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Geometric Distortion in Diffusion-weighted MR Imaging of the Prostate—Contributing Factors and Strategies for Improvement

Posted May 31, 2014 Updated Apr 10, 2023
By Francisco Donato MD , Daniel N. Costa MD , Qing Yuan PhD , Neil M. Rofsky MD , Robert E. Lenkinski PhD , Ivan Pedrosa MD
8 min read

Rationale and Objectives

Image distortion on diffusion-weighted imaging (DWI) of the prostate in 3T endorectal magnetic resonance imaging (MRI) examinations is common. The aim of this study was to determine the degree of distortion on DWI using a state-of-the-art clinical protocol and to explore the main contributors to geometric distortion.

Materials and Methods

Forty consecutive patients underwent 3T MRI of the prostate with an endorectal coil filled with air ( n = 20) or barium sulfate ( n = 20). Distortion was measured as the maximum displacement of the outer boundary of the prostate on DWI relative to T2-weighted imaging. The effects of phase-encoding direction, receiver bandwidth, and parallel imaging were then assessed in a prostate phantom on two MRI scanners from different manufacturers.

Results

There was no statistical difference in the mean displacement of the prostate on DWI between the air cohort (1.8 ± 1.2 mm, range 0–4.2 mm) and barium cohort (1.8 ± 2.2 mm, range 0–9 mm). Displacement of the prostate was observed in the phase-encoding direction. Phantom experiments demonstrated a horizontal displacement of 6.0 mm in the phase-encoding direction, which decreased with the use of parallel imaging and higher bandwidth. Geometric distortion was similar for all b values and across manufacturers.

Conclusions

Geometric distortion on DWI of the prostate is common in the phase-encoding direction and does not improve with inflating the coil with barium sulfate. Strategies to reduce this artifact include the use of higher bandwidth and accelerated imaging. Correction of this phenomenon should improve localization of prostate cancer, particularly important for targeted prostate biopsies or focal therapies.

Diffusion-weighted imaging (DWI) has become a key component of multiparametric magnetic resonance imaging (mpMRI) of the prostate . Although DWI improves prostate cancer detection and apparent diffusion coefficient (ADC) values derived from these acquisitions seem to correlate with tumor grade potentially predicting disease aggressiveness, this technique is prone to image distortion .

DWI relies on multiple acquisitions, probing the rate with which water diffuses in various directions and over different scales. Although single-shot spin-echo echo-planar imaging (SE-EPI) is very sensitive to magnetic field inhomogeneities, such as those caused by air in the rectum or within the endorectal coil (ERC) near the prostate , it is the most commonly used sequence in DWI. With SE-EPI, phase error accumulation results in voxel shifts that distort the image along the phase-encoding direction . New gradient coils capable of extremely fast linear gradient switches have enabled DWI acquisition with higher spatial resolution. However, increasing the resolution results in longer echo trains and further increases the accumulation of errors during the spatial encoding .

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Materials and methods

Patient Selection and MRI Protocol

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Phantom Experiments

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Figure 1, (a) Prostate phantom model (Yezitronix Group Inc., Quebec, Canada) and endorectal coil (Medrad, Warrendale, PA). (b) The coil can be introduced in the phantom through a built-in orifice. (Color version of figure is available online.)

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Image Analysis

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Figure 2, Geometric distortion on diffusion-weighted imaging (DWI) in a patient. (a) The outer contour of the prostate in the T2-FSE image (TR, 8.2 seconds; TE, 120 milliseconds; flip angle, 90°) and (b) DWI image (TR, 6.8 seconds; TE, 76 milliseconds; flip angle, 90°) was delineated manually. The prostate contour from the T2-FSE image was saved and imported into the different b -value DWI images at the same anatomic level. (c) After overlapping the prostate contour from the T2-weighted reference image ( green ) on the DWI image ( yellow ), the geometric distortion was estimated by measuring the displacement between the boundaries in the horizontal axis ( white line ). FSE, fast spin-echo; TE, echo time; TR, repetition time. (Color version of figure is available online.)

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Statistical Analysis

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Results

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Table 1

Mean Displacement of the Prostate Boundaries between the T2-FSE and Diffusion-weighted Images Measured in the Left–Right Direction in Two Cohorts of Patients Undergoing Multiparametric MRI with the ERC Inflated with Air or Barium Sulfate

b Value (s/mm 2 ) Fat-shift TE (milliseconds) TR (milliseconds) FOV (mm 2 ) Reconstruction Matrix Echo Train SENSE Factor BW (Hz/pixel) Displacement (mm) Air ( n = 20) 50 Left 75–78 6776–6955 180 × 180 144 × 140 73 2 1269–1392 1.8 (1.1) 1000 1.7 (1.4) Barium ( n = 20) 50 76–77 6871–6954 1267–1339 1.7 (2.2) 1000 1.8 (2.2)

BW, bandwidth; ERC, endorectal coil; FOV, field of view; FSE, fast spin-echo; TE, echo time; TR, repetition time.

Displacement values within parentheses reflect standard deviations.

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Figure 3, Geometric distortion on diffusion-weighted imaging (DWI) in a phantom model decreases by increasing the acceleration factor. (a) T2-FSE image (TR, 8 seconds; TE, 120 milliseconds; flip angle, 90°) served as anatomic reference for the contour of the prostate ( green ); DWI images with (b) SENSE factor of 1 (TR, 10 seconds; TE, 93 milliseconds; flip angle 90°) and (c) SENSE factor of 2 (TR, 10 seconds; TE, 86 milliseconds; flip angle, 90°) were compared, showing a decrease in the displacement by increasing the acceleration factor. Yellow regions of interest indicate the outer contour of the prostate drawn on the DWI images; white line is the distance between the borders and represents magnitude of displacement (6 mm and 4 mm in b and c , respectively). FSE, fast spin-echo; TE, echo time; TR, repetition time. (Color version of figure is available online.)

Figure 4, Geometric distortion on diffusion-weighted imaging (DWI) in a phantom model decreases by increasing the bandwidth. (a) T2-FSE image (TR, 8 seconds; TE, 120 milliseconds; flip angle, 90°) served as anatomic reference for the contour of the prostate ( green ); DWI images with (b) bandwidth of 751 Hz/pixel (TR, 10 seconds; TE, 86 milliseconds; flip angle, 90°) and (c) 1252 Hz/pixel (TR, 10 seconds; TE, 75 milliseconds; flip angle, 90°) were compared, showing a decrease in the displacement of the contour by increasing the bandwidth. Yellow regions of interest indicate the outer contour of the prostate drawn on the DWI images; white line is the distance between the borders and represents magnitude of displacement (4.0 mm and 2.9 mm in b and c , respectively). FSE, fast spin-echo; TE, echo time; TR, repetition time. (Color version of figure is available online.)

Figure 5, Geometric distortion on diffusion-weighted imaging (DWI) in a phantom model does not change with the b value. (a) T2-FSE image (TR, 8 seconds; TE, 120 milliseconds; flip angle, 90) served as anatomic reference for the contour of the prostate ( green ); DWI images (TR, 10 seconds; TE, 99 milliseconds; flip angle, 90°) with b = 0 (b) , 40 (c) , 500 (d) , 1000 (e) , and 1500 (f) demonstrate the same degree of displacement. Yellow regions of interest indicate the outer contour of the prostate drawn on the DWI images; white line is the distance between the borders and represents magnitude of displacement (6.0 mm in all DWI images). FSE, fast spin-echo; TE, echo time; TR, repetition time. (Color version of figure is available online.)

Figure 6, Geometric distortion on diffusion-weighted imaging (DWI) in a phantom model occurs in the phase-encoding axis and direction. (a) T2-FSE image (TR, 8 seconds; TE, 120 milliseconds; flip angle, 90°) served as anatomic reference for the contour of the prostate ( green ); when the phase-encoding direction on DWI was shifted from right-to-left axis (TR, 10 seconds; TE, 99 milliseconds; flip angle, 90°; b–c) to the anterior-to-posterior axis (TR, 10 seconds; TE, 97 milliseconds; flip angle, 90°; d–e) , the axis of the distortion consistently followed the phase-encoding direction. This effect can also be inferred by assessing the shape of the signal void in the endorectal balloon and the outer contour of the phantom. When the fat-shift direction was inverted, the distortion orientation was inverted ( b , fat-shift left; c , fat-shift right; d , fat-shift posterior; and e , fat-shift anterior). Yellow regions of interest indicate the outer contour of the prostate drawn on the DWI images; white line is the distance between the borders and represents magnitude of displacement. FSE, fast spin-echo; TE, echo time; TR, repetition time. (Color version of figure is available online.)

Table 2

Displacement of the Prostate Boundaries between the T2-FSE and Diffusion-weighted Images in a Phantom Model with the ERC Filled with Barium

Test Fat-shift TE (milliseconds) TR (milliseconds) FOV (mm 2 ) Matrix Echo Train SENSE Factor BW (Hz/pixel) Displacement (mm) 1 Left 99 10,000 180 × 180 144 × 141 141 1 1300 6.0 2 Right 99 10,000 180 × 180 144 × 141 141 1 1300 −4.6 ∗ 3 Posterior 97 10,000 180 × 180 144 × 141 141 1 1492 <1.0 † 4 Anterior 97 10,000 180 × 180 144 × 141 141 1 1526 <1.0 † 5 Left 93 10,000 160 × 160 128 × 124 125 1 1267 6.0 6 Left 86 10,000 160 × 160 128 × 124 65 2 751 4.0 7 Left 75 10,000 160 × 160 128 × 124 65 2 1253 2.9

BW, bandwidth; ERC, endorectal coil; FOV, field of view; FSE, fast spin-echo; TE, echo time; TR, repetition time.

In the tests 5–7, FOV was reduced to 160 × 160 mm 2 to allow broader variations in the BW.

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Discussion

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Δy(x,y,z)=γΔB(x,y,z)FOVy/BWy Δ

y

(

x

,

y

,

z

)

=

γ

Δ

B

(

x

,

y

,

z

)

FOV

y

/

BW

y

where γ is the gyromagnetic ratio, Δ B is the perturbation of the B 0 field, and FOV y and BW y are the field of view and bandwidth, respectively. From this formula, we can conclude that the voxel shift in the phase-encoding direction will be inversely proportional to the effective bandwidth per voxel, which is consistent with our findings in the phantom: distortion was reduced using a higher bandwidth without altering any other acquisition parameter.

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Acknowledgments

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Academic Radiology, Volume 21, Issue 6
Journals General Radiology
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