Fwd: Asking for help about QSM

[Yi Wang]

Best regards,

Yi

Begin forwarded message:

From: 关基景
Date: April 25, 2016 at 11:05:46 AM EDT
To:
Subject: Asking for help about QSM

Dear Professor Wang,

  My name is Jijing Guan, and I am a postgraduate of southern medical university in China, majoring in magnetic resonance imaging. I am interested in quantitative susceptiblity mapping after reading papers published by your group. I tried to do Gd phantom validation experiment using your group code downloaded on web, but I failed. So I am writing to ask for help.

  1. Do the material, size and wall thickness of the vials with Gd solution have an impact on the result?

  2. According to Wikipedia, water susceptibility is -9.035ppm but that is approximately -0.02ppm in your group phantom data. I was wondering if it is because QSM can only measure the relative susceptibility, but how to understand the relative susceptibility.

  Thank you very much, and I look forward to hearing from you. 

Sincerely

Jijing Guan

 

[Alexey Dimov]

Hello Jijing,

1. In short - it does matter. 

Vials' material also generates local magnetic field, however, it usually is disregarded under assumption that Gd-related field is much stronger. Also, as a rule these materials do not have signal at conventionally used echo times which further hinders mapping of their susceptibility.

Thus, to minimize role of the containing material one should use thin-walled vials made of low-susceptibility material (something like a falcon tube should work).

2. Yes, you're correct - values in QSM images are relative. Furthermore, such notions as "para-" and "diamagnetism" are also normally defined in literature relatively to water.

It is just a matter of setting a zero level - you still can convert to absolute susceptibilities by adding known shift (e.g., -9 ppm in case you measure susceptibilities relative to water.)

What was the issue with the phantom you made?

Regards,

-- 

Alexey V. Dimov
PhD Student
Department of Biomedical Engineering
Cornell University

e-mail: av...@cornell.edu

phone: (607) 793 4723

On Mon, Apr 25, 2016 at 11:38 AM, Yi Wang <yiw...@med.cornell.edu> wrote:

Best regards,

Yi

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[jron...@gmail.com]

Hi Alexey,

Thank you for the answer, it is really helpful. I'm working on a ultra-high-field small animal probe, and have found it extremely difficult to run a set of phantoms as the small size of the coil and the high resolution due to the wall thickness. Regarding to the relative susceptibility measurement, without using a CSF as a reference (as in for a phantom measurement), I found the background susceptibility (which was not water) has set to 0 in the MEDI-L1 outcome, therefore the absolute χ should be calculated based on the background material, is that what you meant? 

Thank you!

Best regards,

Jierong