The following text gives a brief introduction to the basic and fundamental physics of MR imaging. The unnecessary physics and technical jargon have been deliberately avoided.
Magnetic Resonance Imaging (MRI) is an imaging modality based on an interaction between transmitted radiofrequency (RF) waves and hydrogen nuclei in human body under the influence of a strong magnetic field.
The simple single steps of an MR examination can be described:
- The patient is placed in a magnet (MRI scanner).
- A radiowave is sent in.
- The radiowave is turned off.
- The patient's body emits a signal.
- The signal is received and used for reconstruction of the image.
- Normally, protons in hydrogen atoms (which are abundant in our body) are moving in a random fashion. Each proton in hydrogen nuclei behave like tiny magnets, always spinning on their axes. The protons behaving like little magnets align themselves when placed in an external magnetic field (magnetic), but still keep spinning. They are aligned in two ways—either parallel or antiparallel to the external magnetic field depending upon their energy states. The magnetic forces of net aligned protons add up their forces in the direction of the external magnetic field. At equilibrium, net magnetization is parallel to the main axis of the external magnetic field. This is called longitudinal magnetization.
- 2A radiofrequency (RF) pulse is now sent into disturb these protons. The spinning protons are disturbed and they wobble a toy spinning ‘top’. When the RF pulse and the protons have same frequency, the protons pick up some energy from the radiowave, by a phenomenon called resonance. Thus, RF pulse tips longitudinal magnetization into the transverse plane, creating “transverse magnetization”. Some of the protons pick up energy, and go from a lower to a higher energy level. The radiofrequency pulse exchanges energy with the protons, and change their energy state.
- When the RF pulse is switched off, the protons begin to lose their excess energy by a process known as relaxation which are determined by two time constants, T1 and T2 which are different and independent processes. The electrical signal given by the relaxing protons is received by an antenna and is used for generating images.
- The number of free hydrogen nuclei determines the exchange of energy, thereby the relaxation time constants (T1 and T2) and contribute to the final signal. The bound hydrogen nuclei have less signal (e.g. cortical bone), while free hydrogen nuclei have more signal (e.g. fat). Different tissues have different T1 and T2 relaxation times under the same magnetic field. The differences in the relaxation times of different tissues are the key to the excellent contrast among them on the MR image created. T1-weighted images (T1W) are used for tissue discrimination, while T2-weighted images (T2W) are very sensitive to the presence of increased water and to differences in susceptibility between tissues.
- T1 or T1 (T-one) (Spin-lattice, thermal, or longitudinal) relaxation time is measured in milliseconds. T1 reflects the characteristic time constant for spins to align themselves with the external magnetic field. T1 weighted (T1W) sequence are designed to distinguish tissues with differing T1 relaxation times. T1W image is one whose contrast is mainly determined by T1 relaxation time. Tissues with a short T1 time (e.g. fat) appear bright, while tissues with a long T1 time (e.g. water) appear dark in T1W images.
MR WORKING IN A NUTSHELL
- 4T2 (T-two) (Spin-spin or transverse): Relaxation time reflects the characteristic time constant for loss of phase coherence among spins, caused by interactions between the spins, resulting in loss of transverse magnetization and MR signal. T2 weighted (T2W) sequence are designed to distinguish tissues with differing T2 relaxation times. T2W image whose contrast depends primarily on T2 relaxation time. Tissues with short T2 time (e.g. water) appear bright, while tissues with long T2 time (e.g. fat) appear dark in T2W images.
How it all Works in a Nutshell?
The basic components of an MRI scanner include the main magnet, gradients, and coils. The patient lies on the table along the direction of the magnetic field. All of the protons within the patient's body align with this main magnetic field. The protons are then excited by a radiofrequency (RF) pulse which elevates the hydrogen atoms to an excited state. These hydrogen atoms then relax to their normal resting state. However, the time that it takes for each hydrogen atom to relax depends on their local tissue environment. Hydrogen atoms in water relax at different rates than hydrogen in soft tissue. It is this difference in relaxation times that enables us to generate an MR image.
How are Images Obtained?
Magnetic resonance imaging (MRI) uses magnetic fields and radiofrequency (RF) waves to create images. It is most often tuned to create images by manipulating protons (hydrogen atoms, high in concentration in water and fat within the body). Differences in proton density and the molecular structure surrounding them leads to protons giving off different RF signal characteristics which results in tissue contrast in the MR image.5
ADVANTAGES OF MRI
- Non-ionizing, for they do not use X-ray as medium for imaging.
- Multiplanar imaging is automatically possible, for images in sagittal, coronal and transverse planes are generated simultaneously.
- Superior contrast in tissue give exquisite anatomical details.
- Certain tissue diagnosis is possible, e.g. lipoma, edema, age of hemorrhage, etc.
- MR myelogram is created without injection of any contrast medium.
- In tumor imaging, it gives exact anatomical details regarding the tumor limits, edema limits, vascularity, etc.
DISADVANTAGES OF MRI (COMPARED TO CT SCAN)
Pertaining to lumbar imaging:
- It has low sensitivity for calcium, therefore cannot diagnose calcification clearly.
- It has low sensitivity for acute hemorrhage.
- Scan time is prolonged.
- Contraindications prevent certain patients from entering the MRI system. Patients with metallic implants such as cochlear implant, steel sutures, pacemakers, etc. are not allowed inside the MR scanner.
- Patients with claustrophobia cannot tolerate the study and some young children may need anesthesia.
- Intravenous contrast agents may be needed.
MULTIPLANAR CAPABILITY IN MR
