Numerous investigations have demonstrated various imaging techniques aimed at optimizing contrast-enhanced MR imaging since the introduction of gadolinium contrast agents in the mid-1980s. In particular, the use of enhanced MR imaging for diagnosing meningeal diseases of the brain represented a significant advance over CT, which is very insensitive to this category of abnormalities. The study by Galassi et al in this issue of AJNR is the latest proposal to improve our ability to detect intracranial meningeal diseases. They suggest that for meningeal diseases, contrast-enhanced T1-weighted imaging with fat suppression is superior to enhanced fluid-attenuated inversion recovery (FLAIR) imaging, which has been advocated by several other authors for these diseases.
Enhanced fat-suppressed T1-weighted imaging represents one of many tools that radiologists can use to optimize detection of enhancing abnormalities and that include increased dosage of gadolinium, delayed imaging, magnetization transfer (MT) saturation, and FLAIR sequences. Galassi et al attribute the success of enhanced T1-weighted imaging with fat suppression to the increased dynamic range of gray-scale contrast achieved by suppressing the high signal intensity from scalp and marrow fat. They neglect to mention the MT effects of chemical shift fat-saturation sequences. For example, the standard fat-saturated T1-weighted sequence used in my practice results in approximately 15% background suppression from off-resonance MT effects compared with 30% background suppression from our standard T1-weighted sequence with MT saturation. The weaker MT saturation achieved with their sequence along with the saturation of high signal intensity from fat may actually produce a more visually appealing MT sequence to some radiologists. One of the complaints about MT imaging is that the images are flat and lack anatomic detail (specifically, gray-white differentiation) and that too much enhancement is seen. Intense vascular enhancement, in particular, is a complaint made by many radiologists when viewing standard MT imaging. Less-intense vascular enhancement and slightly better gray-white differentiation should be produced with fat-suppressed T1-weighted imaging compared with sequences with greater MT saturation, and this may be a beneficial compromise that will appeal to many radiologists.
The fact that vascular enhancement is still emphasized on these images may improve the sensitivity for meningeal diseases as mentioned by Galassi et al, but it also may reduce the specificity of this sequence for these abnormalities. (Sensitivity and specificity could not be determined in the study by Galassi et al because only patients with meningeal disease were evaluated.) The fact that FLAIR imaging does not have vascular enhancement may decrease its sensitivity in some series but potentially could increase its specificity. In a given patient, one cannot predict which sequence will best detect contrast enhancement. Even in Galassi et al’s study, enhanced FLAIR imaging was superior to fat-suppressed T1-weighted imaging in approximately 25% of the studies. Enhanced FLAIR imaging may, therefore, have a complementary role in detecting meningeal diseases.
Whether spin-echo T1-weighted imaging, T1-weighted imaging with MT, or, based on Galassi et al’s study, fat-suppressed T1-weighted imaging is used as the primary sequence after contrast injection will be a choice based on personal preferences and compromises between members of a clinical practice. Whatever the choice, we must keep in mind that there are a number of techniques we can use to improve our detection of enhancing abnormalities, including meningeal disease, and that we should be prepared to offer these techniques to our patients and referring clinicians when needed.
- Copyright © American Society of Neuroradiology
