Susceptibility Weighted Imaging (SWI): A Patient's Guide
Updated: 4 days ago
Susceptibility weighted imaging, or SWI, is a special MRI technique that is very good at showing blood, iron, and calcium in the brain. It works by picking up tiny differences in how tissues react to the MRI's magnetic field. Doctors often use it to look for very small bleeds, blood vessel problems, and certain injuries that may be hard to see on other MRI images.
This guide explains what SWI is, how it works, when it is used, and what it is like for patients.
What is susceptibility weighted imaging?
Every tissue in the body reacts a little differently when it is placed in a strong magnetic field. This reaction is called magnetic susceptibility. Some substances, such as certain forms of blood and iron, change the local magnetic field more than normal brain tissue does.
Most standard MRI images are based mainly on differences in water and fat in the tissues. SWI is different. It is designed to highlight these magnetic differences. As a result, blood products, iron deposits, calcium, and small veins can stand out clearly on the images.
SWI is most often used for the brain. It is usually added as one sequence within a larger brain MRI exam.
How does SWI work?
You don't need to know the physics to have an SWI scan, but here is the basic idea.
The scanner collects two kinds of information. One is the magnitude image, which looks like a typical MRI picture. The other is the phase image, which records tiny shifts in the magnetic field caused by different tissues.
Some substances create bigger shifts. Deoxygenated blood, which is blood that has given up its oxygen, and iron affect the magnetic field more than surrounding tissue.
A computer combines the data. Special processing merges the magnitude and phase information to boost the contrast of those areas.
The final images highlight small details. Small veins, tiny areas of old or new bleeding, and mineral deposits often appear dark and stand out clearly.
When is SWI used?
SWI is often helpful when doctors want to look closely for blood, iron, or blood vessel changes in the brain. Common examples include:
Traumatic brain injury (TBI): SWI is very sensitive to tiny bleeds, called microhemorrhages, that can happen with head injuries. These may be missed on other sequences.
Stroke: SWI can help show whether there is bleeding in or around an area affected by a stroke. That information can matter for treatment decisions.
Vascular malformations: SWI can help show abnormal blood vessels, such as cavernous malformations (also called cavernomas) and some other vessel problems.
Multiple sclerosis (MS): SWI can show iron deposits and small veins in the brain, which researchers and doctors study in MS. Learn more in our guide to MRI and multiple sclerosis.
Brain tumors: SWI can add information about blood vessels and bleeding within a tumor, helping describe it in more detail.
Your doctor decides which MRI sequences are needed based on your symptoms and the question they want answered.
Benefits of SWI
High sensitivity to blood and iron. SWI can show very small bleeds and deposits that are hard to see with other techniques.
Clear view of small veins. It can show small veins in the brain without needing contrast dye.
No radiation. Like all MRI, SWI uses magnets and radio waves, not X-rays. Read more about why MRI scans use no radiation.
Works alongside other sequences. SWI adds information to a standard brain MRI rather than replacing it.
Limits of SWI
SWI is a helpful tool, but it has some limits.
Blood and calcium can look alike. On some SWI images, both can appear dark. Radiologists may use phase images, other MRI sequences, or a CT scan to tell them apart.
Artifacts. Metal, such as dental work, and areas near air-filled spaces like the sinuses can distort the images. Movement can also blur them. See what happens if you move during an MRI.
Findings need context. Small dark spots can have more than one cause. The radiologist reads them along with your history and other images.
Not every exam includes it. SWI requires specific scanner settings and processing, so it is not part of every brain MRI.
SWI compared with other brain MRI techniques
A brain MRI usually includes several sequences. Each one shows the brain in a different way. SWI is one of several specialized options.
Technique | What it highlights | Common uses |
Standard brain MRI sequences | Overall brain structure and many types of lesions | Most brain concerns |
Susceptibility weighted imaging (SWI) | Blood products, iron, calcium, small veins | Tiny bleeds, head injury, vessel problems |
Diffusion tensor imaging (DTI) | Direction of water movement along nerve fibers | White matter pathways and injury |
NeuroQuant | Measured volume of brain structures | Memory concerns, tracking brain volume over time |
Learn more in our guides to diffusion tensor imaging and NeuroQuant imaging.
What is an SWI scan like for patients?
If your brain MRI includes SWI, the experience is the same as a typical brain MRI. You lie on a padded table with your head in a coil. The machine makes loud tapping and knocking sounds, so you'll get ear protection. The SWI sequence is just one part of the exam.
Before the scan, you will be screened for metal, implants, and devices. Wear comfortable clothes with no metal, and leave jewelry at home. Lying very still helps keep the images sharp, which matters for a detail-focused technique like SWI.
After the scan, a board-certified radiologist reviews the images and sends a report to your doctor. Your doctor will explain the results and any next steps. For help with common report words, see how to understand your brain MRI terms.
Frequently asked questions
Is SWI safe?
SWI uses a standard MRI scanner, so it has the same safety profile as any MRI. There is no radiation. People with certain implants or metal in the body may not be able to have an MRI, so safety screening is always done first.
Does SWI need contrast dye?
SWI does not need contrast dye to show blood, iron, and small veins. Your doctor may order contrast for other parts of the exam if needed.
Can SWI replace other MRI sequences?
No. SWI is meant to add information, not replace other sequences. It works best as part of a full brain MRI exam.
How long does SWI add to an MRI?
SWI is one sequence within the exam, so it usually adds only a short amount of time. The total exam length depends on what your doctor ordered.
Do I need to prepare for an SWI scan?
No special preparation is usually needed beyond normal MRI steps, such as removing metal and completing the safety form. Follow any instructions from your doctor or the imaging center.
Related reading
Schedule an MRI at Millenium MRI
SWI is available at our Ocala, Florida and Springdale, Arkansas centers. Millenium MRI has centers in Ocala, Largo, and Tamarac, Florida, and in Jonesboro, Marion, North Little Rock, and Springdale, Arkansas. Our board-certified radiologists and ARRT-certified technologists perform brain MRI and other exams, and same-day appointments are often available. We accept most insurance plans, workers' compensation, personal injury cases, and self-pay. If your doctor has ordered a brain MRI, call your nearest center to ask which exams are offered, or request an appointment online.
This article is for general information and is not medical advice. Talk with your doctor about your specific situation.
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