Bob,
while it naturally would be better to know the ground truth of what blip
dir is correct, I think what you do is a correct empirical approach.
Depending on how "optimized" an EPI sequence is, EPI distortion is not
as obvious as it used to be. As to how to detect the correct blip
direction, you usually also get some elongation in ap-direction with
EPI, the correction of which may be more obvious than the attenuation of
dropouts. I would also recommend subtracting the uncorrected from the
corrected image, so you can appreciate the differences more directly.
Hope this helps,
Marko
Bob Spunt wrote:
> ... apologies for the re-post, still looking for advice ...
>
> Dear SPM experts,
>
> I am using the SPM fieldmap toolbox to compute VDMs/ unwarp EPIs
> (oblique slice prescription, about -25 degrees) based on magnitude
> images and a presubtracted phase image (collected using a Siemens Trio
> 3T). I have determined all protocol-specific parameters except for the
> blip direction. A colleague recommended I run it both ways (Blip +1 and
> Blip -1) and compare the resulting unwarped EPIs to determine which is
> the correct value.
>
> I have done this, however, the differences are not as clear as I had
> expected to see. Attached is the most notable difference among the two,
> showing that Blip +1 seems to smear pixels posteriorly and thus fills in
> areas of dropout (I note again that these images are acquired with a
> pretty steep slice angle). Blip -1 does the opposite (not surprisingly);
> moreover, the co-registration of the Blip -1 unwarped EPIs with the
> hi-res anatomical (not pictured) is slightly (but noticeably) better
> than with the Blip +1.
>
> Any advice on which is correct and/or tips on how to better evaluate the
> results? I of course want to make sure I'm using these maps
> appropriately - thank you in advance for any help!
>
> Cheers,
> Bob
>
> Inline image 1
>
>
>
> On Fri, Dec 21, 2012 at 12:21 PM, Bob Spunt <[log in to unmask]
> <mailto:[log in to unmask]>> wrote:
>
> Dear SPM experts,
>
> I am using the SPM fieldmap toolbox to compute VDMs/ unwarp EPIs
> (oblique slice prescription, about -25 degrees) based on magnitude
> images and a presubtracted phase image (collected using a Siemens
> Trio 3T). I have determined all protocol-specific parameters except
> for the blip direction. A colleague recommended I run it both ways
> (Blip +1 and Blip -1) and compare the resulting unwarped EPIs to
> determine which is the correct value.
>
> I have done this, however, the differences are not as clear as I had
> expected to see. Attached is the most notable difference among the
> two, showing that Blip +1 seems to smear pixels posteriorly and thus
> fills in areas of dropout (I note again that these images are
> acquired with a pretty steep slice angle). Blip -1 does the opposite
> (not surprisingly); moreover, the co-registration of the Blip -1
> unwarped EPIs with the hi-res anatomical (not pictured) is slightly
> (but noticeably) better than with the Blip +1.
>
> Any advice on which is correct and/or tips on how to better evaluate
> the results? I of course want to make sure I'm using these maps
> appropriately - thank you in advance for any help!
>
> Cheers,
> Bob
>
> Inline image 1
>
>
--
____________________________________________________
PD Dr. med. Marko Wilke
Facharzt für Kinder- und Jugendmedizin
Leiter, Experimentelle Pädiatrische Neurobildgebung
Universitäts-Kinderklinik
Abt. III (Neuropädiatrie)
Marko Wilke, MD, PhD
Pediatrician
Head, Experimental Pediatric Neuroimaging
University Children's Hospital
Dept. III (Pediatric Neurology)
Hoppe-Seyler-Str. 1
D - 72076 Tübingen, Germany
Tel. +49 7071 29-83416
Fax +49 7071 29-5473
[log in to unmask]
http://www.medizin.uni-tuebingen.de/kinder/epn/
____________________________________________________
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