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CT Coronary Angiography (CCTA) & CT Calcium Scoring

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Part 1: CT Calcium Scoring (CAC)

What It Is

The Agatston Score

Scores are age-, sex-, and race-dependent, but predict coronary heart disease events independently of standard risk factors (Framingham score). — Goldman-Cecil Medicine, p. 452

Non-Contrast CT: Example Images

Clinical Utility

• Most useful in patients at intermediate Framingham risk — can reclassify them upward or downward
• A CAC = 0 confers excellent prognosis; 12-year ASCVD event rate well below the 7.5% threshold for statin recommendation — making it potentially useful to defer statin therapy
• In women, CAC scoring is especially valuable because global risk scores (e.g., Pooled Cohort Equations) may be imprecise; women with multivessel CAC involvement have nearly a 2-fold higher long-term ASCVD mortality than men with similar scores
• Not useful in already low-risk or high-risk patients (rarely changes management), though very high scores may prompt stress testing

Limitations

• No contrast → tells you how much calcium, not how tight a stenosis is
• Does not correlate the local calcium burden with the physiologic significance of any given stenosis
• High scores in elderly patients and CKD patients can paradoxically limit the usefulness of subsequent CCTA

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Part 2: CT Coronary Angiography (CCTA / CTA)

What It Is

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Acquisition & Technical Requirements

Heart Rate Control

• Coronary arteries exhibit significant translational motion: up to 60 mm/s for the RCA, 20–40 mm/s for the LAD/circumflex
• Image acquisition of <50 milliseconds is required to avoid cardiac motion artifacts
• β-blockers (oral and/or IV) are given pre-scan to:
  • Reduce heart rate and prolong diastolic rest period
  • Reduce premature atrial/ventricular beats

Coronary Vasodilation

• Sublingual nitroglycerin (tablets or spray) is used to maximally dilate coronary vessels immediately before scanning

Contrast & Timing

• Requires IV iodinated contrast (typically 50–100 mL)
• A test bolus or bolus-tracking technique determines the optimal timing of image acquisition relative to peak aortic enhancement
• The scan is performed during a single breath-hold to eliminate respiratory motion

Image Reconstruction Modalities

1. Axial images — standard cross-sectional view
2. Multiplanar reformations (MPR) — coronal, sagittal, oblique planes
3. Maximum-Intensity Projection (MIP) — displays longer vessel segments; limited by overlapping structures
4. Curved MPR — reconstructs the entire vessel in a single plane
5. 3D Volume Rendering — assesses anatomic relationships, selects phases with least motion artifact

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Diagnostic Accuracy

In Acute Coronary Syndromes (ACS)

• Absence of any coronary atherosclerosis (seen in ~50% of ACS-suspected patients) has an excellent Negative Predictive Value (NPV)
• Three landmark RCTs (ACRIN-PA, CT-STAT, ROMICAT II) showed CCTA enables safe rapid discharge in ~50% of chest pain patients, vs. 15–25% with standard care

In Stable CAD

• 64-slice MDCT: pooled sensitivity 94–100%, specificity 89–100%, PPV 93–97%, NPV 96–100%
• Outperforms exercise treadmill, stress MRI, stress nuclear imaging, and stress echo for ACS detection accuracy

Key Limitation

• Presence of significant stenosis on CCTA has only moderate PPV for ACS — especially problematic given low prevalence of ACS (2–8%) in ED chest pain populations
• This can be supplemented by assessment of plaque morphology, wall motion abnormalities, CT perfusion (CTP), or CT-FFR

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Plaque Characterization — High-Risk Features

• Copresence of LAP + PR → 22.5% chance of ACS at 27-month follow-up
• Absence of these features → 0.5% event rate
• At 10-year follow-up: ACS in 23% with two high-risk features vs. 0.6% with no plaque at all
• "Napkin ring" sign + PR + LAP → 40% event rate at 5 years

High-risk plaque features predict ischemia independent of luminal stenosis degree — Fuster & Hurst, p. 140

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CT-FFR (Fractional Flow Reserve by CT)

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Clinical Indications (per 2021 AHA/ACC/ASE/CHEST/SAEM/SCCT/SCMR Guideline)

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Limitations / Contraindications

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Key Comparison: CAC Scoring vs. CCTA

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• Fuster and Hurst's The Heart, 15th Edition (CCTA Acquisition, Accuracy, Plaque Characterization)
• Goldman-Cecil Medicine, International Edition (Calcium Scoring, CTA)
• The Washington Manual of Medical Therapeutics (Coronary CT Angiography, CT-FFR, AHA/ACC Guideline 2021)

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Scanning Protocols: CT Calcium Scoring & CT Coronary Angiography

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PROTOCOL 1: CT Coronary Calcium Scoring (CAC / CACS)

Overview

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Pre-Procedure

Patient Selection / Indications

• Asymptomatic patients at intermediate 10-year ASCVD risk (7.5–20% by Pooled Cohort Equations) where statin decision is uncertain
• Patients requesting additional risk information before deciding on preventive therapy
• Women where global risk scores may underestimate true risk
• Not indicated in: known CAD, very low risk (<7.5%), very high risk (>20%), or symptomatic patients (use CCTA instead)

Patient Preparation

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Scanner Parameters

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Image Acquisition Timing

• Prospective ECG triggering: images acquired at 70–80% of the R-R interval (mid-to-late diastole) — phase of minimal cardiac motion
• If heart rate is irregular or >80 bpm, consider acquiring at 40–50% (end-systole) as a fallback

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Image Analysis & Scoring

• Threshold for calcium: ≥130 HU in an area ≥1 mm²
• Score per lesion = lesion area (mm²) × density factor:
  • 130–199 HU → factor 1
  • 200–299 HU → factor 2
  • 300–399 HU → factor 3
  • ≥400 HU → factor 4
• Total score = sum of all lesion scores across all four coronary vessels (LM, LAD, LCx, RCA)

• Volume score (mm³) — less threshold-dependent than Agatston
• Mass score (mg of calcium) — most reproducible
• Per-vessel scores (LAD score carries highest weight)
• CAC percentile for age/sex/race (MESA reference ranges)

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Reporting Checklist

• Total Agatston score
• Per-vessel scores (LM, LAD, LCx, RCA)
• Volume score and mass score
• MESA percentile for age/sex/race
• Incidental findings on non-cardiac structures (lung nodules, aortic calcification, etc.)

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Limitations

• Quantifies calcium only — no assessment of non-calcified plaque or luminal stenosis
• High inter-scanner variability if non-standard slice thickness used
• Not useful in patients with coronary stents (artifact) or prior CABG
• A CAC of 0 does not exclude the presence of soft (non-calcified) plaque

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PROTOCOL 2: CT Coronary Angiography (CCTA / CTA)

Overview

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Pre-Procedure

Patient Selection / Indications

• Symptomatic patients with low-to-intermediate pre-test probability of obstructive CAD
• Acute chest pain in ED: intermediate-risk patients without known CAD (2021 AHA/ACC Guideline: Class I)
• Equivocal stress test results
• Congenital coronary artery anomalies (excellent technique)
• Pre-operative coronary assessment in selected patients
• Follow-up after coronary bypass surgery (graft patency)

Contraindications

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Patient Preparation

24–48 Hours Before

• Avoid caffeine (coffee, tea, energy drinks) for at least 24 hours — caffeine elevates heart rate and causes ectopic beats
• Stop phosphodiesterase-5 inhibitors (sildenafil, tadalafil) if nitroglycerin will be used (risk of hypotension)
• Metformin: withhold for 48 hours post-procedure if contrast used and renal function borderline

Day of Procedure

Heart Rate Optimization (Critical Step)

• Oral metoprolol 50–100 mg given 60–90 minutes before scan (if HR >65 bpm and no contraindication)
• IV metoprolol 5 mg boluses (up to 15–20 mg) titrated in the scanner room if oral pre-medication insufficient
• Contraindications to β-blockers: severe asthma, significant bradycardia, high-degree AV block, decompensated heart failure, hypotension
• Alternative: ivabradine (heart rate lowering without negative inotropy) in β-blocker contraindicated patients

Coronary Vasodilation

• Sublingual nitroglycerin 0.4–0.8 mg (spray or tablet) administered 3–5 minutes before scanning
• Maximally dilates coronary arteries → improves lumen visualization and diagnostic accuracy
• Hold if SBP <90 mmHg or patient took PDE5 inhibitor within 24–48 hours

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Scanner Parameters

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ECG Gating Modes — Choosing the Right One

Temporal resolution is the single most important technical determinant of CCTA image quality. The reference standard (ICA) has temporal resolution of 5–20 ms; ≤50 ms is desirable for coronary imaging. — Fuster & Hurst's The Heart, 15th ed., p. 137

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Contrast Injection & Timing

1. Test bolus technique: inject 15–20 mL contrast + saline chaser → monitor aortic attenuation over time → calculate time-to-peak → set scan delay accordingly
2. Bolus tracking (SmartPrep / CARE Bolus): ROI placed in descending/ascending aorta; scan triggers automatically when attenuation reaches 100–150 HU threshold above baseline (preferred method)

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Image Reconstruction & Post-Processing

• Mid-diastole (70–75% of R-R): best for most patients with HR <65
• End-systole (35–45% of R-R): better for patients with elevated HR >70

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Stenosis Grading & Plaque Reporting

• Calcified (>130 HU)
• Non-calcified (<30 HU — lipid-rich/necrotic core)
• Mixed (combination)
• High-risk features: Low-attenuation plaque (<30 HU), positive remodeling, spotty calcification, "napkin ring" sign

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Radiation Dose Reduction Strategies

1. Prospective ECG triggering (biggest single reduction)
2. High-pitch dual-source CT when available
3. Tube voltage reduction (100 kVp vs 120 kVp in non-obese) — reduces dose by ~40%
4. Automated tube current modulation
5. Iterative reconstruction algorithms (reduce image noise at lower dose)
6. Careful scan range limitation (only cardiac silhouette)

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Post-Procedure Care

• Monitor patient for 20–30 minutes post-contrast
• Encourage oral hydration (500 mL–1L water) post-procedure
• Nitroglycerin headache counseling (common, resolves in 20–30 min)
• Contrast reaction management protocol on standby
• Renal function re-check at 48–72 hours if borderline eGFR

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Reporting Template (CCTA)

CLINICAL INDICATION:
TECHNIQUE: Retrospective/Prospective ECG-gated CCTA; 
           [X] mL iodinated contrast at [X] mL/sec; 
           Heart rate: [X] bpm; β-blocker given: Y/N; 
           Nitroglycerin given: Y/N

CORONARY DOMINANCE: Right / Left / Co-dominant

CORONARY ARTERIES:
  Left Main (LM): [stenosis %] [plaque type]
  LAD: proximal / mid / distal [stenosis %]
  Diagonal branches: D1, D2
  Left Circumflex (LCx): [stenosis %]
  Obtuse Marginals: OM1, OM2
  RCA: proximal / mid / distal [stenosis %]
  PDA / PLV:

PLAQUE BURDEN: Absent / Mild / Moderate / Severe
HIGH-RISK PLAQUE FEATURES: Present / Absent
  - Low-attenuation plaque: Y/N
  - Positive remodeling: Y/N
  - Spotty calcification: Y/N
  - Napkin ring sign: Y/N

CORONARY CALCIUM SCORE (if non-contrast phase acquired): [Agatston]

CAD-RADS SCORE: [0–5/N] [modifier: +V for vulnerability, +I for ischemia by CTFFR]

CARDIAC STRUCTURES:
  LV function (if retrospective): [visually estimated EF]
  Pericardium: Normal / Abnormal
  Valves: Normal / Abnormal
  Aortic root: [measurement]

EXTRACARDIAC FINDINGS:

IMPRESSION:
RECOMMENDATION:

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Summary Comparison of Both Protocols

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• Fuster and Hurst's The Heart, 15th Edition
• Goldman-Cecil Medicine, International Edition
• The Washington Manual of Medical Therapeutics
• 2021 AHA/ACC/ASE/CHEST/SAEM/SCCT/SCMR Chest Pain Guideline (Class I for CCTA in intermediate-risk acute chest pain)

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Myocardial Imaging by CT & CT Myocardial Perfusion (CTP)

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1. Overview: CT as a Myocardial Imaging Tool

1. Myocardial structure and tissue — wall thickness, masses, thrombus, aneurysm
2. Ventricular function — ejection fraction, wall motion, volumes (LV + RV)
3. Myocardial perfusion — stress CT myocardial perfusion (stress CTP)
4. Coronary-myocardial correlation — combining CCTA anatomy with perfusion physiology

"Stress CT myocardial perfusion may be used for evaluating myocardial perfusion for the detection of functionally significant coronary artery disease." — Grainger & Allison's Diagnostic Radiology

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2. CT Imaging of Myocardial Structure & Function

2.1 Myocardial Tissue Visualisation

2.2 Ventricular Function by CT

• Retrospective ECG gating: images reconstructed at every 5–10% of the R-R interval → allows calculation of:
  • End-diastolic volume (EDV)
  • End-systolic volume (ESV)
  • Ejection fraction (EF)
  • Stroke volume
  • Regional wall motion analysis
• Radiation dose is higher with retrospective gating (12–21 mSv), but can be reduced with ECG-dose modulation
• Low-dose CT data throughout the cardiac cycle can be used if only functional data are needed (not coronary anatomy)

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3. CT Myocardial Perfusion Imaging (Stress CTP)

3.1 Concept and Rationale

Resting myocardial perfusion is normal until luminal diameter narrowing exceeds 90–95%. With maximal coronary hyperemia (pharmacological stress), a progressive decrease in hyperemic response occurs above a threshold of ~50% diameter stenosis. — Fuster & Hurst's The Heart, 15th ed.

• Anatomy (plaque severity, stenosis %) from CCTA
• Physiology (perfusion, flow reserve) from CTP
• In a single examination with iodinated contrast

"Evolving new applications for the adjudication of the hemodynamic significance of stenosis such as CT myocardial perfusion imaging and FFR-CT are now being used in clinical practice." — Fuster & Hurst's The Heart, 15th ed.

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3.2 Types of CT Myocardial Perfusion

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4. Protocol: CT Myocardial Perfusion (CTP)

4.1 Patient Preparation

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4.2 Stress Agents Used in CTP

Vasodilators (First-line)

Dobutamine (Alternative)

• Used in asthma, COPD with bronchospasm, or recent caffeine ingestion
• Protocol: stepwise increments from 5 μg/kg/min up to 40 μg/kg/min, ± atropine if target HR not achieved
• Less ideal because it achieves lower peak coronary blood flow than vasodilators

Contraindications to Vasodilators

• Second- or third-degree AV block
• Sick sinus syndrome (without functioning pacemaker)
• Severe asthma / active bronchospasm
• Systolic BP <90 mmHg
• Caffeine within 12–24 hours

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4.3 Scanner Parameters — Static (First-Pass) CTP Protocol

1. Non-contrast scan → CAC scoring (Agatston score baseline)
2. Stress CTP acquisition → during pharmacological hyperemia
3. Rest CCTA → coronary anatomy 10–15 minutes later (after stress agent wears off and HR stabilises)
4. (Optional) Rest CTP → for comparison with stress to define reversibility

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4.4 Scanner Parameters — Dynamic (4D) CTP Protocol

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5. Image Interpretation in CTP

5.1 Normal vs. Abnormal

• Normal perfusion: uniform myocardial enhancement throughout all segments; attenuation values homogeneous
• Perfusion defect: region of reduced CT attenuation (appears darker/hypodense) compared to surrounding normally perfused myocardium

5.2 Defect Classification

5.3 Quantitative Thresholds (Dynamic CTP)

5.4 17-Segment Model

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6. Combined CCTA + CTP (One-Stop Cardiac CT)

Step 1: Non-contrast scan → CAC Score
Step 2: Pharmacological stress → Stress CTP (functionally significant stenosis?)
Step 3: Wait 10–15 min → Rest CCTA (anatomy: which vessel, stenosis degree, plaque type?)

• CCTA stenosis <50% → low likelihood of hemodynamic significance → no CTP needed
• CCTA stenosis 50–70% (intermediate) → CTP determines if functionally significant
• CTP positive + CCTA significant stenosis → refer for ICA / revascularization
• CTP negative despite stenosis on CCTA → medical management (hemodynamically insignificant)

"In analogy to CMR, CCT can also be used to study myocardial perfusion under stress conditions; in particular, the combination with morphological depiction of the plaque and plaque stenosis severity at coronary CT angiography, this is potentially a very appealing application." — Grainger & Allison's Diagnostic Radiology

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7. Diagnostic Accuracy of Stress CTP

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8. Imaging Markers of Myocardial Viability on CT

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9. Radiation Dose Comparison

• Prospective ECG triggering for rest CCTA component
• 100 kVp or 80 kVp tube voltage
• Weight-based tube current modulation
• Iterative reconstruction algorithms
• Stress-only CTP protocol if rest images not needed

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10. Clinical Indications Summary

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11. Limitations of CT Myocardial Perfusion

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• Fuster and Hurst's The Heart, 15th Edition
• Grainger & Allison's Diagnostic Radiology
• Goldman-Cecil Medicine, International Edition
• Harrison's Principles of Internal Medicine, 22nd Edition