An Echo Shifting Approach for T2Weighted DSCMRI

The T2* decay during the acquisition results in a loss of EPI image resolution (Farzaneh et al. 1992). This effect is stronger for a long acquisition window and a (relatively) short T2*. Extension of the echo train beyond about the T2* value will not increase the resolution any further, in spite of the extra data points acquired as the signal has already decayed. This limits the useful acquisition window to the short T2* values occurring during passage of the bolus. Furthermore, a disadvantage of single shot EPI techniques is the relatively long duration of the data acquisition window, where the entire Fourier domain needs to be acquired within a single repetition time. When motion or mac roscopic susceptibility problems exist, the severity of artifacts is unfortunately related to the repetition time. One method to alleviate these problems is the use of a segmented EPI method with an acquisition window (echo train) that is significantly shorter than the shortest T2* value during bolus passage in order to maintain image resolution throughout the experiment. A high temporal resolution can be maintained by shifting the acquisition into a subsequent TR period. This class of GE echo-shifted MRI results in TE longer than TR by using pulsed gradients flanking the radiofrequency excitation to delay the signal beyond a subsequent excitation, and the resulting PRESTO (principles of echo-shifting with a train of observations) sequence is most conveniently realized as a 3D method, acquiring one complete volume every 1-2 s (Liu et al. 1993)

The effects of the T2* decay during the acquisition on the image can be analyzed using the point spread function (PSF). The PSF shows how signal originating from a single point source is distributed over the actual image, and determines the actual resolution of the image. The wider the PSF, the coarser is the true resolution. Fig. 7.2 demonstrates the PSF for an acquisition period of 50 ms which is typical for a single-

shot EPI method as compared to the 10 ms acquisition period used in the PRESTO method, before (long T2*) and during passage of the bolus (shorter T2*). The results demonstrate the significant loss in resolution in EPI during the bolus passage and the much smaller effect in PRESTO.

The measured frequency shifts close to larger vessels can lead to significant distortions in EPI acquisitions. Limiting the acquisition window as in PRESTO reduces the artifacts from these bolus-induced frequency changes and the blurring effects of the short T2* during bolus passage. This results in an improved image quality, actual resolution, and ultimately more accurate rCBV and delay maps from PRESTO acquisitions (Flacke et al. 2000)

The advantages of the PRESTO sequence are threefold: it allows a reduction of macroscopic susceptibility artifacts, a constant spatial resolution during the passage of the bolus, as well as high temporal resolution (Duyn et al. 1996). In addition, an important advantage is that PRESTO has a more optimal sensitivity and speed, because the gradient echo train is delayed beyond the next excitation pulse (Moonen et al. 1992; Liu et al. 1993; van Gelderen et al. 2000). 3D susceptibility-based perfusion maps have been dem-

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Fig. 7.2a-f. The point spread function (PSF) for the EPI (left) and the PRESTO (right) sequence, before (top row) and during the bolus passage (middle row) including their difference (bottom row), showing the effect of the transient T2* decay due to passage of the bolus on the actual resolution. The horizontal and vertical axes represent voxel number, and signal intensity in arbitrary units, respectively. a,c,e A 50-ms single-shot EPI acquisition. b,d,f A 10-ms PRESTO window. a,b 70-ms T2*; c,d 25-ms T2*. e,f The absolute value of the difference of the PSFs due to the T2* change upon the bolus passage [(a) minus (c) for EPI, and (b) minus (d) for PRESTO, respectively]. The dotted lines indicate the image voxel size as calculated from field-of-view divided by the number of phase encode steps. The relative broadening of the PSF can be expressed as the induced change in area of the PSF within the nominal dimensions of a voxel relative to the total area. For the simulations, changes of 18.7% and 1.8% were found for EPI and PRESTO, respectively. The total intensity of the difference is 44% of the intensity of the PSF without bolus for EPI and 9% for PRESTO. These measurements provide an indication of to what extent signal is displaced during bolus passage for the two methods [see also van Gelderen et al. (2000)]

onstrated in the assessment of acute stroke patients and provided information on perfusion parameters from the entire brain (Fig. 7.3) (Flacke et al. 2000). Although the method is promising, a comparison of obtained rCBV and rCBF needs to be validated against a conventional EPI technique.

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