Motion-robust, blood-suppressed, reduced-distortion diffusion MRI of the liver.

PURPOSE: To evaluate feasibility and reproducibility of liver diffusion-weighted (DW) MRI using cardiac-motion-robust, blood-suppressed, reduced-distortion techniques. METHODS: DW-MRI data were acquired at 3T in an anatomically accurate liver phantom including controlled pulsatile motion, in eight healthy volunteers and four patients with known or suspected liver metastases. Standard monopolar and motion-robust (M1-nulled, and M1-optimized) DW gradient waveforms were each acquired with single-shot echo-planar imaging (ssEPI) and multishot EPI (msEPI). In the motion phantom, apparent diffusion coefficient (ADC) was measured in the motion-affected volume. In healthy volunteers, ADC was measured in the left and right liver lobes separately to evaluate ADC reproducibility between the two lobes. Image distortions were quantified using the normalized cross-correlation coefficient, with an undistorted T2-weighted reference. RESULTS: In the motion phantom, ADC mean and SD in motion-affected volumes substantially increased with increasing motion for monopolar waveforms. ADC remained stable in the presence of increasing motion when using motion-robust waveforms. M1-optimized waveforms suppressed slow flow signal present with M1-nulled waveforms. In healthy volunteers, monopolar waveforms generated significantly different ADC measurements between left and right liver lobes ( p = 0 . 0078 $$ p=0.0078 $$ , reproducibility coefficients (RPC) =  470 × 1 0 - 6 $$ 470\times 1{0}^{-6} $$ mm 2 $$ {}^2 $$ /s for monopolar-msEPI), while M1-optimized waveforms showed more reproducible ADC values ( p = 0 . 29 $$ p=0.29 $$ , RPC = 220 × 1 0 - 6 $$ \mathrm{RPC}=220\times 1{0}^{-6} $$ mm 2 $$ {}^2 $$ /s for M1-optimized-msEPI). In phantom and healthy volunteer studies, motion-robust acquisitions with msEPI showed significantly reduced image distortion ( p < 0 . 001 $$ p<0.001 $$ ) compared to ssEPI. Patient scans showed reduction of wormhole artifacts when combining M1-optimized waveforms with msEPI. CONCLUSION: Synergistic effects of combined M1-optimized diffusion waveforms and msEPI acquisitions enable reproducible liver DWI with motion robustness, blood signal suppression, and reduced distortion.

Characterization and correction of cardiovascular motion artifacts in diffusion-weighted imaging of the pancreas.

PURPOSE: To assess the effects of cardiovascular-induced motion on conventional DWI of the pancreas and to evaluate motion-robust DWI methods in a motion phantom and healthy volunteers. METHODS: 3T DWI was acquired using standard monopolar and motion-compensated gradient waveforms, including in an anatomically accurate pancreas phantom with controllable compressive motion and healthy volunteers (n = 8, 10). In volunteers, highly controlled single-slice DWI using breath-holding and cardiac gating and whole-pancreas respiratory-triggered DWI were acquired. For each acquisition, the ADC variability across volunteers, as well as ADC differences across parts of the pancreas were evaluated. RESULTS: In motion phantom scans, conventional DWI led to biased ADC, whereas motion-compensated waveforms produced consistent ADC. In the breath-held, cardiac-triggered study, conventional DWI led to heterogeneous DW signals and highly variable ADC across the pancreas, whereas motion-compensated DWI avoided these artifacts. In the respiratory-triggered study, conventional DWI produced heterogeneous ADC across the pancreas (head: 1756 ± 173 × 10-6 mm2 /s; body: 1530 ± 338 × 10-6 mm2 /s; tail: 1388 ± 267 × 10-6 mm2 /s), with ADCs in the head significantly higher than in the tail (P < .05). Motion-compensated ADC had lower variability across volunteers (head: 1277 ± 102 × 10-6 mm2 /s; body: 1204 ± 169 × 10-6 mm2 /s; tail: 1235 ± 178 × 10-6 mm2 /s), with no significant difference (P ≥ .19) across the pancreas. CONCLUSION: Cardiovascular motion introduces artifacts and ADC bias in pancreas DWI, which are addressed by motion-robust DWI.

Enhancement of cerebrovascular 4D flow MRI velocity fields using machine learning and computational fluid dynamics simulation data.

Blood flow metrics obtained with four-dimensional (4D) flow phase contrast (PC) magnetic resonance imaging (MRI) can be of great value in clinical and experimental cerebrovascular analysis. However, limitations in both quantitative and qualitative analyses can result from errors inherent to PC MRI. One method that excels in creating low-error, physics-based, velocity fields is computational fluid dynamics (CFD). Augmentation of cerebral 4D flow MRI data with CFD-informed neural networks may provide a method to produce highly accurate physiological flow fields. In this preliminary study, the potential utility of such a method was demonstrated by using high resolution patient-specific CFD data to train a convolutional neural network, and then using the trained network to enhance MRI-derived velocity fields in cerebral blood vessel data sets. Through testing on simulated images, phantom data, and cerebrovascular 4D flow data from 20 patients, the trained network successfully de-noised flow images, decreased velocity error, and enhanced near-vessel-wall velocity quantification and visualization. Such image enhancement can improve experimental and clinical qualitative and quantitative cerebrovascular PC MRI analysis.

Sex Differences in Cardiac Flow Dynamics of Healthy Volunteers.

PURPOSE: The purpose of this study was to further understand the relationship between cardiac function and flow, on the basis of sex, by quantifying cardiac flow characteristics and relating them to cardiac muscle performance in young adults. MATERIALS AND METHODS: In this cross-sectional study, cardiac four-dimensional flow (4D flow) magnetic resonance imaging (MRI) and two-dimensional cine MRI were performed on 20 male and 19 female volunteers aged 20-35. Velocity-based metrics of flow, kinetic energy, vorticity, and efficiency indices were quantified, as well as cardiac strain metrics. RESULTS*: Peak systolic blood kinetic energy (male: 4.76 ± 2.66 mJ; female: 3.36 ± 1.43 mJ; p=0.047) was significantly higher in the male left ventricle (LV) than in the female LV. Peak systolic vorticity index (male: 0.008 ± 0.005 rad-m2/ml-s; female: 0.014 ± 0.007 rad-m2/ml-s; p=0.007), peak diastolic vorticity index (male: 0.007 ± 0.006 rad-m2/ml-s; female: 0.014 ± 0.010 rad-m2/ml-s; p=0.015), and cycle-average vorticity (male: 0.006 ± 0.001 rad-m2/ml-s; female: 0.011 ± 0.002 rad/s; p=0.001) were all significantly higher in the LV of women than they were in the LV of men. Radial, circumferential, and long-axis strain metrics were significantly higher in the female LV than in the male LV (p<0.05). Circumferential systolic and diastolic strain rates displayed moderate correlation to peak systolic (r=-0.38, p=0.022) and diastolic vorticity (r=0.40, p=0.015) values, respectively. *Results are reported as mean ± standard deviation. CONCLUSION: Left ventricular vorticity metrics were observed to be higher in women than in men and displayed moderate correlation to cardiac strain metrics. The methods and results of this study may be used to further understand the sex-based cardiac efficiency relationship between cardiac function and flow.

Modeling Physiological Flow in Fontan Models With Four-Dimensional Flow Magnetic Resonance Imaging, Particle Image Velocimetry, and Arterial Spin Labeling.

The Fontan procedure is a successful palliation for single ventricle defect. Yet, a number of complications still occur in Fontan patients due to abnormal blood flow dynamics, necessitating improved flow analysis and treatment methods. Phase-contrast magnetic resonance imaging (MRI) has emerged as a suitable method for such flow analysis. However, limitations on altering physiological blood flow conditions in the patient while in the MRI bore inhibit experimental investigation of a variety of factors that contribute to impaired cardiovascular health in these patients. Furthermore, resolution and flow regime limitations in phase contrast (PC) MRI pose a challenge for accurate and consistent flow characterization. In this study, patient-specific physical models were created based on nine Fontan geometries and MRI experiments mimicking low- and high-flow conditions, as well as steady and pulsatile flow, were conducted. Additionally, a particle image velocimetry (PIV)-compatible Fontan model was created and flow was analyzed with PIV, arterial spin labeling (ASL), and four-dimensional (4D) flow MRI. Differences, though nonstatistically significant, were observed between flow conditions and between patient-specific models. Large between-model variation supported the need for further improvement for patient-specific modeling on each unique Fontan anatomical configuration. Furthermore, high-resolution PIV and flow-tracking ASL data provided flow information that was not obtainable with 4D flow MRI alone.

Mri-based cancer lesion analysis with 3d printed patient specific prostate cutting guides.

Purpose: MRI methods have improved diagnosis and treatment planning for prostate cancer. However, validation and standardization is needed to encourage widespread adoption of these methods. The purpose of this study was to improve validation methods by creating a prostate cutting guide and to develop a method for 3D comparison between MRI data and post-prostatectomy histological tissue slices. Methods: Prostate Specific Membrane Antigen (PSMA) Positron Emission Tomography (PET)/MRI was performed on 10 patients with prostate cancer before and after chemohormonal treatment. Post-treatment images were used to design patient-specific prostate cutting guides that were used to create uniform thickness sections of surgically removed prostates. The thickness of the prostate tissue slices matched the imaging slice thickness so that comparisons could be made between MRI results and histopathological study results. A method was also developed to compare post-slicing prostate bulk geometry with the predicted MRI prostate geometry. Results: The prostate cutting guides were used to successfully section the prostate for histopathogical evaluation and slice-by-slice MRI comparison. Surface comparison results displayed an average dimensional difference of 1.99 ± 3.19 mm between MRI and post-prostatectomy slice reconstruction prostate geometries. Conclusion: MRI-based prostate cutting guides were designed, fabricated, and implemented in a study examining the utility and accuracy of MRI for the detection of prostate cancer. Furthermore, a three-dimensional part comparison method was developed, which can be used for validation of MRI with pathological and histological data. Future work will analyze more subjects to examine the effectiveness of these guides for histopathological prostate analysis with MRI and PET/MRI.

MRI-based method for lower urinary tract dysfunction in adult male mice.

Benign prostatic hyperplasia (BPH) develops in the majority of men as they age. As a result, lower urinary tract symptoms (LUTS) often develop, which significantly decrease quality of life. One model of studying BPH/LUTS in mice is to use a hormone-induced model of lower urinary tract dysfunction (LUTD), but current methods for studying endpoints require multiple analysis techniques that contribute to an overall lengthy process. However, developments in magnetic resonance imaging (MRI) have opened the door for more accurate and time efficient methods. The purpose of this study was to demonstrate the capabilities of MRI for the analysis of LUTD in mice. To do this, whole and partial urogenital tracts were extracted from mice and imaged on a 9.4 Tesla MRI system. Additionally, a device was designed and fabricated to aid in the imaging of up to 100 mouse urogenital tracts in a single imaging session. Images were processed for both qualitative representation of MRI resolution capabilities and quantitative measurements of urogenital tract components. Even the smallest anatomical structures of the urogenital tracts were resolved and quantified, including the ureters, urethra, ductus deferens, and fine nodules and textures on the seminal vesicles, bladder, and prostatic lobes. The visual representations and urogenital component quantifications demonstrated in this study may be of value in lesion detection, diagnosis, and LUTS symptom progression tracking.

MRI-based modeling of spleno-mesenteric confluence flow.

Characterization of hepatic blood flow magnitude and distribution can lead to a better understanding of the pathophysiology of liver disease. However, the underlying patterns and dynamics of hepatic flow, such as the helical flow structure that often develops following the spleno-mesenteric confluence (SMC) of the hepatic portal vein, have not yet been comprehensively studied. In this study, we used magnetic resonance image (MRI)-based computational models to study the effects of the helical flow structure and SMC geometry on portal blood flow distribution. Additionally, we examined these flow dynamics with four-dimensional (4D) flow MRI in a group of 12 cirrhotic patients and healthy subjects. A validation model was also created to compare computational data to particle image velocimetry (PIV) data. We found significant correlations between flow structure development, vessel geometry, and blood flow distribution in both virtually modified models and in healthy and cirrhotic subjects. However, the direction of these correlations varied among vessel configuration types. Nonetheless, validation model results displayed good qualitative agreement with computational model data.

Analysis of cavopulmonary and cardiac flow characteristics in fontan Patients: Comparison with healthy volunteers.

BACKGROUND: Characterizing the flow of the Fontan circuit, and correlating flow characteristics with the development of complications, is an important clinical challenge. Past work has analyzed the flow characteristics of Fontan circulation on a component-by-component basis. 4D flow MRI with radial projections allows for large volumetric coverage, and therefore can be used to analyze the flow through many codependent cardiovascular components in a single imaging session. PURPOSE: To describe flow characteristics across the entire Fontan circuit and to compare these with the flow characteristics in healthy volunteers. STUDY TYPE: Prospective. SUBJECTS: Eleven single ventricle patients with a Fontan connection and 15 healthy controls. SEQUENCE: Phase contrast with vastly undersampled isotropic projection reconstruction (PC-VIPR) at a field strength of 3 T. ASSESSMENT: Cavopulmonary and ventricular flow distributions, blood flow kinetic energy, vorticities, efficiency indices, and other flow parameters were analyzed using Ensight and MatLab. STATISTICAL TESTS: The results were compared across Fontan subjects, between respiratory phases, and between Fontan subjects and healthy volunteers using a Student's t-test for unequal sample sizes and linear regression. RESULTS: Cava-specific pulmonary flow distributions of Fontan patients varied significantly between respiratory phases (P < 0.05). Ventricular kinetic energy (KE) was significantly higher in Fontan patients than it was in healthy controls, leading to a lower cardiac efficiency metric in the Fontan group. A significant diastolic KE time-shift was also observed in the Fontan patient group. Peak diastolic KE was significantly higher in the single ventricle of patients with right ventricle morphology than it was in left ventricle morphology patients. DATA CONCLUSION: Radial 4D flow MRI can be used for comprehensive analysis of single ventricle Fontan flow characteristics. LEVEL OF EVIDENCE: 2 Technical Efficacy: Stage 2 J. Magn. Reson. Imaging 2019.

Surgical planning for living donor liver transplant using 4D flow MRI, computational fluid dynamics and in vitro experiments.

This study used magnetic resonance imaging (MRI), computational fluid dynamics (CFD) modeling, and in vitro experiments to predict patient-specific alterations in hepatic hemodynamics in response to partial hepatectomy in living liver donors. 4D Flow MRI was performed on three donors before and after hepatectomy and models of the portal venous system were created. Virtual surgery was performed to simulate (1) surgical resection and (2) post-surgery vessel dilation. CFD simulations were conducted using in vivo flow data for boundary conditions. CFD results showed good agreement with in vivo data, and in vitro experimental values agreed well with imaging and simulation results. The post-surgery models predicted an increase in all measured hemodynamic parameters, and the dilated virtual surgery model predicted post-surgery conditions better than the model that only simulated resection. The methods used in this study have potential significant value for the surgical planning process for the liver and other vascular territories.

Impact of image reconstruction parameters when using 3D DSA reconstructions to measure intracranial aneurysms.

BACKGROUND AND PURPOSE: Safe and effective use of newly developed devices for aneurysm treatment requires the ability to make accurate measurements in the angiographic suite. Our purpose was to determine the parameters that optimize the geometric accuracy of three-dimensional (3D) vascular reconstructions. METHODS: An in vitro flow model consisting of a peristaltic pump, plastic tubing, and 3D printed patient-specific aneurysm models was used to simulate blood flow in an intracranial aneurysm. Flow rates were adjusted to match values reported in the literature for the internal carotid artery. 3D digital subtraction angiography acquisitions were obtained using a commercially available biplane angiographic system. Reconstructions were done using Edge Enhancement (EE) or Hounsfield Unit (HU) kernels and a Normal or Smooth image characteristic. Reconstructed images were analyzed using the vendor's aneurysm analysis tool. Ground truth measurements were derived from metrological scans of the models with a microCT. Aneurysm volume, surface area, dome height, minimum and maximum ostium diameter were determined for the five models. RESULTS: In all cases, measurements made with the EE kernel most closely matched ground truth values. Differences in values derived from reconstructions displayed with Smooth or Normal image characteristics were small and had only little impact on the geometric parameters considered. CONCLUSIONS: Reconstruction parameters impact the accuracy of measurements made using the aneurysm analysis tool of a commercially available angiographic system. Absolute differences between measurements made using reconstruction parameters determined as optimal in this study were, overall, very small. The significance of these differences, if any, will depend on the details of each individual case.

Left and right ventricular kinetic energy using time-resolved versus time-average ventricular volumes.

PURPOSE: To measure the effects of using time-resolved (TR) versus time-averaged (TA) ventricular segmentation on four-dimensional flow-sensitive (4D flow) magnetic resonance imaging (MRI) kinetic energy (KE) calculations. MATERIALS AND METHODS: Right (RV) and left (LV) ventricular KE was calculated from 4D flow MRI data acquired at 3.0T in 10 healthy volunteers and five subjects with cardiac disease using TR and TA segmentation. KE was calculated from the mass of blood within the ventricles multiplied by the velocities squared. Differences in TR and TA KE and interobserver variability were quantified with Bland-Altman analysis. RESULTS: In healthy volunteers, peak systolic RV KE (KERV ) were 4.89 ± 1.49 mJ using TR and 5.53 ± 1.62 mJ using TA segmentation (P = 0.016); peak systolic LV KE (KELV ) were 3.29 ± 0.96 mJ and 4.16 ± 1.26 mJ (P = 0.005). Peak diastolic KERV were 3.33 ± 0.90 mJ (TR) and 3.61 ± 1.12 mJ (TA) (P = 0.082), while peak diastolic KELV were 4.90 ± 1.49 mJ and 5.31 ± 1.59 mJ (P = 0.044). In patient volunteers, peak systolic KERV were 4.34 ± 3.78 mJ using TR and 4.88 ± 3.98 mJ using TA segmentation (P = 0.26); peak systolic KELV were 4.39 ± 4.21 mJ and 4.36 ± 3.84 mJ (P = 0.91). Peak diastolic KERV were 3.34 ± 2.08 mJ (TR) and 4.05 ± 1.12 mJ (TA) (P = 0.08), while peak diastolic KELV were 4.34 ± 5.11 mJ and 4.06 ± 3.47 mJ (P = 0.75). Interobserver differences in KELV were greater for TR than TA calculations; bias ranged from 3 ± 30% for TA peak systolic KELV to 36 ± 30% for TR peak diastolic KELV . CONCLUSION: Although qualitatively similar, KE values calculated through TA segmentation were consistently greater than TR KE, with differences more pronounced during systole and in the LV. LEVEL OF EVIDENCE: 2 J. Magn. Reson. Imaging 2017;45:821-828.