Reduced Transit-Time Sensitivity in Noninvasive Magnetic Resonance Imaging of Human Cerebral Blood Flow

Abstract

Herein, we present a theoretical framework and experimental methods to more accurately account for transit effects in quantitative human perfusion imaging using endogenous magnetic resonance imaging (MRI) contrast. The theoretical transit time sensitivities of both continuous and pulsed inversion spin tagging experiments are demonstrated. We propose introducing a delay following continuous labeling, and demonstrate theoretically that introduction of a delay dramatically reduces the transit time sensitivity of perfusion imaging. The effects of magnetization transfer saturation on this modified continuous labeling experiment are also derived, and the assumption that the perfusion signal resides entirely within tissue rather than the arterial microvasculature is examined. We present results demonstrating the implementation of the continuous tagging experiment with delay on an echoplanar scanner for measuring cerebral blood flow (CBF) in normal volunteers. By varying the delay, we estimate transit times in the arterial system, values that are necessary for assessing the accuracy of our quantification. The effect of uncertainties in the transit time from the tagging plane to the arterial microvasculature and the transit time to the tissue itself on the accuracy of perfusion quantification is discussed and found to be small in gray matter but still potentially significant in white matter. A novel method for measuring T1, which is fast, insensitive to contamination by cerebrospinal fluid, and compatible with the application of magnetization transfer saturation, is also presented. The methods are combined to produce quantitative maps of resting and hypercarbic CBF.

Arterial spin labelling MRI for assessment of cerebral perfusion in children with moyamoya disease: comparison with dynamic susceptibility contrast MRI

“…32 axial slices were collected with TR = 5500 ms, TE = 25 ms, slice thickness = 5 mm, FOV = 24 cm, and matrix = 128 x 128. The labeling duration was 1500 ms and a post-labeling delay of 1500 ms was used to reduce errors from transit time effects [15]. Vascular crushing gradients were not applied.…”

“…For ASL, quantitative CBF maps were calculated using the method proposed by Wang et al [16] with an additional factor included to correct for incomplete recovery of the magnetization in the reference image due to a saturation pulse applied two seconds before imaging [17]. The perfusion was calculated according to the following equation [15] [17].…”

Quantifying CBF with arterial spin labeling

“…In PASL with QUIPSS II the image is collected at TI 2 , and, as long as TI 2 Ϫ TI 1 Ͼ ⌬t, all of the bolus will be delivered. For CASL, a similar effect is achieved by inserting a delay after the end of the RF tagging pulse and before data collection (4). If this delay is longer than ⌬t, then again all of the bolus is delivered.…”

Towards quantification of blood-flow changes during cognitive task activation using perfusion-based fMRI

“…Quantitative mapping of CBF changes in the primary motor cortex by use of this technique was shown recently (Mildner et al, 2003). This study had utilized a post-label delay (PLD) of the order of the maximum transit time to reduce the transit-time dependence of the CASL signal change (Alsop and Detre, 1996). If short PLDs are employed, functional changes of the transit time additionally contribute to the CASL signal change upon stimulation (Gonzalez-At et al, 2000;Hernandez-Garcia et al, 2004).…”