Introduction

MPRAGE (1) is commonly used for both pre- and post-contrast imaging to depict the structural anatomy of the brain. While it can provide high resolution, high SNR, and excellent gray/white matter contrast, it does suffer from some downsides for post-contrast imaging. First, the vascular signal is high and can confound visualization of small enhancing lesions. Second, MPRAGE is sensitive to the susceptibility-induced effects of Gadolinium-based contrast agent in the blood, and suffers from signal loss and image distortion as a result. Third, the gray/white matter contrast that is an advantage in pre-contrast images is not always desirable when determining the margins of an enhancing lesion against the brain matter.

T1-weighted 3D fast spin echo imaging with a variable-flip-angle schedule (T1 SPACE; T1-weighted Sampling Perfection with Application optimized Contrasts using different flip angle Evolution) (2-3) has been reported to overcome many of these problems at 3T and allow for better post-contrast visualization of metastatic tumors (4). To our knowledge, post-contrast T1-SPACE imaging at 7T has not yet been reported. The purpose of this work is to report on the feasibility of T1-weighted SPACE imaging for post-contrast brain imaging at 7T and compare to MPRAGE imaging results.

Methods

Imaging was performed on a MAGNETOM Terra 7T system (Siemens Healthcare, Erlangen, Germany) at software version VE12U. All imaging utilized product sequences and was performed with approval from the local institutional review board. Post-contrast imaging was performed in patients (n=3) to assess the utility of both MPRAGE and T1 SPACE for depicting enhancing brain lesions. Because the concentration of Gadolinium in the blood can affect the level of susceptibility-induced artifacts and changes over time after injection, the order of acquisition after contrast injection was varied, with MPRAGE performed first for some patients and second for other. Typical imaging parameters are shown in Table 1.

Results

All imaging was performed successfully. In Figures 1-3, window width and window level were chosen to optimally display the enhancing pathology and were kept constant within each figure. Image results from a patient with a meningioma in the right frontal lobe are shown in Figure 1. In this case, the MPRAGE scan was performed first. Note the improved delineation of the tumor margin on the T1 SPACE images compared to the MPRAGE images. Additionally, the vascular signal is reduced and the susceptibility-induced artifacts are eliminated.

Figure 2 shows images from a patient with a cavernous malformation in the right temporal lobe. In this case, the MPRAGE scan was again performed first. Again, the susceptibility-induced artifacts are reduced and the margins of the malformation are better delineated against the flat gray/white matter contrast in the T1 SPACE. Also of note, the vascular signal in the brain is reduced in the T1 SPACE, making the enhancing malformation more conspicuous.

Images of a midbrain cavernous malformation are shown in Figure 3. In this case, the T1 SPACE was acquired immediately after contrast injection, unlike the other two examples shown in this abstract. Despite the difference in the acquisition order, the T1 SPACE images still show reduced vascular signal, improved margins, and excellent depiction of the internal structure of the malformation.

Discussion

Post-contrast T1 SPACE imaging has a number of advantages over MPRAGE, including reduced vascular signal, a flatter gray/white matter contrast as background for enhancing lesions, and fewer susceptibility-induced artifacts. One disadvantage of using T1 SPACE is reduced signal in the base of the brain and brain stem. This is due to the effect of B1 inhomogeneity that is compounded in T1 SPACE compared to MPRAGE due to the greater number of RF pulses per TR and the higher flip angles of those RF pulses. Nonetheless, the MR signal at the base of the brain appears to be adequate in both sequences, and with the additional vascular suppression offered by the T1 SPACE sequence, enhancing lesions may be visualized in spite of the overall signal loss. This hypothesis will be tested in the future as more patients are recruited.

This study does have some limitations. First, we have yet to encounter any patients with small metastases, so a primary use case of this technique has not yet been tested. Second, the order of acquisition after contrast injection can affect the concentration of Gadolinium in the blood, and thus affect the susceptibility-induced artifacts. As illustrated in Figure 3, order randomization is currently being performed, but more examples are needed to draw a conclusion about the effect it may have on image quality. Third, no radiological evaluation has been performed at this time to determine if the advantages of T1 SPACE will translate to improved diagnosis. Future work will address these limitations through additional patient recruitment and a radiological evaluation.

Conclusion

An optimized T1 SPACE protocol was developed for post-contrast imaging at 7T and compared to the MPRAGE currently used at our institution. T1 SPACE with the parameters described in this work has the advantages of reduced vascular signal, flatter gray/white matter contrast, and reduced susceptibility-induced artifacts compared to MPRAGE post-contrast imaging.

Acknowledgements

No acknowledgement found.