About
111
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1,498
Citations
Citations since 2017
Introduction
Parallel Transmit MRI, RF pulse design, Pulse sequence design, MRI safety.
https://github.com/mriphysics
Skills and Expertise
Additional affiliations
October 2013 - present
Position
- Lecturer
Description
- I lecture second year undergraduate signal processing and electromagnetism
April 2012 - present
November 2007 - March 2012
Education
October 1999 - June 2003
Publications
Publications (111)
Our current understanding of focal epilepsy is evolving as increasing evidence suggests that tissue abnormalities may extend beyond the focus. In adults, widespread structural changes remote from the putative epileptic focus have been demonstrated with MRI, predominantly using morphological markers. However, the underlying pathophysiology of these...
Purpose
The MP2RAGE sequence is typically optimized for either T1-weighted uniform image (UNI) or gray matter–dominant fluid and white matter suppression (FLAWS) contrast images. Here, the purpose was to optimize an MP2RAGE protocol at 7 Tesla to provide UNI and FLAWS images simultaneously in a clinically applicable acquisition time at <0.7 mm isot...
Purpose:
Ultralow-field (ULF) point-of-care MRI systems allow image acquisition without interrupting medical provision, with neonatal clinical care being an important potential application. The ability to measure neonatal brain tissue T1 is a key enabling technology for subsequent structural image contrast optimization, as well as being a potentia...
Fetal Magnetic Resonance Imaging (Fetal MRI) allows insights into human development before birth, complementing conventional Ultrasound imaging with its high resolution and available numerous contrast options. Significant challenges still exist including geometric distortion caused by maternal bowel gas in echo-planar imaging, and restrictions in b...
Purpose
Universal Pulses (UPs) are excitation pulses that reduce the flip angle inhomogeneity in high field MRI systems without subject-specific optimization, originally developed for parallel transmit (PTX) systems at 7 T. We investigated the potential benefits of UPs for single channel (SC) transmit systems at 3 T, which are widely used for clini...
Direct Signal Control (DSC) was combined with a dynamic, fast online-customized (FOCUS) parallel transmit excitation pulse for a 3D TSE sequence using an 8Tx/32Rx head RF coil at 7 Tesla. Whereas DSC could strongly improve the signal, mainly in the cerebellum region, the dynamic, FOCUS excitation pulse achieved better FA and phase homogeneity than...
Purpose:
To evaluate specific absorption rate (SAR) and temperature distributions resulting from pediatric exposure to a 7T head coil.
Methods:
Exposure from a 297-MHz birdcage head transmit coil (CP mode single-channel transmission) was simulated in several child models (ages 3-14, mass 13.9-50.4 kg) and one adult, using time-domain electromagn...
The Developing Human Connectome Project has created a large open science resource which provides researchers with data for investigating typical and atypical brain development across the perinatal period. It has collected 1228 multimodal magnetic resonance imaging (MRI) brain datasets from 1173 fetal and/or neonatal participants, together with coll...
Purpose:
To develop a fully data-driven retrospective intrascan motion-correction framework for volumetric brain MRI at ultrahigh field (7 Tesla) that includes modeling of pose-dependent changes in polarizing magnetic (B0 ) fields.
Theory and methods:
Tissue susceptibility induces spatially varying B0 distributions in the head, which change with...
Purpose:
This work proposes a novel RF pulse design for parallel transmit (pTx) systems to obtain uniform saturation of semisolid magnetization for magnetization transfer (MT) contrast in the presence of transmit field B 1 + inhomogeneities. The semisolid magnetization is usually modeled as being purely longitudinal, with the applied B 1 + field s...
Purpose: This work proposes a novel RF pulse design for parallel transmit (pTx) systems to obtain uniform saturation of semisolid magnetization for Magnetization Transfer (MT) contrast in the presence of transmit field ($B_1^+$) inhomogeneities. The semisolid magnetization is usually modeled as being purely longitudinal, with the applied $B_1^+$ fi...
Objective
Neonatal brain and cardiac imaging would benefit from the increased signal-to-noise ratio levels at 7 T compared to lower field. Optimal performance might be achieved using purpose designed RF coil arrays. In this study, we introduce an 8-channel dipole array and investigate, using simulations, its RF performances for neonatal application...
Purpose: Universal Pulses (UPs) are excitation pulses that reduce the flip angle inhomogeneity in high field MRI systems without subject-specific optimization, originally developed for parallel transmit (PTX) systems at 7T. We investigated the potential benefits of UPs for single channel (SC) transmit systems at 3T, which are widely used for clinic...
Background:
Parallel transmit (pTx) has introduced many benefits to MRI with regard to decreased specific absorption rates and improved transmit field homogeneity, of particular importance in applications at higher magnetic field strengths. PTx has also been proposed as a solution to mitigating dangerous RF induced heating of elongated conductive...
Purpose
Magnetization transfer (MT) and inhomogeneous MT (ihMT) contrasts are used in MRI to provide information about macromolecular tissue content. In particular, MT is sensitive to macromolecules, and ihMT appears to be specific to myelinated tissue. This study proposes a technique to characterize MT and ihMT properties from a single acquisition...
This study presents a comparison of quantitative MRI methods based on an efficiency metric that quantifies their intrinsic ability to extract information about tissue parameters. Under a regime of unbiased parameter estimates, an intrinsic efficiency metric η was derived for fully-sampled experiments which can be used to both optimize and compare s...
Purpose: We present in vivo testing of a parallel transmit (pTx) system intended for
interventional MR-guided cardiac procedures.
Methods: The pTx system was connected in-line with a conventional 1.5T MRI system to
transmit and receive on an 8-coil array. The system used a current sensor for real-time
feedback to achieve real-time current control,...
Purpose: Magnetization transfer (MT) and inhomogeneous MT (ihMT) contrasts are used in MRI to provide information about macromolecular tissue content. In particular, MT is sensitive to macromolecules and ihMT appears to be specific to myelinated tissue. This study proposes a technique to characterize MT and ihMT properties from a single acquisition...
Purpose
To investigate safe limits for neonatal imaging using a 7T head coil, including both specific absorption rate (SAR) and temperature predictions.
Methods
Head‐centered neonate models were simulated using finite‐difference time domain–based electromagnetic and thermal solvers. The effects of higher water content of neonatal tissues compared...
Background: Parallel transmit (pTx) has introduced many benefits to MRI with regard to decreased specific absorption rates and improved transmit field homogeneity, of particular importance in applications at higher magnetic field strengths. PTx has also been proposed as a solution to mitigating dangerous RF induced heating of elongated conductive d...
Purpose:
A framework to design radiofrequency (RF) pulses specifically to minimize the TR of gradient echo sequences is presented, subject to hardware and physiological constraints.
Methods:
Single-band and multiband (MB) RF pulses can be reduced in duration using variable-rate selective excitation (VERSE) VERSE for a range of flip angles; howev...
A Correction to this paper has been published: https://doi.org/10.1038/s41467-020-20353-3
Purpose
To develop a novel gadolinium‐free model‐based quantitative magnetization transfer (qMT) technique to assess macromolecular changes associated with myocardial fibrosis.
Methods
The proposed sequence consists of a two‐dimensional breath‐held dual shot interleaved acquisition of five MT‐weighted (MTw) spoiled gradient echo images, with varia...
MRI is a versatile technique that offers many different options for tissue contrast, including suppressing the blood signal, so‐called black‐blood contrast. This contrast mechanism is extremely useful to visualize the vessel wall with high conspicuity or for characterization of tissue adjacent to the blood pool. In this review we cover the physics...
Prenatal detection of congenital heart disease facilitates the opportunity for potentially life-saving care immediately after the baby is born. Echocardiography is routinely used for screening of morphological malformations, but functional measurements of blood flow are scarcely used in fetal echocardiography due to technical assumptions and issues...
Purpose: This study proposes an efficiency metric to quantify the performance of quantitative MRI methods based on their intrinsic ability to extract information about tissue parameters. The metric can be used to both optimize and compare sequences. Here we compare steady-state sequences with transient measurement methods such as magnetic resonance...
While the observed proton signal in MRI comes primarily from water, it is influenced by the presence of other molecules. The proteins, lipid membranes, and other “semisolids” that comprise human tissue contain NMR-active protons. These protons typically have very short transverse relaxation times making them invisible in MRI, but they can exchange...
Purpose:
To demonstrate that quantitative magnetization transfer (qMT) parameters can be extracted from steady-state free-precession (SSFP) data with no external T1 map or banding artifacts.
Methods:
SSFP images with multiple MT weightings were acquired and qMT parameters fitted with a two-stage elliptical signal model.
Results:
Monte Carlo si...
Purpose:
The widespread clinical application of quantitative MRI has been hindered by a lack of reproducibility across sites and vendors. Previous work has attributed this to incorrect B1 mapping or insufficient spoiling conditions. We recently proposed the controlled saturation magnetization transfer (CSMT) framework and hypothesized that the lac...
Purpose:
MRI-guided cardiovascular intervention using standard metal guidewires can produce focal tissue heating caused by induced radiofrequency guidewire currents. It has been shown that safe operation is made possible by using parallel transmit radiofrequency coils driven in the null current mode, which does not induce radiofrequency currents a...
Purpose:
In this work, we explore the use of multiband (MB) balanced steady-state free precession (bSSFP) with blipped-controlled aliasing in parallel imaging (CAIPI), which avoids the issues of altered frequency response associated with RF phase cycling, and show its application to accelerating cardiac cine imaging.
Methods:
Blipped and RF-cycl...
Purpose:
Inhomogeneous magnetization transfer (ihMT) is an emerging form of MRI contrast that may offer high specificity for myelinated tissue. Existing ihMT and pulsed MT sequences often use separate radiofrequency pulses for saturation and signal excitation. This study investigates the use of nonselective multiband radiofrequency pulses for simu...
Purpose:
This study incorporates a gradient system imperfection model into an optimal control framework for radio frequency (RF) pulse design.
Theory and methods:
The joint design of minimum-time RF and slice selective gradient shapes is posed as an optimal control problem. Hardware limitations such as maximal amplitudes for RF and slice selecti...
Quantitative MRI applications, such as mapping the T1 time of tissue, puts high demands on the accuracy and precision of transmit field () estimation. A candidate approach to satisfy these requirements exploits the difference in phase induced by the Bloch‐Siegert frequency shift (BSS) of 2 acquisitions with opposite off‐resonance frequency radiofre...
Purpose:
To investigate the potential of continuous radiofrequency (RF) shifting (SWEEP) as a technique for creating densely sampled data while maintaining a stable signal state for dynamic imaging.
Methods:
We present a method where a continuous stable state of magnetization is swept smoothly across the anatomy of interest, creating an efficien...
Synopsis The risk of RF-induced currents makes use of electrically conductive guidewires for cardiac catheterization procedures potentially unsafe for MRI. By using a parallel transmit coil array (PTx) system it is possible to generate RF field modes that can be utilized for safe imaging, and also guidewire visualization. As the induced RF currents...
Purpose: To develop an MRI framework for reconstruction of 4D velocity vector blood flow volumes for visualisation and quantification of circulation in the fetal heart and major vessels.
Methods: A novel method of velocity-encoding using multiple non-coplanar stacks of bSSFP phase images was combined with a previous framework for reconstruction of...
Purpose
To develop an MRI acquisition and reconstruction framework for volumetric cine visualization of the fetal heart and great vessels in the presence of maternal and fetal motion.
Methods
Four‐dimensional (4D) depiction was achieved using a highly‐accelerated multi‐planar real‐time balanced steady‐state free precession acquisition combined wit...
Multicomponent driven equilibrium steady-state observation of T 1 and T 2 (mcDESPOT) aims to quantify the Myelin Water Fraction (MWF) using a two-pool microstructural model. The MWF has been used to track neurodevelopment and neurodegeneration and has been histologically correlated to myelin content. mcDESPOT has a clinically feasible acquisition t...
Purpose: Quantitative MRI applications, such as mapping the T1 time of tissue, puts high demands on the accuracy and precision of transmit field (B1+) estimation. A candidate approach to satisfy these requirements exploits the difference in phase induced by the Bloch-Siegert Shift (BSS) of two acquisitions with opposite off-resonance frequency RF p...
FIGURE S1 Comparison of measured signal (top), free pool longitudinal magnetization (middle) and macromolecular longitudinal magnetization (bottom) between pulsed and non‐pulsed bSSFP steady‐state for a fixed free pool flip angle of 68°.
VIDEO S1. Dynamic B0 field map showing the effect of respiratory motion on B0 field homogeneity. Thirty frames with 1.27 s/frame are presented. The magnitude image is presented on the left, and the brain ROI is delineated in orange
VIDEO S2. Dynamic B0 field map showing the effect of fetus motion on B0 field homogeneity. Thirty frames with 1.27 s/frame are presented. The magnitude image is presented on the left, and the brain ROI is delineated in orange
FIGURE S1. One example of good fat suppression where EPI of the fetal brain acquired with L‐IB and optimal SPIR offset is presented in different orientations (L, left; R, right; A, anterior; P, posterior; I, inferior; S, superior). In this example, only water in maternal tissue is contributing to the signal
FIGURE S2. Effect of maternal respiratio...
Purpose: To develop a MRI acquisition and reconstruction framework for volumetric cine visualisation of the fetal heart and great vessels in the presence of maternal and fetal motion. Methods: Four-dimensional depiction was achieved using a highly-accelerated multi-planar real-time balanced steady state free precession acquisition combined with ret...
The emergence of multiparametric diffusion models combining diffusion and relaxometry measurements provides powerful new ways to explore tissue microstructure, with the potential to provide new insights into tissue structure and function. However, their ability to provide rich analyses and the potential for clinical translation critically depends o...
FIGURE S1
B1+ maps (top row) and B0 maps (bottom row) used to produce the predictions in Figure 4. Data are from two separate subjects: the subject whose FLAIR imaging was performed with the 2‐refocussing pulse MP is on the left and the 4‐refocussing pulse MP subject is on the right.
FIGURE S2 Bias corrected and segmented images shown as in Figure...
Purpose
This study demonstrates magnetization transfer (MT) effects directly affect relaxometry measurements and develops a framework that allows single‐pool models to be valid in 2‐pool MT systems.
Methods
A theoretical framework is developed in which a 2‐pool MT system effectively behaves as a single‐pool if the RMS RF magnetic field B1rms is ke...
Purpose
Simultaneous multi‐slice techniques are reliant on multiband RF pulses, for which conventional design strategies result in long pulse durations, lengthening echo‐times so lowering SNR for spin‐echo imaging, and lengthening repetition times for gradient echo sequences. Pulse durations can be reduced with advanced RF pulse design methods that...
Fig. S1. Steady‐state SPGR signal for myelin‐water exchange model as a function of Φ0 and δb (compare with Fig. 2b). As resonance offset δb is varied, no change in signal is observed for values of Φ0 that yield good spoiling (i.e., flat signal that is close to the desired ideal spoiling steady‐state value). However, for the spike values such as Φ0=...
Purpose
Echo planar imaging (EPI) is the primary sequence for functional and diffusion MRI. In fetal applications, the large field of view needed to encode the maternal abdomen leads to prolonged EPI readouts, which may be further extended due to safety considerations that limit gradient performance. The resulting images become very sensitive to wa...
The emergence of multiparametric diffusion models combining diffusion and relaxometry measurements provide powerful new ways to explore tissue microstructure with the potential to provide new insights into tissue structure and function. However, their ability to provide rich analyses and the potential for clinical translation critically depends on...
Fig. S1. Image results in the coronal, sagittal, and tranverse plane from one subject (GA 29 + 0 weeks) scanned using the HighRes protocol. The first row (a–c) shows the images after Multiband reconstruction, (d–f) after pre‐processing including distortion and motion correction. Thereby, the columns show from left to right data acquired with b = 0,...
Table S1. Imaging Protocols Used for the Experiments Shown in the Results Section. Subjects 1–2 Are Adult Volunteers, Subject 3–12 Are Fetal Volunteers. Abbreviations Used: MB, Multiband; PF, Partial Fourier; Res.: Resolution; SB: Singleband
Fig. S1. Acoustic simulation for a conventional EPI readout gradient with trapezoids for the case of flat t...
Purpose:
Image quality obtained for brain imaging at 7T can be hampered by inhomogeneities in the static magnetic field, B0, and the RF electromagnetic field, B1. In imaging sequences such as fluid-attenuated inversion recovery (FLAIR), which is used to assess neurological disorders, these inhomogeneities cause spatial variations in signal that ca...
Video S1. Videos of dynamic and cine image series shown in Figure 7. Real‐time images are shown in full field of view (left) and cropped view (center) at acquired frame rate, with cropped view of cine image series (right) looped for duration of video. The full field of view is 400 × 304 mm and cropped views are 100 × 100 mm. Images are oriented wit...
Video S2. Videos of dynamic and cine image series shown in Figure 8 showing impact of k‐t SENSE regularization in 33 week gestational age fetus. Real‐time image series (left) reconstructed with spatially‐uniform (top) and spatially‐adaptive regularization (bottom) are shown at acquired frame rate. Additional high‐frequency temporal dynamics can be...
Video S3. Videos of dynamic and cine image series shown in Figure 9 comparing cine images reconstructed using some or all steps in the proposed pipeline in a 30 week gestational age fetus with hypoplastic left heart syndrome. Real‐time image series (left) was used to generate cines using (left to right) cardiac synchronization only; cardiac synchro...
Video S1e.
(a) Thirty‐three week gestational age fetus with atrioventricular and ventriculoarterial discordance shown in Figure 7a. (b) Thirty‐three week gestational age fetus with atrioventricular and ventriculoarterial discordance shown in Figure 7b. (c) Twenty‐seven week gestational age fetus shown in Figure 7c. (d) Thirty‐three week gestational...
Fig. S1. The “edge spike” of a minimum‐phase filter can be moved by flipping stop‐band roots. Just as flipping passband roots controls passband energy across the pulse duration, so does flipping stopband roots control the distribution of stopband energy. The top row shows an untouched problematic minimum‐phase filter with all stopband roots outside...
Purpose:
An extended phase graph framework (EPG-X) for modeling systems with exchange or magnetization transfer (MT) is proposed.
Theory:
EPG-X models coupled two-compartment systems by describing each compartment with separate phase graphs that exchange during evolution periods. There are two variants: EPG-X(BM) for systems governed by the Bloc...
Purpose:
To develop a purpose-built quiet echo planar imaging capability for fetal functional and diffusion scans, for which acoustic considerations often compromise efficiency and resolution as well as angular/temporal coverage.
Methods:
The gradient waveforms in multiband-accelerated single-shot echo planar imaging sequences have been redesign...
FIGURE S1. Boxplots of contrast‐to‐noise ratio (CNR) between myocardium and blood within the heart are shown for the images from Figure 5 for all three imaging scenarios. With the extended radiofrequency (RF) shimming regimes, the CNR is increased compared with quadrature, with the minimum squared error (MSE) optimization showing greatest improveme...
Purpose:
Advanced diffusion magnetic resonance imaging benefits from collecting as much data as is feasible but is highly sensitive to subject motion and the risk of data loss increases with longer acquisition times. Our purpose was to create a maximally time-efficient and flexible diffusion acquisition capability with built-in robustness to parti...
