12.2 Cardiac CT — Coronary Angiography, Calcium Scoring, Gating & TAVR

Why This Topic Is Tested

Look closely at how ARRT structures the Chest subcategory outline: cardiac is broken into five separate lettered items (a-e) — coronary artery angiogram, coronary artery calcium scoring, prospective gating, retrospective gating, and TAVR — while every other Chest topic (mediastinum, lungs, airway, chest wall, low-dose lung screening, esophagram) gets only one line each. That level of outline granularity is a direct signal: cardiac CT is the highest-density single topic in the Chest subcategory, and it is tested as a protocol-selection skill, not as cardiology trivia. Expect scenario questions that give you a heart rate, a clinical indication, or a calcium score and ask what acquisition strategy or interpretation follows.

Coronary CT Angiography and Temporal Resolution

Coronary CT angiography (CCTA) images the coronary arteries during a brief, motion-free window of the cardiac cycle, which makes temporal resolution — the time needed to acquire enough data for one reconstructed image — the central technical challenge. Faster gantry rotation directly shortens the acquisition window: with half-scan (partial-scan) reconstruction, a single-source scanner's temporal resolution is approximately half of its gantry rotation time. A worked example: a scanner with a 0.25-second rotation time yields an effective temporal resolution of roughly 125 milliseconds — fast enough to freeze most coronary motion at a controlled heart rate, but not fast enough if the heart rate is elevated or irregular. Dual-source scanners, which use two x-ray tube/detector pairs offset by roughly 90 degrees, can achieve temporal resolution close to a quarter of the rotation time without needing a faster single tube, which is why dual-source systems tolerate higher or less-controlled heart rates better than single-source systems. Because motion blur scales with heart rate, most CCTA protocols target a controlled heart rate under about 65 bpm before scanning, using beta-blockade (e.g., metoprolol, listed in the Patient Care outline's noncontrast medications) to slow and steady the rhythm, plus sublingual nitroglycerin (also a noncontrast medication) to dilate the coronary arteries and improve visualization of small distal vessels.

Prospective vs. Retrospective Gating

This distinction is the single most frequently tested pairing in cardiac CT, and the exam rewards knowing the tradeoff, not just the definition:

A frequent exam trap flips this table: describing a study that captured every cardiac phase and asking which mode was used (retrospective, because only continuous data acquisition permits multiphase reconstruction), or describing a low-dose single-phase acquisition and asking which mode was used (prospective).

Coronary Artery Calcium Scoring

Coronary artery calcium (CAC) scoring quantifies atherosclerotic plaque burden using a noncontrast, prospectively gated, sequential (axial) acquisition, typically at 120 kVp with 3 mm slice thickness. Contrast is deliberately withheld because iodinated contrast raises the CT number of vessel lumens into a range that can be confused with or obscure true calcification, corrupting the score. The Agatston score sums a density-weighted area measurement across every calcified focus (pixels above a 130 HU threshold) in the coronary tree, and the result is interpreted in standard risk bands:

• 0 — no identifiable calcified plaque
• 1-10 — minimal calcification
• 11-100 — mild calcification
• 101-400 — moderate calcification
• Greater than 400 — extensive calcification, high cardiovascular risk; a score in this range can also cause blooming artifact on a subsequent CCTA, where dense calcium appears larger than its true size and can obscure the adjacent lumen

A calcium score is typically obtained before a contrast-enhanced CCTA in the same visit, precisely because a very high score can predict that the CCTA itself will be difficult to interpret due to blooming.

TAVR Planning CT

Transcatheter aortic valve replacement (TAVR) planning is a comprehensive, two-part CT study built to answer two separate clinical questions. The first part is an ECG-gated CTA of the heart and aortic root, used to measure the aortic annulus (both perimeter-derived and area-derived diameters), the sinus of Valsalva and sinotubular junction dimensions, the degree of annular and leaflet calcification, and — critically — the coronary ostial height (the distance from the annular plane to each coronary artery origin). A low coronary ostial height (commonly cited around 10-12 mm or less) raises the risk that the native valve leaflets could be pushed up during deployment and occlude a coronary artery, which can change device selection or add a protective wire strategy. The second part is a non-gated CTA of the chest, abdomen, and pelvis, extending from the aortic arch through the femoral heads, to size and screen the aortoiliofemoral access route for diameter, calcification, and tortuosity — this determines whether the patient is a transfemoral candidate or needs an alternative access site. Skipping either half of this protocol is a common exam distractor; both the annulus/root measurements and the full vascular access run-off are required before valve size and access route can be finalized.

Exam Scenario

A 72-year-old with a heart rate of 58 bpm is scheduled for CCTA, and the scanner's fastest available rotation time is 0.28 seconds. Because the heart rate is already low and regular, a prospectively gated, single-phase acquisition is the dose-appropriate choice — retrospective gating would deliver unnecessary dose for a patient who does not need functional data and whose stable rhythm makes triggering reliable.