Spy glass: radiological imaging to optimize clinical injury management – Part II

Oakland Athletics left fielder Tony Kemp tags out Los Angeles Angels second baseman Luis Rengifo at second base in the 10th inning at Angel Stadium. Mandatory Credit: Kirby Lee-USA TODAY Sports

Magnetic Resonance Imaging (MRI) is often the gold standard or the final modality of choice in diagnosing sports injuries. This is due to its high image contrast, allowing clinicians to differentiate soft tissues and the associated abnormalities easier than other imaging options. In addition, the increased sensitivity to soft tissue differences and the safety due to the use of non-ionizing radiation are key reasons why MRI has surpassed other imaging methods⁽¹⁾.

As MRI is the imaging gold standard, radiologists play a key role in multi-disciplinary teams involved in managing sports injuries. Clinical insight is important when deciding on the best investigation method according to the clinical question and working diagnosis. This enhances clinical outcomes and return to sports timeframes.

How does it work?

Magnetic Resonance Imaging is a highly scientific technique combining magnetic fields and nuclear magnetic resonance properties of protons. The physics involved is outside of the scope of this article. However, it is important to have an overview as it may help understand why each image’s anatomy looks different.

A patient is placed inside a magnetic field (i.e., the tube), generating a pulse of radio waves. Protons are abundant in biological tissues and absorb radio waves in different amounts depending on the tissue type. The tissues then reemit the signal depending on their properties. The antennas around the patient detect the radio waves emitted, and the signal is generated into an image⁽²⁾. 

A set of images is created, depicting slices through a patient in axial, coronal, or sagittal planes. Each point in an image depends on the micromagnetic properties of the tissue corresponding to that point. As discussed above, MRI relies on the different magnetization properties of tissues to create the image. What this means in practice is that tissues will produce different shades of greyscale, ranging from white to black. Amorphous structures contain mobile protons with fast and rapid molecular motion; thus, the characteristics of the bound water protons affect the signal created, and we can use these properties to create the images we want⁽²⁾.

Different Sequences in MSK

To get different clinical images, radiologists alter the parameters that they use. They do this by manipulating the repetition and echo time of the radiofrequency pulses. The standard sequences used in musculoskeletal MRI include anatomical sequences (T1) and fluid-sensitive sequences (T2 and PD). These are the common acronyms radiologists use when writing reports in practice. The sequences are not exclusive and are usually integrated for a complete clinical picture. For example, anatomical sequences assess for collections, scaring, and artifacts, while fluid-sensitive sequences assess muscle injury and architecture(1). In muscle MRI, the images not only display the connective tissue and muscle fibers but also other structure that are involved. This may produce misunderstandings in their interpretation (see figure 1).

T1 imaging

T1-weighted sequences are useful in assessing hematoma, atrophy, and fatty infiltration and detecting scar tissue in chronic injuries. Scar tissue may present as chemical shift artifacts in a fluid-sensitive sequence. Fat has a characteristic high signal in T1 imaging and will appear bright on the image, while water/CSF will appear dark⁽²⁾.

Proton Density (PD) imaging

Proton Density imaging relies on the differences in the number of magnetized protons per unit volume of tissue. Therefore, tissues with large proton densities, such as fat and CSF, will have a characteristic high signal and appear bright⁽²⁾.

T2 imaging
T2 imaging follows from proton density weighting as it, too, is fluid-sensitive. Compared to T1, inversion of tissue and contrast occurs in the image where CSF is bright; and grey and white matter are reversed. Furthermore, fat appears less bright, as in T1⁽²⁾.

Radiologists add water diffusion sequences and gradient echo sequences in some imaging protocols. Still, they are not part of the most used sequences in general and are not for discussion in the article⁽¹⁾.