Technological advances in MRI measurement of brain perfusion

Abstract

Measurement of brain perfusion using arterial spin labeling (ASL) or dynamic susceptibility contrast (DSC) based MRI has many potential important clinical applications. However, the clinical application of perfusion MRI has been limited by a number of factors, including a relatively poor spatial resolution, limited volume coverage, and low signalto-noise ratio (SNR). It is difficult to improve any of these aspects because both ASL and DSC methods require rapid image acquisition. In this report, recent methodological developments are discussed that alleviate some of these limitations and make perfusion MRI more suitable for clinical application. In particular, the availability of high magnetic field strength systems, increased gradient performance, the use of RF coil arrays and parallel imaging, and increasing pulse sequence efficiency allow for increased image acquisition speed and improved SNR. The use of parallel imaging facilitates the trade-off of SNR for increases in spatial resolution. As a demonstration, we obtained DSC and ASL perfusion images at 3.0 T and 7.0 T with multichannel RF coils and parallel imaging, which allowed us to obtain high-quality images with in-plane voxel sizes of 1.5 ϫ 1.5 mm 2 . (2), and diagnosis, treatment planning, and outcome prediction for brain infarction (3). One of the main factors limiting the widespread clinical application of perfusion MRI is its relatively poor spatial resolution. The spatial resolution is primarily limited due to constraints on the signal-to-noise ratio (SNR). An additional constraint is that both arterial spin labeling (ASL) (4) and bolus tracking or dynamic susceptibility contrast (DSC) MRI (5) require rapid image acquisition (i.e., high temporal resolution). In the following we will show how recent methodological developments can increase the temporal and spatial resolution of perfusion MRI of the human brain by improving the SNR and image acquisition speed. SNRThe SNR of perfusion-based MRI is small compared to that achieved by other imaging techniques, such as T 1 /T 2 -weighted and proton density-weighted MRI. In ASL, this is because generally less than 1% of the spins in a given voxel are perfused per second. For an adequate SNR, therefore, ASL requires extensive signal averaging, which leads to long measurement times (typically on the order of 5-10 minutes). In DSC MRI, as a result of the enhancing effect of a contrast injection, a substantially large fraction of the spins contributes to the signal. However, because of the relatively rapid washout of contrast agent (5-10 seconds), SNR of DSC MRI is compromised due to the limited time available for signal averaging. Recent methodological and technical developments have improved the SNR of perfusion MRI in a number of ways, including increased magnetic field strength, the development of multichannel signal detectors, and the optimization of MRI pulse sequences.In MRI the intrinsic SNR scales approximately linearly with the field strength, while at the same time the labeling efficiency of ASL and...

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