MRI is a medical screening technique that uses computer-generated radio waves and magnetic fields to create clear images of the tissues and organs in the body. MRI machines are large and have tube-shaped magnets. MRI scanning is used to diagnose and monitor treatment.
Magnetic resonance imaging (MRI)
Principles and techniques
The magnets in the MRI machine create a strong magnetic field which aligns the hydrogen atoms’ protons (found in the water that makes up living tissues) when exposed to a beam of electromagnetic waves. These electromagnetic waves spin the protons of the body and create a blurry signal. The magnet’s receiver portion detects these signals. The image is produced after processing the receiver’s information by a computer.
MRI takes pictures of the body part that contains water, and the detail comes from the interference of different body tissues with electromagnetic waves. The signals emitted from the electromagnetic waves released from water molecules are tiny and noticeable with keen focus. Initially, water molecules are stimulated into releasing waves and then with high accuracy the locations of those waves are recorded by an MRI device.
The human body is majorly made of water. So, the water molecules have two hydrogen atoms. A single proton is present at the centre of the hydrogen atom. This proton is similar to a bar magnet with “north” and “south” poles. These poles in protons align themselves to the magnetic field of MRI magnets. This alignment will not cause any disruption in the chemical properties of the tissues, and the body continues to function normally while taking MRI.
Parallel Imaging
One of the magnetic resonance techniques is parallel imaging. It is designed to reduce the scanning time. NCBI says that parallel imaging is used for accelerating the acquisition of MRI imaging and has made new applications of MRI. It works by reducing k-space data (MRI’s spatial frequencies representing numbers) with an array of receiver coils.
The MRI Scanner
At present, MRI scanners use cryogenic superconducting magnets. These magnets are extremely cooled (close to zero) to conduct huge currents. NCBI says that the field strength system of magnets in MRI is used from 1.5 Tesla (T) to 3 T.
Improved signal-to-noise ratio (SNR) and higher spatial, spectral and temporal resolution are the advantages of the higher field strength systems. These advanced SNRs can be used to reduce imaging time.
T1- and T2-Weighted MR Imaging
The process of nuclear spin returning to its thermal equilibrium after absorbing radiofrequency (RF) energy is called relaxation. Relaxation is of two types. They are longitudinal and transverse relaxations. They are usually described by time constants T1 and T2, respectively.
T1 is also called spin-lattice relaxation or longitudinal relaxation. To reach thermal equilibrium in spin-lattice relaxation, energy transfer is observed between the nuclear spins and their surroundings.
T2 refers to transverse relaxation or spin-spin relaxation, which measures how fast the spins exchange energy in the magnetic plane (MXY), says NCBI.
Magnetisation Transfer Imaging (MT)
NCBI states that magnetisation Transfer (MT) imaging measures the brain’s compartments. But magnetisation transfer (MT) imaging can be affected by the differences in heavy metal concentration, membrane fluidity and total water content.
MT manipulates tissue contrast and allows the magnetisation transfer ratio (MTR) measurement.
A quantitative tissue characteristic is the magnetisation transfer ratio (MTR). It reflects the behaviour of protons bound to macromolecules which are MR-invisible. The parenchymal changes in the brain are measured using MTR, but they cannot be detected using standard MRI techniques.
The protons in tissues exist in two pools – free and bound. Protons in body water are mobile, which make up a free pool. It has a narrow spectral line with long relaxation times. In contrast, protons in proteins and macromolecules or membranes are MR invisible. It has a broad spectral line, and a shorter relaxation time gives lower SNR.
Through direct interaction between spins, transfer of nuclei, or direct chemical means, magnetisation is transferred between two pools in both directions. The RF pulse excites the protons of the free and bound pool, where the exchange of protons takes place between two pools for saturation. Once saturation is observed, the MR signal gradually decreases.
With the help of magnetisation transfer imaging, the MR invisible protons are also measured with the help of the MT imaging technique.
Diffusion-Weighted Imaging
Brownian motion – random and uncontrolled movement of particles in a fluid constantly collides with other molecules. NCBI says that the differences in the Brownian motion of water molecules generate a contrast signal in human body tissue. This is called the diffusion-weighted imaging (DWI) method. The DWI method is used to evaluate the micro-architecture and molecular function of the human body.
DWI was famous for detecting acute cerebral ischemia, brain tumours and multiple sclerosis. The water molecules moves randomly and equally in all directions, which is called isotropic. When the water molecules move parallel to white matter but perpendicular to them and not equal in all directions. This movement is called anisotropic.
The motion of water molecules is restricted in structured environments due to physical surroundings. In the brain, the water molecules are restricted from moving due to the microstructure of grey and white matter.
Apparent Diffusion Coefficient
According to NCBI, the magnitude of diffusion in tissues’ water molecules is measured by the apparent diffusion coefficient (ADC). This magnitude is measured using MRI and diffusion-weighted imaging (DWI).
Fractional Anisotropy
Fractional anisotropy is a measurement used to measure the isotropic and anisotropic movement of water molecules in the human body. Science Direct says that the measurement of the isotropic and anisotropic movements ranges from 0 to 1, respectively. The movement of cerebrospinal fluid is isotropic, and the movement of the fibre bundle is anisotropic.
Uses of MRI
Medical Industry
Medical industry or healthcare professionals use MRI scans to diagnose various tumours. MRI is useful for diagnosing damage in the brain and spinal cord.
Procedure
How it is performed
MRI scan is a painless process. The duration of this process varies based on the area being scanned and the number of images being taken. Some MRI scans include the process of sedative injection. This may cause dizziness in some patients.
Before the scan
In some cases, patients are allowed to eat, drink and take medicines as usual before an MRI scan. In some other cases, the patients are asked not to eat, drink or take medicines a few hours before the MRI scan.
When you go for an MRI scan, you are supposed to fill a questionnaire form about your medical history and health condition. Once the questionnaire is completed, you are asked to sign a consent form to proceed with the scan.
Contrast agent
A contrast agent (dye) is a part of some MRI scans. These agents are used to get clear and detailed images of certain blood vessels and tissues in your body. In some cases, these contrast agent causes some side effects like dizziness, feeling sick, a headache and skin rashes.
NHS in England says organ and tissue damage may be caused in people with severe kidney disease while injecting a contrast agent. To avoid such risk, people with kidney disease are asked to take a blood test to check the kidneys’ function, and then the physician will determine whether to proceed with an MRI.
Anaesthesia and sedatives
As an MRI scan is painless, anaesthesia and sedatives are unnecessary. But then the doctors will use it for patients with claustrophobia (fear of enclosed spaces).
If a sedative is injected in you during an MRI scan, you are asked to arrange for a family member or a friend to drive you home and accompany you for at least a day.
It is important to stay still during an MRI scan, as any movement will disturb the image. So, babies and children will get a shot of general anaesthesia before an MRI scan.
During the scan
After getting a shot of anaesthesia or sedative, the patient will be taken to the scanner. The MRI technician will communicate with the patient through the intercom to ensure that the patient is comfortable. After ensuring the patient is ready for the scan, the technician will start the scan.
It is perfectly normal to hear a loud clanging sound coming from the MRI machine. If the patient is uncomfortable with the procedure, they can inform the technician through the intercom and request to stop the scanning process for a while.
After the scan
After completing the scanning process, the patients are asked to go home once the radiologist is satisfied with the images after examining them. Then the scanned report will be prepared by the radiologist for the doctor’s consideration.
Effects of MRI
Benefits
A clear differentiation of fat, muscle, water and soft tissues is detected in MRI when compared with a CT scan, says NCBI. The MRI scan images provide information to the physicians to diagnose various diseases.
Adverse effects
- The MRI scanning machines produce a loud knocking noise which may affect hearing if ear protection is not used.
- The loud sound from the MRI machine may cause a twitching sensation.
- The radio waves used during the MRI scan may lead to burns or increase the body temperature.
Risk factors
Patients with implants, external and accessory devices
The presence of metal in your body may distort the image and may be attracted the magnets as powerful magnets are used in MRI machines. Once you enter the scanning room, you are asked to remove the external ornaments for safety concerns.
If the metal affixed in your body (internal and external) are MRI-safe, you will be permissible for an MRI. Your physician will ensure that the metal implants in your body are MRI safe by asking a few questions. Some internal metal devices include
- Artificial heart valves,
- Implanted nerve stimulators,
- Cochlear implants and
- Intrauterine device.
MRI in the Future
There is a progression in the cost and accessibility of MRI nowadays. In the forthcoming years, there may be a reduction in the cost of MRI as the increased availability is expected. In future, advanced magnets with an increased magnetic field strength in MRI machines will be commonly used.
The importance of MRI machines is expected to be enhanced with high resolution and tissue contrast.
Conclusion
MRI scanning is a fast-developing medical screening technique. Through this technique, doctors are able to detect diseases in the organ and tissue in the body clearly. In future, it is believed that there will be an advanced MRI machine used in common for scanning severe ailments.
FAQs
Do MRI's have side effects?
1. Feeling sick,
2. Headache,
3. Dizziness and
4. Skin rashes.
