MR elastography of liver tumours: value of viscoelastic properties for tumour characterisation

Viscoelastic parameters for quantifying liver fibrosis: three-dimensional multifrequency MR elastography study on thin liver rat slices

Objective

To assess in a high-resolution model of thin liver rat slices which viscoelastic parameter at three-dimensional multifrequency MR elastography has the best diagnostic performance for quantifying liver fibrosis.

Materials and Methods

The study was approved by the ethics committee for animal care of our institution. Eight normal rats and 42 rats with carbon tetrachloride induced liver fibrosis were used in the study. The rats were sacrificed, their livers were resected and three-dimensional MR elastography of 5±2 mm liver slices was performed at 7T with mechanical frequencies of 500, 600 and 700 Hz. The complex shear, storage and loss moduli, and the coefficient of the frequency power law were calculated. At histopathology, fibrosis and inflammation were assessed with METAVIR score, fibrosis was further quantified with morphometry. The diagnostic value of the viscoelastic parameters for assessing fibrosis severity was evaluated with simple and multiple linear regressions, receiver operating characteristic analysis and Obuchowski measures.

Results

At simple regression, the shear, storage and loss moduli were associated with the severity of fibrosis. At multiple regression, the storage modulus at 600 Hz was the only parameter associated with fibrosis severity (r = 0.86, p<0.0001). This parameter had an Obuchowski measure of 0.89+/−0.03. This measure was significantly larger than that of the loss modulus (0.78+/−0.04, p = 0.028), but not than that of the complex shear modulus (0.88+/−0.03, p = 0.84).

Conclusion

Our high resolution, three-dimensional multifrequency MR elastography study of thin liver slices shows that the storage modulus is the viscoelastic parameter that has the best association with the severity of liver fibrosis. However, its diagnostic performance does not differ significantly from that of the complex shear modulus.

Tumour biomechanical response to the vascular disrupting agent ZD6126 in vivo assessed by magnetic resonance elastography

BACKGROUND

Magnetic resonance elastography (MRE) is an emerging imaging technique that affords non-invasive quantitative assessment and visualization of tissue mechanical properties in vivo.

METHODS

In this study, MRE was used to quantify (kPa) the absolute value of the complex shear modulus |G*|, elasticity Gd and viscosity Gl of SW620 human colorectal cancer xenografts before and 24 h after treatment with either 200 mg kg(-1) of the vascular disrupting agent ZD6126 (N-acetylcolchinol-O-phosphate) or vehicle control, and the data were compared with changes in water diffusivity measured by diffusion-weighted magnetic resonance imaging.

RESULTS

A heterogeneous distribution of |G*|, Gd and Gl was observed pre-treatment with an intertumoral coefficient of variation of 13% for |G*|. There were no significant changes in the vehicle-treated cohort. In contrast, ZD6126 induced a significant decrease in the tumour-averaged |G*| (P<0.01), Gd (P<0.01) and Gl (P<0.05), and this was associated with histologically confirmed central necrosis. This reduction in tumour viscoelasticity occurred at a time when no significant change in tumour apparent diffusion coefficient (ADC) was observed.

CONCLUSIONS

These data demonstrate that MRE can provide early imaging biomarkers for treatment-induced tumour necrosis.

Magnetic resonance elastography of liver: clinical applications

Magnetic resonance elastography (MRE) has been successfully implemented in the assessment of diffuse liver diseases. Currently, MRE is the most accurate noninvasive technique for detection and staging of liver fibrosis with a potential to replace liver biopsy. Magnetic resonance elastography is able to differentiate isolated fatty liver disease from steatohepatitis with or without fibrosis. Potential clinical applications include the differentiation of benign and malignant focal liver masses and the assessment of treatment response in diffuse liver diseases.

Estimation of the absolute shear stiffness of human lung parenchyma using (1) H spin echo, echo planar MR elastography

PURPOSE

To develop a rapid proton MR elastography (MRE) technique that can quantify the absolute shear stiffness of lung parenchyma, to investigate the ability to differentiate respiration-dependent stiffness variations of the lung, and to demonstrate clinical feasibility.

MATERIALS AND METHODS

A spin-echo echo planar imaging MRE sequence (SE-EPI MRE) with a very short echo time was developed and tested in a series of five healthy volunteers at three different lung volumes: (i) residual volume (RV), (ii) total lung capacity (TLC), (iii) and midway between RV and TLC (MID). At each volume, lung density was quantified using a MR-based density mapping sequence. For reference, data were acquired using the previously described spin-echo lung MRE sequence (SE-MRE). MRE data were also acquired in a patient with proven Idiopathic Pulmonary Fibrosis (IPF) to test clinical feasibility.

RESULTS

The SE-EPIMRE sequence reduced total acquisition time by a factor of 2 compared with the SE-MRE sequence. Lung parenchyma median shear stiffness for the 5 volunteers quantified with the SE-EPI MRE sequence was 0.9 kPa, 1.1 kPa, and 1.6 kPa at RV, MID, and TLC, respectively. The corresponding values obtained with the SE-MRE sequence were 0.9 kPa, 1.1 kPa, and 1.5 kPa. Absolute shear stiffness was also successfully measured in the IPF patient.

CONCLUSION

The results indicate that stiffness variations due to respiration could be measured with the SE-EPIMRE technique and were equivalent to values generated by the previously described SE-MRE approach. Preliminary data obtained from the patient demonstrate clinical feasibility.

MR elastography of the liver: defining thresholds for detecting viscoelastic changes

PURPOSE

To define thresholds for detecting significant change in liver viscoelasticity with magnetic resonance (MR) elastography, both for whole-liver measurements and for voxel-wise measurements in relation to spatial resolution.

MATERIALS AND METHODS

This prospective study was approved by the institutional review board, and all participants provided written informed consent. Thirty participants (16 volunteers and 14 patients with hepatitis B or C; 18 men; median age, 30.4 years; age range, 18.9-58.6 years) underwent imaging twice while in the same position (intraimage reproducibility), after repositioning (within-day reproducibility), and 1-4 weeks later (between-weeks reproducibility). MR elastography parameters comprised elasticity, viscosity, attenuation parameter α, and propagation parameter β. Bland-Altman analysis was used to calculate repeatability indexes for each parameter. Analyses were performed in a region-of-interest and a voxel-by-voxel level. Voxel-wise results were calculated in relation to spatial resolution by applying Gaussian filtering to establish the optimal trade-off point between resolution and reproducibility.

RESULTS

For elasticity, α, and β, within-day and between-weeks results were significantly lower than intraimage results (P ≤ .018 for all). Within-day and between-weeks results did not differ significantly. Over-time changes of more than 22.2% for elasticity, 26.3% for viscosity, 26.8% for α, and 10.1% for β represented thresholds for significant change. The optimal trade-off between spatial resolution and reproducibility was found at a filter size of 8-mm full width at half maximum (FWHM) for elasticity and propagation parameter β and at 16-mm FWHM for viscosity and attenuation parameter α.

CONCLUSION

Repositioning causes a significant decrease in the reproducibility of MR elastography. The propagation parameter β is the most reliable parameter, with an over-time threshold for significant change of 10.1% and the ability to reproduce viscoelasticity up to a resolution of 8-mm FWHM. Online supplemental material is available for this article.

Rapid acquisition of multifrequency, multislice and multidirectional MR elastography data with a fractionally encoded gradient echo sequence

In MR elastography (MRE), periodic tissue motion is phase encoded using motion-encoding gradients synchronized to an externally applied periodic mechanical excitation. Conventional methods result in extended scan time for quality phase images, thus limiting the broad application of MRE in the clinic. For practical scan times, researchers have been relying on one-dimensional or two-dimensional motion-encoding, low-phase sampling and a limited number of slices, and artifact-prone, single-shot, echo planar imaging (EPI) readout. Here, we introduce a rapid multislice pulse sequence capable of three-dimensional motion encoding that is also suitable for simultaneously encoding motion with multiple frequency components. This sequence is based on a gradient-recalled echo (GRE) sequence and exploits the principles of fractional encoding. This GRE MRE pulse sequence was validated as capable of acquiring full three-dimensional motion encoding of isotropic voxels in a large volume within less than a minute. This sequence is suitable for monofrequency and multifrequency MRE experiments. In homogeneous paraffin phantoms, the eXpresso sequence yielded similar storage modulus values as those obtained with conventional methods, although with markedly reduced variances (7.11 ± 0.26 kPa for GRE MRE versus 7.16 ± 1.33 kPa for the conventional spin-echo EPI sequence). The GRE MRE sequence obtained better phase-to-noise ratios than the equivalent spin-echo EPI sequence (matched for identical acquisition time) in both paraffin phantoms and in vivo data in the liver (59.62 ± 11.89 versus 27.86 ± 3.81, 61.49 ± 14.16 versus 24.78 ± 2.48 and 58.23 ± 10.39 versus 23.48 ± 2.91 in the X, Y and Z components, respectively, in the case of liver experiments). Phase-to-noise ratios were similar between GRE MRE used in monofrequency or multifrequency experiments (75.39 ± 14.93 versus 86.13 ± 18.25 at 28 Hz, 71.52 ± 24.74 versus 86.96 ± 30.53 at 56 Hz and 95.60 ± 36.96 versus 61.35 ± 26.25 at 84Hz, respectively).

The shear modulus of the nucleus pulposus measured using magnetic resonance elastography: a potential biomarker for intervertebral disc degeneration

PURPOSE

This study aims to: (1) measure the shear modulus of nucleus pulposus (NP) in intact human vertebra-disc-vertebra segments using a magnetic resonance elastography setup for a 7T whole-body scanner, (2) quantify the effect of disc degeneration on the NP shear modulus measured using magnetic resonance elastography, and (3) compare the NP shear modulus to other magnetic resonance-based biomarkers of dis degeneration.

METHODS

Thirty intact human disc segments were classified as normal, mild, or severely degenerated. The NP shear modulus was measured using a custom-made setup that included a novel inverse method less sensitive to noisy displacements. T2 relaxation time was measured at 7T. The accuracy of these parameters to classify different degrees of degeneration was evaluated using receiver operating characteristic curves.

RESULTS

The magnetic resonance elastography measure of shear modulus in the NP was able to differentiate between normal, mild degeneration, and severe degeneration. The T2 relaxation time was able to differentiate between normal and mild degeneration, but it could not distinguish between mild and severe degeneration.

CONCLUSIONS

This study shows that the NP shear modulus measured using magnetic resonance elastography is sensitive to disc degeneration and has the potential of being used as a clinical tool to quantify the mechanical integrity of the intervertebral disc.

Evaluation of Nonradiative Clinical Imaging Techniques for the Longitudinal Assessment of Tumour Growth in Murine CT26 Colon Carcinoma

Background and Objectives. To determine the most appropriate technique for tumour followup in experimental therapeutics, we compared ultrasound (US) and magnetic resonance imaging (MRI) to characterize ectopic and orthotopic colon carcinoma models. Methods. CT26 tumours were implanted subcutaneously (s.c.) in Balb/c mice for the ectopic model or into the caecum for the orthotopic model. Tumours were evaluated by histology, spectrofluorescence, MRI, and US. Results. Histology of CT26 tumour showed homogeneously dispersed cancer cells and blood vessels. The visualization of the vascular network using labelled albumin showed that CT26 tumours were highly vascularized and disorganized. MRI allowed high-resolution and accurate 3D tumour measurements and provided additional anatomical and functional information. Noninvasive US imaging allowed good delineation of tumours despite an hypoechogenic signal. Monitoring of tumour growth with US could be accomplished as early as 5 days after implantation with a shorter acquisition time (<5 min) compared to MRI. Conclusion. MRI and US afforded excellent noninvasive imaging techniques to accurately follow tumour growth of ectopic and orthotopic CT26 tumours. These two techniques can be appropriately used for tumour treatment followup, with a preference for US imaging, due to its short acquisition time and simplicity of use.

MR elastography derived shear stiffness--a new imaging biomarker for the assessment of early tumor response to chemotherapy

PURPOSE

The overall goal is to develop magnetic resonance elastography derived shear stiffness as a biomarker for the early identification of chemotherapy response, allowing dose, agent type and treatment regimen to be tailored on a per patient basis, improving therapeutic outcome and minimizing normal tissue toxicity. The specific purpose of this study is to test the feasibility of this novel biomarker to measure the treatment response in a well-known chemotherapy model.

METHODS

Tumors were grown in the right flank of genetically modified mice by subcutaneous injection of DoHH2 (non-Hodgkin's lymphoma) cells. Magnetic resonance elastography was used to quantify tumor stiffness before and after injection of a chemotherapeutic agent or saline. Histological tests were also performed on the tumors.

RESULTS

A significant decrease (P < 0.0001) in magnetic resonance elastography-derived tumor shear stiffness was observed within 4 days of chemotherapy treatment, while no appreciable change was observed in saline-treated tumors. No significant change in volume occurred at this early stage, but there were decreased levels of cellular proliferation in chemotherapy-treated tumors.

CONCLUSION

These results demonstrate that magnetic resonance elastography-derived estimates of shear stiffness reflect an initial response to cytotoxic therapy and suggest that this metric could be an early and sensitive biomarker of tumor response to chemotherapy.

Non-invasive characterization of polyurethane-based tissue constructs in a rat abdominal repair model using high frequency ultrasound elasticity imaging

The evaluation of candidate materials and designs for soft tissue scaffolds would benefit from the ability to monitor the mechanical remodeling of the implant site without the need for periodic animal sacrifice and explant analysis. Toward this end, the ability of non-invasive ultrasound elasticity imaging (UEI) to assess temporal mechanical property changes in three different types of porous, biodegradable polyurethane scaffolds was evaluated in a rat abdominal wall repair model. The polymers utilized were salt-leached scaffolds of poly(carbonate urethane) urea, poly(ester urethane) urea and poly(ether ester urethane) urea at 85% porosity. A total of 60 scaffolds (20 each type) were implanted in a full thickness muscle wall replacement in the abdomens of 30 rats. The constructs were ultrasonically scanned every 2 weeks and harvested at weeks 4, 8 and 12 for compression testing or histological analysis. UEI demonstrated different temporal stiffness trends among the different scaffold types, while the stiffness of the surrounding native tissue remained unchanged. The changes in average normalized strains developed in the constructs from UEI compared well with the changes of mean compliance from compression tests and histology. The average normalized strains and the compliance for the same sample exhibited a strong linear relationship. The ability of UEI to identify herniation and to characterize the distribution of local tissue in-growth with high resolution was also investigated. In summary, the reported data indicate that UEI may allow tissue engineers to sequentially evaluate the progress of tissue construct mechanical behavior in vivo and in some cases may reduce the need for interim time point animal sacrifice.