E. Mark Haacke

Susceptibility weighted imaging (SWI)

Susceptibility differences between tissues can be utilized as a new type of contrast in MRI that is different from spin density, T1‐, or T2‐weighted imaging. Signals from substances with different magnetic susceptibilities compared to their neighboring tissue will become out of phase with these tissues at sufficiently long echo times (TEs). Thus, phase imaging offers a means of enhancing contrast in MRI. Specifically, the phase images themselves can provide excellent contrast between gray matter (GM) and white matter (WM), iron‐laden tissues, venous blood vessels, and other tissues with susceptibilities that are different from the background tissue. Also, for the first time, projection phase images are shown to demonstrate tissue (vessel) continuity. In this work, the best approach for combining magnitude and phase images is discussed. The phase images are high‐pass‐filtered and then transformed to a special phase mask that varies in amplitude between zero and unity. This mask is multiplied a few times into the original magnitude image to create enhanced contrast between tissues with different susceptibilities. For this reason, this method is referred to as susceptibility‐weighted imaging (SWI). Mathematical arguments are presented to determine the number of phase mask multiplications that should take place. Examples are given for enhancing GM/WM contrast and water/fat contrast, identifying brain iron, and visualizing veins in the brain. Magn Reson Med 52:612–618, 2004.

Clinical applications of neuroimaging with susceptibility-weighted imaging

Susceptibility‐weighted imaging (SWI) consists of using both magnitude and phase images from a high‐resolution, three‐dimensional, fully velocity compensated gradient‐echo sequence. Postprocessing is applied to the magnitude image by means of a phase mask to increase the conspicuity of the veins and other sources of susceptibility effects. This article gives a background of the SWI technique and describes its role in clinical neuroimaging. SWI is currently being tested in a number of centers worldwide as an emerging technique to improve the diagnosis of neurological trauma, brain neoplasms, and neurovascular diseases because of its ability to reveal vascular abnormalities and microbleeds. J. Magn. Reson. Imaging 2005.

Principles, techniques, and applications of T2*-based MR imaging and its special applications.

T2* relaxation refers to decay of transverse magnetization caused by a combination of spin-spin relaxation and magnetic field inhomogeneity. T2* relaxation is seen only with gradient-echo (GRE) imaging because transverse relaxation caused by magnetic field inhomogeneities is eliminated by the 180 degrees pulse at spin-echo imaging. T2* relaxation is one of the main determinants of image contrast with GRE sequences and forms the basis for many magnetic resonance (MR) applications, such as susceptibility-weighted (SW) imaging, perfusion MR imaging, and functional MR imaging. GRE sequences can be made predominantly T2* weighted by using a low flip angle, long echo time, and long repetition time. GRE sequences with T2*-based contrast are used to depict hemorrhage, calcification, and iron deposition in various tissues and lesions. SW imaging uses phase information in addition to T2*-based contrast to exploit the magnetic susceptibility differences of the blood and of iron and calcification in various tissues. Perfusion MR imaging exploits the signal intensity decrease that occurs with the passage of a high concentration of gadopentetate dimeglumine through the microvasculature. Change in oxygen saturation during specific tasks changes the local T2*, which leads to the blood oxygen level-dependent effect seen at functional MR imaging. The basics of T2* relaxation, T2*-weighted sequences, and their clinical applications are presented, followed by the principles, techniques, and clinical uses of four T2*-based applications, including SW imaging, perfusion MR imaging, functional MR imaging, and iron overload imaging.

Characterizing iron deposition in multiple sclerosis lesions using susceptibility weighted imaging

To investigate whether the variable forms of putative iron deposition seen with susceptibility weighted imaging (SWI) will lead to a set of multiple sclerosis (MS) lesion characteristics different than that seen in conventional MR imaging.

Establishing a baseline phase behavior in magnetic resonance imaging to determine normal vs. abnormal iron content in the brain.

To establish a baseline of phase differences between tissues in a number of regions of the human brain as a means of detecting iron abnormalities using magnetic resonance imaging (MRI).

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.

Correlation of hypointensities in susceptibility-weighted images to tissue histology in dementia patients with cerebral amyloid angiopathy: a postmortem MRI study

Neuroimaging with iron-sensitive MR sequences [gradient echo T2* and susceptibility-weighted imaging (SWI)] identifies small signal voids that are suspected brain microbleeds. Though the clinical significance of these lesions remains uncertain, their distribution and prevalence correlates with cerebral amyloid angiopathy (CAA), hypertension, smoking, and cognitive deficits. Investigation of the pathologies that produce signal voids is necessary to properly interpret these imaging findings. We conducted a systematic correlation of SWI-identified hypointensities to tissue pathology in postmortem brains with Alzheimer’s disease (AD) and varying degrees of CAA. Autopsied brains from eight AD patients, six of which showed advanced CAA, were imaged at 3T; foci corresponding to hypointensities were identified and studied histologically. A variety of lesions was detected; the most common lesions were acute microhemorrhage, hemosiderin residua of old hemorrhages, and small lacunes ringed by hemosiderin. In lesions where the bleeding vessel could be identified, β-amyloid immunohistochemistry confirmed the presence of β-amyloid in the vessel wall. Significant cellular apoptosis was noted in the perifocal region of recent bleeds along with heme oxygenase 1 activity and late complement activation. Acutely extravasated blood and hemosiderin were noted to migrate through enlarged Virchow–Robin spaces propagating an inflammatory reaction along the local microvasculature; a mechanism that may contribute to the formation of lacunar infarcts. Correlation of imaging findings to tissue pathology in our cases indicates that a variety of CAA-related pathologies produce MR-identified signal voids and further supports the use of SWI as a biomarker for this disease.

High-resolution MR venography at 3.0 Tesla.

Purpose The aim of this study was to investigate the visualization of small venous vessels in the normal human brain at a field strength of 3 Tesla. Methods T2*-weighted, three-dimensional gradient-echo images were acquired by exploiting the magnetic susceptibility difference between oxygenated and deoxygenated hemoglobin in the vasculature and microvasculature. The spatial resolution was 0.5 × 0.5 × 1 mm3, and sequence parameters were varied to obtain good vessel delineation. Improved visibility of venous vessels was obtained by creating phase mask images from the magnetic resonance phase images and multiplying these by the magnitude images. Venograms were created by performing a minimum intensity projection over targeted volumes. Results Highly detailed visualization of venous structures deep in the brain and in the superficial cortical areas were obtained without administration of an exogenous contrast agent; compared with similar studies performed at 1.5 T, the echo time could be reduced from typically 40–50 ms to 17–28 ms. Conclusion Imaging at high-field strength offers the possibility of improved resolution and the delineation of smaller vessels compared with lower field strengths.

In vivo measurement of blood oxygen saturation using magnetic resonance imaging: A direct validation of the blood oxygen level-dependent concept in functional brain imaging

A novel noninvasive magnetic resonance imaging (MRI) method was developed to determine in vivo blood oxygen saturation and its changes during motor cortex activation in small cerebral veins. Specifically, based on susceptibility measurements in the resting states, pial veins were found to have a mean oxygen saturation of Yrest = 0.544 ± 0.029 averaged over 14 vessels in 5 volunteers. During activation, susceptibility measurements revealed an oxygen saturation change of ΔYsusc = 0.14 ± 0.02. Independent evaluation from blood flow velocity measurements yielded a value of ΔYflow = 0.14 ± 0.04 for this change. These results validate the blood oxygenation level‐dependent (BOLD) model in functional MRI (fMRI). Hum. Brain Mapping 5:341–346, 1997.

Reducing motion artifacts in two-dimensional Fourier transform imaging.

The effects of motion in two-dimensional Fourier transform imaging (2DFT) are considered. Specific calculations describing the case of periodic motion are presented. The results predict the commonly seen artifact of image replication, sometimes referred to as ghosting. Expressions for both position and amplitude of these ghosts are derived. Simulated examples illustrate the image degradation for pulsatile flow and in plane motion. Several methods of reducing motion artifacts are then suggested. These include: randomization of views, averaging views, matching repeat times to the respiratory period, hybrid imaging, ROPE and COPE. The latter two methods reorder the data acquisition to destroy the coherence of the motion. They do not increase the data acquisition time and promise to be part of the standard approach to remove motion artifacts. The final step in actually recovering ideal resolution can be accomplished by using a model of the motion and a generalized transform inversion technique.