2The 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.
MRI (Magnetic Resonance Imaging) 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 magnetisation is parallel to the main axis of the external magnetic field. This is called longitudinal magnetisation.
- A radiofrequency (RF) pulse is now sent in to disturb these protons. The spinning protons are disturbed and they wobble like 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 (RESONANCE). Thus RF pulse tips longitudinal magnetization into the transverse plane, creating “transverse magnetisation”. 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 (IMAGING).
- 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 4used 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.
T2 or T2 (“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.
- Both T1W and T2W are used in routine spinal imaging. These two TIW, and T2W imaging proctocols are the basic proctocols upon which several new proctocols are created to get more details about disease processes.
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 like 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.
Advantages of MRI
- Non-ionising, 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.