Anatomy for MR Cardiac Stress Imaging (MRCSI), imaging planes and AHA 17-segment model

MRCSI utilizes specific imaging planes and segments to comprehensively evaluate the heart's structure and function. The key anatomical aspects and imaging planes include:

1. Short Axis View:
   

   Fig 2: Figure illustrates MRI heart anatomy and coronary artery distribution in the short-axis view. Images show cross-sectional slices of the heart from basal to apical levels, including chambers (RA, RV, LA, LV) and the papillary muscles (PM). The diagrams correlate the myocardial regions (anterior, lateral, inferior, septal) with coronary artery supply: LAD (left anterior descending), RCA (right coronary artery), and LCX (left circumflex artery). The color-coded rings represent coronary artery territories at different levels (basal, mid, apical). This approach aids in localizing perfusion defects, wall motion abnormalities, and correlating findings with coronary artery disease. Right lower image shows cutting plane
   • Covers basal, mid, and apical levels of the left ventricle
   • Used for stress perfusion imaging and cine imaging
2. Long Axis View:
   

   Fig 3: Figure shows MRI heart anatomy in long-axis views, including 3-chamber, 2-chamber, and 4-chamber views. The 3-chamber view shows the left ventricular (LV) in-and-outflow tract, highlighting the left atrium (LA), LV, aortic valve (Ao), and mitral valve (MV). The 2-chamber view displays the LA, LV, and MV, primarily used to assess the left heart's structure and function. The 4-chamber view includes both atria (RA, LA), both ventricles (RV, LV), and valves (TV, MV), allowing a comprehensive assessment of all chambers. These views are essential for evaluating ventricular function, valvular motion, and chamber morphology.
   • 2-chamber (2ch)
   • 4-chamber (4ch)
   • 3-chamber (3ch)

(AHA) 17-segment model

(AHA) 17-segment model model is used for standardized myocardial segmentation:

• Segments 1-6: Basal short-axis slices
• Segments 7-12: Mid-ventricular short-axis slices
• Segments 13-16: Apical short-axis slices
• Segment 17: Apex (visualized on long-axis images)

This segmentation allows for systematic evaluation of myocardial perfusion, wall motion, and viability across different coronary artery territories.

Key Anatomical structures

During a cardiac stress MRI, several anatomical structures are assessed:

• Left ventricle (LV): Function, volumes, and mass
• Right ventricle (RV): Function and volumes
• Myocardium: Perfusion and viability
• Coronary arteries: Indirectly assessed through perfusion imaging

MRCSI protocol

MRCSI protocol evaluates myocardial ischemia, viability, and function without ionizing radiation. It typically employs vasodilators like adenosine to induce hyperemia, followed by the administration of gadolinium-based contrast agents to visualize myocardial perfusion across various heart segments. 

Vasodilators (e.g., adenosine, dipyridamole, regadenoson): These agents dilate coronary vessels, enhancing blood flow in healthy arteries while revealing ischemic areas (due to stenosis) as regions with less perfusion.

MRCSI with a pharmaceutical stress test, perfusion defects are identified by combining pharmacological stress agents with contrast-enhanced MRI. The process leverages the unique capabilities of MRI to visualize myocardial blood flow and detect areas with impaired perfusion.

Patient Preparation

Fig 7: MRSCI setup involves positioning the patient supine in the MRI scanner with ECG electrodes for cardiac gating and continuous monitoring of heart rhythm. Blood pressure and pulse are tracked via a cuff and oximeter. Adenosine, a vasodilator, is infused intravenously to mimic exercise, while gadolinium contrast is injected through a separate IV line.

• The patient is positioned in the MRI scanner, and ECG leads are placed for cardiac gating to synchronize imaging with the cardiac cycle.

Imaging Workflow

Fig 4: The protocol for MR Cardiac Stress Imaging (MRCSI) involves sequential steps over approximately 40 minutes. It begins with localizer scans to position the heart. Next, adenosine infusion (0.14 mg/kg/min for 3 minutes) is administered to induce stress, followed by myocardial stress perfusion imaging with a short-axis view and gadolinium contrast (0.1 mmol/kg). Optional cine imaging (2-, 3-, or 4-chamber views) is performed for functional assessment. After a 10-minute delay, rest perfusion imaging and viability assessment using late gadolinium enhancement (short-, 2-, 3-, and 4-chamber views) are conducted to evaluate ischemia, perfusion defects, and myocardial viability.

• • A gadolinium-based contrast agent is injected intravenously during the peak effect of the stress agent. Gadolinium highlights blood flow in the myocardium by enhancing the signal in well-perfused areas during first-pass perfusion imaging.

• Stress Imaging

  • Dynamic MRI sequences (T1-weighted fast gradient echo) is used to capture the first pass of gadolinium through the myocardium.
  • Images are taken in short axis plane short axis to assess myocardial perfusion comprehensively.

• Rest Imaging (Baseline Perfusion)

  • After sufficient time has passed for the stress condition to subside, additional gadolinium contrast is administered, and rest perfusion imaging is performed. This provides a comparison to identify reversible ischemia versus scar tissue.

• Analysis for Perfusion Defects

  • Perfusion defects appear as areas with delayed or reduced contrast enhancement during the stress phase compared to the rest phase.
  • A defect seen during stress but not at rest indicates ischemia (reversible perfusion defect).
  • A defect seen during both stress and rest indicates scar tissue or infarction (irreversible perfusion defect).

• Late Gadolinium Enhancement (LGE) Imaging

  • LGE imaging is performed 10–15 minutes after gadolinium injection to assess for myocardial scarring or fibrosis.
  • LGE areas appear bright on T1-weighted images and help distinguish infarction from viable myocardium.

Common Findings

a: MR perfusion scan shows reversible ischemia in septal area (LAD territory)

Fig 5: Stress Perfusion: The top row of images shows reduced perfusion in a region of the myocardium (indicated by arrows) during pharmacological stress, suggesting an area of ischemia. This is often due to stenosis (narrowing) in a coronary artery. Rest Perfusion: The bottom row demonstrates normal perfusion in the same region during rest, which indicates that the reduced perfusion during stress is reversible and likely caused by limited blood flow during increased demand. Followup coronary angiography: The left image shows significant stenosis (narrowing) in the LAD (left anterior descending) artery, corresponding to the ischemic region on the stress perfusion images. Bullseye Plot: The polar map or "bullseye plot" identifies affected segments of the heart and correlates them with the territories of the three coronary arteries (LAD, RCA, and LCX). Here, the LAD territory shows a defect during stress.

b: MR Cine study demonstrates left ventricular aneurysm with contractile abnormality

Fig 6: MR Cine study demonstrates left ventricular aneurysm with contractile abnormality Thin Dyskinetic Wall: The cine MRI shows a segment of the left ventricular wall (marked with red arrows) that is thin and demonstrates dyskinesia (paradoxical outward motion during systole). This finding is characteristic of a ventricular aneurysm. Bulging Outward: The aneurysmal segment bulges outward during systole rather than contracting inward, which contrasts with the normal myocardial contraction seen in other ventricular segments. Loss of Contractility: The affected area shows loss of normal myocardial contractility and is non-functional. Quantitative study confirm impaired left ventricular function (EF: 46.2%) with systolic dysfunction.

c: MR Late Gadolinium Enhancement (LGE) images demonstrate myocardial infarction

Fig 8: Late Gadolinium Enhancement (LGE) highlights areas of fibrosis or scar tissue in the myocardium In normal myocardium, gadolinium contrast washes out quickly, appearing dark. Damaged tissue retains gadolinium longer, appearing bright. The red arrows point to bright areas in the myocardium, indicating infarcted tissue (irreversible damage caused by ischemia). LGE imaging identifies infarction, quantifies scar size, and determines myocardial viability, guiding treatment decisions like revascularization.

d: MR Cine images identify left ventricular thrombus

Fig 9: MRI cine sequences identify a left ventricular (LV) thrombus. Cine MRI provides dynamic, real-time imaging of cardiac motion during the cardiac cycle. The red arrows point to areas within the left ventricle where a thrombus (blood clot) is present. A thrombus appears as a non-moving, hypointense (dark) mass inside the left ventricular cavity, distinct from the surrounding blood flow and myocardium. LV thrombi commonly occur after myocardial infarction or in patients with reduced ejection fraction. They pose a risk for embolism stroke (right lower images) It distinguishes thrombi from other masses (e.g., tumors) based on their lack of enhancement with contrast.

e: LGE images distinguish thrombus from other masses (e.g., tumors) based on their lack of enhancement with contrast

Fig 10: The red arrows point to a hypointense mass within the left ventricle, representing the thrombus. Thrombi lack vascularity and cellular integrity, so they do not take up gadolinium contrast and appear as dark areas on LGE imaging. The green arrows indicate areas of hyperenhancement in the myocardium, consistent with infarction or fibrosis.

Common Technical Challenges and Suggested Solution/Optimization

MRCSI is a powerful diagnostic tool, but it presents several technical challenges. Here are the key challenges and their solutions:

1. Patient respiratory and cardiac motion

Table 1: Table discuss Common Technical Challenges: Motion

• Challenge: Both respiratory motion and cardiac motion can cause blurring and artifacts in images, especially during free-breathing or irregular heart rhythms.
• Suggested Solution/Optimization:
  • Use ECG gating to synchronize image acquisition with the cardiac cycle.
  • Apply motion-correction algorithms (e.g., PSIR HeartFreeze
    

    Fig 12: Phase-Sensitive Inversion Recovery (PSIR) is a post-processing technique designed to enhance contrast between healthy myocardium and areas of myocardial scarring (e.g., fibrosis or infarction). It suppresses normal myocardial signal while preserving the signal from fibrotic tissue, which appears bright after gadolinium contrast administration. Provides higher signal-to-noise ratio (SNR) and improved contrast-to-noise ratio (CNR) compared to standard inversion recovery sequences. HeartFreeze: This feature in Siemens MRI integrates motion correction techniques, making it particularly effective in patients with arrhythmias or difficulty in holding their breath. It freezes cardiac motion by synchronizing with ECG and using robust motion correction algorithms, leading to clearer and more reliable images.
    with free breathing or advanced sequence such as Compressed Sensing
    

    Fig 11: Compressed-Sensing Real-Time Cine: Prospective Gating: Data acquisition is synchronized with the current cardiac cycle, avoiding the need to combine data from multiple heartbeats. Single-Breath-Hold or Free-Breathing: Enables imaging in a single breath-hold or even without breath-holding, making it suitable for patients who cannot hold their breath. Single-Heartbeat Data Acquisition: Captures data in real-time during one cardiac cycle, reducing motion artifacts and errors caused by arrhythmias. Adaptive Triggering: Adjusts to irregular heartbeats, ensuring accurate data capture even in challenging cases.
    to reduce breath-hold time).
  • Use breath-hold techniques or navigator echoes for respiratory motion compensation.

2. ECG gating

Table 2: Table discuss Common Technical Challenges: ECG signal

• Challenge: Disrupted ECG gating and compromise image quality.
• Suggested Solution/Optimization:
  • Use Vector Cardiac Gating (VCG)
  • Always ready peripherial gating (PG)
  • Improve electrode-skin contact

2. Stress Agent Side Effects

• Challenge: Pharmacological agents like adenosine, regadenoson, or dobutamine can cause side effects, such as chest pain, shortness of breath, or arrhythmias.
• Solutions:
  • Continuous monitoring of ECG, blood pressure, and patient symptoms during the test.
  • Have a reversal agent (e.g., aminophylline for adenosine) ready for emergencies.

3. Artifacts in Imaging 

• Challenge: Artifacts such as magnetic susceptibility, chemical shift, or inadequate saturation can obscure important findings. Steady-State Free Precession (SSFP) sequences are more susceptible to metallic artifacts
  

  Fig 13: Metallic artifact caused by a LAD stent done in 3 Tesla MRI
  in cardiac MRI due to their reliance on specific magnetic field properties and their sensitivity to local magnetic field inhomogeneities. 
• Solutions:
  • Use advanced pulse sequences (e.g., SPIR, B1 correction, or shimming) to reduce artifacts.
  • Adjust imaging parameters like field-of-view (FOV), slice thickness, and echo time (TE).
  • Ensure proper patient preparation (e.g., avoiding metallic implants to be scanned in 3 Tesla scanner)

4. Limited Temporal and Spatial Resolution

Table 3: Table discussing Imaging Challenges

• Challenge: Rapid heart rates during stress may reduce temporal resolution, leading to blurred perfusion or cine images.
• Solutions/ Optimisation:
  • Increase the frame rate with faster sequences (e.g., compressed sensing or parallel imaging techniques).
  • Compromise between temperal and spatial resolution 

5. Limited Availability of Expertise and Equipment

Table 4: Table discussing Operator Expertise Challenges

• Challenge: Stress cardiac MRI requires specialized equipment, trained technologists, and experienced radiologists.
• Solutions:
  • Provide additional training for staff in cardiac MRI techniques.
  • Invest in automated or AI-software for easier post-processing and interpretation.

Ongoing advancements in technology, such as motion correction algorithms and AI-driven image processing, are further enhancing the utility of cardiac MRI. Specifically, the development of tools for quantitative Myocardial Reserve Index (MRI)

Fig 14: Figure highlights advancements in software for quantitative evaluation of myocardial perfusion. The top images compare results before and after motion correction, showing significant improvement in image clarity post-correction, essential for accurate analysis. The lower row demonstrates stress and rest perfusion maps using upslope techniques, with the Myocardial Perfusion Reserve Index (MPRI) calculated from the ratio of stress to rest perfusion. Graphs on the right illustrate signal intensity (SI) curves for stress and rest conditions. These advancements enable precise quantification of myocardial blood flow, aiding in the detection of microvascular dysfunction and ischemic heart disease with improved diagnostic accuracy.