Original contributionReduced-FOV excitation decreases susceptibility artifact in diffusion-weighted MRI with endorectal coil for prostate cancer detection
Introduction
Prostate cancer is the second most common type of cancer in the American male population [1]. Because prostate cancer shows a high incidence and low mortality rate in comparison with other cancers [1], an urgent need exists to develop non-invasive imaging approaches for improved prostate cancer patient-specific treatment planning and early assessment of therapeutic failure.
Multiparametric MRI has been studied extensively for identifying prostate cancer [2] through a combination of T2-weighted imaging, dynamic contrast-enhanced imaging, 1H MR spectroscopy, and diffusion-weighted imaging (DWI). To further increase diagnostic capability, the multiparametric MRI can include the use of an endorectal coil in conjunction with a standard pelvic phased-array. Combining endorectal and phased-array coils has proven to increase the signal-to-noise ratio of DWI in prostate exams at 3 T over nine times in comparison to use of a phased-array alone [3].
DWI increases both sensitivity and specificity in prostate cancer detection in multiparametric MR studies [4], [5], [6]. DWI has also been shown to improve the assessment of tumor aggressiveness when combined with conventional T2-weighted imaging, with an inverse relationship between the apparent diffusion coefficient (ADC) map intensity and Gleason score [7]. DWI typically uses the echo-planar imaging (EPI) technique to decrease scan time. However, images acquired with EPI suffer from severe susceptibility artifact at the interfaces of tissue with air, blood, or fecal matter in the rectum. These artifacts are of particular importance because they present at the border of the rectum and the peripheral zone of the prostate, where 70% of prostate cancers are located [8].
In this work, we have utilized a reduced-field-of-view (rFOV) acquisition scheme for prostate DWI that employs a 90° 2D spatially-selective, echo-planar RF pulse to excite a limited extent in the phase field-of-view (FOV) direction [9]. This enables a higher spatial resolution to be achieved in the phase encoding direction than in conventional DWI with a shorter echo-train length, and without obvious aliasing artifacts. The reduced echo-train length can potentially reduce prostate image distortions induced by magnetic-susceptibility differences within the FOV [9]. Additionally, this pulse is designed so that the excited fat profile and the excited water profile do not overlap, so that only the on-resonance water profile can be selected by the subsequent refocusing pulse. This could potentially provide a robust method of periprostatic fat suppression in prostate DWI images [9].
The aim of this study was to determine if image distortion is less in prostate ADC maps generated from the rFOV technique than from a conventional DWI sequence (CONV) and to determine if the rFOV ADC contrast between tumors and healthy-appearing tissue within subjects is as high as or better than that of the CONV sequence.
Section snippets
Subjects
This prospective study was approved by our institutional review board and was compliant with the Health Insurance Portability and Accountability Act. Written, informed consent was obtained from all participants. Fifty patients receiving MR examinations of the prostate were studied between September of 2011 and January of 2013. Patients presented with suspected prostate cancer, as indicated by either elevated levels of serum prostate-specific antigen (PSA) (median = 5, range 0.10–291),
Results
Forty-nine of the fifty patients (98%) assessed showed rectal wall distortion interfering with the peripheral zone of the prostate on either the CONV or rFOV sequence by at least one observer. Examples of each distortion score are shown in Fig. 2. Distortion scores were significantly reduced with the rFOV sequence (p < 0.012 Wilcoxon Signed-Rank Tests, n = 50). Significance and average difference between sequences scored by each observer are shown in Table 1. The measured difference between CONV
Discussion
This study demonstrated that an alternate DWI acquisition scheme, based on using a pulse sequence with reduced-FOV excitation, provided significantly less image distortion and significantly improved contrast between tumor and healthy tissue ADC compared to a conventional DWI sequence acquired with a full phase-direction FOV. To the best of our knowledge, this is the first study to compare either susceptibility-related distortion or tumor contrast in prostate DWI utilizing conventional- and
Conclusion
In conclusion, the rFOV sequence yielded significantly decreased rectal wall susceptibility artifact and provided significantly higher contrast in ADC value between tumor and healthy tissue as compared to the CONV sequence without significantly increasing scan time. This technique shows great promise for improving DWI quality, thereby potentially improving the detection of prostate cancer by MRI.
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