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CT, MRI and PET are the most common imaging tests that doctors would suggest when they suspect a disease or an injury. Find the chunks of capitalized alphabets confusing? You are not alone here. In this article, we will explain the differences between these medical procedures and how doctors select a suitable test for you.
CT is computed tomography, previously known as CAT (computed axial tomography). It uses X-rays and computers to create cross-sectional images of the internal structure of the human body.
The motorized X-ray source rotates around the patient such that multiple projections of X-rays pass through the patient and are picked up by an electronic detector array. Each full rotation creates an image of a “slice” of the object, and the motorized bed moves into the gantry incrementally until the desired number of image slices is obtained. The image slices can be stacked together by a computer and the 3D internal structure of the object can be shown.
MRI stands for Magnetic Resonance Imaging. It produces detailed images of various tissues and organs through the use of powerful magnetic fields, electromagnetic waves and computer programs.
MRI uses powerful magnets to create a strong magnetic field that drives protons in the body to align with it. When a radiofrequency current (low-energy electromagnetic wave) is pulsed through the patient, the protons are excited and spin out of equilibrium, straining against the magnetic field's pull. The MRI sensors can detect the energy produced as the protons realign with the magnetic field when the radiofrequency field is switched off. The quantity of energy released and the time it takes for the protons to realign with the magnetic field vary depending on the environment and the chemical constitution of the molecules. On the basis of these magnetic properties, physicians can distinguish between distinct types of tissues.
PET stands for positron emission tomography and is a type of nuclear medicine imaging. Nuclear medicine uses radiotracers, molecules that are labeled with radioactive material, to pinpoint molecular activity. Therefore, it not only can show the structure of the organs but also highlight the bodily functions.
In PET, the radiotracer is injected into the human body, and its decay produces positrons. A positron is a small particle with roughly the same mass as an electron but positively charged (opposite to an electron). They react with electrons in the body, combining with and annihilating each other, producing energy in the form of 2 photons that eject in opposite directions. The detectors in the PET scanner measure these photons and create images of internal organs.
PET/CT and PET/MRI are imaging techniques that perform 2 tests at once, combining their advantages. They are particularly useful in oncology – detecting tumors and any metastasis, staging the cancers and evaluating the treatments. PET/MRI, in addition, provides great details for applications in cardiology and neurology.
CT | MRI | PET | PET/CT | PET/MRI | |
Good for detecting | Hard Tissues | Soft Tissues | Cell functions | Cancers | Cancers, cadiodiseases, neuro diseases |
Radiation | Higher, taking multiple X-ray images | None | Lower, only the tracer is radioactive | Highest | Lower, only the tracer is radioactive |
Scanning Time | Short | Long | Medium* | Long | Long |
Cost** | Cheapest | Medium | Expensive | Very Expensive | Most Expensive |
Limitations | May not be suitable for pregnant women due to its ionizing radiation dosage |
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| Coregistration error when information is not received simultaneously from PET and CT | The scanner is so expensive that it may not be widely available (Currently not available in public hospitals in Hong Kong) |
*excluding the time needed for the radiotracer to be absorbed
**for the same body part
CT, MRI, PET, PET/CT and PET/MRI are all painless. However, some PET (also PET/CT and PET/MRI) scans may require IV injection of radiotracers, causing slight pain.
The answer depends on whether your mask contains any metal parts (e.g. the nose piece). In CT (and PET/CT) scans, if it overlaps with the area of interest, then it may interfere with the images. In MRI (and PET/MRI) scans, it is very dangerous to have metal on you.
This article was independently written by Healthy Matters. It is informative only and not intended to be a substitute for professional medical advice, diagnosis or treatment. It should not be relied upon for specific medical advice.
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