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Multifrequency MR elastography of the human prostate by multiple surface-based compressed-air drivers: reproducibility and first patient results1Institute of Radiology, Charité, Berlin, Germany, 2Department of Medical Informatics, Charité, Berlin, Germany, 3Department of Urology, Charité, Berlin, Germany
Synopsis
A new setup for prostate MR Elastography is proposed that uses surface-based wave excitation by compressed-air drivers. The post-processing with tomoelastography combining wave fields at drive frequencies of 60, 70, and 80 Hz leads to highly resolved elasticity maps. A study in healthy volunteers demonstrates the good reproducibility of the method. Furthermore, the first patient results show excellent agreement with contrast-enhanced reference images, which motivates future patient studies.
Background
Magnetic resonance Elastography (MRE) is capable of generating image contrast based on the viscoelastic properties of tissue by inducing and detecting time harmonic shear waves in the body.1 MRE of the prostate is an active area of research since prostate cancer is a prevalent disease and noninvasive assessment of the malignant potential is difficult. However, several challenges exist for prostate MRE among which shear wave damping due to the central location of the prostate and resolution of stiffness details due to the high heterogeneity in small volumes are the most persisting ones. Different techniques have been proposed in the literature to overcome these challenges including transurethral 2, endorectal 3 and transperineal drivers4,5. We here propose MRE of the prostate based on multiple actuators which can also be applied to other regions of the abdomen6,7. The new drivers were combined with multifrequency MRE and recently introduced tomoelastography image processing 8 to generate high-resolution maps of shear wave speed as a measure of tissue stiffness.Purpose
To test the reproducibility of in vivo prostate MRE using pressurized-air actuators with improved spatial resolution and to present first patient results.Methods
The
prostate of 12 healthy volunteers (median age: 35, range: 27-55) was
scanned with a 12-channel phased array surface coil at a 1.5T Siemens
Sonata MRI scanner. The scan protocol included a T2 Turbo-Spin-Echo
(TSE) as anatomical reference followed by multifrequency MRE. In order to
assess the reproducibility of the MRE method, the whole protocol was
repeated after the subject left and reentered the scanner.
Additionally, one subject with benign prostatic
hyperplasia (BPH, 62 years old) and one patient with prostate carcinoma
originating from the transitional zone (69 years old) were scanned.
Similar to
previous work [6], spin-echo EPI based shear wave imaging was
combined with three pressurized-air actuators which were placed around the
pelvic region as shown in Figure 1. The actuators were vibrated with
air-pressure pulses of 60, 70 and 80 Hz repetition frequency.
Further
parameters of the MRE scans: 25 transverse slices, 2x2x2 mm³
isotropic voxel size, 8 wave dynamics, 3 wave field components, MEG
amplitude = 32 mT/m, TR=3280 ms, TE=68 ms,
FoV=256×256 mm2,
matrix size: 128×128, GRAPPA factor 2, 2 signal averages,
measurement time per frequency: 2:45 min (8:15 min in total).
In order to
reconstruct the parameter maps depicting the shear wave speed c,
the acquired images were analyzed by tomoelastography processing as
described in 8 and upsampled by a factor of 2. The regions of
interest for the whole prostate, the peripheral zone, and the central
gland (transitional zone + central zone) were drawn manually.
Results
Figure 2 compares the mean c-values of the two measurements for each volunteer in the three regions of interest, clearly showing inter-individual differences. The group c-values and relative deviation between measurement #1 and #2 are given in Table 1. A significant difference between the central (2.22±0.23 m/s) and the peripheral zone (2.27±0.20 m/s) could not be found, as the peripheral zone appeared stiffer only in a few subjects. The relative deviation between measurement #1 and #2 was slightly larger in the peripheral zone (3.7 %) than in the central zone (2.8 %).For the subject with BPH shown in Figure 3, anatomical details such as tissue heterogeneity in the central gland are well displayed in both the T2-TSE scan and the c-map. Figure 4 shows one apical image slice of one carcinoma patient. The tumor area with a mean shear wave speed of 3.23 m/s (Fig. 4c) contrasts strongly the surrounding tissue and closely resembles the enhanced area of the T1 gadolinum image acquired six months prior to the MRE exam (Fig. 4a). The mean c-values of the BPH-case (whole prostate: 2.45 m/s, peripheral zone: 2.45 m/s, central gland: 2.44 m/s) and the carcinoma patient (whole prostate: 2.12 m/s, peripheral zone: 2.09 m/s, central gland: 2.15 m/s) are not significantly different from the volunteers of the reproducibility study.
Discussion and Conclusion
We have
developed prostate MRE utilizing surface-based excitation, which
generated sufficiently high wave amplitudes within the prostate up to 80
Hz. In our current implementation, the acquisition of three separate wave fields including two
averages takes only 8:15 min covering the whole prostate with 2 mm isotropic resolution. Tomoelastography stiffness reconstruction8 provides highly resolved elasticity maps with good test-retest
reproducibility. Further improvements are expected by reducing organ
motion due to intestinal activity-reducing medication. The excellent
agreement of the c-map with the contrast enhanced T1 image in a
carcinoma patient motivates the investigation of more patients to
fully explore the potential of the promising method.Acknowledgements
No acknowledgement found.References
1. Muthupillai R, Ehman RL. Magnetic resonance elastography. Nature Med 1996;2(5):601-603.
2. Arani A, Plewes D, Chopra R. Transurethral Prostate Magnetic Resonance Elastography: Prospective Imaging Requirements. Magnetic Resonance in Medicine 2011;65(2):340-349.
3. Arani A, Da Rosa M, Ramsay E, Plewes DB, Haider Ma, Chopra R. Incorporating endorectal MR elastography into multi-parametric MRI for prostate cancer imaging: Initial feasibility in volunteers. Journal of magnetic resonance imaging : JMRI 2013;38:1251-1260.
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5. Sahebjavaher RS, Nir G, Honarvar M, Gagnon LO, Ischia J, Jones EC, Chang SD, Fazli L, Goldenberg SL, Rohling R, Kozlowski P, Sinkus R, Salcudean SE. MR elastography of prostate cancer: quantitative comparison with histopathology and repeatability of methods. NMR in biomedicine 2015;28:124-139.
6. Dittmann F, Tzschätzsch H, Guo J, Hirsch S, Braun J, Sack I. In vivo multifrequency MR elastography of the human prostate using a surface-based compressed air driver operated in the lower frequency regime. Proc. 24th Annual Meeting ISMRM 2016.
7. Dittmann F, Tzschätzsch H, Hirsch S, Barnhill E, Braun J, Sack I, Guo J. Tomoelastography of the abdomen: Tissue mechanical properties of the liver, spleen, kidney, and pancreas from single MR elastography scans at different hydration states. Magnetic resonance in medicine 2016.
8. Tzschätzsch H, Guo J, Dittmann F, Hirsch S, Barnhill E, Jöhrens K, Braun J, Sack I. Tomoelastography by multifrequency wave number recovery from time-harmonic propagating shear waves. Medical Image Analysis 2016;30:1-10.
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