Diffusion weighted imaging (DWI) is a method of signal contrast generation based on the differences in Brownian motion. DWI is a method to evaluate the molecular function and micro-architecture of the human body. DWI signal contrast can be quantified by apparent diffusion coefficient maps and it acts as a tool for treatment response evaluation and assessment of disease progression. Ability to detect and quantify the anisotropy of diffusion leads to a new paradigm called diffusion tensor imaging (DTI). DTI is a tool for assessment of the organs with highly organised fibre structure. DWI forms an integral part of modern state-of-art magnetic resonance imaging and is indispensable in neuroimaging and oncology. DWI is a field that has been undergoing rapid technical evolution and its applications are increasing every day. This review article provides insights in to the evolution of DWI as a new imaging paradigm and provides a summary of current role of DWI in various disease processes.

Diffusion weighted imaging: Technique and applications

PURPOSE: To compare single-shot echo-planar imaging (SS EPI) diffusion-weighted MRI (DWI) of abdominal organs between 1.5 Tesla (T) and 3.0T in healthy volunteers in terms of image quality, apparent diffusion coefficient (ADC) values, and ADC reproducibility. MATERIALS AND METHODS: Eight healthy volunteers were prospectively imaged in this HIPAA-compliant IRB-approved study. Each subject underwent two consecutive scans at both 1.5 and 3.0T, which included breathhold and free-breathing DWI using a wide range of b-values (0 to 800 s/mm(2)). A blinded observer rated subjective image quality (maximum score= 8), and a separate observer placed regions of interest within the liver, renal cortices, pancreas, and spleen to measure ADC at each field strength. Paired Wilcoxon tests were used to compare abdominal DWI between 1.5T and 3.0T for specific combinations of organs, b-values, and acquisition techniques. RESULTS: Subjective image quality was significantly lower at 3.0T for all comparisons (P = 0.0078- 0.0156). ADC values were similar at 1.5T and 3.0T for all assessed organs, except for lower liver ADC at 3.0T using b0-500-600 and breathhold technique. ADC reproducibility was moderate at both 1.5T and 3.0T, with no significant difference in coefficient of variation of ADC between field strengths. CONCLUSION: Compared with 1.5T, SS EPI at 3.0T provided generally similar ADC values, however, with worse image quality. Further optimization of abdominal DWI at 3.0T is needed.

https://onlinelibrary.wiley.com/doi/pdf/10.1002/jmri.22395

Diffusion-weighted imaging of the abdomen at 3.0 Tesla: image quality and apparent diffusion coefficient reproducibility compared with 1.5 Tesla.

OBJECTIVE The purpose of our study was to compare respiratory-triggered and breath-hold diffusion-weighted liver MRI and to assess the agreement in the apparent diffusion co-efficient (ADC) values between the two sequencesMATERIALS AND METHODS Forty-eight patients (27 men, 21 women; mean age, 452 years) with focal liver lesions underwent respiratory-triggered and breath-hold diffusion-weighted MRI (DWI) in addition to routine MRI Both sequences had identical imaging parameters except for signal averages, which were 6 in respiratory-triggered and 2 in breath-hold sequences A total of 92 lesions (maximum of three lesions per patient; 37 benign, 55 malignant) were evaluated Lesions were confirmed by typical imaging appearance, histopathology, or follow-up Signal-to-noise ratio (SNR) of the liver, contrast-to-noise ratio (CNR), and relative contrast ratio of the lesions were measured in each DWI sequence and were statistically compared using the Mann-Whitney U test The ADC values of normal liver and 

https://www.ajronline.org/doi/pdfplus/10.2214/AJR.08.1260

Respiratory-triggered versus breath-hold diffusion-weighted MRI of liver lesions: comparison of image quality and apparent diffusion coefficient values.

Purpose

The purpose of this study was to develop a multi-step adaptive fitting approach for liver proton density fat fraction (PDFF) and quantification, and to perform an initial validation on a broadly available hardware platform.



Theory and Methods

The proposed method uses a multi-echo three-dimensional gradient echo acquisition, with initial guesses for the fat and water signal fractions based on a Dixon decomposition of two selected echoes. Based on magnitude signal equations with a multi-peak fat spectral model, a multi-step nonlinear fitting procedure is then performed to adaptively update the fat and water signal fractions and values. The proposed method was validated using numeric phantoms as ground truth, followed by preliminary clinical validation of PDFF calculations against spectroscopy in 30 patients.



Results

The results of the proposed method agreed well with the ground truth of numerical phantoms, and were relatively insensitive to changes in field strength, field homogeneity, monopolar/bipolar readout, signal to noise ratio, and echo time selections. The in vivo patient study showed excellent consistency between the PDFF values measured with the proposed approach compared with spectroscopy.



Conclusion

This multi-step adaptive fitting approach performed well in both simulated and initial clinical evaluation, and shows potential in the quantification of hepatic steatosis. Magn Reson Med 72:1353–1365, 2014. © 2013 Wiley Periodicals, Inc.

Liver fat quantification using a multi-step adaptive fitting approach with multi-echo GRE imaging

Clinically established thermal therapies such as thermoablative approaches or adjuvant hyperthermia treatment rely on accurate thermal dose information for the evaluation and adaptation of the ther...

/pdf/magnetic-resonance-thermometry-methodology-pitfalls-and-3qtfp0b990.pdf

Magnetic resonance thermometry: Methodology, pitfalls and practical solutions

In this work, a new two-point method for water–fat imaging is described and explored. It generalizes existing two-point methods by eliminating some of the restrictions that these methods impose on the choice of echo times. Thus, the new two-point method promises to provide more freedom in the selection of protocol parameters and to reach higher scan efficiency. Its performance was studied theoretically and was evaluated experimentally in abdominal imaging with a multigradient-echo sequence. While depending on the choice of echo times, it is generally found to be favorable compared to existing two-point methods. Notably, water images with higher spatial resolution and better signal-to-noise ratio were attained with it in single breathholds at 3.0 T and 1.5 T, respectively. The use of more accurate spectral models of fat is shown to substantially reduce observed variations in the extent of fat suppression. The acquisition of in- and opposed-phase images is demonstrated to be replaceable by a synthesis from water and fat images. The new two-point method is finally also applied to autocalibrate a multidimensional eddy current correction and to enhance the fat suppression achieved with three-point methods in this way, especially toward the edges of larger field of views. Magn Reson Med, 2010. © 2010 Wiley-Liss, Inc.

https://onlinelibrary.wiley.com/doi/pdf/10.1002/mrm.22578

Dual-echo Dixon imaging with flexible choice of echo times.

Purpose
To assess cardiac motion-induced signal loss in diffusion-weighted magnetic resonance imaging (DWI) of the liver using dynamic DWI

https://link.springer.com/content/pdf/10.1007%2Fs10334-009-0183-1.pdf

Influence of cardiac motion on diffusion-weighted magnetic resonance imaging of the liver

Purpose

To identify and quantify the potential sources of motion in carotid artery imaging.



Materials and Methods

Two healthy volunteers and 12 patients (20–75 years old) with atherosclerotic disease were scanned on a Philips Intera 1.5T system. A single-shot balanced-fast field echo (bFFE) sequence was used to acquire real-time axial views of the carotid artery wall (three images per second). A three-step acquisition protocol was performed to analyze the three types of motion (arterial pulsation, breathing, and swallowing) separately. The isocenter carotid artery motion amplitude in either the x or y direction was measured. Radial variation in the carotid lumen between the systolic and diastolic phases was analyzed. Motion frequency was reported for each patient.



Results

Significant motion related to arterial pulsation (amplitude = 0.27–0.93 mm, mean = 0.6, SD = 0.19), breathing (amplitude = 0.5–3.6 mm, mean = 1.56, SD = 0.99)), and swallowing (amplitude = 1.4–9.2 mm, mean = 4.7, SD = 2.4) were visualized.



Conclusion

Pulsation, breathing, and swallowing are sources of significant motion in the carotid artery wall. Such motion should be considered in the future to improve carotid artery image quality. J. Magn. Reson. Imaging 2006. © 2006 Wiley-Liss, Inc.

https://onlinelibrary.wiley.com/doi/pdf/10.1002/jmri.20525

Swallowing, arterial pulsation, and breathing induce motion artifacts in carotid artery MRI

The invention relates to a method of MR imaging of at least a portion of a body (10) placed in a main magnetic field Bo within the examination volume of a MR device. It is an object of the invention to enable MR imaging with reliable motion detection and high image quality. The method of the invention comprises the following steps: - subjecting the portion of the body (10) to an imaging sequence comprising one or more RF pulses and switched magnetic field gradients for acquiring imaging signals; - subjecting the portion of the body (10) to a navigator sequence applied at least once before, during, or after the imaging sequence, the navigator sequence comprising one or more RF pulses and switched magnetic field gradients controlled in such a manner that navigator signals are acquired by means of a single-point or multi-point Dixon technique; - deriving translation and/or rotation and/or shear data from the navigator signals, which translation and/or rotation and/or shear data reflect motion occurring within the body (10) during the acquisition of the imaging signals; - reconstructing a MR image from the imaging signals, wherein the translation and/or rotation and/or shear data are used for adapting the imaging sequence and/or for motion correction during reconstruction of the MR image. Moreover, the invention relates to a MR device (1) and to a computer program to be run on a MR device (1).

MRI with motion correction using navigators acquired using a Dixon technique

Online MR temperature monitoring during radiofrequency (RF) ablation of cardiac arrhythmias may improve the efficacy and safety of the treatment. MR thermometry at 1.5 T using the proton resonance frequency (PRF) method was assessed in 10 healthy volunteers under normal breathing conditions, using a multi-slice, ECG-gated, echo planar imaging (EPI) sequence in combination with slice tracking. Temperature images were post-processed to remove residual motion-related artifacts. Using an MR-compatible steerable catheter and electromagnetic noise filter, RF ablation was performed in the ventricles of two sheep in vivo. The standard deviation of the temperature evolution in time (TSD) was computed. Temperature mapping of the left ventricle was achieved at an update rate of approximately 1 Hz with a mean TSD of 3.6 ± 0.9 °C. TSD measurements at the septum showed a higher precision (2.8 ± 0.9 °C) than at the myocardial regions at the heart–lung and heart–liver interfaces (4.1 ± 0.9 °C). Temperature rose maximally by 9 °C and 16 °C during 5 W and 10 W RF applications, respectively, for 60 s each. Tissue temperature can be monitored at an update rate of approximately 1 Hz in five slices. Typical temperature changes observed during clinical RF application can be monitored with an acceptable level of precision. Copyright © 2011 John Wiley & Sons, Ltd.

/pdf/feasibility-of-fast-mr-thermometry-during-cardiac-37w88vx6u2.pdf

Gwenael Herigault

Papers

Dual-echo Dixon imaging with flexible choice of echo times.

Influence of cardiac motion on diffusion-weighted magnetic resonance imaging of the liver

Swallowing, arterial pulsation, and breathing induce motion artifacts in carotid artery MRI

MRI with motion correction using navigators acquired using a Dixon technique

Feasibility of fast MR-thermometry during cardiac radiofrequency ablation.