Windowing and greyscale inversionĪs if the variability in handedness was not sufficiently annoying, how phase images are presented is also variable. This will in turn affect whether paramagnetic and diamagnetic materials will appear as white or black. If clockwise, this is referred to as a left-handed system because when you curl your left hand into a fist the fingers curl in a clockwise direction. This is referred to as 'handedness' depending on the direction a positive change in phase is shown. Unfortunately, how phase information is shown is not uniform and varies from vendor to vendor and sequence 5. It can be helpful to reconstruct your images in the coronal plane to accentuate this effect 6. In this instance looking at the superior-most and inferior-most images is the key as they will be imaging not the lesion itself, but the magnetic dipole that matches the direction of phase shift of the central lesion, without the effect of aliasing. In many instances, however, the lesion will have both black and white components due to aliasing. If it is the opposite, then it will be diamagnetic and therefore most likely dystrophic calcification. If it is the same as veins it is paramagnetic and therefore contains blood products. If you are lucky it will appear uniformly either black or white, in which case you are finished. Once you have established this, then look at the lesion. If the patient has pineal or choroid calcification this can also be helpful, however, due to aliasing (see below) this can be more challenging. Generally, the internal cerebral veins are readily identified. Thus the first step is to find a reliable internal control to establish how paramagnetic (e.g. This does not, however, prevent the images being greyscale-inverted. Interpretationĭespite the aforementioned difficulties, there are a number of tricks that enable you to correctly interpret whether a lesion is composed of dystrophic calcification or blood products on phase images in most cases.įirstly, you will probably become familiar with your scanners and know if they are right or left-handed systems. These are discussed below in the "pitfalls" section. size and degree to which a lesion causes a phase shift (aliasing).This is, however, not without its own complications as whether a lesion appears black or white on phase imaging depends on numerous factors: veins/hemorrhage and calcification will appear of opposite signal intensity) 3. The filtered phase images are, however, able to (in principle) distinguish between the two as diamagnetic and paramagnetic compounds will affect phase differently (i.e. Radiographic features MRIĭistinguishing between calcification (made up primarily of calcium phosphate, but also contain very small amounts of copper (Cu), manganese (Mn), zinc (Zn), magnesium (Mg), and iron (Fe)) 3 and blood products is not possible on the post-processed SWI images as both demonstrate signal drop out and blooming. They are also well suited to assess veins as deoxyhemoglobin results in both a loss in magnitude and a shift in phase 4. The most common use of SWI is for the identification of small amounts of hemorrhage/blood products or calcium, both of which may be inapparent on other MRI sequences. Often a fourth set of images is provided, minimum intensity projection (minIP) which is just a thick slab of the conventional SWI images and is better able to demonstrate venous anatomy. SWI (combined post-processed magnitude and phase).Paramagnetic compounds include deoxyhemoglobin, ferritin and hemosiderin 1.ĭiamagnetic compounds include bone minerals and dystrophic calcifications 1.įollowing the acquisition, post-processing takes place which includes a high-pass filter, to remove background inhomogeneity of the magnetic field, and the application of a phase map to accentuate the directly observed signal loss 2,4. Unlike most other conventional sequences, SWI takes advantage of the effect on phase as well as magnitude 4.Ĭompounds that have paramagnetic, diamagnetic, and ferromagnetic properties all interact with the local magnetic field distorting it and thus altering the phase of local tissue which, in turn, results in a change of signal 2. SWI is a 3D high-spatial-resolution fully velocity corrected gradient-echo MRI sequence 1-3.
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