Quantitative susceptibility mapping

Magnetic resonance imaging technique

Quantitative susceptibility mapping (QSM) is an advanced magnetic resonance imaging (MRI) technique used to measure and map the magnetic susceptibility of tissues in the human body. This technique provides valuable insights into the composition and structure of tissues, particularly in the brain, by quantifying the magnetic properties of different substances such as iron, calcium, and myelin.

Principles of QSM

QSM is based on the principle that different tissues and substances within the body have distinct magnetic susceptibilities, which affect the local magnetic field when placed in an external magnetic field, such as that used in MRI. By measuring the phase shifts in MRI signals caused by these local field variations, QSM can reconstruct a map of magnetic susceptibility.

3D visualization of susceptibility distribution

The process of QSM involves several steps:

1. Phase Image Acquisition: MRI scans are performed to acquire phase images, which contain information about the local magnetic field variations.
2. Background Field Removal: The background magnetic field, which is uniform and affects the entire image, is removed to isolate the local field variations.
3. Local Field Calculation: The local magnetic field is calculated from the phase images, providing a map of field variations due to tissue susceptibility.
4. Susceptibility Inversion: Mathematical algorithms are used to invert the local field map to obtain a quantitative susceptibility map.

Applications of QSM

QSM has a wide range of applications in both clinical and research settings. It is particularly useful in the study of neurological disorders, where it can provide insights into the distribution of iron and other paramagnetic substances in the brain.

Local field map used in QSM

Neurological Disorders

QSM is used to study conditions such as multiple sclerosis, Parkinson's disease, and Alzheimer's disease. In multiple sclerosis, QSM can help visualize demyelination and iron deposition in lesions. In Parkinson's disease, it can be used to assess iron accumulation in the substantia nigra, which is associated with the disease's progression.

Vascular Imaging

QSM can also be used to visualize and quantify venous blood oxygenation and to detect microbleeds and calcifications in the brain. This makes it a valuable tool in the assessment of stroke and other vascular conditions.

Iron and calcium distribution in the brain

Technical Challenges

While QSM offers many advantages, it also presents several technical challenges. The inversion of the local field map to obtain susceptibility values is an ill-posed problem, meaning that small errors in the data can lead to large errors in the results. Advanced algorithms and regularization techniques are often required to produce accurate susceptibility maps.

Related pages

• Magnetic resonance imaging
• Multiple sclerosis
• Parkinson's disease
• Alzheimer's disease
• Stroke

Gadolinium-enhanced QSM image

Conclusion

Quantitative susceptibility mapping is a powerful MRI technique that provides unique insights into the magnetic properties of tissues. Its ability to quantify and visualize the distribution of substances like iron and calcium makes it an invaluable tool in both clinical diagnostics and research.