Introducing MRI: Multiecho Spin Echo Imaging (33 of 56) 

+Curt Sagraves - Dr. Lipton responds: Thank you for your questions and feedback! “I understand multiple 180 degree pulses to get different information (different contrast info) at TE. You demonstrated 2 times to make two different images. Each 180 degree pulse and corresponding TE within a certain TR would create information used toward a unique image, correct?” Dr. Lipton: YES “For example, if I were to use 10 180 degree pulses and 10 TE's within a single TR, I would generate image information for 10 unique images. Same slice, different contrast. Correct?” Dr. Lipton: YES “Also, Does not TR contribute to image contrast as a function of T1 recovery?” Dr. Lipton: It does, but TR is the same for every signal collected (i.e., at each TE) and therefore for every image generated in this scenario. Remember that TR is the time from one excitation to the next. In the context of multi-spin echo, the “excitation” is always the same. T1 contrast is the difference in Mz present at the time of excitation (i.e., when the signal - read coherent Mt - is generated). After this time, no additional T1 contrast can be added to the signal. “I ask this because, and I think it would apply to T2 decay also, isn't contrast different all along TR if we sample at different places because of the constant T1 recovery and T2 decay along the timeline of TR?” Dr. Lipton: I think this is mostly covered by the previous answer. The progressive T2 relaxation does create a progressive change in T2 contrast at each subsequent TE. As above, TR is always the same and T1 contrast does not change.

+Albert Einstein College of Medicine Thank you Dr. Lipton: So....I think I'm trying to ask a question that I don't know how to formulate! Let's assume we are acquiring images for, let's say, a head and we start a multi-spin echo sequence to acquire a T2 contrast weighted series of images with 10 TE's within each TR. After the sequence is finished we may end up with 12 axial images of the brain generated by that particular sequence. All of the images look the same, as in each one looks to be similarly T2 weighted. Each TE within the TR of that particular sequence acquires information with different contrast as answered above. I suppose the answer may be that the 10 TE's are just averaged out each time for each slice. Is this the right way of understanding this? So we are, indeed, limited on how many TE's we can acquire within a TR, not only because of time, but because of the average weighting we are trying to acquire, right? Too many TE's on either side of a point may dilute and/or overpower a particular optimum T2 contrast weight? I hope I'm at least slightly coherent with my thoughts on this. I suppose this kind of thought process would apply to any (or most) multi- TE sequence, yes?

just my 2 cents: you will have 12 x 10 = 120 slices. usually, the first 2 echoes (TEs ) are considered proton density weighted, while the remaining 8 echos (TEs) are T2 weighted.

From Dr. Lipton: "Sorry for the long delay in getting back on this one! In multi-echo imaging AND in multi-slice imaging AND when multi-echo and multi-slice are combined, each TE contributes a single line of k-space to a single image per TR. In the following example: Excite Slice #1 >> 180 >> TE-a >> 180 >> TE-b | Excite Slice #2 >> 180 >> TE-a >> 180 >> TE-b | …….TR Excite Slice #1…. --The above is repeated at TR for the number of phase encoding steps required (Np) We will generate a single line of data for the following 4 images: Slice #1/TE-a Slice #1/TE-b Slice #2/TE-a Slice #2/TE-b Slices 1, 2 with TE-b will represent the same anatomy, at greater T2 contrast, compared to Slices 1 and 2 with TE-a. Hope that clarifies."

SS gradients to be applied each time during RF excitation. It doesn't matter which pulse sequence you use (as far as you want to excite a specific area). Same with FE gradients but in terms of spatial distribution of the signal.

From Dr. Lipton: "You are correct that in a properly designed spin echo pulse sequence, where sampling occurs at the moment 2*(TE/2), fat and water will be in phase. This is because the fat/water chemical shift IS a T2’ effect that may be compensated by the spin echo. To achieve out of phase images, the timing of TE would have to be altered. GRE is most widely used for Dixon imaging, but spin echo-based methods have also been created."