Residency · Residency · Cardiology

Cardiac CT and MRI

Coronary CT Angiography (CCTA)

Technical Aspects

Coronary CT angiography relies on ECG-gated acquisition, using either prospective triggering, which delivers lower radiation and is the preferred approach, or retrospective gating, which provides higher radiation exposure but enables functional analysis of wall motion. Modern scanners achieve spatial resolution of approximately 0.3 to 0.5 mm and temporal resolution of 66 to 175 milliseconds depending on scanner technology. Heart rate optimization is essential for image quality, targeting a rate below 65 beats per minute through oral metoprolol at 50 to 100 mg administered 1 hour before the scan, with intravenous metoprolol available in the imaging suite if needed. Sublingual nitroglycerin at 0.4 to 0.8 mg is administered immediately before acquisition for coronary vasodilation.

The examination requires iodinated intravenous contrast at volumes of 60 to 80 mL, with bolus tracking to optimize timing of arterial opacification. Contraindications to contrast include contrast allergy, which can be managed with a premedication protocol, and severe renal impairment with an estimated glomerular filtration rate below 30, which is a relative contraindication. Radiation dose has decreased substantially with modern technology. Prospective triggering achieves doses below 3 mSv, and high-pitch spiral acquisition on dual-source scanners can deliver doses below 1 mSv. Iterative reconstruction algorithms further reduce dose requirements.

Clinical Applications

Several landmark trials have validated coronary CT angiography for clinical use. The NICE-RCTA, PROMISE, and SCOT-HEART trials collectively support its use as a first-line test for evaluating stable chest pain. The SCOT-HEART trial notably demonstrated a 41% reduction in myocardial infarction at 5 years with a CCTA-based diagnostic strategy, attributed to more accurate diagnosis leading to more appropriate initiation of preventive therapies. In the acute chest pain setting, the ROMICAT-II and CT-STAT trials demonstrated that CCTA reduces time to diagnosis and cost in patients at low-to-intermediate risk, capitalizing on its exceptionally high negative predictive value exceeding 99%.

For coronary stenosis assessment, CCTA achieves a sensitivity exceeding 95% and specificity of approximately 85% for detecting stenoses greater than 50%. Its high negative predictive value makes it an excellent "rule-out" test for excluding significant coronary artery disease. Limitations include blooming artifact from heavy coronary calcification, which may overestimate stenosis severity, and reduced accuracy in evaluating small-caliber vessels.

CT-Derived FFR (FFRCT)

CT-derived fractional flow reserve applies computational fluid dynamics to standard coronary CT angiography datasets, estimating FFR at each point along the coronary tree without additional imaging, contrast, or pharmacologic stress agents. The HeartFlow FFRCT platform is FDA-cleared and was validated in the NXT trial, which demonstrated diagnostic accuracy of 86% compared to invasive FFR as the reference standard. The PLATFORM trial demonstrated that FFRCT reduced unnecessary invasive angiography by 61%, representing a significant reduction in procedural risk and cost. The primary clinical role of CT-derived FFR is in the identification of hemodynamically significant stenoses noninvasively, with particular value for intermediate stenoses in the 40 to 70% range seen on coronary CT angiography.

Coronary Plaque Characterization

Beyond stenosis assessment, coronary CT angiography enables detailed characterization of plaque morphology and composition. High-risk plaque features include low-attenuation plaque below 30 Hounsfield units, indicating lipid-rich composition, positive remodeling with a remodeling index exceeding 1.1, spotty calcification, and the napkin-ring sign, characterized by a low-attenuation core surrounded by a ring of higher attenuation. The pericoronary fat attenuation index represents an emerging biomarker of coronary inflammation, with values above negative 70 Hounsfield units indicating pericoronary inflammation. The CRISP-CT study demonstrated its potential for identifying vulnerable plaques at risk of future events. Quantitative assessment of total plaque burden, including total stenosis, plaque volume, and composition, enables serial imaging to assess progression or regression with medical therapy.

Non-Coronary CT Applications

Cardiac CT has essential roles beyond coronary evaluation. Transcatheter aortic valve replacement planning relies on CT for annulus sizing using the perimeter-derived diameter, measurement of coronary artery height and sinus of Valsalva dimensions, assessment of left ventricular outflow tract calcification, and evaluation of the access route including iliofemoral caliber, calcification, and tortuosity. Left atrial appendage occlusion planning uses CT to characterize appendage morphology, classified as chicken wing, cactus, windsock, or cauliflower, and to measure the landing zone diameter and depth. Pre-atrial fibrillation ablation CT maps pulmonary vein anatomy including ostial diameters, branching patterns, and anomalies such as common ostia or supernumerary veins, along with left atrial volume and fibrosis assessment.

Aortic assessment by CT provides rapid evaluation of dissection including identification of the intimal flap, true and false lumens, and branch vessel involvement, as well as aneurysm sizing, coarctation anatomy, and intramural hematoma. For pericardial disease, CT is the best modality for detecting pericardial calcification and can assess pericardial thickening exceeding 4 mm, with dynamic imaging evaluating for constrictive physiology.

<image> A comprehensive cardiac CT imaging panel showing four key applications. Panel 1 (top left): CCTA showing a coronary artery cross-section with color-coded plaque characterization -- calcified plaque (white, >350 HU), fibrous plaque (yellow-green, 130-350 HU), and lipid-rich/necrotic core (red, <30 HU), with measurement annotations. A curved multiplanar reformation (MPR) of the LAD showing a mixed plaque causing 70% stenosis, with corresponding FFRCT value of 0.72 displayed as a color map along the vessel (green transitioning to red at the stenosis). Panel 2 (top right): TAVR planning CT showing the aortic annulus in a short-axis view with perimeter measurement (82mm), area measurement (510mm^2), and the derived annular diameter. Include a 3D volume-rendered image of the aortic root showing the coronary ostial heights (left main 14mm, RCA 16mm) measured from the annular plane. Panel 3 (bottom left): CAC scoring image showing calcified deposits in LAD, LCx, and RCA highlighted with colored overlays, with Agatston score calculation (total score 485, with individual vessel scores). Panel 4 (bottom right): CT showing constrictive pericarditis with thickened calcified pericardium (>6mm thickness marked with calipers) surrounding the heart, with arrows pointing to calcification along the right AV groove. Each panel labeled with the application name and key measurements. </image>

Cardiac MRI

Core Sequences and Applications

Cine Imaging (Balanced SSFP)

Balanced steady-state free precession cine imaging represents the gold standard for quantification of ventricular volumes and ejection fraction, providing superior reproducibility compared to echocardiography for both left and right ventricular assessment. Normal values include a left ventricular ejection fraction of 52% or greater in males and 54% or greater in females, indexed left ventricular end-diastolic volume of approximately 74 mL/m squared in males and 66 mL/m squared in females, and a right ventricular ejection fraction of 45% or greater. Wall motion assessment encompasses all 17 left ventricular segments, with the ability to detect subtle regional abnormalities and perform quantitative strain analysis. The standard acquisition protocol includes a short-axis stack from base to apex for volumetric analysis, supplemented by 2-chamber and 4-chamber long-axis views for qualitative assessment and cross-referencing.

LGE Patterns and Diagnostic Significance

Late Gadolinium Enhancement (LGE)

Late gadolinium enhancement imaging is performed 10 to 15 minutes after intravenous gadolinium injection, exploiting the accumulation of contrast in areas of increased extracellular space such as fibrosis, necrosis, or infiltration. Myocardial nulling techniques maximize contrast between normal dark myocardium and abnormal bright tissue.

The distribution pattern of late gadolinium enhancement is the single most diagnostically useful finding for differentiating cardiomyopathy etiology. Ischemic patterns follow a coronary territory distribution with subendocardial or transmural involvement, where the extent of transmural enhancement predicts viability. Enhancement involving more than 50% of wall thickness indicates non-viable myocardium unlikely to recover function with revascularization. Non-ischemic patterns carry specific diagnostic implications: mid-wall enhancement is characteristic of idiopathic dilated cardiomyopathy, present in 30% of cases and predictive of arrhythmias and sudden cardiac death, and healed myocarditis. Subepicardial enhancement suggests active myocarditis, particularly in the inferolateral wall, sarcoidosis, and Anderson-Fabry disease. Diffuse or transmural enhancement occurs in amyloidosis and severe fibrosis. Patchy enhancement is seen in hypertrophic cardiomyopathy at the right ventricular insertion points and areas of maximal hypertrophy, and in sarcoidosis. Right ventricular insertion point enhancement occurs in hypertrophic cardiomyopathy and pulmonary hypertension from pressure overload.

T1 Mapping and Extracellular Volume (ECV)

Native T1 mapping reflects tissue properties including water content, fibrosis, edema, and infiltration. Elevated native T1 values are found in myocarditis, amyloidosis, and fibrosis, while reduced values characterize Fabry disease from lipid storage and iron overload. Extracellular volume fraction, calculated from pre- and post-contrast T1 maps with hematocrit correction, represents the extracellular space fraction. Normal values are approximately 25%, with marked elevation in amyloidosis often exceeding 40 to 50%, diffuse fibrosis, myocarditis, and edema. The clinical utility of these parametric maps lies in their ability to detect diffuse myocardial disease that conventional late gadolinium enhancement may miss, their correlation with amyloid burden for treatment monitoring, and their capacity for serial quantitative assessment.

T2 Mapping/T2-Weighted Imaging

T2-weighted imaging and T2 mapping detect myocardial edema through elevated T2 signal in areas of acute inflammation or injury. Clinical applications include differentiating acute from chronic myocardial infarction, as edema is present only in the acute phase, identifying active myocarditis with concurrent T2 edema and late gadolinium enhancement, and detecting acute sarcoid flares with active granulomatous inflammation. The Modified Lake Louise Criteria for myocarditis require both a T2-based criterion reflecting edema and a T1-based criterion including late gadolinium enhancement or elevated T1 and extracellular volume to achieve high diagnostic accuracy.

T2* Mapping

T2 mapping quantifies cardiac iron content based on the paramagnetic properties of iron, which shorten the T2 relaxation time. A T2 value below 20 milliseconds indicates cardiac iron overload, while values below 10 milliseconds indicate severe overload associated with heart failure and arrhythmia risk. This sequence is essential for managing patients with thalassemia and other transfusion-dependent anemias, hereditary hemochromatosis, and other iron overload states. Serial monitoring guides chelation therapy intensity, with improvement in T2 expected with effective chelation.

Stress CMR

Vasodilator stress cardiac MRI using adenosine or regadenoson enables first-pass perfusion imaging to detect subendocardial perfusion defects indicating ischemia, with simultaneous assessment of both perfusion and wall motion. Diagnostic performance is excellent, with sensitivity of approximately 89% and specificity of approximately 87%, and superior to single-photon emission computed tomography for detecting multivessel disease because balanced ischemia is not missed. The MR-INFORM trial demonstrated that a CMR-guided management strategy was non-inferior to an FFR-guided strategy for clinical outcomes while reducing the need for invasive angiography. Key advantages include the absence of ionizing radiation, excellent spatial resolution, and enhanced sensitivity for subendocardial ischemia compared to nuclear imaging.

Disease-Specific CMR Protocols

Ischemic Heart Disease

In ischemic heart disease, cardiac MRI provides definitive viability assessment through late gadolinium enhancement transmurality analysis. Enhancement involving less than 25% of wall thickness indicates a high likelihood of functional recovery after revascularization, 25 to 50% transmurality represents intermediate likelihood, and enhancement exceeding 50% indicates recovery is unlikely. In acute myocardial infarction, microvascular obstruction appears as a dark zone within the area of late gadolinium enhancement, indicating severe microvascular injury despite epicardial reperfusion and predicting adverse remodeling and worse outcomes. Intramyocardial hemorrhage, detected as a T2* dark zone within edematous myocardium, represents the strongest prognostic marker in acute myocardial infarction.

Cardiomyopathies

For hypertrophic cardiomyopathy, cardiac MRI precisely quantifies wall thickness and mass, assesses left ventricular outflow tract obstruction, and determines late gadolinium enhancement extent. Enhancement involving 15% or more of left ventricular mass constitutes a sudden cardiac death risk factor. Apical hypertrophic cardiomyopathy is better visualized on cardiac MRI than echocardiography. In dilated cardiomyopathy, mid-wall septal late gadolinium enhancement is present in 30% of patients and serves as an independent predictor of ventricular tachycardia and sudden cardiac death, informing implantable defibrillator decisions beyond the ejection fraction threshold of 35% or less alone. Arrhythmogenic right ventricular cardiomyopathy assessment evaluates right ventricular free wall fat infiltration, regional wall motion abnormalities, and fibrofatty replacement through late gadolinium enhancement, with biventricular assessment increasingly recognized as important. The cardiac MRI protocol for amyloidosis demonstrates diffuse late gadolinium enhancement with inability to null the myocardium, elevated native T1, and markedly elevated extracellular volume fraction, as detailed in the amyloidosis chapter. Sarcoidosis evaluation relies on identification of patchy mid-wall or transmural late gadolinium enhancement in a non-coronary distribution, with T2 edema indicating active inflammation that guides immunosuppressive therapy decisions.

Myocarditis

The Modified Lake Louise Criteria for myocarditis diagnosis require T2-based edema criteria combined with T1-based criteria including late gadolinium enhancement or elevated T1 and extracellular volume fraction. When both criteria are present, diagnostic accuracy is high. The subepicardial pattern of late gadolinium enhancement is the classic myocarditis pattern, in contrast to the subendocardial or transmural pattern of myocardial infarction. The non-coronary territory distribution provides an additional differentiating feature.

CMR Safety and Contraindications

Most modern pacemakers and implantable cardioverter-defibrillators are MRI-conditional, and scanning can be performed following manufacturer-specific protocols that address specific absorption rate limits, scan duration restrictions, and post-scan device interrogation. Patients with non-MRI-conditional devices represent a relative contraindication, though scanning can be performed at experienced centers with appropriate precautions when the clinical benefit outweighs the risk, as outlined in the 2017 HRS Expert Consensus. Absolute contraindications include certain metallic implants such as some cochlear implants, specific cerebral aneurysm clips, and metallic foreign bodies in the orbit. Severe claustrophobia may require sedation or general anesthesia.

Gadolinium-based contrast agents should be avoided in patients with an estimated glomerular filtration rate below 30 due to the risk of nephrogenic systemic fibrosis, which is primarily associated with older linear agents. Newer macrocyclic agents such as gadobutrol and gadoterate carry a much lower risk and are preferred when contrast is necessary in patients with chronic kidney disease.

<image> A reference panel showing five classic LGE patterns in cardiac MRI, each displayed as a short-axis LV image. Panel 1 (Ischemic - Subendocardial): bright subendocardial enhancement in the LAD territory (anteroseptal wall) with a clear dark-to-bright transition at the subendocardial border, sparing the epicardium. Labeled "Ischemic MI - subendocardial, follows coronary territory." Panel 2 (Myocarditis - Subepicardial): bright enhancement in the subepicardial (outer) layer of the inferolateral wall, with the subendocardium appearing dark/normal. Labeled "Myocarditis - subepicardial, often inferolateral." Panel 3 (DCM - Mid-Wall): thin linear bright enhancement in the mid-wall of the interventricular septum, sandwiched between dark normal myocardium on both sides. Labeled "Dilated CM - mid-wall septal, predicts VT/SCD." Panel 4 (HCM - RV Insertion): bright spots at the anterior and posterior RV insertion points into the interventricular septum, with patchy enhancement in the hypertrophied septum. Labeled "HCM - RV insertion points + areas of maximal hypertrophy." Panel 5 (Amyloidosis - Diffuse): diffuse transmural or subendocardial enhancement with difficulty nulling the myocardium (myocardium and blood pool appear similar signal intensity). Labeled "Amyloidosis - diffuse, unable to null myocardium." Each panel should be a clearly labeled short-axis view with arrows pointing to the LGE pattern. Use consistent image formatting with the LV cavity in black (nulled) and LGE in bright white. </image>

Choosing Between CT and MRI

When to Choose CT vs MRI

CT Preferred

Cardiac CT is the preferred modality for coronary artery evaluation including stenosis assessment, calcium scoring, and CT angiography for chest pain evaluation. It is essential for pre-procedural planning for transcatheter aortic valve replacement, left atrial appendage occlusion, and electrophysiology procedures. In aortic emergencies including dissection and rupture, CT provides the speed advantage necessary for acute decision-making. CT is superior to MRI for detecting pericardial calcification. Faster acquisition times, often under 10 minutes, make CT more suitable for claustrophobic patients. CT is also the appropriate choice when patients have non-MRI-conditional devices.

MRI Preferred

Cardiac MRI is preferred for tissue characterization, including differentiation of myocarditis from myocardial infarction, cardiomyopathy etiology determination, amyloidosis, sarcoidosis, and iron overload assessment. It is the gold standard for ventricular volume and function quantification of both the left and right ventricles. Viability assessment through late gadolinium enhancement transmurality guides revascularization decisions. Valvular regurgitation can be quantified more accurately through flow measurements than by echocardiography in selected cases. Complex congenital heart disease evaluation benefits from the multiplanar capabilities and shunt quantification possible with MRI. The absence of ionizing radiation makes MRI preferred for young patients and those requiring serial surveillance. In patients with renal impairment, macrocyclic gadolinium agents are generally safer than iodinated contrast used for CT.

Key Clinical Pearls

• CAC = 0 provides a powerful "warranty period" against CV events -- in an asymptomatic intermediate-risk patient, a zero calcium score has a ~10-year event rate < 1% and may justify deferring statin therapy
• CCTA with FFRCT can replace invasive angiography for intermediate coronary lesions -- PLATFORM trial showed 61% reduction in unnecessary invasive procedures; this is a paradigm shift in chest pain evaluation
• The LGE pattern on CMR is the single most useful finding for differentiating cardiomyopathy etiology: subendocardial = ischemic, subepicardial = myocarditis, mid-wall = DCM/sarcoid, diffuse = amyloid, RV insertion = HCM/PH
• Mid-wall LGE in DCM independently predicts VT and SCD even when LVEF is only mildly reduced -- this finding should be incorporated into ICD decision-making beyond the EF <= 35% threshold
• Native T1 mapping can differentiate cardiac amyloidosis (high T1) from Fabry disease (low T1) and iron overload (low T1/T2*) -- these are key discriminators when LVH of unknown etiology is present
• In acute MI, microvascular obstruction (dark zone within LGE) and intramyocardial hemorrhage (dark zone on T2*) are the strongest predictors of adverse LV remodeling -- these findings should prompt more aggressive HF management and closer surveillance

References

• Knuuti J, et al. 2019 ESC Guidelines for the Diagnosis and Management of Chronic Coronary Syndromes. Eur Heart J. 2020;41:407-477.
• Rajiah P, et al. Cardiac CT: Updated Recommendations for Cardiovascular CT. Radiology. 2024.
• Kramer CM, et al. Standardized Cardiovascular Magnetic Resonance Imaging (CMR) Protocols: 2020 Update. JCMR. 2020;22:17.
• Ferreira VM, et al. Cardiovascular Magnetic Resonance in Nonischemic Myocardial Inflammation: Expert Recommendations. JACC. 2018;72:3158-3176.
• SCOT-HEART Investigators. Coronary CT Angiography and 5-Year Risk of Myocardial Infarction. NEJM. 2018;379:924-933.

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