Readout-segmented Echo-planar Imaging for Diffusion-weighted Imaging in the Pelvis at 3T—A Feasibility Study

Rationale and Objectives

Diffusion-weighted imaging (DWI) of the pelvis at 3T is prone to artifacts that diminish the image quality. Readout-segmented echo-planar imaging (RS-EPI) is a new DWI technique that can reduce the artifacts associated with standard single-shot echo-planar imaging (SS-EPI) DWI. The purpose of this study was to evaluate the feasibility and image quality of RS-EPI in pelvic DWI compared to SS-EPI on a 3T imaging system.

Materials and Methods

Thirty patients underwent pelvic DWI on a 3T scanner with SS-EPI and RS-EPI techniques. Two blinded readers independently assessed each set of images for geometric distortion, image blurring, ghosting artifacts, lesion conspicuity, and overall image quality on a 7-point scale. Qualitative image scores were compared using paired Wilcoxon signed rank test. Interreader correlation was assessed by Spearman rank correlation.

Results

Geometric distortion, imaging blurring, ghosting artifacts, lesion conspicuity, and overall image quality were rated significantly better by both readers for RS-EPI technique ( P < .01 for all parameters). There was moderate–high correlation between the readers ( r = 0.649–0.752) for all parameters apart from lesion conspicuity ( r = 0.351). Both readers preferred the RS-EPI set of DWI images in most of the cases (reader 1: 0.87, 95% CI 0.74–0.99; reader 2: 0.77, 95% CI 0.61–0.93). Mean difference and limits of agreement between apparent diffusion coefficient (ADC) values obtained from the two methods were 0.01 (−0.08, 0.10) × 10 −3 mm 2 /s.

Conclusions

RS-EPI DWI images showed improved image quality compared to SS-EPI technique at 3T. RS-EPI is a feasible technique in the pelvis for producing high-resolution DWI.

Diffusion-weighted imaging (DWI) has emerged in the past decade as an important functional imaging technique in extracranial oncologic imaging. In the pelvis, DWI has been applied to imaging of rectal, prostate, endometrial, and cervical cancers . DWI provides an excellent contrast mechanism that is useful for tumor detection and delineating disease extent.

Currently the most widely used sequence for clinical DWI in the pelvis is single-shot echo-planar imaging (SS-EPI), whereby all the lines in k-space are filled by multiple gradient reversals in a single acquisition after a single radiofrequency pulse. However, the SS-EPI technique suffers from significant artifacts that reduce the image quality of DWI. It is vulnerable to T2*-induced blurring and geometric distortions due to magnetic field inhomogeneities that cause accumulation of phase errors . In the pelvis, the presence of gas-containing viscera, such as the rectum, small and large bowel, and vagina, frequently exacerbates these artifacts. Distortion and blurring artifacts worsen with higher field strength and higher resolution, limiting the achievable resolution with SS-EPI before these effects become prohibitive . The functional image quality and resolution achieved with DWI thus presently lags considerably behind the high-resolution T2-weighted sequences used for morphologic assessment and anatomic correlation in the pelvis.

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Materials and methods

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MR Imaging

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Table 1

Sequence Parameters for Single-shot and Readout-segmented Echo-planar Imaging

Sequence Parameter Single-shot Echo-planar Imaging Readout-segmented Echo-planar Imaging Diffusion directions Three-direction trace Three-direction trace Diffusion encoding scheme Monopolar, Stejskal-Tanner Monopolar, Stejskal-Tanner b-value (s/mm 2 ) 0, 100, 1000 0, 1000 Fat suppression Inversion recovery, gradient reversal Inversion recovery, gradient reversal TR (millisecond) 5300–6900 7800–8200 TE (millisecond) 62–79 (minimum) 63–80 (minimum) Field of view (mm) ∗ 260 240 Matrix ∗ 160 × 120 192 × 164 Number of sections 30–34 30–34 Section thickness (mm) 4 4 Number of readout segments 1 7 Number of signals acquired 6 1 Parallel imaging GRAPPA GRAPPA Acceleration factor 2 2 Acquisition time (minute:second) 3:40 4:17

GRAPPA, generalized autocalibrating partially parallel acquisition; TE, echo time; TR, repetition time.

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Image Assessment

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Statistical Analysis

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Results

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Qualitative Image Assessment

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Table 2

Comparison of Qualitative Scores of DWIs Using SS-EPI Technique and RS-EPI Technique

Parameter Reader 1 (Mean Score ± SD)P † Reader 2 (Mean Score ± SD)P † Spearman Correlation ( r__s ) ‡ SS-EPI RS-EPI SS-EPI RS-EPI Geometric distortion 3.77 ± 0.57 5.93 ± 0.25 <.01 ∗ 3.90 ± 0.55 5.13 ± 0.68 <.001 ∗ 0.752 ( P < .001) ∗ Image blurring 3.93 ± 0.37 6.00 ± 0.26 <.001 ∗ 4.00 ± 0.26 4.93 ± 0.83 <.001 ∗ 0.649 ( P < .001) ∗ Ghosting artifacts 3.87 ± 0.78 5.60 ± 0.62 <.001 ∗ 3.93 ± 0.64 5.13 ± 0.73 <.001 ∗ 0.711 ( P < .001) ∗ Lesion conspicuity 4.58 ± 1.26 5.74 ± 1.24 .002 ∗ 4.05 ± 0.40 5.05 ± 0.91 .004 ∗ 0.351 ( P = 1) Overall image quality 3.97 ± 0.41 5.83 ± 0.38 <.001 ∗ 4.10 ± 0.31 5.10 ± 0.61 <.001 ∗ 0.737 ( P < .001) ∗

DWI, diffusion-weighted imaging; RS-EPI, readout-segmented echo-planar imaging; SD, standard deviation; SS-EPI, single-shot echo-planar imaging.

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ADC and SNR Analysis

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Discussion

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References

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Readout-segmented Echo-planar Imaging for Diffusion-weighted Imaging in the Pelvis at 3T—A Feasibility Study

Rationale and Objectives

Materials and Methods

Results

Conclusions

Materials and methods

MR Imaging

Image Assessment

Statistical Analysis

Results

Qualitative Image Assessment

ADC and SNR Analysis

Discussion

References

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