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The Evolution of Magnetic Resonance Imaging: 3T MRI in Clinical Applications

Until recently, 3 Tesla magnetic resonance imaging (3T MRI) was only used in research applications. However, as MRI technology evolves, 3T MRI studies (as opposed to 1.5T) are increasingly common in the clinical setting. The higher field strength of 3T MRI results in an increase in signal-to-noise ratio, spatialresolution, and speed, all of which may provide substantial benefits. However, radiologists familiar with 3T MRI have cited several limitations to the increased field strength, such as a greater amount of noise, imaging contrast issues, and safety concerns. This article will discuss the present challenges, benefits, and limitations of 3T MRI in the clinical setting in specific clinical applications,including imaging studies of the brain, spine, chest, abdomen, pelvis, extremities, cardiac system, vascular system, and breast.
Introduction
Magnetic resonance imaging (MRI) is an imaging technique that uses a magnetic field and radio waves to image the body. The MRI modality differs from X-ray imaging because MRI does not use iodizing radiation to produce images. The advent of MRI technology hasresulted in considerable medical advances, because clinicians have been able to arrive at more precise diagnoses and provide more focused disease management in many therapeutic areas, including orthopedics, oncology, and neurology. The imaging field is constantly advancing, and radiologists may soon have the option to switch from traditional MRI machines to those that offer greater fieldstrength. As the availability of the stronger 3 Tesla (3T) MRI appears on the horizon, radiologists are faced with the process of weighing the pros and cons of adopting this newer technology. This article will review the benefits and limitations of using 3T MRI in the clinical setting, addressing important issues in specific clinical applications.
A Brief History of MRI
The MRI modality is based on aphysical phenomenon called nuclear magnetic resonance (NMR), which was discovered in 1931 by Isidor Rabi and his colleagues.1 The NMR phenomenon is observed when a substance is placed in a magnetic field and radio waves are applied. As a result of this process, the atoms of the substance will emit tiny, detectable radio signals.2 The strength of the magnetic field is measured in the unit referred toas the Tesla (T). Today's clinical MRI scanners typically operate at a strength of between .35T to 3T. MRI systems in operation today are classified as either low field (.35T), mid field (.5-.7T), high field (1-1.5T), or ultra high field (≥3T). In contrast to MRI field strengths, the strength of the Earth's gravitational pull is approximately 0.00005T. Consequently, a 1T MRI scanner uses amagnetic field that is 20 000 times the gravitational pull of the Earth, and a 3T MRI scanner operates at a strength of 60 000 times the gravitational pull of the Earth.
Magnetic resonance imaging uses a magnetic field and radiofrequencies (RFs) to create images. The RF used in the MRI is determined by the strength of the magnetic field. The Larmor equation determines the frequency of the RF based onthe field strength of the magnetic field. This frequency is directly proportional to the applied magnetic field strength. The Larmor equation is as follows:
ω0 = γ B0
The symbol ω0 represents the angular frequency of the precession of protons in an external magnetic field, the symbol γ is a proportionality constant called the gyromagnetic ratio, and B0 is the strength of the external magneticfield.3
At a magnetic strength of 1T, the proportionality constant or gyromagnetic ratio (γ) is equal to 42.56 MHz. To find the Larmor frequency for different magnetic field strengths, one must use this equation. For example, the Larmor frequency for a 1.5T magnet is 1.5(T)*42.56 (MHz T-1) = 63.8 MHz. For a 3T magnet the Larmor frequency would be 127.6 MHz. Stronger magnetic fields require...