Correcting for the echo‐time effect after measuring the cerebral blood flow by arterial spin labeling

Purpose:

To take into account the echo time (TE) influence on arterial spin labeling (ASL) signal when converting it in regional cerebral blood flow (rCBF). Gray matter ASL signal decrease with increasing TE as a consequence of the difference in the apparent transverse relaxation rates between labeled water in capillaries and nonlabeled water in the tissue (δR). We aimed to measure ASL/rCBF changes in different parts of the brain and correct them.

Materials and Methods:

Fifteen participants underwent ASL measurements at TEs of 9.7–30 ms. Decreases in ASL values were localized by statistical parametric mapping. The corrections assessed were a subject‐per‐subject adjustment, an average δR value adjustment, and a two‐compartment model adjustment.

Results:

rCBF decreases associated with increasing TEs were found for gray matter and were corrected using an average δR value of 20 s^?1. Conversely, for white matter, rCBF values increased with increasing TEs (δR = ?23 s^?1).

Conclusion:

Our correction was as good as using a two‐compartment model. However, it must be done separately for the gray and white matter rCBF values because the capillary R values are, respectively, larger and smaller than those of surrounding tissues. J. Magn. Reson. Imaging 2011;. © 2011 Wiley‐Liss, Inc.

     

k‐TE generalized autocalibrating partially parallel acquisition (GRAPPA) for accelerated multiple gradient‐recalled echo (MGRE) R2* mapping in the abdomen

Multiple gradient‐recalled echo (MGRE) methods are commonly used for abdominal R₂* mapping. Accelerated MGRE acquisitions would offer the potential to shorten requisite breathhold times and/or increase spatial resolution and coverage. In both phantom and normal volunteer studies, view‐sharing (VS) methods, generalized autocalibrating partially parallel acquisition (GRAPPA) methods, and newly proposed k–echo time (k‐TE) GRAPPA methods were compared for the purpose of accelerating MGRE acquisitions. Utilization of water‐selective spatial spectral excitation pulses reduced artifact levels for both VS and k‐TE GRAPPA approaches. VS approaches were found to be highly sensitive to off‐resonance effects, particularly at increasing acceleration rates. k‐TE GRAPPA significantly reduced residual artifact levels compared to GRAPPA approaches while improving the accuracy of accelerated abdominal R₂* measurements. These initial feasibility studies demonstrate that k‐TE GRAPPA is an effective method to reduce scan times during abdominal R₂*‐mapping procedures. Magn Reson Med, 2009. © 2008 Wiley‐Liss, Inc.     

Assessment of brain maturation in the preterm infants using diffusion tensor imaging (DTI) and enhanced T2 star weighted angiography (ESWAN)

Purpose

To assess the brain maturation of preterm infants using diffusion tensor imaging (DTI) and enhanced T2 star weighted angiography (ESWAN).

Materials and methods

Conventional magnetic resonance imaging (MRI), DTI and ESWAN were performed in 60 preterm infants and 21 term controls. 60 preterm infants were subgrouped to two groups according to the age at imaging: before and at term-equivalent age (TEA). Fractional anisotropy (FA), apparent diffusion coefficient (ADC) map from DTI, T₂* and R₂* maps from ESWAN were post-processed at an off-line workstation. The values of FA, ADC, T₂* and R₂* from the posterior limb of internal capsule (PLIC), frontal white matter (FWM), occipital white matter (OWM) and lentiform nuclei (LN) were determined. These parameters were compared between preterm and term infants. Correlations of DTI and ESWAN parameters with the gestational age, postmenstrual age and postnatal age were analyzed.

Results

ADCs of FWM, OWM and LN, and T₂* values of the PLIC and LN were higher in the preterm infants at TEA compared with the term controls.The correlations were existed between the postmenstrual age and the values of FA, ADC, T₂*, R₂* from the PLIC, values of ADC, T₂*, R₂* from the LN, T₂* value from the OWM. The correlations were also found between the postnatal age and the values of FA, ADC, T₂* from the PLIC, and T₂* value from the LN.

Conclusion

The maturity of preterm brain around TEA was different from that of term controls and appeared to be independent of the prematurity at birth. T₂* was one of valuable indices to evaluate brain maturation in preterm infants.