D6 Advanced Imaging

Topic

Advanced imaging provides structural and functional information about the interior of the musculoskeletal system, with spatial and temporal resolution sufficient for quantitative diagnosis. Each advanced imaging modality exploits a distinct physical phenomenon: magnetic resonance imaging (MRI) exploits the precession of atomic nuclei around a static magnetic field; computed tomography (CT) measures the extent to which tissues attenuate an X-ray beam passing through them; high-frequency ultrasound measures the time it takes for an ultrasound pulse reflected within the body to return; and intravital microscopy uses optical fluorescence to image living tissue directly.

In MRI, the body's hydrogen nuclei—which behave like tiny magnets—precess around the equipment's static magnetic field at a frequency specific to the field strength. A radiofrequency pulse emitted at precisely that frequency tips the nuclei's magnetization away from its equilibrium position; the signal emitted by the nuclei as they progressively return to that equilibrium position is what is measured during relaxation. This return to equilibrium occurs according to two distinct time constants specific to each tissue: one describing how quickly magnetization recovers in the direction of the field, and another describing how quickly magnetization is lost in the plane perpendicular to that direction. It is the difference between these two time constants across different tissues that generates image contrast.

A variant of MRI also allows for the measurement—within each image volume element—of how water diffuses through the tissue in various spatial directions. From this measurement, one can derive both the average rate of water diffusion at that point and the degree to which diffusion occurs preferentially in a specific direction rather than being distributed equally in all directions. This second value—the degree of diffusion directionality—quantifies the integrity of muscle and nerve fiber bundles, because water diffuses more easily along intact, aligned fibers than across damaged or disorganized ones.

In the high-frequency ultrasound mentioned earlier, each interface between two distinct tissues reflects a fraction of the ultrasound pulse passing through it; this fraction depends on the difference in acoustic impedance between the tissues on either side of the interface. The resolution with which the system distinguishes structures at varying depths is inversely proportional to the frequency of the ultrasound-emitting transducer: a higher-frequency transducer yields better resolution, but that same higher frequency penetrates less deeply into the tissue.

Ultrasound can also be used to measure tissue stiffness via a technique called elastography: a shear wave is mechanically induced within the tissue, and the wave's propagation speed is measured using ultrasound. This propagation speed is directly proportional to the square root of the tissue's shear stiffness; consequently, stiffer tissues transmit the shear wave faster than soft tissues.

The behavior observed across these advanced imaging modalities confirms that each one detects actual tissue changes: in the MRI variant that tracks fiber bundles, the degree of water diffusion directionality decreases in peripheral nerves affected by neuropathy; in elastography, the shear stiffness of a healthy tendon is ten to one hundred times greater than that of inflamed tissue; and computed tomography, when performed dynamically during locomotion rather than at rest, reveals joint-loading patterns that the same study performed at rest fails to show. Taken together, these three observations explain why advanced imaging succeeds in delivering the quantitative structural and functional information with which this description began.

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