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Biomedical subjects

E M Haacke

Publications and source records attributed to E M Haacke.

At least 19 recordsLinked to original sources
PubMed2004

Diffuse axonal injury in children: clinical correlation with hemorrhagic lesions.

An inception cohort of 40 children and adolescents with traumatic brain injury and suspected diffuse axonal injury were studied using a new high-resolution magnetic resonance imaging susceptibility-weighted technique that is very sensitive for hemorrhage. A blinded comparison was performed between the extent of parenchymal hemorrhage and initial clinical variables as well as outcomes measured at 6 to 12 months after injury. Children with lower Glasgow Coma Scale scores (< or =8, n = 30) or prolonged coma (>4 days, n = 20) had a greater average number (p = 0.007) and volume (p = 0.008) of hemorrhagic lesions. Children with normal outcomes or mild disability (n = 30) at 6 to 12 months had, on average, fewer hemorrhagic lesions (p = 0.003) and lower volume (p = 0.003) of lesions than those who were moderately or severely disabled or in a vegetative state. Significant differences also were observed when comparing regional injury to clinical variables. Because susceptibility-weighted imaging is much more sensitive than conventional T2*-weighted gradient-echo sequences in detecting hemorrhagic diffuse axonal injury, more accurate and objective assessment of injury can be obtained early after insult, and may provide better prognostic information regarding duration of coma as well as long-term outcome.

Adolescent↗
PubMed2001

Quantitative differentiation between BOLD models in fMRI.

Several gradient-echo fMRI blood oxygenation level-dependent (BOLD) effects are described in the literature: extravascular spin dephasing around capillaries and veins, intravascular phase changes, and transverse relaxation changes of blood. This work considers a series of tissue compartmentalized models incorporating each of these effects, and tries to determine the model which is most consistent with the data. To isolate the different tissue contributions, a series of multi-echo inversion recovery (IR) fMRI scans were performed. Visual stimulation experiments were performed at 1.5 T, one interleaved six-echo and two IR six-echo EPI scans (the latter to suppress gray matter (GM) and cerebrospinal fluid (CSF)). The tissue and vascular composition of activated areas was analyzed using independent spin-echo IR MRI experiments and MR venography, respectively. This information was used to fit the multi-echo fMRI data to the BOLD models. The activated areas almost always included a venous vessel visible on the venogram and consisted of GM and CSF. The fMRI signal changes were best described by extravascular dephasing effects in both GM and CSF around a venous vessel, in combination with intravascular effects. The role of spin dephasing around capillaries in GM appears to be insignificant. Magn Reson Med 45:233-246, 2001.

Body Fluid Compartments↗
PubMed2001

Partial Fourier imaging in multi-dimensions: a means to save a full factor of two in time.

We present an improvement to the traditional one-dimensional partial Fourier method by extending the method to multi-dimensions. The modified method allowed a full factor of two savings in time with much better coverage of the central k-space information and, because of this, smaller reconstruction artifacts. The residual magnitude error was found to correlate strongly with the residual phase error. Numerical simulation also indicated that with a priori perfect phase information, the original magnitude image could be perfectly reconstructed with half of the k-space data points in the multi-dimensional case. Simulated, phantom, and human data sets were tested with edge differences ranging from 10% (consistent with variable Gibbs ringing) to 25% (consistent with a blurred version of the object). The method was found to be a valuable adjunct to human imaging for short TR, T1-weighted three-dimensional gradient-echo imaging and magnetic resonance (MR) angiographic methods, especially when short echo times were used.

Algorithms↗
PubMed2001

Reducing oblique flow effects in interleaved EPI with a centric reordering technique.

Segmented interleaved echo planar imaging offers a fast and efficient approach to magnetic resonance angiography. Unfortunately, this technique is particularly sensitive to oblique flow in the imaging plane. In this work, a mathematical analysis of oblique flow effects for several types of k-space coverage is presented. The conventional linear acquisition scheme, an alternating centric and a nonalternating centric encoding scheme are compared with respect to their flow properties. It is shown both by simulations and imaging experiments that artifacts from oblique in-plane flow are effectively reduced by both centric reordered phase-encoding schemes. The nonalternating centric acquisition scheme is preferred to the alternating centric scheme due to the smoother phase error transition in k-space in the presence of obliquely-angled flow. Magn Reson Med 45:623-629, 2001.

Artifacts↗
PubMed2001

High-resolution blood oxygen-level dependent MR venography (HRBV): a new technique.

Our purpose of this study was to demonstrate the clinical potential and spatial resolution of a new MRI technique: high-resolution blood oxygen-level dependent venography (HRBV), in well-known intracranial vascular lesions, such as cavernous and venous angiomas, and venous sinus thrombosis. HRBV provides unique high-resolution information on veins without administration of contrast medium. The data are independent of conventional findings on MRI and potentially useful in characterising and demonstrating the architecture of vascular lesions of the brain.

Adult↗
PubMed2001

An exact form for the magnetic field density of states for a dipole.

We present an analytical form for the density of states for a magnetic dipole in the center of a spherical voxel. This analytic form is then used to evaluate the signal decay as a function of echo time for different volume fractions and susceptibilities. The decay can be considered exponential only in a limited interval of time. Otherwise, it has a quadratic dependence on time for short echo times and an oscillatory decaying behavior for long echo times.

Humans↗
PubMed2000

Artery and vein separation using susceptibility-dependent phase in contrast-enhanced MRA.

In magnetic resonance angiography, contrast agents are frequently used to help highlight arteries over background tissue. Unfortunately, enhancing veins hamper the visualization of arteries when data are collected over a long period of time after the arterial phase of the contrast agent. To overcome this problem, we have developed a novel imaging and postprocessing method that is capable of eliminating veins by utilizing the susceptibility difference between veins and surrounding tissue. This method was applied in the peripheral vasculature where the vessels are predominantly parallel to the main field and where the blood oxygen level-dependent effect is most pronounced. Results are presented for both long (15.8 msec) and short echo times (7.8 msec) and for sequential and centrally reordered acquisition schemes. The short echo scan approach appears to be the most promising, making it possible to obtain good suppression of the venous signal even when the timing is not perfect or when repeat scans are necessary.

Arteries↗
PubMed2000

High-resolution segmented EPI in a motor task fMRI study.

A high-resolution gradient echo, multi-slice segmented echo planar imaging method was used for functional MRI (fMRI) using a motor task at 1.5 Tesla. Functional images with an in-plane resolution of 1 mm and slice thickness of 4 mm were obtained with good white-gray matter contrast. The multi-shot approach, combined with a short total readout period of 82 ms, limits blurring effects for short T(2)(*) tissues (such as gray matter), assuring truly high-resolution images. In all subjects, motor functions were clearly depicted in the contralateral central sulcus over several slices and sometimes activation was detected in the supplementary motor area and/or ipsilateral central sulcus. The average signal change of 11+/-3% was much higher than in standard low-resolution fMRI EPI experiments, as a result of larger relative blood fractions.

Adult↗
PubMed2000

Noise in polymer gel measurements using MRI.

With the development of conformal radiotherapy, particularly intensity modulated radiation therapy (IMRT), there is a clear need for multidimensional dosimeters. A commercial polymerizing gel, BANG-2 gel (MGS Research, Inc., Guilford, CT), has recently been developed that shows potential as a multi-dimensional dosimeter. This study investigates and characterizes the noise and magnetic resonance (MR) artifacts from imaging BANG-2 gels. Seven cylindrical vials (4 cm diam, 20 cm length) were irradiated end on in a water bath and read using MRI (B0=1.5 T, TE=20 ms/100 ms, TR=3000 ms). The gel calibration compared the measured depth-dose distributions in water against the change in solvent-proton R2 relaxivity of the gel. A larger vial (13 cm diam, 14 cm length) was also irradiated to test the calibration accuracy in a vial of sufficient volume for dose distribution measurements. The calibration curve proved accurate to within 1.3% in determining the depth dose measured by the larger vial. An investigation of the voxel-to-voxel (IXIX 3 mm3) noise and sensitivity response curve showed that the voxel-to-voxel variation dominated the dose measurement uncertainty. The voxel-to-voxel standard deviation ranged from 0.2 Gy for the unirradiated gel to 0.7 Gy at 20 Gy. Slice-to-slice R2 magnitude deviations were also observed corresponding to 0.2 Gy. These variations limited the overall accuracy of the gel dose measurements and warrant an investigation of more accurate MR readout sequences.

Calibration↗
PubMed1999

Sub-millimeter fMRI at 1.5 Tesla: correlation of high resolution with low resolution measurements.

Functional magnetic resonance imaging of the visual cortex with an in-plane resolution of 0.4 x 0.4 mm2 was performed using a simple visual stimulus resulting in clear maps of activation. A collapsing filter was used to compare these high-resolution images with low-resolution images collected during the same session. A good correspondence between the high- and low-resolution functional maps was found with respect to the center of localization of activation. However, only 20% of the size of activated areas in the low-resolution experiment was observed at high resolution, which was partly caused by the difference in signal-to-noise ratio. The high-resolution images produce signal changes much higher than the low-resolution images due to reduced partial volume effects. Additionally, the high-resolution functional maps were compared with detailed anatomical and venous information. The activated areas were predominantly observed at venous vessels within the sulci with a diameter on the order of the pixel size.

Adult↗
PubMed1999

Improving high-resolution MR bold venographic imaging using a T1 reducing contrast agent.

Recently, a new imaging method was proposed by Reichenbach et al (Radiology 1997;204:272-277) to image small cerebral venous vessels specifically. This method, referred to as high-resolution blood oxygen level-dependent venography (HRBV), relies on the susceptibility difference between the veins and the brain parenchyma. The resulting phase difference between the vessels and the brain parenchyma leads to signal losses over and above the usual T2* effect. At 1.5 T, a rather long TE (roughly 40 msec) is required for this cancellation to become significant, leading to enhanced susceptibility artifacts and a long data acquisition time. In this study, we examine the utility of incorporating a clinically available T1 reducing contrast agent, Omniscan (Sanofi Winthrop Pharmaceuticals, NY, NY), with the HRBV imaging approach to reduce susceptibility artifacts and imaging time while maintaining the visibility of cerebral veins. Using a double-dose injection of Omniscan, we were able to reduce TE from 40 to 25 msec. This led to a decrease in TR from 57 to 42 msec, allowing a 26% reduction in data acquisition time while maintaining the visibility of cerebral venous vessels and reducing susceptibility artifacts. J. Magn. Reson. Imaging 1999;10:118-123, 1999.

Artifacts↗
PubMed1999

Accuracy of T1 measurements at high temporal resolution: feasibility of dynamic measurement of blood T1 after contrast administration.

The purpose of this work was to optimize a technique to measure blood T1 dynamically after contrast agent administration with a high temporal resolution. This technique uses a 90 degrees prepared gradient-echo sequence and has a temporal resolution of one T1 measurement per cardiac cycle. The non-ideal excitation slice profiles on the estimation of T1 were evaluated by theoretical simulations and used to obtain corrected blood T1 values. The technique was validated on phantom and in vivo pig studies, which demonstrated significant improvement on the accuracy of the dynamic T1 measurement method after slice profile correction. This technique may find important applications in studying the dynamic blood T1 after injection of various contrast agents. J. Magn. Reson. Imaging 1999;10:576-581.

Animals↗
PubMed1999

Efficacy of slow infusion of gadolinium contrast agent in three-dimensional MR coronary artery imaging.

The impact of the contrast infusion scheme on contrast-enhanced coronary artery imaging was investigated. The imaging technique was a retrospective respiratory-gated gradient-echo sequence. To improve the contrast between blood vessels and myocardium, an inversion pulse was applied to null the myocardial signal. Extravascular contrast agent was infused slowly during data acquisition. Two contrast infusion schemes were compared in animal and volunteer studies and with simulations. One scheme enhanced blood signals during acquisition of both peripheral and central k-space data, which resulted in a relatively flat low-pass k-space filter. The other scheme mainly enhanced blood signals during acquisition of central k-space data, which resulted in a sharp low-pass k-space filter. It is concluded that the former scheme improves the signal-to-noise ratio and edge definition of coronary arteries compared with the latter scheme. J. Magn. Reson. Imaging 1999;10:800-805.

Adult↗
PubMed1999

Geometric distortion correction in gradient-echo imaging by use of dynamic time warping.

Distortion caused by local static field variations in magnetic resonance imaging can be corrected by using a method developed by Chang and Fitzpatrick (CF) that requires two acquisitions identical except for the polarity of the read gradient. This approach has previously been applied to spin-echo imaging. In this report, its role in correcting geometric distortion in gradient-echo imaging is investigated using a dynamic programming algorithm. This approach appears to successfully handle many of the edge artifacts otherwise inherent in the CF method. Once the correction has been made, it is then possible to extract the local magnetic field itself. Magn Reson Med 42:585-590, 1999.

Algorithms↗
PubMed1999

High-resolution, multiple gradient-echo functional MRI at 1.5 T.

A multiple gradient echo, high resolution imaging method is proposed to better visualize different sources of activation in functional magnetic resonance imaging (fMRI) experiments. Eight echoes are collected from 30 ms to 205 ms with an echo spacing of 25 ms. All echoes show significant activation, but each echo reveals its own pattern of activation. From this variability, it appears that large vessel contributions can be separated from small vessel contributions using a fuzzy cluster analysis across echo times. The results demonstrate the importance of a multiple gradient echo data acquisition approach in localizing various vascular contributions to brain activation in fMRI.

Adult↗
PubMed1999

The correlation between phase shifts in gradient-echo MR images and regional brain iron concentration.

The purpose of this study was to investigate the relationship between the magnetic susceptibility of brain tissue and iron concentration. Phase shifts in gradient-echo images (TE = 60 ms) were measured in 21 human subjects, (age 0.7-45 years) and compared with published values of regional brain iron concentration. Phase was correlated with brain iron concentration in putamen (R2 = 0.76), caudate (0.72), motor cortex (0.68), globus pallidus (0.59) (all p < 0.001), and frontal cortex (R2 = 0.19, p = 0.05), but not in white matter (R2 = 0.05,p = 0.34). The slope of the regression (degrees/mg iron/g tissue wet weight) varied over a narrow range from -1.2 in the globus pallidus and frontal cortex to -2.1 in the caudate. These results suggest that magnetic resonance phase reflects iron-induced differences in brain tissue susceptibility in gray matter. The lack of correlation in white matter may reflect important differences between gray and white matter in the cellular distribution and the metabolic functions of iron. Magnetic resonance phase images provide insight into the magnetic state of brain tissue and may prove to be useful in elucidating the relationship between brain iron and tissue relaxation properties.

Adolescent↗
PubMed1999

Increased-contrast, high-spatial-resolution, diffusion-weighted, spin-echo, echo-planar imaging.

An insertable head gradient coil with a maximum gradient amplitude of 45 mT/m and a rise time of 150 musec along all three major axes was used to collect high-spatial-resolution, single-shot, spin-echo, echo-planar, diffusion-weighted magnetic resonance images with b values ranging from 0 to 2,200 sec/mm2. Improvements in spatial resolution allowed better visualization of large white matter tracts and their relation to adjacent anatomic structures. Excellent contrast and anatomic detail were revealed for most structures in the brain when a sufficient number of acquisitions were collected.

Adult↗