Download Advancements in Multichannel Coils and Parallel Imaging in MRI and more Slides Radiology in PDF only on Docsity! Multichannel Coil (cont.) 15 Advancements in multi-element/multichannel technology (to 32 elements and beyond) will continue to play a role in the development of imaging techniques with higher spatial resolution, faster scan times, and increased diagnostic quality. 1) MCG has an 8-element GE 3T scanner (Sept. 2005) 2) UGA has GE 3T research scanner (8-16 channels โ summer 2006). docsity.com Multichannel Coil (cont.) 16 Advancements in multi-element/multichannel coils: New 96-channel head coil (Wald, MGH) High-field imaging with 8-channel coil docsity.com Parallel Imaging 19 How fast can we go? If we have M coil elements covering the FOV, we can skip up to M-1 lines for each line in k-space we sample. The number of lines โskippedโ: acceleration factor (R). This can be fractional as well: # of phase-encodes to cover k-space R = โโโโโโโโโโโโโโโโโโโโโโโโโ # of phase-encodes used in acquisition Names for acceleration factors: iPAT factor (Siemens) SENSE factor (Philips) ASSET factor (GE) Parallel systems: iPAT (Siemens) SENSE (Philips) ASSET (GE) docsity.com Parallel Imaging 20 Example #1: We have an 8-element phased-array head coil. We want an acquisition matrix of 256 x 256. What is the maximum acceleration factor we can achieve? Answer: If we have M elements, we can skip up to M-1 lines in k-space. So, M=8, and M-1=7. In the case of this acceleration, for each 8 lines within k-space, we are acquiring only 1 of these line. # of phase-encodes to cover k-space R = โโโโโโโโโโโโโโโโโโโโโโโโโ # of phase-encodes used in acquisition = 256 phase โencodes / { (1 acquired line/8 lines) * 256 } = 8 docsity.com Parallel Imaging 21 Increasing acceleration leads to decreasing SNR. However, the benefits may be greater than saving time as well. For EPI images, which are greatly affected by susceptibility differences, parallel imaging can improve geometric distortion and/or image voids. Because the gradients are switching so quickly for an EPI image, one can accrue errors that lead to distortion. These are alleviated using parallel imaging, where the sequence requires less lines in k-space to be read out. docsity.com Parallel Imaging (cont.) 24 Spatial coil sensitivity = function describing the sensitivity of the coil element at any particular position in the FOV. (Ref. #2) C1 Total C2 R L Both types of parallel imaging techniques rely on this function. docsity.com Parallel Imaging (cont.) 25 How is the spatial sensitivity measured? a Method #1: Acquire quick images from each element (a) and reconstruct the full image using all elements (b). Image (a) divided by (b) gives a noisy sensitivity map (c). Filtering smoothes out the noise, yielding our sensitivity map (d). In short, with this method, one must acquire a map before running a parallel imaging sequence. Takes a minute or so. If one uses the summed image from all elements as a reference, this technique is called โAuto-SENSEโ. b c d docsity.com Parallel Imaging (cont.) 26 Method #2: During the parallel scan, we can acquire extra data in the very center of k-space, using the number of phase encodes in this region that we would have used without parallel imaging. Because the center of k-space is responsible for low spatial resolution, this will also give you spatial sensitivity maps for each coil element. (This is the basis of โAUTO-SMASHโ, โVD-AUTO- SMASHโ, and โGRAPPAโ). Key: White=filled part of k-space Black=unfilled k-space docsity.com
