High-resolution distortion-free diffusion imaging of the prostate using stimulated echo based turbo spin echo (DPsti-TSE) sequence
Qinwei Zhang1, Bram F. Coolen1, Gustav J. Strijkers2, Laurens van Buuren3, Uulke van der Heide3, Oliver J. Gurney-Champion 1, Sónia I. Gonçalves4, and Aart J. Nederveen1

1Department of Radiology, Academic Medical Center, University of Amsterdam, Amsterdam, Netherlands, 2Biomedical Engineering and Physics, Academic Medical Center, University of Amsterdam, Amsterdam, Netherlands, 3Department of Radiation Oncology, The Netherlands Cancer Institute, Amsterdam, Netherlands, 4Institute for Biomedical Imaging and Life Sciences, University of Coimbra, Coimbra, Portugal


Diffusion imaging is part of the standard MR imaging protocol for prostate cancer diagnosis. Conventional echo planar imaging (EPI) diffusion sequence has limitation on image resolution and additionally suffers from image distortion. The present study introduces a new stimulated echo based 3D diffusion preparation turbo spin echo sequence (DPsti-TSE) to achieve high-resolution and distortion free image. The sequence is also proved to be immune to eddy currents.


Diffusion MRI (DWI) is considered part of the standard imaging protocol for evaluating prostate cancer. It has been used to distinguish benign and malignant tissue as well as to evaluate cancer aggressiveness [1,2]. However, conventional diffusion-weighted echo planar imaging (DW-EPI) allows limited spatial resolution and additionally suffers from image distortions[3]. Several methods were proposed to combine diffusion-preparation (DP) with turbo-field-echo (DP-TFE) and turbo-spin-echo acquisitions (DP-TSE) to achieve high-resolution and distortion-free DWI [4,5]. However, one of the challenges is that eddy currents from the strong diffusion gradients cause additional, unpredictable, non-diffusion-related signal losses. This seriously hinders the determination of correct diffusion quantities. In this study we introduce a new stimulated-echo-based 3D DP-TSE sequence (DPsti-TSE) that is insensitive to eddy currents. We demonstrate in phantom experiment that the new sequence yields correct diffusion values and have applied the sequence for high-resolution distortion-free DWI of the prostate.


Figure-1 depicts the proposed DPsti-TSE sequence. Every TSE shot is preceded by a DP module, which consists of a 90°x-180°y-180°y-90°-x RF pulse train, with diffusion weighting achieved by 4 gradients (blue). Diffusion gradient moments m0 and m1 are nulled to reduce motion sensitivity. Stimulated echoes are acquired by applying a strong dephasing gradient (red) in the slice-selection direction before tip-up in combination with rephasing gradients in the TSE readout. Rephasing gradients were balanced to satisfy the CPMG condition. The proposed sequence was validated at 3T (Ingenia, Philips). A homogeneous gel phantom was scanned by DW-EPI, DP-TSE, DPsti-TSE, and DPsti-TSE with flip-angle (FA) sweep. Seven b-values (0-800s/mm2) were applied. For validation purposes, phase encoding gradients were disabled to obtain transient signals. ADC values were obtained at each echo location by fitting the transient signals Sb,t to Sb,t = S0,t ×e(-b×ADCt). The prostate of a healthy volunteer (23y-M) was scanned. Low- and high-resolution DW images were obtained at 4 different b-values using both 3D DPsti-TSE with FA sweep and 2D DW-EPI sequences. From these images, pixel-wise ADC and corresponding R2 maps were calculated. Anatomical T2w images with equal resolutions were used to assess image distortion in DWI. Detailed scan parameters are listed in Table 1.


In the phantom experiment (Figure-2), the ADCt from DW-EPI was stable throughout the EPI readout with a value of (1.99±0.02)×10-3mm2/s. For the DP-TSE sequence, the ADCt was severely overestimated at (3.64±0.05)×10-3mm2/s, caused by eddy currents induced signal losses. As expected, ADC values for the DPsti-TSE (2.03±0.03)×10-3mm2/s and DPsti-TSE with FA sweep (2.11±0.03)×10-3mm2/s were close to the DW-EPI reference values. For the latter, the ADCt was constant for the full TSE shot of 990ms, whereas ADC values were constant for only 20ms, 345ms, and 420ms in DW-EPI, DP-TSE, and DPsti-TSE sequences, respectively. For the vivo DW-EPI scans (Figure-3), the prostate as well as other surrounding tissues were distorted in comparison to the T2w scans. In contrast, the DPsti-TSE did not suffer from such geometrical distortions. Low-resolution ADC maps were similar between DW-EPI and DPsti-STE, with mean ADC values in the prostate transition zone of (1.5±0.2)×10-3mm2/s and (1.4±0.2)×10-3mm2/s, respectively. More importantly, at higher resolution ADC values from DPsti-TSE remained unchanged (1.5±0.2)×10-3mm2/s while for DW-EPI, high-resolution ADC maps were characterized by lower R2 values and a lower mean ADC of (1.3±0.3)×10-3mm2/s.


Phantom experiments demonstrated that the DPsti-TSE sequence provides correct ADC estimations in the presence of eddy currents. Furthermore, correct diffusion-weighting is maintained during the entire FA sweep, enabling time-efficient high-resolution 3D DWI. Eddy currents induced spin dispersion by the strong diffusion gradients is a problem for both the DW-EPI and DP-TSE sequences. Whereas in DW-EPI eddy currents lead to the well-known geometrical image distortions, in DP-TSE they cause signal losses due to the flip-up RF pulse and spoiler gradient and the end of the DP module. In the DPsti-TSE sequence eddy currents in the DP module (collectively indicated by the shaded gray gradient surface in Figure 1) cause neither distortions nor signal losses, because the eddy currents merely cause a shift of the echo position which has no influence on image quality. Because of the stimulated-echo readout signal is reduced by a constant factor ½. Nevertheless, SNR was still sufficient for acquisition of high resolution 3D diffusion-weighted images of the prostate.


In this study, a stimulated echo based diffusion-prepared 3D TSE sequence was presented. The sequence can be applied for high-resolution distortion-free DWI of the prostate. We are currently implementing this sequence into a standard prostate cancer patient imaging protocol to further assess its performance.


No acknowledgement found.


[1]Koh et al. AJR, 188.6(2007):1622-1635. [2]Miao et al. EJR, 61.2(2007):297-302. [3]D. Le Bihan et al. JMRI, vol.24,no.3,pp.478–488,2006. [4]U. Sinha et al. JMRI, vol.6,no.4,pp.657–666,1996. [5]Y. Xie et al. JCMR, vol.16,no.1,pp.1–10,2014.


Figure 1: 3D DPsti-TSE sequence. Rectangle and Sinc waveform denote non-selective and selective RF pulses respectively.

Table 1: Major scan parameters for in vivo prostate experiments

Figure 2: Phantom experiment results. (a-d): Transient echo signals as function of echo time in the EPI/TSE readout for varying b-values. (e-h): Fitted ADCt values as function of echo time.

Figure 3: In vivo prostate imaging. Top row: Corresponding, low resolution anatomical, DPsti-TSE, and DW-EPI b-value=0s/mm2 images (a-c); Bottom row: high resolution anatomical, DPsti-TSE and DW-EPI b-value=0s/mm2 images (d-f). Insets are ADC and goodness-of-fit R2 maps.

Proc. Intl. Soc. Mag. Reson. Med. 24 (2016)