However, detector size, rotation time and temporal resolution vary between CT systems and should not be presented as a single universal scanner specification.
Contrast Enhancement
Coronary CT angiography requires strong and consistent opacification of the coronary arteries. A high-flow iodinated contrast injection is therefore used, with the injection rate selected according to the scanner, contrast concentration, IV access and local protocol. Accurate contrast timing is essential to obtain adequate coronary artery enhancement.
Coronary Calcium Scoring
Coronary artery calcium scoring is a non-contrast CT examination used to detect and quantify calcified plaque within the coronary arteries. The Agatston score is a widely used method for quantifying coronary artery calcium. Calcium scoring can also provide calcium mass or other quantitative measures depending on the technique and software. The purpose of coronary calcium scoring includes quantifying the burden of coronary artery calcification, supporting cardiovascular risk assessment, identifying patients with coronary calcification, and helping determine the need for further cardiovascular assessment in appropriate clinical pathways. Calcium scoring may also be used as part of the clinical decision-making process regarding whether a patient should proceed to further coronary assessment, including coronary CT angiography, depending on the clinical indication and local pathway.
Coronary CT Angiography
Coronary CT angiography (CCTA) provides detailed anatomical assessment of the coronary arteries following intravenous contrast administration. It can demonstrate coronary artery anatomy, coronary artery narrowing, atherosclerotic plaque, coronary calcification, coronary artery anomalies, and selected postoperative or post-interventional anatomy.
Major Clinical Applications
Cardiac CT may be used for coronary CT angiography, coronary artery calcium scoring, assessment of suspected coronary artery disease, coronary artery anatomical assessment, selected structural cardiac assessment, pre-procedural planning, and selected assessment of cardiac phases and function.
Radiation Dose
Radiation dose optimisation is an important consideration. Prospective ECG-triggered techniques can provide substantially lower radiation exposure in appropriate patients because irradiation is limited to selected portions of the cardiac cycle. Retrospective ECG-gated acquisition can provide information from multiple cardiac phases but may result in higher radiation exposure. Modern scanners may reduce dose through ECG-triggered acquisition, tube current modulation, tube voltage optimisation, iterative or advanced image reconstruction, appropriate scan range, and patient-specific protocol selection. The objective is to obtain diagnostic image quality while maintaining radiation exposure as low as reasonably achievable.
Factors Affecting Image Quality
Cardiac CT image quality may be affected by high heart rate, irregular cardiac rhythm, cardiac motion, respiratory motion, poor ECG signal, heavy coronary artery calcification, metallic artefact, poor contrast enhancement, incorrect contrast timing, and inadequate breath-hold.
Conclusion
Cardiac CT combines high-resolution CT imaging with ECG synchronisation to minimise the effects of cardiac motion. High temporal resolution, appropriate spatial resolution, accurate contrast timing and reliable ECG synchronisation are central to successful cardiac CT. Its major clinical applications include coronary CT angiography, coronary calcium scoring, coronary artery assessment and selected structural and pre-procedural cardiac examinations.