Magnetic resonance imaging (MRI) is a noninvasive diagnostic technique that is used to visualize internal body structures. It is based on nuclear magnetic resonance (NMR) and uses high-flux magnets.
Tissues that contain free hydrogen nuclei generate varying signals when bathed in pulses of radiofrequency (RF) energy. These signals are mathematically transformed into images.
What is MRI?
MRI uses high-powered magnets to scan the body, enabling doctors to see tissue structures in greater detail than other imaging methods. It’s particularly good for soft tissues such as muscles, ligaments and tendons. It’s also very good for scanning bones and cartilage, allowing doctors to detect herniated discs, spinal cord compression or bone disease.
Inside the scanner, you lie on a motorised bed that slides into a tube open at both ends. As the machine starts to scan, protons in the water that makes up your tissues begin to align with the magnetic field and send out radio signals. These are picked up by receivers and turned into images by a computer. The time it takes for the protons to re-align and the energy released during this process tells doctors about the type of tissues they’re looking at.
The radiographer who operates the computer will be in a separate room to keep their own magnetic field away from yours. The MRI scanner is quite noisy, and it’s important to stay still because movement can blur the images. Some patients need a sedative to help them stay still. You can choose to have music playing or earplugs in place to block the noise. Depending on the scan, you might have leads placed on your chest to monitor your heartbeat or have a plastic tube inserted into a vein to give you a small dose of dye (gadolinium) that helps certain tissue and blood vessels show up more clearly.
You’ll be asked to change into a gown and to leave any metal objects, including jewellery, in a locker. You might also need to remove anything with metal clips, buttons, fasteners or buckles, as these can interfere with the scanning process.
The Birth of MRI
Aside from X-rays, Magnetic resonance imaging (MRI) is one of the biggest medical breakthroughs of the 20th century. MRI uses powerful magnets, radio waves and computers to create clear, black-and-white images of the body that cannot be seen with other techniques. It is so accurate that it can diagnose some medical problems without any surgery, or even with a needle.
The physics behind MRI dates back nearly a century to the 1920s, when physicists like Nils Bohr and Isidor Isaac Rabi discovered distinct lines in the absorption spectra of atoms. These were attributed to the discrete magnetic moments of certain particles. It was later found that these moments could be measured by applying a magnetic field to the atoms.
In the 1950s, a Stony Brook University Ph.D. student named Raymond Damadian developed the first MR scanner. He experimented with a heavy water sample and was the first to produce a one-dimensional NMR spectrum.
The next step was to apply the technology to living tissue. To do this, it was necessary to understand how differences in the chemical environment of water protons affects their ability to relax back to their natural state after being induced with a radio-pulse kick. This is how the contrasts and anatomic detail of MRI are created.
Mansfield built on this knowledge by using a gradient to image liquids with high water content, then moving on to live tissues. His work was a success, and in 1978, he produced the first MRI scan of a human body part, a cross-section of a finger.
By the 1980s, Siemens in Erlangen was developing and producing the MRI scanners that are now used worldwide. A pioneering development was the fusion of MRI’s millimeter-precise soft tissue anatomy capabilities with positron emission tomography (PET)’s molecular imaging. This gave doctors the best of both worlds and opened up new fields of diagnostics.
The First MRI Scan
An MRI scan uses powerful magnets, radio waves and a computer to produce pictures of organs and tissues without using ionizing radiation. It helps doctors diagnose disease and injury. It is safe and painless. It’s important to lie still during the scan, because movement can distort the images. You may be given drugs or a buzzer to press when you are supposed to stop moving. You’ll be given earplugs or headphones, because the scanner can make loud tapping noises (which are caused by the electric current in the gradient coils being turned on and off).
An image of the body is produced when hydrogen proton, which are found in water molecules in all tissues, are scanned with a magnetic field inside the body-shaped MRI machine. When the magnetic field is switched on and off in quick pulses, different types of tissue protons realign at a different rate and give off unique signals that are detected by the MRI scanner and recorded by the computer. The radiologist then constructs detailed pictures of the body part being studied.
Unlike X-rays, which image calcium and are only useful for bones, MRI is sensitive to water in the body, so it can detect many different diseases. A radiologist can also identify certain tissues, such as fat or muscle, by their specific signal. They can then select a sequence that will suppress the signal from these tissues to better see abnormalities.
Some people, particularly those with claustrophobia, have difficulty tolerating long scan times in the narrow tunnel of an MRI scanner. However, NIBIB-funded researchers are working to improve the procedure to reduce the discomfort for patients. This may include techniques to help them relax during the scan, such as visual imagery and relaxation strategies, as well as sedation or anesthesia.
The Second MRI Scan
MRI scanners have become much faster and more accurate over the years, allowing doctors to get clearer images of body structures. They can even use a contrast agent (such as gadolinium) to help the tissues and organs show up more clearly in the image. This helps them make a diagnosis, plan treatment, or find out how well a treatment is working.
But MRI can be difficult to interpret, because it shows the same structure in different ways. A tumor may appear different on two separate scans because it has moved, or it might appear to have grown even though the growth is actually shrinking. This can lead to a lot of discussion and debate between radiologists over whether there really is a difference or not.
The way MRI works is that it uses a powerful magnetic field to create images of the body. Its strength is that it can detect very small changes in the structure of tissue, and because it does not use radiation, it is one of the safest imaging techniques available.
MRI can also be used to spot some kinds of abnormal growths, such as brain tumours. They can be hard to spot when they are surrounded by healthy tissue, but MRI makes it possible to see them clearly. The high resolution of MRI also allows doctors to distinguish between the various types of growth, such as solid or fluid-filled ones.
Before an MRI scan, patients should tell their healthcare provider about any metal objects or implanted electronic medical devices they have in the body. The strong magnets and radio wave signals used during an MRI can cause heating or movement of some implanted medical devices, which could result in health or safety issues.
The Third MRI Scan
MRI scanners create detailed images of the body’s internal organs and tissue. These images are three-dimensional and can be viewed from many different angles. They produce more information than other diagnostic imaging tests, such as computed tomography (CT) and X-rays. They can also be used in conjunction with a procedure called magnetic resonance spectroscopy, which is used to identify tumors based on their chemical makeup. In addition, an MRI scan can detect and evaluate problems such as torn ligaments from sports injuries, disk abnormalities in the spine, and tumors of the bones or soft tissues.
During an MRI scan, the magnetic field causes hydrogen atoms in the body to align with a north and south pole, similar to a bar magnet. When these atoms are scanned with a second magnetic field, their axes are then uniformly aligned. This allows the radiologist to create a map of the body’s structures and tissues. This map can be analyzed by computer to reveal important details about a disease, such as detecting tumors, locating damaged blood vessels, and determining the presence or extent of an injury.
One of the main challenges in MRI scanning is balancing the scan speed with maintaining the body’s temperature, because the magnetic field absorbs electromagnetic energy and produces heat. This can be difficult for patients with implanted or accessory medical devices such as artificial joints, stents, or pacemakers. To overcome this challenge, a new type of MRI scanner known as an open MRI was created to provide more comfort for patients with anxiety or who have trouble sitting still for the longer scan times usually required by conventional closed MRI machines. Additionally, higher-field MRI scanners have been developed, with 3T and 7T MRI machines being cleared for clinical use. The higher magnetic field strengths allow for superior image quality and a higher signal-to-noise ratio, resulting in more detail for MRA and enhanced contrast between gray and white matter.
