Intravoxel incoherent motion model-based liver lesion characterisation from three b-value diffusion-weighted MRI

Combined diffusion and perfusion index maps from simplified intravoxel incoherent motion imaging enable visual assessment of breast lesions

The aim was to evaluate visual breast lesion assessment using single binary index maps (IDf) in comparison to the use of combined regions of interest (ROI) analysis of estimated diffusion coefficient (D') AND perfusion fraction (f'), which proved to be the best method in a previous simplified intravoxel incoherent motion DWI, if diffusion-weighted imaging (DWI) is used as stand-alone tool. IDf, was constructed voxel-wise from cut-off values of D' and f'. The cut-off values, the data of 105 malignant and 86 benign lesions and the ROIs were re-used. For visual assessment, IDf was displayed as two-colour b800 overlay with red representing "malignant" and green "benign" voxels. A lesion was rated as "malignant", if a red hot spot was found within translucent hyperintensity on b800, otherwise as "benign". Intraindividual comparison of quantitative analysis and visual assessment of IDf showed comparable accuracy, both to each other and to combined ROI-analysis of D' and f' maps (0.927 vs. 0.937, p = 0.157, and 0.921 vs. 0.937, p = 0.157, respectively). Thus, visual assessment of IDf can replace combined ROI analysis of D' and f' without loss in accuracy enabling a considerable facilitation in clinical routine.

The Relationship Between Conventionally Obtained Serum-Based Liver Function Indices and Intravoxel Incoherent Motion Diffusion-Weighted Imaging and Magnetic Resonance Elastography in Patients With Hepatocellular Carcinoma

OBJECTIVES

To investigate the relationship between conventionally obtained serum-based biochemical indices and intravoxel incoherent motion imaging (IVIM) parameters compared with magnetic resonance elastography (MRE).

METHODS

Patients with hepatocellular carcinoma who underwent ≥2 liver magnetic resonance imaging (MRI) scan, including IVIM and MRE, between 2017 and 2020 and biochemical testing within 1 week before or after MRI were included in this study. Biochemical tests were performed to determine the albumin-bilirubin (ALBI) score and modified ALBI (mALBI) grade, aspartate aminotransferase to platelet ratio index (APRI), and fibrosis-4 index (FIB-4). The diffusion coefficient ( D ), pseudo-diffusion coefficient ( D *), fractional volume occupied by flowing spins ( f ), and apparent diffusion coefficient were calculated for IVIM. The correlations between (1) the imaging parameters and biochemical indices and (2) the changes in mALBI grades and imaging parameters were evaluated.

RESULTS

This study included 98 scans of 40 patients (31 men; mean age, 67.7 years). The correlation analysis between the biochemical and IVIM parameters showed that ALBI score and D* had the best correlation ( r = -0.3731, P < 0.001), and the correlation was higher than that with MRE ( r = 0.3289, P < 0.001). However, among FIB-4, APRI, and MRI parameters, MRE outperformed IVIM parameters (MRE and FIB-4, r = 0.3775, P < 0.001; MRE and APRI, r = 0.4687, P < 0.001). There were significant differences in the changes in MRE among the 3 groups (improved, deteriorated, and unchanged mALBI groups) in the analysis of covariance ( P = 0.0434). There were no significant changes in IVIM.

CONCLUSIONS

Intravoxel incoherent motion imaging has the potential to develop into a more readily obtainable method of liver function assessment.

Simplified intravoxel incoherent motion DWI for differentiating malignant from benign breast lesions

Background

To evaluate simplified intravoxel incoherent motion (IVIM) diffusion-weighted imaging (DWI) for differentiating malignant versus benign breast lesions as (i) stand-alone tool and (ii) add-on to dynamic contrast-enhanced magnetic resonance imaging.

Methods

1.5-T DWI data (b = 0, 50, 250, 800 s/mm²) were retrospectively analysed for 126 patients with malignant or benign breast lesions. Apparent diffusion coefficient (ADC) ADC (0, 800) and IVIM-based parameters D₁′ = ADC (50, 800), D₂′ = ADC (250, 800), f₁′ = f (0, 50, 800), f₂′ = f (0, 250, 800) and D*′ = D* (0, 50, 250, 800) were voxel-wise calculated without fitting procedures. Regions of interest were analysed in vital tumour and perfusion hot spots. Beside the single parameters, the combined use of D₁′ with f₁′ and D₂′ with f₂′ was evaluated. Lesion differentiation was investigated for lesions (i) with hyperintensity on DWI with b = 800 s/mm² (n = 191) and (ii) with suspicious contrast-enhancement (n = 135).

Results

All lesions with suspicious contrast-enhancement appeared also hyperintense on DWI with b = 800 s/mm². For task (i), best discrimination was reached for the combination of D₁′ and f₁′ using perfusion hot spot regions-of-interest (accuracy 93.7%), which was higher than that of ADC (86.9%, p = 0.003) and single IVIM parameters D₁′ (88.0%) and f₁′ (87.4%). For task (ii), best discrimination was reached for single parameter D₁′ using perfusion hot spot regions-of-interest (92.6%), which were slightly but not significantly better than that of ADC (91.1%) and D₂′ (88.1%). Adding f₁′ to D₁′ did not improve discrimination.

Conclusions

IVIM analysis yielded a higher accuracy than ADC. If stand-alone DWI is used, perfusion analysis is of special relevance.

Application of intravoxel incoherent motion diffusion-weighted imaging in hepatocellular carcinoma

The morbidity and mortality of hepatocellular carcinoma (HCC) rank 6th and 4th, respectively, among malignant tumors worldwide. Traditional diffusion-weighted imaging (DWI) uses the apparent diffusion coefficient (ADC) obtained by applying the monoexponential model to reflect water molecule diffusion in active tissue; however, the value of ADC is affected by microcirculation perfusion. Using a biexponential model, intravoxel incoherent motion (IVIM)-DWI quantitatively measures information related to pure water molecule diffusion and microcirculation perfusion, thus compensating for the shortcomings of DWI. The number of studies examining the application of IVIM-DWI in patients with HCC has gradually increased over the last few years, and many results show that IVIM-DWI has vital value for HCC differentiation, pathological grading, and predicting and evaluating the treatment response. The present study principally reviews the principle of IVIM-DWI and its research progress in HCC differentiation, pathological grading, predicting and evaluating the treatment response, predicting postoperative recurrence and predicting gene expression prediction.

Simplified intravoxel incoherent motion diffusion-weighted MRI of liver lesions: feasibility of combined two-colour index maps

Background

To evaluate the feasibility of two-colour index maps containing combined diffusion and perfusion information from simplified intravoxel incoherent motion (IVIM) for liver lesion malignancy assessment.

Methods

Diffusion-weighted data from a respiratory-gated 1.5-T magnetic resonance sequence were analysed in 109 patients with liver lesions. With three b values (0, 50, 800 s/mm²) estimated diffusion coefficient D′, perfusion fraction f′, and apparent diffusion coefficient (ADC) maps were calculated and analysed for regions of interest (ROIs). D′ and f′ cutoff values were determined by differentiating haemangiomas from other lesions and focal nodular hyperplasias from other lesions, respectively. Combined I_Df index maps were generated with a voxel value set to 100, if both D′ and f′ voxel values were lower than their cutoff values (1,529.4 × 10^-6 mm²/s and 114.4 × 10^-3, respectively), otherwise to 0. Moreover, I_ADC index maps were generated from ADC cutoff value (1,338.5 × 10^-6 mm²/s) obtained by differentiating benign from malignant lesions. Discriminatory power was assessed for both I_Df and I_ADC. Index maps were displayed as two-colour overlays to b-800 images and visually assessed within the translucent hyperintense areas.

Results

For I_Df, the same diagnostic accuracy was achieved as for the combined use of parameters D′ and f′ (93.6%). Compared to I_ADC, I_Df showed a higher diagnostic accuracy. Visual judgment of I_Df yielded an accuracy (95.4%) similar to that of quantitative analysis (93.6%).

Conclusion

Voxel-wise combined two-colour index maps I_Df provide similar diagnostic accuracy as ROI-based combination of estimated IVIM parameters D′ and f′ and are suitable for visual assessment of liver lesion malignancy.

Statistical Evaluation of Different Mathematical Models for Diffusion Weighted Imaging of Prostate Cancer Xenografts in Mice

Purpose

To evaluate fitting quality and repeatability of four mathematical models for diffusion weighted imaging (DWI) during tumor progression in mouse xenograft model of prostate cancer.

Methods

Human prostate cancer cells (PC-3) were implanted subcutaneously in right hind limbs of 11 immunodeficient mice. Tumor growth was followed by weekly DWI examinations using a 7T MR scanner. Additional DWI examination was performed after repositioning following the fourth DWI examination to evaluate short term repeatability. DWI was performed using 15 and 12 b-values in the ranges of 0-500 and 0-2000 s/mm², respectively. Corrected Akaike information criteria and F-ratio were used to evaluate fitting quality of each model (mono-exponential, stretched exponential, kurtosis, and bi-exponential).

Results

Significant changes were observed in DWI data during the tumor growth, indicated by ADC_m, ADC_s, and ADC_k. Similar results were obtained using low as well as high b-values. No marked changes in model preference were present between the weeks 1−4. The parameters of the mono-exponential, stretched exponential, and kurtosis models had smaller confidence interval and coefficient of repeatability values than the parameters of the bi-exponential model.

Conclusion

Stretched exponential and kurtosis models showed better fit to DWI data than the mono-exponential model and presented with good repeatability.

Restricted Water Diffusion in Diffusion-Weighted Magnetic Resonance Imaging in Pancreatic Cancer is Associated with Tumor Hypoxia

Simple Summary

Pancreatic cancer is characterized by a dense network of connective tissue surrounding clusters of cancer cells, the so-called stroma. This ubiquitous connective tissue impairs the delivery of oxygen to cancer cells. This results in hypoxia, which renders the cancer more aggressive and more resistant to treatment. In the present study, we investigated whether the extent of hypoxia in pancreatic cancer can be predicted by magnetic resonance imaging (MRI), a widely used medical imaging technique. More specifically, we used an MRI sequence which can quantitate the random motion (i.e., diffusion) of water molecules within the cancer tissue, namely diffusion-weighted (DW) MRI. We found that the random motion of water molecules is lower in cancer lesions with high hypoxia compared to those with low hypoxia. The findings from our study imply that DW-MRI can be used to identify pancreatic cancer lesions with high hypoxia which are at high risk for treatment failure.

Abstract

Hypoxia is a hallmark of pancreatic cancer (PDAC) due to its compact and extensive fibrotic tumor stroma. Hypoxia contributes to high lethality of this disease, by inducing a more malignant phenotype and resistance to radiation and chemotherapy. Thus, non-invasive methods to quantify hypoxia could be helpful for treatment decisions, for monitoring, especially in non-resectable tumors, or to optimize personalized therapy. In the present study, we investigated whether tumor hypoxia in PDAC is reflected by diffusion-weighted magnetic resonance imaging (DW-MRI), a functional imaging technique, frequently used in clinical practice for identification and characterization of pancreatic lesions. DW-MRI assesses the tissue microarchitecture by measuring the diffusion of water molecules, which is more restricted in highly compact tissues. As reliable surrogate markers for hypoxia, we determined Blimp-1 (B-lymphocyte induced maturation protein), a transcription factor, as well as vascular endothelial growth factor (VEGF), which are up-regulated in response to hypoxia. In 42 PDAC patients, we observed a close association between restricted water diffusion in DW-MRI and tumor hypoxia in matched samples, as expressed by high levels of Blimp-1 and VEGF in tissue samples of the respective patients. In summary, our data show that DW-MRI is well suited for the evaluation of tumor hypoxia in PDAC and could potentially be used for the identification of lesions with a high hypoxic fraction, which are at high risk for failure of radiochemotherapy.

Associations between IVIM histogram parameters and histopathology in rectal cancer

PURPOSE

Histogram analysis can better reflect tumor heterogeneity than conventional imaging analysis. The present study analyzed possible correlations between histogram analysis parameters derived from Intravoxel-incoherent imaging (IVIM) and histopathological features in rectal cancer (RC).

METHODS

Seventeen patients with histopathologically proven rectal adenocarcinomas were retrospectively acquired. In all cases, pelvic MRI was performed. Diffusion weighted imaging was obtained using a multi-slice single-shot echo-planar imaging sequence with b values of 0, 50, 200, 500 and 1000 s/mm². Simplified IVIM analysis was performed using the IntelliSpace portal, version 10 and the following images were generated: f (perfusion fraction) map, D (true diffusion coefficient) map, and ADC map utilizing all b-values. Histogram based analysis of signal intensities was performed for every IVIM map using an in-house matlab tool. Histopathology was investigated using Ki 67 specimens with calculation of Ki 67-index and cellularity. CD31 stained specimens were used for calculation of microvessel density (MVD).

RESULTS

There were statistically significant correlations between Ki 67 index and mode derived from ADC as well as entropy from f, r=-0.50, p=.04 and r=-0.55, p=.02, respectively. MVD correlated well with parameters derived from f.

CONCLUSION

IVIM histogram analysis parameters can reflect histopathology in RC. ADC and D values are associated with proliferation potential. Perfusion fraction f is associated with MVD.

Progress of intravoxel incoherent motion diffusion-weighted imaging in liver diseases

Traditional magnetic resonance (MR) diffusion-weighted imaging (DWI) uses a single exponential model to obtain the apparent diffusion coefficient to quantitatively reflect the diffusion motion of water molecules in living tissues, but it is affected by blood perfusion. Intravoxel incoherent motion (IVIM)-DWI utilizes a double-exponential model to obtain information on pure water molecule diffusion and microcirculatory perfusion-related diffusion, which compensates for the insufficiency of traditional DWI. In recent years, research on the application of IVIM-DWI in the diagnosis and treatment of hepatic diseases has gradually increased and has achieved considerable progress. This study mainly reviews the basic principles of IVIM-DWI and related research progress in the diagnosis and treatment of hepatic diseases.

Intravoxel Incoherent Motion of Colon Cancer Liver Metastases for the Assessment of Response to Antiangiogenic Treatment: Results from a Pilot Study

OBJECTIVE

This study was aimed at evaluating the intravoxel incoherent motion (IVIM) parameter alterations of liver metastases of colorectal carcinoma (CRC) during antiangiogenic bevacizumab combination therapy.

METHODS

Twenty-five patients with CRC liver metastases treated with bevacizumab in combination with FOLFOX-or-FOLFIRI protocols were enrolled in the study. MRI was performed using a 1.5-tesla scanner pre-treatment (PT) and at 3, 6, and 9 months of therapy. Routine abdominal MRI sequences and an IVIM-DWI (diffusion-weighted imaging) sequence were obtained. The IVIM-DWI sequence was executed with 16 b-values varying from 0 to 1,400 s/mm2. The mean values of apparent diffusion coefficient (ADC), true diffusion (D), pseudodiffusion (D*), and perfusion fraction (f) of each metastasis were obtained for all b-values, and the time-related changes were recorded to analyze the chronologic responses to antiangiogenic therapy. The RECIST 1.1 criteria were used for the evaluation of treatment response.

RESULTS

The diameters of the metastases diminished significantly at 9 months when compared with PT (p = 0.03). The D (p = 0.10) and ADC (p = 0.21) values of the metastases increased at 9 months of therapy. D* was the highest at 3 months (p =0.24); it decreased at 6 (p =0.97) and 9 months (p =0.87) of therapy. The f value had peaked at 3 months (p =0.51) and started to decrease thereafter. At 6 months, f decreased to the lowest values (p =0.12).

CONCLUSION

IVIM parameters, particularly the perfusion fraction, may quantitatively reflect the response to antiangiogenic treatment. The antiangiogenic response manifests after 3 months of therapy before the RECIST-related response.

Topics on quantitative liver magnetic resonance imaging

Liver magnetic resonance imaging (MRI) is subject to continuous technical innovations through advances in hardware, sequence and novel contrast agent development. In order to utilize the abilities of liver MR to its full extent and perform high-quality efficient exams, it is mandatory to use the best imaging protocol, to minimize artifacts and to select the most adequate type of contrast agent. In this article, we review the routine clinical MR techniques applied currently and some latest developments of liver imaging techniques to help radiologists and technologists to better understand how to choose and optimize liver MRI protocols that can be used in clinical practice. This article covers topics on (I) fat signal suppression; (II) diffusion weighted imaging (DWI) and intravoxel incoherent motion (IVIM) analysis; (III) dynamic contrast-enhanced (DCE) MR imaging; (IV) liver fat quantification; (V) liver iron quantification; and (VI) scan speed acceleration.

Potentials and challenges of diffusion-weighted magnetic resonance imaging in radiotherapy

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Discussion of DW imaging protocols and imaging setup.

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Discussion of mono- and bi-exponential models for quantitative parameter extraction.

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Review of recent publications investigating potential benefits of using DWI in RT, including detailed synoptic table.

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Detailed discussion of geometric and quantitative accuracy of DW imaging and DW-derived parameters.

Purpose

To review the potential and challenges of integrating diffusion weighted magnetic resonance imaging (DWI) into radiotherapy (RT).

Content

Details related to image acquisition of DWI for RT purposes are discussed, along with the challenges with respect to geometric accuracy and the robustness of quantitative parameter extraction. An overview of diffusion- and perfusion-related parameters derived from mono- and bi-exponential models is provided, and their role as potential RT biomarkers is discussed. Recent studies demonstrating potential of DWI in different tumor sites such as the head and neck, rectum, cervix, prostate, and brain, are reviewed in detail.

Conclusion

DWI has shown promise for RT outcome prediction, response assessment, as well as for tumor delineation and characterization in several cancer types. Geometric and quantification robustness is challenging and has to be addressed adequately. Evaluation in larger clinical trials with well designed imaging protocol and advanced analysis models is needed to develop the optimal strategy for integrating DWI in RT.

Determination of Hepatocellular Carcinoma and Characterization of Hepatic Focal Lesions with Adaptive Multi-Exponential Intravoxel Incoherent Motion Model

PURPOSE: To distinguish hepatocellular carcinoma (HCC) from other types of hepatic lesions with the adaptive multi-exponential IVIM model. METHODS: 94 hepatic focal lesions, including 38 HCC, 16 metastasis, 12 focal nodular hyperplasia, 13 cholangiocarcinoma, and 15 hemangioma, were examined in this study. Diffusion-weighted images were acquired with 13 b values (b = 0, 3, …, 500 s/mm²) to measure the adaptive multi-exponential IVIM parameters, namely, pure diffusion coefficient (D), diffusion fraction (f_d), pseudo-diffusion coefficient (D_i*) and perfusion-related diffusion fraction (f_i) of the ith perfusion component. Comparison of the parameters of and their diagnostic performance was determined using Mann-Whitney U test, independent-sample t test, one-way analysis of variance, Z test and receiver-operating characteristic analysis. RESULTS: D, D₁* and D₂* presented significantly difference between HCCs and other hepatic lesions, whereas f_d, f₁ and f₂ did not show statistical differences. In the differential diagnosis of HCCs from other hepatic lesions, D₂* (AUC, 0.927) provided best diagnostic performance among all parameters. Additionally, the number of exponential terms in the model was also an important indicator for distinguishing HCCs from other hepatic lesions. In the benign and malignant analysis, D gave the greatest AUC values, 0.895 or 0.853, for differentiation between malignant and benign lesions with three or two exponential terms. Most parameters were not significantly different between hypovascular and hypervascular lesions. For multiple comparisons, significant differences of D, D₁* or D₂* were found between certain lesion types. CONCLUSION: The adaptive multi-exponential IVIM model was useful and reliable to distinguish HCC from other hepatic lesions.

Meta-analysis of intravoxel incoherent motion magnetic resonance imaging in differentiating focal lesions of the liver

INTRODUCTION

Accurate detection and characterization of focal liver lesions, including differentiation between malignant and benign lesions, are particularly important. The objective of this meta-analysis was to evaluate the parameters of intravoxel incoherent motion (IVIM), including apparent diffusion coefficient (ADC), pure molecular diffusion coefficient (D), perfusion-related diffusion coefficient (D*), and perfusion fraction (f) in differentiating focal liver lesions.

METHODS

IVIM method employed for focal liver lesion and the quality assessment of diagnostic studies were evaluated. Standardized mean differences and 95% confidence intervals were calculated. The heterogeneity was quantified with the I statistic.

RESULTS

The difference between groups was analyzed according to the I values from 6 different studies using fixed effects or random effects models. Significant differences in ADC (P < .001) and D (P < .001) were observed between benign and malignant lesions. Moreover, significant differences in ADC (P < .001), D (P < .001), and f (P = .01) were found between hemangioma and hepatocellular carcinoma (HCC). In addition, no significant difference was observed between the metastases and HCC.

CONCLUSIONS

D and ADC values were useful for the differentiation between benignity and malignancy; higher values of ADC, D, and f were observed in hemangioma compared to HCC. Nevertheless, IVIM did not result as the optimal approach for differentiation between the metastases and HCC.

Accurate IVIM model-based liver lesion characterisation can be achieved with only three b-value DWI

OBJECTIVE

The objective of this study was to evaluate a simplified intravoxel incoherent motion (IVIM) approach of diffusion-weighted imaging (DWI) with four b-values for liver lesion characterisation at 1.5 T.

METHODS

DWI data from a respiratory-gated MRI sequence with b = 0, 50, 250, 800 s/mm² were retrospectively analysed in 173 lesions and 40 healthy livers. The apparent diffusion coefficient ADC = ADC(0,800) and IVIM-based parameters D₁' = ADC(50,800), D₂' =ADC(250,800), f₁', f₂', D*', ADC_low = ADC(0,50), and ADC_diff=ADC_low-D₂' were calculated voxel-wise without fitting procedures. Differences between lesion groups were investigated.

RESULTS

Focal nodular hyperplasias were best discriminated from all other lesions by f₁' with an area under the curve (AUC) of 0.989. Haemangiomas were best discriminated by D₁' (AUC of 0.994). For discrimination between malignant and benign lesions, ADC(0,800) and D₁' were best suited (AUC of 0.915 and 0.858, respectively). Discriminatory power was further increased by using a combination of D₁' and f₁'.

CONCLUSION

IVIM parameters D and f approximated from three b-values provided more discriminatory power between liver lesions than ADC determined from two b-values. The use of b = 0, 50, 800 s/mm² was superior to that of b = 0, 250, 800 s/mm². The acquisition of four instead of three b-values has no further benefit for lesion characterisation.

KEY POINTS

• Diffusion and perfusion characteristics are assessable with only three b-values. • Association of b = 0, 50, 800 s/mm²is superior to b = 0, 250, 800 s/mm². • A fourth acquired b-value has no benefit for differential diagnosis. • For liver lesion characterisation, simplified IVIM analysis is superior to ADC determination. • Simplified IVIM approach guarantees numerically stable, voxel-wise results and short acquisition times.

Diffusion Quantification in Body Imaging

Diffusion-weighted imaging (DWI) is increasingly incorporated into routine body magnetic resonance imaging protocols. DWI can assist with lesion detection and even in characterization. Quantitative DWI has exhibited promise in the discrimination between benign and malignant pathology, in the evaluation of the biologic aggressiveness, and in the assessment of the response to treatment. Unfortunately, inconsistencies in DWI acquisition parameters and analysis have hampered widespread clinical utilization. Focusing primarily on liver applications, this article will review the basic principles of quantitative DWI. In addition to standard mono-exponential fitting, the authors will discuss intravoxel incoherent motion and diffusion kurtosis imaging that involve more sophisticated approaches to diffusion quantification.

Relative enhanced diffusivity: noise sensitivity, protocol optimization, and the relation to intravoxel incoherent motion

OBJECTIVE

To explore the relationship between relative enhanced diffusivity (RED) and intravoxel incoherent motion (IVIM), as well as the impact of noise and the choice of intermediate diffusion weighting (b value) on the RED parameter.

MATERIALS AND METHODS

A mathematical derivation was performed to cast RED in terms of the IVIM parameters. Noise analysis and b value optimization was conducted by using Monte Carlo calculations to generate diffusion-weighted imaging data appropriate to breast and liver tissue at three different signal-to-noise ratios.

RESULTS

RED was shown to be approximately linearly proportional to the IVIM parameter f, inversely proportional to D and to follow an inverse exponential decay with respect to D*. The choice of intermediate b value was shown to be important in minimizing the impact of noise on RED and in maximizing its discriminatory power. RED was shown to be essentially a reparameterization of the IVIM estimates for f and D obtained with three b values.

CONCLUSION

RED imaging in the breast and liver should be performed with intermediate b values of 100 and 50 s/mm², respectively. Future clinical studies involving RED should also estimate the IVIM parameters f and D using three b values for comparison.

Intravoxel incoherent motion MRI as a means to measure in vivo perfusion: A review of the evidence

The idea that in vivo intravoxel incoherent motion magnetic resonance signal is influenced by blood motion in the microvasculature is exciting, because it suggests that local and quantitative perfusion information can be obtained in a simple and elegant way from a few diffusion-weighted images, without contrast injection. When the method was proposed in the late 1980s some doubts appeared as to its feasibility, and, probably because the signal to noise and image quality at the time was not sufficient, no obvious experimental evidence could be produced to alleviate them. Helped by the tremendous improvements seen in the last three decades in MR hardware, pulse design, and post-processing capabilities, an increasing number of encouraging reports on the value of intravoxel incoherent motion perfusion imaging have emerged. The aim of this article is to review the current published evidence on the feasibility of in vivo perfusion imaging with intravoxel incoherent motion MRI.

Standard-b-Value Versus Low-b-Value Diffusion-Weighted Imaging in Hepatic Lesion Discrimination: A Meta-analysis

OBJECTIVE

We sought to determine the comparative diagnostic performance of standard-b-value (500-1000s/mm) versus low-b-value (≤500 s/mm) diffusion-weighted imaging (DWI) in the discrimination of hepatic lesions.

METHODS

A total of 1775 hepatic malignant lesions and 1120 benign hepatic lesions from 21 studies were included.

RESULTS

(1) The global sensitivity was 0.86 (95% confidence interval [CI], 0.847-0.879), the specificity was 0.82 (95% CI, 0.797-0.842), the positive likelihood ratio (PLR) was 6.234 (95% CI, 4.260-9.123), the negative likelihood ratio (NLR) was 0.175 (95% CI, 0.135-0.227), and diagnostic odds ratio (DOR) was 42.836 (95% CI, 24.134-76.031). The area under the curve (AUC) and Q* index were 0.93 and 0.87. Publication bias was not present (P > 0.05). (2)The sensitivity of a subgroup meta-analysis of standard-b-value DWI was 0.858 (95% CI, 0.835-0.880), the specificity was 0.836 (95% CI, 0.807-0.863), the PLR was 6.527 (95% CI, 3.857-11.046), the NLR was 0.168 (95% CI, 0.123-0.239), and the DOR was 49.716 (95% CI, 22.897-107.98). The AUC and Q* index were 0.941 and 0.88. (3)The sensitivity of a subgroup meta-analysis of low-b-value DWI was 0.87 (95% CI, 0.84-0.89), the specificity was 0.80 (95% CI, 0.76-0.83), the PLR was 6.22 (95% CI, 3.29-11.76), the NLR was 0.19 (95% CI, 0.12-0.29), and the DOR was 37.14 (95% CI, 14.80-93.18). The AUC and Q* index were 0.922 and 0.86.

CONCLUSIONS

Hepatic DWI is useful in differentiating between malignant and benign hepatic lesions. Standard-b-value DWI displayed an overall superior diagnostic accuracy over low-b-value DWI. Further trials needed to determine whether increasing b values beyond 1000 s/mm affects the diagnostic accuracy of hepatic lesion discrimination.

Differentiation of pancreatic carcinoma and mass-forming focal pancreatitis: qualitative and quantitative assessment by dynamic contrast-enhanced MRI combined with diffusion-weighted imaging

Differentiation between pancreatic carcinoma (PC) and mass-forming focal pancreatitis (FP) is invariably difficult. For the differential diagnosis, we qualitatively and quantitatively assessed the value of dynamic contrast-enhanced MRI (DCE-MRI) and diffusion-weighted imaging (DWI) in PC and FP in the present study. This study included 32 PC and 18 FP patients with histological confirmation who underwent DCE-MRI and DWI. The time-signal intensity curve (TIC) of PC and FP were classified into 5 types according to the time of reaching the peak, namely, type I, II, III, IV, and V, respectively, and two subtypes, namely, subtype-a (washout type) and subtype-b (plateau type) according to the part of the TIC profile after the peak. Moreover, the mean and relative apparent diffusion coefficient (ADC) value between PC and FP on DWI were compared. The type V TIC was only recognized in PC group (P < 0.01). Type IV b were more frequently observed in PC (P = 0.036), while type- IIa (P < 0.01), type- Ia (P = 0.037) in FP. We also found a significant difference in the mean and relative ADC value between PC and FP. The combined image set of DCE-MRI and DWI yielded an excellent sensitivity, specificity, and diagnostic accuracy (96.9%, 94.4%, and 96.0%). The TIC of DCE-MRI and ADC value of DWI for pancreatic mass were found to provide reliable information in differentiating PC from FP, and the combination of DCE-MRI and DWI can achieve a higher sensitivity, specificity, and diagnostic accuracy.

Intravoxel incoherent motion model-based analysis of diffusion-weighted magnetic resonance imaging with 3 b-values for response assessment in locoregional therapy of hepatocellular carcinoma

Purpose

The aim of this study was to evaluate an intravoxel incoherent motion (IVIM) model–based analysis of diffusion-weighted imaging (DWI) for assessing the response of hepatocellular carcinoma (HCC) to locoregional therapy.

Patients and methods

Respiratory-gated DWI (b=0, 50, and 800 s/mm²) was retrospectively analyzed in 25 patients who underwent magnetic resonance imaging at 1.5 T before and 6 weeks following the first cycle of transarterial chemoembolization therapy, transarterial ethanol-lipiodol embolization therapy, and transarterial radioembolization therapy. In addition to the determination of apparent diffusion coefficient, ADC(0,800), an estimation of the diffusion coefficient, D′, and the perfusion fraction, f′, was performed by using a simplified IVIM approach. Parameters were analyzed voxel-wise. Tumor response was assessed in a central slice by using a region of interest (ROI) covering the whole tumor. HCCs were categorized into two groups, responders and nonresponders, according to tumor size changes on first and second follow ups (if available) and changes of contrast-enhanced region on the first follow up.

Results

In total, 31 HCCs were analyzed: 17 lesions were assigned to responders and 14 were to nonresponders. In responders, ADC(0,800) and D′ were increased after therapy by ~30% (P=0.00004) and ~42% (P=0.00001), respectively, whereas f′ was decreased by ~37% (P=0.00094). No significant changes were found in nonresponders. Responders and nonresponders were better differentiated by changes in D′ than by changes in ADC(0,800) (area under the curve =0.878 vs 0.819 or 0.714, respectively).

Conclusion

In patients with HCCs undergoing embolization therapy, diffusion changes were better reflected by D′ than by conventional ADC(0,800), which is influenced by counteracting perfusion changes as assessed by f′.

Intravoxel Incoherent Motion Diffusion-Weighted MR Imaging for Prediction of Early Arterial Blood Flow Stasis in Radioembolization of Breast Cancer Liver Metastases

PURPOSE

To retrospectively evaluate predictive value of intravoxel incoherent motion (IVIM) diffusion-weighted imaging (DWI) for early arterial blood flow stasis during transarterial radioembolization (TARE) of liver dominant breast metastases (LdBM).

MATERIALS AND METHODS

Preinterventional 1.5T DWI (b0, b1, b2 = 0, 50, 800 s/mm(2)) data for 28 liver lobes of 18 female patients treated by resin-based radioembolization (10 bilobar and 8 unilobar treatments) were analyzed. Apparent diffusion coefficient (ADC) (0, 800) and an estimation of the true diffusion coefficient D' and of the perfusion fraction f' were calculated for the 2 largest metastases. Response rate at 3 months and survival were analyzed. Procedures without full dose application because of early stasis were assigned to group A (n = 15), and procedures with full dose application were assigned to group B (n = 13).

RESULTS

Metastases in group A showed significantly lower f' (0.035 ± 0.018 vs 0.076 ± 0.015, P < .0001) and a trend toward lower ADC(0, 800) with values given in 10(-6) mm(2)/s (1,066 ± 141 vs 1,189 ± 176, P = .051); no group difference was shown for D'. Groups were best discriminated by weighted mean f' values of the 2 largest metastases with accuracy of 100%. Mean tumor diameter before and after TARE was 51 mm ± 18 and 50 mm ± 24 in group A and 47 mm ± 27 and 48 mm ± 32 for group B. Imaging response did not differ between groups (P = .545). Overall survival did not differ significantly between group A (230 d) and B (155 d) (P = .124).

CONCLUSIONS

Perfusion-sensitive IVIM parameter f' may predict early blood flow stasis in patients undergoing TARE for LdBM. Determination of this parameter before intervention may increase awareness of the interventionalist and increase safety of microsphere administration.

The value of intravoxel incoherent motion model-based diffusion-weighted imaging for outcome prediction in resin-based radioembolization of breast cancer liver metastases

Purpose

To evaluate prognostic values of clinical and diffusion-weighted magnetic resonance imaging-derived intravoxel incoherent motion (IVIM) parameters in patients undergoing primary radioembolization for metastatic breast cancer liver metastases.

Subjects and methods

A total of 21 females (mean age 54 years, range 43–72 years) with liver-dominant metastatic breast cancer underwent standard liver magnetic resonance imaging (1.5 T, diffusion-weighted imaging with b-values of 0, 50, and 800 s/mm²) before and 4–6 weeks after radioembolization. The IVIM model-derived estimated diffusion coefficient D’ and the perfusion fraction f’ were evaluated by averaging the values of the two largest treated metastases in each patient. Kaplan–Meier and Cox regression analyses for overall survival (OS) were performed. Investigated parameters were changes in f’- and D’-values after therapy, age, sex, Eastern Cooperative Oncology Group (ECOG) status, grading of primary tumor, hepatic tumor burden, presence of extrahepatic disease, baseline bilirubin, previous bevacizumab therapy, early stasis during radioembolization, chemotherapy after radioembolization, repeated radioembolization and Response Evaluation Criteria in Solid Tumors (RECIST) response at 6-week follow-up.

Results

Median OS after radioembolization was 6 (range 1.5–54.9) months. In patients with therapy-induced decreasing or stable f’-values, median OS was significantly longer than in those with increased f’-values (7.6 [range 2.6–54.9] vs 2.6 [range 1.5–17.4] months, P<0.0001). Longer median OS was also seen in patients with increased D’-values (6 [range 1.6–54.9] vs 2.8 [range 1.5–17.4] months, P=0.008). Patients with remission or stable disease (responders) according to RECIST survived longer than nonresponders (7.2 [range 2.6–54.9] vs 2.6 [range 1.5–17.4] months, P<0.0001). An ECOG status ≤1 resulted in longer median OS than >1 (7.6 [range 2.6–54.9] vs 1.7 [range 1.5–4.5] months, P<0.0001). Pretreatment IVIM parameters and the other clinical characteristics were not associated with OS. Classification by f’-value changes and ECOG status remained as independent predictors of OS on multivariate analysis, while RECIST response and D’-value changes did not predict survival.

Conclusion

Following radioembolization of breast cancer liver metastases, early changes in the IVIM model-derived perfusion fraction f’ and baseline ECOG score were predictive of patient outcome, and may thus help to guide treatment strategy.

Advanced imaging techniques in the therapeutic response of transarterial chemoembolization for hepatocellular carcinoma

Hepatocellular carcinoma (HCC) is one of the major causes of morbidity and mortality in patients with chronic liver disease. Transarterial chemoembolization (TACE) can significantly improve the survival rate of patients with HCC and is the first treatment choice for patients who are not suitable for surgical resections. The evaluation of the response to TACE treatment affects not only the assessment of the therapy efficacy but also the development of the next step in the treatment plan. The use of imaging to examine changes in tumor volume to assess the response of solid tumors to treatment has been controversial. In recent years, the emergence of new imaging technology has made it possible to observe the response of tumors to treatment prior to any morphological changes. In this article, the advances in studies reporting the use of computed tomography perfusion imaging, diffusion-weighted magnetic resonance imaging (MRI), intravoxel incoherent motion, diffusion kurtosis imaging, magnetic resonance spectroscopy, magnetic resonance perfusion-weighted imaging, blood oxygen level-dependent MRI, positron emission tomography (PET)/computed tomography and PET/MRI to assess the TACE treatment response are reviewed.

Evaluation of a Simplified Intravoxel Incoherent Motion (IVIM) Analysis of Diffusion-Weighted Imaging for Prediction of Tumor Size Changes and Imaging Response in Breast Cancer Liver Metastases Undergoing Radioembolization: A Retrospective Single Center Analysis

To investigate the value of a simplified intravoxel incoherent motion (IVIM) analysis for evaluation of therapy-induced tumor changes and response of breast cancer liver metastases (mBRC) undergoing radioembolization.In 21 females (mean age 54 years, range 43-72) with mBRC tumor size changes and response evaluation criteria in solid tumors (RECIST) response to 26 primary radioembolization procedures were analyzed. Standard 1.5-T liver magnetic resonance imaging including respiratory-gated diffusion-weighted imaging (DWI) with b0 = 0 s/mm, b1 = 50 s/mm, b2 = 800 s/mm before and 6 weeks after each treatment was performed. In addition to the apparent diffusion coefficient (ADC)(0,800), the estimated diffusion coefficient D' and the perfusion fraction f' were determined using a simplified IVIM approach. For each radioembolization, the 2 largest treated metastases (if available) were analyzed. Lesions were categorized according to size changes into group A (reduction of longest diameter [LD]) and group B (LD increase) after 3 months. Radioembolization procedures were further categorized into "response" (partial response and stable disease) and "nonresponse" (progressive disease) according to RECIST after 3 months. ADC and D' are given in 10 mm/s.Forty-five metastases were analyzed. Thirty-two lesions were categorized as A; 13 as B. Before therapy, group A lesions showed significantly larger f'-values than B (P = 0.001), but ADC(0,800) and D' did not differ. After therapy, in group A lesions the ADC(0,800)- and D'-values increased and f' decreased (P < 0.0001); in contrast in group B lesions f' increased (P = 0.001). Groups could be differentiated by preinterventional f' and by changes of D' and f' between pre and postinterventional imaging (area under the curve [AUC] of 0.903, 0.747 and 1.0, respectively).Preinterventional parameters did not differ between responders and nonresponders according to RECIST. ADC(0,800)- and D'-values showed a larger increase in responders compared with nonresponders (P = 0.013 and P = 0.001, respectively). After therapy f'-values decreased significantly in responders (P = 0.001). Good to excellent prediction of long-term RECIST response was possible by therapy-induced changes in LD, D', and f' (AUC 0.903, 0.879, and 0.867, respectively).A simplified IVIM model-based analysis of early post-treatment DWI can deliver additional information on tumor size changes and long-term RECIST response after radioembolization of mBRC. The estimated perfusion fraction f' is better suited for response assessment than the conventional ADC(0,800) or D'. This can be useful to guide further treatment strategy.

Multiparametric MR Imaging in Abdominal Malignancies

Modern MR imaging protocols can yield both anatomic and functional information for the assessment of hepatobiliary and pancreatic malignancies. Diffusion-weighted imaging is fully integrated into state-of-the-art protocols for tumor detection, characterization, and therapy monitoring. Hepatobiliary contrast agents have gained ground in the evaluation of focal liver lesions during the last years. Perfusion MR imaging is expected to have a central role for monitoring therapy in body tumors treated with antivascular drugs. Approaches such as Magnetic resonance (MR) elastography and (1)H-MR spectroscopy are still confined to research centers, but with the potential to grow in a short time frame.

Diffusion-weighted magnetic resonance imaging in cancer: Reported apparent diffusion coefficients, in-vitro and in-vivo reproducibility

There is considerable disparity in the published apparent diffusion coefficient (ADC) values across different anatomies. Institutions are increasingly assessing repeatability and reproducibility of the derived ADC to determine its variation, which could potentially be used as an indicator in determining tumour aggressiveness or assessing tumour response. In this manuscript, a review of selected articles published to date in healthy extra-cranial body diffusion-weighted magnetic resonance imaging is presented, detailing reported ADC values and discussing their variation across different studies. In total 115 studies were selected including 28 for liver parenchyma, 15 for kidney (renal parenchyma), 14 for spleen, 13 for pancreatic body, 6 for gallbladder, 13 for prostate, 13 for uterus (endometrium, myometrium, cervix) and 13 for fibroglandular breast tissue. Median ADC values in selected studies were found to be 1.28 × 10(-3) mm(2)/s in liver, 1.94 × 10(-3) mm(2)/s in kidney, 1.60 × 10(-3) mm(2)/s in pancreatic body, 0.85 × 10(-3) mm(2)/s in spleen, 2.73 × 10(-3) mm(2)/s in gallbladder, 1.64 × 10(-3) mm(2)/s and 1.31 × 10(-3) mm(2)/s in prostate peripheral zone and central gland respectively (combined median value of 1.54×10(-3) mm(2)/s), 1.44 × 10(-3) mm(2)/s in endometrium, 1.53 × 10(-3) mm(2)/s in myometrium, 1.71 × 10(-3) mm(2)/s in cervix and 1.92 × 10(-3) mm(2)/s in breast. In addition, six phantom studies and thirteen in vivo studies were summarized to compare repeatability and reproducibility of the measured ADC. All selected phantom studies demonstrated lower intra-scanner and inter-scanner variation compared to in vivo studies. Based on the findings of this manuscript, it is recommended that protocols need to be optimised for the body part studied and that system-induced variability must be established using a standardized phantom in any clinical study. Reproducibility of the measured ADC must also be assessed in a volunteer population, as variations are far more significant in vivo compared with phantom studies.

Technical advancements and protocol optimization of diffusion-weighted imaging (DWI) in liver

An area of rapid advancement in abdominal MRI is diffusion-weighted imaging (DWI). By measuring diffusion properties of water molecules, DWI is capable of non-invasively probing tissue properties and physiology at cellular and macromolecular level. The integration of DWI as part of abdominal MRI exam allows better lesion characterization and therefore more accurate initial diagnosis and treatment monitoring. One of the most technical challenging, but also most useful abdominal DWI applications is in liver and therefore requires special attention and careful optimization. In this article, the latest technical developments of DWI and its liver applications are reviewed with the explanations of the technical principles, recommendations of the imaging parameters, and examples of clinical applications. More advanced DWI techniques, including Intra-Voxel Incoherent Motion (IVIM) diffusion imaging, anomalous diffusion imaging, and Diffusion Kurtosis Imaging (DKI) are discussed.

[Modern magnetic resonance imaging of the liver]

Magnetic resonance imaging (MRI) of the liver has become an essential tool in the radiological diagnostics of both focal and diffuse diseases of the liver and is subject to constant change due to technological progress. Recently, important improvements could be achieved by innovations regarding MR hardware, sequences and postprocessing methods. The diagnostic spectrum of MRI could be broadened particularly due to new examination sequences, while at the same time scanning time could be shortened and image quality has been improved. The aim of this article is to explain both the technological background and the clinical application of recent MR sequence developments and to present the scope of a modern MRI protocol for the liver.

A comparative study of apparent diffusion coefficient and intravoxel incoherent motion-derived parameters for the characterization of common solid hepatic tumors

BACKGROUND

The performance of diffusion-weighted imaging parameters for characterizing hepatic tumors is controversial.

PURPOSE

To compare the performances of apparent diffusion coefficient (ADC) and intravoxel incoherent motion (IVIM)-derived parameters, including the pure diffusion coefficient (D), perfusion coefficient (D*), and perfusion fraction (f), in the characterization of common solid hepatic tumors.

MATERIAL AND METHODS

Twelve healthy volunteers and 43 patients underwent free-breath diffusion-weighted magnetic resonance imaging (DW-MRI) of the liver using eight b values (10-800 s/mm(2)). Twelve regions of interest (ROIs) of normal liver tissue in healthy volunteers and 49 hepatic lesions (23 hepatocellular carcinomas [HCCs], 16 hemangiomas, and 10 metastases) were measured. Conventional ADC(0,500) and ADCtotal obtained by the mono-exponential model, as well as D, D*, and f were calculated. Student t-tests and receiver operating characteristic (ROC) analysis were also performed.

RESULTS

ADC(0,500), ADCtotal, and D were significantly lower in the malignant group ([1.48 ± 0.35] × 10(-3) mm(2)/s; [1.35 ± 0.30] × 10(-3) mm(2)/s; [1.18 ± 0.33] × 10(-3) mm(2)/s) compared to the hemangioma group ([2.74 ± 1.03] × 10(-3) mm(2)/s; [2.61 ± 0.81] × 10(-3) mm(2)/s; [1.97 ± 0.79] × 10(-3) mm(2)/s]. D* did not differ among multiple comparisons. For the area under the ROC curve (AUC-ROC), the maximum value was attained with ADCtotal (0.983) and was closely followed by ADC(0,500) (0.967), with lower values obtained for D (0.837), f (0.649), and D* (0.599). Statistically significant differences were found between the AUC-ROC of both ADCs (ADCtotal and ADC(0,500)) and D. There was no statistically significant difference between the AUC-ROC of ADCtotal and ADC(0,500).

CONCLUSION

ADCs showed superior diagnostic performance compared to IVIM-derived parameters in detecting differences between the malignant group and hemangioma group.

Whole-body intravoxel incoherent motion imaging

OBJECTIVES

To investigate the technical feasibility of whole-body intravoxel incoherent motion (IVIM) imaging.

MATERIALS AND METHODS

Whole-body MR images of eight healthy volunteers were acquired at 3T using a spin-echo echo-planar imaging sequence with eight b-values. Coronal parametrical whole-body maps of diffusion (D), pseudodiffusion (D*), and the perfusion fraction (Fp) were calculated. Image quality was rated qualitatively by two independent radiologists, and inter-reader reliability was tested with intra-class correlation coefficients (ICCs). Region of interest (ROI) analysis was performed in the brain, liver, kidney, and erector spinae muscle.

RESULTS

Depiction of anatomic structures was rated as good on D maps and good to fair on D* and Fp maps. Exemplary mean D (10(-3) mm(2)/s), D* (10(-3) mm(2)/s) and Fp (%) values (± standard deviation) of the renal cortex were as follows: 1.7 ± 0.2; 15.6 ± 6.5; 20.9 ± 4.4. Inter-observer agreement was "substantial" to "almost perfect" (ICC = 0.80 - 0.92). The coefficient of variation of D* was significantly lower with the proposed algorithm compared to the conventional algorithm (p < 0.001), indicating higher stability.

CONCLUSION

The proposed IVIM protocol allows computation of parametrical maps with good to fair image quality. Potential future clinical applications may include characterization of widespread disease such as metastatic tumours or inflammatory myopathies.

KEY POINTS

• IVIM imaging allows estimation of tissue perfusion based on diffusion-weighted MRI. • In this study, a clinically suitable whole-body IVIM algorithm is presented. • Coronal parametrical whole-body maps showed good depiction of anatomic details. • Potential future applications include detection of widespread metastatic or inflammatory disease.

Simple and reliable determination of intravoxel incoherent motion parameters for the differential diagnosis of head and neck tumors

Intravoxel incoherent motion (IVIM) imaging can characterize diffusion and perfusion of normal and diseased tissues, and IVIM parameters are authentically determined by using cumbersome least-squares method. We evaluated a simple technique for the determination of IVIM parameters using geometric analysis of the multiexponential signal decay curve as an alternative to the least-squares method for the diagnosis of head and neck tumors. Pure diffusion coefficients (D), microvascular volume fraction (f), perfusion-related incoherent microcirculation (D*), and perfusion parameter that is heavily weighted towards extravascular space (P) were determined geometrically (Geo D, Geo f, and Geo P) or by least-squares method (Fit D, Fit f, and Fit D*) in normal structures and 105 head and neck tumors. The IVIM parameters were compared for their levels and diagnostic abilities between the 2 techniques. The IVIM parameters were not able to determine in 14 tumors with the least-squares method alone and in 4 tumors with the geometric and least-squares methods. The geometric IVIM values were significantly different (p<0.001) from Fit values (+2±4% and −7±24% for D and f values, respectively). Geo D and Fit D differentiated between lymphomas and SCCs with similar efficacy (78% and 80% accuracy, respectively). Stepwise approaches using combinations of Geo D and Geo P, Geo D and Geo f, or Fit D and Fit D* differentiated between pleomorphic adenomas, Warthin tumors, and malignant salivary gland tumors with the same efficacy (91% accuracy = 21/23). However, a stepwise differentiation using Fit D and Fit f was less effective (83% accuracy = 19/23). Considering cumbersome procedures with the least squares method compared with the geometric method, we concluded that the geometric determination of IVIM parameters can be an alternative to least-squares method in the diagnosis of head and neck tumors.

Comparison of fitting methods and b-value sampling strategies for intravoxel incoherent motion in breast cancer

PURPOSE

To compare fitting methods and sampling strategies, including the implementation of an optimized b-value selection for improved estimation of intravoxel incoherent motion (IVIM) parameters in breast cancer.

METHODS

Fourteen patients (age, 48.4 ± 14.27 years) with cancerous lesions underwent 3 Tesla breast MRI examination for a HIPAA-compliant, institutional review board approved diffusion MR study. IVIM biomarkers were calculated using "free" versus "segmented" fitting for conventional or optimized (repetitions of key b-values) b-value selection. Monte Carlo simulations were performed over a range of IVIM parameters to evaluate methods of analysis. Relative bias values, relative error, and coefficients of variation (CV) were obtained for assessment of methods. Statistical paired t-tests were used for comparison of experimental mean values and errors from each fitting and sampling method.

RESULTS

Comparison of the different analysis/sampling methods in simulations and experiments showed that the "segmented" analysis and the optimized method have higher precision and accuracy, in general, compared with "free" fitting of conventional sampling when considering all parameters. Regarding relative bias, IVIM parameters fp and Dt differed significantly between "segmented" and "free" fitting methods.

CONCLUSION

IVIM analysis may improve using optimized selection and "segmented" analysis, potentially enabling better differentiation of breast cancer subtypes and monitoring of treatment.

Diffusion-weighted MRI and optimal b-value for characterization of liver lesions

BACKGROUND

Diffusion-weighted imaging (DWI) is commonly used to distinguish between benign and malignant liver lesions. However, different b-values are recommended.

PURPOSE

To determine the most suitable b-value in DWI for differentiation of benign and malignant liver lesions.

MATERIAL AND METHODS

A total of 124 lesions in 89 consecutive patients (43 men, 46 women; age, mean ± standard deviation, 58 ± 14 years) with a pathological or radiological diagnosis of malignant or benign focal liver lesions after magnetic resonance imaging (MRI) were included in this study. Routine abdominal MRI and DWI were performed using seven b-values (0, 50, 200, 400, 600, 800, 1000 s/mm(2)). Lesions were analyzed for benignity/malignity using apparent diffusion coefficient (ADC) values with 10 b-value combinations and by measuring the lesion/normal parenchyma ADC ratio.

RESULTS

Mean ADC values were significantly different between malignant and benign lesions for all b-value combinations (P=0.000). The best b-value combination was 0 and 800 (Az=0.935). Using lower b-values such as 0 and 50 together with higher b-values ≥ 600 s/mm(2) was beneficial (Az=0.928 and 0.927). Mean ADC values were approximately 13% (1-15%) higher in total when b=0 and b=50 s/mm(2) were included in multiple b-value combinations.

CONCLUSION

In DWI, we recommend the use of b-values of 0 and 800 s/mm(2) as two b-values, or b=0, 50, 600, 800, and 1000 s/mm(2) as multiple b-values for distinguishing between benign and malignant liver lesions. Mean ADC value is 13% higher in total by additional use of b=0 and b=50 s/mm(2) in multiple b-value combinations.

Non-Gaussian analysis of diffusion weighted imaging in head and neck at 3T: a pilot study in patients with nasopharyngeal carcinoma

PURPOSE

To technically investigate the non-Gaussian diffusion of head and neck diffusion weighted imaging (DWI) at 3 Tesla and compare advanced non-Gaussian diffusion models, including diffusion kurtosis imaging (DKI), stretched-exponential model (SEM), intravoxel incoherent motion (IVIM) and statistical model in the patients with nasopharyngeal carcinoma (NPC).

MATERIALS AND METHODS

After ethics approval was granted, 16 patients with NPC were examined using DWI performed at 3T employing an extended b-value range from 0 to 1500 s/mm(2). DWI signals were fitted to the mono-exponential and non-Gaussian diffusion models on primary tumor, metastatic node, spinal cord and muscle. Non-Gaussian parameter maps were generated and compared to apparent diffusion coefficient (ADC) maps in NPC.

RESULTS

Diffusion in NPC exhibited non-Gaussian behavior at the extended b-value range. Non-Gaussian models achieved significantly better fitting of DWI signal than the mono-exponential model. Non-Gaussian diffusion coefficients were substantially different from mono-exponential ADC both in magnitude and histogram distribution.

CONCLUSION

Non-Gaussian diffusivity in head and neck tissues and NPC lesions could be assessed by using non-Gaussian diffusion models. Non-Gaussian DWI analysis may reveal additional tissue properties beyond ADC and holds potentials to be used as a complementary tool for NPC characterization.

Hepatocellular carcinoma: short-term reproducibility of apparent diffusion coefficient and intravoxel incoherent motion parameters at 3.0T

PURPOSE

To evaluate short-term test-retest and interobserver reproducibility of IVIM (intravoxel incoherent motion) diffusion parameters and ADC (apparent diffusion coefficient) of hepatocellular carcinoma (HCC) and liver parenchyma at 3.0T.

MATERIALS AND METHODS

In this prospective Institutional Review Board (IRB)-approved study, 11 patients were scanned twice using a free-breathing single-shot echo-planar-imaging, diffusion-weighted imaging (DWI) sequence using 4 b values (b = 0, 50, 500, 1000 s/mm(2)) and IVIM DWI using 16 b values (0-800 s/mm(2)) at 3.0T. IVIM parameters (D: true diffusion coefficient, D*: pseudodiffusion coefficient, PF: perfusion fraction) and ADC (using 4 b and 16 b) were calculated. Short-term test-retest and interobserver reproducibility of IVIM parameters and ADC were assessed by measuring correlation coefficient, coefficient of variation (CV), and Bland-Altman limits of agreements (BA-LA).

RESULTS

Fifteen HCCs were assessed in 10 patients. Reproducibility of IVIM metrics in HCC was poor for D* and PF (mean CV 60.6% and 37.3%, BA-LA: -161.6% to 135.3% and -66.2% to 101.0%, for D* and PF, respectively), good for D and ADC (CV 19.7% and <16%, BA-LA -57.4% to 36.3% and -38.2 to 34.1%, for D and ADC, respectively). Interobserver reproducibility was on the same order of test-retest reproducibility except for PF in HCC. Reproducibility of diffusion parameters was better in liver parenchyma compared to HCC.

CONCLUSION

Poor reproducibility of D*/PF and good reproducibility for D/ADC were observed in HCC and liver parenchyma. These findings may have implications for trials using DWI in HCC.

Diffusion-weighted imaging with acquisition of three b-values for response evaluation of neuroendocrine liver metastases undergoing selective internal radiotherapy

OBJECTIVES

To evaluate diffusion-weighted MRI with acquisition of three b-values and calculation of fractioned ADCs for response evaluation of neuroendocrine liver metastases undergoing selective internal radiotherapy (SIRT).

METHODS

Ten consecutive patients with neuroendocrine liver metastases underwent MRI before and following SIRT. Diffusion-weighted imaging included acquisition of the b-values 0, 50 and 800 s/mm(2) and calculation of ADC(50,800), ADC(0,50) and ADC(0,800) maps. According to therapy response, lesions were categorised into group A [≥20% reduction of the longest diameter (LD) in comparison to baseline MRI] and group B (<20% reduction of the LD).

RESULTS

Twelve out of 31 metastases were categorised as group A and 19 out of 31 metastases were categorised as group B. Pretherapeutic values of ADC(0,800) and ADC(50,800) did not differ significantly between the two groups; however, ADC(0,50) was 32% lower in group A (P = 0.049). ADC(0,800) and ADC(50,800) increased significantly after therapy in both groups, however, group differences were not statistically significant. Conversely, the increase in ADC(0,50) was about a factor of 7 larger in group A than in group B (P = 0.023).

CONCLUSIONS

Our study showed that the ADC(0,50) is a promising biomarker for response assessment of neuroendocrine liver metastases following SIRT.

KEY POINTS

• Diffusion-weighted MRI offers new information about neuroendocrine hepatic metastases. • Evaluation of perfusion and diffusion components requires fractioned apparent diffusion coefficients (ADCs). • Perfusion effects represented by ADC (0.50) can be observed in neuroendocrine metastases. • Pretherapeutic ADC (0.50) was significantly lower in metastases with a response ≥20%. • Such biomarkers may help evaluate liver metastases in patients undergoing therapy.