Susceptibility-Weighted Imaging: Technical Aspects and Clinical Applications, Part 1|
AJNR 2009 - 30;19-30
Susceptibility-weighted imaging (SWI) is a new neuroimaging technique, which uses tissue magnetic susceptibility differences to generate a unique contrast, different from that of spin density, T1, T2, and T2*. In this review (the first of 2 parts), we present the technical background for SWI. We discuss the concept of gradient-echo images and how we can measure local changes in susceptibility. Armed with this material, we introduce the steps required to transform the original magnitude and phase images into SWI data. The use of SWI filtered phase as a means to visualize and potentially quantify iron in the brain is presented. Advice for the correct interpretation of SWI data is discussed, and a set of recommended sequence parameters for different field strengths is given.
Susceptibility-Weighted Imaging: Technical Aspects and Clinical Applications, Part 2|
AJNR 2009 - 30;232-252
Susceptibility-weighted imaging (SWI) has continued to develop into a powerful clinical tool to visualize venous structures and iron in the brain and to study diverse pathologic conditions. SWI offers a unique contrast, different from spin attenuation, T1, T2, and T2* (see Susceptibility-Weighted Imaging: Technical Aspects and Clinical Applications, Part 1). In this clinical review (Part 2), we present a variety of neurovascular and neurodegenerative disease applications for SWI, covering trauma, stroke, cerebral amyloid angiopathy, venous anomalies, multiple sclerosis, and tumors. We conclude that SWI often offers complementary information valuable in the diagnosis and potential treatment of patients with neurologic disorders.
Settling Properties of Venous Blood Demonstrated in the Peripheral Vasculature Using Susceptibility-Weighted Imaging (SWI)|
JMRI 2009 - 29;1465–1470
To evaluate the settling properties of venous blood in the peripheral vasculature during periods of immobility. Susceptibility-weighted imaging (SWI) was performed for nine subjects at two time points: within 10 minutes of entering the magnet and after 40 minutes spent stationary in the magnet. Changes in the phase and in the distribution of phase of the veins were used to draw conclusions about the separation of red blood cells from plasma over time. Settling was observed to occur in eight of the nine subjects, the only exception being the youngest subject (18 years old). The bottom half of some veins was seen to darken while the top half showed little change often with a clear dividing line between the two. Phase values measured in the bottom layer were consistent with the layer consisting entirely of red blood cells. Settling was seen to increase with time spent stationary and to correlate with the size of veins in the calf. Older subjects tended to have larger veins and consequently more settling of the red blood cells. Our results show that even 40 minutes of rest can easily lead to settling of the blood depending on the position of the leg.
Characterizing the Mesencephalon Using Susceptibility-Weighted Imaging|
AJNR 2009 - 30;569–574
The mesencephalon is involved in a number of human neurodegenerative disorders and has been typically imaged with T1-, T2- and T2*-weighted methods. Our aim was to collect high-contrast susceptibility-weighted imaging (SWI) data to differentiate among and within the basic mesencephalic structures: namely, the red nucleus, substantia nigra, and crus cerebri. High-resolution SWI, 3D T1-weighted, and T2-weighted data were collected to study contrast in the mesencephalon at 1.5T and 4T. Contrast between structures was calculated for SWI high-pass (HP)-filtered-phase, T1 gradient-echo, and spin-echo T2-weighted data. SWI HP-filtered-phase data revealed similar contrast for the red nucleus and substantia nigra when compared with T2-weighted imaging. However, SWI was able to show structures within the red nucleus, substantia nigra, and medial geniculate body that were invisible on T2-weighted imaging. T1-weighted imaging, on the other hand, did not reveal measurable contrast for any of the structures of interest. SWI HP-filtered-phase data at 4T agreed well with india ink-stained cadaver brain studies, which appear to correlate with capillary density. With SWI, it is possible to create better anatomic images of the mesencephalon, with improved contrast compared with conventional T1- or T2-weighted sequences.
Susceptibility-Weighted Imaging: Clinical Angiographic Applications|
MRI Clinical N Am 2009 - 17;47-61
Susceptibility-weighted imaging (SWI) provides a new means to enhance contrast in MR imaging. Conventional imaging relies on the magnitude information to generate the image; the phase information, however, has typically been discarded except for a few applications in flow imaging. Historically, phase images have been difficult to interpret, because the valuable information about susceptibility changes between tissues was hidden by background field inhomogeneities caused by air-tissue interfaces and main magnetic field effects. It has been shown, however, that by using a special high-pass filter it is possible to remove most of these unwanted effects, leaving behind only the valuable information about susceptibility changes between tissues.2 The contrast in the phase image is complimentary to the magnitude contrast and the two can be combined to create what is now referred to as "susceptibilityweighted" images. This triplet of images (magnitude, phase, and susceptibility-weighted images) has now become part of the standard clinical neuroimaging protocol in at least one manufacturer’s product.