Understanding Phase Contrast Imaging for Cardiac MRI Technologists and Its Unique Advantages

Phase contrast imaging is a powerful tool in cardiac MRI that provides unique insights into blood flow and velocity within the heart and vessels. For cardiac MRI technologists, mastering this technique can significantly enhance the diagnostic value of scans and improve patient outcomes. This post explores what phase contrast imaging is, why it stands apart from other sequences like cine imaging or mapping, when to use it, and the best anatomical locations for acquiring these images.

Phase contrast image 3ch inplane shown above.

Magnitude image showing through plane at level of STJ. Videos below showing phase images with and without aliasing.

https://video.wixstatic.com/video/51892d_4140ef750e1b4a28854662a9bcdd397a/720p/mp4/file.mp4

Phase image showing aliasing.

https://video.wixstatic.com/video/51892d_c3e2e4e9916448d79613dba6e6ea3ae1/720p/mp4/file.mp4

Phase image showing no aliasing.

What Is Phase Contrast Imaging?

Phase contrast imaging is a MRI technique that measures the velocity of moving fluids, primarily blood, by encoding velocity information into the phase of the MRI signal. Unlike standard anatomical imaging, which focuses on tissue structure, phase contrast imaging captures dynamic flow data. This allows visualization and quantification of blood flow patterns, direction, and speed.

The method works by applying bipolar gradient pulses that cause moving spins (such as flowing blood) to accumulate phase shifts proportional to their velocity. Stationary tissues do not accumulate this phase shift, enabling clear differentiation between moving and static structures.

Why Phase Contrast Imaging Is Different and Important

Phase contrast imaging offers several advantages that make it essential in cardiac MRI:

• Quantitative Flow Measurement

It provides precise measurements of blood flow volume and velocity, which are critical for assessing valve function, shunts, and vessel patency.

• Directionality of Flow

Unlike cine imaging, which shows motion but not flow direction, phase contrast imaging reveals the exact direction of blood movement, helping identify regurgitation or abnormal flow patterns.

• Non-Invasive Hemodynamic Assessment

It allows evaluation of pressure gradients and flow dynamics without catheterization, reducing patient risk.

• Complementary to Other Sequences

While cine MRI excels at showing cardiac wall motion and function, and mapping sequences provide tissue characterization, phase contrast imaging uniquely quantifies flow, offering a more complete cardiac assessment.

What Information Does Phase Contrast Imaging Provide Compared to Cine and Mapping Sequences?

Phase contrast imaging provides unique quantitative flow data compared to cine and mapping sequences:

• Primary Focus: Quantifies blood flow velocity and volume, while cine imaging visualizes cardiac wall motion and function, and mapping sequences characterize tissue properties (T1, T2)

• Flow Direction: Phase contrast can determine the exact direction of blood flow, while cine and mapping sequences cannot

• Quantitative Measurements: Provides precise flow volume, velocity, and regurgitant fraction calculations, whereas cine offers limited qualitative motion data and mapping provides no flow data

• Hemodynamic Assessment: Enables non-invasive evaluation of pressure gradients and flow dynamics, which cine and mapping sequences cannot provide

• Valve Visualization: Indirectly visualizes valve function through flow patterns, while cine provides direct anatomical visualization

This complementary information makes phase contrast imaging essential for comprehensive cardiac assessment alongside cine and mapping sequences.

Phase contrast imaging is especially useful for evaluating:

• Valve regurgitation severity by measuring backward flow volume.

• Shunts such as atrial or ventricular septal defects by quantifying flow across defects.

• Vessel stenosis by assessing velocity changes.

• Cardiac output by measuring flow in the ascending aorta or pulmonary artery.

  
  

When to Perform Phase Contrast Imaging

Deciding when to include phase contrast imaging depends on clinical questions and initial scan findings. Consider phase contrast imaging when:

• Valve Disease Is Suspected

If cine images show valve abnormalities or regurgitation, phase contrast imaging quantifies the severity.

• Shunt or Congenital Heart Disease

When septal defects or abnormal connections are suspected, phase contrast imaging measures shunt volumes.

• Assessment of Cardiac Output

To calculate stroke volume and cardiac output non-invasively.

• Vascular Pathology

To evaluate flow in the aorta, pulmonary arteries, or veins for stenosis or aneurysms.

• Unexplained Symptoms

When symptoms suggest abnormal flow dynamics not visible on standard imaging.

Technologists should review cine images and clinical history before deciding to add phase contrast sequences. If abnormal flow or valve dysfunction is visible, phase contrast imaging can provide critical quantitative data.

Best Locations for Phase Contrast Imaging Acquisition

Selecting the correct anatomical location is key to obtaining meaningful phase contrast data. Common sites include:

• Ascending Aorta

For measuring cardiac output and aortic valve flow.

• Pulmonary Artery

To assess pulmonary blood flow and valve function.

• Mitral and Aortic Valves

To quantify regurgitation or stenosis.

• Superior and Inferior Vena Cava

For venous flow assessment in complex congenital cases.

• Descending Aorta

To evaluate flow distal to coarctation or other vascular lesions.

Positioning the imaging plane perpendicular to the vessel or flow direction ensures accurate velocity encoding. Technologists should carefully plan slice orientation based on scout images and cine views.

Practical Tips for Cardiac MRI Technologists

• Velocity Encoding (VENC) Settings

Choose VENC values based on expected flow velocities. Too low causes aliasing; too high reduces sensitivity. Common VENC ranges include:

- Ascending/descending aorta: 150-200 cm/s

- Aortic valve (stenosis): 250-400 cm/s

- Mitral valve: 80-120 cm/s

- Pulmonary artery: 80-150 cm/s

- IVC/SVC: 40-80 cm/s

- Great veins: 30-60 cm/s

• Breath-Holding

Use breath-hold sequences when possible to reduce motion artifacts.

• ECG Gating

Synchronize acquisition with cardiac cycle for accurate temporal resolution.

• Quality Checks

Review images for aliasing, misalignment, or artifacts before concluding the scan.

• Communication

Collaborate with radiologists and cardiologists to tailor phase contrast imaging protocols to clinical needs.

Summary

Phase contrast imaging is an indispensable tool in the cardiac MRI technologist's skillset. Unlike cine imaging, which visualizes cardiac anatomy and motion, and mapping sequences, which characterize tissue properties, phase contrast imaging provides unique quantitative data about blood flow velocity, direction, and volume.

For cardiac MRI technologists, understanding when and how to apply phase contrast imaging is crucial for comprehensive patient assessment. This technique excels in evaluating valve regurgitation severity, quantifying shunt volumes in congenital heart disease, measuring cardiac output, and assessing vascular pathology, all without invasive catheterization.

Key technical considerations include selecting appropriate VENC values (ranging from 30-60 cm/s for great veins to 250-400 cm/s for stenotic aortic valves), ensuring proper slice orientation perpendicular to flow, utilizing breath-holding when possible, and maintaining ECG synchronization for temporal accuracy. Always review images for aliasing artifacts before completing the scan.

By integrating phase contrast imaging with cine and mapping sequences, technologists can provide physicians with a complete hemodynamic picture, ultimately improving diagnostic accuracy and patient care. As cardiac MRI continues to evolve, proficiency in phase contrast imaging remains essential for delivering high-quality cardiovascular assessments.