Voxel size 0.3 mm isotropic, acceleration factor 7.2, 3 segments, acquisition time 5 min.
Time-of-Flight (TOF) magnetic resonance angiography (MRA) enables non-invasive visualization of blood vessel without intravenous contrast agents. Thus, also patients for whom the use of contrast agents is contraindicated (e.g. due to renal insufficiency) can be examined. In today’s clinical setting, TOF-MRA, used at 1.5 or 3 T, suffers from low spatial resolution (> 0.5 mm) or long acquisition times (TA) (>10 min). The depiction of small vessels would be highly desirable, for example, for examining the vascular component of neurodegenerative diseases such as Alzheimer’s, Parkinson, multiple sclerosis, migraine, and depression.
TOF-MRA benefits multifold from ultra-high magnetic field (UHF) strengths (B0 ≥ 7 T) . UHF MRI results in increased signal-to-noise ratio (SNR) and prolonged T1 relaxation times of brain tissue, which enables higher spatial resolution (< 0.5 mm) and results in improved image contrast, respectively. Larger matrix sizes at 7 T might can moreover enable higher acceleration factors than at 3 T. However, the specific absorption rate (SAR) increases at higher field strength. Thus, energy-intensive pulses for venous saturation limit the choice of parameter settings due to SAR limitations.
To improve acquisition time for TOF imaging, it has already been demonstrated that Compressed Sensing (CS) at 3 T can be used without loss of image quality. We adapted and optimized this method for 7 T. In a cooperation project , additional Variable-Rate Selective Excitation (VERSE) pulses are used for saturation pulses to overcome SAR problems. Segments are applied to the saturation to shorten the acquisition further.
The combination of those independent techniques (VERSE, CS, segments) enables the application of high-resolution (0.3 mm isotropic) TOF-MRA with venous saturation at 7 Tesla in clinical time settings (TA ≈ 5 min, Fig. 1). With these settings, the acquisition time could be reduced by a factor of 2.4 compared to a conventional accelerated sequence. This higher acceleration can be used to acquire ultra-high resolution TOF imaging (< 0.3 mm) in reasonable acquisition times (≈ 10 min, Fig. 2) .
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 Schmitter S, Bock M, Johst S, Auerbach EJ, Ugurbil K, Van de Moortele PF.
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High resolution time-of-flight MR-angiography at 7T exploiting VERSE saturation, compressed sensing and segmentation.
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