<br><div class="gmail_quote"><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div style="line-height:1.7;font-size:14px;font-family:arial"><div>Hi , professor, I have three questions: </div>
<div>(1) can dtistudio analyze high angular resolution diffusion imaging(hardi) data?</div></div></blockquote><div><br></div><div>Sorry, NO.</div><div> </div><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">
<div style="line-height:1.7;font-size:14px;font-family:arial"><div>(2) can we process hardi data as diffsuion weighted data? Will the data quality be weaken if we do so?</div></div></blockquote><div><br></div><div>Yes, you can process HARDI data just as a plane DTI data. Diffusion "Tensor" Imaging tries to fit the 6-parameter tensor to the observation (DWIs); effectively reducing all the information into the 6 numbers. You theoretically need only 6 DWIs for the calculation but to enhance SNR, 10 - 50 DWIs are routinely acquired. If you acquire a HARDI data with, for example, 128 DWIs, you can use all of them for simple DTI calculation. Then you may think HARDI inherently include DTI data and would provide more by using advanced non-tensor analysis.</div>
<div><br></div><div>This is, in theory, true, but in practice, there is an issue because HARDI requires two things; more directions AND high b-values.</div><div>The former (more direction) is a matter of scanning time. 128 direction should take about 15 min of scanning time.</div>
<div>The latter (higher b-value) is more problematic. Typical DTI uses b = 700-1,000. HARDI usually requires 3,000 - 30,000. </div><div>The high-b-value leads to large signal loss, leading to poorer SNR. It also requires longer TE, which further reduces SNR. It causes more motion artifacts and eddy current distortion, which are difficult to correct because each raw DWI has poor SNR. As a result, HARDI tends to use lower spatial resolution. Modern 3T scanners should provide DTI data with 2-2.2mm resolution with decent SNR with 5 min scan time. I am not sure how long the scan you would need if you want similar resolution and SNR using the HARDI protocol.</div>
<div><br></div><div>Because of this issue, HARDI data is not routinely used as a substitute of regular DTI, which can provide higher resolution and higher SNR with shorter scan time.</div><div> </div><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">
<div style="line-height:1.7;font-size:14px;font-family:arial"><div>(3) What's the measurement of hardi data, FA, ADC or some others?</div><div><br></div></div></blockquote><div>Well , this is the most important question. It has been shown that HARDI can retrieve more anatomical information than DTI. However, I think the community is struggling to provide quantitative numbers out of it. These numbers need to carry anatomical meanings which are different from what DTI can provide. We can think of indices like the number of fiber populations, crossing angles, FA of each population, etc, but I'm not sure how many of them have been adopted in the community. Such indices need to be reasonably robust against noise and hopefully not much dependent on the algorithm or imaging parameters. At this moment, HARDI seems to be used mostly as a prior for advanced tractography, but not for quantitative image analysis.</div>
<div><br></div><div>We have been closely monitoring the development of HARDI-type data acquisition and analysis but haven't decided whether we should be committed to any of the available approaches. At present, spherical deconvolution approach by Tournier et al seems the closest for quantitative analysis and I encourage you to read their papers as well as trying MRtrix software.</div>
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