What is chemical shift MRI (Magnetic Resonance Imaging)?

Chemical Shift MRI: Principles and Clinical Applications

Chemical shift MRI is a specialized imaging technique that exploits the difference in resonance frequencies between water and fat protons to differentiate tissues containing only water from those containing both water and fat, enabling accurate characterization of various lesions, particularly adrenal adenomas. 1

Basic Principles

Chemical shift occurs because hydrogen nuclei in different chemical environments (such as water versus fat) experience slightly different magnetic field strengths due to the shielding effects of surrounding electrons. This results in:

• Hydrogen nuclei in water molecules experience a stronger magnetic field and rotate at a higher resonance frequency
• Hydrogen nuclei in fat have more surrounding electrons, experience a weaker field, and rotate at a lower frequency 1

The difference in resonance frequencies between water and fat protons allows MRI to create:

1. In-phase images: Acquired when water and fat signals are aligned, resulting in additive signal intensity
2. Out-of-phase (opposed-phase) images: Acquired when water and fat signals are in opposite directions, causing signal cancellation in voxels containing both water and fat 1

Technical Considerations

For optimal diagnostic accuracy:

• Images should be acquired using the earliest possible echo times
• The opposed-phase echo should be acquired before the in-phase echo
• Both sequences should be obtained during a single breath hold 2
• Fat saturation techniques should be avoided as they interfere with chemical shift imaging 1

Clinical Applications

1. Adrenal Mass Characterization

Chemical shift MRI excels at differentiating benign adrenal adenomas from malignant lesions:

• Adenomas contain intracytoplasmic (microscopic) lipid that causes signal drop on opposed-phase images compared to in-phase images 2
• When performed properly, chemical shift MRI has over 90% specificity for diagnosing adrenal adenomas 3
• Signal intensity loss can be assessed qualitatively (visual assessment) or quantitatively (using signal intensity ratios) 2
• Using spleen as reference tissue, a signal intensity ratio of <0.73 is diagnostic of adenoma with no overlap with non-adenomas 4

The American College of Radiology rates chemical shift MRI as highly appropriate (rating: 8/9) for evaluating indeterminate adrenal masses, particularly when:

• A lesion is identified only on contrast-enhanced CT and further characterization is required
• The mass is discovered incidentally on MRI 1

2. Hepatic Lesion Evaluation

Chemical shift imaging can help identify focal fat deposition or fatty infiltration in the liver that may not be apparent on conventional MRI sequences 5, 6.

3. Renal Mass Characterization

For renal masses:

• The combination of low T2 signal and signal drop on chemical-shift imaging is specific for lipid-poor angiomyolipomas (AMLs) 1
• Chemical shift imaging helps differentiate lipid-poor AMLs from renal cell carcinomas 1

4. Other Applications

• Brain: Aids in diagnosis of lipid-containing lesions such as lipomas, dermoids, and teratomas 5
• Mediastinum: Helps distinguish normal/hyperplastic thymus from thymic tumors and lymphoma in adults 1
• Accentuates visceral margins (e.g., kidney and liver) to improve visualization 5

Limitations and Pitfalls

1. Technical pitfalls:

   • Incorrect order of echo acquisition
   • Not using a single breath-hold
   • Using fat saturation techniques 3

2. Interpretive pitfalls:

   • Some lipid-poor adenomas may be non-diagnostic on chemical shift MRI (CT washout studies may diagnose these) 3
   • Other lesions containing lipid or gross fat may mimic adenomas (e.g., myelolipoma) 3
   • Heterogeneity within a lipid-containing lesion could represent myelolipoma, lipomatous metaplasia, or collision tumor 3

3. Clinical context:

   • Always correlate imaging findings with clinical history and laboratory investigations 3
   • In patients with a history of cancer, adrenal lesions are more likely to be malignant and require more aggressive evaluation 7

Chemical shift MRI represents a powerful diagnostic tool that has evolved from what was initially recognized as an imaging artifact to become an essential technique for tissue characterization, particularly in adrenal imaging where it provides critical information for distinguishing benign from potentially malignant lesions.