What is the protocol for coronary CT (Computed Tomography) angiography, including premedication and potential caveats?

Coronary CT Angiography Protocol

Coronary CTA requires intravenous iodinated contrast (50-160 mL at 4-6 mL/s), premedication with beta-blockers to achieve heart rate ≤60 bpm, and sublingual nitroglycerin for coronary vasodilation, with critical attention to radiation dose optimization and patient-specific contraindications. 1

Contrast Administration Protocol

The standard protocol involves 60-100 mL of iodinated contrast medium (typically 320 mg Iodine/mL for coronary CTA) injected at 4-6 mL/s through an 18-gauge or larger antecubital IV cannula. 1, 2

Contrast Timing and Delivery

• Accurate timing is the major determinant of image quality—use either a test bolus (10-20 mL) to measure time to peak aortic root enhancement, or automated bolus tracking that triggers scanning when aortic enhancement exceeds a predetermined threshold 1
• For CCTA specifically, administer 50-150 mL at 4-7 mL/s with bolus tracking or test bolus technique 2
• Weight-adjusted protocols (1.0 mL/kg with 15-second injection duration) provide more consistent enhancement across different body weights compared to fixed-dose protocols, with significantly better image quality and fewer beam-hardening artifacts 3
• Even reduced volumes (60 mL vs 80 mL) achieve adequate enhancement (>300 HU) in >96% of coronary segments when using 370 mg I/mL contrast 4

Contrast-Related Caveats

• Exclude patients with compromised renal function due to nephrotoxicity risk, as they may subsequently require invasive angiography 1
• Risk of anaphylactoid reactions to iodinated contrast 1
• Do not mix contrast with other drugs or total nutritional admixtures in IV lines 2
• Warm contrast to body or room temperature before administration 2
• Maximum recommended total iodine dose for adults is 80 grams 2

Pharmacological Premedication

Beta-Blocker Administration (Primary Heart Rate Control)

Administer beta-blockers (oral 60-90 minutes before scan or IV immediately before) to achieve heart rate ≤60 bpm, which is essential for minimizing motion artifacts on scanners with temporal resolution ≥250 ms. 1

• Target heart rate depends on scanner temporal resolution:
  • ≤60 bpm for scanners with 250 ms temporal resolution 1
  • ≤75 bpm acceptable for scanners with ≥167 ms temporal resolution 1
  • Up to 90 bpm acceptable for dual-source CT scanners (may not require pharmacological control) 1
• Metoprolol is the beta-blocker of choice, effective in achieving goal heart rate and minimizing heart rate variability 5

Beta-blocker contraindications and cautions:

• Absolute contraindications: hypotension, high-degree AV block 5
• Use with extreme caution in: asthma/obstructive pulmonary disease, decompensated heart failure, vasospastic or vasoocclusive disease 5
• May be contraindicated in aortic stenosis—consult referring cardiologist 6
• For patients with asthma/bronchospastic disease or sympathetically denervated orthotopic heart transplants, consider diltiazem (calcium channel blocker) as alternative 5

Nitroglycerin Administration (Coronary Vasodilation)

Administer sublingual nitroglycerin (0.3-0.4 mg) immediately before scanning to achieve coronary vasodilation and improve image quality, particularly for distal vessel visualization. 1, 6

• Shown to improve image quality in coronary CTA studies 1
• Especially useful for dilating distal arteries to improve stenosis visibility 5

Nitroglycerin contraindications and cautions:

• Absolute contraindications: recent phosphodiesterase inhibitor use (erectile dysfunction medications), severe anemia 6, 5
• Use cautiously in: aortic stenosis or other preload-dependent cardiac pathologies 6, 5
• Can cause systemic hypotension leading to reflex tachycardia and worsening myocardial ischemia 6
• Do NOT routinely administer for non-coronary cardiac CT (endocarditis, device evaluation) in hemodynamically unstable patients 6
• Remove all transdermal nitroglycerin patches from chest before defibrillation to prevent arcing 6

Technical Acquisition Parameters

Patient Preparation

• Hydrate patients before and after contrast administration 2
• ECG monitoring equipment required for gating 7
• Patients must be able to hold breath for 8-20 seconds during acquisition (though high-pitch dual-source CT can be performed during free breathing in patients with heart rate ≤60 bpm without substantial image quality loss) 1, 8
• Exclude patients with irregular cardiac rhythm or heart rate >100 bpm 2

Image Acquisition

• Spatial resolution: 0.4 mm with 64-slice MDCT (submillimeter resolution achieved with 16-320 slice scanners) 1
• Retrospective ECG gating used to reconstruct images during phase of cardiac cycle with least motion 1
• Scan duration: 8-20 seconds depending on scanner generation 1

Critical Caveats and Limitations

Image Quality Limitations

• Heavy coronary calcification causes false-positive findings and may render segments non-evaluable for stenosis assessment 1
• Coronary stents create artifacts that preclude adequate lumen visualization 1
• Motion artifacts from high heart rates or arrhythmias significantly degrade image quality 1
• Poor acoustic windows or respiratory motion can compromise diagnostic quality 1

Radiation Exposure

Coronary CTA exposes patients to considerably higher radiation than standard radiographs, calcium scoring, or invasive angiography (effective dose 10-21 mSv depending on technique and patient sex). 1

• Radiation dose reduction strategies:
  • ECG-controlled tube-current modulation (reduces dose 25-45%) 1
  • Reduce tube current during anterior-posterior vs lateral scanning 1
  • Use lowest dose necessary for adequate visualization 2
  • Adjust tube current/voltage based on patient body size 1
• Particularly concerning in pediatric patients due to higher projected lifetime cancer mortality risk—use bismuth breast shielding in female children (reduces breast exposure 40%) 1
• Inappropriate for very low-risk populations where radiation risk may exceed benefit 1

Patient Selection Exclusions

• Compromised renal function (contrast nephrotoxicity) 1
• Inability to cooperate with breath-holding (though free-breathing protocols exist for low heart rates) 8
• Severe obesity may require increased radiation dose for adequate image quality 1
• Avoid extravasation, especially in patients with severe arterial/venous disease 2