Pulmonary Embolism & Cardiac CT Protocols

Acquisition settings and contrast timings below are illustrative adult protocol examples. Select the authorized protocol for the indication, scanner, body size, access device, and clinical condition. Treatment decisions belong to the responsible clinical team.

Overview of Thoracic Cardiovascular Imaging

Thoracic cardiovascular computed tomography represents one of the most technologically demanding frontiers in medical imaging. Scanning the heart and pulmonary vasculature requires overcoming extreme physiological motion: the gross respiratory excursion of the diaphragm and thoracic wall, combined with the rapid, violent cycloid contractions of the myocardium ($60\text{ to }100+\text{ beats per minute}$).

Modern cardiovascular CT protocols integrate rapid gantry rotation times ($0.25\text{ to }0.35\text{ seconds}$), submillimeter detector arrays, prospective and retrospective electrocardiographic (ECG) gating, and high-flow dual-syringe contrast dynamics. Technologists must master the physics of scan direction, physiological artifact prevention, right ventricular strain metrics, the Agatston calcium scoring algorithm, and the pharmacotherapy governing Coronary CTA.

---

CT Pulmonary Angiography (CTPA) for Acute Pulmonary Embolism

Acute pulmonary embolism (PE) is a life-threatening cardiovascular emergency resulting from the dislodgement of deep vein thrombi (DVT) that transit into the pulmonary arterial bed, causing acute mechanical vascular obstruction and potential right ventricular failure. CT Pulmonary Angiography (CTPA) is the universally established diagnostic gold standard for acute PE.

Scan direction and complete coverage

Some CTPA protocols scan caudocranially to obtain the motion-prone bases early; others use a validated craniocaudal technique. Neither direction guarantees where motion will occur. Emboli can involve upper or lower lobe arteries, so include and evaluate the complete pulmonary arterial tree. Do not justify missing apical detail with an unsupported claim that 80–90% of significant emboli occur basally. Tailor the breathing instruction and speed to the patient and scanner.

Contrast Injection Protocol & Bolus Optimization

Diagnostic CTPA requires dense, uniform vascular opacification of the pulmonary arterial tree ($\ge 250\text{ to }300\text{ HU}$) to distinguish intraluminal thrombi ($+30\text{ to }+50\text{ HU}$) from surrounding contrast:

• Catheter & Injection Rate: An 18-gauge or 20-gauge peripheral IV catheter placed in an antecubital fossa vein is one possible access choice for an injection rate of $4.0\text{ to }5.0\text{ mL/s}$.
• Contrast Volume: $50\text{ to }75\text{ mL}$ of high-concentration non-ionic contrast ($350\text{--}370\text{ mg I/mL}$).
• The Saline Chaser Mandate: The contrast bolus must be followed immediately by a $30\text{ to }50\text{ mL}$ saline flush injected at the identical high flow rate ($4.0\text{--}5.0\text{ mL/s}$). The saline chaser serves two indispensable physical functions:
  1. It pushes the trailing contrast bolus out of the peripheral and central veins into the pulmonary circulation, maximizing peak arterial attenuation while potentially improving contrast utilization under the chosen protocol.
  2. It washes out hyperdense, undiluted contrast from the right subclavian vein, brachiocephalic vein, and superior vena cava (SVC). This reduces perivenous beam hardening and streak artifacts that radiate directly across the right pulmonary artery, which otherwise causes false-positive interpretations.
• Bolus Tracking Protocol: An automated region of interest (ROI) is positioned in the main pulmonary artery (pulmonary trunk). The scan triggers automatically once the CT number reaches a threshold between $+100\text{ and }+150\text{ HU}$, with a short post-threshold diagnostic delay ($2\text{ to }4\text{ seconds}$) to allow contrast to populate the distal segmental arteries.

---

The Transient Interruption of Contrast (TIC) Artifact

One of the most perplexing and frustrating artifacts in thoracic imaging is the Transient Interruption of Contrast (TIC) artifact, also termed the flow-related contrast dilution artifact:

• Mechanism: When a patient is instructed to "take a deep breath and hold it," an anxious or tachypneic patient often takes a violent, maximal inspiratory gasp. This deep inspiration acts as a physiological bellows, precipitating a massive drop in intrathoracic pleural pressure. The negative pressure draws a sudden surge of unopacified venous blood out of the inferior vena cava (IVC) and splanchnic circulation into the right atrium, washing out and diluting the incoming dense contrast stream entering from the superior vena cava. Consequently, the pulmonary arterial trunk is flooded with unopacified blood during data acquisition, causing vascular attenuation to plummet below $+150\text{ HU}$ and completely invalidating the study despite perfect injector operation.
• Technologist Prevention Protocol: Technologists must coach the patient never to take a deep inspiratory gasp. The breathing instruction should be delivered calmly: "Take a gentle, quiet breath in, and hold your breath." Alternatively, scanning during quiet end-tidal breath-holding or suspended expiration reduces the negative thoracic pressure gradient, which may reduce TIC under an appropriate protocol.

---

Diagnostic Findings of Acute PE & Signs of Right Heart Strain

Intraluminal Filling Defects on CTPA

On thin-section contrast-enhanced images viewed on wide vascular settings (WW 700, WL 100), acute pulmonary emboli manifest as:

1. The "Polo Mint" Sign: A complete cylindrical cross-section of a vessel demonstrating a central, non-enhancing low-attenuation thrombus surrounded by a circumferential ring of bright contrast.
2. Eccentric / Partial Filling Defect: The thrombus abuts the vessel wall, forming acute angles with the wall, surrounded partially by contrast.
3. Complete Luminal Cutoff: Total vascular obstruction with complete non-enhancement of the vessel branch distally.
4. Saddle Embolus: A thrombus lodged across the bifurcation of the main pulmonary artery, extending into both the right and left main pulmonary branches.

CT Markers of Right Ventricular (RV) Strain (Cor Pulmonale)

In acute pulmonary embolism, mortality is driven by acute right ventricular failure resulting from sudden pulmonary vascular afterload. The CT technologist and radiologist must immediately evaluate three cardinal cross-sectional signs of acute RV strain:

• RV/LV diameter ratio: enlargement of the right ventricle relative to the left can suggest strain. A ratio around or above 1 is commonly assessed, but method, preexisting disease and the clinical condition matter. It is not a universal prediction of imminent collapse.
• Septal flattening or leftward bowing: can reflect altered ventricular pressure relationships.
• Reflux into the IVC or hepatic veins: can accompany elevated right-sided pressures, but also depends on injection and other factors. It is not specific to acute PE.

Integrate these signs with symptoms and hemodynamics. Saddle location alone does not define physiologic severity or prescribe thrombolysis.

Coronary Artery Calcium (CAC) Scoring & The Agatston Methodology

Coronary artery calcification is an exquisite, characteristic marker of coronary atherosclerosis. The total quantity of coronary calcium correlates directly with overall atherosclerotic plaque burden and future cardiovascular events.

Standardized calcium-score acquisition

Conventional Agatston scoring uses an unenhanced ECG-synchronized acquisition, usually at 120 kVp with approximately 2.5–3 mm reconstructed sections and a validated kernel. The conventional 130 HU detection threshold and score calibration depend on that technique. Validated alternative acquisition and calibration methods exist; do not transfer the conventional threshold blindly to another voltage or reconstruction. Iodinated blood-pool enhancement confounds conventional calcium scoring, so routine contrast CTA is not a substitute for its dedicated unenhanced acquisition.

The Mathematical Agatston Scoring Formula

To qualify as a calcified coronary atherosclerotic plaque, an anatomical focus must satisfy two criteria:

1. A minimum lesion area of $\ge 1\text{ mm}^2$ (typically at least 3 contiguous pixels);
2. A peak CT x-ray attenuation of $\ge 130\text{ HU}$.

The Agatston score for each individual lesion is calculated by multiplying the lesion area (in $\text{mm}^2$) by a Peak Attenuation Weighting Factor (1 to 4) determined by the highest CT number measured within the lesion:

$$\text{Agatston Score} = \text{Area (mm}^2\text{)} \times \text{Peak Attenuation Factor}$$

The total Agatston Calcium Score (CAC) is the mathematical sum of the scores of all individual lesions identified across the entire coronary tree:

$$\text{Total CAC Score} = \text{Score}_{LM} + \text{Score}_{LAD} + \text{Score}_{LCx} + \text{Score}_{RCA}$$

What the score does and does not establish

The Agatston score combines calcified lesion area with a factor determined by peak attenuation. For example, a 5 mm² lesion with a peak of 250 HU has factor 2 and contributes 10 score units. Add contributions across the prescribed coronary lesions. A score of zero means no coronary calcium detected by that method; it does not prove that there is no noncalcified plaque or no coronary disease.

Increasing calcium burden can refine cardiovascular risk assessment alongside age, symptoms and other risk factors. It does not automatically mandate aspirin, a stress test or one treatment for every patient. Preventive medication decisions belong to the clinical team, and aspirin has a bleeding-risk tradeoff. The technologist ensures standardized acquisition and accurate series identification.

Coronary CT Angiography (CCTA): Clinical Principles & Patient Preparation

Coronary CT Angiography (CCTA) provides high-resolution, non-invasive cross-sectional anatomical evaluation of coronary artery luminal stenosis, plaque morphology, and congenital coronary anomalies. Performing motion-free CCTA requires meticulous pharmacological patient preparation.

Heart-rate preparation and nitroglycerin

A beta-blocker may be ordered to reduce heart rate when appropriate for the scanner and patient. Review baseline pulse, blood pressure, relevant conduction disease, bronchospasm and contraindications under the authorized protocol. A commonly desired low heart rate is not a universal threshold that makes all higher-rate scans impossible; temporal resolution and rhythm matter.

Nitroglycerin may improve coronary luminal visualization. Follow the ordered dose and route, allow the protocol's interval—SCCT guidance generally uses about five minutes—and monitor blood pressure and symptoms. Recent phosphodiesterase-5 inhibitor use and other contraindications require review before administration. Do not describe hypotension as invariably fatal or assume that one list replaces the patient's medication assessment.

Reference: SCCT coronary CTA acquisition guidance.

ECG Gating Architectures: Prospective Triggering vs. Retrospective Gating

1. Prospective ECG triggering: exposure occurs in a selected cardiac window. Supported implementations include sequential axial and selected high-pitch techniques. The window can be diastolic or systolic; padding can provide additional phase flexibility at additional dose. Reduced exposure time can lower dose compared with broad retrospective acquisition, but no universal 70–90% reduction applies. Irregular rhythm can impair timing.
2. Retrospective ECG gating: ECG is recorded during acquisition so data can be reconstructed at selected phases. Supported protocols can provide functional information, but dose depends on pitch, output, phase coverage and ECG-based modulation. A low-output phase may be less suitable for a detailed coronary assessment. The ability to reconstruct a phase depends on available data, not merely on choosing a percentage afterward.

Contrast Protocol for CCTA

• Injection rate: $5.0\text{ to }6.0\text{ mL/s}$ via an 18-gauge IV catheter.
• Contrast volume: $60\text{ to }80\text{ mL}$ of high-concentration iodinated contrast ($350\text{--}370\text{ mg I/mL}$) followed by a $40\text{ to }50\text{ mL}$ saline flush.
• Triphasic Contrast Bolus Protocol: Many advanced centers utilize a triphasic injection protocol: (1) $100\%$ contrast ($50\text{--}60\text{ mL}$); (2) $50/50$ contrast/saline mixture ($20\text{--}30\text{ mL}$); followed by (3) $100\%$ saline flush ($30\text{ mL}$). The $50/50$ blend prevents beam hardening artifacts from dense contrast in the right ventricle while maintaining sufficient right heart attenuation to delineate the Right Coronary Artery (RCA) and interventricular septum.
• Bolus tracking ROI is placed in the ascending aorta, triggering at $+100\text{ to }+150\text{ HU}$.

---

Coronary Artery Anatomy, Stenosis Quantification & Vulnerable Plaque Morphology

Major Coronary Artery Branches

1. Left Main Coronary Artery (LM): Arises from the left coronary sinus of Valsalva and bifurcates into the LAD and LCx.
2. Left Anterior Descending (LAD): Travels in the anterior interventricular groove toward the cardiac apex. Gives off diagonal branches (supplying the anterolateral left ventricular wall) and septal perforators (supplying the anterior two-thirds of the interventricular septum).
3. Left Circumflex (LCx): Travels in the left atrioventricular groove. Gives off obtuse marginal (OM) branches supplying the posterolateral left ventricle.
4. Right Coronary Artery (RCA): Arises from the right coronary sinus, travels in the right atrioventricular groove, gives off the conus branch, sinus node branch, acute marginal branches, and continues to the crux of the heart.

• Coronary Dominance: Defined by the origin of the Posterior Descending Artery (PDA) and posterolateral branches:
  • Right Dominant ($85\%$ of population): PDA originates from the distal RCA.
  • Left Dominant ($10\%$ of population): PDA originates from the distal LCx.
  • Codominant ($5\%$ of population): PDA originates from RCA; posterolateral branch originates from LCx.

CAD-RADS Stenosis Classification

Luminal diameter stenosis is clinically categorized using the Coronary Artery Disease - Reporting and Data System (CAD-RADS):

• CAD-RADS 0: $0\%$ (Documented absence of plaque and stenosis).
• CAD-RADS 1: $1\%\text{ to }24\%$ (Minimal non-obstructive stenosis).
• CAD-RADS 2: $25\%\text{ to }49\%$ (Mild non-obstructive stenosis).
• CAD-RADS 3: $50\%\text{ to }69\%$ (Moderate stenosis; borderline hemodynamic significance).
• CAD-RADS 4A: $70\%\text{ to }99\%$ in single or double vessel; CAD-RADS 4B: Left Main $\ge 50\%$ or three-vessel obstructive stenosis $\ge 70\%$.
• CAD-RADS 5: $100\%$ (Complete total occlusion).

High-Risk Vulnerable Plaque Features

Beyond percentage luminal stenosis, CCTA possesses the unique capability to evaluate qualitative morphological features of vulnerable plaques prone to rupture and acute myocardial infarction:

1. Low-Attenuation Plaque ($<30\text{ HU}$): Represents a large lipid-rich necrotic core.
2. Positive Arterial Remodeling (Remodeling Index $>1.1$): Outward compensatory expansion of the vessel wall to maintain lumen diameter despite massive plaque accumulation.
3. Spotty Microcalcifications ($<3\text{ mm}$): Small, punctate calcium deposits within the fibrous cap (unlike dense, stable sheet-like calcifications).
4. The "Napkin-Ring" Sign: A central low-attenuation plaque core surrounded by a high-attenuation peripheral ring, reflecting a thin fibrous cap overlying a necrotic lipid core.

---

Cardiovascular Protocol Comparison: CTPA vs. CAC vs. CCTA

Reference: CAD-RADS 2.0, including plaque burden and applicable modifiers. A stenosis category alone does not establish physiologic ischemia.