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From: peter@hpl.hp.com (Peter Webb)
Subject: Re: Wrap-around effect in MR images
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Date: Fri, 19 May 1995 16:27:16 GMT
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Bart Schraa (bart@xecho.xs4all.nl) wrote:

: In article <D8qE7A.AyA@hpl.hp.com> peter@hpl.hp.com (Peter Webb) writes:

: > The only exception is in the readout direction, where, by oversampling,
: > the FOV can effectively be made very large.  The extra FOV is thrown
: > away by digital filtering or by cropping the output of the
: > reconstruction.  This technique cannot be applied in the phase encode
: > directions, because oversampling this way requires more excitations, and
: > thus takes longer (another exception -- multiple average scans are
: > usually performed as one average with extended FOV.  The scanner
: > prescription software hides this from the user).

: It is in fact also possible in phase encoding direction. The oversampling
: in the phase encoding direction can be varied from 0-100%. Depending on which
: FOV you choose you can choose either a small oversampling rate, or a larger
: one. It's possible to scan small FOV's without any aliasing. Because the
: FOV can be reduced to let's say half of the large one, it's possible to get
: a higher resolution with a 256 matrix in the small FOV in stead of the
: 512 matrix with the large FOV, and that in about the same scan time. Due to
: this fact you won't need more excitations of course.

We are probably agreeing here, and using different words...

Let's assume, to be concrete, that we want a 256 mm FOV, and the object
is 400 mm across in the phase encode direction.  If we use 256 PEs,
256 mm FOV, then we will get aliasing.  If we use 512 PEs, 512 mm FOV,
we won't.  If we specify 256 matrix, 2 excitations, then behind our back
the scanner can actually perform 512 PEs, 1 excitation, reconstruct a 
512 mm FOV, then throw away 128 columns of pixels on each edge, resulting
in the 256 matrix, 256 mm FOV we wanted.  

My statement that "this technique cannot be applied in the PE direction"
was intended to mean that, for this example, you *cannot* acquire 1 mm
resolution in the PE direction in only 256 excitation without aliasing.
There are multiple ways of prescribing a scan that uses (in this case)
more than 400 excitation to achieve the desired result.  

Peter

--------------------------------
Peter Webb, HP Labs Medical Dept
E-Mail: peter_webb@hpl.hp.com        
Phone: (415) 813-3756               
