Prostate lesions characterization using diffusion-weighted spatiotemporal encoded MRI: Feasibility and initial assessment

PURPOSE

To assess the feasibility and reliability of a DWI protocol based on spatiotemporally encoding (SPEN), to target prostate lesions along guidelines normally used in EPI-based DWI clinical practice.

METHODS

Prostate Imaging-Reporting and Data System recommendations underlying clinical prostate scans were used to develop a SPEN-based DWI protocol, which included a novel, local, low-rank regularization algorithm. These DWI acquisitions were run at 3 T under similar nominal spatial resolutions and diffusion-weighting b-values as used in EPI-based clinical studies. Prostates of 11 patients suspected of clinically significant prostate cancer lesions were therefore scanned using the two methods, with the same number of slices, same slice thickness, and same interslice gaps.

RESULTS

Of the 11 patients scanned, SPEN and EPI provided comparable information in 7 of the cases, whereas EPI was deemed superior in a case for which SPEN images had to be acquired with a shorter effective TR owing to scan-time constraints. SPEN provided reduced susceptibility to field-derived distortions in 3 of the cases.

CONCLUSIONS

SPEN's ability to provide prostate lesion contrast was most clearly evidenced for DW images acquired with b ≥ 900 s/mm² . SPEN also succeeded in decreasing occasional image distortions in regions close to the rectum, affected by field inhomogeneities. EPI advantages arose when using short effective TRs, a regime in which SPEN-based DWI was handicapped by its use of nonselective spin inversions, leading to the onset of an additional T₁ weighting.

Motion compensated renal diffusion weighted imaging

PURPOSE

To assess the effect of respiratory motion and cardiac driven pulsation in renal DWI and to examine asymmetrical velocity-compensated diffusion encoding waveforms for robust ADC mapping in the kidneys.

METHODS

The standard monopolar Stejskal-Tanner pulsed gradient spin echo (pgse) and the asymmetric bipolar velocity-compensated (asym-vc) diffusion encoding waveforms were used for coronal renal DWI at 3T. The robustness of the ADC quantification in the kidneys was tested with the aforementioned waveforms in respiratory-triggered and breath-held cardiac-triggered scans at different trigger delays in 10 healthy subjects.

RESULTS

The pgse waveform showed higher ADC values in the right kidney at short trigger delays in comparison to longer trigger delays in the respiratory triggered scans when the diffusion gradient was applied in the feet-head (FH) direction. The coefficient of variation over all respiratory trigger delays, averaged over all subjects was 0.15 for the pgse waveform in the right kidney when diffusion was measured in the FH direction; the corresponding coefficient of variation for the asym-vc waveform was 0.06. The effect of cardiac driven pulsation was found to be small in comparison to the effect of respiratory motion.

CONCLUSION

Short trigger delays in respiratory-triggered scans can cause higher ADC values in comparison to longer trigger delays in renal DWI, especially in the right kidney when diffusion is measured in the FH direction. The asym-vc waveform can reduce ADC variation due to respiratory motion in respiratory-triggered scans at the cost of reduced SNR compared to the pgse waveform.

Radiomics improves the utility of ADC for differentiation between renal oncocytoma and chromophobe renal cell carcinoma: Preliminary findings

OBJECTIVE

Differentiation between renal oncocytoma (RON) and chromophobe renal cell carcinoma (chRCC) remains challenging. We aimed to assess the accurate apparent diffusion coefficient (ADC) radiomics features in differentiating these tumors.

MATERIALS AND METHODS

This single-center retrospective study included 14 patients with histopathologically proven RON (n = 6) and chRCC (n = 8) who underwent magnetic resonance imaging. Features were extracted from ADC maps. Features with an intraclass correlation coefficient >0.90, an intergroup p < 0.01 and interrater differences with normal distribution underwent agreement and receiver operating characteristic curve analyses.

RESULTS

Overall, 6 features qualified for further analysis and Bland-Altman plots revealed acceptable agreement for all. Only 1 first order feature and 5 high order texture features successfully predicted RON with more than 90% sensitivities and specificities more than 80%.

CONCLUSION

Squared mean ADC and certain gray level run length matrix features extracted by radiomics of ADC mapping provide quite high diagnostic precision in terms of distinguishing between RON and chRCC.

Imaging Evaluation of Intervertebral Disc Degeneration and Painful Discs-Advances and Challenges in Quantitative MRI

In recent years, various quantitative and functional magnetic resonance imaging (MRI) sequences have been developed and used in clinical practice for the diagnosis of patients with low back pain (LBP). Until now, T2-weighted imaging (T2WI), a visual qualitative evaluation method, has been used to diagnose intervertebral disc (IVD) degeneration. However, this method has limitations in terms of reproducibility and inter-observer agreement. Moreover, T2WI observations do not directly relate with LBP. Therefore, new sequences such as T2 mapping, T1ρ mapping, and MR spectroscopy have been developed as alternative quantitative evaluation methods. These new quantitative MRIs can evaluate the anatomical and physiological changes of IVD degeneration in more detail than conventional T2WI. However, the values obtained from these quantitative MRIs still do not directly correlate with LBP, and there is a need for more widespread use of techniques that are more specific to clinical symptoms such as pain. In this paper, we review the state-of-the-art methodologies and future challenges of quantitative MRI as an imaging diagnostic tool for IVD degeneration and painful discs.

Gradient nonlinearity correction in liver DWI using motion-compensated diffusion encoding waveforms

OBJECTIVE : To experimentally characterize the effectiveness of a gradient nonlinearity correction method in removing ADC bias for different motion-compensated diffusion encoding waveforms.

METHODS

The diffusion encoding waveforms used were the standard monopolar Stejskal-Tanner pulsed gradient spin echo (pgse) waveform, the symmetric bipolar velocity-compensated waveform (sym-vc), the asymmetric bipolar velocity-compensated waveform (asym-vc) and the asymmetric bipolar partial velocity-compensated waveform (asym-pvc). The effectiveness of the gradient nonlinearity correction method using the spherical harmonic expansion of the gradient coil field was tested with the aforementioned waveforms in a phantom and in four healthy subjects.

RESULTS

The gradient nonlinearity correction method reduced the ADC bias in the phantom experiments for all used waveforms. The range of the ADC values over a distance of ± 67.2 mm from isocenter reduced from 1.29 × 10^-4 to 0.32 × 10^-4 mm²/s for pgse, 1.04 × 10^-4 to 0.22 × 10^-4 mm²/s for sym-vc, 1.22 × 10^-4 to 0.24 × 10^-4 mm²/s for asym-vc and 1.07 × 10^-4 to 0.11 × 10^-4 mm²/s for asym-pvc. The in vivo results showed that ADC overestimation due to motion or bright vessels can be increased even further by the gradient nonlinearity correction.

CONCLUSION

The investigated gradient nonlinearity correction method can be used effectively with various motion-compensated diffusion encoding waveforms. In coronal liver DWI, ADC errors caused by motion and residual vessel signal can be increased even further by the gradient nonlinearity correction.

Quantitative MRI Changes During Weekly Ultra-Hypofractionated Prostate Cancer Radiotherapy With Integrated Boost

Purpose: Quantitative MRI reflects tissue characteristics. As possible changes during radiotherapy may lead to treatment adaptation based on response, we here assessed if such changes during treatment can be detected.

Methods and Materials: In the hypoFLAME trial patients received ultra-hypofractionated prostate radiotherapy with an integrated boost to the tumor in 5 weekly fractions. We analyzed T2 and ADC maps of 47 patients that were acquired in MRI exams prior to and during radiotherapy, and performed rigid registrations based on the prostate contour on anatomical T2-weighted images. We analyzed median T2 and ADC values in three regions of interest (ROIs): the central gland (CG), peripheral zone (PZ), and tumor. We analyzed T2 and ADC changes during treatment and compared patients with and without hormonal therapy. We tested changes during treatment for statistical significance with Wilcoxon signed rank tests. Using confidence intervals as recommended from test-retest measurements, we identified persistent T2 and ADC changes during treatment.

Results: In the CG, median T2 and ADC values significantly decreased 12 and 8%, respectively, in patients that received hormonal therapy, while in the PZ these values decreased 17 and 18%. In the tumor no statistically significant change was observed. In patients that did not receive hormonal therapy, median ADC values in the tumor increased with 20%, while in the CG and PZ no changes were observed. Persistent T2 changes in the tumor were found in 2 out of 24 patients, while none of the 47 patients had persistent ADC changes.

Conclusions: Weekly quantitative MRI could identify statistically significant ADC changes in the tumor in patients without hormonal therapy. On a patient level few persistent T2 changes in the tumor were observed. Long-term follow-up is required to relate the persistent T2 and ADC changes to outcome and evaluate the applicability of quantitative MRI for response based treatment adaptation.

Retrieved cerebral thrombi studied by T2 and ADC mapping: preliminary results

Background

Recent advances in MRI technology makes it increasingly more competitive to CT also in the field of interventions. Multi-parametric MRI offers a significant amount of data relevant for characterization of human cerebral thrombi.

Patients and methods

Cerebral thrombi of 17 patients diagnosed with acute stroke were acquired by mechanical thrombectomy. The thrombi were subsequently scanned using a high spatial-resolution 3D T₁-weighted MRI to obtain morphological characteristics of the thrombi and also by apparent diffusion coefficient (ADC) and transversal nuclear magnetic resonance (NMR) relaxation time (T₂) mapping. The MRI results were analysed for possible correlations between thrombectomy procedure parameters (recanalization time and number of passes) and MR-measurable parameters (sample-mean ADC and T₂, within-sample coefficient of variation of ADC and T₂, and thrombus length).

Results

Both MRI mapping techniques enabled a good discrimination among thrombi regions of different water mobility and compaction. Within-sample coefficient of variation of ADC was found most sensitive for discrimination between the thrombi where thrombectomy procedure was performed in a single pass and those where is was performed in two or more passes (p = 0.03). Interestingly, negative correlation was found between the recanalization time and thrombus length (ρ = -0.22).

Conclusions

Preliminary results of presented study shows that pretreatment MRI assessment of thrombi in stroke patients could potentially ease stroke treatment planning. In this study it is shown that within-sample coefficient of variation of ADC could serve for prediction of possible complications during thrombectomy procedures.

Dynamic diffusion-weighted hyperpolarized 13 C imaging based on a slice-selective double spin echo sequence for measurements of cellular transport

PURPOSE

To develop a pulse sequence to dynamically measure the ADC of hyperpolarized substrates during their perfusion, metabolic conversion, and transport.

METHODS

We proposed a slice-selective double spin echo sequence for dynamic hyperpolarized ¹³ C diffusion-weighted imaging. The proposed pulse sequence was optimized for a high field preclinical scanner through theoretical analysis and simulation. The performance of the method was compared to non-slice-selective double spin echo via in vivo studies. We also validated the sequence for dynamic ADC measurement in both phantom studies and transgenic mouse model of prostate cancer studies.

RESULTS

The optimized pulse sequence outperforms the traditional sequence with smaller saturation effects on the magnetization of hyperpolarized compounds that allowed more dynamic imaging frames covering a longer imaging time window. In pre-clinical studies (N = 8), the dynamic hyperpolarized lactate ADC maps of 6 studies in the prostate tumors showed an increase measured ADC over time, which might be related to lactate efflux from the tumor cells.

CONCLUSIONS

The proposed sequence was validated and shown to improve dynamic diffusion weighted imaging compared to the traditional double spin echo sequence, providing ADC maps of lactate through time.

Diffusion weighted imaging demystified: the technique and potential clinical applications for soft tissue imaging

Diffusion-weighted imaging (DWI) is a fast, non-contrast technique that is readily available and easy to integrate into an existing imaging protocol. DWI with apparent diffusion coefficient (ADC) mapping offers a quantitative metric for soft tissue evaluation and provides information regarding the cellularity of a region of interest. There are several available methods of performing DWI, and artifacts and pitfalls must be considered when interpreting DWI studies. This review article will review the various techniques of DWI acquisition and utility of qualitative as well as quantitative methods of image interpretation, with emphasis on optimal methods for ADC measurement. The current clinical applications for DWI are primarily related to oncologic evaluation: For the assessment of de novo soft tissue masses, ADC mapping can serve as a useful adjunct technique to routine anatomic sequences for lesion characterization as cyst or solid and, if solid, benign or malignant. For treated soft tissue masses, the role of DWI/ADC mapping in the assessment of treatment response as well as recurrent or residual neoplasm in the setting of operative management is discussed, especially when intravenous contrast medium cannot be given. Emerging DWI applications for non-neoplastic clinical indications are also reviewed.

Whole lung morphometry with 3D multiple b-value hyperpolarized gas MRI and compressed sensing

Purpose

To demonstrate three‐dimensional (3D) multiple b‐value diffusion‐weighted (DW) MRI of hyperpolarized ³He gas for whole lung morphometry with compressed sensing (CS).

Methods

A fully‐sampled, two b‐value, 3D hyperpolarized ³He DW‐MRI dataset was acquired from the lungs of a healthy volunteer and retrospectively undersampled in the k_y and k_z phase‐encoding directions for CS simulations. Optimal k‐space undersampling patterns were determined by minimizing the mean absolute error between reconstructed and fully‐sampled ³He apparent diffusion coefficient (ADC) maps. Prospective three‐fold, undersampled, 3D multiple b‐value ³He DW‐MRI datasets were acquired from five healthy volunteers and one chronic obstructive pulmonary disease (COPD) patient, and the mean values of maps of ADC and mean alveolar dimension (Lm_D) were validated against two‐dimensional (2D) and 3D fully‐sampled ³He DW‐MRI experiments.

Results

Reconstructed undersampled datasets showed no visual artifacts and good preservation of the main image features and quantitative information. A good agreement between fully‐sampled and prospective undersampled datasets was found, with a mean difference of +3.4% and +5.1% observed in mean global ADC and Lm_D values, respectively. These differences were within the standard deviation range and consistent with values reported from healthy and COPD lungs.

Conclusions

Accelerated CS acquisition has facilitated 3D multiple b‐value ³He DW‐MRI scans in a single breath‐hold, enabling whole lung morphometry mapping. Magn Reson Med 77:1916–1925, 2017. © 2016 The Authors Magnetic Resonance in Medicine published by Wiley Periodicals, Inc. on behalf of International Society for Magnetic Resonance in Medicine. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

Rapid in vivo apparent diffusion coefficient mapping of hyperpolarized (13) C metabolites

PURPOSE

Hyperpolarized (13) C magnetic resonance allows for the study of real-time metabolism in vivo, including significant hyperpolarized (13) C lactate production in many tumors. Other studies have shown that aggressive and highly metastatic tumors rapidly transport lactate out of cells. Thus, the ability to not only measure the production of hyperpolarized (13) C lactate but also understand its compartmentalization using diffusion-weighted MR will provide unique information for improved tumor characterization.

METHODS

We used a bipolar, pulsed-gradient, double spin echo imaging sequence to rapidly generate diffusion-weighted images of hyperpolarized (13) C metabolites. Our methodology included a simultaneously acquired B1 map to improve apparent diffusion coefficient (ADC) accuracy and a diffusion-compensated variable flip angle scheme to improve ADC precision.

RESULTS

We validated this sequence and methodology in hyperpolarized (13) C phantoms. Next, we generated ADC maps of several hyperpolarized (13) C metabolites in a normal rat, rat brain tumor, and prostate cancer mouse model using both preclinical and clinical trial-ready hardware.

CONCLUSION

ADC maps of hyperpolarized (13) C metabolites provide information about the localization of these molecules in the tissue microenvironment. The methodology presented here allows for further studies to investigate ADC changes due to disease state that may provide unique information about cancer aggressiveness and metastatic potential.

Demonstration of nonlinearity bias in the measurement of the apparent diffusion coefficient in multicenter trials

PURPOSE

Characterize system-specific bias across common magnetic resonance imaging (MRI) platforms for quantitative diffusion measurements in multicenter trials.

METHODS

Diffusion weighted imaging (DWI) was performed on an ice-water phantom along the superior-inferior (SI) and right-left (RL) orientations spanning ± 150 mm. The same scanning protocol was implemented on 14 MRI systems at seven imaging centers. The bias was estimated as a deviation of measured from known apparent diffusion coefficient (ADC) along individual DWI directions. The relative contributions of gradient nonlinearity, shim errors, imaging gradients, and eddy currents were assessed independently. The observed bias errors were compared with numerical models.

RESULTS

The measured systematic ADC errors scaled quadratically with offset from isocenter, and ranged between -55% (SI) and 25% (RL). Nonlinearity bias was dependent on system design and diffusion gradient direction. Consistent with numerical models, minor ADC errors (± 5%) due to shim, imaging and eddy currents were mitigated by double echo DWI and image coregistration of individual gradient directions.

CONCLUSION

The analysis confirms gradient nonlinearity as a major source of spatial DW bias and variability in off-center ADC measurements across MRI platforms, with minor contributions from shim, imaging gradients and eddy currents. The developed protocol enables empiric description of systematic bias in multicenter quantitative DWI studies.

Is the quantitative Diffusion-Weighted MR Imaging and ADC mapping with b-values of 50, 400, and 800 sec/mm(2) a reliable method for evaluation of meniscal tears in the knee?

BACKGROUND

Our aim was to evaluate the efficacy of Diffusion-Weighted Imaging (DWI) and Apparent Diffusion Coefficient (ADC) mapping with different b-factors in visualisation of meniscal tears.

MATERIAL/METHODS

Seventy-four patients; 30 males and 44 females, 37 left and 37 right knees with meniscal tears were involved in this study. Eleven of them were lateral meniscal tears and 63 - medial meniscal tears. DWI was obtained by 3D-SE Echo-planar Imaging (EPI) in coronal and sagittal planes. ADC mapping was carried out in coronal planes with b-factors of 50, 400, and 800 sec/mm(2). The statistical analysis of DWI and ADC mapping results was performed with the use of the Fisher's test and the chi-square test.

RESULTS

1. For both menisci and 74 tears: DWI revealed 86% sensitivity and 100% specificity (p=0.149) with a positive predictive value (PPV) of 1 and a negative predictive value (NPV) of 0.09. ADC mapping with b-value of 400 sec/mm(2) had 78% sensitivity, and 100% specificity, with PPV of 1 and NPV of 0.06 (p=0.230). 2. For the lateral meniscal tears: DWI and ADC mapping with b-value of 800, for the medial meniscal tears: DWI and ADC mapping with b-factor of 400 s/mm(2) revealed higher sensitivity and specificity than other methods.

CONCLUSIONS

Quantitative DWI and ADC mapping, especially with b-factor of 400 sec/mm(2), may be an alternative to routine MR imaging sequences for the visualisation of meniscal tears in the knee.