Stuart Crozier

University of Queensland, Brisbane, Queensland, Australia

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Publications (478)599.88 Total impact

  • Xia Kong · Minhua Zhu · Ling Xia · Qiuliang Wang · Yi Li · Xuchen Zhu · Feng Liu · Stuart Crozier
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    ABSTRACT: The uniformity of the static magnetic field B0 is of prime importance for an MRI system. The passive shimming technique is usually applied to improve the uniformity of the static field by optimizing the layout of a series of steel shims. The steel pieces are fixed in the drawers in the inner bore of the superconducting magnet, and produce a magnetizing field in the imaging region to compensate for the inhomogeneity of the B0 field. In practice, the total mass of steel used for shimming should be minimized, in addition to the field uniformity requirement. This is because the presence of steel shims may introduce a thermal stability problem. The passive shimming procedure is typically realized using the linear programming (LP) method. The LP approach however, is generally slow and also has difficulty balancing the field quality and the total amount of steel for shimming. In this paper, we have developed a new algorithm that is better able to balance the dual constraints of field uniformity and the total mass of the shims. The least square method is used to minimize the magnetic field inhomogeneity over the imaging surface with the total mass of steel being controlled by an L1-norm based constraint. The proposed algorithm has been tested with practical field data, and the results show that, with similar computational cost and mass of shim material, the new algorithm achieves superior field uniformity (43% better for the test case) compared with the conventional linear programming approach.
    No preview · Article · Feb 2016 · Journal of Magnetic Resonance
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    ABSTRACT: An MRI-LINAC system combines a magnetic resonance imaging (MRI) system with a medical linear accelerator (LINAC) to provide image-guided radiotherapy for targeting tumors in real-time. In an MRI-LINAC system, a set of split gradient coils is employed to produce orthogonal gradient fields for spatial signal encoding. Owing to this unconventional gradient configuration, eddy currents induced by switching gradient coils on and off may be of particular concern. It is expected that strong intra-coil interactions in the set will be present due to the constrained return paths, leading to potential degradation of the gradient field linearity and image distortion. In this study, a series of gradient coils with different track widths have been designed and analyzed to investigate the electromagnetic interactions between coils in a split gradient set. A driving current, with frequencies from 100 Hz to 10 kHz, was applied to study the inductive coupling effects with respect to conductor geometry and operating frequency. It was found that the eddy currents induced in the un-energized coils (hereby-referred to as passive coils) positively correlated with track width and frequency. The magnetic field induced by the eddy currents in the passive coils with wide tracks was several times larger than that induced by eddy currents in the cold shield of cryostat. The power loss in the passive coils increased with the track width. Therefore, intra-coil interactions should be included in the coil design and analysis process.
    No preview · Article · Jan 2016
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    ABSTRACT: Background: MRI has developed into one of the most important medical diagnostic imaging modalities, but it exposes staff to static magnetic fields (SMF) when present in the vicinity of the MR system, and to radiofrequency and switched gradient electromagnetic fields if they are present during image acquisition. We measured exposure to SMF and motion-induced time-varying magnetic fields (TVMF) in MRI staff in clinical practice in the UK to enable extensive assessment of personal exposure levels and variability, which enables comparison to other countries. Methods: 8 MRI facilities across National Health Service sites in England, Wales and Scotland were included, and staff randomly selected during the days when measurements were performed were invited to wear a personal MRI-compatible dosimeter and keep a diary to record all procedures and tasks performed during the measured shift. Results: 98 participants, primarily radiographers (71%) but also other healthcare staff, anaesthetists and other medical staff were included, resulting in 149 measurements. Average geometric mean peak SMF and TVMF exposures were 448 mT (range 20-2891) and 1083 mT/s (9-12 355 mT/s), and were highest for radiographers (GM=559 mT and GM=734 mT/s). Time-weighted exposures to SMF and TVMF (GM=16 mT (range 5-64) and GM=14 mT/s (range 9-105)) and exposed-time-weighted exposures to SMF and TVMF (GM=27 mT (range 11-89) and GM=17 mT/s (range 9-124)) were overall relative low-primarily because staff were not in the MRI suite for most of their shifts-and did not differ significantly between occupations. Conclusions: These results are comparable to the few data available from the UK but they differ from recent data collected in the Netherlands, indicating that UK staff are exposed for shorter periods but to higher levels. These data indicate that exposure to SMF and TVMF from MRI scanners cannot be extrapolated across countries.
    No preview · Article · Dec 2015 · Occupational and environmental medicine
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    ABSTRACT: This article proposes a numerical formulation for handling mixed-dimensional elements embedded in a standard three-dimensional (3d) mesh, avoiding thus the volume meshing of filaments and strips. The method is then applied to predict the temperature heating and cooling profile of gradient coils in magnetic resonance imaging. These coils are typically constructed from copper wires or tracks and embedded in an epoxy layer. It was found that the new method significantly reduces the computational time of steady-state and transient simulations, with speedups in the range of 3.5-5. The method proved to be accurate, with relative errors below 0.5% for steady-state simulations and 1.5% with respect to a complete 3d simulation. 2016
    No preview · Article · Dec 2015 · Numerical Heat Transfer Applications
  • Y. Wang · F. Liu · S. Crozier
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    ABSTRACT: Purpose: To evaluate the acoustic noise levels induced by the split-gradient coils of a MRI-LINAC system, and also propose a scheme for acoustic noise reduction.
    No preview · Article · Dec 2015 · Medical Physics
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    Full-text · Dataset · Oct 2015
  • Elliot Smith · Fabio Freschi · Maurizio Repetto · Stuart Crozier

    No preview · Article · Sep 2015 · IEEE transactions on bio-medical engineering
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    ABSTRACT: To develop an automated approach for 3D quantitative assessment and measurement of alpha angles from the femoral head-neck (FHN) junction using bone models derived from magnetic resonance (MR) images of the hip joint.Bilateral MR images of the hip joints were acquired from 30 male volunteers (healthy active individuals and high-performance athletes, aged 18-49 years) using a water-excited 3D dual echo steady state (DESS) sequence. In a subset of these subjects (18 water-polo players), additional True Fast Imaging with Steady-state Precession (TrueFISP) images were acquired from the right hip joint. For both MR image sets, an active shape model based algorithm was used to generate automated 3D bone reconstructions of the proximal femur. Subsequently, a local coordinate system of the femur was constructed to compute a 2D shape map to project femoral head sphericity for calculation of alpha angles around the FHN junction. To evaluate automated alpha angle measures, manual analyses were performed on anterosuperior and anterior radial MR slices from the FHN junction that were automatically reformatted using the constructed coordinate system.High intra- and inter-rater reliability (intra-class correlation coefficients > 0.95) was found for manual alpha angle measurements from the auto-extracted anterosuperior and anterior radial slices. Strong correlations were observed between manual and automatic measures of alpha angles for anterosuperior (r = 0.84) and anterior (r = 0.92) FHN positions. For matched DESS and TrueFISP images, there were no significant differences between automated alpha angle measures obtained from the upper anterior quadrant of the FHN junction (two-way repeated measures ANOVA, F < 0.01, p = 0.98).Our automatic 3D method analysed MR images of the hip joints to generate alpha angle measures around the FHN junction circumference with very good reliability and reproducibility. This work has the potential to improve analyses of cam-type lesions of the FHN junction for large-scale morphometric and clinical MR investigations of the human hip region.
    Full-text · Article · Sep 2015 · Physics in Medicine and Biology
  • Phong Thanh Nguyen · Amin Abbosh · Stuart Crozier
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    ABSTRACT: A technique for noninvasive microwave hyperthermia treatment for breast cancer is presented. In the proposed technique, electromagnetic focusing on patient-specific breast models is implemented to concentrate the power at the tumor position while keeping the power levels at other positions (healthy tissues) at minimum values. This step is realized by optimizing phase excitations of the utilized antenna elements. In the next step, a thermal analysis is used to determine the scaling factor of the antennas’ excitation amplitudes to realize the required temperature at the tumor position. A closed-loop procedure ensures that there are no hot spots in any of the healthy tissues. The technique is tested in a realistic environment which includes three-dimensional breast models and antenna arrays. The presented results on fatty and dense breast models using two types of directional antennas validate the proposed technique. The investigations across wide frequency bands indicate that the frequencies around 4.2 GHz and 4.5 GHz are optimum values for the hyperthermia treatment of dense and fatty breasts, respectively.
    No preview · Article · Aug 2015 · IEEE Transactions on Antennas and Propagation
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    ABSTRACT: This corrigendum presents modifications to the results presented in Schaap, K., Christopher-De Vries, Y., Crozier, S., De Vocht, F., and Kromhout, H. (2014) Exposure to static and time-varying magnetic fields from working in the static magnetic stray fields of MRI scanners: A comprehensive survey in the Netherlands. Annals of Occupational Hygiene; 58(9): 1094-110. Although several results were modified, these changes did not affect the conclusions of the article.
    No preview · Article · Jul 2015 · Annals of Occupational Hygiene
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    ABSTRACT: In some MRI-LINAC (Magnetic Resonance Imaging and Linear Accelerator) hybrid systems, the MRI scanner is split into two parts to form a central gap for the accommodation of the patient or a LINAC. Little is known about the acoustic characteristics of the split gradient coil structure needed for this system; however, it is believed to be very different from its typical configurations. It is important to develop dedicated numerical methods for the characterization of the unique acoustic properties, to provide engineering solutions for the noise attenuation for such a new system. In this article, we modeled the acoustic fields of a split MRI system and traditional gradient structures using the finite element method. The models were validated against acoustic experimental results obtained from a conventional MRI scanner. The acoustic field distribution analysis showed that the average sound pressure levels in the central gap were lower than those in the cylindrical tunnels of the split MRI system at most frequencies. This was also true when both the x coils or z coils were energized independently. Thus, if the patient bed is placed perpendicular to the axis of the main magnet of the split MRI system, the patient will be subjected to relatively lower acoustic intensities compared with that if the patient bed is placed parallel to the axis of the main magnet. Further work is planned to reduce the sound level in the central gap where the patient bed may be placed for this split system. © 2015 Wiley Periodicals, Inc. Concepts Magn Reson Part B (Magn Reson Engineering), 2015
    No preview · Article · Jun 2015 · Concepts in Magnetic Resonance Part B Magnetic Resonance Engineering
  • Xia Kong · Minhua Zhu · Ling Xia · Stuart Crozier · Qiuliang Wang · Zhipeng Ni · Feng Liu
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    ABSTRACT: This paper presents a novel passive shimming method for the effective correction of static magnetic field (B0) inhomogeneities in Magnetic Resonance Imaging (MRI) systems. Passive shimming is used to find an optimum configuration for the placement of iron pieces applied to improve the B0 uniformity in the predefined imaging region referred to as the diameter of spherical volume (DSV). However, most passive shimming methods neglect to recognize that the space under the patient bed is not in use for imaging. In this work, we present a new algorithm that attempts to avoid the unnecessary shimming of the space under the patient bed. During implementation, the B0 field is still measured over the DSV surface and then mapped onto the effective imaging volume surface; a dedicated sensitivity matrix is generated only for the imaging area above the patient bed. A linear programming optimization procedure is performed for the determination of thicknesses and locations the shim pieces. Our experimental results showed that by revising the shimming target area, the new method provides superior optimization solutions. Compared to a conventional approach, the new method requires smaller amount of iron to correct the B0 inhomogeneities in the imaging area which has the effect of improving thermal stability to the B0 field. It also reduces the complexity of the optimization problem. Our new shimming strategy helps to improve the magnetic field homogeneity within the realistic imaging space, and ultimately improve image quality. Copyright © 2015 Elsevier Inc. All rights reserved.
    No preview · Article · May 2015 · Journal of Magnetic Resonance
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    ABSTRACT: Many medical image processing techniques rely on accurate shape modeling of anatomical features. The presence of shape abnormalities challenges traditional processing algorithms based on strong morphological priors. In this work, a sparse shape reconstruction from a statistical shape model is presented. It combines the advantages of traditional statistical shape models (defining a 'normal' shape space) and previously presented sparse shape composition (providing localized descriptors of anomalies). The algorithm was incorporated into our image segmentation and classification software. Evaluation was performed on simulated and clinical MRI data from 22 sciatica patients with intervertebral disc herniation, containing 35 herniated and 97 normal discs. Moderate to high correlation (R=0.73) was achieved between simulated and detected herniations. The sparse reconstruction provided novel quantitative features describing the herniation morphology and MRI signal appearance in three dimensions (3D). The proposed descriptors of local disc morphology resulted to the 3D segmentation accuracy of 1.07±1.00mm (mean absolute vertex-to-vertex mesh distance over the posterior disc region), and improved the intervertebral disc classification from 0.888 to 0.931 (area under receiver operating curve). The results show that the sparse shape reconstruction may improve computer-aided diagnosis of pathological conditions presenting local morphological alterations, as seen in intervertebral disc herniation. Crown Copyright © 2015. Published by Elsevier Ltd. All rights reserved.
    No preview · Article · May 2015 · Computerized Medical Imaging and Graphics
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    ABSTRACT: To simulate the delivery of proton beams to the treatment zone inside a split-bore MRI-guided proton therapy system. Field maps from a split-bore 1 T MRI system are used as input to Monte Carlo simulations which model the trajectory of proton beams towards isocentre. Both inline (along the MRI bore) and perpendicular (through the split-bore gap) orientations are simulated. Monoenergetic diverging beams of energy 90 MeV, 195 MeV and 300 MeV starting from 1.5 m above isocentre were modelled. A phase space file detailing a 2D calibration pattern is used to set the particle starting positions, and their spatial location as they cross isocentre recorded. Inline Orientation: The radial symmetry of the solenoidal style fringe field acts to rotate the protons around the beam's central axis. For protons starting at 1.5 m from isocentre this rotation is 17° (90 MeV), and 8° (300 MeV). Perpendicular Orientation: Isocentre shifts of 135 mm (90 MeV) and 65 mm (300 MeV) were observed in the direction perpendicular to the main imaging field. Off-axis protons are also slightly deflected towards or away from the central axis in the direction perpendicular to the main deflection direction. This leads to a distortion of the phase space pattern, not just a shift. The distortion increases from zero at the central axis to 10 mm (90 MeV) and 5 mm (300 MeV) for a proton 150 mm off-axis. The complexity and energy-dependent nature of the magnetic deflection and distortion indicates the pencil beam scanning method will be the only choice for delivering a therapeutic proton beam inside a potential MRI-guided proton therapy system. Significant correction strategies will be required to account for the MRI fringe fields. The authors acknowledge funding from NHMRC Program Grant No. 1036078 and ARC Discovery Grant No. DP120100821.
    Preview · Article · May 2015 · Medical Physics
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    ABSTRACT: The paper presents a computational study for the estimation of the temperature elevation occurring in a human subject carrying metallic hip prostheses when exposed to the magnetic field produced by gradient coils. The simulations are performed through validated numerical codes, which solve the electromagnetic and thermal equations applied to a high-resolution anatomical human model. Three different sets of gradient coils (traditional, split and uniplanar) are considered to evaluate the maximum steady-state temperature elevation in the human body. This result is then rescaled to take into account the waveform of the signal, the duty-cycle and the duration of the scan. Several exposure situations obtained by changing the patient's position are analyzed, finding temperature elevations on the order of some degrees. The results are of possible concern and provide evidence of the need for further specific investigations aimed at assuring the safety of potential patients carrying metallic hip implants. Magn Reson Med, 2015. © 2015 Wiley Periodicals, Inc. © 2015 Wiley Periodicals, Inc.
    No preview · Article · Apr 2015 · Magnetic Resonance in Medicine
  • Yang Yang · Feng Liu · Zhaoyang Jin · Stuart Crozier
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    ABSTRACT: Random phase-encode undersampling of Cartesian k-space trajectories is widely implemented in compressed sensing (CS) magnetic resonance imaging (MRI). However, its one-dimensional (1-D) randomness inherently introduces large coherent aliasing artefacts along the undersampled direction in the reconstruction, and thus degrades the image quality. This paper proposes a novel reconstruction scheme to reduce the 1-D undersampling induced aliasing artefacts. The proposed reconstruction progress is separated into two steps in our new algorithm. In step one, we transfer the original two-dimensional (2-D) image reconstruction into a parallel 1-D signal reconstruction procedure, which takes advantage of the superior incoherence property in the phase direction. In step two, using the new k-space data obtained from the 1-D reconstructions, we implement a follow-up 2-D CS reconstruction to produce a better solution, which exploits the inherent correlations between the adjacent lines of 1-D reconstructed signals. We evaluated the performance on various cases of typical MR images, including cardiac cine, brain, foot and angiogram at the reduction factor up to 10 and compared the results with the conventional CS method. Experiments using the proposed method demonstrated faithful reconstruction of the MR images. Compared with conventional method, the new method achieves more accurate reconstruction results with 2~5dB gain in peak signal-to-noise ratio and higher structural similarity index. The proposed method improves image quality of the reconstructions and suppresses the coherent artefacts introduced by the random phase-encode undersampling.
    No preview · Article · Apr 2015 · IEEE transactions on bio-medical engineering
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    ABSTRACT: IntroductionAccurate knowledge of O6-methylguanine methyltransferase (MGMT) gene promoter subtype in patients with glioblastoma (GBM) is important for treatment. However, this test is not always available. Pre-operative diffusion MRI (dMRI) can be used to probe tumour biology using the apparent diffusion coefficient (ADC); however, its ability to act as a surrogate to predict MGMT status has shown mixed results. We investigated whether this was due to variations in the method used to analyse ADC.Methods We undertook a retrospective study of 32 patients with GBM who had MGMT status measured. Matching pre-operative MRI data were used to calculate the ADC within contrast enhancing regions of tumour. The relationship between ADC and MGMT was examined using two published ADC methods.ResultsA strong trend between a measure of ‘minimum ADC’ and methylation status was seen. An elevated minimum ADC was more likely in the methylated compared to the unmethylated MGMT group (U = 56, P = 0.0561). In contrast, utilising a two-mixture model histogram approach, a significant reduction in mean measure of the ‘low ADC’ component within the histogram was associated with an MGMT promoter methylation subtype (P < 0.0246).Conclusion This study shows that within the same patient cohort, the method selected to analyse ADC measures has a significant bearing on the use of that metric as a surrogate marker of MGMT status. Thus for dMRI data to be clinically useful, consistent methods of data analysis need to be established prior to establishing any relationship with genetic or epigenetic profiling.
    Full-text · Article · Apr 2015
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    ABSTRACT: Recent studies have consistently shown that amongst staff working with MRI, transient symptoms directly attributable to the MRI system including dizziness, nausea, tinnitus, and concentration problems are reported. This study assessed symptom prevalence and incidence in radiographers and other staff working with MRI in healthcare in the UK. One hundred and four volunteer staff from eight sites completed a questionnaire and kept a diary to obtain information on subjective symptoms and work practices, and wore a magnetic field dosimeter during one to three randomly selected working days. Incidence of MRI-related symptoms was obtained for all shifts and prevalence of MRI-related and reference symptoms was associated to explanatory factors using ordinal regression. Incident symptoms related to working with MRI were reported in 4 % of shifts. Prevalence of MRI-related, but not reference symptoms were associated with number of hours per week working with MRI, shift length, and stress, but not with magnetic field strength (1.5 and 3 T) or measured magnetic field exposure. Reporting of prevalent symptoms was associated with longer duration of working in MRI departments, but not with measured field strength of exposure. Other factors related to organisation and stress seem to contribute to increased reporting of MRI-related symptoms. • Routine work with MRI is associated with increased reporting of transient symptoms • No link to the strength of the magnetic field was demonstrated. • Organisational factors and stress additionally contribute to reporting of MRI-related symptoms.
    No preview · Article · Mar 2015 · European Radiology
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    ABSTRACT: PurposeTo validate a fully automated scheme to extract biochemical information from the hip joint cartilages using MR T2 mapping images incorporating segmentation of co-registered three-dimensional Fast-Spin-Echo (3D-SPACE) images.Methods Manual analyses of unilateral hip (3 Tesla) MR images of 24 asymptomatic volunteers were used to validate a 3D deformable model method for automated cartilage segmentation of SPACE scans, partitioning of the individual femoral and acetabular cartilage plates into clinically defined sub-regions and propagating these results to T2 maps to calculate region-wise T2 value statistics. Analyses were completed on a desktop computer (∼10 min per case).ResultsThe mean voxel overlap between automated A and manual M segmentations of the cartilage volumes in the (clinically based) SPACE images was 73% . The automated and manual analyses demonstrated a relative difference error <10% in the median “T2 average signal” for each cartilage plate. The automated and manual analyses showed consistent patterns between significant differences in T2 data across the hip cartilage sub-regions.Conclusion The good agreement between the manual and automatic analyses of T2 values indicates the use of structural 3D-SPACE MR images with the proposed method provides a promising approach for automated quantitative T2 assessment of hip joint cartilages. Magn Reson Med, 2015. © 2015 Wiley Periodicals, Inc.
    No preview · Article · Jan 2015 · Magnetic Resonance in Medicine
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    ABSTRACT: We present a statistical shape model approach for automated segmentation of the proximal humerus and scapula with subsequent bone-cartilage interface (BCI) extraction from 3D magnetic resonance (MR) images of the shoulder region. Manual and automated bone segmentations from shoulder MR examinations from 25 healthy subjects acquired using steady-state free precession sequences were compared with the Dice similarity coefficient (DSC). The mean DSC scores between the manual and automated segmentations of the humerus and scapula bone volumes surrounding the BCI region were 0.926 ± 0.050 and 0.837 ± 0.059, respectively. The mean DSC values obtained for BCI extraction were 0.806 ± 0.133 for the humerus and 0.795 ± 0.117 for the scapula. The current model-based approach successfully provided automated bone segmentation and BCI extraction from MR images of the shoulder. In future work, this framework appears to provide a promising avenue for automated segmentation and quantitative analysis of cartilage in the glenohumeral joint.
    No preview · Article · Jan 2015 · Physics in Medicine and Biology

Publication Stats

4k Citations
599.88 Total Impact Points

Institutions

  • 1989-2015
    • University of Queensland
      • • School of Information Technology and Electrical Engineering
      • • School of Mathematics and Physics
      Brisbane, Queensland, Australia
  • 2012
    • The Florey Institute of Neuroscience and Mental Health
      • Stroke Division
      Melbourne, Victoria, Australia
  • 2010
    • Qingdao University
      Tsingtao, Shandong Sheng, China
  • 2005
    • National High Magnetic Field Laboratory
      Tallahassee, Florida, United States