Tie-Qiang Li

Image-based method for retrospective correction of physiological motion effects in fMRI: RETROICOR

Respiration effects and cardiac pulsatility can induce signal modulations in functional MR image time series that increase noise and degrade the statistical significance of activation signals. A simple image‐based correction method is described that does not have the limitations of k‐space methods that preclude high spatial frequency correction. Low‐order Fourier series are fit to the image data based on time of each image acquisition relative to the phase of the cardiac and respiratory cycles, monitored using a photoplethysmograph and pneumatic belt, respectively. The RETROICOR method is demonstrated using resting‐state experiments on three subjects and compared with the k‐space method. The method is found to perform well for both respiration‐ and cardiac‐induced noise without imposing spatial filtering on the correction. Magn Reson Med 44:162–167, 2000.

High-field MRI of brain cortical substructure based on signal phase

The ability to detect brain anatomy and pathophysiology with MRI is limited by the contrast-to-noise ratio (CNR), which depends on the contrast mechanism used and the spatial resolution. In this work, we show that in MRI of the human brain, large improvements in contrast to noise in high-resolution images are possible by exploiting the MRI signal phase at high magnetic field strength. Using gradient-echo MRI at 7.0 tesla and a multichannel detector, a nominal voxel size of 0.24 × 0.24 × 1.0 mm3 (58 nl) was achieved. At this resolution, a strong phase contrast was observed both between as well as within gray matter (GM) and white matter (WM). In gradient-echo phase images obtained on normal volunteers at this high resolution, the CNR between GM and WM ranged from 3:1 to 20:1 over the cortex. This CNR is an almost 10-fold improvement over conventional MRI techniques that do not use image phase, and it is an ≈100-fold improvement when including the gains in resolution from high-field and multichannel detection. Within WM, phase contrast appeared to be associated with the major fiber bundles, whereas contrast within GM was suggestive of the underlying layer structure. The observed phase contrast is attributed to local variations in magnetic susceptibility, which, at least in part, appeared to originate from iron stores. The ability to detect cortical substructure from MRI phase contrast at high field is expected to greatly enhance the study of human brain anatomy in vivo.

Layer-specific variation of iron content in cerebral cortex as a source of MRI contrast

Recent advances in high-field MRI have dramatically improved the visualization of human brain anatomy in vivo. Most notably, in cortical gray matter, strong contrast variations have been observed that appear to reflect the local laminar architecture. This contrast has been attributed to subtle variations in the magnetic properties of brain tissue, possibly reflecting varying iron and myelin content. To establish the origin of this contrast, MRI data from postmortem brain samples were compared with electron microscopy and histological staining for iron and myelin. The results show that iron is distributed over laminae in a pattern that is suggestive of each region’s myeloarchitecture and forms the dominant source of the observed MRI contrast.

Susceptibility Contrast in High Field MRI of Human Brain as a Function of Tissue Iron Content

Magnetic susceptibility provides an important contrast mechanism for MRI. Increasingly, susceptibility-based contrast is being exploited to investigate brain tissue microstructure and to detect abnormal levels of brain iron as these have been implicated in a variety of neuro-degenerative diseases. However, it remains unclear to what extent magnetic susceptibility-related contrast at high field relates to actual brain iron concentrations. In this study, we performed susceptibility weighted imaging as a function of field strength on healthy brains in vivo and post-mortem brain tissues at 1.5 T, 3 T and 7 T. Iron histology was performed on the tissue samples for comparison. The calculated susceptibility-related parameters R(2)(*) and signal frequency shift in four iron-rich regions (putamen, globus pallidus, caudate, and thalamus) showed an almost linear dependence (r>or=0.90 for R(2)(*); r>or=0.83 for phase, p<0.01) on field strength, suggesting that potential ferritin saturation effects are not relevant to susceptibility-weighted contrast for field strengths up to 7 T. The R(2)(*) dependence on the putative (literature-based) iron concentration was 0.048 Hz/T/ppm. The histological data from brain samples confirmed the linear dependence of R(2)(*) on field strength and showed a slope against iron concentration of 0.0099 Hz/T/ppm dry-weight, which is equivalent to 0.05 Hz/T/ppm wet-weight and closely matched the calculated value in vivo. These results confirm the validity of using susceptibility-weighted contrast as an indicator of iron content in iron-rich brain regions. The absence of saturation effects opens the way to exploit the benefits of MRI at high field strengths for the detection of iron distributions with high sensitivity and resolution.

Extensive heterogeneity in white matter intensity in high-resolution T2*-weighted MRI of the human brain at 7.0 T

MRI at high magnetic field strength potentially allows for an increase in resolution and image contrast. The gains are particularly dramatic for T(2)(*)-weighted imaging, which is sensitive to susceptibility effects caused by a variety of sources, including deoxyhemoglobin, iron concentration, and tissue microstructure. On the other hand, the acquisition of high quality whole brain MRI at high field is hampered by the increased inhomogeneity in B(o) and B(1) fields. In this report, high-resolution gradient echo MRI was performed using an 8-channel detector to obtain T(2)(*)-weighted images over large brain areas. The high SNR achieved with the multi-channel array enabled T(2)(*)-weighted images of the brain with an unprecedented spatial resolution of up to 0.2 x 0.2 x 0.5 mm(3). This high resolution greatly facilitated the detection of microscopic susceptibility effects. In addition to the expected contrast between gray, white matter, cerebral spinal fluid, and veins, a large degree of heterogeneity in contrast was observed throughout the white matter of normal brain. The measured T(2)(*) values in white matter varied as much as 30% with some of the variation apparently correlating with the presence of large fiber bundles.

Noise considerations in the determination of diffusion tensor anisotropy

In this study the noise sensitivity of various anisotropy indices has been investigated by Monte-Carlo computer simulations and magnetic resonance imaging (MRI) measurements in a phantom and 5 healthy volunteers. Particularly, we compared the noise performance of indices defined solely in terms of eigenvalues and those based on both the eigenvalues and eigenvectors. It is found that anisotropy indices based on both eigenvalues and eigenvectors are less sensitive to noise, and spatial averaging with neighboring pixels can further reduce the standard deviation. To reduce the partial volume effect caused by the spatial averaging with neighboring voxels, an averaging method in the time domain based on the orientation coherence of eigenvectors in repeated experiments has been proposed.

Functional Magnetic Resonance Imaging of Regional Cerebral Blood Oxygenation Changes During Breath Holding

BACKGROUND AND PURPOSE Recently, noninvasive MRI methods have been developed that are now capable of detecting and mapping regional hemodynamic responses to various stress tests, which involve the use of vasoactive substances such as acetazolamide or inhalation of carbon dioxide. The aim of this study was to assess regional cerebral blood oxygenation changes during breath holding at 1.5 T. METHODS In 6 healthy volunteers, T2*-weighted gradient echo images were acquired for a total dynamic scanning time of 10 minutes during alternating periods of breath holding and normal breathing at 40-second intervals after inspiration, at 30-second intervals after expiration, and at 18 seconds after expiration. To quantify the relative signal changes, 2.5-minute baseline image sampling with normal breathing was carried out. RESULTS Repeated challenges of breath holding of various durations induced an overall rise in blood oxygen level-dependent (BOLD) signal intensities. In general, BOLD signal intensity increases were greatest in gray matter and nonsignificant in white matter. Depending on the breath-holding duration and techniques, BOLD signal intensity increases of all activated pixels varied from 0.8% to 3.5%. CONCLUSIONS The present study demonstrates that cerebral blood oxygenation changes during breath holding can be detected by means of fMRI at 1.5 T. The breath-holding test, a short and noninvasive method to study cerebral hemodynamics with fMRI, could become a useful alternative to the acetazolamide or CO2 test.

Functional MRI of Human Brain during Breath Holding by BOLD and FAIR Techniques

BOLD (blood oxygenation level-dependent) and FAIR (flow-sensitive alternating inversion recovery) imaging techniques were used to investigate the oxygenation and hemodynamic responses of human brain during repeated challenges of breath holding and prolonged single breath holding. The effects of different breathing techniques on BOLD and FAIR image contrasts were carefully examined. With a periodic breath-holding paradigm of 30 s, global changes in gray matter were observable both in T*2-weighted and FAIR images. T*2-weighted images showed 1-4% relative signal intensity increases, while FAIR images demonstrated relative cerebral blood flow (CBF) increase up to 30-70%. The activated pixels depicted in FAIR images were about three times less than those seen in T*2-weighted images. With prolonged breath holding, it was observed that signal intensities in T*2-weighted and FAIR images were dependent on the breathing techniques used. Breath holding after expiration gave rise to immediate signal intensity increases in T*2-weighted and FAIR images, whereas breath holding performed after deep inspiration signals showed a biphasic change both in flow and T*2-weighted. T*2-weighted and FAIR signals showed a transient decrease before rising above the baseline level.

Increased extracellular brain water after coronary artery bypass grafting is avoided by off-pump surgery

OBJECTIVE To determine if coronary artery bypass graft (CABG) surgery without cardiopulmonary bypass (CPB) avoids the brain swelling known to occur after CPB, to quantify these brain water compartment changes, and to identify the water shifts as due to intracellular or extracellular water. DESIGN Prospective, controlled, and blinded. SETTING Cardiac surgical unit in a university teaching hospital. SUBJECTS Patients scheduled for CABG who were assigned to conventional (n = 10) or off-pump (n = 7) surgery according to their coronary anatomy. INTERVENTIONS Magnetic resonance imaging (MRI) examinations were performed 1 day before surgery and 1 hour and 1 week after CABG surgery. MAIN OUTCOME MEASURES Extracellular and intracellular water homeostasis was described quantitatively by calculating the averaged apparent diffusion coefficient of brain water using diffusion-weighted MRI. Blinded visual ordering of the images from the three examinations was performed according to brain size using conventional MRI. RESULTS The average diffusion coefficient of brain water increased 4.7%+/-1.5% immediately after CABG with CPB and normalized after 1 week but did not change after CABG without CPB. No focal ischemic changes were seen in either group, and no gross neurologic deficits were observed. Visual analysis showed consistent brain swelling after CPB and variable changes in those operated without CPB. CONCLUSION Changes consistent with increased extracellular brain water seen after CABG with CPB were not observed in patients undergoing CABG without CPB. The clinical significance of brain water changes and increased brain water content after surgery with CPB remains undefined.

The contribution of chemical exchange to MRI frequency shifts in brain tissue.

Recent high‐field MRI studies based on resonance frequency contrast have revealed brain structure with unprecedented detail. Although subtle magnetic susceptibility variations caused by iron and myelin seem to be important to this contrast, recent research on protein solutions suggests that chemical exchange between water and macromolecular protons may contribute substantially to the observed gray‐white matter frequency contrast. To investigate this, we performed spectroscopic MRI experiments at 14 T on samples of fixed human visual cortex and fresh pig brain. To allow direct observation of any exchange‐induced frequency shifts, these samples were soaked in reference chemicals (TSP and dioxane) that are assumed not to be involved in exchange. For both fresh and fixed tissues and with both reference chemicals, substantial negative exchange‐induced gray‐white matter frequency contrast (–6.3 to –13.5 ppb) was found, whereas intracortical contrast was negligible. The sign of the gray‐white matter exchange‐induced frequency difference was opposite to the overall gray‐white matter frequency difference observed in vivo. This suggests that exchange contributes to, but is not sufficient to explain, the frequency contrast in vivo and tissue susceptibility differences may have a greater contribution than previously thought. The exchange‐dependent contribution may report on tissue chemical composition and pH. Magn Reson Med, 2010.