Friday, May 16, 2008

Disk Herniations and the Hereschmuk Sign

Take a look at this picture and tell me what you see:

Did you recognize the Mona Lisa? How much easier this would have been with the appropriate image contrast...

Image contrast is also important in MRI. Findings can be obscure and hidden on one pulse sequence, and obvious on another, simply due to differences in the contrast of the tissues.

Sixty year-old male with two weeks of severe, non-radiating low back pain after a lifting injury:

Axial T1 and T2-weighted images at the L2-3 disk space reveal a broad-based, left-sided far lateral disk protrusion (herniation). Easy to miss, especially if you are reading through a large stack of cases.

Let's examine sagittal images from the same patient:

The abnormal disk (yellow arrows) can be seen on both T2 and STIR images, but note how the presence of soft-tissue edema (red arrows) on the STIR makes it much easier to pick up this abnormality.

It's an excellent idea to include some form of fluid-sensitive, fat-suppressed sequence in orthopedic MRI. The accentuation of soft tissue edema often gives rise to the famed radiologic "Hereschmuck sign", as in "Here schmuck, don't miss this finding".
Two major types of fat suppression are used in MRI:

1) STIR (short-tau inversion recovery)

2) Chemical selective fat suppression- radiofrequency pulses and gradients are used to suppress the signal from fat protons (the chemical species affected by this technique)

Each technique has its advantages and drawbacks. The major appeal of STIR is uniform, robust fat-suppression, while its most important drawback is relatively poor signal. Chemical-selective fat suppression, with its better signal images, depicts anatomic features better, but nonuniform fat suppression can be a problem. For example, here are sagittal STIR and T2 fatsat images of the ankle:

The T2 fatsat image uses chemical-selective fat suppression, and the uniformity of fat suppression has deliberately been disrupted for this example. Normal fatty bone marrow should be dark on these pulse sequences, but the nonuniform fat suppression results in artifactually high signal in the bone marrow
. Compare this with the robust, uniform suppression of the bone marrow on the STIR images (green asterisks). The nonuniform fat suppression on the T2 fatsat image makes it impossible to identify the pathologic marrow edema in the posterosuperior calcaneus (white arrows).

Also note the water suppression that is evident on the T2 fatsat image- the chemical selective pulse intended to suppress signal from fat is instead of suppressing signal from water protons in some areas, including the flexor hallucis longus muscle (red arrows) and cartilage within an enchondroma (pink arrows). This example illustrates the importance of uniform fat suppression in orthopedic MRI. As long as fat suppression is uniform, however, chemical selective fat suppression is typically preferred over STIR, due to superior depiction of anatomic detail.

This case also illustrates the occasional importance of the "scout" images obtained during an MRI. Scout (also known as "localizer") images are obtained by the MRI technologist at the beginning of the MRI examination. These preliminary, large field-of-view images are used to prescribe the MRI sequences used to study the area in question. In some cases, these scout images can give additional information. For example, a coronal scout image from this examination confirmed the presence of the far lateral disk:

Finally, here is another image of the Mona Lisa from the beginning of this post. This version has the appropriate contrast. I think you will agree that the Mona Lisa is a bit easier to see with the proper contrast, just like pathologic findings on MRI:

Vic David MD

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