High-resolution intracranial MRA at 7T using autocalibrating parallel imaging: initial experience in vascular disease patients
ABSTRACT Greater spatial resolution in intracranial three-dimensional time-of-flight (TOF) magnetic resonance angiography (MRA) is possible at higher field strengths, due to the increased contrast-to-noise ratio (CNR) from the higher signal-to-noise ratio and the improved background suppression. However, at very high fields, spatial resolution is limited in practice by the acquisition time required for sequential phase encoding. In this study, we applied parallel imaging to 7T TOF MRA studies of normal volunteers and patients with vascular disease, in order to obtain very high resolution (0.12 mm(3)) images within a reasonable scan time.
Custom parallel imaging acquisition and reconstruction methods were developed for 7T MRA, based on generalized autocalibrating partially parallel acquisition (GRAPPA). The techniques were compared and applied to studies of seven normal volunteers and three patients with cerebrovascular disease.
The technique produced high resolution studies free from discernible reconstruction artifacts in all subjects and provided excellent depiction of vascular pathology in patients.
7T TOF MRA with parallel imaging is a valuable noninvasive angiographic technique that can attain very high spatial resolution.
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ABSTRACT: Purpose To investigate intracranial microvascular images with transceiver RF coils at ultra-high field 7 T MRI. Materials and Methods We designed several types of RF coils for the study of 7 T MRA and analyzed quantitatively each coil’s performance in terms of the SNR profiles to evaluate the usefulness of RF coils for microvascular imaging applications. We also obtained the microvascular images with different resolutions and parallel imaging technique. Results The overlapped 6-channel transceiver coil exhibited the highest performance for angiographic imaging. Although other multi-channel coils, such as 4- or 8-channel, were also suitable for fast imaging, these coils performed poorly in homogeneity or SNR for angiographic imaging. Furthermore, the 8-channel coil was poor in SNR at the center of the brain, while it had the highest SNR at the periphery. Conclusion The present study has demonstrated that the overlapped 6-channel coil with large-size loop coils provided the best performance for microvascular imaging or angiography with the ultra-high-field 7 T MRI, mainly because of its long penetration depth together with high SNR.Magnetic Resonance Imaging 11/2014; DOI:10.1016/j.mri.2014.07.006 · 2.02 Impact Factor
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ABSTRACT: The visibility of the vasculature in time-of-flight (TOF) magnetic resonance angiography (MRA) highly profits from increased magnetic field strengths. However, the application of additional saturation pulses for suppression of the venous system is often not possible at 7 T; to remain within the regulatory specific absorption rate (SAR) limits, the repetition time (TR) needs to be prolonged, preventing the acquisition of high-resolution MRA data sets within clinically acceptable acquisition times. In this work, saturation pulses were modified regarding flip angle and duration to meet SAR constraints and minimize total measurement time. To ameliorate SAR restrictions, the variable-rate selective excitation (VERSE) algorithm was used for both excitation and saturation radio frequency pulses. In this way, saturation pulses (executed every TR) become applicable in high-resolution TOF MRA protocols but still lengthen total measurement time notably. In this work, saturation pulses were further modified in terms of flip angle and duration to meet SAR constraints and minimize total measurement time. In the considered parameter range for excitation flip angle α of 15° to 35° and TR of 20 ms to 35 ms, sufficient saturation flip angles (αSAT) were 30° to 50°. This work shows that by lowering the flip angle αSAT, saturation pulses can be applied in high-resolution clinical TOF protocols using a TR as short as 20 ms. An αSAT of α + 15° is sufficient for suppression of the venous system in TOF MRA protocols in the parameter range normally used at 7 T. Instead of the standard 90° saturation pulse, only half the flip angle (or even less) is necessary, substantially ameliorating SAR constraints and enabling acquisition of high resolution in acceptable imaging time.Investigative radiology 08/2012; 47(8):445-50. DOI:10.1097/RLI.0b013e31824ef21f · 4.85 Impact Factor
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ABSTRACT: Surgical or endovascular approaches have proved effective for large-vessel diseases over the past decade. However, approaches for small vessel diseases are unlikely to be accomplished by those for large vessels and only few have been applied, because it is hard to access to those small vessels and one could not directly delineate the affected small vessels due to a lack of detection modalities. This study is to examine patients with vascular diseases using ultra-high field 7T MRI with conventional time-of-flight (TOF) sequence, 3D fast low-angle shot (FLASH) gradient-echo. We have evaluated several radio-frequency (RF) coils to find the optimal one for 7T magnetic resonance angiography (MRA), especially for micro-vascular imaging. We have conducted several comparison studies with vascular disease patients. The results showed that micro-vessels such as lenticulostriate arteries in the subjects with risk factors like hypertension or stroke patients were significantly less than in the healthy subjects. 7T MRA images in steno-occlusive patients also showed clearly numerous collateral vessels not visible by 1.5T or 3T MRA. Furthermore, 7T MRA images were comparable to those obtained by digital subtraction angiography (DSA), particularly for micro-vascular imaging. In this article, we would like to share the clinical experiences on 7T MRA that vascular images of 7T MRA were superior to conventional angiography images including 1.5T and 3T MRA, and even comparable to DSA. We also expect that further technical development and clinical applications of 7T MRA would be a clinically important diagnostic tool, in terms of an early detection of the stroke in a totally non-invasive manner. © 2014 Wiley Periodicals, Inc. Int J Imaging Syst Technol, 24, 121–128, 2014International Journal of Imaging Systems and Technology 06/2014; 24(2). DOI:10.1002/ima.22085 · 0.77 Impact Factor