Scanning of axial CE-T1WI and axial CE-FLAIR imaging was started at 2 minutes 40 seconds, and 5 minutes after the injection of contrast material, respectively. Axial CE-FLAIR imaging in all patients was performed immediately after the routine CE-coronal and axial T1WI. The other parameters were as follows: section thickness of 5 mm with a 2 mm gap, field of view of 193 × 220 mm, number of excitations of 2 and the acquisition time was 2 minutes 33 seconds and 2 minutes 42 seconds, respectively. For each patient, the MR imaging was performed using 1.5T scanner (Avanto Siemens Medical Solution, Erlangen, Germany) or 3T scanner (Skyra Siemens Medical Solution, Erlangen, Germany). Postcontrast images were obtained shortly after contrast material administration. In our study, contrast agent (gadobutrol Bayer Healthcare, Berlin, Germany) was administered at the standard dose of 0.1 mmol/kg of body weight. Therefore, the FLAIR sequence should be performed with both pre- and post-contrast scans ( 4, 5, 6). However, in CE-FLAIR imaging alone, the observed hyperintensity lesion may be due to either T2 lengthening or T1 shortening, thus limiting the usefulness of the FLAIR sequence. Therefore, CE-FLAIR images are highly effective in the detection of sulcal or meningeal infection, inflammation and metastases that abut the border of the CSF. In addition, unlike CE-T1WI, CE-FLAIR images do not show contrast enhancement in normal vascular structures and normal meninges ( 4, 5, 14). This indicates that faintly enhancing lesions on CE-T1WI might be depicted more clearly on CE-FLAIR images, but marked enhancing lesions with large Gd accumulation show no enhancement on FLAIR images because the signal-reducing T2 effects obscure the signal-enhancing T1 effects. 1) showed that, the FLAIR sequence was more sensitive to T1 shortening than T1WI at lower concentrations of Gd, while the FLAIR sequence was sensitive to T2 effects at high Gd concentrations. Although Gd concentration alone cannot explain all the phenomena of intracranial enhancement in vivo, our phantom study ( Fig. The differences in enhancement characteristics between CE-T1WI and CE-FLAIR images have been shown in previous studies, and can be explained by a combination of a different T1-shortening effect at a certain concentration of Gd and a different T2 effect according to the vascularity of a lesion ( 5, 10, 12, 13). Although T1WI is typically used for post contrast examinations, CE-FLAIR is increasingly used currently. Contrast enhancement in the CNS is the result of a combination of 3 processes: for intra-axial brain lesions, the blood brain barrier (BBB) must be disrupted for Gd to enter the extracellular space for extra-axial lesions, enhancement is observed in lesions with relatively high vascularity and for leptomeningeal regions, contrast leakage occurs from vessels into the CSF ( 8, 9, 10, 11). However, lesion contrast enhancement is caused predominantly by the T1-shortening effect at clinical doses ( 4, 5, 6, 7). The commonly used contrast agent, Gd, shortens both the T1 and T2 relaxation times of tissues in which it has accumulated. Intravenous magnetic resonance (MR) contrast agents are frequently used to improve lesion detection and characterization of central nervous system (CNS) disorders. Underlying Mechanism of Gadolinium Enhancement Additionally, some distinctive conditions detected following gadolinium (Gd) administration such as the hyperintense acute reperfusion marker (HARM) and Gd encephalopathy related to renal failure will be discussed. This article describes the diagnostic importance of CE-FLAIR imaging for various intracranial pathologic conditions, as well as normally enhancing structures on CE-FLAIR imaging. Many clinical studies have shown that CE-FLAIR offers more information than CE-T1WI alone.
Although FLAIR images are heavily T2-weighted images (T2WI), contrast enhancement on FLAIR imaging is the result of a mild T1 effect that is produced by the long TI thus, lesions that show enhancement on contrast-enhanced T1-weighted imaging (CE-T1WI) also show enhancement on contrast-enhanced FLAIR (CE-FLAIR) images. Fluid-attenuated inversion recovery (FLAIR) is a special inversion recovery pulse sequence with a long repetition time (TR) and echo time (TE), and an inversion time (TI) that effectively nulls signals from the cerebrospinal fluid (CSF) ( 1, 2, 3).