Introduction

Distortions in the phase-encoding direction increase with larger fields-of-views in the phase-encoding direction and with longer echo spacing for diffusion weighted imaging (DWI) using echo planar imaging (EPI) technique. Conventional abdominal DWI therefore often suffers from distortion artifacts. Small field-of view echo planar imaging (Zoomit_EPI) acquires fewer k-space lines and therefore uses shorter EPI echo train length, leading to less distortions. In this study, we compared Zoomit_EPI and conventional single shot EPI (SS_EPI) for DWI with matched scan time in the evaluation of renal cancer through qualitative image quality, lesion detection, and apparent diffusion coefficient (ADC) values.

Materials and Methods:

This prospective study was approved by the Ethics Committee of our hospital. Thirteen patients (n=13, 5 females, age = 53 +/- 4.6 years) previously diagnosed with renal cancer were recruited, and informed written consent was obtained from all patients. All patients underwent renal MR examinations, including conventional coronal T1w, T2w, SS_EPI, and a prototype Zoomit_EPI DWI sequence, on a 3T MRI scanner (MAGNETOM Skyra, Siemens Healthcare, Erlangen, Germany) with 18-channel abdominal phase array coil and 32-channel Tim spinal array coil. Sequence parameters for SS_EPI and Zoomit_EPI are listed in Table 1. Two blinded radiologists with 5-7 years of experience assessed the image quality in terms of image blur, sharpness of boundaries, distortion artifacts, and overall image quality using a 5-point Likert scale. For quantitative analysis, three regions of interest (ROI) were drawn on the tumor, right, and left renal healthy tissue for the same location of SS_EPI and Zoomit_EPI, respectively. The mean ADC value of each ROI was calculated. Paired t-test, Pearson coefficient, and Bland-Altman analysis were used to assess the difference, correlation, and agreement of ADC between SS_EPI and Zoomit_EPI methods. p value <0.05 was considered statically significant.

Results

Image quality scores are shown in Table 2. Zoomit_EPI image quality scores were significantly higher than those of SS_EPI (p<0.01). Zoomit_EPI also showed less blur, increased sharpness of boundaries, and fewer distortion artifacts (Figure 1). ADC values of SS_EPI and Zoomit_EPI were highly correlated (R2=0.91, p<0.05). Bland-Altman analysis showed that ADC values of Zoomit_EPIs were higher than that of SS_EPI, with a bias of 80.4 ×10-6 s/mm2. This difference between ADC values was statistically significant as determined by paired-sample t-test (p<0.05).

Discussion

Results indicate that Zoomit_EPI can significantly improve image quality compared with conventional SS_EPI. Less blur, sharper boundaries, and fewer distortions were observed from Zoomit_EPI, consistent with findings in previous studies performed in the pancreas, prostate, and kidney.1-3 In addition, because Zoomit_EPI had higher resolution and more averages compared with SS_EPI in the same scan time, DWI images with b=800 s/mm2 for Zoomit_EPI improved appearance compared with SS_EPI images.
The findings of good correlation with significant bias between techniques was inconsistent with previous studies,1,2 which found no significant difference between the two methods. However, in a previous prostate cancer study using Zoomit_EPI, ADC tended to be higher in Zoomit_EPI, though without statistical significance.3 The reason for our findings may be that the ADC map of SS_EPI had less homogeneity and more regions with low ADC, meanwhile, complex averaging in Zoomit _EPI may increase the ADC value.

Conclusions

Zoomit_EPI DWI technique showed significant improvements in image quality compared to conventional SS_EPI DWI, and it may be useful for the detection of kidney lesions.

Acknowledgements

No acknowledgement found.