Choosing the Right CT or MRI system

Magnetic resonance imaging (MRI) and computed tomography (CT) systems are the two types of equipment most commonly used to visualize an anatomical structure in 3D. Below we summarize the characteristics of each.

• Magnetic resonance imaging (MRI) systems: These are medical imaging systems, also known as nuclear magnetic resonance (NMR) systems. There are two examination techniques:
  • Anatomical magnetic resonance imaging: This technique consists of exposing the hydrogen atoms in the human body to a magnetic field, mainly in the water and fat contained in the tissues. This type of MRI is therefore particularly suitable for observing organs and soft tissues such as the spinal cord, brain, muscles, tendons, liver, kidneys, spleen, etc.
  • Functional magnetic resonance imaging (fMRI): This technique makes it possible to assess brain activity based on changes in blood flow and oxygenation. It is widely used in cases of cerebrovascular accident (CVA) or, in neurology, in the diagnosis of multiple sclerosis or Alzheimer’s disease, for example.

• Computed tomography (CT) scanners: These are also called “computerized tomography” or “CAT scans” (computerized axial tomography). These use special X-ray equipment to generate cross-sectional images (slices) of tissues and organs in the body. The sequence of images is then processed by computer to obtain a three-dimensional reconstruction of the anatomical structure under study. Tomographic images are generally more detailed than conventional 2D radiographs.

GE Healthcare MRI system

A conventional MRI system consists of superconducting coils that generate a strong magnetic field (cooled by liquid nitrogen), RF antennas, which capture radiofrequency waves, and a computerized image reconstruction system.

The main hybrid systems with MRI are PET/MRI and SPECT/MRI. We summarize their characteristics below.

• PET/MRI system: As the name suggests, this combines conventional magnetic resonance imaging and positron emission tomography (PET) in a single device and examination. Positron emission tomography is a technique that maps the metabolic activity of an organ in 3D by detecting the radiation generated by the positrons emitted by a radiopharmaceutical previously injected into the patient. It is based on the principle of scintigraphy, which consists of injecting a radioactive marker, whose behavior and biological properties are known, in order to obtain an image of the functioning of an organ.
• SPECT/MRI system: Combines conventional magnetic resonance imaging and single photon emission computed tomography (SPECT). This hybrid technology is used in certain pre-clinical applications, particularly in neurology or for imaging tumors. Single photon emission computed tomography (SPECT) is a nuclear imaging technique that combines computed tomography with gamma-emitting radioisotopes.

Siemens Healthineers PET/CT system

The tomograph, or CT scanner, is an X-ray machine made up of an X-ray tube and detectors arranged on a circular support. These detectors measure the difference in intensity of the X-ray beam before and after it passes through the part of the body under study. During the examination, the tube that emits the X-rays and the detectors rotate continuously around the patient’s body while the examination table moves forward. The attenuation of the X-ray beam is measured at different rotation angles. This data is then transmitted to a computer, which reconstructs the images by assigning gray scales according to the amount of radiation absorbed by the different structures.

The main hybrid systems with computed tomography are PET/CT and SPECT/CT systems. Below we summarize the characteristics of each.

• PET/CT system: This hybrid system combines the anatomical images from computed tomography with the functional images obtained from the PET module. This combination of images makes it possible to locate a lesion or functional anomaly with even greater precision. As such, it is possible to optimize a radiotherapy protocol, for example, avoid unnecessary surgery, make surgery more effective, or reduce the need for invasive procedures such as biopsies.
• SPECT/CT system: This combination achieves precise alignment and total integration of the computed tomography system and single photon emission tomography. With this system, it is possible to obtain images with a generally even higher resolution to identify and monitor the evolution of a disease over time (for example, certain cancers).

Hitachi Medical Systems open MRI system

Magnetic resonance imaging systems have several advantages and disadvantages over computed tomography systems.

• Advantages of MRI systems:
  • Definition / Image quality: MRI produces sharper images than CT scans. In particular, better-quality images of organs and soft tissues are obtained, allowing torn ligaments, herniated discs, etc. to be seen in greater detail.

• Disadvantages of MRI systems:
  • Cost: The higher cost of the equipment and its maintenance is naturally reflected in the price of an MRI scanner.
  • Long examinations: MRI scans take much longer (sometimes over an hour) than CT scans (around ten minutes on average).
  • Noise level: MRI machines are very noisy during image acquisition. Patients are usually provided with earplugs or headphones, which muffle the noise, to reduce their discomfort.
  • Claustrophobia: During the MRI scan, which can take a long time, patients lie on a table inside a tunnel, which can cause anxiety and a feeling of claustrophobia for some people. There are, however, some open MRI machines.
  • Motion artifacts: MRI scans require precision. Any movement from the patient can therefore create movement artifacts and result in blurred images. In order to avoid these situations, the patient must control their breathing according to the instructions given to them during the examination.

Philips large diameter CT system

The main advantages and disadvantages of computed tomography systems compared to magnetic resonance imaging systems are described below.

• Advantages of CT systems:
  • Comfort: Computed tomography systems are much quieter than MRI systems, so there is no need for hearing protection during the procedure. In addition, they have a more open configuration than MRI systems and may be more suitable for overweight patients.
  • Speed: This test provides results much more quickly than an MRI. Therefore, doctors most often choose this type of test when they need to make an urgent diagnosis.
• Disadvantages of CT systems:
  • Radiation absorbed: The main disadvantage of this test is the radiation absorbed by the patient, which is non-existent in magnetic resonance imaging. The amount of radiation absorbed with a CT scan is around a thousand times greater than with a conventional X-ray.

Several factors can make it difficult or even impossible to carry out an MRI scan, especially when examining the spine or the whole body. MRIs are not suited for those with an implantable cardiac device or a metallic implant, for example. These are some of the main reasons why an MRI would not be suitable:

• Implantable cardiac device: In general, magnetic resonance imaging is not recommended for people with pacemakers, cardio-defibrillators, or other implantable cardiac devices.
• Metal fragments or implants: Patients with metal fragments in their body or with a metal implant (pedicle screws, bone compression plates, or aneurysm clips) cannot undergo an MRI scan. The magnetic field emitted during the examination can dislodge the metal object.
• Claustrophobia: People suffering from severe claustrophobia are not advised to have an MRI scan, as they may not be able to withstand the conditions of the examination. There are, however, open MRI systems that are less likely to cause anxiety. The solution may also involve sedating the patient.
• Tattoos: Patients with large, dark-colored tattoos should not undergo an MRI scan, as the ink in the tattoos may contain metals, which are sensitive to the strong electromagnetic fields of the MRI.

There are two situations in which computed tomography is highly inadvisable or even impossible:

• Pregnancy: Pregnant women should not undergo CT scans, particularly during the first trimester of pregnancy. Exposure of the fetus to radiation can cause serious developmental problems.
• Previous exposure to large doses of radiation: The amount of radiation absorbed during a computed tomography scan represents a potential danger for patients who have already had several examinations with radiation throughout their lives.