In this blog post, we’ll break down the principles, pros and cons, and clinical roles of MRI and CT in an easy-to-understand way.

Why Are Imaging Tests Important?

In modern society, medical technology continues to advance, and people want to live longer, healthier lives. Against this backdrop, the demand for and research into reliable diagnostic technologies have become increasingly important, and among these, diagnostic testing and treatment form the very foundation of healthcare. CT and MRI are two representative imaging tests that the general public most commonly encounters when visiting a hospital, yet their principles and processes are often poorly understood.

Principles and Components of CT (Computed Tomography)

CT imaging is performed by entering a large, circular machine; unlike simple X-ray images, it produces cross-sectional images of the human body. This reduces the overlap of structures and allows for a clear view of lesions or anatomical structures.
The core of a CT machine is the X-ray generator, which consists of a high-voltage generator and an X-ray tube. The X-ray tube is the device that actually produces X-rays, while the high-voltage generator produces the tens of kilovolts of voltage required to power the tube. In the early days, X-rays were generated using simple gas tubes (Crookes tubes), but efficiency improved significantly with the introduction of high-vacuum tubes containing tungsten filaments, known as Coolidge tubes.
Furthermore, the development of rotating anode tubes, which feature a rotating anode, reduced heat generation and enabled the production of stronger X-rays. This allowed for the creation of various voltage and current conditions required for examinations, making it possible to perform prolonged scans and acquire high-quality images.
High-voltage generators typically receive an input voltage of 100–200 V (from a single-winding transformer, etc.) and step it up to 35–95 kV in the subsequent process; depending on the application, a wide range of voltages from 20 kV (for soft X-ray imaging) to 150 kV can be used. Voltage affects the “quality” of X-rays; higher voltages result in shorter wavelengths and greater penetrating power, while current is proportional to the “intensity” of the X-rays. A control panel for adjusting the exposure time is also essential.

Principles and Imaging Characteristics of MRI (Magnetic Resonance Imaging)

MRI works by placing the patient inside a strong magnetic field and briefly exposing them to a radiofrequency (RF) signal of a specific frequency to excite hydrogen nuclei. The system then uses a computer to calculate the signals emitted as these nuclei return to their stable state (relaxation), thereby creating an image. The amplitude of the measured signal varies depending on the density of hydrogen atoms in the tissue and the unique relaxation time of each tissue type.
MRI images are typically obtained as T1-weighted and T2-weighted images. Simply put, T1 and T2 refer to the different time scales over which hydrogen nuclei lose energy after excitation; most lesions tend to appear as lower signal intensity (darker) than normal brain tissue on T1-weighted images and higher signal intensity (brighter) on T2-weighted images. By utilizing this difference, MRI offers excellent contrast in soft tissues and superior ability to distinguish lesions.

Clinical Differences and Applications

Although both CT and MRI produce cross-sectional images, they operate on different physical principles. CT uses X-rays to create images, while MRI uses magnetic fields and radiofrequency signals. This difference leads to distinct advantages and disadvantages for each.
The advantages of CT include relatively low cost and shorter examination time, making it rapid and useful in emergency situations or for conditions such as fractures and lung diseases. In contrast, MRI offers high resolution and contrast in soft tissues, making it the standard test when examining soft tissues such as muscles, ligaments, and brain disorders.
MRI produces fewer artifacts caused by bone, allowing it to clearly visualize areas such as brain tumors located near the occipital bone (back of the head).
Furthermore, while occlusive cerebrovascular lesions such as cerebral infarction are often difficult to detect with early-stage CT, MRI can detect them within a few hours of onset and is capable of identifying small cerebral infarcts measuring 5 mm or less, as well as demyelinating lesions associated with multiple sclerosis (MS).
The ability to “freely obtain any desired cross-section,” once considered a unique advantage of MRI, has become less distinct due to the recent widespread adoption of multi-channel (multi-detector) CT, which allows for the reconstruction of images after scanning to produce various cross-sections and three-dimensional (3D) images. To put it simply, this can be understood as a reconstruction capability similar to converting a flat print into a three-dimensional object using a 3D printer.

Current Status and Future Outlook

MRI and CT are fundamental and essential tools for diagnosing lesions in today’s medical field. Doctors use these images to diagnose patients’ illnesses, determine treatment plans, and protect the lives of patients and their families.
While there is a high likelihood that alternative technologies will be developed in the future to reduce the risk of radiation exposure, CT and MRI have played a transitional and essential role in making such advancements possible. Although the two technologies have distinct differences and applications, they ultimately share the common goal of detecting abnormalities in our bodies and ensuring our safety.