Magnetic resonance imaging (MRI) relies on the intrinsic spin of protons. When protons are placed in a magnetic field, they tend to align their magnetic poles along the axis of the magnetic field. They can also absorb and then re-emit electromagnetic radiation in the form of radiofrequency signals. The nuclei of cells absorb energy from radiofrequency pulses and may resonate from the pulses. This resonance induces orientation to the magnetic field. The frequency of the pulse required to generate resonance of the target is determined by the strength of the magnetic field and the chemical properties of the target.
When the radiofrequency signal is removed, the absorbed energy is released. This energy can be detected and can be used to create images, with the strength of the emission corresponding to the signal intensity of a given area. This signal intensity depends on the concentration of protons and the longitudinal and transverse relaxation times, which are intrinsic properties of the given tissue and depend on the properties of the water molecules within it.
Two relaxation times are important for MRI. The T1 (longitudinal) relaxation time describes the return of protons back to equilibrium after a radiofrequency pulse. The T2 (transverse) relaxation time describes the loss of phase coherence between individual protons immediately after the pulse. Different pulse sequences can be used to enhance the differences between T1 and T2, thus creating image contrast. Sequences with short repetition times (TR) (<800 milliseconds) and short echo times (TE) (<30 milliseconds) are termed T1-weighted sequences. T1-weighted images provide good anatomic detail. Sequences with long TR (>2000 milliseconds) and long TE (>60 milliseconds) are termed T2-weighted sequences. T2-weighted sequences are useful for evaluating pathology. Sequences with intermediate TR (>1000 milliseconds) and short TE (<30 milliseconds) are termed proton density sequences. These provide good anatomic detail and maximal signal-to-noise ratios at the cost of impaired tissue contrast.
In musculoskeletal imaging, suppression of fat signal can often be useful for evaluating pathology. Using the short tau inversion recovery (STIR) technique, the effects of prolonged T1 and T2 relaxation times are cumulative, leading to the suppression of fat signal (“fat saturation”). Fat suppression can also be performed using frequency-selective (chemical) techniques that improve spatial resolution.
Faster imaging techniques such as gradient-recalled echo (GRE) have become popular because they shorten imaging time. With GRE, pulse sequences are performed using variable flip angles of less than 90 degrees, which shortens imaging time since the low flip-angle radiofrequency pulses destroy only a portion of the longitudinal magnetization with each pulse cycle. In musculoskeletal imaging, GRE sequences are useful for imaging ligaments, tendons, and cartilage.
The musculoskeletal system is ideally suited for evaluation by MRI because different tissues have different signal intensities on T1- and T2-weighted images. For example, fat displays high signal intensity on T1-weighted images and intermediate signal intensity on T2-weighted images. Air, cortical bone, ligaments, ...