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Distortion-free diffusion imaging with TSE

White Paper
van de Ven, Kim, Ph.D. Philips Healthcare • Netherlands
Nijenhuis, Marco Philips Healthcare • Netherlands

Diffusion weighted imaging (DWI) produces images where tissues or lesions with different diffusion characteristics stand out, such as for instance, oncologic lesions. For DWI in the brain, EPI read-out is frequently used because of the high speed and SNR it provides. However, in areas such as the skull base, the inner ear and the head/neck area, susceptibility differences can result in artifacts on gradient echo EPI images.

As TSE sequences are inherently less sensitive to susceptibility differences, TSE-based diffusion can be a good alternative for these areas. At Philips we recently improved our Diffusion TSE method to better meet needs in clinical use.


Diffusion TSE provides high quality DWI in challenging areas

A TSE acquisition can be made sensitive to diffusion by using a gradient before and after the 180 degree refocusing pulse, as shown in the figure. This method is known as the Alsop method for Diffusion TSE.

 

A main challenge in Diffusion TSE imaging, is to get sufficient SNR while maintaining sharp images. With the Alsop method as a starting point, we applied several optimizations to provide pristine image quality.
  

   Alsop DC, Phase insensitive preparation of single-shot RARE: applications to diffusion imaging
   in humans. Magn Reson Med 1997;38:527-5331 
  

Phase artifacts addressed by averaging

Using modulus averaging in image space is important for good image
quality. When complex averaging is used, phase artifacts can deteriorate
image quality. The images below show image quality differences between
the two methods, especially in areas of high susceptibility.
Complex averaging <br>Modulus averaging <br>
Complex averaging
Modulus averaging


RF pulse shape adapted for faster scanning and reduced blurring

To produce an optimal slice profile, Diffusion TSE by default involves a long RF pulse that produces
a very nice refocusing slice profile. We have implemented a shorter RF pulse in the Diffusion TSE method to obtain optimal SNR. This short pulse produces a refocusing slice profile that is somewhat less perfect, but allows to reduce the length of the TSE train.
 <br><br>


Short RF pulse, short echo spacing, short shot length, high SNR

The short RF pulse will reduce blurring because the echo spacing will become shorter and
therefore the shot length will become shorter. Additionally, the short RF pulse will slightly
increase SNR due to the shorter shot length enabled. Note that your sequence parameter
combination automatically determines which RF pulse will be applied.
 TE 79 ms, echo spacing 7.4 ms<br>
SENSE factor 2<br> <br> TE 69 ms, TSE echo spacing 4.0 ms<br>
SENSE factor 2<br>
TE 79 ms, echo spacing 7.4 ms
SENSE factor 2

TE 69 ms, TSE echo spacing 4.0 ms
SENSE factor 2

Compatible with SENSE to benefit quality

Adding SENSE to Diffusion TSE further reduces the shot length, which makes the pulse train shorter, which has a positive effect on the blurring. It also enables single shot Diffusion TSE, avoiding the need for phase navigation and providing robust image quality.
 TE 69 ms, TSE echo spacing 4.0 ms<br> <br> TE=69 ms, TSE echo spacing 4.0 ms<br> <br>
TE 69 ms, TSE echo spacing 4.0 ms

TE=69 ms, TSE echo spacing 4.0 ms

Advantages in clinical practice

The clinical examples demonstrate the performance of Diffusion TSE in an area with large susceptibility differences. The Diffusion TSE images show better image quality than EPI diffusion in this anatomy.
  
Diffusion EPI Diffusion TSE
Diffusion EPI
Diffusion TSE
Courtesy of Dr. R.K. Gupta, Fortis Memorial Research Institute, Gurgaon, India.
  
  
Diffusion EPI Diffusion TSE
Diffusion EPI
Diffusion TSE

In the EPI diffusion it is difficult to discriminate the lesions from artifacts. In the Diffusion TSE images the lesions are clearly visible.

Courtesy of Dr. J. Casselman, AZ St. Jan Brugge, Belgium.


  

Summary

- Single shot Diffusion TSE in combination with SENSE delivers robust image quality. 

- Preset protocols for transverse skull base imaging and for inner ear imaging.


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White Paper
Achieva 1.5T dStream, Achieva 3.0T dStream, Ingenia 1.5T, Ingenia 3.0T
DWI, Head/neck, Neuro, Oncology, SENSE, skull base
 

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