Zoom DWI – DWI without Aliasing Artifact

Yuki Takai

When scanning small organs like prostate and spinal cord, implementation of a small field of view (FOV) and high spatial resolution is required. However, if FOV of phase encoding direction (PE-FOV) is small, it can cause aliasing artifacts. Therefore, some special methods are suggested to prevent these types of aliasing artifacts ^1-7.
Canon Medical Systems Corporation implements Zoom diffusion-weighted imaging (DWI) applications to remove aliasing artifacts. In Zoom DWI, the excitation pulses are rotated by a certain angle related to the refocus pulses to selectively excite the acquisition FOV, making it possible to eliminate aliasing artifacts in the PE direction. The rotation angle of the excitation pulses depends on the gap between the acquisition, FOV and the slice. Therefore, it is recommended to set a large gap or perform multi-coverage acquisition such as Coverage Interleave or Double Coverage Interleave. To support multi-slice acquisition, the signals in the transition FOVs (regions where the excitation pulse interacts with the refocus pulse outside the PE-FOV) are suppressed by the outer volume suppression (OVS) pulses. Aliasing artifacts caused by the side lobes of the radio frequency (RF) pulses generated by the OVS pulses are suppressed by setting NoWrap.
This makes it possible to narrow PE-FOV, which also makes it possible to reduce distortion in principle.

This article demonstrates the effectiveness of Zoom DWI.
Firstly, aliasing artifacts were evaluated.

Target:	ACR phantom
Scan conditions:	PE × RO matrix = 160 × 160 FOV = 12 × 24 cm (Zoom DWI), 24 × 24 cm (Conventional DWI) ST = 4 mm, Slice = 30 slices, Plane = Axial, Averaging = 1, Echo Space = 0.9 ms SPEEDER acceleration factor = 2.0 (Exsper)

Conventional DWI and Zoom DWI were acquired under the FOV 12 × 24 cm, where aliasing artifacts were found in Conventional DWI.
Below is the comparison image between Zoom DWI and Conventional DWI.
It is demonstrated that Zoom DWI enables the ability to reduce aliasing artifacts while Conventional DWI does not (red arrow).

Secondly, distortion was evaluated.
Conventional DWI (FOV 24 x 24 cm) and Zoom DWI (FOV12 x 24 cm) were acquired.
The amount of distortion is measured by setting a straight-line region of interest (ROI) with respect to the grid at the center of the grid phantom and measuring the amount of deviation between the grid and the straight-line ROI.
The amount of distortion on Zoom DWI is 2.2 mm and on Conventional DWI is 4.4 mm, demonstrating that the amount of distortion is halved under the half PE-FOV condition.
This demonstrates that Zoom DWI enables the ability to reduce distortion correctly.

Finally, volunteer images were evaluated.

Scan conditions:

TR = 4400 ms, TE = 72 ms PE × RO =96 × 96 (Zoom DWI), PE × RO = 112 × 112 (Conventional DWI) FOV=13 × 13 cm (Zoom DWI), 24 × 24 cm (Conventional DWI) ST = 3 mm, Slice = 30 slices, Plane = Axial, Averaging = 7, EchoSpace = 0.7 ms SPEEDER acceleration factor = 2.0 (Exsper) Fatsat = SPAIR (Zoom DWI), PASTA+SPAIR (Conventional DWI)

It is shown that aliasing artifacts do not occur, and the amount of distortion is reduced even when the PE-FOV is reduced. In addition, small PE-FOV makes it possible to eliminate signals from unwanted tissues outside the ROI. While resolution is increased, signal-to-noise ratio (SNR) is decreased by narrowing PE-FOV.
In conclusion, Zoom DWI has been shown to reduce aliasing artifacts and the amount of distortion by narrowing PE-FOV. //

References

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2. Feinberg DA, Hoenninger JC, Crooks LE, et al. Inner volume MR imaging: technical concepts and their application. Radiology. 1985;156:743–747. (early method using intersection of two perpendicularly excited slabs to limit imaged volume)
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6. Saritas EU, Cunningham CH, Lee JH, Han ET, Nishimura DG. DWI of the spinal cord with reduced FOV single-shot EPI. Magn Reson Med 2008; 60:468-473.
7. Wilm BJ, Svensson J, Henning A, et al. Reduced field-of-view MRI using outer volume suppression for spinal cord diffusion imaging. Magn Reson Med 2007; 57: 625–630.