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CT Scan Vs. MRI Scan


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CT Scan Vs. MRI Scan: What is the difference and which one is best?

The Queen Square Imaging Centre is privileged to own and operate two types of scanner; a state-of-the-art CT scanner and a wide-bore 3T MRI scanner.  Both CT and MRI scanners have similar uses in that they produce images of structures inside the human body.  However, we often get asked what the difference is between the two.  In this article, we will discuss the difference between CT scans and MRI scans, how they create detailed pictures and highlight some of their common uses. We will also talk about safety and the reasons why a CT scan or MRI scan may not be suitable for some patients.

The basics: CT Scan Vs MRI

MRI scans (short for Magnetic Resonance Imaging) and CT scans (short for Computed Tomography) are now used routinely in modern medicine to allow doctors to examine structures inside the human body. (CT is sometimes referred to as a CAT scan but the two terms mean the same thing.)   Both types of scans allow for the assessment of anatomy and the identification of any changes which may suggest the presence of disease or injury.  Imaging is one of the key tools available to clinicians to diagnose illness and decide on the most appropriate treatment.   The choice of which diagnostic modality to use for a particular diagnosis will always depend on the benefits of one modality over the other and the specific clinical indications being presented.

A CT scanner and MRI scanner create detailed images using very different techniques. The main difference is that whilst an MRI machine use radio waves and a strong magnetic field, a CT scanner uses X-rays.  Both types of scan are considered relatively low risk and safe when used appropriately by skilled radiographers.  However, as we shall discuss later in this article, the unique process each type of scanner uses in creating detailed images gives rise to special considerations for safety.

What is each type of scan used for?

As we mentioned earlier, the choice of one type of scan over the other will always depend on a number of factors, including the clinical indications being presented, the benefits of one scan over the other and safety implications. In addition, CT scans are less expensive and often more readily available than MRI scans in some hospitals, which can also influence the decision-making process.  However, the unique processes involved in each type of scan makes each modality particularly good for specific common uses.

When are CT Scans helpful?

In general, the use of x-rays in CT Scanning makes it a very fast and particularly accurate method for examining bones and looking for signs of bleeding and the development of cancers.  Here at the Queen Square Imaging Centre, we will commonly use CT scanning to take a detailed look at the bones of the skull, inner ear and spine to look for bone fractures or degenerative change. We also perform many CT scans to assess the soft tissue organs in the chest, abdomen and pelvis. Contrast dyes can often be used, which show up on the x-ray pictures and ‘enhance’ the appearance of soft tissues to reveal more detail.  Our CT Contrast Dye patient guide available here provides some more information. We can also use images created by the CT scanner to guide the placement of a needle in the spine with high accuracy for either tissue biopsy or the injection of pain-relieving medication.

When are MRI Scans helpful?

The use of radio waves and a magnetic field for MRI scanning make this type of scan extremely useful for looking at soft tissues, such as organs, muscles, ligaments, nerves, and blood vessels.  We will commonly use MRI to diagnose problems in the brain, spinal cord and peripheral nerves, and other organs such as the heart, liver, and prostate.  While MRI does not show bone particularly well (compared to a CT scan), it is used to take a detailed look at the soft tissues in joints like the knee, ankle, and shoulder. In fact, MRI is considered to be a definitive diagnostic tool for problems in the muscles, ligaments, and tendons which do not show up on CT scans.  Again, MRI contrast agents are frequently used during MRI scans to achieve more detail on certain scans.  Our article “MRI Contrast: Is there a need to worry?” provides some more information about MRI contrast dyes.

What is it like having a CT or MRI scan?

There are many similarities in the experience of having an MRI or CT scan.  In each case, the patient will be asked to lie down on a comfortable couch, which will then be moved into the scanner.  The patient will need to lie very still during the scan so that the images are as clear and free from movement as possible.  The radiographer operating the scanner will not be in the room whilst the images are being taken. Still, they will be able to see the patient and talk to them throughout the scan.

There are also some key differences between the two scan procedures.  Compared to CT scans that typically take no more than 5 minutes, an MRI scan will take longer.  Depending on the area being scanned, an MRI scan take anything from 20 minutes to 60 minutes to complete.  Despite having no moving parts, MRI scans produce more noise than CT, with the machine making various clicking and buzzing noises whilst the images are being taken.  These sounds are perfectly normal but loud enough that patients will always be given ear protection to avoid any risk of damage to their hearing.  CT machines do have moving parts but create only a tiny amount of noise that is not loud enough to be uncomfortable.

One key difference is that claustrophobic patients may be more anxious about MRI scanners compared to CT scanners. This is because MRI scanners are larger and, depending on the area being scanned, more enclosed.  We will always do everything we can to make even the most nervous patient comfortable. Our MRI is designed to be comfortable and much less claustrophobia-inducing than others.  There are also many things that patients can do to help themselves if they are nervous.  Our article “MRI Scans and Claustrophobia: Dispelling the myths and managing anxiety” provides some useful tips.

Are they safe?

CT and MRI are both very safe procedures, and our highly skilled radiographers are trained to be able to reduce any risk to the minimum possible level.  In general, the safety implications of each type of scan relate to how images are created.

As we mentioned earlier, a CT scanner is a large x-ray machine that uses x-rays to rapidly scan and create detailed images.  During a CT scan, a patient will subsequently receive a very small amount of ionizing radiation, which could potentially affect biological tissues.  CT scanners and the scanning techniques used by radiographers are designed to reduce radiation dose to the absolute minimum, and CT scans are only used when the exposure can be justified.  In practice, the risk of damage, such as developing a cancer, from exposure to radiation from a medical x-ray is minimal.  However, CT scans and x-rays are typically not used during pregnancy or for children unless necessary.

MRI scans do not use ionizing radiation, and therefore do not have the potential to damage biological tissues.  However, the use of strong magnetic fields does pose a different potential risk, which radiographers are always very careful to recognise and minimise.  MRI scans may be unsafe for some patients with particular medical devices and metallic implants, so an MRI scan visit will always start with a detailed safety questionnaire so that the radiographer can assess whether or not a patient is safe to be scanned.  In practice, the number of unsafe implants is reducing. Imaging departments like our own Chenies Mews Imaging Centre can now offer MRI scans safely to patients with cardiac pacemakers, who historically have always been prevented from having MRI. Our article, “Is MRI Safe?” provides some more information. If medical implants are unsafe to be scanned, the patient can often have a CT scan instead quite safely.

Choosing the right scan

Your doctor will always choose and recommend the most appropriate scan for you.  As we have discussed, making the decision whether a CT scan or MRI scan will be most helpful and effective depends on several factors, including:

The medical reason for the scan
The level of detail that is necessary for the images
Whether the patient is safe to have a particular type of scan
Whether the patient is claustrophobic or any other factors might make tolerating an MRI scan more difficult.

Some patients may require both types of scan if the clinical condition warrants.

If you have any further questions about either scan type, please do not hesitate to speak to your doctor or contact the Queen Square Imaging Centre at imaging@qsprivatehealthcare.com.

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