<div dir="ltr">The generic table is only a generic table.<div style>In old days when the gradient information is not stored in DICOM files, the generic tables are the only information we had.</div><div style>Usually, the generic table is what the pulse programs for DTI were using inside the scanner. The pulse programs have only one internal generic table. Then you specify patient position (head first / foot first), patient orientation (face up / face down), image orientation (cor/sag/ax), oblique information, etc. Based on these study-specific parameters, the pulse program dynamically generate RF and gradient waveforms. During this dynamic generation of waveforms, the generic table is translated into actual time sequences of the physical X, Y, Z gradients. If you record what actual X, Y, Z gradient do and compared to x, y, z components in the generic gradient table, you may find X, Y, Z and x, y, z are not the same; they could be permutated, the +/- sign of X and x may be flipped, or X, Y, Z is rotated with respect to x, y, z.</div>
<div style><br></div><div style>Therefore, in old days, we have to figure out the "correct" table from the generic table. For example, x, y, z of a old generic gradient table was Y, X, -Z in Philips. In DICOM files, you can find gradient information after a study-specific gradient table is generated. Therefore, X, Y, Z permutation and +/- sign are supposed to be correct. </div>
<div style><br></div><div style>As I mentioned in a previous email, the oblique information is still tricky because you may still have to rotate the gradient table in the DICOM based on the oblique angle.</div></div><div class="gmail_extra">
<br><br><div class="gmail_quote">On Fri, Apr 12, 2013 at 7:48 PM, Santosh Yadav <span dir="ltr"><<a href="mailto:santoshyadav20076@gmail.com" target="_blank">santoshyadav20076@gmail.com</a>></span> wrote:<br><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">
<div dir="ltr"><div><div><div>Thanks Dr. Susumu.<br><br></div>So for taking the exact correct gradient table, for example I should extract gradient table from DICOM files using DTI studio having the option called (Get gradient from DICOM file header) or gradient file from the scanner. Since i have the discrepancy between gradient file obtained from scanner provided by manufacturer and extracted from DICOM file header using DTIstudio. Which one is more accurate?<br>
<br></div>With best Regards,<br><br></div>Santosh<br><div><div><br><br><div><br><br></div></div></div></div><div class="gmail_extra"><br><br><div class="gmail_quote">On Fri, Apr 12, 2013 at 4:10 PM, susumu mori <span dir="ltr"><<a href="mailto:susumu@mri.jhu.edu" target="_blank">susumu@mri.jhu.edu</a>></span> wrote:<br>
<blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div dir="ltr">Hi Santosh,<div><br></div><div>If the gradient table is wrong, all calculation would be wrong.</div><div>
There is a mistake that could be corrected after the calculation but it is only when the +/- sign of entire column (say, y-component column) of the table is wrong. In this case, I think FA/MD are not affected but the vector signs are flipped. So you can flip the sign of the y-components before tracking. This rule is, however, no longer valid when you do gradient table rotation for oblique-imaging or you rotate the image for image registration.</div>
<div> </div></div><div class="gmail_extra"><br><br><div class="gmail_quote"><div><div>On Fri, Apr 12, 2013 at 6:28 PM, Santosh Yadav <span dir="ltr"><<a href="mailto:santoshyadav20076@gmail.com" target="_blank">santoshyadav20076@gmail.com</a>></span> wrote:<br>
</div></div><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div><div><div dir="ltr">
<p style="margin-bottom:12pt;line-height:normal"><span style="font-size:12pt;font-family:"Times New Roman","serif"">Hi Dr. Susumu,</span></p>
<p style="margin-bottom:12pt;line-height:normal"><span style="font-size:12pt;font-family:"Times New Roman","serif"">I have one question regarding gradient table in calculation
of DTI metric and fiber tractography.</span></p>
<p style="margin-bottom:0.0001pt;line-height:normal"><span style="font-size:12pt;font-family:"Times New Roman","serif"">Is gradient table have only effect on
fiber tractography and doesn’t affect ADC and FA values, Is this correct? <br></span></p>
<p style="margin-bottom:0.0001pt;line-height:normal"><span style="font-size:12pt;font-family:"Times New Roman","serif"">With best Regards, <br></span></p>
<p style="margin-bottom:0.0001pt;line-height:normal"><span style="font-size:12pt;font-family:"Times New Roman","serif"">Santosh</span></p>
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