Biomedical subjects

Enrico De Vita

Publications and source records attributed to Enrico De Vita.

10 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

Delayed whole-body cooling to 33 or 35 degrees C and the development of impaired energy generation consequential to transient cerebral hypoxia-ischemia in the newborn piglet.

OBJECTIVES: Fundamental questions remain about the precise temperature providing optimal neuroprotection after perinatal hypoxia-ischemia (HI). Furthermore, if hypothermia delays the onset of the neurotoxic cascade and the secondary impairment in cerebral energy generation, the "latent phase" may be prolonged, thus extending the period when additional treatments may be effective. The aims of this study were to investigate the effects of delayed systemic cooling at either 33 degrees C or 35 degrees C on the following: (1) latent-phase duration, and (2) cerebral metabolism during secondary energy failure itself, in the 48-hour period after transient HI. METHODS: Piglets were randomly assigned to the following: (1) HI-normothermic (HI-n) rectal temperature (Trectal; n = 12), (2) HI-Trectal 35 degrees C (HI-35; n = 7), and (3) HI-Trectal 33 degrees C (HI-33; n = 10). Groups were cooled to the target Trectal between 2 and 26 hours after HI. Serial magnetic resonance spectroscopy was performed over 48 hours. The effect of cooling on secondary energy failure severity (indexed by the nucleotide triphosphate/exchangeable phosphate pool [NTP/EPP] and phosphocreatine/inorganic phosphate [PCr/Pi] ratios) was assessed. RESULTS: Compared with HI-n, HI-35 and HI-33 had a longer NTP/EPP latent phase and during the entire study duration had higher mean NTP/EPP and PCr/Pi. The latent phase (both PCr/Pi and NTP/EPP) and the whole-brain cerebral energetics were similar for HI-35 and HI-33. During the hypothermic period, compared with HI-n, PCr/Pi was preserved in the cooled groups, but this advantage was not maintained after rewarming. Compared with HI-n, HI-35 and HI-33 had higher NTP/EPP after rewarming. CONCLUSIONS: Whole-body hypothermia for 24 hours at either 35 or 33 degrees C, commenced 2 hours after resuscitation, prolonged the NTP/EPP latent phase and reduced the overall secondary falls in mean PCr/Pi and NTP/EPP during 48 hours after HI. Reducing the temperature from 35 to 33 degrees C neither increased mean PCr/Pi and NTP/EPP nor further lengthened the latent phase.

Animals↗

PubMed2005

Depth of delayed cooling alters neuroprotection pattern after hypoxia-ischemia.

Hypothermia after perinatal hypoxia-ischemia (HI) is neuroprotective; the precise brain temperature that provides optimal protection is unknown. To assess the pattern of brain injury with 3 different rectal temperatures, we randomized 42 newborn piglets: (Group i) sham-normothermia (38.5-39 degrees C); (Group ii) sham-33 degrees C; (Group iii) HI-normothermia; (Group iv) HI-35 degrees C; and (Group v) HI-33 degrees C. Groups iii through v were subjected to transient HI insult. Groups ii, iv, and v were cooled to their target rectal temperatures between 2 and 26 hours after resuscitation. Experiments were terminated at 48 hours. Compared with normothermia, hypothermia at 35 degrees C led to 25 and 39% increases in neuronal viability in cortical gray matter (GM) and deep GM, respectively (both p < 0.05); hypothermia at 33 degrees C resulted in a 55% increase in neuronal viability in cortical GM (p < 0.01) but no significant increase in neuronal viability in deep GM. Comparing hypothermia at 35 and 33 degrees C, 35 degrees C resulted in more viable neurons in deep GM, whereas 33 degrees C resulted in more viable neurons in cortical GM (both p < 0.05). These results suggest that optimal neuroprotection by delayed hypothermia may occur at different temperatures in the cortical and deep GM. To obtain maximum benefit, you may need to design patient-specific hypothermia protocols by combining systemic and selective cooling.

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↗

PubMed2004

B0 dependence of the on-resonance longitudinal relaxation time in the rotating frame (T1rho) in protein phantoms and rat brain in vivo.

On-resonance longitudinal relaxation time in the rotating frame (T1rho) has been shown to provide unique information during the early minutes of acute stroke. In the present study, the contributions of the different relaxation mechanisms to on-resonance T1rho relaxation were assessed by determining relaxation rates (R1rho) in both protein phantoms and in rat brain at 2.35, 4.7, and 9.4 T. Similar to transverse relaxation rate (R2), R1rho increased substantially with increasing magnetic field strength (B0). The B0 dependence was more pronounced at weak spin-lock fields. In contrast to R1rho, longitudinal relaxation rate (R1) decreased as a function of increasing B0 field. The present data argue that dipole-dipole interaction forms only one pathway for T1rho relaxation and the contributions from other physicochemical factors need to be considered.

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↗