Biomedical subjects

Roger J Ordidge

Publications and source records attributed to Roger J Ordidge.

At least 19 recordsLinked to original sources

PubMed2006

Magnetic resonance imaging of neonatal encephalopathy at 4.7 tesla: initial experiences.

OBJECTIVES: The goals were to develop safe 4.7-T MRI examination protocols for newborn infants and to explore the advantages of this field strength in neonatal encephalopathy. METHODS: Nine ventilated newborn infants with moderate or severe encephalopathy were studied at 4.7 T, with ethical approval and informed parental consent. The custom-made, 4.7-T-compatible, neonatal patient management system included acoustic noise protection and physiologic monitoring. An adult head coil was used. Acquisition parameters for T2-weighted fast spin echo MRI and a variety of T1-weighted methods were adapted for MRI of neonatal brain at 4.7 T. The pulse sequences used had a radiofrequency specific absorption rate of <2 W/kg. RESULTS: Physiologic measures were normal throughout each scan. T2-weighted fast spin echo imaging provided better anatomic resolution and gray/white matter contrast than typically obtained at 1.5 T; T1-weighted images were less impressive. CONCLUSIONS: With appropriate safety precautions, MRI of newborn infants undergoing intensive care is as feasible at 4.7 T as it is at 1.5 T; our initial studies produced T2-weighted fast spin echo images with more detail than commonly obtained at 1.5 T. Although T1-weighted images were not adequately informative, additional pulse sequence optimization may be advantageous. A smaller neonatal head coil should also permit greater flexibility in acquisition parameters and even more anatomic resolution and tissue contrast. In neonatal encephalopathy, interpretation of the T2-weighted pathologic detail in combination with comprehensive neurodevelopmental follow-up should improve prognostic accuracy and enable more patient-specific therapeutic interventions. In addition, more precise relationships between structural changes and functional impairment may be defined.

Brain Diseases↗

PubMed2006

Improving whole brain structural MRI at 4.7 Tesla using 4 irregularly shaped receiver coils.

Both higher magnetic field strengths (> or =3 T) and multiple receiver "array coils" can provide increased signal-to-noise ratio (SNR) for MRI. This increase in SNR can be used to obtain images with higher resolution, enabling better visualisation of structures within the human brain. However, high field strength systems also suffer from increased B(1) non-uniformity and increased power deposition, reaching specific absorption rate (SAR) limits more quickly. For these problems to be mitigated, a careful choice of both the pulse sequence design and transmit RF coil is required. This paper describes the use of a prototype array coil consisting of 4 irregularly shaped coils within a standard configuration for neuroimaging at 4.7 T (a head transmit/receive volume coil to minimise SAR and a head gradient insert for maximum gradient performance). With a fast spin echo (FSE) pulse sequence optimised for 4.7 T, this provides dramatically increased quality and resolution over a large brain volume. Using the array coil, a SNR improvement relative to the volume coil of 1-1.5 times in central brain areas and 2-3 times in cortical regions was obtained. Array coil images with a resolution of 352 x 352 x 2000 mum had a SNR of 16.0 to 26.2 in central regions and 19.9 to 34.8 in cortical areas. Such images easily demonstrate cortical myeloarchitecture, while still covering most of the brain in a approximately 12 min scan.

Algorithms↗

PubMed2006

EPI distortion correction from a simultaneously acquired distortion map using TRAIL.

PURPOSE: To develop a method for shot-by-shot distortion correction of single-shot echo-planar imaging (EPI) that is capable of correcting each image individually using a distortion measurement performed during acquisition of the image itself. MATERIALS AND METHODS: The recently-introduced method known as two reduced acquisitions interleaved (TRAIL) was extended to measure the distribution of the main magnetic field B0 with each shot. This corresponded to a map of distortion, and allowed distortion to be corrected in the acquired images. RESULTS: Distortion-corrected images were demonstrated in the human brain. The distortion field could be directly visualized using the "stripe" distribution imposed by the TRAIL pulse sequence. This confirmed the success of the correction. Over a time-course measurement of 10 images, variance was reduced by using shot-by-shot distortion correction compared to correction with a constant field map. CONCLUSION: Shot-by-shot distortion correction may be performed for EPI images acquired using an extension of the TRAIL technique, ensuring that the correction reflects the actual distortion pattern and not merely a previously measured, but possibly no longer valid, distortion field. This avoids errors due to changes in the distortion field or misregistration of a previously measured distortion map resulting from subject motion.

Brain Mapping↗

PubMed2006

In vivo measurement of the longitudinal relaxation time of arterial blood (T1a) in the mouse using a pulsed arterial spin labeling approach.

A novel method for measuring the longitudinal relaxation time of arterial blood (T1a) is presented. Knowledge of T1a is essential for accurately quantifying cerebral perfusion using arterial spin labeling (ASL) techniques. The method is based on the flow-sensitive alternating inversion recovery (FAIR) pulsed ASL (PASL) approach. We modified the standard FAIR acquisition scheme by incorporating a global saturation pulse at the beginning of the recovery period. With this approach the FAIR tissue signal difference has a simple monoexponential dependence on the recovery time, with T1a as the time constant. Therefore, FAIR measurements performed over a range of recovery times can be fitted to a monoexponential recovery curve and T1a can be calculated directly. This eliminates many of the difficulties associated with the measurement of T1a. Experiments performed in vivo in the mouse at 2.35T produced a mean value of 1.51 s for T1a, consistent with previously published values.

Animals↗

PubMed2006

Regional variation of cerebral blood flow and arterial transit time in the normal and hypoperfused rat brain measured using continuous arterial spin labeling MRI.

Continuous arterial spin labeling (CASL) is a noninvasive magnetic resonance (MR) method for measuring cerebral perfusion. In its most widely used form, CASL incorporates a postlabeling delay to minimize the sensitivity of the technique to transit time effects, which otherwise corrupt cerebral blood flow (CBF) quantification. For this delay to work effectively, it must be longer than the longest transit time present in the system. In this work, CASL measurements were made in four coronal slices in the rat brain using a range of postlabeling delays. By doing this, direct estimation of both CBF and arterial transit time (delta(a)) was possible. These measurements were performed in the normal brain and during hypoperfusion induced by occlusion of the common carotid arteries. It was found that, in the normal rat brain, significant regional variation exists for both CBF and delta(a). Mean values of CBF and delta(a) in the selected gray matter regions of interest were 233 mL/100 g min and 266 ms, respectively, with the latter ranging from 100 to 500 ms. Therefore, use of a 500-ms postlabeling delay is suitable for any location in the normal rat brain. After common carotid artery occlusion, CBF decreased and delta(a) increased by regionally dependent amounts. In the sensory cortex, delta(a) increased to a mean value of 740 ms, significantly greater than 500 ms. These results highlight the importance of either (a) determining delta(a) as part of the CASL measurement or (b) knowing the approximate range of values delta(a) is likely to take for a given application, so that the parameters of the CASL sequence can be chosen appropriately.

Animals↗

PubMed2006

Comparative prognostic utilities of early quantitative magnetic resonance imaging spin-spin relaxometry and proton magnetic resonance spectroscopy in neonatal encephalopathy.

OBJECTIVE: We sought to compare the prognostic utilities of early MRI spin-spin relaxometry and proton magnetic resonance spectroscopy in neonatal encephalopathy. METHODS: Twenty-one term infants with neonatal encephalopathy were studied at a mean age of 3.1 days (range: 1-5). Basal ganglia, thalamic and frontal, parietal, and occipital white matter spin-spin relaxation times were determined from images with echo times of 25 and 200 milliseconds. Metabolite ratios were determined from an 8-mL thalamic-region magnetic resonance spectroscopy voxel (1H point-resolved spectroscopy; echo time 270 milliseconds). Outcomes were assigned at age 1 year as follows: (1) normal, (2) moderate (neuromotor signs or Griffiths developmental quotient of 75-84), (3) severe (functional neuromotor deficit or developmental quotient <75 or died). Predictive efficacies for differentiation between normal and adverse (combined moderate and severe) outcomes were compared by receiver operating characteristic curve analysis and logistic regression. RESULTS: Thalamic and basal ganglia spin-spin relaxation times correlated positively with outcome and predicted adversity. Although thalamic and basal ganglia spin-spin relaxation times were prognostic of adversity, magnetic resonance spectroscopy metabolite ratios were better predictors, and, of these, lactate/N-acetylaspartate was most accurate. CONCLUSIONS: Deep gray matter spin-spin relaxation time was increased in the first few days after birth in infants with an adverse outcome. Proton magnetic resonance spectroscopy was more prognostic than spin-spin relaxation time, with lactate/N-acetylaspartate the best measure. Nevertheless, both techniques were useful for early prognosis, and the potential superior spatial resolution of spin-spin relaxometry may define better the precise anatomic pattern of injury in the early days after birth.

Brain Chemistry↗

PubMed2005

Spin-echo MRS in humans at high field: LASER localisation using FOCI pulses.

Significant improvements in spin-echo MRS are possible when voxel localisation is performed using high bandwidth frequency offset corrected inversion (FOCI) pulses as opposed to more conventional lower bandwidth pulses. The reduced chemical shift displacement errors result in a spectrum that more accurately reflects the actual metabolite distribution within any region of interest that is selected graphically on a series of scout images, and can lead to improved metabolite detection in the case of homonuclear J-coupled spins. At 4.7T, FOCI pulses with a 20 kHz bandwidth result in extremely sharp and uniform selection profiles, and negligible contamination from outside of the voxel of interest, for all signals in the 1H spectral range that is normally studied. A 'FOCI' adiabatic half-passage is observed to provide good excitation over the 1H spectral range. Single shot performance with echo-time (TE)48 ms is reported using a four-port drive birdcage head coil. GAMMA simulations show that, for many detectable metabolites at 4.7 T, LASER localisation using FOCI pulses with TE=48 ms results in 1H anti-phase spectral components that are the same order as would be obtained from a symmetric PRESS sequence with TE=32 ms. Timing schemes are proposed to enable good measurement of lactate with very little signal loss arising from chemical shift displacement errors at TE=144 and 288 ms.

Algorithms↗

PubMed2005

Assessment of magnetic field (4.7 T) induced forces on prosthetic heart valves and annuloplasty rings.

PURPOSE: To assess the magnetic field interactions on 11 heart valve prostheses and 12 annuloplasty rings subjected to a 4.7 T MR system. MATERIALS AND METHODS: Ex vivo testing was performed to evaluate translational and rotational forces using previously described techniques. RESULTS: Seventeen out of 23 prostheses showed zero interaction with the magnetic field. Translational forces with deflection angles of 2-20 degrees were demonstrated in six prostheses. Only two heart valves and two annuloplasty rings demonstrated rotational forces. The Carpentier Edwards (CE) Physio Ring, which contains Elgiloy, demonstrated deflection angles three times greater than those previously measured at 1.5 T. Furthermore, there was a direct relationship between increasing prosthesis size and increasing translational force. All heart valve prostheses attracted to the magnetic field were slightly paramagnetic/weakly ferromagnetic. CONCLUSION: Twenty-three heart valve prostheses evaluated for MRI are considered safe in static fields up to 4.7 T based on current safety criteria. However, the CE Physio Ring appeared to develop an increasing magnetism upon re-entry into the MR system. We conclude that prostheses made from Elgiloy may not be acceptable for patients in an MR environment of > or =4.7 T. Further investigations are required to confirm the safety of Elgiloy.

Bioprosthesis↗

PubMed2005

Gradual changes in the apparent diffusion coefficient of water in selectively vulnerable brain regions following brief ischemia in the gerbil.

Although selective vulnerability and delayed neuronal death following global ischemia have been recognized in both the human and animal brain, the underlying mechanisms of cell damage are not fully understood. In this study we investigated the time-dependent changes of the apparent diffusion coefficient (ADC) of water and cerebral blood flow (CBF) in a classic animal model of selective vulnerability and delayed neuronal death, using magnetic resonance (MR) diffusion- and perfusion-weighted imaging. CBF was monitored using the noninvasive MR arterial spin labeling method called flow-sensitive alternating inversion recovery (FAIR). Bilateral common carotid occlusion was induced for 5 min, followed by 10 hr of reperfusion in a gerbil model. The most notable finding was that the lateral portion of the striatum in the basal ganglia exhibited a prolonged and gradual ADC decrease throughout the study following reperfusion. This pattern was not exhibited within the cortex. It is suggested that regions known to exhibit so-called delayed cell death progress to infarction via a gradual process that can be monitored by MR diffusion-weighted imaging (DWI).

Animals↗

PubMed2005

3D MDEFT imaging of the human brain at 4.7 T with reduced sensitivity to radiofrequency inhomogeneity.

A modification to the 3D modified driven equilibrium Fourier transform (MDEFT) imaging technique is proposed that reduces its sensitivity to RF inhomogeneity. This is especially important at high field strengths where RF focusing effects exacerbate B(1) inhomogeneity, causing significant signal nonuniformity in the images. The adiabatic inversion pulse used during the preparation period of the MDEFT sequence is replaced by a hard (nonadiabatic) pulse with a nominal flip angle of 130 degrees. The spatial inhomogeneity of the hard pulse preparation compensates for the inhomogeneity of the excitation pulses. Uniform signal intensity is obtained for a wide range of B(1) amplitudes and the high CNR characteristic of MDEFT is retained. The new approach was validated by numerical simulations and successfully applied to human brain imaging at 4.7 T, resulting in high-quality T(1)-weighted images of the whole human brain at high field strength with uniform signal intensity and contrast, despite the presence of significant RF inhomogeneity.

Brain Mapping↗

PubMed2005

Common SENSE (sensitivity encoding using hardware common to all MR scanners): a new method for single-shot segmented echo planar imaging.

A new method is presented that enables image acquisition to be segmented into two readouts. This is achieved using a new pulse sequence that creates two components of magnetization with different spatial profiles. Each component of the magnetization is measured in one of the readouts. This produces two images with complimentary "sensitivity profiles" and near identical contrast. The images can be acquired with a reduced data matrix that corresponds to shorter periods of data acquisition. The reduced matrix images are then combined to produce a full matrix image using reconstruction methods previously applied to images from multiple RF coils in the sensitivity encoding (SENSE) technique. The most promising application for this technique is in improving the performance of echo planar imaging (EPI) at high field. In this application, common SENSE obtains two segments of data in a single excitation of the magnetization (i.e., two readouts are performed per shot). The combination of these segments in image space avoids the difficulties normally associated with segmented EPI methods, namely, increased ghosting from discontinuities in the k-space data. The main advantages are a reduction in distortion and blurring. Common SENSE is compatible with parallel imaging and partial Fourier methods.

Echo-Planar Imaging↗

PubMed2005

Understanding and optimizing the amplitude modulated control for multiple-slice continuous arterial spin labeling.

Multiple-slice perfusion imaging by continuous arterial spin labeling (CASL) is made possible by amplitude modulation (AM) of the labeling RF pulse, but perfusion sensitivity is reduced relative to the single-slice technique. A computer model of the Bloch equations for velocity driven adiabatic fast passage was developed to elucidate the compromised sensitivity to perfusion of the AM control technique for CASL. Calculations were performed over ranges of RF pulse amplitude, B1; magnetic field gradient, G; phase, phi, and frequency, f, of the modulation function; velocity, v, and relaxation times, T1 and T2, of blood. It was found that unless f>2piB1, phi determines the performance of the AM control; excessively high B1 or v reduces the efficiency of the AM control; and T1 relaxation dominates if f is too great. In vivo, in rat brain (n=5) at 2.35 T, the sensitivity of the AM technique to perfusion was 70% of the sensitivity of single-slice CASL.

Animals↗

PubMed2004

TurboFLASH FAIR imaging with optimized inversion and imaging profiles.

Optimal implementation of pulsed arterial spin labeling (PASL) methods such as flow-sensitive alternating inversion recovery (FAIR), require the minimization of interactions between the inversion and imaging slabs. For FAIR, the inversion:imaging slice thickness ratio (STR) is usually at least 3:1 in order to fully contain the extent of the imaging slice. The resulting gap exacerbates the transit time. So far, efforts to minimize the STR have concentrated on the inversion profile. However, the imaging profile remains a limiting factor especially for rapid sequences such as turbo fast low-angle shot (TurboFLASH) which uses short pulses. This study reports the implementation of a TurboFLASH sequence with optimized inversion and imaging profiles. Slice-selection is achieved with a preparation module incorporating a pair of identical adiabatic frequency offset corrected inversion (FOCI) pulses. The optimum radiofrequency (RF) and gradient scheme for this pulse combination is described, and the relaxation characteristics of the slice-selection scheme are investigated. Phantom experiments demonstrate a reduction in the STR to approximately 1.13:1. Implementation in an animal model is described, and the benefit of the improved profile in probing the sensitivity of the flow signal to tagging geometry is demonstrated. Sensitivity to transit time effects can be minimized with this sequence, and ASL methodologies can be better explored as a result of the improved conformance with the ideal of square slice profiles.

Animals↗

PubMed2004

Method for spatially interleaving two images to halve EPI readout times: two reduced acquisitions interleaved (TRAIL).

A new MRI method is presented that can generate images using half the normal readout time or, more usefully, half the number of phase-encode steps, combining two readouts per excitation. However, the corresponding data are interleaved in image space-not in k-space, as in many other fast techniques. This gives a resilience to the phase-related artifacts that can occur in many other techniques due to subject motion. A modified stimulated-echo experiment is used to create two low-resolution images from a single sequence. The magnetization that contributes to these images is nonuniformly distributed within each pixel, forming two sinusoidal waves in quadrature, with an oscillation period of exactly two pixels. Since only half of each pixel contributes significant signal, the two images can be interleaved to create a full image with twice as many pixels and double the resolution. When the technique is used in the phase-encode direction, the effective imaging time is halved, though with two readouts per TR period. When two half-length echo-planar readouts are used, the method can also reduce blurring and distortion by halving the effective readout time for echo-planar imaging (EPI). For even further improvements, the technique can be combined with partial Fourier or parallel imaging.

Echo-Planar Imaging↗

PubMed2004

High-resolution fast spin echo imaging of the human brain at 4.7 T: implementation and sequence characteristics.

In this work, a number of important issues associated with fast spin echo (FSE) imaging of the human brain at 4.7 T are addressed. It is shown that FSE enables the acquisition of images with high resolution and good tissue contrast throughout the brain at high field strength. By employing an echo spacing (ES) of 22 ms, one can use large flip angle refocusing pulses (162 degrees ) and a low acquisition bandwidth (50 kHz) to maximize the signal-to-noise ratio (SNR). A new method of phase encode (PE) ordering (called "feathering") designed to reduce image artifacts is described, and the contributions of RF (B(1)) inhomogeneity, different echo coherence pathways, and magnetization transfer (MT) to FSE signal intensity and contrast are investigated. B(1) inhomogeneity is measured and its effect is shown to be relatively minor for high-field FSE, due to the self-compensating characteristics of the sequence. Thirty-four slice data sets (slice thickness = 2 mm; in-plane resolution = 0.469 mm; acquisition time = 11 min 20 s) from normal volunteers are presented, which allow visualization of brain anatomy in fine detail. This study demonstrates that high-field FSE produces images of the human brain with high spatial resolution, SNR, and tissue contrast, within currently prescribed power deposition guidelines.

Adult↗

PubMed2003

Magnetic resonance proton spectroscopy and diffusion weighted imaging of chick embryo brain in ovo.

Metabolic compensatory mechanisms may partly account for the decreased vulnerability to hypoxia observed in the developing brain. We used proton magnetic resonance spectroscopy and diffusion-weighted imaging to measure the cerebral concentrations of lactate and other metabolites, as well as the apparent diffusion coefficient (ADC) of tissue water, before, during and after hypoxia in anaesthetised chick embryos in ovo. Reducing the inspired oxygen concentration to 8% for 40 min caused a significant rise in both mean (+/-S.D.) lactate:creatine and alanine:creatine ratios from 0.58 (0.41) to 1.56 (0.56) and 0.14 (0.14) to 0.29 (0.17), respectively. Under similar hypoxic conditions, ADC did not change from a mean baseline value of 0.95 (0.09), but did fall to 0.40 (0.12) x 10(9) mm(2) s(-1) with further stepwise reductions in oxygenation. Moderate hypoxia increases lactate concentration in the developing chick brain without compromising cellular energy metabolism.

Adaptation, Physiological↗

PubMed2003

Velocity-driven adiabatic fast passage for arterial spin labeling: results from a computer model.

Velocity-driven adiabatic fast passage (AFP) is commonly employed for perfusion imaging by continuous arterial spin labeling (CASL). The degree of inversion of protons in blood determines the sensitivity of CASL to perfusion. For this study, a computer model of the modified Bloch equations was developed to establish the optimum conditions for velocity-driven AFP. Natural variations in blood velocity over the course of the cardiac cycle were found to result in significant variations in the degree of inversion. However, the mean degree of inversion was similar to that for blood moving at a constant velocity, equal to the time-averaged mean, at peak velocities and heart rates within normal ranges. A train of RF pulses instead of a continuous RF pulse for labeling was found to result in a highly nonlinear dependence of the degree of inversion on RF duty cycle. This may have serious implications for the quantification of perfusion.

Animals↗

PubMed2003

Comparative study of the FAIR technique of perfusion quantification with the hydrogen clearance method.

Arterial spin labeling magnetic resonance methods, including flow-sensitive alternating inversion recovery (FAIR), are becoming increasingly common for the noninvasive quantification of cerebral blood flow (CBF). This report compares the FAIR method with hydrogen clearance. The latter is an established, invasive technique for CBF measurement in animals. Paired readings of CBF were obtained in gerbils to maximize the degree of spatial and temporal correspondence between methods. Flow-sensitive alternating inversion recovery (50 averages, 6.7-minute measurement time) and hydrogen clearance measurements were made concurrently. Cerebral blood flow values measured by both techniques displayed an initial decrease because of the injurious effects of electrode insertion and subsequent recovery. Mixed model regression analysis, structural equations modeling, and a simple concordance correlation coefficient analysis were performed. No evidence of a marked systematic bias in the FAIR measurements was found; mixed model regression analysis yielded relative bias estimates of 0.4 (confidence interval: 3.0, 3.9) mL. 100 g-1. min-1 and -3.7 (-12.1, 4.7) mL. 100 g-1. min-1 at 20 and 100 mL. 100 g-1. min-1, respectively. The principal limitation of the FAIR technique was the magnitude of the random measurement error (imprecision), which had a standard deviation on the order of 10 mL. 100 g-1. min-1.

Animals↗