Physicians are often faced with diagnostic situations for which it is important to order the right type of neuroimaging. One of the most common decisions is whether to use MRI or CT scans.

Both types of imaging are capable of producing high-quality and diagnostically useful images, but each operates differently and has merits or weaknesses, depending on where it is being used and for what.

Factors that go into the decision include; suspected diagnosis, availability of imaging, the anatomical location of the suspected abnormality and cost. In some areas, you may not have a lot of choice, or you may be restricted by cost. Patient factors (such as affordability) may give you only one answer.

Here are the basics of when to use each and why:

CT scans: The basics

A CT scan (Computed Tomography) works by taking multiple X-rays at different angles, then combining them with a computer to create 360 degree, cross-sectional views of the body.

A CT will image blood, bone and soft tissues all at the same time. Typically, CT scans are quicker than MRI scans (five to twenty minutes), and less uncomfortable for those who are claustrophobic or sensitive to noise.

CT scans and equipment tend to be more affordable than MRI. If it’s a question of cost and either option is possible, CT will usually be less expensive for the patient.

What CT scans are best used for

CT scans can be used on any part of the body, although they don’t typically provide as much detail in imagery of soft tissues as MRI. There is a long list of conditions for which CT is used, including:

• Monitoring and detection of tumors
• Monitoring and detection of heart disease or liver masses
• Detection of internal injuries following an accident
• Detection of blood clots, excess fluid or infections.
• Pinpointing issues with bones and joints

In some cases, CT scans may be used on patients for whom MRI is not suitable.

When not to use CT scans

CT scans avoid some of the absolute no-go conditions for MRI, however, due to the radiation, CT is not usually recommended during pregnancy. There is potential risk of harm to the unborn child.

MRI: The basics

The biggest difference between MRI and CT is that MRI uses radio waves and there is no radiation involved. MRI combines a powerful magnetic field with an advanced computer system and radio waves to produce accurate, detailed pictures of organs, soft tissues, bone and other internal body structures.

In general, MRI creates pictures that can show differences between healthy and unhealthy or abnormal tissues. It provides detailed imagery across a range of bodily structures. A scan typically takes from 15 minutes to one hour as images are taken in “slices” of the body being scanned. Timing can also depend upon the type of technology being used.

What MRI scans are best used for

MRI is particularly good at providing detailed images of inner organs and soft tissues such as the brain, reproductive system, skeletal system and other organs. MRI will show the difference between normal and abnormal tissue in these structures.

MRI is a non-invasive procedure, but it can be very loud and the bore can be uncomfortable for any claustrophobic patients. It may not be suitable for those patients, or for those with chronic sensory issues.

When not to use MRI

Due to its use of a strong magnet, MRI may not be recommended on patients with:

• Implanted pacemakers which are not MRI conditional
• Intracranial aneurysm clips which are not MRI conditional
• Cochlear implants
• Certain prosthetic devices
• Implanted drug infusion pumps
• Neurostimulators
• Bone-growth stimulators
• Certain contraceptive devices; or
• Any other type of iron-based metal implants.

MRI should also no be used on patients who have ferrous metal in locations of the body which might pose a risk of injury. For example, gunshot victims where the bullet is near a sensitive area, shrapnel in areas where injuries might occur, and certain medical devices.

Additionally, there has been some debate among medical researchers as to the safety of MRI on pregnant women. Several studies have concluded no significant risk, some mention a risk that the temperature of the amniotic fluid may rise harmfully, while one Canadian study concludes:

“Exposure to MRI during the first trimester of pregnancy compared with non-exposure was not associated with increased risk of harm to the fetus or in early childhood. Gadolinium MRI at any time during pregnancy was associated with an increased risk of a broad set of rheumatological, inflammatory, or infiltrative skin conditions and for stillbirth or neonatal death. The study may not have been able to detect rare adverse outcomes.”

Final thoughts

Both CT and MRI are capable of providing high-quality images for a range of conditions and areas of the body. While the former brings a small risk from the radiation involved, the latter is a bigger risk to patients who have ferromagnetic implants or other materials on or inside their bodies.

If there are no risk factors or cost barriers involved, then MRI will give more detailed images of soft tissue and bone structure than CT. Both systems can show the presence of masses for cancer detection and give oncologists a good idea of whether and where cancer has metastasized. Neither is a final diagnostic tool for cancer.

It really comes down to the situation that necessitates medical imaging. For example, with speed being of the essence, CT scans are commonly used in the ER. MRI tends to be used in non-emergency situations where a longer look can be taken and patients can be screened ahead for suitability.