In Susceptibility Weighted Imaging
(SWI) (1), T2*-weighted gradient-echo magnitude and filtered
phase images are multiplied together to create a distinctive tissue contrast
that highlights variations in tissue magnetic susceptibility. In this short
lecture, I will give an overview of the physical principles of SWI and explain
the practical steps and parameters needed to generate susceptibility weighted
images. I will briefly review the history of SWI and its clinical applications
and discuss its limitations and possible ways these may be overcome. I will
compare SWI with the newer technique of quantitative susceptibility mapping
(QSM) and touch on newer SWI-inspired combinations.
The original aim of SWI was to
enhance susceptibility-induced contrast by exploiting both the MRI signal magnitude
and its phase which had historically been discarded. Early work (2) suggested a simple method for combining
magnitude and corrected phase data to improve the sensitivity of images to an
injected paramagnetic contrast agent. A few years later, focusing on emphasising
endogenous venous vessels, Reichenbach et al (3) proposed the technique which was developed
further and named “Susceptibility Weighted Imaging” by Haacke et al. (4).
Conventional gradient-echo MRI uses only the signal magnitude, but exploiting
the phase of the signal improves tissue visualisation by enhancing the contrast
between tissues with different magnetic susceptibilities. There are several
steps required to create SWI images. First, complex (magnitude and phase) data
must be acquired using a T2*-weighted gradient-echo imaging sequence (5,6). Phase is an angle
defined between ±p therefore phase images are affected by wraps or aliasing. These wraps
must be removed using an unwrapping technique before it can be further
processed for SWI. The unwrapped phase contains contributions from background
fields arising from susceptibility sources outside the region of interest. In
brain imaging these are primarily due to air-tissue susceptibility differences
and are assumed to be mostly of low spatial frequency. This means that
background phases can often be removed by high-pass filtering (7). There are now a large variety of sophisticated techniques for removing
background fields or phases as these have been developed for QSM (8,9). However, one of the
simplest techniques, Homodyne filtering (10,11) is still commonly
used to perform both phase unwrapping and background field removal in a single
step. In standard SWI, the filtered phase is rescaled between ±p and then further
processed to emphasise venous vessels or structures with large phase
differences relative to the surrounding parenchyma. For example, negative phase
values corresponding to paramagnetic tissues are mapped to a range between 0
and 1 and positive phase values are set to unity (12). This produces a phase mask which is then multiplied several - usually
four (4,13) - times with the
magnitude image to produce the final SWI image which has increased contrast
between tissues of different magnetic susceptibility. Increasingly, MRI scanner
manufacturers provide SWI as a “product” which performs all the processing
steps described above to automatically reconstruct SWI images with no user
intervention.
SWI acquisitions can be 2D or 3D gradient-echo sequences which tend to
be more efficient. Traditionally SWI used single echo acquisitions but more
recently multi-echo sequences have been used (14,15) and this can allow
for reduced background field inhomogeneity artifacts (15). SWI can be acquired at any magnetic field strength but higher field
strengths offer both increased signal-to-noise ratio that can be traded for
higher resolution, and higher time efficiency by allowing shorter TE and TR (13,16). Specific imaging
parameters including voxel geometries (17), TE, TR, flip angle and bandwidth can be optimised for the specific
application (e.g. venography) and field strength (16). To avoid unwanted contributions to the phase from flowing spins in
vessels, it is necessary to apply flow compensation along all three encoding
directions (18). When multiple radio-frequency
coils are used in an array, care needs to be taken to ensure that the complex
data from each coil channel is combined properly (19-22), taking the complex
coil sensitivities into account to avoid open-ended fringe lines or
singularities in the phase images which cannot be unwrapped. It is also
important to be aware that MRI systems from different manufacturers may produce
phase images of opposite sign (23). This needs to be taken into account in the production of phase masks
designed to emphasise either paramagnetic or diamagnetic tissues (24). Finally, SWI images are often displayed as minimum intensity
projections (mIPs) and may be further post-processed to emphasise vascular
structures (25).
SWI has become a widely used
clinical tool (26,27), particularly for neurovascular and neurodegenerative disease
applications (28-30). SWI is highly sensitive to pathologies that lead to a change in
tissue magnetic susceptibility including microbleeds and haemorrhages, iron
deposition and calcifications. This means that SWI has made significant impact
in the diagnosis and clinical management of cerebral amyloid angiopathy (31), traumatic brain injuries (32), vascular malformations and anomalies (33), stroke (34), multiple sclerosis (35) and tumours (36). SWI has a growing range of applications
outside the brain for visualising breast calcifications (37,38), liver cirrhosis and other liver pathologies (39,40), renal carcinomas (41), and even musculoskeletal applications (42,43).
SWI images must be very carefully
interpreted, taking into account the contrast contributions and behaviour in
both the magnitude and the processed phase images used to produce SWI images.
One further drawback of SWI images is that hypointense regions with very
similar appearance may have a number of different sources including, for
example, both small veins and microbleeds. The contrast in SWI is “qualitative”
and does not relate directly to tissue properties. SWI suffers from the
orientation-dependence and non-local nature of phase image contrast and cannot enable
positive (e.g. deoxygenated blood) and negative susceptibilities (e.g.
calcifications) to be distinguished. Quantitative susceptibility mapping (QSM) (8,9,44-46) overcomes many of these limitations, providing maps of an intrinsic
tissue property (susceptibility) that is closely related to tissue composition.
QSM overcomes the orientation-dependent (47) and non-local properties of SWI phase
contrast to allow quantitative information to be obtained and offers both
positive and negative contrast so that microbleeds can be distinguished from
calcifications (48). Unlike SWI, QSM is calculated from the
phase information and does not involve multiplicative combination of magnitude
and phase images. As QSM is a more recently developed technique, it is yet to
achieve the widespread clinical uptake and growing clinical applications of
SWI.
In addition to QSM, newer
techniques are available which are inspired by SWI and build on the idea of
multiplying together images with different contrast to provide more information
in a single image (49) . One example is “true-SWI” (tSWI) or QSM-WI
(50,51) which creates a mask from a QSM image and multiplies this with
the corresponding T2*-weighted magnitude image to overcome some of the
limitations of traditional SWI.
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