Biomedical subjects

Derek L G Hill

Publications and source records attributed to Derek L G Hill.

At least 19 recordsLinked to original sources

PubMed2006

Cerebral atrophy measurements using Jacobian integration: comparison with the boundary shift integral.

We compared two methods of measuring cerebral atrophy in a cohort of 38 clinically probable Alzheimer's disease (AD) subjects and 22 age-matched normal controls, using metrics of zero atrophy, consistency, scaled atrophy and AD/control group separation. The two methods compared were the boundary shift integral (BSI) and a technique based on the integration of Jacobian determinants from non-rigid registration. For each subject, we used two volumetric magnetic resonance (MR) scans at baseline and a third obtained 1 year later. The case of zero atrophy was established by registering the same-day baseline scan pair, which should approximate zero change. Consistency was established by registering the 1-year follow-up scan to each of the baseline scans, giving two measurements of atrophy that should be very similar, while scaled atrophy was established by reducing one of the same-day scans by a fixed amount, and rigidly registering this to the other same-day scan. Group separation was ascertained by calculating atrophy rates over the two 1-year measures for the control and AD subjects. The results showed the Jacobian integration technique was significantly more accurate in calculating scaled atrophy (P < 0.001) and was able to distinguish between control and AD subjects more clearly (P < 0.01).

Aged↗

PubMed2006

Nonlinear phase correction of navigated multi-coil diffusion images.

Cardiac pulsatility causes a nonrigid motion of the brain. In multi-shot diffusion imaging this leads to spatially varying phase changes that must be corrected. A conjugate gradient based reconstruction is presented that includes phase changes measured using two-dimensional navigator echoes, coil sensitivity information, navigator-determined weightings, and data from multiple coils and averages.A multi-shot echo planar sequence was used to image brain regions where pulsatile motion is not uniform. Reduced susceptibility artifacts were observed compared to a clinical single-shot sequence. In a higher slice, fiber directions derived from single-shot data show distortions from anatomical scans by as much as 7 mm compared to less than 2 mm for our multi-shot reconstructions. The reduced distortions imply that phase encoding can be applied in the shorter left-right direction, enabling time savings through the use of a rectangular field of view. Higher resolution diffusion imaging in the spine permits visualization of a nerve root.

Algorithms↗

PubMed2006

Interactive MR imaging and tracking of catheters with multiple tuned fiducial markers.

PURPOSE: The lack of magnetic resonance (MR) safe catheters and guide wires remains an important obstacle to widespread clinical use of MR-guided endovascular procedures. The authors looked at the feasibility of using multiple tuned fiducial markers (TFM) and novel imaging sequences to track catheters reliably under MR and to evaluate the safety of such markers in terms of heating. MATERIALS AND METHODS: The visualization and tracking of a catheter with six quadrature tuned fiducial coils was carried out in a special designed in-vitro setup within a 1.5-T MR imager simulating an MR-guided endovascular intervention. The fiducial markers were also tested for heating. RESULTS: The excellent signal contrast between the fiducial and the background when using novel interleaved real time and interactive sequences allowed for rapid and reliable identification of the fiducial markers and therefore the catheter. No significant heating of the marker was noted. CONCLUSIONS: The authors have shown that catheters with multiple tuned fiducial markers are superior to passive catheter designs in terms of visualization and do not carry the risk of heating that is commonly associated with active catheters.

Cardiac Catheterization↗

PubMed2006

Generalized overlap measures for evaluation and validation in medical image analysis.

Measures of overlap of labelled regions of images, such as the Dice and Tanimoto coefficients, have been extensively used to evaluate image registration and segmentation algorithms. Modern studies can include multiple labels defined on multiple images yet most evaluation schemes report one overlap per labelled region, simply averaged over multiple images. In this paper, common overlap measures are generalized to measure the total overlap of ensembles of labels defined on multiple test images and account for fractional labels using fuzzy set theory. This framework allows a single "figure-of-merit" to be reported which summarises the results of a complex experiment by image pair, by label or overall. A complementary measure of error, the overlap distance, is defined which captures the spatial extent of the nonoverlapping part and is related to the Hausdorff distance computed on grey level images. The generalized overlap measures are validated on synthetic images for which the overlap can be computed analytically and used as similarity measures in nonrigid registration of three-dimensional magnetic resonance imaging (MRI) brain images. Finally, a pragmatic segmentation ground truth is constructed by registering a magnetic resonance atlas brain to 20 individual scans, and used with the overlap measures to evaluate publicly available brain segmentation algorithms.

Algorithms↗

PubMed2006

Phenomenological model of diffuse global and regional atrophy using finite-element methods.

The main goal of this work is the generation of ground-truth data for the validation of atrophy measurement techniques, commonly used in the study of neurodegenerative diseases such as dementia. Several techniques have been used to measure atrophy in cross-sectional and longitudinal studies, but it is extremely difficult to compare their performance since they have been applied to different patient populations. Furthermore, assessment of performance based on phantom measurements or simple scaled images overestimates these techniques' ability to capture the complexity of neurodegeneration of the human brain. We propose a method for atrophy simulation in structural magnetic resonance (MR) images based on finite-element methods. The method produces cohorts of brain images with known change that is physically and clinically plausible, providing data for objective evaluation of atrophy measurement techniques. Atrophy is simulated in different tissue compartments or in different neuroanatomical structures with a phenomenological model. This model of diffuse global and regional atrophy is based on volumetric measurements such as the brain or the hippocampus, from patients with known disease and guided by clinical knowledge of the relative pathological involvement of regions and tissues. The consequent biomechanical readjustment of structures is modelled using conventional physics-based techniques based on biomechanical tissue properties and simulating plausible tissue deformations with finite-element methods. A thermoelastic model of tissue deformation is employed, controlling the rate of progression of atrophy by means of a set of thermal coefficients, each one corresponding to a different type of tissue. Tissue characterization is performed by means of the meshing of a labelled brain atlas, creating a reference volumetric mesh that will be introduced to a finite-element solver to create the simulated deformations. Preliminary work on the simulation of acquisition artefacts is also presented. Cross-sectional and longitudinal sets of simulated data are shown and a visual classification protocol has been used by experts to rate real and simulated scans according to their degree of atrophy. Results confirm the potential of the proposed methodology.

Alzheimer Disease↗

PubMed2006

Automatic quantification of changes in bone in serial MR images of joints.

Recent innovations in drug therapies have made it highly desirable to obtain sensitive biomarkers of disease progression that can be used to quantify the performance of candidate disease modifying drugs. In order to measure potential image-based biomarkers of disease progression in an experimental model of rheumatoid arthritis (RA), we present two different methods to automatically quantify changes in a bone in in-vivo serial magnetic resonance (MR) images from the model. Both methods are based on rigid and nonrigid image registration to perform the analysis. The first method uses segmentation propagation to delineate a bone from the serial MR images giving a global measure of temporal changes in bone volume. The second method uses rigid body registration to determine intensity change within a bone, and then maps these into a reference coordinate system using nonrigid registration. This gives a local measure of temporal changes in bone lesion volume. We detected significant temporal changes in local bone lesion volume in five out of eight identified candidate bone lesion regions, and significant difference in local bone lesion volume between male and female subjects in three out of eight candidate bone lesion regions. But the global bone volume was found to be fluctuating over time. Finally, we compare our findings with histology of the subjects and the manual segmentation of bone lesions.

Algorithms↗

PubMed2006

Factors influencing the accuracy of biomechanical breast models.

Recently it has been suggested that finite element methods could be used to predict breast deformations in a number of applications, including comparison of multimodality images, validation of image registration and image guided interventions. Unfortunately knowledge of the mechanical properties of breast tissues is limited. This study evaluated the accuracy with which biomechanical breast models based on finite element methods can predict the displacements of tissue within the breast in the practical clinical situation where the boundaries of the organ might be known reasonably accurately but there is some uncertainty on the mechanical properties of the tissue. For two datasets, we investigate the influence of tissue elasticity values, Poisson's ratios, boundary conditions, finite element solvers and mesh resolutions. Magnetic resonance images were acquired before and after compressing each volunteer's breast by about 20%. Surface displacement boundary conditions were derived from a three-dimensional nonrigid image registration. Six linear and three nonlinear elastic material models with and without skin were tested. These were compared to hyperelastic models. The accuracy of the models was evaluated by assessing the ability of the model to predict the location of 12 corresponding anatomical landmarks. The accuracy was most sensitive to the Poisson's ratio and the boundary condition. Best results were achieved for accurate boundary conditions, appropriate Poisson's ratios and models where fibroglandular tissue was at most four times stiffer than fatty tissue. These configurations reduced the mean (maximum) distance of the landmarks from 6.6 mm (12.4 mm) to 2.1 mm (3.4 mm) averaged over all experiments.

Algorithms↗

PubMed2005

Generalised overlap measures for assessment of pairwise and groupwise image registration and segmentation.

Effective validation techniques are an essential pre-requisite for segmentation and non-rigid registration techniques to enter clinical use. These algorithms can be evaluated by calculating the overlap of corresponding test and gold-standard regions. Common overlap measures compare pairs of binary labels but it is now common for multiple labels to exist and for fractional (partial volume) labels to be used to describe multiple tissue types contributing to a single voxel. Evaluation studies may involve multiple image pairs. In this paper we use results from fuzzy set theory and fuzzy morphology to extend the definitions of existing overlap measures to accommodate multiple fractional labels. Simple formulas are provided which define single figures of merit to quantify the total overlap for ensembles of pairwise or groupwise label comparisons. A quantitative link between overlap and registration error is established by defining the overlap tolerance. Experiments are performed on publicly available labeled brain data to demonstrate the new measures in a comparison of pairwise and groupwise registration.

Algorithms↗

PubMed2005

Inter-breath-hold registration for the production of high resolution cardiac MR volumes.

High resolution MRI images of the beating heart permit observation of detailed anatomical features and enable quantification of small changes in metrics of cardiac function. To obtain approximately isotropic sampling with an adequate spatial and temporal resolution, these images need to be acquired in multiple breath-holds. They are, therefore, often affected by through-plane discontinuities due to inconsistent breath-hold positions. This paper presents a method to correct for these discontinuities by performing breath-hold-by-breath-hold registration of high resolution 3D data to radial long axis images. The corrected images appear free of discontinuities, and it was found that they could be delineated more reproducibly than uncorrected images. This reduces the sample size required to detect systematic changes in blood pool volume by 57% at end systole and 78% at end diastole.

Algorithms↗

PubMed2005

An inverse problem approach to the estimation of volume change.

We present a new technique for determining structure-by-structure volume changes, using an inverse problem approach. Given a pre-labelled brain and a series of images at different time-points, we generate finite element meshes from the image data, with volume change modelled by means of an unknown coefficient of expansion on a per-structure basis. We can then determine the volume change in each structure of interest using inverse problem optimization techniques. The proposed method has been tested with simulated and clinical data. Results suggest that the presented technique can be seen as an alternative for volume change estimation.

Algorithms↗

PubMed2005

Impact of electromagnetic field exposure limits in Europe: is the future of interventional MRI safe?

The European Union (EU) has recently introduced a directive that aims to protect workers from adverse effects of exposure to electromagnetic fields. All countries within the EU are required to incorporate this directive into their national law by 2008. This legislation applies to all types of occupational exposure to electromagnetic fields with frequencies from 0 Hz to 300 GHz. It has dramatic implications for interventional magnetic resonance (MR) imaging, because workers who are close to the MR scanner while scanning is taking place are exposed at levels substantially above the exposure limits. This is especially the case for time-varying magnetic fields in the 110 Hz-5 kHz range, which includes the field from the imaging gradients. In this article, the scientific evidence on which the limits are based is brought into question. It is concluded that an urgent review of the directive is required and that more research, especially at MR gradient frequencies, is needed.

Electromagnetic Fields↗

PubMed2005

A system for real-time XMR guided cardiovascular intervention.

The hybrid magnetic resonance (MR)/X-ray suite (XMR) is a recently introduced imaging solution that provides new possibilities for guidance of cardiovascular catheterization procedures. We have previously described and validated a technique based on optical tracking to register MR and X-ray images obtained from the sliding table XMR configuration. The aim of our recent work was to extend our technique by providing an improved calibration stage, real-time guidance during cardiovascular catheterization procedures, and further off-line analysis for mapping cardiac electrical data to patient anatomy. Specially designed optical trackers and a dedicated calibration object have resulted in a single calibration step that can be efficiently checked and updated before each procedure. An X-ray distortion model has been implemented that allows for distortion correction for arbitrary c-arm orientations. During procedures, the guidance system provides a real-time combined MR/X-ray image display consisting of live X-ray images with registered recently acquired MR derived anatomy. It is also possible to reconstruct the location of catheters seen during X-ray imaging in the MR derived patient anatomy. We have applied our registration technique to 13 cardiovascular catheterization procedures. Our system has been used for the real-time guidance of ten radiofrequency ablations and one aortic stent implantation. We demonstrate the real-time guidance using two exemplar cases. In a further two cases we show how off-line analysis of registered image data, acquired during electrophysiology study procedures, has been used to map cardiac electrical measurements to patient anatomy for two different types of mapping catheters. The cardiologists that have used the guidance system suggest that real-time XMR guidance could have substantial value in difficult interventional and electrophysiological procedures, potentially reducing procedure time and delivered radiation dose. Also, the ability to map measured electrical data to patient specific anatomy provides improved visualization and a path to investigation of cardiac electromechanical models.

Cardiac Catheterization↗

PubMed2004

Novel method of quantifying pulmonary vascular resistance by use of simultaneous invasive pressure monitoring and phase-contrast magnetic resonance flow.

BACKGROUND: Pulmonary vascular resistance (PVR) quantification is important in the treatment of children with pulmonary hypertension. The Fick principle, used to quantify pulmonary artery flow, may be a flawed technique. We describe a novel method of PVR quantification by the use of magnetic resonance (MR) flow data and invasive pressure measurements. METHODS AND RESULTS: In 24 patients with either suspected pulmonary hypertension or congenital heart disease requiring preoperative assessment, PVR was calculated by the use of simultaneously acquired MR flow and invasive pressure measurements (condition 1). In 19 of the 24 patients, PVR was also calculated at 20 ppm nitric oxide +30% (condition 2) and at 20 ppm nitric oxide +100% oxygen (condition 3), with the use of the MR method. This method proved safe and feasible in all patients. In 15 of 19 patients, PVR calculated by Fick flow was compared with the MR method. At condition 1, Bland-Altman analysis revealed a bias of 2.3% (MR > Fick) and limits of agreement of 50.2% to -45.5%. At condition 2, there was poorer agreement (bias was 28%, and the limits of agreement were 151.3% to 95.2%). At condition 3, there was very poor agreement (bias was 54.2%, and the limits of agreement were 174.4% to -66.0%). CONCLUSIONS: We have demonstrated the feasibility of using simultaneous invasive pressure measurements and MR flow data to measure PVR in humans.

Adolescent↗

PubMed2004

Visualization and tracking of an inflatable balloon catheter using SSFP in a flow phantom and in the heart and great vessels of patients.

Passive catheter tracking involves direct interaction between the device and its surroundings, creating a local signal loss or enhancement of the image. Using only standard balloon catheters filled with CO(2) and imaged with a steady-state free precession sequence, it was possible to visualize and passively track catheters in a flow phantom and in the heart and great vessels of 20 patients without any additional image processing. The phantom work demonstrated that it was advantageous to sacrifice spatial resolution in order to increase temporal resolution. Frame rates greater than 10/sec were necessary for ease of catheter manipulation. Although only the tip of the catheter was visualized, this technique proved to be effective in patients undergoing cardiac catheterization.

Adolescent↗

PubMed2004

Coil-based artifact reduction.

Multiple MRI receiver coils provide extra information and can enable the reconstruction of multiple images using data from different combinations of coils. Comparison of these images shows that artifacts due to motion or flowing blood appear with different intensities due to the differing coil sensitivities. Typically, the artifact appears amplified in regions of low coil sensitivity. An optimization routine was developed to correct for the artifact by comparing reconstructions from various coil combinations and favoring a self-consistent solution. It is demonstrated that images artifacted by blood flowing in the aorta, or translational motion of the head, can be improved.

Algorithms↗

PubMed2004

Free-breathing radial acquisitions of the heart.

There is considerable interest in performing free-breathing acquisitions of the heart in order to obtain high-quality images without the need for multiple, long breathholds. In this article a 3D motion-correction method is described that is based on image registration of in-plane data and through-plane slice tracking. A number of fast radial undersampled images are acquired, each of which is free of motion artifacts. Initially, in-plane translational and rotational motion between each image was corrected before combining the data to give a fully sampled image. At the next stage, correction of in-plane deformation, in addition to translations and rotations, was performed in the image domain. Through-plane translational motion was compensated using a navigator echo to move the acquisition plane. Using this method, information on the motion of the heart was captured at the same time as acquiring the image data. No motion model, assumptions about the motion, or training data are required. The method is demonstrated on phantom data and cardiac images acquired on free-breathing volunteers.

Algorithms↗

PubMed2003

Cardiac catheterisation guided by MRI in children and adults with congenital heart disease.

BACKGROUND: Fluoroscopically guided cardiac catheterisation is an essential tool for diagnosis and treatment of congenital heart disease. Drawbacks include poor soft tissue visualisation and exposure to radiation. We describe the first 16 cases of a novel method of cardiac catheterisation guided by MRI with radiographic support. METHODS: In our cardiac catheterisation laboratory, we combine magnetic resonance and radiographic imaging facilities. We used MRI to measure flow and morphology, and real-time MRI sequences to visualise balloon angiographic catheters. 12 patients underwent diagnostic cardiac catheterisation, two had interventional cardiac catheterisations, and for two patients, MRI was used to plan radiofrequency ablation for treatment of tachyarrhythmias. FINDINGS: In 14 patients, some or all of the cardiac catheterisation was guided by MRI. In two patients undergoing radiofrequency ablation, catheters were manipulated with use of fluoroscopic guidance and outcome was assessed with MRI. All patients received lower amounts of radiation than controls. There was some discrepancy between pulmonary vascular resistance calculated by flow derived from MRI and the traditional Fick method. We were able to superimpose fluoroscopic images of electro physiology electrode catheters on the three dimensional MRI of the cardiac anatomy. INTERPRETATION: We have shown that cardiac catheterisation guided by MRI is safe and practical in a clinical setting, allows better soft tissue visualisation, provides more pertinent physiological information, and results in lower radiation exposure than do fluoroscopically guided procedures. MRI guidance could become the method of choice for diagnostic cardiac catheterisation in patients with congenital heart disease, and an important tool in interventional cardiac catheterisation and radiofrequency ablation.

Adult↗

PubMed2003

Reconstruction after rotational motion.

Patient rotational motion during a scan causes the k-space sampling to be both irregular and undersampled. Conventional regridding requires an estimate of the sampling density at each measured point and is not strictly consistent with sampling theory. Here, a 2D problem is converted to a series of 1D regriddings by exact interpolation along the measured readouts. Each 1D regridding, expressed in matrix form, requires a matrix inversion to gain an exact solution. Undersampled regions make the matrix ill-conditioned but summing the matrix columns (without inversion) indicates the undersampled regions. The missing data could be reacquired, but in this study it is estimated using a Delaunay triangle-based linear interpolation on the original 2D data. The matrix conditioning is improved, leading to images with reduced artifacts compared to other regridding schemes. Furthermore, there is no requirement to estimate a density compensation function at each of the measured points.

Head↗