When I go ahead and order imaging for my patients, one of the most common questions I'm asked is:
“What's the difference between an ultrasound and an MRI?”
Both are incredibly useful tools in gynaecology, but they work in very different ways and are used for different reasons.
What is an ultrasound?
Ultrasound uses sound waves to create images of the pelvic organs. It is usually the first test requested because it is quick, widely available and very effective at assessing the uterus and ovaries. There are two types:
● Abdominal ultrasound, where a probe is moved over the lower abdomen. For this, you are often asked to have a full bladder, as it helps improve the images.
● Transvaginal ultrasound, where a slim probe is gently inserted into the vagina. This gives much clearer, more detailed images and does not require a full bladder.
It is important to note that a transvaginal scan is not appropriate for everyone. For patients who are not sexually active—particularly teenagers—a transabdominal ultrasound or, in some cases, an MRI may be more suitable. Most women find ultrasound slightly uncomfortable rather than painful. It usually takes around 10–20 minutes, and you can return to normal activities immediately afterwards. Painkillers are not usually necessary. Importantly, ultrasound does not involve radiation and is considered very safe.
What is an MRI?
MRI (Magnetic Resonance Imaging) uses a powerful magnetic field and radio waves to produce highly detailed images. It is often used when we need more information after an ultrasound, or when assessing more complex conditions such as fibroids, endometriosis or pelvic masses.
During an MRI, you lie on a table that moves into a tunnel-like scanner, usually feet first. The scan typically takes 30–45 minutes. It is painless, although some people find the enclosed space challenging—particularly those with claustrophobia or high levels of anxiety. If this is a concern, it is important to mention it beforehand, as there may be ways to make the experience more comfortable. You do not usually need a full bladder for an MRI, and painkillers are not required. Like ultrasound, MRI does not use radiation.
So which is better?
This is a common question—but the answer is that they are complementary rather than competing tests.
Ultrasound is usually the first step because it is quick and effective for most common problems, such as ovarian cysts, fibroids or assessing the lining of the womb. MRI is used when we need more detailed information, for surgical planning, if the cause of pain is suspected to be non-gynaecological in nature or a clearer map of what is happening on inside the pelvis is needed.
Why has my doctor requested this scan?
Imaging helps us understand symptoms such as pelvic pain, heavy periods, irregular bleeding or fertility concerns. It allows us to look directly at the uterus, ovaries and surrounding structures, helping guide diagnosis and treatment.
When will I get the scan?
Another common question is whether scans can be done on the same day as your consultation. In reality, this is usually not always possible. Ultrasound and MRI appointments often need to be scheduled separately, depending on availability and the type of scan required.
When will I get my results?
A common misconception is that scan results are available immediately. In reality, the images need to be carefully reviewed by a consultant radiologist, who specialises in interpreting these scans. They will produce a detailed report, which is then sent to your gynaecologist.
Your gynaecologist will review this report alongside your symptoms and clinical history. This step is important, as it allows them to interpret the findings in context and formulate a sensible and personalised management plan. While this process can take a little time, it ensures that decisions about your care are based on a thorough and accurate assessment.
Is it dangerous?
Both ultrasound and MRI are considered very safe. Neither involves radiation exposure, which is often a concern for patients. MRI is not suitable for people with certain metal implants, but this is carefully checked beforehand.
What should I expect?
In most cases, these scans are straightforward, well tolerated and provide valuable reassurance or clarity. While the idea of imaging can feel daunting, understanding what to expect often makes the experience much easier.
Ultimately, these tests are not just about pictures—they are about helping us understand your symptoms better and aiding your gynaecologist to formulate a robust and comprehensive management plan for you.